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PREPARATION AND EVALUATION OF POLYMER COMPOSITE MULTILAYERS
ON Si02 FOR USE IN MEDICAL SYSTEMS
HEATHER ANN TROTTER
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
Heather Ann Trotter
I first would like to acknowledge my mother and family, who are my life support.
They are the reason I am here. I would like to recognize the staff in the Department of
Materials Science and Engineering at the University of Florida. This program has given
me opportunities I never imagined. I would like to give special thanks to my advisor, Dr.
Abbas Zaman. His patience and guidance have made these accomplishments possible. I
would also like to thank my co-advisor, Dr. Richard Partch. His drive and commitment
to research have inspired and motivated me throughout this work. My other committee
members, Dr. Ron Baney and Dr. Karl-Johan Soderholm, are recognized for their
insightful discussions and positive attitudes.
I am also grateful for the financial support provided by the University of Florida
Particle Engineering Research Center (NSF Grant No. EEC-94-02989) and the industrial
partners of the Particle Engineering Research Center. Nissan Chemical Industries, Ltd.,
is acknowledged for the donation of the nano-sized Si02 particles. Help from Mr. Eric
Hughes for conducting some of the experiments and Mr. Scott Brown is greatly
acknowledged. Miss Jennifer Brandt is acknowledged for valuable discussions and
TABLE OF CONTENTS
A C K N O W L E D G M E N T S ................................................................................................. iii
LIST OF TABLES ............. ...................... ................................. .......... .. vi
LIST OF FIGURE S ......... ..................................... ........... vii
ABSTRACT ........ .............. ............. ...... ...................... ix
1 LITERATURE REVIEW AND INTRODUCTION................................1
Introduction to Polyelectrolyte M ultilayer Research...................................................1
Applications for Polyelectrolyte Multilayered Particles ......................................3
Current Characterization Techniques for Polyelectrolyte Multilayers................5
N ovelty and Scope of Study ............................................................................. 6
G oal of Study ............................................................... .... ..... ........ 8
2 M ATERIALS AND M ETHOD S ........................................... ........................ 10
Introduction to Materials: Biomedical Applications ..............................................10
M a te ria ls ...................................................................................................1 0
Study of PDADMAC Adsorption.... ............................ ..................11
K inetics of A dsorption .......................................................... ......... .............. 12
A dsorption Isotherm .................... ...... .. ............. ............ .... .. .............. 13
Electrophoretic Mobility (EPM) Measurements ...........................................13
D esorption Study .................................................... .... ........... 15
Study of P SS A dsorption ................. .............. .................................. ............... 16
Rheology .................. ..... .... ... ....... ...... .. .............. 16
Properties of Suspensions for Rheological Studies..................................17
Preparation of Suspensions for Parallel Plate Method .......................................18
Parallel Plate Rheological M easurem ents ................................ .....................19
Measurement of Adsorption Layer Thickness of PDADMAC ........................ 19
Preparation of suspensions for glass capillary method .............................20
Glass capillary theological measurements ...............................................21
Effective volume fraction (EVF) calculations .......................................... 23
Rg (Radius of Gyration) Calculations.............................. ...............23
Study of N anoSiO 2 Layer ........................................................................ 24
3 RESULTS AND DISCU SSION ........................................... .......................... 26
Adsorption of PDADMAC .............. ..................... ............... 26
Desorption of PDADMAC and PSS ................... .................. ................ 31
Z eta P potential ................................................................... 32
A dsorbed Layer Thickness ................................................. ............................ 35
Rheology of Dispersions of Multilayered Particles .................................................39
N an o S iO 2 ...................................................................4 1
4 SUMMARY AND CONCLUSIONS.......................................................................44
5 FU TU RE W O RK ........................ .......................... .. .... ....... ........ 47
L IST O F R E FE R E N C E S ....................................................................... ... ...................50
B IO G R A PH IC A L SK E TCH ..................................................................... ..................57
Table 3-1: PDADMAC adsorbed layer thickness values obtained by various methods. ..38
LIST OF FIGURES
1-1: Schematic of forces influencing properties of layer-by-layer films, and the
applications achieved by controlling or manipulating interactions . ...................4
2-1: Chemical structures of PDADMAC and PSS repeat units............... ...............11
2-2: A schematic of the double layer and ion distribution on a particles surface (upper
part of figure), and a depiction of the zeta potential at the shear (lower part of
2-3: Schematic showing the layer-by-layer adsorption of PDADMAC and PSS on
SiO 2 core particle. ........................ .... .................. .. .... .. .. .. ........... 16
2-4: Cannon-Fenske glass capillary viscom eter. ........................................ .................21
3-1: Adsorption density as a function of time for SiO2-PDADMAC system (250C, pH
= 4 .5 8 ). ........................................................................... 2 7
3-2: Adsorption density as a function of equilibrium concentration of PDADMAC
onto the surface of SiO2 particles (250C, pH = 4.58) ............................................ 29
3-3: Zeta potential as a function of polymer dosage for SiO2-PDADMAC system
(25C pH = 4.58) .......................................................................30
3-4: Desorption study of PDADMAC on SiO2 particles after centrifuging and
washing with deionized water. ........................................ ............................ 32
3-5: Zeta potential measurements showing the change in surface charge as
PDADMAC (Layer Number 1), PSS (Layer Number 2), PDADMAC (Layer
Number 3), and PSS (Layer Number 4) are added (250C, pH = 4.35, in the
absence of salt). ........................................................................33
3-6: Relative viscosities of dispersions versus volume fraction of SiO2 in water. .........36
3-7: Relative viscosity of SiO2 dispersion versus 200,000 molecular weight
PD AD M A C dosage ......................................... ....... ........ .. ........ .... 37
3-8: Adsorbed layer thickness of PDADMAC on surface of 1.5 [m SiO2 particles,
using effective volume fraction calculations, as concentration of PDADMAC
3-9: Viscosity versus shear rate at increasing dosage of PDADMAC on core SiO2
p articles. .......................................................... ................ 4 0
3-10: Viscosity versus dosage of PDADMAC on core SiO2 particles as a function of
increasing shear rate. .......................... ...................... ... .. ...... .... ...........40
3-11: Viscosity versus shear rate of core SiO2 particles, PDADMAC-coated SiO2, and
P SS-PD A D M A C -coated Si 2...................................................................... .... ..41
3-12: SEM of 1.5 m SiO 2 core particles. ............................................... .....................42
3-13: SEM of 1.5 [tm SiO2 particles completely covered with PDADMAC (MW =
2 0 0 ,0 0 0 )............................. ............................................................. ............... 4 2
3-14: SEM of nanoSiO2 adsorbed onto PDADMAC-coated Si2. ..................................43
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
PREPARATION AND EVALUATION OF POLYMER COMPOSITE MULTILAYERS
ON Si02 FOR USE IN MEDICAL SYSTEMS
Heather Ann Trotter
Chair: Abbas A. Zaman
Major Department: Materials Science and Engineering
Poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium 4-
styrenesulfonate) (PSS) have been consecutively adsorbed onto 1.5-rm charged silica
(Si02) particles to model the assembling of multilayered particles for use in time released
drug delivery particle systems. Time dependent adsorption studies indicate that, due to
the strong ionic charge of the dissociated polycation in water, adsorption is complete in
less than 30 minutes. Indications of the maximum adsorption density, changes in surface
charge, and stability of the layered particles are demonstrated through adsorption
isotherms and electrophoretic mobility (EPM) measurements. Further stability of the
PDADMAC layer is demonstrated through multiwashing with ultra pure deionized water.
Preliminary desorption studies of the PSS layer also illustrate a stabilized two-layer
Also, in this study, a systematic investigation on the viscosity behavior of
concentrated dispersions of silica particles with adsorbed PDADMAC and PSS layers has
been performed. The variation of shear viscosity and storage and loss moduli as a
function of layer number was investigated. The study illustrates that the adsorption of
PDADMAC increases the viscosity. Ideal conditions for maximum bridging flocculation
were also observed. From viscosity measurements of the PSS layer, a structural
formation was observed at low shear rates that broke down as shear rate increased.
Viscosity measurements showed that adding polymer layers to create multilayered
composite particles affects the theological properties of the particles.
Adsorbed layer thickness of PDADMAC was determined using viscosity data for
dispersions of bare SiO2 particles and dispersions of PDADMAC-coated SiO2 particles.
A comparison shows that experimental values, using relative viscosity, were comparable
to values recorded in literature using different methods.
After adding SiO2 nanoparticles (nanoSiO2) to the mixture of PDADMAC-coated
SiO2, EPM and SEM showed a change in surface charge as well as surface roughness,
indicating that the nanoSiO2 has been adsorbed onto the layered particles. Future work
includes surface and bulk property changes due to the nanoSiO2 on the surface, as well as
stabilizing this layer.
LITERATURE REVIEW AND INTRODUCTION
Introduction to Polyelectrolyte Multilayer Research
In recent years, there has been increasing interest surrounding the fabrication of
composite micro- and nano-structured materials using the self-assembly of polymers. A
number of novel possibilities arise from using self-assembly processes of polymers: by
involving electrostatic interactions, multilayered materials with unique and tailored
properties can be built. The pioneering work on synthetic nanoscale heterostructures of
organic molecules was carried out by the Kuhn group  in the late 1960s using the
Langmuir-Blodgett (LB) technique, in which monolayers of polymer are formed on a
water surface and then transferred onto a solid support as a single layer of molecular
chains on the surface. Their experiments with donor and acceptor dyes in different layers
of LB films provided direct proof of distance-dependent Forster energy transfer on the
nanoscale. These experiments were also the first true nanomanipulations, as they allowed
for the mechanical handling of individual molecular layers (such as separation and
contact formation) with Angstrom precision. However, the LB technique requires special
equipment and has severe limitations with respect to substrate size and topology as well
as film quality and stability.
Hong and Decher first proved the concept of alternating exposure of a charged
substrate to solutions of positive or negative polyelectrolytes . Provided that each
adsorption step leads to charge inversion of the surface, the subsequent deposition finally
results in a layered complex, stabilized by strong electrostatic forces, so-called self-
assembled polyelectrolyte multilayers. Since the electrostatic interactions are a very
general principle, the process is very versatile with respect to the incorporation of
different charged compounds or nanoobjects. As building blocks, for example, inorganic
nanoparticles such as gold colloids , functional polymers such as temperature-sensitive
compounds , orientable chromophores [5, 6], and mesogenic units inducing local order
 have been employed. Further work involves the deposition of proteins into
This principle of layer formation has not only been applied to achieve adsorption
onto planar substrates but has even been applied to colloidal particles [13-16], a
development which had a major impact in the field. The use of colloidal surfaces is
particularly attractive, since not only can the core particle be controlled but also the
particles within the layers. Micro- and nanoparticles are being studied for use in drug
delivery systems  as well as removing toxins during water treatment . Layered
particles can maximize the amount of drug carried and help control diffusion rates as well
as concentrations of toxins adsorbed, whether in the body or the environment. And
recently, core particles that have been removed to produce hollow shell materials have
promising structures for future applications [4, 19-22].
A number of external parameters, such as ionic strength of the solutions, the
polyion concentration, the charge density of the polyions, and the molecular weight, are
known to influence the resulting layer structure. By varying these parameters during the
deposition process, there are an infinite number of structures and, thus, properties for
these tailored formations.
Multilayered films of organic compounds on solid surfaces have been studied for
more than 60 years because they allow fabrication of multicomposite molecular
assemblies of tailored architecture [2, 23-25]. It has been well documented that the
physisorption or chemisorption of polyelectrolytes onto suface-functionalized substrates
can lead to the deposition of molecularly thin surface films. The controlled and selective
surface modification of colloidal particles allows the fabrication of composite materials
with tailored and unique properties for various applications in the areas of coating,
electronics, photonics, catalysis, sensing, and separations. Composite particles that
contain an inner core covered by a shell exhibit significantly different properties from
those of the core itself (for example, surface chemical composition, increased stability,
higher surface area, as well as different magnetic and optical properties). The surface
properties are governed by the characteristics of the shell coating . The interest in the
fabrication of layer-by-layer assembled multicomposite particles has increased in the last
few years as evidenced by the increase in the number of papers dealing with this issue.
Polyelectrolytes bearing dissociated ionic groups are one type of matter that can be
used as the multilayered shell of these composite particles. Their unique properties,
dominated by strong long-range electrostatic interactions, have been studied extensively
over the past few decades [27, 28], and due to their ability to adsorb strongly onto
oppositely charged surfaces, polyelectrolytes make good candidates for creating the
multilayered shells on core particles. The concept of electrostatically driven assembly of
multilayer structures allows for the incorporation of a wealth of different materials .
Applications for Polyelectrolyte Multilayered Particles
There is a wide application of natural and synthetic polymers in medicine, paper
making, mineral separation, paint and food industries, cosmetics and pharmacy, water
treatment processes, and soil remediation . A thorough understanding of
polyelectrolytes has become increasingly important in biochemistry and molecular
biology. The reason is that virtually all proteins, as well as DNA, are polyelectrolytes.
Their interactions with each other and with the charged cell membrane are still very much
As shown schematically in Fig. 1-1, electrostatic interactions between the polyion
in solution and the surface are the key to the final structure of the polyion layered thin
film; however, secondary, shorter range forces also play a role in determining the film
thickness, the final morphology of the film, the surface properties, and in some cases, can
determine whether or not stable multilayers can form at all.
Separations Drug Delivery
Biosensors Dental Composites
Selective Patterning Selective Membranes
Figure 1-1: Schematic of forces influencing properties of layer-by-layer films, and the
applications achieved by controlling or manipulating interactions .
These secondary interactions can also play a role in the selective deposition of polymers
on surfaces, the formation of acentric polar structures, and the nature of permeation and
ion transport within the film . An understanding of these interactions, as well as an
ability to combine polyions with other charged systems, dimensional polymer structures
and patterns, selective membranes, and a range of functional organic and organic-organic
hybrid composite thin films can be produced.
Ionic interactions are the most versatile; they permit the use of water as a solvent,
which is both environmentally attractive and allows the use of charged biopolymers such
as DNA, as well as polyelectrolytes, proteins, colloids, and many other charged or
chargeable materials. One potential property of such devices is a simple dynamic
structure in which the distance between two layers of "hard" objects colloidss or proteins)
is adjusted by controlling the degree of swelling in an intermediate "soft" layer
polyelectrolytee) simply by changing, for example, humidity. The physiochemical
properties of the resulting architectures can be largely modified by varying the number of
deposited layers, by changing the nature of the polyelectrolytes, the pH or the ionic
strength. The large versatility of the concept allows numerous applications in the
biomedical field ranging from modification of biomaterial surfaces to the construction of
enzymatic nanoreactors in which a cascade of reactions can be induced.
Current Characterization Techniques for Polyelectrolyte Multilayers
There are many ways to determine layer thickness as well as surface roughness.
Some of the techniques that have been used are transmission electron microscopy (TEM)
, single-particle light scattering (SPLS) , scanning electron microscopy (SEM)
, atomic force microscopy (AFM) , nuclear magnetic resonance (NMR) , IR-
spectroscopy, x-ray reflectivity , and using tagged molecules (e.g., 14C).
Considerable published evidence exists [35-39] on the ability of polyelectrolytes to
associate on the surface of particles, but the stability of such single or multilayer
assemblies has not been thoroughly evaluated. The major advantages of layer-by-layer
adsorption from solution are that multiple unique materials can be incorporated in
individual layers on core particles and that the final particle architectures and properties
are completely determined by the deposition sequence.
Novelty and Scope of Study
In this thesis, the basic principles of layer formation and internal properties of
multilayer formation was studied, starting from simple concepts describing the initial
adsorption process, and then advancing towards the bulk properties of the system, such as
flow properties. The focus was on fundamental physical properties and the evolution of
more realistic models from first simple model ideas for a drug delivery particulate
system. Oppositely charged polyelectrolytes were used as multilayers on SiO2 core
particles. The layers of polyelectrolytes represent layers of oppositely charged species
such as proteins and biological drugs. SiO2 was used as the core because it has been used
in many biomedical composites and is the main component in dental composites. If
layered particles are to delivery drugs or absorb toxins from the body or the environment,
a core material like SiO2 is a practical choice. As layers of oppositely charged
polyelectrolytes are sequentially adsorbed, the surface properties of the particles
change-surface chemistry, charge, size-affecting bulk properties such as the
conditions for flocculation and viscosity. The layers were prepared and characterized
using kinetics of adsorption, adsorption studies, as well as electrophoresis and desorption
studies. The mechanism of adsorption can be described as the self-assembly process
involving electrostatic interactions. The internal dynamics and stability of the interacting
layers were expected to be strongly dependent on the coupling of the charges between
subsequently deposited layers.
For biopolymers such as DNA and proteins, the association between counterions
and the backbone chain is electrostatic in origin. In order to design new technologies
with charged polymers the fundamental time scales and length scales of polymer and
counterion association need to be quantified. Charged polymers present a challenge due
to the long-range electrostatic interactions and coupled dynamics between small, fast-
moving counterions and polymers. Practical methods such as Langmuir adsorption
isotherms and electrophoretic mobility measurements were used to characterize the
change of chemistry and charge on the surface. The thickness of the polymer layer which
gives rise to steric stability depends on the conformation of the polymer at the interface.
In order to determine the adsorbed layer thickness of the first polymer layer, conditions
were kept natural with no salt or pH adjustments.
At first, polyelectrolytes were mainly used as rheology modifiers, and one of the
most interesting uses of these materials has been the stabilization of a wide variety of
colloidal systems. The specificity of interactions between the particles as a function of
polymer layer is studied mainly through zeta potential measurements and theological
methods. In the case of inorganic oxides dispersed in water, due to the interaction of the
solid colloidal particles with the dispersing phase, there is the development of charged
surface sites (depending on the dispersing phase pH) according to these reactions :
oxide MOH + H+ <- oxide MOH2+ (1)
<- oxide M+ + H20 (2)
oxide MOH + OH- -> oxide M(OH)2- (3)
-> oxide MO- + H20 (4)
An ionic surface is formed and, because of these charges, ions of opposite charge tend to
During the past forty years, theories have been developed to describe the adsorption
and conformation of polymer at the solid-liquid interface and also, theories have been
developed to explain particle-particle interactions in the presence of polymers [16, 41-
43]. Several important factors such as Brownian motion of the particles, particle size,
particle size distribution, particle shape, volume fraction of the particles, the viscosity of
the suspending media, and the range of particle-particle interactions govern the
Goal of Study
In order to tailor specific properties, a basic understanding of the structure and the
control of the process of layer formation is required. There is thus a demand for further
fundamental studies and for basic physical understanding. Specific properties of
polyelectrolyte multilayers, which are of fundamental physical interest, include the fact
that polyelectrolyte multilayers form two-dimensionally stratified layers, which grow step
by step in three dimensions. This leads to a behavior being dominated by internal
interfaces, and differing from the corresponding volume properties of the material.
In order to use these particles as a model for drug delivery or medical composite
systems, the appropriate methods for preparation and characterization must be performed.
After successfully creating multilayers of nanoscale films on SiO2 core particles,
nanoscale SiO2 was adsorbed onto rthe positively charged surface.
Recently, shear thinning and shear thickening have been predicted and observed
[44, 45]. Shear thickening is believed to be correlated to the loss of close packed layer
ordering . Addition of a polymer to a colloidal dispersion is also found to change the
structural ordering dramatically and often leads to a phase separation [43, 47]. Behavior
of charged colloids as a function of volume fraction (4) and ionic strength has been
extensively studied both experimentally  as well as theoretically ; however, the
stability of colloidal dispersions as a function of surface charge density has been
examined only recently [50, 51].
The importance of the change in theological properties was examined as layer
number increases. As the number of polymer layers on the surface increases, the
effective volume fraction, and thus the viscosity also increases. The conditions for
flocculation also change. It is important to understand how the theological properties can
be affected by adding polyelectrolyte layers if the particles are to be used as drug delivery
devices or for the adsorption of toxins.
MATERIALS AND METHODS
Introduction to Materials: Biomedical Applications
SiO2 is used for a variety of biomedical applications. This material is used as filler
in dental composites, in nanostructured materials and coatings for biomedical sensors
, for coatings on hip replacements, and coatings with antibacterial activity.
Nanoporous SiO2 in also used in biomedical applications. SiO2 particles are known to
have a high binding capacity for DNA , and it is possible to put entire genes into
nanoporous Si02. One can envision using these SiO2 particles as vectors for targeted
drug or DNA delivery . Since Si02 has a negative surface charge at pH > 3, this
material is a good candidate to model a drug delivery system with electrostatic
The Si02 particles used in this study were nearly monodisperse of approximately
1.5 tm in diameter (dso value) with a specific surface area of 2.19 m2/g . The purity
and density of the powder were 99.9% and 2.1 g/cm3, respectively. Si02 particles were
provided by Geltech Corporation and were used as received. Sizing of the Si02 particles
was performed using the Coulter LS230 at pH = 9 so that the Si02 particles were
completely dispersed. Poly(diallyldimethylammonium chloride) (PDADMAC), Mw <
200,000, and poly(sodium 4-styrenesulfonate) (PSS), Mw = 70,000, were purchased from
Aldrich and were used as received, at 20% wt and 30% wt in water, respectively.
PDADMAC and PSS repeat units are shown in Fig. 3-1. As stated earlier, the charged
polymers model charged species such as proteins, DNA, and some biological drugs used
to remove toxins. The SiO2 nanoparticles (19% wt SiO2 suspension in water, diameter =
13 nm as determined from TEM) were provided by from Nissan Chemical Industries,
Ltd. The water used in all experiments was high-purity deionized water prepared in a
three-stage Millipore Milli-Q Plus 185 purification system and had a resistivity of 18
MQ/cm. All experiments in this work were conducted in the absence of any added salt
and under the natural pH of the system. All centrifugations were conducted for 15
minutes at 8000 rpm (14400g).
--CH2 CH -)
(-CH2 CH CH2H
CH + CH, SO3Na
Figure 2-1: Chemical structures of PDADMAC and PSS repeat units.
Study of PDADMAC Adsorption
The multilayers of charged polymers were formed by alternating adsorption of
polycations and polyanions. Since SiO2 has a negative surface charge at the conditions
used, the PDADMAC was the first layer to be adsorbed. In order to understand the
properties and structure of what is made, an investigative study was done. This study
began with determining the adsorption development of PDADMAC on SiO2 particles.
All adsorption experiments were conducted at room temperature (250C) using
suspensions of 2% vol SiO2, with no salt added. The SiO2 particles were added to
varying polyelectrolyte dosages using a 1-g/L aqueous PDADMAC stock solution.
Depending upon the polymer dosage, the PDADMAC stock solution was diluted with
high purity deionized water to the desired concentration and used as the suspending fluid.
The required mass of dry particles was slowly added to the PDADMAC solution. After
addition of particles, the suspensions were ultrasonicated for eight minutes and left on a
Burrell Model 75 wrist shaker for 15 minutes in order for equilibrium and maximum
adsorption to be reached. Ultrasonicating and shaking ensures a uniformly charged
surface on the core SiO2 particles by exposing the entire surface to the polycation. After
equilibration, the samples were centrifuged. The supernatant was stored in a refrigerator
and later used for analysis by Tekmar-Dorhman Phoenix 8000 Total Organic Carbon
(TOC) analyzer. TOC measurements were made on the Tekmar-Dorhman Phoenix 8000
TOC Analyzer utilizing UV light and chemical oxidation techniques to break down
species containing organic carbon (such as polymers or surfactants) to CO2 which is then
analyzed using a non-dispersive infra-red (NDIR) detector for quantification. The
instrument can analyze samples containing as little as 2 ppb carbon.
Kinetics of Adsorption
Another important part of preparing multilayered particles for use in drug delivery
is the time of adsorption. Whether the layered particles will by adsorbing toxins in the
body or releasing drugs, kinetics and diffusion are very important to understand. In
biomedical applications, it is vital to know how long the drugs or other materials will be
in contact with the cell membranes or how quickly the particles can adsorb substances
harmful to the body. To study the kinetics of adsorption, all dispersions were prepared at
a polymer dosage of 6 mg/(g solids). The required mass of dry particles was slowly
added to the PDADMAC solution, and the suspensions were sonicated for five minutes
and left on the shaker for times ranging from 0 to 180 minutes. The samples were then
centrifuged and the supernatant removed. The residual polymer solutions were stored in
a refrigerator and later used for residual carbon analysis. The TOC helped to determine
how much polymer was adsorbed on the surface of the particles, and then a correlation
between time and concentration were made.
Also important is to determine the adsorption density of the polymer on the surface.
The molecular weight as well as chemical structure of the polyion can affect the density
of the polymer on the surface. It has been well established that adsorbed concentration as
well as ionic strength can affect the morphology on the surface [7, 16, 35, 55]. After
centrifugation, the supernatants were analyzed for organic carbon. Using a dilution factor
of 100, high-purity deionized water was used to dilute the residual solutions. The TOC
calibration produced an acceptance criteria of R2 = 0.99972 for aqueous solutions of
Electrophoretic Mobility (EPM) Measurements
A practical method for determining if the surface charge of the SiO2 particles has
changed after adding the cationic polyelectrolyte into the system is to use electrophoretic
mobility (EPM) measurements. Electrophoretic mobility is the rate of migration per unit
of electric field strength of a charged particle in a solution under the influence of an
applied electric field. From the EPM measurements, the zeta potential, or electric
potential at the shear plane, can be determined. The shear plane (slipping plane) is an
imaginary surface separating the thin layer of liquid bound to the solid surface and
showing elastic behavior from the rest of liquid showing normal viscous behavior. The
net charge at the particle surface affects the ion distribution in the nearby region,
increasing the concentration of counterions close to the surface. Thus, an electric double
layer is formed in the region of the particle-liquid interface. Figure 2-2 shows this double
layer and where the zeta potential originates. This double layer (upper part of figure)
consists of two parts: an inner region that includes ions bound relatively tightly to the
surface, and an outer region where a balance of electrostatic forces and random thermal
motion determines the ion distribution. The potential in this region, therefore, decays
with increasing distance from the surface until, at sufficient distance, it reaches the bulk
solution value, conventionally taken to be zero. This decay is shown by the lower part of
the figure and the indication is given that the zeta potential is the value at the surface of
Surface of sbear
,e e *
I s I
-- Stern layer
Figure 2-2: A schematic of the double layer and ion distribution on a particles surface
(upper part of figure), and a depiction of the zeta potential at the shear (lower
part of figure).
Electrophoretic mobilities of the bare SiO2 and PDADMAC-coated SiO2 particles
were measured using a Zeta Reader Mark 21 at 250C with no salt added. The apparatus
transforms the electrophoretic mobility u (m2/V-s) into a (-potential (mV) by using the
C U/ (5)
where r (Pa-s) and e (C/m-V) are the viscosity and permittivity of the solution,
respectively. Measured amounts of dry SiO2 were added the 1-g/L PDADMAC stock
solution to achieve a range of dosages. The 0.05% vol dispersions of the bare SiO2 and
the 0.05% vol dispersions of the PDADMAC-coated SiO2 were sonicated for three
minutes to break up any aggregates and left on the shaker for 30 minutes to ensure
equilibrium and complete PDADMAC adsorption before the measurements were made.
For either drug delivery or toxin removal, desorption of the polymer layers is
important to understand. If drugs are to be incorporated into a multilayered particulate
system, they must remain adsorbed until the external parameters change and they can be
released. Desorption behavior is a crucial part of achieving safe and effective time
released drug delivery systems. For toxin removal, the multilayers must stay intact while
in the body and adsorbing toxins. For these desorption studies, dispersions of 2% vol
were prepared at a polymer dosage of 15 mg/(g solids), ultrasonicated for five minutes
and left on the shaker for one hour. The samples were centrifuged and the supernatant
removed. The residual polymer solutions were stored in a refrigerator until analyzed by
TOC. High-purity deionized water was added to redisperse the samples in 2% vol
dispersions. The samples were ultrasonicated for five minutes, left on the shaker for one
hour, and centrifuged again. The multiwashing steps were repeated up to five times.
Study of PSS Adsorption
In order to achieve multilayers, an anionic polyelectrolyte is used, PSS. After
centrifugation and removal of residual PDADMAC, the particles were suspended in a
solution containing 15 mg/(g solids) of PSS polymer. After addition of the particles, the
2% vol suspensions were ultrasonicated for eight minutes and left on a Burrell Model 75
wrist shaker for one hour to ensure complete polyanion adsorption onto the PDADMAC-
coated SiO2 particles. Excess polyelectrolyte was removed after centrifugation.
o II L
1U 'P* t'' I I
Figure 2-3: Schematic showing the layer-by-layer adsorption of PDADMAC and PSS on
SiO2 core particle.
While the rheology of suspensions of hard spheres seems to be rather well
understood by now [39, 44, 57, 58], the flow behavior of particles stabilized by
multilayers of long polymer chains is still in need of further investigation. Rheological
studies of multilayered particles is a novel idea. Depending on the application of the
particles, they will be influenced by external forces. One of these forces, shear, is easily
induced in processing as well as for quality purposes. The rheology measurements can
give an idea of the physical properties of the particles. Environments where the
multilayered particles will experience shear are the bloodstream, processing conditions,
and perhaps, while being injected into the body. No one has shown the effects of shear as
layer number and composition changes. The theological study performed involves each
the PDADMAC and PSS layers. Both polyions have different molecular weights, chain
lengths, and chemical structures. These factors will have an affect on the particle-particle
interactions as well as over long-range hydrostatic interactions. Rheological
measurements are how to quantify these affects.
Properties of Suspensions for Rheological Studies
Since the theological properties of suspensions vary over an extremely wide range
depending upon volume fraction of the particles, shear rate, and particle-particle
interactions , different kinds of theological instruments must be used to determine the
viscosity of the suspensions at different shear rates and volume fraction of the particles.
Since deviations from homogeneity (e.g., phase separation) and sedimentation of the
particles can lead to serious errors, it is most important to make sure the samples have
been ultrasonicated and left on the shaker for a suitable time period. The method used to
determine the theological properties of suspensions depends on the characteristics of the
material to be studied. From the theological point of view, suspensions can be classified
as follows :
1. Dilute, low viscosity stable suspensions (viscous fluids)
2. Concentrated, high viscosity stable suspensions viscoelasticc fluids)
3. Solid suspensions (elastic solids)
4. Flocculated and coagulated suspensions viscoelasticc fluids with time
The state of a suspension at rest is determined by a balance between the Brownian motion
of the particles and interparticle forces. The suspension is considered to be dilute if the
Brownian motion is dominant and concentrated if particle-particle interactions
Preparation of Suspensions for Parallel Plate Method
All SiO2 suspensions were prepared at room temperature (250C) with 50% vol of
solids, with no salt added. PDADMAC was the first layer to be deposited on the SiO2
core particles. The SiO2 particles were added to varying polyelectrolyte dosages using a
30- and 60-g/L aqueous PDADMAC stock solution. Depending upon the polymer
dosage, the PDADMAC stock solution was diluted with high purity deionized water to
the desired concentration and used as the suspending fluid. The required mass of dry
particles was slowly added to the PDADMAC solution. After addition of particles, the
suspensions were ultrasonicated for 30 minutes and left on a Burrell Model 75 wrist
shaker for 24 hours in order for equilibrium and maximum adsorption to be reached.
Ultrasonicating and shaking ensures a uniformly charged surface on the core silica
particles by exposing the entire surface to the polycation. Using Paar Physica UDS 200
rheometer, at 250C, viscosity as a function of shear rate was measured at increasing
PDADMAC dosage, 0 to 20 mg/(g solids).
PSS was added as the second layer onto the PDADMAC-coated SiO2 particles.
After centrifugation and removal of residual PDADMAC, the particles were suspended in
a solution containing 15 mg/(g solids) of PSS polymer. After addition of the particles,
the 50% vol suspensions were sonicated for 30 minutes and left on a Burrell Model 75
wrist shaker for 24 hours to ensure complete polyanion adsorption onto the PDADMAC-
coated SiO2 particles. Excess polyelectrolyte was removed after centrifugation. All
samples were sonicated for five minutes prior to running the rheometer. Only 15 mg/(g
solids) PSS was used for viscosity measurements.
Parallel Plate Rheological Measurements
The Paar Physica UDS 200 is a rotational rheometer that can be used to measure
shear viscosity, viscoelastic functions, creep, and yield stress of materials using different
geometries such as cone-and-plate, parallel-plate, and concentric cylinder. Tests can be
performed under controlled "shear rate" or controlled "stress." In this study, the parallel-
plate geometry is used and the temperature controlled at 250C. The sample is loaded
into the space between the two plates. One of the confining surfaces is held stationary,
while the other one is made to rotate. The applied torque required to turn the rotating
plate is measured.
This method will be used to determine differences in viscosity and storage and loss
moduli as the layer composition changes. Changes in viscosity should be seen since the
effective volume fraction of the particles is increasing. Changes in storage and loss
modulus will give an idea of any structural organization, such as gelation, that may be
Measurement of Adsorption Layer Thickness of PDADMAC
Adsorbed layer thickness, 6h, is dependent on polymer chemistry, chain length,
ionic strength of the solvent, pH, and the ionic strength of the polyion. The thickness of
the polymer layer changes the properties of the particles, such as viscosity, and can affect
the applications and processing limitations. The modification of the viscosity by an
adsorbed layer of a surfactant or a polymer can be modeled in terms of an appropriate
increase 6h of the radius a of the particles. This leads to the increase in the volume
fraction, 4, of the particles, which is related to the viscosity of the suspension through
Einstein's equation :
|/o = 1 + k) (6)
where [t (Pa-s) is the viscosity of the suspension and [to (Pa-s) is the viscosity of the liquid
phase. Einstein determined a k coefficient equal to 2.5, which is only valid for an
infinitely dilute suspension. The Si02 particles in this study have a radius of 0.75 |tm,
and are considered fine, so it is necessary to determine the value ofk, Einstein's
Other methods of determining the adsorbed layer thickness of a polymer on
colloids are reported in the literature. Small-angle neutron scattering (SANS) , small-
angle x-ray scattering (SAXS) ,TEM , x-ray reflectivity , and uv-vis
absorbance . All of these methods require the knowledge of complicated equipment
and derivations. In this work, the relationship between changes in relative viscosity due
to the addition of the polyions is used (Einstein's equation) as well as radius of gyration
Preparation of suspensions for glass capillary method
Two sets of suspensions were prepared for the glass capillary method. The first, a
series of stable SiO2 suspensions (Si02 and deionized water) exhibiting various volume
fractions up to 20% vol Si02 were prepared. After ultrasonicating and shaking for 24
hours, the viscosities ([t) were determined.
The second set of suspensions contained Si02 particles and varying PDADMAC
concentrations up to 4% vol Si02. The Si02 particles were added to varying PDADMAC
concentrations. Depending on the dosage, the PDADMAC stock solution was diluted
with high purity deionized water to the desired concentration and used as the suspending
fluid. Several suspensions were prepared at a natural pH of 3 with various PDADMAC
concentrations (MW < 200000). After ultrasonicating and shaking for 24 hours, the
viscosity of the suspensions ([t) and of the corresponding mother liquors ([o) were
Glass capillary theological measurements
The glass capillary method was used to determine the adsorbed layer thickness of
PDADMAC on SiO2 particles. The glass capillary method has been shown to be an
effective method of determining the viscosity of dilute stable suspensions over a wide
range by varying the capillary diameter . In this simple technique, the time required
for a given volume of sample to flow through the length of the capillary, from point A to
B, under its own hydrostatic head is measured.
Figure 2-4: Cannon-Fenske glass capillary viscometer.
Flow effluxx) times, t (s), are related to the viscosity of the sample by an equation of the
[t/p = v = at + b/t (7)
where [t (cP) is the dynamic viscosity, p (g/cm3) is the density of the fluid, v (is the
kinematic viscosity (cSt), and a (cSt/s) and b are instrument constants. The last term in
the above equation is related to the kinetic energy correction which is negligible for flow
times over about two minutes.
The liquid in the bulb above the capillary provides the driving force, and since the
height change is relatively small, pressure changes are small during the test. Glass
capillary viscometers are usually used for low viscosity samples. Capillaries of different
diameters are used for different viscosity ranges to keep flow time through the capillary
in the range of 2-5 minutes. In this case, one capillary diameter was used.
Before the measurement was made, the capillary was cleaned of all debris and dried
with compressed air. After the sample was suctioned into the glass capillary up to the
appropriate line, the glass capillary was immersed in a constant temperature bath of
25 C. After waiting five minutes to allow for temperature equilibration and no bubbles
were present, the timer was started and stopped when the sample reached the appropriate
marks on the glass capillary. This procedure was repeated three times. Since all the
readings were within 180-360 seconds, only one glass capillary was used.
The visocmeter calibration constant was multiplied by the average time (in
seconds) to determine the kinematic viscosity, v, of the sample by using the above
equation with b = 0. The kinematic vsicosity was then multiplied by the density of the
suspension, p, shown in Eqn. 8. to determine the relative viscosity, [t.
v = [/p (8)
Effective volume fraction (EVF) calculations
The adsorbed polymer creates a coating on the surface of the particle with a
significant thickness and therefore, the particles have an effective radius, aef, that is
larger than the radius of the core particles. These systems may be treated as hard spheres
if an effective volume fraction is used instead of the core volume fraction of the particles
. Effective volume fraction (EVF) is defined as 
4eff = (aff/a)3 (9)
where 4eff is effective volume fraction, and 4 is the core volume fraction of the particles.
In the case of sterically stabilized suspensions, aef may be written as 
aff = a + 6 (10)
where 6 (nm) is the adsorbed layer thickness.
Rg (Radius of Gyration) Calculations
One method to calculate the adsorbed layer thickness without making experimental
measurements, is to use the radius of gyration, Rg, of the polymer. The Rg is the measure
of the end-to-end distance a polymer chain. Therefore, the thickness of the monolayer of
polymer on the surface of the SiO2 particles should be close in value to the Rg. Rg,
however, depends on the solvent, the temperature, the molecular weight, and chemical
composition of the polymer. Depending on the conformation of the polymer on the
surface, the Rg may or may not be a good predictor of adsorbed layer thickness. By using
r2 = NL2 (11)
Rg2 = r2/6 (12)
the Rg can be estimated. N is the number of segments in the polymer chain and depends
on the molecular weight and chemical structure. L is the length of each segment, or the
persistence length. Persistence length is the average projection of the end-to-end vector
on the tangent to the chain contour at a chain end in the limit of infinite chain length. The
persistence length is the basic characteristic of the worm-like chain . Tricot 
reports a persistence length of 27 A for PDADMAC (MW = 250,000 g/mol). This is not
the molecular weight of the PDADMAC used in this work, so the calculated Rg will be an
Study of NanoSiO2 Layer
In this work, the nanoSiO2 represents charged drugs that can be delivered to the
body's cells. For toxin removal, by incorporating nanoparticles as one of the layers, there
is an increase in surface area as well as surface energy. Adsorption of toxins should
increase due to the addition of a monolayer of nanoparticles. After removing excess
PDADMAC from the PDADMAC-coated SiO2 suspension, a 25-g/L solution of ultra
pure water and nanoSiO2 was added. In order to confirm that the nanoSiO2 was adsorbed
onto the positively charged surface, zeta potential measurements were made using the
Colloidal Dynamic Acoustosizer IIs. The Colloidal Dynamics Acoustosizer uses the
Electrokinetic Sonic Amplitude (ESA) effect. Charged particles in suspension are forced
to move by an oscillating electric field to produce acoustic compression waves (i.e.,
sound waves at the frequency of the excitation). Changes in density as the particles move
towards and away from the electrodes generate acoustic waves at the surface of the
electrodes as an echo of the excitation. The amplitude of the acoustic response is
proportional to the velocity of the particle caused by the electric field. The velocity is
proportional to the electric field amplitude. The "constant" of proportionality is the
dynamic mobility, which can be shown to be a function of particle size, zeta potential,
frequency, and phase lag of the particles velocity relative to the excitation frequency.
This relationship is used to extract the particle size and surface charge (zeta potential)
from measurement of the ESA. The advantage of this technique is that it can be applied
to very concentrated suspensions (up to 40 vol%).
After the zeta potential measurements were conducted, scanning electron
microscopy (SEM) was used to determine any changes in surface structure of the layered
RESULTS AND DISCUSSION
Adsorption of PDADMAC
Kinetics of adsorption was studied to determine the time it takes for complete
adsorption of PDADMAC onto the surface of the SiO2 particles to occur. Fig. 3-1 shows
the results from this kinetics study. Adsorption density of the polymer was determined as
a function of time for samples prepared using a polymer dosage of 6 mg/(g solids). After
mixing the particles with the polymer solution, samples were ultrasonicated for five
minutes and then left on a shaker for a given period of time, ranging from 0 to 180
minutes. An adsorption density of 0.4 mg/m2 is reached in less than 30 minutes. The
time of centrifugation and ultrasonication are included in the final time of adsorption.
The adsorption density varies by less than 0.1 mg/m2, but shows a steady plateau around
0.4 mg/m2. The shape of the kinetic curve is typical: a linear increase, which ends rather
abruptly. This study shows that the polycation completely adsorbs to the SiO2 surface
very quickly. In the fabrication of multilayers adsorbed onto colloids, additional aspects
have to be considered: competing with the formation of a defined monolayer around the
particle is the process of coagulation of partly covered particles. The corresponding
timescales of adsorption and coagulation are crucial for successful multilayer formation
on single colloids, as seen from the following: the rate of polymer chain adsorption (s/m2)
onto a particle is given by :
kads = 47lRpaDpolCpol (13)
using the assumption of an irreversible and diffusion controlled adsorption. Rpa (A) is the
radius of the particle Dpol (m2/s) is the polymer diffusion coefficient, cpoi (mg/mL) the
polymer concentration. The competing process, the rate of collisions (s/m2) between
particles, can be described
kcoll = 4l7Rpa2Dpacpa (14)
with Dpa (m2/s) and Cpa (mg/mL) as the diffusion coefficient and concentration of
particles, respectively. The factor 2 takes into account the fact that both objects are
diffusing. The requirement of a fast chain adsorption is thus fulfilled if the polymer
concentration is large compared to the particle concentration.
For medical composite systems like drug delivery processes, if charged species
are used and electrostatic interaction is the mechanism of adsorption, it is important to
understand the kinetics of this charged process, whether it be for the process of adsorbing
toxins or processing the multilayered systems. For removing toxins, the process of
adsorbing charged chemicals or molecules must be quick in order to avoid the death of
4 0.4 *
0 i- -
0 50 100 150 200
Figure 3-1: Adsorption density as a function of time for SiO2-PDADMAC system (250C,
pH = 4.58).
For the 2% vol SiO2 suspension containing PDADMAC (Mw < 200,000 g/mol), an
adsorption isotherm was determined by analyzing the initial solution and the residual
solution removed after centrifuging. The adsorption isotherm for SiO2 suspension
containing PDADMAC can be seen in Fig. 3-2. This curve has a typical shape with a
very steep increase at extremely low Ceq, followed by a saturation plateau where F is
nearly independent of concentration. F is the adsorption density of the polycation
adsorbed on the surface of the particles. The residual solution was tested for organic
carbon by TOC measurements. In Fig. 3-2, coverages approaching 0.5 mg/m2 were
observed at high concentrations of PDADMAC, on the apparent plateau of the isotherm.
At the surface of the SiO2, the average interfacial conformation of the PDADMAC chains
may be changing with coverage to include some segments that are loosely attached to the
surface, as small tails or loops. This may result from repulsions within the adsorbed
layer, which can be long range at the low ionic strength . When increasing the
amount of polymer added to the system, giving higher coverages above 0.6 mg/m2, the
ionic strength is increased by ions brought along with the polymer. The increase in ionic
strength resulting from the additional polymer in the system may screen long-range
electrostatic repulsions, allowing chains to approach more closely or adsorb more flatly
with decreased mobility . The amount of polyelectrolyte adsorbed depends on the
charge density of the surface, c, as well as the pH and ionic strength of the solution. The
c and charge density along the chain, and the pH and ionic strength are dominating
parameters for polyelectrolyte adsorption to hard surfaces. In this study, the pH and ionic
strength were not adjusted. Therefore, the flexibility, morphology, thickness, and
transport properties of the monolayer depends on the chemical structure, MW, and
amount of polyelectrolyte adsorbed. Error analysis of this study included preparing three
samples for each point and each sample was analyzed three times. The error calculated
was negligible on the plot presented.
0.0 0.2 0.4 0.6 0.8 1.0
Figure 3-2: Adsorption density as a function of equilibrium concentration of PDADMAC
onto the surface of SiO2 particles (250C, pH = 4.58).
The zeta potential is a measure of electrostatic interactions between solid particles
covered by polymer layers, and its magnitude depends on the adsorbed concentrations.
The magnitude of the zeta potential gives an indication of the potential stability of the
colloidal system. Literature reports that the SiO2 surface charge is dependent on the pH
value and the electrolyte concentration. At pH > 3, SiO2 has a negative surface charge,
and the surface becomes more negatively charged with increasing pH. In the case of bare
SiO2, there is a plateau in basic solution, and the point of zero charge, near pH = 3.0 3.5
. The bare SiO2 particles are well dispersed above pH = 3, and the negative charge of
the SiO2 surface will allow for electrostatic adsorption of the PDADMAC onto the SiO2
core particles. At a pH < 3, the surface of the SiO2 particles will have a positive charge,
and a PDADMAC monolayer will not form based on electrostatic forces.
Fig. 3-3 represents zeta potential as a function of PDADMAC dosage and, as can
be observed, the magnitude of zeta potential increases as more polymer is adsorbed onto
the surface of SiO2 particles. As shown in Fig. 3-3, zeta potential will reach a plateau at a
polymer dosage of 10 mg/(g solids). This dosage corresponds with the adsorption density
of 0.5 mg/m2, shown in Fig. 3-2.
60 f *
n 40- 4
N > 20
S0 ,-- i ---,-- i --- --
S5 10 15 20
PDADMAC Dosage, mg/(g solids)
Figure 3-3: Zeta potential as a function of polymer dosage for SiO2-PDADMAC system
(25C, pH = 4.58).
Both figures indicate that maximum saturation of the 1.5-[tm SiO2 particles, in natural pH
and ionic strength conditions, occurs at an adsorption density of 0.5 mg/m2 of
PDADMAC. Increasing the amount of polymer stabilizes the dispersion, and none or
only very weak repulsive forces exist in such a case. The zeta potential also increases but
begins to level off around +62 mV. Optimal flocculation in salt-free environments is
observed at the isoelectric point (C = 0). The plateau indicates a complete saturation of
SiO2 core particles with the polycation above 10 mg/(g solids). For error analysis four
samples for each point were analyzed four times due to the sensitivity of the TOC
analysis. The error was negligible the on the plot presented. Particles with zeta
potentials more positive than +30 mV are normally considered stable, and at a zeta
potential of +62 mV, the surface of SiO2 is completely covered with the PDADMAC.
Desorption of PDADMAC and PSS
To study the desorption of PDADMAC from the surface of SiO2 particles, samples
were centrifuged and re-dispersed in pure water several times. The supernatants were
analyzed for desorbed carbon using a TOC analyzer. The bare SiO2 particles were mixed
with a concentration of 0.63 g/L of PDADMAC. The supernatant removed after the first
centrifugation contained 0.57 g/L of PDADMAC, shown in Fig. 3-4, leaving 0.06 g/L
PDADMAC on the surface of the SiO2 particles. After adding deionized water,
redispersing the PDADMAC-coated particles, and centrifuging again, the desorbed
PDADMAC in the supernatant is measured using TOC up to five times. This study
indicates that after centrifuging and washing with water, the PDADMAC was not
removed from the surface of the SiO2 particles. Preliminary TOC measurements on the
desorption of PSS produce similar results. After centrifugation, not all of the PSS was
removed from the solution. PSS remained on the surface of the PDADMAC-coated SiO2
particles. EPM measurements also supported this observation.
The polymer layers must remain intact throughout the processing procedure.
Excess polyion in solution is removed through physical means and washing with pure
water. The electrostatic interaction between the layers and the surface of the SiO2 is very
strong. For a drug delivery system, however, the layers of polymer should be released
due to changes in external parameters, such as pH, glucose concentration (for diabetes),
or even high toxin levels. These parameters will have to be looked at in regards to the
diffusion of the polymers away from the surface of the particles.
E 0.2- \
0 1 2 3 4 5
Number of Washes
Figure 3-4: Desorption study of PDADMAC on SiO2 particles after centrifuging and
washing with deionized water.
EPM measurements were conducted to follow the assembly of the
PDADMAC/PSS multilayers onto the surface of SiO2 particles. In the absence of
polymers, SiO2 samples exhibited a zeta potential of nearly equal to -23 mV in pure water
(pH = 5). The zeta potential of the SiO2 particles can be altered, depending on whether a
polycation or a polyanion is adsorbed as the outermost layer . The zeta potential of
each layer is shown in Fig. 3-5. The zeta potential of the PDADMAC layer was +64 mV,
a value consistent with the outermost layer being a polycation. The assembly of the
second layer of PSS caused a reversal in sign of the zeta potential, from +64 mV to -42
mV, an indication that the outermost layer is a polyanion. This reversal in sign occurred
up to four total layers on the surface of the SiO2 core particles, ending with a negative
zeta potential, or PSS.
0 40 /
u( 0 1 2 3 4
Figure 3-5: Zeta potential measurements showing the change in surface charge as
PDADMAC (Layer Number 1), PSS (Layer Number 2), PDADMAC (Layer
Number 3), and PSS (Layer Number 4) are added (250C, pH = 4.35, in the
absence of salt).
The zeta potential measurements were conducted at a pH 5.3 (above the
isoelectric point of SiO2), bare SiO2 bears an overall negative charge at this pH. As
subsequent adsorption of PDADMAC and PSS layers occurred, the zeta potential
exhibited changes from positive to negative, respectively. The zeta potential values were
+62 mV when PDADMAC formed the outermost layer and about -42 mV when PSS
formed the outer layer on the SiO2 core particles. Caruso et al.  have reported a value
of +45 mV for zeta potential when PDADMAC forms the outermost layer in the presence
of 0.5 M NaCl in the system. The difference between the zeta potential values is due to
the presence of salt in the system used by these investigators. The alternating sign of zeta
potentials observed are characteristic of stepwise growth of multilayer films on colloids
. A similarly regular zeta potential has been observed after deposition of various
polyion pairs. An interesting feature is the fact that the zeta potential oscillates between
the same values, with saturation occurring around 60 mV after polycation adsorption, and
around -40 mV for polyanion adsorption. This implies that saturation occurs when a
fixed potential is reached, independently of material employed. It provides evidence for
the electrostatic repulsion by the outer layer being the mechanism controlling the
adsorbed amount, and limiting further adsorption, and not the polyion stoichiometry with
respect to the underlying layer.
The silica-water interface contains a variety of silanol groups (Si-O-H) that can
ionize to produce negative charges (Si-O-). Although the density of surface charges on
silica at pH = 5 is quite low, the density of potential counterion sites is high because the
silanol protons can exchange for other adsorbed cations . This allows for the
polycation adsorption. Additionally, the protonation of the oligomer itself contributes to
the ionic strength of the solution. The influence of ions on the coverage in Fig. 3-2,
however, cannot be observed since no salt was added to the system.
The increase in ionic strength resulting from the addition of polymer to the system
may screen long-range electrostatic repulsions, allowing chains to approach more closely
or adsorb more flatly with decreased mobility. The charge of the polyions leads to
enlarged polymer coils due to electrostatic repulsion between charged segments.
Furthermore, adsorption of polyions onto oppositely charged particles is dominated by
electrostatic forces whereby the adsorbed amount depends on shape, size, and
conformation of the polyion coils. At low ionic strength, polyelectrolytes with high
charge density, such as PDADMAC, tend to adsorb in a planar structure with a large
portion of adsorbed trains .
As suggested by Schwarz et al. , the question remains that to what extent zeta
potential and adsorption measurements can be applied to characterize stability or
instability of dispersions in the presence of polyelectrolytes. As shown in Fig. 3-4, the
magnitude of zeta potential increases by increasing the amount of polymer in the system.
The repulsive forces which are caused by the positive charges of the polycations increase
with increasing concentration of polyelectrolyte. This difference in magnitude of the zeta
potential can be ascribed to variations in the conformation of the polyelectrolytes at the
surface of quartz , which may arise from the differences in underlying surface
morphology of the particles. The alternating values do qualitatively demonstrate a
successful recharging of the particles surface with each polyelectrolyte deposition .
Adsorption of polyelectrolytes on oppositely charged surfaces depends strongly on
the electrostatic interaction, so the nature and concentration of the salt in solution and the
pH of solution may have a significant role. It has been observed that PDADMAC
adsorption on the surface of SiO2 particles increases with KC1 concentrations .
Bremmell et al.  measured forces between SiO2 surfaces in the presence of a cationic
copolymer and 0.1 mM NaC1. They found that the force was purely attractive at low
concentrations and repulsive above 50 ppm.
Adsorbed Layer Thickness
Adsorbed layer thickness can be calculated using many methods, as stated earlier.
For example, when using UV-vis adsorption spectroscopy or x-ray reflectivity after the
fabrication of multilayered particles [2, 24], the observed interference pattern, called
Kiessig fringes, can be analyzed to give the overall film thickness as a function of layer
number [24, 73]. As with many other systems prepared by this technique, they show that
the total thickness of the multilayer assemblies increases linearly with the number of
adsorbed layers, indicating a stepwise and regular deposition process. However, using
these methods can be difficult and time consuming.
Einstein's equation relates linearly the viscosity of the suspension to the volume
fraction of particles in suspension. For dilute solutions (< 4% vol) a k coefficient is equal
to 2.5. Fig. 3-6 shows that above 4% vol the suspension begins to deviate from
S1.60 r = 4.5763(p + 0.9331
pr = 2.5(p + 1
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Figure 3-6: Relative viscosities of dispersions versus volume fraction of SiO2 in water.
In order to determine adsorbed layer thickness for PDADMAC (MW < 200,000),
increasing concentrations of PDADMAC were added to the SiO2 core particles. As
expected at a dosage of 10 mg/(g solids) a plateau begins, as shown in Fig. 3-7. From the
previous adsorption isotherms and EPM measurements, this value makes sense.
Complete coverage of the core particles was occurring. To determine the adsorbed layer
thickness of the PDADMAC, a relative viscosity located on the plateau was used for
calculations. For a PDADMAC dosage of 12 mg/(g solids), the relative viscosity
recorded, 1.1989, corresponds to a volume fraction of 0.050, which amounts to 1.25
times the initial volume fraction, 0.04. As the SiO2 particle has a radius of 0.75 ptm, if
the volume fraction is multiplied by 1.25, the radius will be multiplied by (1.25)1/3, which
is an increase in radius that corresponds to a polymer thickness of 58 nm.
Fig. 3-7 shows the increase in relative viscosity as a function of polymer
concentration. It is clear that above 12 mg/(g solids) a monolayer of PDADMAC has
adsorbed. The relative viscosity values at PDADMAC concentrations above 12 mg/(g
solids) were used to plot the adsorbed layer thickness versus dosage of PDADMAC in
0 2 4 6 8 10 12 14 16 18
Dosage of PDADMAC, mg/(g solids)
Figure 3-7: Relative viscosity of SiO2 dispersion versus 200,000 molecular weight
8 10 12 14 16
Dosage of PDADMAC, mg/(g solids)
Figure 3-8: Adsorbed layer thickness of PDADMAC on surface of 1.5 [mm SiO2 particles,
using effective volume fraction calculations, as concentration of PDADMAC
It can be seen that at a critical dosage, a monolayer of PDADMAC is formed. By using
the glass capillary and calculating the relative viscosities of the samples, the thickness of
the adsorbed layer can be determined.
The persistence length of the 250,000 molecular weight polymer and equations (6)
and (7) were used to determine the adsorbed layer thickness. These values are listed in
Table 3-1 and are compared with adsorbed layer thickness values obtained using other
techniques. The radius of gyration of a polymer is dependent on molecular weight, ionic
strength of the solution, temperature, and chain length. Caruso and Mohwald  have
used single particle light scattering (SPLS), scanning electron microscopy (SEM), and
transmission electron microscopy (TEM) to determine adsorbed layer thickness of
polymer/particle multilayers. They also report an error of 10%.
Table 3-1: PDADMAC adsorbed layer thickness values obtained by various methods.
Layer Thickness (nm)
Layer Number SPLS  SEM  TEM  1r Rg
1 24 38 30 58 49
Due to error involved using the instruments and differences in the composition of
the core particles, ionic strength, and pH of the systems, deviations from Caruso and
Mohwald's work are expected. Caruso and Mohwald used negatively charged
polystyrene latex particles of diameter 640 nm, and they adjusted the ionic strength of
their PDADMAC solution to 10-3 M NaCl and the pH to 5.6. In this work, SiO2 is the
core particle (d = 1.5 [tm), and the ionic strength and pH (3-4) are not adjusted. Since the
values are within the same magnitude, it can be concluded that the glass capillary method
can be fairly accurate when determining the adsorbed layer thickness of the PDADMAC
on the surface of SiO2 core particles.
The glass capillary method was used to determine the kinematic viscosity, v. This
method is very sensitive to temperature and dispersion of the particles in the suspending
fluid. The samples were ultrasonicated and the particles dispersed well. The error,
however, calculated using this method was very small. Many dispersions were run
multiple times in order to exclude error in this data.
Rheology of Dispersions of Multilayered Particles
Rheology measurements were made using the parallel plate geometry as
PDADMAC concentration increases. Fig. 3-9 shows a distinct increase in viscosity
initially at low shear rates. This makes sense because as the PDADMAC is adsorbing
onto the SiO2 surface, the adsorbed layer thickness, and thus the effective volume also
increases. At low shear rates the viscosity increases by a factor of 10. In the case of 5
mg/(g solids), the viscosity has the maximum value. In order to explain this behavior, it
is necessary to note that complete coverage of the SiO2 particles does not occur until 10
mg/(g solids) PDADMAC solution is added. Therefore, it can be concluded that at this
dosage only part of the SiO2 particle surfaces are covered. When the particles are only
partly covered with polymer, the adsorbed polymer can form macromolecular bridge by
adsorbing onto two particles simultaneously. A dosage of 5 mg/(g solids) appears to be
the condition for maximum bridging flocculation.
At high shear rates, the bridging structures are broken down due to the high shear.
At high shear, the double layer deforms and the effective volume fraction is affected. In
Fig. 3-9, the viscosity in all cases shows a decrease at higher shear rates. At low shear
rates interparticle forces dominate, whereas at high shear rates hydrodynamic forces
1 -*-0 mg/(g solids)
---5 mg/(g solids)
-- F C10 mg/(g solids)
a. 12.5 mg/(g solids)
0.1 15 mg/(g solids)
C --0-20 mg/(g solids)
1 10 100 1000 10000
Shear Rate, s1
Figure 3-9: Viscosity versus shear rate at increasing dosage of PDADMAC on core SiO2
In Fig. 3-10 at a dosage of 5 mg/(g solids) a peak can been seen at low shear rates.
As the shear rate increases, this peak gradually disappears, another indication of the
optimal conditions of bridging flocculation. At higher shear rates, the particles are
dominated by long range hydrodynamic forces. For error analysis, three samples for each
point were analyzed three times. The error was negligible to the plot presented.
S-- 5.90 s-1
20 5 s-1
0 5 10 15 20
Dosage, mg/(g solids)
Figure 3-10: Viscosity versus dosage of PDADMAC on core SiO2 particles as a function
of increasing shear rate.
After the second layer, PSS, is added to the surface the effective volume fraction
increases again. At low shear rates, the viscosity is 100 times higher, but at high shear
rates the viscosities converge. This indicates structure formation at low shear rates. This
structure may be a gel formation. At low shear rates, the interaction between the
suspended particles is relatively weak and the system is predominantly viscous in nature,
while in the region of higher shear, the system becomes predominantly elastic and the
adsorbed polymer layers may interpenetrate or be compressed .
100 g PDADMAC layer
B g _A Si02
0.01 A A A AAAAAAAAAAAAA
1 10 100 1000
Shear Rate, s1
Figure 3-11: Viscosity versus shear rate of core SiO2 particles, PDADMAC-coated SiO2,
and PSS-PDADMAC-coated SiO2.
Fig. 3-12 is an SEM picture of 1.5 ptm core SiO2 particles. This picture exhibits
well the monodispersity of the core particles. After adding the appropriate concentration
of PDADMAC to the dispersion, the particles appear more ordered and less aggregated
(Fig. 3-13). The particles become more disperse due to the polyelectrolyte acting as a
dispersant. The particles are still monodisperse and appear to have a smooth surface.
Figure 3-12: SEM of 1.5 |tm SiO2 core particles.
Figure 3-13: SEM of 1.5 |tm SiO2 particles completely covered with PDADMAC (MW =
Fig. 3-14 shows the nanoSiO2 adsorbed onto PDADMAC-coated SiO2 particles.
The surface of the particles appears "rough." This "roughness" is the nanoSiO2 particles.
The particles are also attached and are flocculated. This may be due to the increase in
surface energy of the system. As the surface area of the particles increases due to the
addition of nanoSiO2, the surface energy also increases. Future work indicates that this
will be a problem for drug delivery and toxin removal systems. The layered particles
need to be able to circulate throughout the environment they are in and have maximum
surface exposure to be successful in their application. Another reason for the flocculation
is that the nanoSiO2 may have been added too quickly. The particles did not have enough
time for steric stabilization to occur. The exposed positive charges of the PDADMAC
caused the particles to stick together.
Figure 3-14: SEM of nanoSiO2 adsorbed onto PDADMAC-coated SiO2.
SUMMARY AND CONCLUSIONS
In this work, the stability of polycation and polyanion multilayers on SiO2 were
examined, both as a fundamental exercise and as a basis for future quantitative
interpretations of adsorbed high-molecular-weight cationic and anionic polyelectrolytes
for use in colloid drug delivery systems or toxin removal. The ionic strength and pH was
not adjusted, and for all the conditions studied, data for coverages above 0.5 mg/m2 were
found to show complete polyelectrolyte coverage on the Si02 particles for both
PDADMAC and PSS. For the natural pH range of our experiments (pH = 4-5), Si02
particles had a negative charge. EPM measurements confirmed this behavior. An
adsorption isotherm of adsorbed polycation as a function of equilibrium concentration
showed a characteristic plateau at 0.5 mg/m2. At and above this concentration, the
PDADMAC is completely covering the surface of the SiO2 core particles. The kinetics
study further showed that complete coverage and stability of the particle-polymer system
occurs in less than 30 minutes. This quick adsorption time makes sense since the
PDADMAC has a strong ionic charge when dissociated in water.
This work demonstrates that a homogenous and stable polyelectrolyte, both
polycationic and polyanionic, can be adsorbed onto micron-sized Si02 core particles in a
controlled, stepwise adsorption using electrostatic forces as the basis of motivation. Zeta
potential reversal with deposition of each layer was observed. Desorption studies under
multiwashing conditions shows stable layered structures. Adsorption isotherms and EPM
measurements both verify adsorption of polyelectrolytes onto SiO2 core particles. The
employment of colloidal particles as templates for the assembly of multilayer shells of
inorganic-organic materials through solution adsorption provides a viable route to the
production of tailored new materials with unique properties for drug delivery or
adsorption of toxins.
Control of stability and bulk theological properties is an important part of colloid
systems. It is also of fundamental interest to understand the relation between the
colloidal properties and factors such as interparticle forces, hydrodynamic interactions,
and physical and chemical characteristics of the system that govern the dispersion
properties . The theological measurements conducted showed that as the layer
number increases, the viscosity also increases. For the PSS layer, there may be a
structural organization of the particles at low shear rates that is disrupted at higher shear
rates. These measurements showed a large change in the bulk properties of the particles.
By adding one and two layers of polymer on the surface of the SiO2 particle, changes can
After a stable PDADMAC layer was demonstrated, nanoSiO2 was added into the
system. With no pH adjustments, the nanoSiO2 have a negative surface charge. It was
hypothesized that the nanoSiO2 should adsorbed onto the PDADMAC-coated SiO2
particles. Zeta potential measurements confirmed that the nanoSiO2 was adsorbed onto
the surface, but did not exhibit a stable structure on the surface of the layered particles.
At a pH of 4.3, the zeta potential of the particles was -14.23 mV. Further
characterizations are needed at this point.
SEM pictures indicated that there was a definite change in surface structure of the
layered particles. This structure is described as a "rougher" surface as compared to the
PDADMAC-coated SiO2. The PDADMAC-coated SiO2 micrographs exhibit very
smooth surfaces of evenly dispersed particles. After adding nanoSiO2 into the layered
structure, the surfaces appear "rough," and the particles are not well-dispersed. The
layered particles are sticking together. This indicates that the system cannot be
considered stable. The particles cannot be used as single drug delivery systems if they
are agglomerated. Further research must be done in order to resolve this issue.
For a number of applications, such as in controlled release or in separation
technology, it is of interest whether the properties of polyelectrolyte multilayers can be
changed by varying the external conditions, since controllable and tunable properties are
required. This concerns, for example, the control of permeation by external parameters.
In addition to this, the response of polyelectrolyte multilayers to external parameters can
be analyzed in terms of basic information on their internal material properties. For
example, swelling experiments provide insight into the interactions within polyelectrolyte
multilayers, and thus for example the internal hydrophobicity.
Predicting overall material properties as a consequence of the internal
composition and local interactions remains a challenging task. Polyelectrolyte
multilayers are complex materials with interesting properties on several length scales.
Each layer has its own unique properties, and the multilayered structure creates a three-
dimensional structure in which the layers combined create a particle with it's own unique
The growth of polyelectrolyte multilayers on colloid particles has been
demonstrated and shown to be a fairly simple process. However, from previous SEM
pictures, the surface layer on the 13-nm SiO2 particles does not appear to be stable, in the
sense that the layered particles appear to be aggregating. Zeta potential measurements
show that the surface charge is consistent with the nanoSiO2 covering the surface
completely (-14.3 mV, pH = 4.3). In order to stabilize this system, the layered particles
cannot be sticking together. Consideration into adjusting the parameters of adsorption
should be considered.
Once the nanoSiO2 is stabilized on the surface, experiments involving toxins
found in water as well as other classified bio-toxins. There has also been some work in
using nanosupsensions as nanoparticulate drug delivery systems.  It would be
promising work to begin incorporating drugs into the layers using electrostatics. In order
to use the systems is drug deliver, there will also have to be diffusion studies of the
polymer layers. The external parameters that affect diffusion rates will have to be
determined and stated. Some work regarding the diffusion of polyions in polyelectrolyte
multilayers has begun .
Further desorption studies in changing pH environments should also be
considered. It is known that pH has an important role in the layer thickness, morphology,
and surface properties of the polyelectrolyte structures, but it is unknown how is can
affect the stability of the layers.
In principle all polyelectrolytes should be suitable for incorporation into
multilayer assemblies; and in addition to this, this study has shown the incorporation of
nanoparticles as a layer. This implies that there is no principle restriction to
polyelectrolyte, and that the construction of multilayer assemblies should also be possible
by using charged particles. As an example of such particles we have chosen nanoSiO2.
For future work, the nanoparticles bind to the charged PDADMAC surface and partially
penetrates it. It is not known how the stable these nanoparticles are as a layer, as well,
whether or not multilayers can be formed on nanoparticles. It may be possible that the
next deposition layer, whether it be PDADMAC, a silanol (for silylating the surface), or
another positively charged polymer species (protein, DNA, etc), fills holes between the
nanoparticles, and the layer surface restored. Uniform layers of large biomolecules or
biomolecular assemblies under controlled conditions can be achieved.
The future, however, of multilayered SiO2 particles lies in adsorption of toxins,
heavy metal ions, and corrosive molecules. By using an adsorbent and layering that
adsorbent with nanoparticles increases the surface area as well as the surface energy.
These particles would be ideal for adsorption processes and can be studied further. There
is much potential in medical composite systems for these layered particles.
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I was born in 1980 and grew up in Tampa, Florida. I graduated from Berkeley
Preparatory School, Tampa, Florida, in June 1999. I was accepted into the University of
Florida and finished my Bachelor of Science in materials science and engineering,
specializing in polymers, in the summer of 2003. I continued with my Master of Science,
also in materials science and engineering, at the University of Florida. I plan to receive
my Master of Science in May 2005.
I met my future husband, Chad Macuszonok, in 1999, and we are planning our
wedding for August 2005. This is the beginning of the rest of our lives.