Molecular weight characterization and rheology of lignins for carbon fibers

MISSING IMAGE

Material Information

Title:
Molecular weight characterization and rheology of lignins for carbon fibers
Physical Description:
xix, 169 leaves : ill., photos ; 29 cm.
Language:
English
Creator:
Schmidl, Gerald Wolfgang, 1961-
Publication Date:

Subjects

Subjects / Keywords:
Rheology   ( lcsh )
Lignin   ( lcsh )
Molecular weights   ( lcsh )
Carbon fibers   ( lcsh )
Chemical Engineering thesis Ph. D
Dissertations, Academic -- Chemical Engineering -- UF
Genre:
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1992.
Bibliography:
Includes bibliographical references (leaves 159-168).
Statement of Responsibility:
by Gerald Wolfgang Schmidl.
General Note:
Typescript.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 029634518
oclc - 28997462
System ID:
AA00013563:00001


This item is only available as the following downloads:


Full Text








MOLECULAR WEIGHT CHARACTERIZATION AND RHEOLOGY
OF LIGNINS FOR CARBON FIBERS















By

GERALD WOLFGANG SCHMIDL


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

1992


UMVi.iiTy OF FLORIDA LiERARIS


























Copyright 1992

by

Gerald Wolfgang Schmidl




























To my parents, Hans and Hilda, and to my wife, Viana










ACKNOWLEDGEMENTS


The author wishes to thank Dr. A.L. Fricke for his guidance and friendship

throughout the many years required to complete this work. His extensive knowledge

and experience, and his hard driving work ethic, have been very inspiring. He also

wishes to thank Dr. C.L. Beatty for his friendship and advice, and for the use of his

equipment. The author would also like to thank Dr. R.S. Drago, Dr. G. Hoflund,

and Dr. C.W. Park for their willingness to participate in the review and critique of

this dissertation, and Mr. Stan Sobczynski at the Department of Energy for providing

ample funding for this project.

The members of Dr. Fricke's research group: Daojie Dong, Allan Preston,

Barbara Speck, and Abbas Zaman, and fellow suffering graduate students, also

deserve the author's sincere appreciation for friendship and support. The author also

thanks Dr. Bill Toreki for performing the fiber carbonization work, David Bennett

for his invaluable help in measuring tensile properties of the carbonized lignin fibers,

and Ron Baxley, Tracey Lambert, and the office staff, for their help in solving the

numerous mechanical and bureaucratic problems that frequently arose.

Finally, the author wishes to thank Tito and Adela Ostrea, his loving parents

Hans and Hilda, and his wife and best friend, Viana, for their love and support

during this long and arduous endeavor.










TABLE OF CONTENTS




ACKNOW LEDGEM ENTS ............................................................................ iv

LIST OF TABLES ...................................................................................................... ix

LIST OF FIGURES ................................................................................................... x

KEY TO SYM BOLS ................................................................................................ x iii

KEY TO ABBREVIATIONS ............................................................................... xvi

ABSTRACT ................................................................................................................. xviii

CHAPTERS

1 INTRODUCTION .................................................................................... 1

1.1 Overview ...................................................................................... 1
1.2 Research Objectives .................................................................. 2
1.3 Lignin ................................................................................. 2
1.3.1 Occurrence in W ood ................................................ 2
1.3.2 Structure ...................................................................... 3
1.3.3 Lignin Utilization and Applications ...................... 5
1.4 Pulping Processes ...................................................................... 7
1.4.1 Kraft Process .............................................................. 7
1.4.2 Organosolv Process ................................................... 8
1.5 Carbon Fibers ........................................................................... 9
1.5.1 Properties and Applications ................................... 9
1.5.2 Precursor Materials and Commercial Fibers ....... 11
1.5.3 Processing Steps ....................................................... 12
1.5.4 Carbon Fibers from Lignin ................................... 14
1.6 Fiber Spinning .......................................................................... 14
1.7 Need for Lignin Characterization ...................................... 15
1.8 Overview of Subsequent Chapters ..................................... 16






2 LIGNIN SELECTION AND PURIFICATION ................................. 17

2.1 General Considerations .......................................................... 17
2.2 Lignin Selection ....................................................................... 18
2.3 Lignin Purification .................................................................. 20
2.3.1 Kraft Lignins ............................................ ..... 20
2.3.2 Organosolv Lignins .................................................. 20
2.3.3 Storage ...................................................................... 23

3 MOLECULAR WEIGHT CHARACTERIZATION .................... 24

3.1 Introduction .............................................................................. 24
3.2 SEC Theory ............................................................................. 25
3.2.1 Separation Mechanism ............................................ 25
3.2.2 Detection ................................................................... 27
3.2.3 Calibration ................................................................ 28
3.2.4 Nonsize Exclusion Effects ..................................... 30
3.3 Background and Literature Review ....................................... 31
3.3.1 Introduction ............................................................... 31
3.3.2 Traditional SEC Analyses ..................................... 33
3.3.3 Association and Adsorption ..................................... 34
3.3.4 Column Calibration ................................................. 36
3.3.5 Multidetection and Absolute MWD .................... 40
3.4 Experimental Work and Data Analysis .............................. 44
3.4.1 Instrumentation ....................................................... 44
3.4.2 Mobile Phase Selection and Preparation ........... 46
3.4.3 Sample and Standards Preparation ..................... 46
3.4.4 SEC Runs and Data Analysis ............................... 49
3.5 Results and Discussion ........................................................... 50
3.5.1 General Comments on Mobile Phase Evaluation 50
3.5.2 Lignin Analysis in THF ............................................ 51
3.5.3 Lignin Analysis in DMF and DMF Mixed Mobile
Phases ..................................................................... 52
3.5.4 Lignin Analysis in NaOH Solutions .................... 55
3.5.5 Lignin Analysis in DMSO + LiBr Solutions ........ 57
3.5.6 Column Calibration ................................................. 66
3.5.7 Comparison of SEC Results with Previous Work 68
3.6 Conclusions and Recommendations ................................... 70
3.6.1 Conclusions ................................................................ 70
3.6.2 Recommendations for Future Work ................... 71

4 LIGNIN THERMAL ANALYSIS ......................................................... 72

4.1 Introduction .............................................................................. 72






4.2 Theory ........................................................................................ 73
4.2.1 Glass Transition ....................................................... 73
4.2.2 Effect of Plasticizer on Tg ..................................... 75
4.2.3 DSC Principles of Operation ................................... 76
4.3 Background and Literature Review .................................... 78
4.3.1 Introduction .......................................................... 78
4.3.2 Early Work: Characteristic Softening
Temperatures ....................................................... 79
4.3.3 Lignin Tg Studies ..................................................... 80
4.3.4 Enthalpy Relaxation ............................................... 82
4.3.5 Glass Transition Behavior of Plasticized Lignins 83
4.4 Experimental Work and Data Analysis .............................. 85
4.4.1 Instrumentation ....................................................... 85
4.4.2 Sample Selection and Preparation ...................... 86
4.4.3 DSC Experimental Methods ................................. 87
4.4.4 Data Analysis ........................................................... 89
4.5 Results and Discussion .......................................................... 91
4.5.1 Glass Transition Temperatures for Dry Lignins .. 91
4.5.2 Tg s for Solvent Plasticized Indulin AT ............... 95
4.6 Conclusions and Recommendations ................................... 100
4.6.1 Conclusions ............................................................... 100
4.6.2 Recommendations for Future Work ................... 101

5 LIGNIN RHEOLOGY ......................................................................... 103

5.1 Introduction ............................................................................. 103
5.2 Rheometry Theory ................................................................. 104
5.2.1 Viscometric Flows and Material Functions ....... 104
5.2.2 Steady Shear Operation ......................................... 105
5.2.3 Dynamic Shear Operation and Linear
Viscoelasticity ...................................................... 108
5.3 Background and Literature Review .................................... 111
5.3.1 Black Liquor Rheology .......................................... 111
5.3.2 Polymer Rheology ...................................................... 111
5.4 Experimental Work ................................................................ 113
5.4.1 Sample Preparation ................................................... 113
5.4.2 Rheometer ................................................................... 115
5.4.3 Testing Procedures ................................................. 116
5.5 Results and Discussion ............................................................. 118
5.5.1 General Observations ............................................. 118
5.5.2 Steady Shear Behavior ........................................... 119
5.5.3 Dynamic Shear Rheometry ...................................... 121
5.6 Conclusions and Recommendations ...................................... 125
5.6.1 Conclusions .................................................................. 125






5.6.2 Recommendations for Future Work ......................


6 LIGNIN FIBER SPINNING AND CARBONIZATION .............. 127

6.1 Introduction ............................................................................. 127
6.2 Background and Literature Review .................................... 127
6.2.1 Early Japanese Development Work .................... 127
6.2.2 West German Process ............................................ 130
6.2.3 Carbon Fibers from Black Liquor .......................... 131
6.2.4 Fiber Microstructure .............................................. 132
6.2.5 Recent Development Work .................................. 133
6.3 Experimental Work ................................................................ 134
6.3.1 Lignin Fiber Spinning ............................................. 134
6.3.2 Fiber Carbonization ................................................ 136
6.3.3 Fiber Analysis .......................................................... 137
6.4 Results and Discussion .......................................................... 140
6.4.1 Thermogravimetric Analysis ................................. 140
6.4.2 Surface Morphology ............................................... 142
6.4.3 Elemental Composition ......................................... 146
6.4.4 Mechanical Properties ............................................ 147
6.5 Conclusions and Recommendations ................................... 151
6.5.1 Conclusions .................................................................. 151
6.5.2 Recommendations for Future Work ................... 152

7 OVERALL CONCLUSIONS AND RECOMMENDATIONS ....... 154

7.1 Sum m ary ...................................................................................... 154
7.2 Conclusions .............................................................................. 155
7.3 Recommendations for Future Work ................................... 157

REFERENCES ........................................................................................................ 159

BIOGRAPHICAL SKETCH ................................................................................... 169


viii


125










LIST OF TABLES


Table pa

1-1 Performance Properties and Application Areas of Lignin
Products ................................................................................................ 6

1-2 Physical Properties and Applications of Carbon Fibers............ 10

2-1 Lignins Selected for this Study...................................................... 19

3-1 SEC Mobile Phase Selection.......................................................... 47

3-2 Lignin Molecular Weights from SEC in DMSO + 0.1M LiBr
at 85 C ............................................................................................... 64

3-3 Comparison of SEC Results for Mixed Hardwood Kraft and
Organosolv Lignins with Literature Values.............................. 69

4-1 Hansen Solubility Parameters for Lignin Solvents.................... 87

4-2 Temperature Program for DSC Analysis of Dry and Solvent
Plasticized Lignins............................................................................ 89

4-3 Glass Transition Temperatures for Dry Lignins........................ 93

6-1 Lignin Fiber Spinning Conditions................................................. 137

6-2 Lignin Fiber Carbonization Conditions....................................... 138

6-3 Elemental Composition of Lignin Carbon Fibers..................... 147

6-4 Mechanical Properties of Lignin-Based and PAN-Based Carbon
Fibers .................................................................................................. 150










LIST OF FIGURES


Figure page

1-1 Representative Model for Native Softwood Lignin
Structure ............................................................................................... 4

1-2 Lignin Monomers: p-Coumaryl Alcohol (I), Coniferyl Alcohol
(II), and Sinapyl Alcohol (11) ...................................................... 5

2-1 Kraft Lignin Isolation and Purification Scheme............... 21

3-1 Typical SEC Chromatogram for a Softwood Kraft Lignin Run
in DMF at 85 *C on Jordi Gel Mixed Bed + 103 A Columns.. 53

3-2 SEC Chromatogram for a Softwood Kraft Lignin Run in
DMF/EGMPE (98/2) at 85"C on Jordi Gel Mixed Bed + 103
A C olum ns............................................................................................ 55

3-3 SEC Chromatograms for Indulin AT Run in DMSO with
Various Concentrations of Lithium Bromide at 85 C on the
Jordi Gel 103 A GBR Column......................................................... 58

3-4 SEC Chromatograms for Selected UF Kraft Softwood Lignins
Run in DMSO + 0.1M LiBr at 85 C on the Jordi Gel 103 A
G BR C olum n....................................................................................... 60

3-5 SEC Chromatograms for Selected UF Kraft Softwood Lignins
Run in DMSO + 0.1M LiBr at 85"C on the Jordi Gel 103 +
104 A GBR Column Set................................................................... 61

3-6 SEC Chromatograms for Indulin AT, Maple, and Organosolv
Lignins Run in DMSO + 0.1M LiBr at 85 C on the Jordi Gel
103 A G BR Column........................................................................... 62

3-7 SEC Calibration Curve with Narrow MWD Polysaccharide
Standards for the Jordi Gel 103 + 104 A GBR Column Set
Running DMSO + 0.1M LiBr at 85 C....................... 67






4-1 Experimental Definition for the Onset Glass Transition
Temperature...................................................................................... 90

4-2 DSC Scan for S.D. Warren Birch Kraft Lignin. Heating Rate
= 10 C/min in Nitrogen................................................................. 92

4-3 Effect of Lignin Polydispersity on the Breadth of the Glass
Transition Region............................................................................ 96

4-4 Glass Transition Depression for Solvent Plasticized Indulin AT
Lignin.................................................................................................. 98

5-1 Cone and Plate Geometry. (a) Steady Shear Flow; and (b)
Dynamic Oscillatory Shear Flow..................................................... 106

5-2 Steady Shear Rheometry of Indulin AT + 28% NMP at 80 and
1000C .................................................................................................... 120

5-3 Dynamic Oscillatory Shear Strain Sweeps of Indulin AT + 28%
NMP. Frequencies were 1.0 rad/sec at 80 C, and 10 rad/sec
at 1000 C............................................................................................... 122

5-4 Dynamic Oscillatory Shear Rheometry of Indulin AT +
28%NMP at 80 and 100 C............................................................ 123

5-5 A Comparison of First Normal Stress Differences and Storage
Moduli, from Steady Shear and Dynamic Shear Rheometry,
Respectively...................................................................................... 124

6-1 Lignin Fiber Spinning Apparatus.................................................... 135

6-2 Carbonized Lignin Fiber Tensile Testing Apparatus............... 139

6-3 Thermogravimetric Analysis of Fibers Spun from Indulin AT
+ 28% NMP. Normal TGA Curve for Softwood Kraft Lignin
(----) by Masse [62]. Heating Rate = 100C/min in Nitrogen.. 141

6-4 SEM Micrographs for Lignin Fiber. (a) Uncarbonized "Green"
Fiber; (b) Carbonized "B" Fiber................................................... 143

6-5 SEM Micrographs for "B" Carbonized Lignin Fiber................. 144

6-6 Tensile Test for Carbonized Lignin Fiber "A".............................. 148





6-7 Tensile Test for Carbonized Lignin Fiber "B"............... 149










KEY TO SYMBOLS


Symbol Definition

a Mark-Houwink constant

Cp Heat capacity at constant pressure, J/(g- C)

F Total normal force, N

G* Complex shear modulus, Pa

G' Storage modulus, Pa

G" Loss modulus, Pa

K Distribution coefficient of solute;
Mark-Houwink constant

M Molecular weight in Mark-Houwink relationship

1RIn Number average molecular weight

Mp Peak molecular weight

/M Weight average molecular weight

N1 First normal stress difference, Pa

N2 Second normal stress difference, Pa

R Cone, plate radius, mm

r radial position

T Torque, N-m;
Temperature, C


xiii






Tg Glass transition temperature, C

Tg Glass transition temperature for pure polymer, C

Tm Onset melting temperature, "C

Ts Softening temperature, C

t Time, sec

tR Solute retention time, min

v Velocity, m/sec

Vi Pore volume, ml

Vo Interstitial (dead) volume of SEC column, ml

VR Retention volume of solute, ml

VT Total column volume, ml

W2 Weight fraction of diluent, g/g



a Cone angle, rad

y Strain

Yo Strain amplitude

? Shear rate, sec"1

6 Phase shift, rad;
general solubility parameter, (cal/cm3)05

so Overall Hansen solubility parameter, (cal/cm3)0-5

64d Hansen dispersion (nonpolar) parameter, (cal/cm3)05

6h Hansen hydrogen bonding parameter, (cal/cm3)05s

6, Hansen polar parameter, (cal/cm3)05






?7, rlapp Steady shear apparent viscosity, Pa-sec

17o Zero shear rate viscosity, Pa

[N] Intrinsic viscosity, cm3/g

?" Complex viscosity, Pa-sec

r7' Dynamic viscosity (real component of '*), Pa-sec

r7" Imaginary component of 17', Pa-sec

0 Spherical coordinate direction

T Shear stress, Pa

To Shear stress amplitude, Pa

0 Spherical coordinate direction

TY1 First normal stress coefficient, Pa-sec2

2 Second normal stress coefficient, Pa-sec2

n Angular velocity, rad/sec

SFrequency, rad/sec










KEY TO ABBREVIATIONS


ACS

DMF

DMSO

DRI

DSC

DV

DVB

EDS

EG

EGDME

EGMME

EGMPE

FRT

GPC

HPLC

HPSEC

LAILS

MW


American Chemical Society

N,N-Dimethylformamide

Dimethylsulfoxide

Differential refractive index

Differential scanning calorimetry

Differential viscometry

Divinylbenzene

Energy dispersive x-ray spectroscopy

Ethylene glycol

Ethylene glycol dimethyl ether

Ethylene glycol monomethyl ether

Ethylene glycol monopropyl ether

Force rebalance transducer

Gel permeation chromatography

High pressure liquid chromatography

High pressure size exclusion chromatography

Low angle laser light scattering

Molecular weight






MWD

NMP

PAN

PEG

PEO

PID

PMMA

PRT

PS

PSS

PVA

SEC

SEM

TBA

TCE

TEA

TGA

THF

UF

UV/Vis

VPO


xvii


Molecular weight distribution

N-Methyl pyrrolidinone

Polyacrylonitrile

Polyethylene glycol

Polyethylene oxide

Proportional, integral, and derivative

Polymethyl methacrylate

Platinum resistive thermosensor

Polystyrene

Polystyrene sulfonate

Polyvinyl alcohol

Size exclusion chromatography

Scanning electron microscopy

Torsional braid analysis

1,1,1-Trichloroethane

Triethylamine

Thermogravimetric analysis

Tetrahydrofuran

University of Florida

Ultraviolet/visible

Vapor pressure osmometry










Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

MOLECULAR WEIGHT CHARACTERIZATION AND RHEOLOGY
OF LIGNINS FOR CARBON FIBERS

By

Gerald Wolfgang Schmidl

December 1992

Chairperson: Arthur L. Fricke
Major Department: Chemical Engineering

This investigation was initiated to (1) characterize purified lignins, from a

statistically designed pulping experiment, and from commercial sources, for molecular

weights (MW s) and molecular weight distribution (MWD) by size exclusion

chromatography (SEC), to support a larger overall study of kraft black liquor physical

properties, and to (2) study the feasibility of producing lignin-based carbon fibers as

an alternative high value use for lignins. To support the lignin fiber spinning work,

glass transition temperatures (Tg s) for dry and solvent plasticized lignins were

determined by differential scanning calorimetry, and rheological properties of solvent

plasticized lignins were measured by steady and oscillatory shear rheometry. Kraft

softwood, kraft hardwood, and organosolv lignins were studied.

A new SEC method for comparative lignin MWD characterization was

developed which consists of dimethyl sulfoxide + 0.1M LiBr running at 85"C in a


xviii






custom made "deactivated" column, and overcomes persistent lignin association and

adsorption problems. Accurate column calibration methods, such as resolution of

moments, must still be investigated because calculated weight average MW s differed

from fully corrected absolute values by a factor of 3-15.

Lignin Tg s ranged from 130 to 1700 C, which reflect the effect of differences

in pulping conditions on MW. The glass transitions were very broad, and correlated

linearly with polydispersity of MW. The Tg depression for solvent plasticized Indulin

AT (a kraft softwood lignin) was greater with N-methyl pyrrolidinone (NMP), a

weaker hydrogen bonding solvent, than with dimethyl formamide, a stronger one.

The rheological properties of Indulin AT plasticized with NMP were measured

at 80 and 100 C with a cone and plate rheometer. This material exhibited shear

thinning behavior and some degree of viscoelasticity. Apparent viscosity and complex

viscosity both decreased with increasing shear rate or frequency, and first normal

stress difference and storage modulus both increased with increasing shear rate or

frequency. These trends are the same as for synthetic polymer melts and solutions.

Single fibers of Indulin AT + 28% NMP were spun at 100 m/min at 1300C,

and carbonized at 1,0000 C under argon. These fibers had a carbon content of 91%,

and mechanical properties--diameter, tensile strength, modulus, and elongation--of

103 3.5 /m, 150 20 MPa, 49.1 14.4 GPa, and 0.32 0.11%, respectively.

Producing carbon fibers from kraft lignins is currently not a viable alternative

application, but these results were encouraging, and further work in this area is

recommended.


xix












CHAPTER 1
INTRODUCTION


1.1 Overview


lignin is a complex, amorphous, heterogeneous natural polymer, which, after

cellulose, is the most abundant and important natural polymeric substance in the

plant world. It is extracted from wood during pulping operations for papermaking

and is the primary organic component of the black liquor byproduct. Although its

primary use is as a fuel in the pulping process, other applications could include

carbon fiber manufacture. In order to develop alternative applications, a thorough

understanding of lignin structure/property relationships, including molecular weight

characterization and rheological behavior, is necessary.

This chapter identifies the objectives of this research (Section 1.2), and briefly

discusses the structure, properties, and current utilization of lignins in Section 1.3.

A brief description of the dominant kraft pulping process and a newer organosolv

pulping process are given in Section 1.4. An introduction to carbon fibers is given

in Section 1.5 followed by a brief description of the fiber spinning process in Section

1.6. Finally, the justification for this characterization work, and a brief description

of the remaining chapters, is discussed in Sections 1.7 and 1.8, respectively.






2
1.2 Research Objectives


This experimental study has two principal objectives: (1) to characterize

purified lignins, from a statistically designed pulping experiment, and from

commercial sources, for molecular weights and molecular weight distribution by SEC,

and (2) to investigate the feasibility of producing carbon fibers from these lignins.

These two objectives are semi-independent and reflect the dual nature of this work:

basic lignin material properties characterization, and applications development for

purified lignins.

The molecular weight characterization work will support a much larger overall

study of kraft black liquor physical and chemical properties which will benefit the

pulp and paper industry in its long term plan to more efficiently process black

liquors. The development of lignin-based carbon fibers could provide an alternative

high value use for lignins, as compared to its current predominantly low value fuel

use. Three primary types of lignins were studied: kraft softwood, kraft hardwood,

and organosolv lignins.


1.3 Lignin


1.3.1 Occurrence in Wood


Wood is a three-dimensional cellular composite structure consisting of

cellulose, hemicelluloses, lignin, small amounts of extractives such as phenols,

terpenes, and organic acids; and ash. Wood is not a homogeneous material; its






3
chemical constituents are not uniformly distributed, and there are also various types

of cells. Lignin comprises approximately 18-35 weight % of wood, and is

concentrated in the thickest layer of the cell wall. It provides strength to wood by

serving as a matrix to hold the cellulose fibers together. There are two main

categories of wood: gymnosperms (softwoods), such as spruce, fir, pine, and cedar;

and angiosperms (hardwoods) such as oak, maple, and birch [71, 81].


1.3.2 Structure


Lignin has a very complex, heterogeneous, highly branched, amorphous

structure which can vary significantly with morphology (location in cell), cell type

(vessel versus fiber), wood type (softwood versus hardwood), and species. A

representative model for this complex structure is shown in Figure 1-1. In different

cell regions, lignin can be a random three-dimensional network polymer, or a

nonrandom two-dimensional network polymer. Upon delignification, the properties

of the solubilized macromolecules reflect the properties of the network from which

they are derived [22, 37, 81] .

Three phenylpropane monomers, differing only in the number of methoxyl

substituents, polymerize to form lignin. These monomers are p-coumaryl alcohol,

coniferyl alcohol, and sinapyl alcohol, and are shown in Figure 1-2. Lignification is

initiated when a phenolic hydroxyl hydrogen atom is abstracted by the enzyme

peroxidase to form a phenoxy free radical. This phenoxy free radical can be

delocalized to both aromatic and side chain carbon atoms. Because of this


























r 1.-





Figure 1-1. Representative Model for Native Softwood Lignin Structure.
Source: Obst [71].

delocalization, coupling of these radicals can form ether linkages, carbon-carbon
bonds, and bonds to more than one other phenyl propane unit. This results in the
complicated lignin polymer having a crosslinked and three dimensional structure [71].








CH2OH

JH
II
HC




OH

I


CH2OH

CH

J


CH2OH

CH
IIHC
HC


OCH3 H3CO


OH

II


OCH3


OH


Figure 1-2. Lignin Monomers: p-Coumaryl Alcohol (I), Coniferyl Alcohol
(II), and Sinapyl Alcohol (IM). Source: Obst [71].

1.3.3 Lignin Utilization and Applications

Total worldwide lignin production is approximately 100 million tons/year',

and there are currently four main areas of commercial utilization: (1) as a remaining

component in mechanical, high yield semi-chemical, and unbleached chemical pulps,

e.g. in newsprint, (2) as a fuel, (3) as a polymeric product, and (4) as a source of low

molecular weight chemicals [22]. The predominant use for lignin today is as a fuel,

because recovery of the process chemicals in the dominant kraft pulping process is

based on incineration of the spent black liquor, and due to the high heating value of

the organic material in the spent liquor: 23.4 MJ/kg (10,070 Btu/lb) [22].


* Extrapolated from data presented by Glasser and Kelley [33].








Table 1-1. Performance Properties and Application Areas of
Lignin Products.


Performance property


Application areas


1. Dispersing Dispersants for carbon black,
pigments, dyestuffs, clays,
pesticides; cement grinding,
concrete superplasticizer, gypsum
wallboard, oil well drilling muds
2. Complexing/dispersing Boiler and cooling water
treatments, micronutrients,
corrosion inhibition, industrial
cleaners, and protein precipitation
3. Binding Adhesives for board and veneer,
animal feed pellets, printing inks,
foundry sands, ore and coal
briquettes; phenolic resin
substitute, ceramics and
refractories, soil conditioning
4. Emulsion stabilizing Asphalt, waxes, soaps, fire foam
5. Adsorption/interfacial Enhanced oil recovery
tension
6. Adsorption/desorption Control release pesticides
7. Mechanical strength Rubber reinforcing

Sources: Fengel and Wegener [22], and Lin [56].


The utilization of polymeric purified lignins and lignin derivatives comprises

only about 1-2 % of total lignin production and is generally based on the dispersing,

adhesive, and surface active properties of the lignin products [22]. A summary of

these diverse applications is provided in Table 1-1. High fractionation and

modification costs, due to its inherent chemical and molecular weight inhomogeneity,

have limited the utilization of lignin for the production of low molecular weight






7
chemicals and as a raw material for polymers and structural plastics [57]. At present,

only vanillin and related substituted phenols are derived from lignin [22]. A potential

application for lignin is as a raw material for the production of low to medium

strength carbon fibers.


1.4 Pulping Processes


In pulping processes for paper manufacture, the objective is to delignify the

wood and liberate the cellulose fibers from the wood cell structure. The cellulose

remains behind in the pulp which is then made into paper. Lignin and other organic

extractables, such as hemicelluloses and sugars, reduce the mechanical properties and

optical quality of paper and are thus not desirable. Pulping of wood can be

accomplished by chemical means, mechanical means, or a combination of the two.


1.4.1 Kraft Process


The dominant pulping process in use today is the kraft process which accounts

for 74% of all chemical pulp production, and 58% of total pulp production [22]. In

the kraft process, wood is reacted in an aqueous solution of sodium hydroxide and

sodium sulfide at temperatures of 160 to 180 C for 45 to 120 minutes in either batch

or continuous digesters. The sulfide acts to promote and accelerate the dissolution

of lignin while minimizing condensation reactions [1, 22].

Following digestion (pulping), the spent liquor, known as black liquor, which

consists of lignin and other dissolved organic in an aqueous sodium salt solution, is






8
concentrated in multiple effect evaporators to increase the solids content, and then

incinerated in a Tomlinson-type recovery furnace. This chemical recovery stage is

an integral part of the kraft process, because it provides for recovery of the process

cooking chemicals and utilization of the high heating value of the dissolved organic

(especially lignin) for steam production [1, 89].

The advantages of the kraft process attest to its widespread use: it works for

virtually all softwood and hardwood species, has superior delignification selectivity,

results in a strong pulp, and includes a well established and relatively simple

chemical recovery and regeneration system [1, 22]. Some of the main drawbacks of

this process are the relatively low yields (usually 45-50%), the dark color of the

unbleached pulps, the pollution problems and associated abatement costs (especially

the foul odor vented to the surroundings), and the enormous capital costs for

installation of a new mill [1, 22, 89]. These economic factors have been the driving

force for the development of new or modified pulping processes.


1.4.2 Organosolv Process


Organosolv pulping processes encompass the use of a wide range of organic

solvents, such as alcohols, glycol, phenol, organic acids, and amines, as pulping

chemicals [47]. They have been actively investigated for at least the last fifty years,

but none have been fully commercialized because of economic considerations.

Recently, Repap Technologies, Inc., started up a 30 ton/day commercial scale pilot





9

plant to evaluate its ALCELLT process [103]. It is described here primarily because

it has the potential to become a major new pulping process.

In the ALCELLT process, wood is reacted with aqueous ethanol solution

containing an undisclosed catalyst. Pulping and washing take place in an extractor

with three successively cleaner cooking liquors under temperature and pressure

conditions of 200"C and 34 bar, respectively [103]. The spent pulping liquor is

recovered and recycled for subsequent extraction, and the byproducts--lignin, wood

sugars, and volatile components--are separated and concentrated for particular end

uses.

The chief advantage of this process over the kraft process is that it is sulfur

free, resulting in a significant reduction in environmental pollution. Capital costs for

a fully commercialized system would be low compared to a kraft mill because it does

not require a recovery boiler, brownstock washer, or a lime cycle. Operating costs

would be comparable, however, and bleached pulps have strength properties

comparable to those of kraft pulps. The primary disadvantage is that the process

appears to work well only for hardwoods [59, 103].


1.5 Carbon Fibers


1.5.1 Properties and Applications


Carbon and graphite fibers have been developed over the past thirty years

primarily as low density, high modulus (high Young's modulus) reinforcing elements

for plastic composite materials [48]. Although originally developed for aerospace








Table 1-2. Physical Properties and Applications of Carbon Fibers.


Physical property


Applications


1. Physical strength, specific Aerospace: wings, control surfaces;
toughness, light weight automotive: springs, tire cords;
sporting goods: skis, tennis rackets
2. High dimensional Missiles, aircraft brakes, aerospace
stability, low coefficient antenna and support structures, large
of thermal expansion, telescopes, optical benches,
and low abrasion waveguides for stable high-frequency
(GHz) precision measurement frames
3. Good vibration damping, Audio equipment, loudspeakers, voice
strength, and toughness coils, pickup arms, musical
instruments, robot arms
4. Electrical conductivity Automobile hoods, novel tooling,
casings and bases for electronic
equipment, EMI and RF shielding,
brushes, conductive papers and
plastics, electrodes, heating elements,
superconducting cables
5. Biological inertness Blood filters, prosthetic devices,
surgery and x-ray equipment, implants,
tendon/ligament repair
6. Fatigue resistance, self- Textile machinery, general
lubrication, high engineering, high stress bearings,
damping flywheels
7. Chemical inertness, high Chemical industry; nuclear field;
corrosion resistance valves, seals, gaskets, and pump
components in process plants
8. Electromagnetic Large generator retaining rings,
properties radiological equipment

Sources: Donnet and Bansal [19], Dresselhaus et al. [20], and Sittig [88].


applications, where high strength and light weight are of paramount importance, they

have since been widely applied in less demanding areas, as shown in Table 1-2.





11
The diversity of applications for carbon fibers is a direct reflection of some

of their very unique properties. The theoretical Young's modulus of graphite is

estimated to be about 1,000 GPa and a representative selection of commercially

available carbon fibers exhibit a range of moduli from 200 to 800 GPa, tensile

strengths from 1.8 to 7.1 GPa, and strain to failure from 0.2 to 2.4% [48]. Generally,

high modulus fibers have low tensile strengths and low strain to failure, and vice

versa.

The high modulus of all carbon fibers is due to good orientation of the

turbostratic graphite layer planes which constitute the material and also give rise to

good thermal and electrical conductivity. The stability of carbon fiber reinforced

structures is enhanced by a very low coefficient of thermal expansion, excellent

damping characteristics, chemical inertness, and biocompatibility.


1.5.2 Precursor Materials and Commercial Fibers


Carbon fibers have been produced from a wide variety of organic precursor

materials ranging from natural ones, such as wool and lignin, to synthetic polymers,

such as poly methylmethacrylate (PMMA), and high performance fibers, such as

Kevlar [48, 88]. Cellulosics, especially rayon, were the first material from which

carbon fibers were made in the U.S. in the 1960's. Ex-rayon fibers were not

competitive, however, because of very low yield and poor mechanical properties of

the carbonized rayon. Today, only two precursor materials are of any commercial

significance: polyacrylonitrile (PAN), a second generation material first used to make






12
carbon fibers in the United Kingdom in the 1960's, and mesophase petroleum pitch

introduced in the 1970's [48, 88].

The commercially available carbon and graphite fibers range in price from

about $20 per kg for low modulus ex-PAN fibers to over $2,000 per kg for ultra-high

modulus ex-pitch fibers [20]. Most of the current applications for carbon fibers

utilize high strength, low modulus ex-PAN fibers costing $20-60 per kg. Despite

rapid growth in consumption in recent years, the price has not dropped significantly.

This is due to the fact that the PAN precursor fiber is relatively expensive, and the

yield is less than 50% [20].

Ex-pitch precursor fibers were expected to be ultimately much cheaper than

those made from PAN because of lower raw material costs and higher yields. This

has not happened, however, because of difficulties in preparing and spinning pitch

which lead to significantly higher costs. For both ex-PAN, and ex-pitch fibers, the

price increases rapidly with increasing modulus. This is partly due to the cost of heat

treatment of any material near 3,000 *C, and partly due to the small market for high

modulus fibers. From an economic standpoint, applications requiring very high

modulus fibers necessitate even more performance advantages than those which use

low modulus fibers [20].


1.5.3 Processing Steps


The processing of carbon fibers has several steps which are common to all

fibers made from polymeric precursors [18, 20]: (1) spinning--extrusion of polymer






13
melt or solution into fine fibers, (2) stabilization--conversion of fibers into a chemical

form which will prevent melting or fusion of the fiber so that it can withstand higher

temperature heat treatments, (3) carbonization at temperatures of approximately

1,000 "C to eliminate noncarbon elements and form a material made up primarily of

hexagonal networks of carbon, and (4) graphitization--further heat treatment to

temperatures of up to 3,000"C to increase the degree of order in fibers and thereby

achieve the ultimate mechanical properties, especially very high modulus, in the final

carbon fibers.

Carbonization and graphitization stages are similar for almost all organic;

the major difference being the degree of orientation and crystallinity which can be

achieved at a given temperature. During one of the stages of the pyrolysis process,

the precursor fibers are given a stretching treatment in order to achieve a preferred

orientation along the fiber axis [18].

A high carbon yield is important for an economical process, and the significant

factors in obtaining one are (1) the nature of the polymeric precursor, (2) the nature

of the degradation process, (3) the capacity of the precursor for cyclization, ring

fusion, and coalescence, and (4) the nature of the stabilizing pretreatment.

Degradation of the precursor should involve cyclization of a mesophase type of

mechanism, and the glass transition temperature of the precursor, or its stabilized

intermediate form, is a critical parameter during the carbonization and graphitization

processes [18].








1.5.4 Carbon Fibers from Lignin


As a raw material for carbon fibers, lignins present some distinct advantages

over PAN and pitch. They are readily available, relatively inexpensive, and are

structurally rich in aromatic rings. For most applications, low to medium strength

carbon fibers are sufficient, and lignins could be suitable for this category of fibers.

The utilization of lignins as carbon fiber precursors would be a high value added

application.


1.6 Fiber Spinning


Fiber spinning is a unique polymer processing operation in which a fluid is

continuously extruded through an orifice to form an extrudate of usually circular

cross section. Further downstream of the die, the extrudate is contacted such that

the filaments can be pulled and conveyed to further processing steps, such as

stretching and carbonization in the case of carbon fibers [64].

The determination that a fluid is fiber forming is a necessary, but not

sufficient, condition for the development of a spinning process [64]. The

"spinnability" of a polymer melt or solution depends not only on its viscosity values,

but also on its viscoelastic properties, its ability to undergo large degrees of

stretching, and its mass transfer characteristics in the case of dry and wet spinning

[94].

The three primary spinning processes are melt spinning, dry spinning, and wet

spinning. In melt spinning, the molten polymer is simply extruded through a






15
spinneret die. In dry spinning, the polymer is extruded as a solution and the filament

is formed by evaporation of the solvent. In wet spinning, the polymer solution is

extruded into a nonsolvent which causes the filaments to coagulate [6]. Melt

spinning is primarily a uniaxial extensional flow; the extensional viscosity is related

to the spinning behavior. The spinning process involves a complex strain history,

which, starting in the die, consists of shear, recoil (swell), and finally uniaxial

stretching at a variable rate [15].

Rheological material properties thus play an important role in analyzing the

spinning process. A thorough rheological characterization of the lignins is therefore

necessary to investigate the feasibility of spinning fibers.


1.7 Need for Lignin Characterization


The characterization of lignins for molecular weight and rheological properties

is very significant for investigating the feasibility of spinning fibers. In addition, such

a database of lignin material properties would be very valuable to the pulp and paper

industry because there is a great need for improvement in the recovery process, but

the database required for the design of such improvements is generally lacking [26].

Lignin molecular weight has a significant effect on the physical properties of

concentrated lignin solutions, e.g., black liquors, such as viscosity, boiling point

elevation, and low temperature thermodynamic transitions, and these parameters are

very important for improving the processing, concentration, and incineration of black

liquor solutions [26].






16
Ongoing research on black liquor physical properties characterization is based

on the premise that kraft black liquor can be treated as a polymer solution,

particularly at high solids, with the behavior dominated by the lignin present. This

allows the application of a wealth of polymer science theory and analytical

techniques.


1.8 Overview of Subsequent Chapters


In chapter 2, the criteria for lignin selection, and the different purification

schemes, are discussed. Chapter 3 covers the molecular weight characterization work

with an emphasis on the development of a new analytical method for SEC. A study

of glass transition temperatures for purified dry lignins and solvent plasticized lignins

is presented in chapter 4, and a study of rheological properties, specifically

viscoelastic properties of solvent plasticized lignins, is covered in chapter 5. Both the

lignin thermal analysis, and the rheological characterization work, were performed

to support the lignin fiber spinning and carbonization work. Chapter 6, then, covers

some preliminary development work on lignin-based carbon fibers. Finally, overall

conclusions and recommendations for this work are presented in chapter 7.










CHAPTER 2
LIGNIN SELECTION AND PURIFICATION


2.1 General Considerations


Several important criteria were considered in choosing the particular lignins

for the various aspects of this study. These factors included the wood species,

availability of the black liquor raw material or purified lignin, pulping method, and

the suitability of commercial and special research lignins.

The importance of choosing lignins from a variety of both hardwood and

softwood species is self-evident. Numerous species of trees are pulped for

papermaking in different parts of the U.S. In the Northeast and North Central U.S.,

major hardwoods include birch, maple, beech, aspen, poplar, and oak; and major

softwoods include pines, balsam fir, spruce, and hemlock. In the Western U.S., alder

is the major hardwood, and douglas fir, ponderosa, sugar, and lodgepole pines, cedar,

firs, spruce, larch, and hemlock are the major softwoods. Finally, in the Southeastern

U.S., the major hardwoods are gums, tulip poplar, sycamore, oaks, and hickory; and

the major softwoods are yellow, loblolly, slash, longleaf, and shortleaf pines [84].

The kraft process is by far the dominant pulping process, and kraft lignins,

from raw kraft black liquors, are therefore of significant commercial importance, and

are readily available from pulp and paper companies and from a specially designed






18
and constructed pilot plant in the Department of Chemical Engineering at the

University of Florida. Lignins from organosolv pulping could also be investigated

and are readily available from Repap Technologies, Inc. and its pilot plant scale

ALCELLM organosolv pulping process.

Many researchers, however, have used special, noncommercial lignins, such

as those obtained by steam explosion followed by organic solvent extraction, ball mill

grinding, and other methods, for analytical studies such as this one [e.g. 10]. These

lignins are not readily available, however, and are not very representative of

industrial lignins. Therefore, because of the commercial nature of this project, the

emphasis should be on studying kraft lignins.


2.2 Lignin Selection


In consideration of the above discussion, three distinct types of lignins were

chosen for this study: softwood kraft lignins, hardwood kraft lignins, and an

organosolv lignin which consisted of mixed hardwoods. Table 2-1 lists all of the

lignins studied, their wood species, sources, and pulping conditions. Identification

codes for each of these lignins are listed in column one and will be used in

subsequent chapters. In general, detailed information regarding the pulping

conditions for the industrially obtained lignins was not available.

The lignins obtained from pulping activities at the University of Florida Pulp

and Paper pilot plant in our own research group form part of a controlled,

statistically designed pulping experiment in which the four parameters of cooking









Table 2-1. Lignins Selected for this Study


Lignin (Code)


Form' Sourceb


Species


Pulping Conditionse


Indulin AT (IND) L W Loblolly pine x# = 95-100
Mixed hardwood L W Mixed hardwood: K# = 25
kraft (WHK) oak, sweet gum
Birch kraft (WBK) BL SDW Somersett paper x# = 14.7, H = 1,400, EA =
birch 13.0%, S = 30%
Maple kraft (WMK) BL SDW Michigan sugar x# = 15.0, H = 1,414, EA =
maple 13.5%, S = 30%
ABAFX011,012 BL UF Southern slash x# = 107, t = 40 min, T =
(FX11) pine 330 F, EA = 13%, S = 20%
ABAFX015,016 BL UF Southern slash K# = 61.1, t = 80 min, T =
(FX15) pine 3300 F, EA = 16%, S = 20%
ABAFX025,026 BL UF Southern slash x# = 18.5, t = 80 min, T =
(FX25) pine 3500 F, EA = 16%, S = 35%
ABAFX027,028 BL UF Southern slash K# = 77.5, t = 80 min, T =
(FX27) pine 330" F, EA = 13%, S = 20%
ABAFX037,038 BL UF Southern slash x# = 433, t = 80 min, T =
(FX37) pine 350*F, EA = 13%, S = 35%
ABAFX043,044 BL UF Southern slash K# = 51.1, t = 60 min, T =
(FX43) pine 340-F, EA = 14.5%, S = 27.5%
ABAFX055,056 BL UF Southern slash K# = 29.4, t = 60 min, T =
(FX55) pine 3400 F, EA = 17.5%, S = 27.5%
Organosolv (RO) L R Mixed hardwood: See ALLCELLM process
50% maple, 25% description, section 1.4.2
aspen, 25% birch


Notes: a L
bw


= lignin, BL = black liquor.
= Westvaco, North Charleston, SC; SDW = S.D. Warren, Westbrooke, ME; UF =


University of Florida pulp and paper pilot plant, Gainesville, FL; R = Repap
Technologies, Inc., Valley Forge, PA.
c K# = Kappa number: a numerical value representing the amount of residual lignin in the
pulp.
H .= H-factor: a numerical value that represents time and temperature as a single variable
in the kraft (alkaline) cooking process [89].
EA = effective alkali: NaOH + Y2Na2S, expressed as equivalent weight of Na20 [89].
S = sulfidity: the percentage ratio of Na2S to NaOH + Na2S, expressed as equivalent
weight of Na20 [89].






20
time, temperature, effective alkali (EA), and sulfidity (S) are investigated. The effect

of varying these parameters on the physical properties of the resulting black liquors

forms the basis of the industrially important black liquor physical properties

characterization work [26]. The pilot plant is described in detail by Fricke [28].


2.3 Lignin Purification


2.3.1 Kraft Lignins


Most of the kraft lignins in this study had to be isolated and purified from

kraft black liquors which are very complex mixtures of fibrous materials, dissolved

organic (lignins, hemicelluloses, sugars, acids, resins, and other extractables), and

inorganic salts. The purification scheme developed by D.J. Dong* is shown in detail

in Figure 2-1 and involves a lengthy series of acid precipitation, redissolution,

washing, and drying steps. The final dried lignin obtained is then approximately

98+ % pure with low molecular weight organic acids and bound sulfur as its major

remaining impurities. Lignins that were already obtained as dried powders were

further purified by performing only the last few steps of the purification scheme.


2.3.2 Organosolv Lignins


The purity of the organosolv lignin, as received, was 97-98%, and a suggested

purification scheme to remove the major impurities (low molecular weight sugars and


' Dong, D.J. Personal Communication (1992).







Kraft Black Liquor


Dilution to 10% Solids & Filtration

Precipitation with 1.ON H2SO4 to
pH 2; Centrifuge & Separation -


Washing & Separation
Washing & Separation


,> Particulates


-4 Supernate

-' Supernate


Redissolving in 0.1N NaOH


> Non-Lignin
Solids


Precipitation with 1.ON H2SO4 t
pH 2; Centrifuge & Separation

Washing with Deionized Water

Washing with 0.01N H2SO4 (2 t

Washing with D.I. Water (2 tim


Supernate

Supernate


>- Supernate

'> Supernate

> Water


Freeze Drying


Hexane Extraction
Hexane Extraction


> Organic
Impurities


Freeze Drying

Lignin Sample


-> Hexane


Figure 2-1. Kraft Lignin Isolation and Purification Scheme.






22
resin acids) consisted of a graded solvent extraction progressing from completely

nonpolar to very polar: petroleum ether, ethyl ether, ethyl acetate, acetone,

anhydrous methanol, and 90% methanol/10% water'. This extraction scheme was

modified by the author to the following: n-hexane, 1,1,1-trichloroethane, acetone, and

methanol (all from Fisher Scientific, Inc., Orlando, FL) based on their ready

availability and higher boiling temperatures.

The graded solvent extraction was performed on only one organosolv lignin

sample using a Soxhlet apparatus according to standard procedures [85]. A porous

alumina thimble was initially charged with 26.5 g of vacuum dried lignin. Each

extraction step was run for 4-5 hours, and the lignin remaining in the thimble was

then vacuum dried to remove residual solvent prior to moving on to the next solvent.

Qualitative observations, such as color changes in the extracting solvents,

indicate that a multitude of organic compounds were extracted from the lignin.

Initially, all of the solvents were clear. In the first extraction, n-hexane turned yellow,

and an orange-yellow solid precipitated when the solution cooled. The TCE in the

second extraction turned a deep reddish brown, and large flocs of precipitate formed

after several days. The acetone in the third extraction became cloudy and turned

dark brown, and in the fourth extraction, the methanol turned dark reddish brown.

The masses of lignin remaining after each step were not consistent, but did indicate

that very little was extracted in the n-hexane step, and substantial amounts were

extracted in each of the remaining three steps. Although samples of extracting


'Cronlund, M., Repap Tech., Inc. Personal Communication (3 April 1991).






23
solvent from each step were retained for future chemical analysis, this has not yet

been done. An overall yield for this extraction was only on the order of 10%.


2.3.3 Storage


The purified lignins were stored in the dark in capped glass sample vials

sealed with Parafilm*, and over a two year period, no color changes in the lignin

samples were noticed. The raw black liquors were kept refrigerated at close to 0 C

to minimize degradation reactions.










CHAPTER 3
MOLECULAR WEIGHT CHARACTERIZATION


3.1 Introduction


Lignin has been extensively studied and characterized [e.g. 34]. However,

molecular weights determined by a large number of investigators exhibit an extremely

wide range of values. This can be attributed to the multiplicity of extraction

techniques, the wide variety of wood species, different purification procedures, and

different analytical techniques that have been employed.

Analytical techniques for measuring molecular weights of polymers fall into

two general classes: "absolute" methods such as vapor pressure osmometry (VPO)

and low angle laser light scattering (LALLS), and "secondary" methods such as size

exclusion chromatography (SEC), also known as gel permeation chromatography

(GPC). Absolute methods allow the determination of true values for the number

average molecular weight (Mn), and the weight average molecular weight (NM), from

VPO and LALLS, respectively. Size exclusion chromatography is much more

versatile and allows the determination of all the molecular weight averages, as well

as the molecular weight distribution (MWD). However, these values have only

relative meaning because they are dependent on the calibration scheme employed.






25
Although SEC can only provide relative molecular weight values, it is a very

rapid and convenient technique as compared to VPO and LALLS which are very

laborious and time consuming methods. Both VPO and LALLS require very careful

experimental technique and numerous corrections for nonideal behavior. For

example, Kim [52] demonstrated that measurements of lignin M, by LALLS must be

made at or above the Theta temperature for the lignin-solvent pair and that nonideal

optical phenomena significantly affect the results. One experimental determination

of M by LALLS requires six separate measurements: the effect of polymer

concentration on solution refractive index, the effect of polymer concentration on

light absorption at the particular wavelength used, light scattering of the solvent,

excess light scattering of the solution, light polarization, and scattered light

flourescence. From these data, corrections for optical effects can be made and M1

determined.

In this study, SEC was primarily used to determine the average molecular

weights and the MWD of lignins, and a novel calibration procedure was investigated

to overcome the limitations mentioned above.


3.2 SEC Theory


3.2.1 Separation Mechanism


In SEC, separation is accomplished by injecting the polymer solution into a

continuously flowing solvent stream which passes through one or more columns

packed with highly porous, sub 10 um rigid gel particles and then detecting the






26

fractionated sample as it elutes from the column. The polymer molecules are

separated in the column packing according to their molecular size or hydrodynamic

volume in solution. The degree of retention of the polymer molecules in the pores

is the phenomenon which affects the separation. Smaller molecules are retained to

a greater degree than larger ones, and, as a result, the largest size molecules elute

from the column first followed by successively smaller molecules [55, 105].

This fractionation process is entropy driven and based on the concentration

gradient of solute that exists between the stationary mobile phase within the pores

of the gel particles and the interstitial flowing mobile phase. Solute permeation into

the pores is associated with a decrease in entropy because solute mobility becomes

more limited inside the pores of the column packing. The SEC separation is

controlled by the differential extent of permeation, not the differential rate of

permeation. Solute diffusion in and out of the pores is rapid enough with respect to

the flow rate to maintain an equilibrium solute distribution. SEC is an equilibrium

entropy controlled size exclusion process [105].

The volume of solvent at which a solute elutes from the column or the volume

of liquid corresponding to the retention of a solute on a column is known as the

retention volume. This can be related to the physical parameters of the column as

follows:

V, = V. + K V (3-1)

where VR is the retention volume of the solute, Vo is the interstitial volume (dead

volume) of the column, Vi is the pore volume, and K is the distribution coefficient






27
based on the relative concentrations between the two phases. The total column

volume VT is given by

VT = V. + V (3-2)

Therefore, the retention volume is expressible in terms of the two measurable

quantities Vo and VT as

VR = V0(1-K) + KVT for 0
The void volume corresponds to the total exclusion of solute molecules from

the pores. Between Vo and V., solute molecules are selectively separated based on

their molecular size in solution. If molecules elute beyond VT, corresponding to K

> 1, separation is no longer achieved by a size exclusion mechanism, but rather,

solute is retained on the column support by an affinity mechanism such as adsorption.


3.2.2 Detection


The fractionated sample is usually detected by means of a mass concentration

detector such as a differential refractive index (DRI) detector, or an

ultraviolet/visible (UV/Vis) absorption spectrophotometer. Both of these detectors

continuously monitor the mass of sample eluting from the column set by measuring

the difference in refractive index, or light absorption, respectively, between the

fractionated sample solution and pure solvent (or air for UV/Vis). This differential

property is then directly proportional to the mass of sample present.

UV/Vis detectors generally operate in the wavelength range of 190-600 nm

and are significantly more sensitive than DRI detectors. However, UV/Vis detection






28
requires the sample to have an ultraviolet or visibly active chromophore which is not

active at the same wavelength as the solvent.


3.2.3 Calibration


Calibration in SEC involves converting a chromatogram into a molecular

weight distribution curve. Narrow standard calibration has traditionally been the

method of choice, but universal calibration and broad standard calibration have also

been used, especially with the development of sophisticated computer software for

data analysis. Finally, resolution of moments, which is a numerically demanding

method, also appears very promising.

In narrow standard calibration, narrow MWD polymer standards, with

polydispersities less than 1.1, are used to generate volume retention curves. A one-

to-one correspondence of peak retention volume with peak molecular weight (Mp)

of the standard is made, and a plot of log Mp versus retention volume generates a

primary molecular weight calibration curve which is usually cubic in form:

logMp = a + bV + cV2 + dV9 (3-4)

where a, b, c, and d are constants that usually differ by at least an order of

magnitude.

The chromatogram for the unknown sample is then divided up into discrete

volume (or time) intervals and molecular weight values, Mi, are assigned to each

sample slice as a function of the elution volume (or time) in accordance with (3-4).

The various molecular weight averages are then calculated by the usual formulas






29
[105]. A serious limitation of this method is the lack of well characterized narrow

MWD standards for many polymers such as lignin. Thus, only an apparent MWD

curve for the sample polymer is possible.

Universal calibration is an empirical method utilizing the concept of

hydrodynamic volume which can be expressed in terms of the product of the intrinsic

viscosity, [17], and the molecular weight, M, of the polymer sample. When plotted as

log [ri]M versus elution volume, SEC calibration curves for different types of

polymers merge into a single plot. This behavior is theoretically sound. When

separation occurs strictly by size exclusion involving only entropy changes, polymers

of different chemical structures, but the same hydrodynamic volume, will elute at the

same retention volume from any given SEC column set. However, significant

deviations between experiment and theory, due to possible reversible adsorption,

crosslinking, and extensive branching, for example, can exist [41, 43, 105].

The relationship between molecular weight and intrinsic viscosity is given by

the empirical Mark-Houwink equation:

[-q] = KM' (3-5)

where K, and a are the Mark-Houwink constants. These constants vary with polymer

type, temperature, and solvent, and accurate values are difficult to obtain

experimentally. For polymers with a three dimensional network structure, such as

lignin is believed to have, universal calibration is not valid [41, 105].

Broad standard calibration can be an integral MWD method, which utilizes

the complete MWD curve of the polymer standard, or linear calibration methods






30
which use only the average molecular weight values of the polymer standard but

assume a linear approximation of the calibration curve [105]. Although both

approaches are valid, the linear calibration methods are more versatile and pose no

restrictions on the MWD shape of the standards.

In the linear method, an iterative procedure is used to determine values for

the coefficients a and b in (3-4) (c and d are zero) such that computed molecular

weight values are in agreement with the known values for the polymer standard.

The resolution of moments method is a generalization of the integral broad standard

calibration technique except that no set form for the distribution is assumed [26, 66].

The objective is to generate a third order calibration equation such as (3-4) by

determining values of the constants a, b, c, and d such that M1 and M. computed

from the chromatogram match two known values of M. and M, from absolute

measurements, specified for the sample polymer. This technique requires calculation

of the moments of the distribution and involves a complex and iterative numerical

optimization procedure. The calibration equation obtained by this method will be

valid for a specific type of polymer and set of operating conditions.


3.2.4 Nonsize Exclusion Effects


The separation mechanism described above applies only to ideal size exclusion

behavior. Since solute-solvent-matrix interactions govern SEC elution behavior,

nonsize exclusion effects must frequently be taken into account or eliminated in

order to achieve ideal SEC behavior [4].






31
There are a multitude of possible nonsize exclusion effects which can lead to

nonideal SEC behavior. These include solute/packing enthalpic interactions,

intermolecular solute association, intramolecular electrostatic effects, concentration

effects, polymer shear degradation, ultrafiltration, hydrodynamic effects, polymer

chain orientation and deformation, and peak dispersion [4]. Further nonideal effects

can arise from the use of mixed mobile phases such as preferential solvation of the

polymer [4].

Enthalpic interactions that can occur between polymer and packing can result

in polymer adsorption to the gel matrix. These interactions include ion exchange, ion

inclusion, ion exclusion, hydrophobic interactions, hydrogen bonding, dispersion

(London) forces, dipole interactions, and electron-donor-acceptor interactions [4].

The mobile phase is usually chosen to eliminate these effects so that it is a

good solvent for the polymer and whose solubility parameter, 6, is close to that of

the gel. This results in both polymer and packing being well solvated and potential

adsorptive sites on both being deactivated. If 6S, > 6sovnt, normal phase adsorption

will occur, and if S6O < sovnt, the packing will act as a reversed phase packing. If

6gel = 6solvnt, size exclusion will be the dominant separation mechanism [4].


3.3 Background and Literature Review


3.3.1 Introduction


Since it was first developed in the 1960's, SEC has been applied to the

characterization of lignins. Consequently, an extensive body of work exists which






32
encompasses a wide range of mobile phases, column chemistries, and lignins.

Likewise, a very broad range of lignin molecular weights has been reported: from less

than 1,000 for some kraft lignins, to over 100,000 for some lignin sulphonates [24,

25]. The diversity of this research effort is a direct reflection of the inherent

molecular complexity of lignin and the difficulty in counteracting unfavorable lignin-

column-solvent interactions in order to achieve true size exclusion behavior.

The main advantage of SEC is its ease of use and rapid sample analysis, and

the main limitation is that it provides only relative molecular weight data. Various

calibration techniques have therefore been employed in an attempt to overcome this

limitation and achieve absolute molecular weight characterization for lignins. A

discussion of these calibration procedures is therefore a very significant and integral

part of the overall picture of lignin SEC characterization work.

It is difficult to make direct one-to-one comparisons among the many studies

in the literature because of the unique character of each lignin-column-solvent set.

The interactions among each of the three components govern lignin's elution

behavior and therefore the particular mobile phases, column packing materials, and

lignins and their method of preparation, that each group of investigators have

employed, are very significant. Because of the extensive nature of this topic, a

thorough review of the available literature is not practical. Therefore, only

significant highlights are discussed below.








3.3.2 Traditional SEC Analyses


Traditional SEC analyses of kraft lignins, organosolv lignins, and

lignosulphonates have been carried out on a variety of gel packing materials

including polysaccharide, or more specifically, polydextran based gels, on acrylate

polymer based gels, on silica based columns, and on polystyrene divinylbenzene (PS-

DVB) copolymer gel columns.

The polydextran columns (Sepharose, or Sephadex type by Pharmacia) have

been used with aqueous mobile phases [25, 97], and polar organic mobile phases such

as DMF [11, 12, 13, 54, 70]. The acrylate gels (PW series by Toyo Soda

Manufacturing Co.) are semi-rigid high performance gels and have been used with

aqueous mobile phases [73]. The silica based packing (Waters Associates Bondagel

column) has been used with polar organic mobile phases [98], and the PS-DVB

copolymer gel columns (Waters p-Styragel, Ultrastyragel for example) have been

used with polar organic mobile phases, principally THF [10, 40, 51, 74].

For high pressure (high performance) SEC, PS-DVB gels, with THF as mobile

phase and polystyrene narrow MWD standards for calibration, have become the most

widely used SEC system. This is probably due to the good compatibility between

THF and the PS-DVB gel (in terms of solubility parameters) [4]. Sample detection

is usually by means of differential refractive index or ultraviolet absorption at 280

nm.








3.3.3 Association and Adsorption


Lignin association in the mobile phase and reversible adsorption to the gel

packing have been widespread and troublesome nonsize exclusion effects. Both of

these two phenomena involve complex and often little understood interactions among

the lignins, mobile phases, and column gels. The nonideal SEC behavior

accompanying these effects results in erroneously high apparent MW's for

association, and erroneously low MW's for adsorption.

Previous investigators have almost universally used chemically modified lignin

samples to minimize both adsorption and association effects, and added salts to polar

organic mobile phases, such as DMF, to minimize association effects. These

derivatized lignins have been methylated, acetylated, silylated, or hydroxypropylated

at the free phenolic hydroxyl positions where hydrogen bonding interactions are

believed to occur. The main concern with this procedure is that quantitative

derivitization is difficult, and derivitized lignins have altered conformations and

different elution profiles than nonderivitized ones.

Association can occur in both aqueous solutions at pH < 12-13, and in organic

mobile phases at temperatures below the Theta or Flory temperature for the

respective lignin-solvent pair [52]. Many investigators recognized this phenomenon

[10, 13, 74.]. In higher fractional polarity solvents such as DMF, lignin-lignin

associative interactions are high, resulting in bimodal or multimodal elution profiles.

These associative effects produce peaks of very high apparent molecular weight with

some elution beyond the exclusion limit of the column set [10].






35
Many investigators have been limited to ambient temperature conditions for

lignin SEC experiments with DMF and have therefore been unable to overcome the

association effects solely by operating above the Theta temperature for this system

(about 80 "C for kraft softwood lignins in DMF [52]). They have therefore resorted

to adding lithium salts (0.1M LiBr or LiCI) to DMF mobile phases which effectively

broke up lignin association complexes and changed the multimodal elution profiles

to a single broad peak profile.

Connors et al. [13] using Sephadex columns at ambient temperature, showed

that molecular association was disrupted for LiCI concentrations in DMF of between

0.0001M and 0.001M. The added salt was theorized to prevent association by

shielding dipoles in the individual molecules. Further studies showed that when the

fractions from the bimodal molecular weight distribution of lignins were collected

and rechromatographed, the materials from the higher and lower end of the

distribution were chemically different though not vastly different in molecular

weights. Since acetylated lignins displayed similar elution patterns, molecular

association was not due to hydrogen bonding [13].

Pellinen and Salkinoja-Salonen [74] ran derivatized and underivatized lignin

samples and model compounds in THF on PS-DVB based columns. They believed

that polymeric lignins would not associate because they observed that underivatized

model compounds neither absorbed on to the gel nor underwent intermolecular

association. Free hydroxyl groups in the lignins and the model compounds were

derivatized to eliminate hydrogen bonding between the target molecules.






36
Adsorption of lignins to the column gel has been a common observation for

PS-DVB based columns with DMF mobile phases. Because of its structure with

many free phenolic hydroxyl groups, lignin is attracted to the aromatic rings of the

gel through the unshared electron pairs on the oxygen atoms. Adsorption can be

hydrophilic or hydrophobic and leads to an underestimation of the MW's. In

aqueous mobile phases, ionic interactions are due to the polyelectrolytic nature of

lignins [74].


3.3.4 Column Calibration


Column calibration has been a persistent problem which has limited the

applicability of SEC for obtaining accurate and realistic MW values for lignins. The

primary calibration methods that have been employed are the use of narrow MWD

polymer standards, principally polystyrene, the use of lignin model compounds, and

the use of narrow fractions of lignin samples whose molecular weights have been

determined by ultracentrifugation. Absolute MWD determination by multidetection

and universal calibration methods will be discussed in Section 3.3.5.

Column calibration with narrow MWD polymer standards, such as polystyrene,

poly methyl methacrylate (PMMA), polyethylene oxide (PEO) or others, is the most

straightforward technique and has been widely used [e.g. 10]. Polystyrene standards

in relatively nonpolar mobile phases, such as THF, are ideally suited for PS-DVB

gels. However, in polar mobile phases such as DMF, polystyrenes reversibly adsorb

to the PS-DVB gel matrix resulting in increased retention times [10, 31, 51]. More






37
polar polymer standards such as PEO and PMMA adsorb to a lesser extent and are

more suitable for DMF. In aqueous mobile phases, polystyrene sulfonates have been

used [73, 97].

Regardless of the standard used, there is a common limitation to this

technique: the structure and conformation of the standard is very different from that

of the sample lignins; all of the commercially available narrow MWD standards are

linear polymers, whereas lignin is highly branched and spherical. This results in

lignin molecular weights as determined by narrow standard calibration that are as

much as an order of magnitude too low as compared to values determined by

absolute methods.

In addition to polystyrene standards, many investigators have used

monodisperse lignin model compounds to calibrate their column sets [11, 12, 40, 51,

54, 73, 74]. Connors [11], and Connors et al. [12] used 15 different lignin model

compounds to calibrate Sephadex columns in DMF. These model compounds

spanned the molecular weight range of 168 to 1,076 and consisted of various

substituted and derivitized phenyl propane oligomers which represent some of the

functional groups of lignin. They found a good correlation between molecular weight

and elution volume or partition coefficient.

Kristersson et al. [54] investigated the elution properties of lignin model

compounds (guaiacylglycerol, pinoresinol, dihydrodehydrodiisoeugenol),

carbohydrates, and low molecular weight lignin carbohydrate compounds which

spanned the molecular weight range of 180 to 990. These were run in dioxane-water






38
(1:1), and DMF on Sephadex columns. They found that all of the compounds eluted

essentially according to molecular size in DMF, but not in dioxane-water.

Using both lignin model compounds and polystyrene standards, Himmel et al.

[40] calibrated their column set in terms of hydrodynamic radius by determining the

effective hydrodynamic radius as a function of molecular weight. They analyzed

steam exploded aspen lignins in a dioxane/chloroform mixed mobile phase on PS-

DVB based columns, and concluded that the relationship of molecular weight to

hydrodynamic radius, specific for each polymer-solvent system, must still be

determined by a direct method.

Pellinen and Salkinoja-Salonen used low molecular weight lignin model

compounds such as vanillin and vanillic acid [73], and various substituted methoxy

phenols in the molecular weight range of 154 to 638 that were representative of

different structures and functional groups typical for lignin [74]. These model

compounds were run underivitized and as acetylated and silylated versions.

Calibration with the model compounds gave somewhat higher values of Mn

and lower values of M, than PS calibration, but both calibrations gave similar low

values of Mn for the underivitized samples. The elution volume depended on MW

as well as on the derivitization of the lignin model compounds, and the polydispersity

was smaller when the model compound calibration was used. The chief limitation

is the lack of high MW lignin model compounds for calibration [74].

Johnson et al. [51] compared the elution behavior of lignin model compounds

and model polymers in THF and DMF on PS-DVB based gel columns. The lignin






39
samples were organosolv aspen lignins that had been quantitatively acetylated. In

high fractional polarity solvents made with DMF, the derivitized lignin model

compounds and lignin model polymers adsorbed less than the PS standards.

Linear lignin model polymers, and derivitized and underivitized lignins,

exhibited similar associative behavior in polar solvents (e.g. DMF) which decreased

with the addition of 0.1M LiBr. None of the low MW lignin model compounds,

derivitized or not, clearly exhibited associative behavior in polar solvents.

Chromatograms of mixtures of well defined low MW lignin model compounds, ether

bonded lignin model polymers and acetylated lignins in polar solvents appeared to

be merely additive [51].

A calibration technique that circumvents the vexing problem of structural and

conformational inhomogeneity between the sample lignins and the polymer standards

is the use of narrow fractions of sample lignins whose molecular weights have been

determined by some absolute method such as LALLS or ultracentrifugation. In this

way, the elution behavior of both the standards and the samples should be identical,

and this method should theoretically provide absolute MW values.

Obiaga and Wayman [70], Forss et al. [25], and Wagner et al. [97], among

others, have used this method. Obiaga and Wayman [70] analyzed a spruce

lignosulfonate in dimethyl sulfoxide (DMSO) on a Sephadex column which they

calibrated with only three lignin fractions whose molecular weights had been

measured by ultracentrifugation. This calibration curve was shifted and rotated to

correct for skewing and axial dispersion. For the sample, 1, as determined by SEC






40
and sedimentation equilibrium differed by only 4%. Forss et al. [25] calibrated

Sephadex columns with both kraft lignin fractions and lignosulfonate fractions, which

had been characterized by light scattering, for analysis in aqueous mobile phases.

The serious disadvantage of this method is the inordinate amount of time

required to determine the molecular weights of the lignin fractions for calibration.

Both LALLS and ultracentrifugation are laborious and tedious procedures.


3.3.5 Multidetection and Absolute MWD


Several groups of investigators have utilized a dual detection system for SEC

that incorporates both a DRI detector and a LALLS detector [29, 53, 87], or a DRI

detector and a differential viscosity (DV) detector for universal calibration [42, 43,

86, 87]. Both approaches are sophisticated attempts to obtain absolute MW values

while bypassing the use of unsuitable calibration standards.

The on-line SEC-LALLS system makes it possible to overcome the calibration

problem and continuously calculate the molecular weight of the molecules eluting

from the column set. However, complex problems are associated with this method

that make its application to lignin analysis difficult. All three groups of investigators

encountered experimental difficulties with LALLS detection, particularly optical

effects such as sample flourescence, absorption, and polarization, which must be

corrected for.

Kolpak et al. [53] analyzed several softwood lignins from spent kraft pulping

liquors in THF on PS-DVB gel columns. They compared their online results with






41
static (stand alone) LALLS measurements and found that static LALLS

measurements for one of the lignins were much higher than SEC/LALLS MW

values: M, = 17,300 for static LALLS versus ,, = 10,650 for SEC/LALLS. They

attributed this large discrepancy to sample aggregation in THF.

Froment and Pla [29] studied acetylated derivatives of dioxane extracted

spruce lignin, alkali black cottonwood lignin, and organosolv black cottonwood lignin

in THF on PS-DVB gel columns. In order to correct for the optical effects

mentioned above, at least three recorder traces were made for each sample: vertical

and horizontal components of the scattered light (polarization correction),

transmitted light (absorption correction), and the concentration profile (DRI scan);

and a flourescence filter was used. Froment and Pla [29] recognized that this method

was very promising, but also full of difficulties.

In the third highlighted study, Siochi et al. [87] analyzed four hydroxypropyl

derivatives of organosolv red oak, and aspen hardwood lignins, and a Westvaco

mixed kraft hardwood lignin. Their system consisted of a Waters 150C HPSEC with

a DRI detector in series with a Chromatix KMX-6 LALLS detector and in parallel

with a Viscotek Model 100 DV detector. Their mobile phase/column system was the

same as in the other two studies: THF at 300 C and PS-DVB gel columns. Siochi et

al. [87] also concluded that in order to use LALLS detection, corrections for sample

absorbance, flourescence, and beam polarization must be made; optical effects gave

them erroneously high calculated M, s from SEC/LALLS, as compared to values

measured directly by VPO.






42
Absolute molecular weight determination by universal calibration is a well

established technique, and with the recent development of differential viscosity

detectors for SEC, the molecular weight and intrinsic viscosity measurements can

now be made online in real time.

Himmel et al. [43] studied four different acetylated aspen hardwood lignins

that had been obtained by ball milling and solvent extraction, steam explosion

followed by alkaline extraction, organosolv pulping followed by water extraction of

the associated sugars, and dilute sulfuric acid hydrolysis followed by sodium

hydroxide extraction. These samples were run in THF at ambient temperature on

PS-DVB gels. Narrow MWD standards such as polystyrenes, polybutadienes,

PMMA's, and low molecular weight lignin model compounds (synthetic phenyl

tetramers) were found to fit universal calibration.

They concluded that differential viscosity was a valuable detection method, but

that the MW values for these lignins needed to be compared to absolute values

obtained from LALLS and VPO. A limitation of these SEC-based "absolute" MW

measurements is the narrow concentration window available for analysis. Also, due

to the lack of available appropriate MW, composition, and branched polymer

standards, the limits of fit for universal calibration to complex biopolymers such as

lignin could not be judged [43].

Siochi et al. [86, 87] investigated the feasibility of using SEC/DV for absolute

molecular weight determination of hydroxypropylated derivatives of red oak, aspen,

and hardwood kraft lignins. These were run in THF at 300C on Waters






43
Ultrastyragel columns in a Waters 150C HPSEC with both DRI and DV detectors.

Narrow MWD polystyrene calibration standards were used, and "absolute" reference

1M values were obtained from VPO to check the validity of the universal calibration

method.

All the lignins had M. values in the range of 1,100 to 2,000, and values

obtained from SEC/DV compared favorably to those obtained from VPO. These

lignins also demonstrated time dependent association in THF at 30 *C: M,, increased

by 20% in two days. Changes in the absolute molecular weight distributions in all

the experiments confirmed that time dependent association occurs in lignin

derivatives in THF. They concluded that SEC/DV is a reliable and convenient

technique for obtaining average molecular weights and the absolute MWD for lignins

[86, 87].

Himmel et al. [42] used three hydrodynamic methods to determine unknown

lignin MW's: SEC, universal calibration, and sedimentation equilibrium. They

analyzed acetylated aspen hardwood lignins in THF on a set of p-Spherogel columns

(PS-DVB based) with pore sizes of 104, 103, and 500 A.

Conventional SEC with polystyrene calibration produced the lowest MW

estimates for the four lignins, whereas both universal calibration and sedimentation

equilibrium produced similar MW estimates that were 1.5-2.5 fold higher. The

higher apparent MW's from universal calibration, relative to SEC, are consistent with

the concept of lignin being a branched polymer, because branched polymers of higher

MW may occupy the same hydrodynamic volume as linear polymers of lower






44
molecular weight. These low MW acetylated aspen lignins appeared to fit universal

calibration [42].


3.4 Experimental Work and Data Analysis


3.4.1 Instrumentation


The experimental setup for the SEC work consists of a Waters 150C

ALC/GPC integrated high pressure liquid chromatography system, and an outboard

Waters 486 UV/Vis tunable absorbance detector (Waters Division of Millipore

Corp., Milford, MA), interfaced with a NEC APC IV computer workstation which

runs the Maxima 820 software for HPLC and GPC data acquisition and processing

(Dynamic Solutions Division of Millipore Corp., Ventura, CA). The mobile phase

is supplied by a Kontes integrated HPLC mobile phase handling system (Kontes,

Vineland, NJ) which has a five liter capacity and is capable of solvent filtration,

degassing by sparging with an inert gas, and mobile phase storage.

The 150C is a fully programmable, self contained unit which includes a high

pressure pump, 16 sample carousel, automatic injector, DRI detector, and column

oven. It has complete temperature control to 150 C over the full analysis sequence

of sample injection, fractionation, and detection. The UV/Vis detector was installed

at a later date in series with, and upstream from, the DRI detector. This unit is a

single channel detector with a wavelength range of 190-600 nm. For more detailed

information, the reader is referred to the respective operator's manuals [99, 100].






45
Three sets of analytical columns, employing different chemistries, were used

to investigate a wide variety of solvents as possible lignin mobile phases. For THF,

a set of three Ultrastyragel columns (Waters Division of Millipore Corp., Milford,

MA): 104 + 103 + 100 A pore sizes, were connected in series. For DMF and other

polar organic mobile phases, we used a set of two Jordi Gel columns: Mixed Bed +

103 A pore sizes connected in series, and for aqueous and polar organic mobile

phases, we used a set of Jordi Gel 10W + 104 A GBR columns (Jordi Associates, Inc.,

Bellingham, MA).

The Ultrastyragel columns contain a highly crosslinked styrene divinylbenzene

copolymer gel and measure 30 cm long by 7.8 mm internal diameter (i.d.). The Jordi

Gel columns contain a highly crosslinked poly-DVB gel and measure 50 cm long by

10 mm i.d., except the 10 A GBR column which is 25 cm long by 10 mm i.d.

Although all of these columns are temperature stable up to 150 "C, the polymer gel

bed in all of the Jordi Gel columns does not shrink or swell appreciably upon solvent

changeover, whereas the Ultrastyragel ones may if the difference in solvent polarities

is significant. This limits the application of the Ultrastyragel columns to mobile

phases with similar polarities. In the GBR column, the crosslinked poly-DVB gel has

been modified by adding glucose amine groups to the aromatic rings and the alkane

chains. This deactivates the aromatic rings toward adsorption interactions and makes

the gel compatible with both aqueous and polar organic mobile phases.






46
3.4.2 Mobile Phase Selection and Preparation


Selection of the proper mobile phase and column chemistry for lignin analysis

has been the major emphasis in the development of an effective SEC method. Table

3-1 lists the wide variety of pure and mixed solvents, together with several column

chemistries, that have been investigated in order to overcome the nonsize exclusion

behavior, particularly adsorption to the column gel in polar organic mobile phases,

that lignins exhibit with most common mobile phase/column systems.

Preparation of the mobile phase was a straightforward process: solvents were

vacuum filtered through 0.45 or 0.50 pm pore size nylon or teflon membrane filters

(Gelman Sciences, Inc., Ann Arbor, MI), then degassed by sparging with helium for

15-20 minutes while pulling a vacuum, and then stored under a helium blanket of 1-2

psig in the Kontes mobile phase reservoir. All of the solvents were either HPLC

grade or Certified ACS grade and were obtained from Fisher Scientific Co. (Orlando,

FL), except ethylene glycol monopropyl ether (EGMPE) and NMP, which were

obtained from Eastman Kodak (Rochester, NY). Mixed solvents were prepared on

a volume basis prior to filtration. For each new mobile phase, the column set was

equilibrated (usually overnight) at 0.1 or 0.2 ml/min until at least three column

volumes had eluted.


3.4.3 Sample and Standards Preparation


For the lengthy methods development process of mobile phase evaluation and

selection, several older softwood kraft lignins [27], Indulin AT, and organosolv lignin








Table 3-1. SEC Mobile Phase Selection


Mobile Phase'


Temp. ("C) Column Set"


1 THF 30,45 U
2 DMF 50, 80, 85 JG
3 DMF + LiBr (0.05, 0.1M) 80, 85 JG
4 DMF + 2% TEA 85, 100 JG
5 DMF / DMSO (95/5) 85 JG
6 DMF / EGMPE (90/10, 95/5, 85, 100 JG
98/2, 99/1)
7 DMF / EG (95/5, 97/3, 98/2) 85, 100 JG
8 DMF + 2% EGDME 85 JG
9 DMF / EGMME (95/5, 98/2) 85, 100 JG
10 EGMME 85 JG
11 DMF + 10% Pyridine 85 JG
12 DMF / N-butanol (90/10) 85 JG
13 Pyridine 60, 85 JG
14 DMF / TCE (50/50, 90/10) 55, 60 JG
15 DMF / Toluene (91/9) + 0.05M 85 JG
LiBr
16 DMF / NMP (95/5) 85 JG
17 DMF + 1.1% Pyrogallol 85 JG
18 KOH (0.1, 1.0M) 40, 50, 60 GBR
19 DMF / 1.OM KOH (50/50) 40, 80 GBR
20 NaOH (0.1, 0.2, 0.3, 0.5M) 40, 50 GBR
21 DMSO 85 GBR
22 DMSO + LiBr (0.01, 0.05, 0.1, 85 GBR
0.15, 0.2M)

Note: aSolvent abbreviations defined in Key to Abbreviations.
bU = Ultrastyragel, JG = Jordi Gel, and GBR = Jordi Gel GBR.


No.






48
were used as test samples. For promising mobile phases, e.g. 0.2M NaOH, and

DMSO + 0.1M LiBr, all of the lignins listed in Table 2-1 were prepared and

analyzed. Because of the wide variety of mobile phases and column chemistries that

have been investigated, several different narrow MWD polymer standards were

required for effective SEC calibration. For THF, polystyrene standards were used,

and for DMF based mobile phases, polyethylene oxide, polyethylene glycol and poly

methyl methacrylate standards were used. For aqueous mobile phases and DMSO

+ LiBr, polysaccharide standards (linear polymaltotrioses) were used. All of the

standards were obtained from Pressure Chemical Co. (Pittsburgh, PA).

Lignins were vacuum dried for several hours prior to preparing the sample

solutions, and both samples and standards were weighed out on a Sartorius electronic

balance with 0.1 mg resolution (G6ttingen, Germany). Solutions of lignins and

standards were prepared in the respective mobile phase in 25 ml volumetric flasks

at approximate concentrations of 1-2 g/L (0.1-0.2% w/v), and 1 g/L (0.1% w/v),

respectively. Lignins normally dissolved within one hour, while standards were

allowed to thoroughly dissolve overnight. Usually, two or three standards, differing

by at least a factor of five in nominal molecular weight, were combined in one flask.

Samples and standards were filtered through 0.45 pm pore size nylon or teflon

Acrodisc syringe filters (Gelman Sciences, Inc., Ann Arbor, MI) into 4 ml sample

vials for loading into the sample carousel for automatic injection in the 150C. As a

precaution against association in some mobile phases at room temperature, lignin






49
samples were filtered 'hot', at close to the SEC run temperature, by preheating both

the sample solutions and the glass syringes.


3.4.4 SEC Runs and Data Analysis


Final operating conditions for the 150C were established for running DMSO

+ 0.1M LiBr, as the preferred mobile phase, on the Jordi Gel GBR columns.

Initially, only the 103 A column was used, but to complete this study, the 104 A

column was also installed. A nominal flow rate of 1.1 ml/min (actual flow rate

approximately 1.03 ml/min), analysis temperature of 850 C, injection volumes of 50

pl and 100 Ml for lignins and standards, respectively, and two injections per sample

were used. Run times were 30 minutes for the 103 A column, and 45 minutes for the

103 + 104 A column set Proper injection volumes for both samples and standards

were determined by monitoring the peaks' retention time shifts with respect to

decreasing injection volume until no further shift occurred, or until the signal-to-noise

ratio became unacceptably low.

The UV/Vis and DRI detectors were connected in series which enabled dual

detection of the lignin samples and the polymer standards. However, only the lignins

displayed any UV absorbance in the transparent range of the mobile phase, and their

mass distributions were therefore monitored by UV at 280 nm, while the polymer

standards were monitored by DRI. The greater sensitivity of the UV/Vis detector

allowed for lower lignin injection volumes, and the 0.15 min time lag between the

two detectors was accounted for in the standards' retention times. Lignin molecular






50
weights were then calculated from the sample chromatograms by means of third

order narrow standard calibration curves with correlation coefficients of 0.995 or

greater. These MW calculations were cutoff on the low side at a calibration MW of

50.


3.5 Results and Discussion


3.5.1 General Comments on Mobile Phase Evaluation


The mobile phases listed in Table 3-1 represent a systematic approach to

developing a suitable mobile phase/column combination. This lengthy evaluation

process became the main emphasis in the development of an effective SEC analytical

method for lignins because of the experimental problems that were encountered.

The complex chemical interactions in lignin-column-mobile phase systems frequently

resulted in nonideal SEC elution behavior for lignins.

The experimental difficulties, such as association and adsorption, that many

previous investigators experienced, have also been observed in this study. General

results for the preliminary evaluation (mobile phases 1-17 in Table 3-1) were often

very inconsistent and not reproducible. This merely adds to the wealth of seemingly

contradictory and confusing SEC analyses of lignins. Derivitization of the lignins is

a common procedure to minimize some of these undesirable interactions; however,

this was not done in this study. The key element was selecting the proper column

chemistry (stationary phase) in combination with a compatible and effective lignin

solvent system.






51
The discussion of the experimental difficulties encountered in the evaluation

of the various mobile phases is an important aspect of this method development

because it addresses the major problems inherent in the SEC analysis of lignins.

Three mobile phase/column groupings were considered: THF, DMF and DMF mixed

mobile phases, and aqueous mobile phases (principally NaOH); run on Ultrastyragel,

Jordi Gel, and Jordi Gel GBR columns, respectively. Results for lignins run in

DMSO + LiBr on the Jordi Gel GBR columns are discussed separately.


3.5.2 Lignin Analysis in THF


THF was a logical mobile phase to start with because it has been frequently

used in the past by many other investigators. In addition, our set of Ultrastyragel

columns, which were purchased with the 150C, came packed in THF. This system

of THF with PS-DVB column chemistry and calibration with narrow MWD

polystyrene standards is widely used for nonpolar polymers, but was not satisfactory

for our analysis of lignins.

The major problem with this system was the very poor discrimination among

different molecular weight lignin samples (as determined by VPO and LALLS). All

of the lignins had essentially the same elution profiles with the same retention times.

Consequently, based on the polystyrene calibration, they all had nearly identical

average molecular weights. Another problem was the sometime limited solubility of

lignins in THF. The causes of this inconsistent behavior were not pursued, but it was






52
later surmised that the solubility of lignins in THF is strongly affected by the amount

of water present as an impurity.


3.5.3 Lignin Analysis in DMF and DMF Mixed Mobile Phases


DMF has also been widely used as a mobile phase for lignins--it is more polar

than THF and is a very good lignin solvent. However, the Ultrastyragel columns

could not be run in DMF because of the appreciable shrinkage in the gel, especially

for the 100 A column, which would result from a THF to DMF solvent changeover.

Therefore, the Jordi Gel columns were purchased for running DMF, and subsequent

organic mobile phases, because of the greater versatility of this gel for running

different polarity solvents.

The common result for these DMF based mobile phases has been lignin

adsorption on to the poly-DVB stationary phase resulting in abnormally long

retention times and unrealistically low molecular weights. An example of this

behavior is shown in Figure 3-1 for a typical softwood kraft lignin. Note how the

elution profile of the polymer peak is interrupted by the sharp negative peak which

is probably water and identifies the total permeation limit (low MW resolution limit)

of the column. Occasionally, normal looking chromatograms were observed,

however, these were not reproducible.

The mechanism for lignin adsorption probably involves attraction by the r

electrons of the aromatic rings in the gel for unshared electron pairs in hydroxyl and

ether groups in lignin. During kraft pulping, lignin undergoes significant structural











I 0.13-
0.12-
0.11-
0.10-
25 0.09-
0.08-
0.07-
0.06-
M 0.05-
0.04-
0.03-
0.02-
0.01-
0.00 1, -,-, -,
25.0 30.0 35.0 40.0 45.0 50.0 55.0
Retention Time (min)


Figure 3-1. Typical SEC Chromatogram for a Softwood Kraft Lignin Run
in DMF at 85" C on Jordi Gel Mixed Bed + 10W A Columns.


degradation followed by condensation reactions which partially counterbalance the

degradation and result in a structure which is very rich in phenolic hydroxyl and

methoxy groups [9]. Numerous adsorption sites for interaction with the aromatic

rings in the gel are therefore available.

The polarity of DMF, relative to that of lignin and the gel, must also play a

role in this elution behavior, because in THF, adsorption was not observed. The

overall Hansen solubility parameters, S. s, for DMF, THF, and the PS-DVB gel are

12.1, 9.5, and 9.1 (cal/cm3)), respectively [4, 21,]. We expect that 6. for the poly-






54
DVB gel is very similar to that for the PS-DVB gel. THF has thus approximately the

same 60 value as the gel, whereas DMF is significantly more polar than the poly-

DVB gel, and this polarity difference can promote lignin adsorption on to the gel.

Our approach for overcoming lignin adsorption has been to investigate mixed

mobile phases where a minor solvent possessing unshared electron pairs is added to

DMF so that it will preferentially adsorb to the gel instead of lignin. These

cosolvents are listed in Table 3-1 and include ethylene glycol (EG) and several of its

derivatives--ethylene glycol monopropyl ether (EGMPE), ethylene glycol dimethyl

ether (EGDME), and ethylene glycol monomethyl ether (EGMME); and others such

as triethyl amine (TEA), n-butanol, and pyrogallol (1,2,3-trihydroxy benzene). This

strategy differs from the more common approach of derivatizing lignins to tie up free

phenolic hydroxl groups as many previous investigators have done.

Chromatograms for lignins in these mobile phases generally also show

adsorption behavior, but on occasion have demonstrated a combination of both

adsorption and association behavior as seen in Figure 3-2 for a softwood kraft lignin

run in DMF/EGMPE (98/2) at 850 C. Note the sharp main peak, small secondary

peak, low MW tail, and small adsorbing peak, which is due to low MW lignin

fragments that are rich in phenolic hydroxy and methoxy groups. However, for all

of the mobile phases run on the Jordi Gel column set, any elution profiles that

appeared normal, and were relatively free from nonideal effects, were not

reproducible.











1 0.15-
o 0.14-
> 0.13-
U 0.12-
S 0.11-
4 0.10-
'0.09-
S0.08 -
0.07-
- 0.06-
0.05-
o0.04-
t 0.03-
3 0.02-
0.01-
0.00 ,i i i i i ,
15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0
Retention Time (min)


Figure 3-2. SEC Chromatogram for a Softwood Kraft Lignin Run in
DMF/EGMPE (98/2) at 85 C on Jordi Gel Mixed Bed +
103 A Columns.



3.5.4 Lignin Analysis in NaOH Solutions


Aqueous SEC provided a different, and potentially promising analytical

approach since lignins are readily soluble in strong alkaline solutions, and do not

associate in solution above a pH of 13. This switch to aqueous mobile phases,

though, required an entirely new column chemistry, and Jordi Associates, Inc.

provided us with a specially modified poly-DVB column which had been specifically






56
designed to minimize sample adsorption and be compatible with aqueous mobile

phases.

Elution profiles for lignins run in aqueous NaOH on the Jordi Gel GBR

column show excellent reproducibility and are characterized by very sharp and

narrow peaks, but with no resolution among the different MW lignins. As with THF,

all of the samples had nearly identical retention times, and based on polysaccharide

calibration, they had essentially identical molecular weights. For the seven lignins

from the University of Florida pulping experiment, M. s were only 3,537 to 3,698 as

compared to Mw s from LALLS measurements of 18,920 to 83,000.

In the strong alkaline solutions that were investigated: 0.1-0.5M NaOH (pH

13.0 to 13.7), both lignins and gel have electrolytic character. Hydroxide groups on

both must be partially or completely ionized at these high pH s. As the NaOH

concentration was increased, the eluting lignin peaks (both Indulin AT and

organosolv) were systematically shifted to longer retention times and gradually

broadened and lost their distinctive sharpness. Lignin molecules must become more

compact and assume a progressively more spherical shape as the solution ionic

strength is increased. This minimizes their hydrodynamic volume and leads to longer

retention times. Lignins were probably more ionized, and had a higher charge

density, than the gel, and therefore experienced a salting in effect and were trapped

in the pores of the gel for progressively longer times as the NaOH concentration was

increased due to charge repulsion from the mobile phase.






57
Aside from the poor MW resolution discussed above, these strong aqueous

NaOH solutions were also unsuitable from the perspective of equipment

compatibility. The quartz cell in the DRI detector, and the quartz windows in the

UV/Vis detector, were attacked by NaOH and resulted in a costly failure of the DRI

detector. The refractometer cell, especially the reference side where the solution is

stagnant, had to be flushed regularly with deionized water to slow down this

degradation process. Another, less serious problem, was persistent leakage from

tubing connections and fittings. It was exceedingly difficult to maintain tight

connections that were repeatedly broken and reassembled.


3.5.5 Lignin Analysis in DMSO + LiBr Solutions


Polar organic solvents, such as DMSO, were once more investigated, but this

time using the GBR columns where lignin adsorption was not a problem. Dimethyl

sulfoxide is a very good lignin solvent, but we discovered that lignins associate very

strongly in it. Lithium bromide salt was added to the mobile phase to break up these

associated complexes, and Figure 3-3 shows some examples of the dramatic changes

in elution profiles for Indulin AT in DMSO with various concentrations of LiBr.

The sharp bimodal distribution for Indulin AT in DMSO changes to a single,

more rounded, and nearly symmetrical peak in DMSO + 0.1M LiBr, and in DMSO

+ 0.2M LiBr, the peak exhibits retarded elution behavior, and is skewed to the low

MW end. At this salt concentration, lignin molecules are being trapped in the pores







0.40


0




0


4)



0


0.10 1 0 1 1. .
8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0


Retention Time (min)


Figure 3-3.


SEC Chromatograms for Indulin AT Run in DMSO with Various Concentrations of Lithium
Bromide at 85 C on the Jordi Gel 103 A GBR Column.


0.35



0.30



0.25



0.20



0.15






59
of the gel by a 'salting in' mechanism. Based on this comparison, DMSO + 0.1M

LiBr was selected as the appropriate mobile phase.

Association of lignins in polar organic solvents is a complex and regrettably

common phenomenon. It is dependent on temperature, as well as other parameters,

and can be eliminated by raising the analysis temperature to above the solute-solvent

system's Theta temperature. For example, Kim [52] established that the Theta

temperature for softwood kraft lignin-DMF systems is 80 C. Below this temperature,

association becomes progressively more pronounced.

The significant lignin association in DMSO at 850 C is quite unexpected.

However, the actual temperature of the UV/Vis detector is not 85" C, but room

temperature, because it is outside the 150C SEC and the mobile phase exits the

temperature controlled cabinet of the 150C SEC, passes through the UV/Vis

detector, and then returns into the 150C SEC to pass through the DRI detector

before going to waste. The solution cools very rapidly, and then heats back up and

equilibrates rapidly because there is no visible drift in the baseline signal from the

DRI detector. The association kinetics must be more rapid than the dissociation

kinetics because the bimodal elution behavior was also observed at the DRI detector.

The mode of association appears to be one of smaller molecules associating to form

much larger conglomerates resulting in a distinctive bimodal distribution.

Representative chromatograms for selected lignins are presented in Figures

3-4, 3-5, and 3-6, and demonstrate the versatility of this mobile phase-column

combination in separating a wide variety of lignins.







0.35


0.30




0.25




0.20




0.15


0.10 0.0 12.0 14.0 16.0 180 20.0 22.0
8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0


Retention Time (min)


Figure 3-4.


SEC Chromatograms for Selected UF Kraft Softwood Lignins Run in DMSO + 0.1M LiBr
at 85 C on the Jordi Gel 103 A GBR Column.


24.0







0.30


0.28
S_ FX27
^ 0.26 /
o FX11
0.24 FX25

S 0.22 0 o

N 0.20

0 0.184 -




C .12 -
0.14

S0.12 V IW6

0.10

0 .08 I I I I- I | I I -l I I I -
15.0 20.0 25.0 30.0 35.0 40.0

Retention Time (min)

Figure 3-5. SEC Chromatograms for Selected UF Kraft Softwood Lignins Run in DMSO + 0.1M LiBr at
850 C on the Jordi Gel 103 + 104 A GBR Column Set.






0.34

0.32 ----- Indulin AT
S0-.30 Maple
4 0.30 \
---- Organosolv
0.28 /

0.26 /

0 0.24

_, 0.22 /

0 0.20 -

0.18 / \ o

o 0.16 /- / \

41 0.14-
S0.12

0.10

0.08 I l i I n i-
8.0 10.0 12.0 14.0 16.0 18.0 20.0 22.0 24.0

Retention Time (min)


Figure 3-6. SEC Chromatograms for Indulin AT, Maple, and Organosolv Lignins Run in DMSO + 0.1M
LiBr at 85 C on the Jordi Gel 103 A GBR Column.






63
In Figure 3-4, elution profiles for four softwood kraft lignins from the UF

pulping experiment--FX11, FX25, FX27, and FX43--are very similar and have the

same general shape, but different retention times. There are some subtle differences

though: FX43 is slightly skewed to the high MW side, and FX25 is noticeably skewed

to the low MW side. All four have a small shoulder at tR = 20 min where

oligomeric lignin fragments are eluting. These characteristics are representative of

of all the lignins studied and reflect the different molecular weight distributions

resulting from the different pulping conditions. All of the kraft softwood lignins from

the UF pulping experiment are from the same species of wood, and therefore should,

on average, be the same chemically. Molecular weights for the complete set of

lignins are summarized in Table 3-2.

All of the lignins also exhibit the same sharp initial peak at tR = 11.3 min in

Figure 3-4. This was thought to be due to some small amount of very high MW

unresolved material that was being excluded from the 10W A GBR column. With

both 103 + 104 A GBR columns in use, the shapes of the elution profiles for the

same four lignins, presented in Figure 3-5, are essentially identical to those in Figure

3-4, except that the sharp initial peaks, which are now definitely being separated,

have decreased in magnitude relative to the main peaks.

In Figure 3-6, the elution profiles for three different types of lignins: kraft

softwood (Indulin AT), kraft hardwood, and organosolv, are significantly different.

The Indulin AT has a nearly symmetric profile, whereas the maple lignin is skewed

to the low MW side, and the organosolv lignin is skewed to the high MW side. All








Table 3-2. Lignin Molecular Weights from SEC in DMSO + 0.1M LiBr
at 85"C.


Lignin


nW


M1/Mn 19ybS


Indulin AT 1,582 6,058 3.829 49,380 8.15
1,332 5,142 3.859 9.60
Mixed Hardwood 997 3,357 3.360
Birch 1,148 3,128 2.725 29,710 9.50
Maple 1,196 3,229 2.700 12,900 4.00
ABAFX011 & 012 1,483 6,263 4.224 19,630 3.13
1,387 6,411 4.621 3.06
ABAFX015 & 016 1,750 8,519 4.868 83,000 9.74
1,521 8,687 5.711 9.55
ABAFX025 & 026 1,217 3,951 3.246 58,880 14.9
1,155 3,910 3.385 15.1
ABAFX027 & 028 1,672 7,298 4.365 21,930 3.00
1,519 7,960 5.242 2.76
ABAFX037 & 038 1,368 5,352 3.912 18,920 3.54
1,251 5,589 4.468 3.39
ABAFX043 & 044 1,696 8,677 5.116 42,930 4.95
1,543 9,672 6.269 4.44
ABAFX055 & 056 1,581 6,552 4.144
1,516 7,149 4.718
Organosolv
As received 809 2,403 2.970
N-hexane fraction 840 2,477 2.948
TCE fraction 977 2,905 2.973
Acetone fraction 944 2,763 2.926
Methanol fraction 954 2,907 3.047 __

Note: a For lignins with two MW entries, the upper number corresponds to runs on
the Jordi 103A GBR column only, and the lower entry corresponds to runs
on the Jordi 103 + 104 A GBR column set.
Calibration with narrow MWD polysaccharide standards; MW calculations
were cut off at a calibration MW of 50.
b Fully corrected M1 values were determined by Daojie Dong (unpublished
data) from LALLS.






65
three lignins also have a slight shoulder on the low MW side at tR = 19.5 min, and

the maple lignin has a second, and very substantial, shoulder at tR = 18.8 min.

These different profiles are primarily due to the different pulping conditions, and to

the general structural differences between softwood and hardwood lignins.

The extent of delignification (pulping) was much greater for the maple lignin

than for Indulin AT, based on their respective Kappa numbers: 15.0 versus 95-100,

respectively. Consequently, the molecular weights of the maple lignin would be

significantly lower than for the Indulin AT, as seen in Table 3-2. Pulping conditions

for the organosolv are not known, but based on the low MW s, the delignification

was probably very complete. Softwood and hardwood lignins have different

concentrations of primary and secondary ether bonds in their structures, and these

experience different depolymerization kinetics during the pulping reactions.

The three kraft hardwood lignins, despite being from different species--mixed

hardwood (oak and sweet gum), birch, and maple--have nearly identical elution

profiles, such as the one for maple displayed in Figure 3-6, and very similar average

molecular weights, as seen in Table 3-2. This is not surprising because both the

maple and birch lignins were pulped under identical conditions, as listed in Table 2-1.

All five of the organosolv samples exhibit the same elution profiles as the one

displayed in Figure 3-6, and only modest increases (about 17%) in both 1M and M.,

between the first two fractions: original, and n-hexane, and the remaining three

fractions: TCE, acetone, and methanol. Thus, the lengthy purification/extraction






66
procedure for the organosolv lignin did not significantly alter its average molecular

weights and MWD, as seen by the values listed in Table 3-2.


3.5.6 Column Calibration


The MW data presented in Table 3-2 is based on a third order calibration of

the Jordi Gel GBR columns using narrow MWD polysaccharide standards, which are

unique and have not yet been used by others for lignin analysis. A sample

calibration curve for the 103 + 104 A column set is presented in Figure 3-7.

The polysaccharide standards are linear molecules, and unfortunately, this

calibration method suffers from some of the same limitations that have plagued

previous investigators using polystyrenes and other linear polymers: the molecular

structure of the polysaccharides, and hence their elution behavior, are very different

from that of the highly branched lignins. Consequently, calculated MW values based

on this calibration can vary significantly from absolute values as seen by the

comparison of M, values from SEC and LALLS in Table 3-2.

The ,, values determined by LALLS have been fully corrected for optical

effects--sample flourescence, anisotropy, and absorption--and are 3-15 times greater

than the corresponding values from SEC, as seen in the last column in Table 3-2.

More significantly, there is also no correlation between the two sets of values, and

no constant factor that can be used to relate the SEC values to the LALLS values.

We believe that these 1M values from LALLS are accurate because in a forthcoming

study by Dong [17], M,, values for several kraft softwood lignins measured in three











106
log M 41.24 3.978tR + 0.1439tR2 0.001786t
r2 0.9967

105



100



103






102
15 20 25 30 35 40

Retention Time, tR (min)

Figure 3-7. SEC Calibration Curve with Narrow MWD Polysaccharide
Standards for the Jordi Gel 10W + 104 A GBR Column Set
Running DMSO + O.1M LiBr at 85 C.


different solvents: 0.1N NaOH, DMF, and pyridine, were within 10% of each other.

Narrow standard calibration, is thus only suitable for determining relative MW data

for lignins, unless well characterized lignin MW fractions are used as standards,

which is a very tedious approach.

An alternative calibration procedure, such as resolution of moments, which

was briefly described in section 3.2.3, is therefore needed. This calibration procedure





68
was the intended extension of this SEC study, but has not yet been investigated

because the necessary absolute M. values for these lignins have not yet been

measured by VPO. Resolution of moments should be pursued for the whole set of

kraft softwood lignins from the UF pulping experiment where an entire range of

carefully controlled pulping conditions has been investigated.


3.5.7 Comparison of SEC Results with Previous Work


Comparing calculated MW values for lignins from different studies is difficult,

and not very meaningful, because of the uniqueness of the lignin-column-solvent

conditions in each study. Our thoroughly executed, statistically designed pulping

experiment has not been duplicated by other investigators, and therefore, results for

these UF kraft softwood lignins cannot be compared directly with those from other

studies. Two of the commercially available lignins that we have studied--the mixed

hardwood kraft lignin from Westvaco, Inc., and the hardwood organosolv lignin from

Repap Technologies, Inc.--have also been analyzed by Siochi et al. [87], albeit using

a different mobile phase, column chemistry, and calibration procedure. Selected

results from these two studies are compared in Table 3-3.

Siochi et al. [87] derivatized their lignins to avoid nonideal interactions, and

ran them in THF on PS-DVB gel columns and used narrow MWD polystyrene

standards to construct a universal calibration curve. Their calculated NM, and M,

values from SEC/DV are 60% and 37% higher than our respective values, for the

Westvaco mixed hardwood kraft lignin, and 97% and 99% higher than our M, and








Table 3-3. Comparison of SEC Results for Mixed Hardwood Kraft
and Organosolv Lignins with Literature Values.

Lignin M M/_____
Mixed Hardwood (WHK) 997 3,357 3.36
HPL Mixed Hardwooda
(VPO)b 1,499
(SEC/LALLS) 3,711 17,120 4.61
(SEC/DV) 1,597 4,589 2.87
Organosolv (RO) 809 2,403 2.97
HPL Aspena
(VPO) 1,393
(SEC/LALLS) 4,004 24,070 6.01
(SEC/DV) 1,591 4,783 3.01

Note: a HPL Mixed Hardwood, and HPL Aspen are the hydroxypropyl deriv-
atives of the Westvaco mixed hardwood kraft lignin, and the aspen
organosolv lignin from Repap Technologies, Inc., respectively.
VPO and SEC runs were performed in THF.
Data is from Siochi et al. [87].
b Method abbreviations are defined in Key to Abbreviations.


1M values, respectively, for the organosolv lignin. Their SEC/LALLS results are too

high because they did not perform a beam polarization correction, but the

polydispersities for the two lignins from the two studies agree very well. These large

discrepancies are not serious, and this comparison should be viewed as having only


relative value.






70
3.6 Conclusions and Recommendations


3.6.1 Conclusions


In this study, after a lengthy mobile phase/column selection process, a new,

and relatively simple, SEC characterization method for kraft lignins has been

developed which does not require derivatization of the lignins to overcome

adsorption interactions. The following conclusions were then reached:

1. The elution behavior of lignins is complex and reflects its peculiar and

complicated chemistry. Selection of the proper mobile phase and

column chemistries is critical to achieving good elution behavior and

minimizing nonideal effects, such as adsorption interactions.

2. The preferred mobile phase is DMSO + 0.1M LiBr running at 85 C

on Jordi Gel GBR columns with sample detection by UV at 280 nm.

3. The GBR series of columns, with their deactivated gel structures, has

been an important development in this work because unfavorable

lignin adsorption interactions have been minimized, and consequently,

the need to derivatize lignins, in order to overcome these interactions,

has been eliminated.

4. Accurate and convenient column calibration methods must still be

investigated. The narrow MWD polysaccharide standard calibration

procedure, while convenient, resulted in M., s for kraft lignins being






71
lower by a factor of 3-15 as compared to fully corrected 1t, values

determined by LALLS.


3.6.2 Recommendations for Future Work


This SEC method is still not fully functional because the calibration procedure

only yields relative MW data. Several experimental problems still need to be

addressed in future work, and based on the discussion above, the following

recommendations were made:

1. The remaining UF kraft softwood lignins should be run in DMSO +

0.1M LiBr to determine their MWD s.

2. Once absolute 1M. values for these kraft softwood lignins have been

measured by VPO, the resolution of moments calibration procedure

should be pursued in order to develop more accurate column

calibrations.










CHAPTER 4
LIGNIN THERMAL ANALYSIS


4.1 Introduction


Thermal analysis is a very broad area, and within the scope of this study, we

have restricted it to the measurement of glass transition temperatures for purified

lignins and for solvent plasticized lignins. Lignins are amorphous polymers and upon

heating undergo a glass transition which is due to the onset of chain segment motion.

Glass transition behavior is characteristic of any amorphous polymer and is

accompanied by abrupt changes in physical properties, such as free volume, heat

capacity, and thermal expansion coefficient [68].

Differential scanning calorimetry, (DSC), is the method of choice for

measuring glass transition temperatures. Although it is not as accurate as a good

adiabatic calorimeter (1-2% vs. 0.1%), DSC's accuracy is adequate for most uses, it

is a very rapid and convenient technique, and is the method used in this study [6].

In the remainder of this chapter, the theory of the glass transition

phenomenon, and the operating principles for DSC, are discussed in section 4.2.

Previous investigations into the glass transition behavior of lignins, including

plasticized lignins, are discussed in section 4.3. The experimental work and data

analysis are described in section 4.4, and the results and discussion are presented in






73
section 4.5. Finally, conclusions and recommendations for future research are given

in section 4.6.


4.2 Theory


4.2.1 Glass Transition


The glass transition for amorphous polymers corresponds to the onset of

liquidlike motion of long segments of molecules, characteristic of the rubbery state,

as the material is heated. Conversely, as the material is cooled through the glass

transition, molecular configurations are frozen into a glassy state [6]. These

phenomena occur at the glass transition temperature, T9.

Below the Tg, amorphous polymers exhibit many of the properties associated

with ordinary inorganic glasses, including hardness, stiffness, brittleness, and

transparency, and demonstrate only local molecular motion, such as vibration and

rotation. Above the Tg, large scale segmental chain motion is evident [6]. Because

polymers are generally polydisperse materials, the glass transition is not sharp and

occurs over a range of temperature. The Tg is then defined as some intermediate

temperature within this range.

The glass transition phenomenon is usually explained by considering theories

based on free volume concepts and thermodynamics. In free volume theory, the

degree of molecular mobility is considered dependent on the intermolecular void

spaces, i.e. free volume, between polymer chains present in the material. This free






74
volume decreases with decreasing temperature until the glass transition is reached

where molecular mobility is no longer allowed [6, 62].

Below the Tg, the amount of free volume remains constant as the temperature

is decreased. The large scale molecular motion of polymers above the Tg requires

more free volume than the short range excursions of atoms in the glassy state. This

rise in relative free volume with increasing temperature leads to the abrupt change

in observed volume expansion coefficient at the glass transition [6, 62].

From a thermodynamic perspective, the glass transition phenomenon is often

referred to as a second order or apparent second order transition because of the

discontinuity that exists in the second derivative of the Gibbs free energy at the

transition temperature:


C = -Ti-G) (4-1)


where Cp is the heat capacity at constant pressure, G is the Gibbs free energy, T is

temperature, and P is pressure.

This discontinuity exists because the heat capacity of the glass is always lower

than that of the liquid at the same temperature and because there is no latent heat

in stopping translational molecular motion [104]. DSC provides a measure of the

heat capacity, and therefore it can readily measure this transition temperature [62].

However, this analogy with thermodynamic second order transitions is a poor one

because it implies more thermodynamic significance than the transition warrants [6].






75
In practice, the glass transition is very much a kinetically dominated event

which reflects the temperature region where the time scale for molecular motion

becomes comparable to that of the experiment. The Tg therefore does not have a

unique value, but occurs over a range of temperature and depends on the rate of

heating or cooling, e.g. annealing versus quenching [79]. Other factors which affect

the Tg are the material's MQ, and the molecular weight distribution. Plasticization

lowers the Tg by incorporating low molecular weight diluents. Chain branching

lowers the Tg (higher concentration of chain ends increases free volume), but

crosslinking raises the Tg because it lowers the free volume [6].


4.2.2 Effect of Plasticizer on T.


It is well established that adding a low molecular weight diluent or external

plasticizer to an amorphous polymer lowers its T. This phenomenon occurs because

the free volume of a low molecular weight liquid is very large relative to that of a

polymer at the same temperature and pressure. The overall free volume of the

mixture is therefore increased resulting in a reduction of the Tg [63]. Plasticizers can

also reduce secondary polymer-polymer bonding and can themselves form secondary

bonds to the polymer molecules thus increasing the free volume available for

polymer mobility and thereby lowering the Tg [80].

The lowering of the Tg for most systems is directly proportional to the diluent

concentration in the polymer. The widely accepted empirical equation relating the

Tg depression to the diluent content is given by Ferry [23]:






76

T = T kW2 (4-2)

where Tg is the Tg for the pure polymer, W2 is the weight fraction of diluent (g/g),

and k is an empirical constant. This linear relationship is valid at relatively low

dilution (<20%) if diluent and polymer are compatible, whereas a parabolic function

is required to cover the entire range of diluent concentrations [82]. Fujita and

Kishimoto [30] derived an analagous equation based on the iso-free volume concept:

T = T- W (4-3)


where a is the difference in thermal expansion coefficient above and below the

transition temperature and has a constant value of 4.8 x 104 per degree, and B is a

parameter representing the contribution of the diluent to the increase in free volume.

For various low molecular weight solvents in several common synthetic polymers,

values of 8 ranged from approximately 0.10 to 0.30 [30].


4.2.3 DSC Principles of Operation


DSC is a comparative analytical technique in which the differential thermal

behavior between a sample and a reference is continuously monitored and controlled

according to a time or temperature program. For the Perkin-Elmer DSC 7

instrument used in this study, this general operational principle is known as power

compensated DSC.






77
This instrument contains two control loops, one for average temperature

control and the other for differential temperature control. The average temperature

circuit measures and controls the temperature of the sample and the reference

holders to conform to a predetermined temperature program. The temperature

difference circuit compares the temperatures of the sample and reference holders

and proportions power to the heater in each holder so that the temperatures remain

equal. Thus, when the sample undergoes a thermal transition, power is supplied to

the two heaters as necessary to correct any temperature difference between them,

and a signal proportional to this differential power is plotted versus the time or the

temperature [6, 75, 101].

Platinum resistance heaters and thermometers are used in the DSC 7 to

accomplish the temperature and energy measurements which are made directly in

energy units (milliwatts) providing a true electrical energy measurement of the peak

areas [75]. The area under a peak is then directly proportional to the thermal energy

absorbed or released in the transition [101]. Some of the physical transitions that

therefore can be observed by DSC are crystallization, crystalline orientation, melting,

heat capacity, glass transition, heat of reaction, and polymer structure [68].

Numerous factors affect the characteristics of thermograms. Some of these

are instrument related and fixed, such as the design characteristics of sample and

reference holders, and others are operator adjustable such as the sample size and

mass and the heating rate. Other sample related factors include the heat capacity,

packing density, particle size, and thermal conductivity.






78
The two main factors of sample mass and heating rate must be varied in order

to strike an optimum balance between the two opposing criteria of resolution and

sensitivity. Increasing the sample mass increases the sensitivity, but decreases the

resolution and vice versa. Slower heating rates result in increased resolution of

minor thermal effects, while faster heating rates yield a larger signal-to-noise ratio

and greater sensitivity, resulting in larger peaks. Reversible transition temperatures,

such as fusion temperatures, are essentially independent of heating rate, whereas

irreversible transformation temperatures, such as glass transition temperatures, are

heating rate dependent [68].

For a given set of operating conditions, i.e. heating rate, type of sample pan,

and cooling medium in the reservoir, the energy axis (y-axis), and the temperature

axis (x-axis) must be calibrated with a standard material, such as indium, having a

known transition temperature (melting temperature), and a known energy of

transition (heat of melting).


4.3 Background and Literature Review


4.3.1 Introduction


A modest body of work covering the thermal analysis of purified lignins for

glass transition temperatures, and reporting a wide range of Tg s exists in the

literature. Lignin is an inherently complex material, and this range of T. values can

be attributed to the variety of wood species that have been studied, the various

extraction and purification techniques that have been employed, and the different






79
analytical procedures that were followed. Because of these experimental differences,

direct comparisons of Tg s from different studies are not very meaningful.

Although DSC is currently the method of choice, other techniques that have

been used include measuring softening temperatures by monitoring the collapse of

a column of powdered lignin in a capillary, and torsional braid analysis (TBA). In

TBA, a glass braid impregnated with the lignin sample is subjected to free torsional

oscillations during programmed heating. From these oscillations, changes in the

relative rigidity and damping and damping index reveal primary and secondary

transitions, such as melting or glass transitions, in the polymer [101].


4.3.2 Early Work: Characteristic Softening Temperatures


Goring [35] was one of the first to investigate lignin's glass transition behavior

by measuring a characteristic softening temperature (T,) for various softwood and

hardwood lignins and lignin sulphonates. His apparatus consisted of a capillary with

a weighted plunger in which a sample of lignin powder was compressed under a

constant load. The entire apparatus was immersed in an oil bath, and the extent and

rate of collapse of the column of powdered lignin were measured as a function of

temperature. The softening temperature was then defined as the temperature at

which the powder collapsed into a solid plug.

These lignins displayed softening temperatures in the range of 130 to 1900C

and were plasticized by water which decreased the T. and to some extent also

sharpened the transition. Two of the lignins were also plasticized by absorbed






80
organic solvents such as ethanol, benzene, pyridine, and dimethyl sulfoxide [35]. The

Ts also increased with increasing lignin molecular weight: T, = 127 "C for M. =

4,300, and T, = 176 C for M, = 85,000 [36]. This behavior is analogous to that for

synthetic amorphous polymers.

Lignin softening temperatures, while indicative of a physical transition in the

lignin, are not exactly analagous to the glass transition temperature. Softening

temperatures may be more closely associated with the onset of rubbery flow, which

is caused by the slippage of long range entanglements of molecular chains, rather

than the glass transition, which would normally commence at somewhat lower

temperatures [44].


4.3.3 Lignin Tg Studies


In a review by Nguyen, et al. [68], values of Tg for different lignins varied from

80 C for spruce dioxane lignin to 235 C for a softwood sodium lignosulphonate. For

several kraft softwood lignins, organosolv lignins, and lignin sulphonates, Tg s were

affected by thermal history. Two Tg s were observed for heat treatments below

132 C, but only one was observed for heat treatments above 1320 C. For two

observed Tg s, the lower Tg increased with increasing heat treatment temperature.

The Tg s for a fractionated thiolignin varied almost linearly with molecular weight

from 109 "C to 124"C, and the presence of methoxy groups decreased the T.

whereas the presence of hydroxyl groups increased it [68].






81
In his master's thesis, Masse [62] used DSC to determine Tg s of several kraft

softwood lignins obtained from a statistically designed pulping experiment. As with

previous work, he found the glass transition region to be very broad: generally 50 C

from 120 to 170" C. The Tg values were 144-148" C and were determined graphically

as the midpoint of the transition region. Since there is significant experimental error

in estimating the endpoints of the transition region, there was no significant

difference in the glass transition temperatures determined above.

In a comparative study, Yoshida et al. [106] used both DSC and TBA to

investigate the glass transition behavior of a softwood kraft lignin which was

fractionated by successive extraction with organic solvents. Molecular weights of the

lignin fractions were determined by SEC on an acetylated sample. For the

unfractionated lignin, M = 1,400, and M, = 39,000; for the lignin fractions, Mg =

450-5,800, and M,, = 620-180,000. The DSC experiments were run with 10 mg disc

shaped samples at a heating rate of 10 C/min under nitrogen, and the TBA samples

were run at a heating rate of 2" C/min for thermal pretreatment and analysis [106].

The Tg increased with increasing molecular weight from 32 to 173 "C, and the

temperature range of the glass transition increased significantly with an increase in

molecular weight and molecular weight distribution [106]. This is well known

behavior for many polymers. The Tg values estimated from TBA agreed closely with

those measured by DSC. However, results obtained by TBA may be influenced by

the macrostructure of the material [44].




Full Text
xml version 1.0 encoding UTF-8
REPORT xmlns http:www.fcla.edudlsmddaitss xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.fcla.edudlsmddaitssdaitssReport.xsd
INGEST IEID E3XFZPS85_LOST0R INGEST_TIME 2013-03-21T14:12:57Z PACKAGE AA00013563_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES