Hydrophilic surface modification of polymers for improved biomaterials

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Hydrophilic surface modification of polymers for improved biomaterials
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Thesis (Ph. D.)--University of Florida, 1995.
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Includes bibliographical references (leaves 220-229).
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by Tung-Liang Lin.
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HYDROPHILIC SURFACE MODIFICATION OF POLYMERS FOR IMPROVED
BIOMATERIALS
















BY

TUNG-LIANG LIN


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


1995














ACKNOWLEDGEMENTS


I would like to express my deepest gratitude and

appreciation to my research advisor and doctoral committee

chairman, Dr. Eugene P. Goldberg, for his guidence,

encouragement, generous support, and assistance during

preparation of this manuscript. My sincere thanks are also

extended to Dr. Batich, Dr. Adair, Dr. Whitney, and Dr.

Duran for their participation on the doctoral committee.

My sincere thanks must go to Drs. Hiroyuke Hattori,

Eric Luo, Ali Yahiaoui, and Jeanne Quigg for their advice

and support. Sincere appreciation is also extended to Paul

Martin, Emmanuel Biagtan, and Drew Amery for their

assistance and friendship.

In addition, I would like to thank all my fellow

students, for their friendship and cooperation.

I would also like to thank my parents, my brothers and

my sister for their support and encouragement. Most of all

I am grateful to my wife and my children for their love,

patience, and support during those years.















TABLE OF CONTENTS


ACKNOWLEDGEMENTS .......................................... ii

ABSTRACT ................................................... v

CHAPTERS

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

2 BACKGROCUID ................................................. 6

2.1 Biomaterials and Related Problems ..................6
2.2 Radio-Frequency (RF) Glow Discharge Plasma..........14
2.3 Surface Modification of Polymers ...................25

3 MATERIALS AND METHODS ...................................39

3.1 Materials................................................ 39
3.2 Methods.................................................. 42

4 RESULTS AND DISCUSSION ..................................72

4.1 Initial Studies of the PVP/NVP System...............72
4.2 RF Plasma Treatment of PMMA, PDMS, PP, PC, PVDF,
and FEP ....................................... ... .89
4.3 Surface Graft Copolymerization of PVP/NVP System
onto PMMA, PDMS, PP, PC, PVDF, and FEP Using
"Plasma/Gamma" Method .............................104
4.4 Surface Graft Copolymerization of PDMA onto
PMMA, PDMS, PP, PC, PVDF, and FEP Using
"Plasma/Gamma" Method .............................158
4.5 Surface Graft Copolymerization of PVP/NVP onto
PMMA Using "Two-step" Method ...................... 190
4.6 In Vitro Studies on the Hydrophilic Surface
Modified Substrates ................................194

5 SUMMARY AND SUGGESTED FUTURE WORK ......................206

5.1 Summary and Conclusions...... ......................206
5.2 Future Work ...................................... 209

APPENDICES








A HYDROPHILIC SURFACE MODIFICATION OF SILICONE
COPOLYMER CONTACT LENSES............................... 210

B QUlNJTITI NATIVE IAALYSIS OF MIGRATORY SILICONE IN THE
TISSUE OF IMPLANT SURROUNDING BY FT-IR/ATR LIQUID
CELL ... ..............................................216

LIST OF REFERENCES .......................................220

BIOGRAPHICAL SKETCH ......................................230














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


HYDROPHILIC SURFACE MODIFICATION OF POLYMERS FOR IMPROVED
BIOMATERIALS

By

Tung-Liang Lin

May, 1995





Chairman: Dr. Eugene P. Goldberg
Major Department: Materials Science and Engineering


In the biomaterials industry, polymeric materials are

widely used as medical or prosthetic devices. However, most

polymers currently used for these applications do not fully

satisfy the biocompatibility requirements. Although the

interactions between medical implants and living tissues or

cells are very complex and are still not fully understood,

it is well known that the interfacial properties of implants

play an important role in the interactions. Therefore, the

synthesis of biocompatible interfaces using surface

modification techniques were considered as one of the most









promising solutions in improving the biocompatibility of

implants.

This research was devoted to the synthesis and

characterization of hydrophilic surface grafts onto several

major biomedical polymers. These biomedical polymers

include PMMA, PDMS, PC, PP, FEP, and PVDF. Monomers,

including N-vinylpyrrolidone (NVP) and dimethylacrylamide

(DMA), were utilized in this study. Also, a new PVP/NVP

monomer system was employed for the surface modification.

A novel technique termed the "Plasma/Gamma Method" was

used to graft hydrophilic vinyl monomers onto polymeric

substrates. The "Plasma/Gamma Method" involves two

important steps: radio frequency (RF) water plasma treatment

and gamma-induced graft polymerization. The purpose of

plasma treatment is to revitalize the inert polymer surface,

thereby enhancing interfacial interactions and diffusion of

monomer to the substrate surface.

Surfaces were characterized using gravimetric analysis,

ellipsometry, contact angle measurements, Fourier transform

infrared by attenuated total reflectance (FT-IR/ATR), X-ray

photoelectron spectroscopy (XPS), and scanning electron

microscopy (SEM).

Preliminary in-vitro evaluations carried out with

polymer slabs indicated that PVP-grafted polymers by the

"Plasma/Gamma Method" significantly reduced the incidence of

cell adhesions.














CHAPTER 1
INTRODUCTION



In the biomaterials industry, polymeric materials are

widely used as medical or prosthetic devices. These

applications include heart valve stents, artificial heart

pumps, vascular grafts, sutures, artificial ligaments,

syringes, catheters, contact lenses, intraocular lenses,

viscoelastic solutions, orthopedics, wound dressings,

cosmetic implants, drug delivery systems, parenteral

packagings, and prosthetic fixatives, etc. [1].

Unfortunately, most polymers currently used for these

applications do not fully satisfy biocompatibility

requirements. The interactions between medical implants and

living tissues or cells are very complex and are still not

fully understood. However, it is well known that the

interfacial properties of implants play an important role in

the interactions. Consequently, the synthesis of

biocompatible interfaces using surface modification

techniques is considered to be one of the most promising

approaches to the problem of poor biocompatibility of

implants. Surface modification is a technique that only

changes the surface properties of substrate without altering

the bulk properties. The advantages of surface modification









techniques include maintaining bulk properties of implants

such as mechanical strength or geometry, combining graft and

substrate properties to form a superior composite structure,

and meeting the biological compatibility requirements.

Since Wichterle and Lim [2,3] established that poly(2-

hydroxyethyl methacrylate) hydrogels are excellent

candidates for contact lens applications, hydrogels have

received tremendous attention for medical and pharmaceutical

applications. Hydrogels are crosslinked networks of

hydrophilic homopolymers or copolymers. They are similar to

natural tissues and possess the properties of high water

content, softness, low interfacial free energy under aqueous

environments, and high permeability to oxygen, ions, and

small molecules, all of which may contribute to their

biocompatibility [4]. The major disadvantage of hydrogels

is their relatively low mechanical strength. To combine the

advantages of hydrogels with biomedical polymers,

hydrophilic surface modifications have been applied to

various implants. The hydrophilic surface graft serves as a

"slippery" water cushion while the bulk part of implants

provides needed strength to maintain mechanical integrity.

Therefore, hydrophilic surface modification of polymeric

materials has been recognized as one of the most feasible

approaches to improve the biocompatibility of medical

implants or devices.

Hydrophilic surface modification of intraocular lens

(IOL) has been intensely studied in this laboratory for more









than one decade [5,6]. The previous results suggested that

the hydrophilic surface modified IOL has the potential

advantage of reducing tissue and cell adhesion, thereby

improving long-term biocompatibility in the ocular

environment [5,7,8].

Surface modifications can be carried out by various

techniques. The most popular techniques include

conventional chemical reaction, UV induced graft

polymerization, gamma induced graft polymerization, and RF-

plasma induced graft polymerization [9-11]. Gamma induced

graft polymerization of hydrophilic vinyl monomers onto

polymeric IOLs has been developed and investigated in this

laboratory [5,6]. This is a heterogeneous reaction and is

carried out at the solid/aqueous interface by free radical

formation. Thermodynamic incompatibility between

hydrophilic monomers and hydrophobic substrates usually

hinders the heterogeneous grafting reaction. Swelling

agents to enhance monomer penetration [12] or use of radical

inhibitors to control excess homopolymerization [13] have

been employed to improve the grafting reaction. A "Presoak

Method," developed by Yahiaoui in this laboratory, was used

to control the diffusion of monomers into substrates and

resulted in a significant improvement in the grafting yield

[6].

This thesis describes a novel technique termed the

"Plasma/Gamma Method," also developed by Yahiaoui [6], that

was used to graft hydrophilic vinyl monomers onto polymeric









substrates. The "Plasma/Gamma Method" involves two

important steps: radio frequency (RF) water plasma treatment

and gamma induced graft polymerization. The purpose of the

RF water plasma treatment is to activate the inert

hydrophobic substrate surface. The non-equilibrium cold

plasma oxidizes and may generate peroxides on the surface.

After plasma treatment, the substrate surface changes from

apolar hydrophobic to polar hydrophilic. This change

decreases the thermodynamic incompatibility barrier between

the monomer and substrate surfaces. Eventually, these

generated weak peroxides could dissociate and form free

radicals which would enhance the grafting reaction during

gamma induced graft polymerization.

The objective of this research was to investigate the

graft process and the properties of grafted surfaces

prepared by the "Plasma/Gamma Method." Polymeric substrates

selected for this research included poly(methyl

methacrylate) (PMMA), poly(dimethyl siloxane) (PDMS),

polyproplyene (PP), polycarbonate (PC), poly(vinylidene

fluoride) (PVDF), and fluorinated ethylene-propylene

copolymer (FEP). The hydrophilic monomers primarily used in

this study were N-Vinyl pyrrolidone (NVP) and

dimethylacrylamide (DMA). A new monomer system containing

poly(vinyl pyrrolidone) (PVP) with NVP monomer was also

employed for the surface modification. Poly(vinyl

pyrrolidone) is a linear hydrophilic polymer and a good

wetting agent. Methods to characterize the grafted surfaces









included gravimetric analysis, contact angle measurement,

FTIR/ATR, XPS, SEM, ellipsometry, and optical microscopy.

Shear viscosity of polymer solutions was measured with a

Brookfield digital viscometer model DV II.

In addition, a two-step gamma method was also used to

synthesize PVP-g-PMMA. This method involved two consecutive

steps of gamma-induced graft polymerization. The purpose of

first gamma-induced graft polymerization is to increase the

wettability of PMMA surface and function similar to the RF

plasma treatment in the plasma/gamma method. The monomers

used were NVP and the PVP/NVP system.

Rabbit lens epithelial cell adhesion and spreading was

used to evaluate the biocompatibility of unmodified, plasma

treated, and hydrophilic surface modified substrates.














CHAPTER 2
BACKGROUND


2.1 Biomaterials and Related Problems



2.1.1 IOL Implants and Related Problems


Polymethylmethacrylate (PMMA) IOL was introduced into a

physiological environment in 1949 by Dr. H. Ridley [14]

after his observation that the fragments of Spitfire

canopies made of PMMA remained inert within the eyes of

pilots during the Second World War. PMMA is an optically

clear, rigid, and completely amorphous thermoplastic which

has shown relatively good ocular tolerance, biostability and

biological inertness [15]. Because of this, most IOL optics

today are made of PMMA. Figure 2-1 shows one-piece PMMA

lenses and three-piece lenses made of PMMA optics and

polypropylene haptic. Some drawbacks do exist with PMMA

IOLs, however. The material tends to be rigid and

hydrophobic which can lead to ocular tissue trauma, corneal

endothelium damage, iris erosion, secondary glaucoma,

intermittent hyphema. Residual lens epithelial cell

adhesion and associated postoperative inflammation are also

possibly related to the hydrophobicity and rigidity of PMMA
































(a)


0.15


0.30

0



6.5 13.5










(b)

Figure 2-1. Typical IOL Lenses
(a) one-piece PMMA lens,
(b) three-piece PMMA lens with PP haptic









surfaces [16-18]. Improvements in cataract surgery

techniques and PMMA IOL design have not completely

eliminated those problems. Also, another disadvantage of

PMMA is that it can not be sterilized by autoclaving because

of its relatively low glass transition temperature (Tg=105

C). Because of this, other polycarbonate (PC) polymers

having higher glass transition temperature (Tg=150 C) have

been considered as replacements for PMMA [19].

Presently, in cataract surgery, a new technology called

phacoemulsification has been adopted to remove the natural

lenses through a small incision (<4 mm).

Phacoemulsification uses high-frequency ultra-sound waves to

emulsify the natural cataractous lens [20]. Since this

method permits a smaller incision to be made than in the

past, it can minimize complications such as wound-induced

astigmatism and achieve early rehabilitation. However, the

diameter of PMMA IOL optics, usually 5-7 mm, is larger than

the size of the incision. This led to the use of foldable

IOLs which can be inserted through the small incision in

their folded state and then expand to full size. Two

foldable materials, polydimethylsiloxane (PDMS) and

polyhydroxyethylmethacrylate (PHEMA), have been utilized in

intraocular lens and have been evaluated for two

decades[21]. Both polymers can be sterilized by autoclaving

which is more convenient and is safer than ethylene oxide

gas sterilization. The structures of PMMA, PC, PDMS, and

PHEMA are shown in Figure 2-2.













CH3
I
---CH --C --

COOCH

PMMA





CH
--(-S
I
-f-si-o- -n
CH3
PDMS


CH3 0
^-- -< -o I-I

CH3

PC




CH3

--+CH C

COO-CH-CH-OH
2 2
PHEMA


Figure 2-2. Chemical structure of IOL materials.








Silicone (PDMS) IOLs were approved by the Food and Drug

Administration (FDA) and are currently commercially

available [22]. Silicone is a hydrophobic, soft, and

optically clear elastomer with a glass transition

temperature of -123 C. It forms a smooth surface without

polishing [23]. Silicone is biologically inert and has a

long history of implantation into biological tissues [24].

In clinical studies, silicone IOLs showed good stability and

tissue tolerance [25,26]. However, several problems have

been associated with silicone IOLs. These problems include

tissue irritation, adhesion and damage to living tissues due

to hydrophobic interactions, and loss of IOL optical

properties due to high lipid absorption and impurities [27-

29].

Polyhydroxyethylmethacrylate (PHEMA) has also been

evaluated as a potential material for flexible IOL design

[30,31]. PHEMA is a cross-linked hydrogel polymer which

will swell in water but does not dissolve. Hydrated PHEMA

is a hydrophilic, soft, and optically clear polymer. EHEILA

IOLs showed good tissue tolerance, stability, and visual

acuity [31]. In vitro studies indicate that PHEMA has lower

lens epithelial cell adhesion than PMMA and PDMS [32].

However, PHEMA lens, like many hydrogels, could result in

untimely unfolding and lens tears if it was not properly

handled [33].

In contrast, PMMA and PDMS exhibit good mechanical

strength, but lack a gentle hydrophilic surface. It is








obvious that a better combination can be created by surface

modification of PMMA or PDMS with hydrophilic hydrogel

monomers. whilee the PMMA or PDMS substrates can provide the

necessary mechanical strength, the hydrophilic surface layer

will supply a gentle interface to the living tissue.

Hydrophilic surface modification of PMMA IOLs has been

extensively studied in this research group for the past

decade. PMMA IOL surfaces have been covalently grafted with

PVP and other hydrogels using the gamma-induced graft

polymerization technique. In vitro studies showed that PVP

modified PMMA IOLs can significantly reduce lens epithelial

cell adhesion, corneal endothelium damage, and iris abrasion

[7,8]. Also, in rabbit lens cortex-induced inflammation

models, PVP modified PMMA IOLs yielded clean, cell-free lens

surfaces 3 weeks after surgery [34,35]. These results

indicate that the biocompatibility of PMMA IOLs can be

significantly improved by hydrophilic surface modification.


2.1.2 Interfacial Phenomena and Biomaterials


In considering the interfacial phenomena between

implant surfaces and biological environment, two major

events must be considered: 1) biomolecular interactions such

as protein adsorption; and 2) cellular interactions such as

platelet adhesion and lens epithelial cell adhesion and

spreading [36]. The first event is protein adsorption onto

the biomaterial surfaces, which occurs instantaneously when









biomaterials are placed in contact with biological

environments and continues until a protein film has covered

the biomaterial surface [37,38]. The second event, adhesion

of cellular components, may occur during formation of the

protein film or after this film has built up to a critical

thickness, which varies for different substrates, depending

on the surface properties of the materials [38].

Protein adsorption plays an important role in

determining the biocompatibility of implants. The amount of

protein adsorbed onto a biomaterial surface is strongly

dependent upon the implant surface properties such as

wettability, critical surface energy, and interfacial free

energy. Several studies have demonstrated that the extent

of protein adsorption decreases from hydrophobic surfaces to

hydrophilic surfaces [38-41]. Absolom et al. [37],

comparing several different proteins, also showed that the

most hydrophobic protein, fibrinogen, adsorbs to the

greatest extent to all surfaces tested, whereas the most

hydrophilic protein, bovine serum albumin, adsorbs to the

least extent to all the surfaces. Upon adsorption, the

first layer of proteins may denature on the solid surface

due to surface-induced conformational changes [38].

Therefore, the influence of the substrate is conveyed to the

cellular components through the denatured protein film.

Biocompatibility or biotolerability of implants is

measured by the degree of their interaction with the

biological environment. The biological response of








biomaterials is an extremely complex phenomenon and is

influenced by the surface properties of the biomaterials,

interfacial properties of the cellular components,

biomolecule adsorption, and surface-induced biomolecule

conformation changes. Adhesion of cellular components onto

biomaterial surfaces may cause inflammation or blood

clotting if the biomaterial surface is exposed to the blood.

The empirical relation between surface properties with the

biological environment have been described in a variety of

situations. Several hypotheses, based upon general

physicochemical considerations, have been formulated to

explain the interfacial properties of these biomaterials.

Some of the most interesting of these theories include the

moderate surface energy model of Baier [42], the minimum

interfacial tension hypothesis originally proposed by

Andrade [39], and modifications of these ideas [38].

Unfortunately, none of these theories adequately predict the

long-term biological response of biomaterials. It is very

difficult to use a single hypothesis to predict these

complex interactions between biomaterials and biological

environments. Furthermore, various hypotheses based on in

vitro observations have often conflicted with long term in

vivo studies.

Despite the contradictions, hydrophilic surfaces are

still very promising in terms of biocompatibility. Soft and

slippery hydrophilic surfaces are more similar to the

natural tissue, and their nonsticking characteristics can









reduce protein and cell adhesion [38-44]. Looking for

unique hydrophilic structures to improve biocompatibility or

blood compatibility is a challenging task.


2.2 Radio-Frequency (RF) Glow Discharge Plasma


Plasma treatments are widely used to improve the

wettability, bonding capability, and biocompatibility of

polymers. Three major effects are observed in plasma

treatments: ablation, cross-linking, and oxidation to a

depth of typically 50-500 A [45]. Wettability can be

improved by introducing polar oxygen and nitrogen groups

onto polymer surfaces via nonpolymer-forming plasma (e.g.,

plasma of water, helium, argon, nitrogen, oxygen). The

detailed mechanisms in plasma reactions are complex and are

not well understood. Energy can be transferred from a

plasma to a polymer surface through optical radiation,

through neutral particle fluxes, and through ionic particle

fluxes [46]. Yasuda [47] suggested two major energy

transferring reactions occur during plasma treatment. These

reactions include 1) direct reaction of active species with

polymer, and 2) reaction of polymer free radicals with

oxygen and nitrogen upon exposure to the air after

treatments [47]. Therefore, plasma treatments often

introduce polar groups onto the polymer surface even when

the plasma is an inert gas. The results of plasma

experiments strongly depend on the experimental parameters








such as pressure, field strength, gas velocity, and geometry

of reactor [48].

Plasmas can be classified into thermal plasma, cold

plasma, and hybrid plasma. Cold plasma (glow discharge in

reduced pressure) and hybrid plasma (corona discharge at

atmospheric pressure) are mainly used for plasma treatments.

Glow discharge plasmas can be generated between point-plate,

or wire-plate electrodes, using dc or ac (low frequency; 60

Hz) voltages of 500 to several thousand. On the other hand,

electrodeless glow discharges are generated by high

frequency oscillations introduced into the gases by means of

coils wound around reactors. High frequency oscillations

are supplied by a spark-gap generator (10-50 kHz), radio-

frequency generator (1.5-50 MHz), or microwave generator

(150-10,000 MHz) [49]. Radio-frequency (RF) plasma has

received the most attention in biomedical engineering, and

is currently used industrially also.

RF plasmas are ionized gases that contain UV radiation

ions, electrons, radicals, excited molecules, and atoms

[46,49]. Typical examples are


He----*hu + He + He'+ e- (2-1)

02---hu+ 0 0+ 0 + 0 (2-2)



where an asterisk denotes a metastable neutral species. The

gaseous ions and molecules are at ambient temperature,

whereas the electrons are at tens of thousands degrees








Kelvin. Chemical reactions are mainly induced by UV

radiation, free radicals, and the metastable neutrals

[50,51]. In the initiation step, polymer radicals are

formed by several different processes [46]:



By UV radiation:


hv
RF--- R- + F. (2-3)

By excited noble gas atoms:


RF* + He (2-4)
He*
RF R" + F- + He (2-5)

Ri + R2 + He (2-6)

By oxygen radicals:


O. R' + F' + 02 (2-8)
RF ---)
Ri + R20. (2-9)

By hydrogen radicals:



RF -H' R- + HF (2-10)



The polymer radicals undergo chain scission, radical

transfer, oxidation, disproportionation, and recombination.

These reactions give rise to a combination of degradation,

ablation, oxidation, and cross-linking in the surface layer

[45]. Thus, RF plasma is not only able to etch or cross-

link the surface of polymers, but also can introduce









functional groups or polar groups in the top surface layer.

The introduction of functional groups onto a biomaterial

surface will influence protein adsorption and cell adhesion

to that surface. In addition, these groups may provide

sites for the subsequent covalent coupling of biomolecules

or polymers. However, RF plasmas have several

disadvantages. These drawbacks include the following: RF

plasma has to be carried out under reduced pressure; the

life of polar or functional groups on polymer surface is

short (from minutes to months) depending on the surface

dynamic properties of polymer and the chemical-physical

environment; the kinetic mechanisms are very complex and

difficult to predict; and the stoichiometry of plasma

products is often not related to the chemical structure of

starting reactants [47].


2.2.1 The effects of Plasma Treatment on Polymers


RF plasma possesses the following characteristic

features: 1) the radiation effect is limited to the surface,

so that the depth of penetration is much smaller than with

other penetrating radiation and 2) the intensity at the

surface is generally stronger than with the more penetrating

radiation [47]. Therefore, the plasma discharge process

provides a unique and powerful means for altering the

surface properties of biomaterials without changing their

bulk properties.








2.2.1.1 The Effects of Plasma on PP

Plasma treatment of PP was first studied by Bamford and

Ward [52]. The electron spin resonance (ESR) studies on

plasma treated PP have shown that the radical formation

occurs mainly at the tertiary carbon. Peroxy-radicals (also

detected by ESR) were introduced when the plasma treated PP

was admitted to air. The peroxy-radicals in PP appear to be

stable with little decay in concentration after 17 hrs at

room temperature. Hall et al. [53] showed that adhesive-

bond strength was dramatically improved when PP was pre-

treated with oxygen plasma. Blais and Wiles [54] also

showed that the adhesion between nylon and PP was markedly

improved by a brief corona discharge treatment of the films

in nitrogen prior to coating. Attachment of amino groups to

PP surface was investigated by Hollahan et al. [55]. Amino

groups were attached to PP surfaces either by NH3 plasma or

by N2/H2 mixture plasma. The reaction for amino group

attachment is described as in equation (2-11).




Plasma *NH2 H2
-CH2 HCH2- --CH--CH-2 1-CHt -CH2 (2-11)
CH3 CH3 CH3



Following the plasma treatment, heparin was attached to PP

surface through ionic bonding to the quaternary sites

produced from the amino groups.







2.2.1.2 The Effects of Plasma on PMMA

Plasma treatment of PMMA has been extensively studied

by Vargo et al.[56]. The surfaces of PMMA films were

treated by 02/H20 and H20 radio-frequency glow discharge

(RFGD) plasmas and characterized with XPS, ISS (low energy

ion scattering), FTIR/ATR, and critical surface energy from

contact angle measurements. XPS and ISS show that the depth

of treatment for O2/H20 and H20 plasmas is less than 20 A.

Their results for XPS, ISS, and FTIR/ATR also confirm that

an 02/H20 RFGD is more reactive than the pure H20 discharge

because the C/O ratio on the 02/H20 plasma treated surface

is less than the ratio on the pure H20 plasma treated

surface. However, the contact angle measurements show that

the H20 plasma treated surfaces have a lower contact angle

(a more polar surface) than the 02/H20 plasma treated

surfaces. Based on the experimental results, Vargo et al.

proposed two simple models for the surface modification of

PI-II by 02/H20 and H20 plasmas. These models are illustrated

in Figures 2-3 and 2-4. Hollahan et al. have also tried to

attach heparin to PMMA surfaces by first plasma treating

with N2/H2 and NH3 plasmas [55].

2.2.1.3 The Effects of Plasma on PDMS

Hollahan and Carlson [57] have studied the surface

oxidation of PDMS using RFGD plasmas. Following the plasma

treatments, the PDMS surfaces were examined by FTIR

(internal reflection infrared) spectroscopy. The results

from IR spectra indicate that RF oxygen plasma and corona











CH3 CH3 Q\OCH
3 OCH3

-HC- --CH- --H--HC --C ---CH -
S CH
SOCH c OCH3 HCO
3

0o/Hp RFGD

--HC HC C OCH,

CH3C
C3 V HH H
-HC- CH- --H --C-CH,--

OCH, cOCH CH3 HCO
3
Modification
Product


CH OH 9 OCH,
C-HC-,-O-CH-,C-C-HC -C--HC -CH-

SOCH OH H \\O


Figure 2-3. 0/H20 RFGD modification of PMMA model. [56]








.OCH
CH3 CH3 3 CHOC
-H2C- -CH-- -- H2Y --HC -C -CH
C H
d'fOCH ;'oCH /\o
3 3 HCO
3
H20 RFGD

IF .OCH
CH3 CH, OCH CH3
-H- -CH-- --C2 -C -HC --C---CH--

C 3 OCH3 OCH HCO
3

Modification
Product


H 9H mHi 3 H
-HC- -CH2- --HC-C -H--C H9-

OCH H H R O


Figure 2-4. H20 RFGD modification of PMMA model. [56]









treated PDMS exhibit a high density of hydroxyl groups and

extensive hydrogen bonding. The IR data also suggested that

-OH formation in PDMS by RF oxygen plasma treatment is

predominantly of a -CH2OH structure rather than -SiOH. The

development of -SiCH OH structure takes place by the

sequences in Figure 2-5.

In the case of corona treatment, the localized high

temperature at a few plasma surface regions could strip off

methyl groups and produce -SiOH groups. Cross-linking

processes from radical recombination also possibly occurs

via the mechanisms in Figure 2-6.

Similar results were reported by Triolo and Andrade

[58] on surface modification of PDMS using RFGD helium

plasma. The shift of XPS Cls peak revealed that the carbon

became more bound to oxygen rather than to silicon alone as

a function of treatment time.

Sowell et al. [59] also studied surface modification of

RTV silicone using oxygen and argon RF plasma. The

wettability and bond strength were drastically improved by

plasma treatments. However, they did not address chemical

analysis in their studies.

2.2.1.4 The Effects of Plasma Treatment on PC

Surface modification of PC using RFGD oxygen plasma was

first studied by Hansen et al. [60]. Rate of weight loss

was monitored during plasma treatment. The oxidized surface

layers had remarkably low contact angles with water.






















Figure 2-5.


Oxygen RFGD modification


of PDMS model.


and


CH2 CH -CH
+ 5> cH-/f<--




CH-Si
2
S i rP


Figure 2-6. Cross-linking model of corona treated PDMS.


CH CH2
S + O* + OH-



CH2' CHOH

i + OH- 2


[57]


CH
/I<


[57]









The results suggested that the initiation stage during

oxidation by atomic oxygen is a direct and rapid attack on

the polymer which accounts for a great portion of the

overall oxidation of the polymer.

An ESCA (XPS) investigation on surface modification of

PC using hydrogen and oxygen RFGD plasmas has been conducted

by Clark and Wilson [61]. The data indicate that the effect

of hydrogen plasma treatment is to reduce the level of

oxygen functionality. On the other hand, oxygen plasma

treatment results in the formation of carbonyl and

carboxylate structural features. In addition, cross-linking

processes might occur during oxygen plasma treatment because

there is an increase in the number average of carbon atoms

in the oxygen-plasma-treated samples.

2.2.1.5 The Effects of Plasma Treatment on FEP

Fluorinated ethylene-propylene copolymer (FEP, Teflon)

is well known for its chemical inertness and thermal

stability. Surface treatments of FEP are therefore more

difficult than other polymers. Plasma treatments of FEP

have been studied by several authors [62-64]. FEP surfaces

were treated with helium and oxygen plasmas to improve their

wettability and bond strength [62,63]. Oxygen plasma

treatment led to noticeably weaker bond strength than helium

plasma treatment. The contact angle measurements showed

that helium plasma decreased the contact angle of FEP while

oxygen plasma had no effect on this property. This could








explain the difference in bond strength between oxygen and

helium treatments.

Triolo and Andrade [58] also did extensive work on

surface modifications of FEP using RFGD helium plasma

treatment. The FEP surfaces were characterized utilizing

XPS, SEM, and contact angle measurements before and after

plasma treatment. The results showed that the carbon and

oxygen contents at the surface increased with increasing

RFGD treatment time, and the relative amount of fluorine

decreased. The Cls peak located between the C-H and C-F2

peaks in the XPS spectra of RFGD plasma treated FEP

indicated the formation of carbon-oxygen and C-FH

functionalities.


2.3 Surface Modification of Polymers


Biocompatibility of medical implants or devices is

critically dependent upon the interactions between the

living tissue and the surface of implants. Surface

modification of polymers has therefore become one of the

most important approaches for improving the

biocompatibility. A variety of techniques for surface

modification of polymers has been investigated over the past

years. These techniques include gamma or e-beam induced

graft polymerization, chemical means, UV coating, and plasma

coating. The utilization of gamma induced graft

polymerization for biomedical applications has several









advantages over other techniques. These advantages include

the following: 1) no chemical initiators or external heat is

needed to initiate or assist the reaction; 2) gamma

radiation can generate the free radicals in monomers and

activate polymeric substrates simultaneously; 3) complex

geometries can be uniformly modified; 4) extremely clean

processes can minimize the presence of toxic residual

materials, and 5) grafting procedures are simple, making

production cost very low.

Besides, gamma induced graft copolymerization has been

used to increase surface wettability, dyeability, solvent

resistance, light resistance, and other properties

considered useful for many industrial applications [65].

The two major techniques generally used are indirect or

preirradiation methods, and simultaneous or mutual

irradiation of substrate and monomer. One-step simultaneous

irradiation is probably the simplest technique and was the

method of choice in this research.


2.3.1 Gamma Induced Surface Modification of Polymers


2.3.1.1 Radiation Effects in PMMA

The effects of radiation on PMMA have been extensively

studied and reported in various publications [9,66,67]. The

primary effect of gamma irradiation on PMMA is chain

scission degradation with the accompanying formation of free

radicals [68,69]. Color changes may be noticeable at








moderate radiation doses (< 3 Mrad)[9]. Several mechanisms

have been proposed by Kircher et al. and also by Todd

[68,69]. These mechanisms are illustrated in Figure 2-7.

Each main chain break liberates approximately 0.8 ester side

groups [67]. In addition, hydrogen abstraction from polymer

chain or formation of CH3- also initiates the main chain

scission [68]. Electron spin resonance (ESR) spectra

indicated that the primary free radical (IV) appears mainly

in the gamma irradiated PMMA, but the free radical (V) has

never been observed [68,70,71]. If the free radical (V)

forms, it may decay by a hydrogen atom abstraction from a

neighboring chain [67]. The G value (sessions per 100 eV of

energy absorbed) for PMMA is about 1.6 [66].

2.3.1.2 Radiation Effects in PP

When the irradiation of PP takes place in the presence

of air, it tends to undergo a very marked oxidative

degradation, even at fairly low doses [9]. The degradation

includes discoloration and oxidative embrittlement [72].

The results of ESR indicate that the predominant type of

radical formed at low temperature and low dose (< 5 Mrad.)

is most likely the free radical (I), or the free radical

(II) or both [73-75]. At room temperature, the thermally

stable radicals are probably either allylic radicals (III)

or (IV), or alkyl radicals (I) [73,76]. The sensitivity of

PP to oxidative degradation results from the large number of

tertiary hydrogens within the polymers [9].


















H 3 gamma
-C- CH---C- CH- -
COOCH- COOCH3 rays


CH3 CH3
I I
---+-CH-C-CH -C-CH--
COOCH

I

CH3 CH3
--- CH-C- CH -C- CH,-
SOOCH, OOCH3
II
CH3
-CH -C- CH,--C- CH,-
SCOOCH COOCH,


III


CH3
I -CH-C=CH


_I I +
II -OHOC-CH =C
2LCHOO3HJOOCH3


III -CH--2 =CH2
COOCH3


CH
I
+ C--CH--
cOOCH3
IV


CH3
-CH& -
OOCH3
V


CH3
+ C--CH,-
OOCH
IV


Figure 2-7. Schematic diagram of PMMA degradation
mechanisms. [68,69]


+ .CH3


















H
-CH -C- CH-
CH 2
CH3


gamma

rays


--CH -C- CH-
21 2
CH3
I

H
-- -CH2-C- CH
I




--- -C-CH-C=CH--
CH3 CH3 CH3
III


S-C- CH-C=CHC-
II2






CH3 CH3
IV


Figure 2-8. Schematic diagram of PP degradation mechanisms.
[73-75]









Hydroperoxides may be formed at the tertiary carbon sites,

when the PP is irradiated in air or oxygen [9].


2.3.1.3 Radiation Effects in PDMS

Poly(dimethyl siloxane) (PDMS) is relatively radiation

stable but tends to cross-link when it is subjected to a

high gamma irradiation dose [9]. The yield of cross-links

(G) is about 2.5 4.5 cross-links per 100 eV absorbed

[77,78]. Hydrogen, methane and ethane gases are evolved

[9]. Studies using ESR indicate that the C-H and Si-C bonds

are easy to fracture, leading to the formation of hydrogen

atoms and methyl radicals [79,80]. The schematic diagrams

of radical formation are illustrated as follows:






CH
CH_ CH; + --Si- O--
Ij gamma CH
--Si-O- --
I rays
CH3

H- + --Si-O-
I
CH3



Figure 2-9. Schematic diagram of PDMS degradation
mechanisms. [79,80]



2.3.1.4 Radiation Effects in PC

Polycarbonate (PC) contains aromatic rings in the main

chain structure and shows good radiation resistance up to

high radiation dose (ca. 100 Mrad) [81]. Acierno et al.








[82] have studied the radiation effects on PC and measured

the melt flow index and intrinsic viscosity of irradiated PC

samples. All the data show that a cross-linking effect

predominates at small doses (up to 3 Mrad), while main chain

scission occurs at higher doses (> 3 Mrad). The value of G

(scissions per 100 eV of energy absorbed) is 0.14 in oxygen

and 0.09 in vacuum [83]. The mechanisms of cross-linking

and main chain scission in PC are not well established, but

may occur as indicated in Figures 2-10 and 2-11.

2.3.1.4 Radiation Effects in Fluoropolymers

Fluorinated polymers can be extremely sensitive to

radiation and are among the poorest known polymers in terms

of radiation stability [9,81]. For example, PTFE shows

significant radiation damage even at a fairly low dose of

0.04 Mrad. [81]. However, FEP polymer is more radiation

resistant than PTFE and can be sterilized without extensive

damage. Other fluoropolymers, such as poly(vinyl fluoride),

and poly(vinylidene fluoride), show much less sensitivity to

radiation. Poly(vinylidene fluoride) (PVDF) were found to

cross-link at high gamma doses [68].

ESR spectra of PTFE indicate that the free radical (I)

and the free radical (II) are yielded from the main chain

scission [84,85].
F FF F
--C. ---C-C--
i I I
F F F

(I) (II)
















CH O CH 0
-- 11- gamma C, 3 O-
-- -- o-- -o-Ic-o--






CH O

CH,
CH,
-0- -- -0--
CH 0


Figure 2-10. Schematic diagram of
mechanisms.


PC cross-linking









CH, 0 CH3 0
--o- -^>-o--o0- 0 -,-o- -
CH3 CH3

gamma rays



CH 0





CH3 0
-0- -1 -0
CH3


CH3 0
-o- _-c- o-o--
CHI3-


Figure 2-11. Schematic diagram of PC degradation mechanisms.









The presence of double bonds of -CF=CF- and -CF=CF2 in

irradiated PTFE was suggested by Ryan [86] on the basis of

IR analysis.


2.3.2 Gamma-Induced Graft Copolymerization of Vinyl Monomers
onto Polymeric Substrates


Radiation induced grafting is a very efficient method

for preparing polymers with specific surface properties.

This method has been used in several studies for the

synthesis of polymers for biomedical applications. The

grafting reaction can either be carried out homogeneously

through thick layers of polymer, or limited to a surface

zone of any desired thickness. Among the various methods

for radiation grafting, four have received special

attention. These are 1) the direct radiation grafting of a

vinyl monomer onto a polymer; 2) grafting onto radiation-

peroxidized polymers; 3) grafting initiated by trapped

radicals; and 4) the intercross-linking of two different

polymers [9]. It was shown that most radiation induced

grafting proceeds by free radical mechanisms [87].

Grafting onto polymers that have been radiation-

peroxidized or via trapped radicals usually requires a high

radiation dose to generate the radicals or peroxides on the

polymer. However, surface uniformity of the graft and

reproducibility are often poor with either methods.

Furthermore, the high dose of radiation may cause permanent

damage to polymers.









Very low dose (< 0.2 Mrad) direct radiation grafting of

vinyl monomers onto polymers has been developed in this

laboratory. This is particularly beneficial with bioactive

molecules involved in medical applications. Grafting is

accomplished in only a single step, and the graft surfaces

are much more uniform than with the other methods mentioned

above. However, one problem with this method can be the

gelation or homopolymerization of the monomer solution

before grafting is completed [9,88]. Gelation or

homopolymerization of monomer limits the grafting by

restricting monomer diffusion to the polymer surface.

Gelation also makes sample handling and washing difficult.

The use of selective inhibitors has been suggested to

minimize homopolymerization in this method [89,90].

Swelling agents have been used to assist monomer diffusion

to the polymer and open the physical structure of the

polymer [91]. Since the residues of chemical inhibitors and

swelling agents may cause unnecessary biological responses

after implantation, the use of these chemicals may be

problematic for biomedical applications.

Yahiaoui [6], in this laboratory, has studied the

presoakk method" in order to help monomer diffusion into the

polymer and to increase the grafting yield without using

chemical inhibitors or swelling agents. In the presoakk

method" [6], polymeric substrates are first soaked in

aqueous monomer solutions at various concentrations,

temperatures, and times. Soon after presoaking, samples are









transferred to fresh monomer solutions and irradiated by

Cobalt-60 gamma radiation or electron beam in an argon

atmosphere. Results indicate that the presoak method

improves grafting yields by creating a monomer-rich

interface and allowing the monomer to diffuse into the

polymeric substrate. An interpenetrating network (IPN) type

of graft is obtained by this method.

The use of polyvinylpyrrolidone (PVP) in graft

copolymerization of N-vinylpyrrolidone onto polymeric

substrates was investigated in this research. Poly(vinyl

pyrrolidone) (PVP) is a linear and water soluble polymer.

PVP can wet most hydrophobic polymeric surfaces and serves

as an interfacial mediator between a polymeric surface and

the monomer solution. Consequently, interfacial

thermodynamic compatibility between polymeric surface and

the monomer solution may be improved by adding PVP into the

monomer solution. In addition, the presence of both PVP and

NVP molecules in the same solution may inhibit the solution

gelation during gamma-induced graft copolymerization. The

large molecules of PVP may also intercross-link onto

polymeric surface to make the graft more stable. Mentak

[92], in this laboratory, has shown that the PVP/NVP

modified PDMS surface is more stable than the NVP modified

PDMS surface.








2.3.3 Plasma/Gamma Induced Graft Copolymerization Vinyl
Monomers onto Polymeric Substrates


Plasma treatment of polymeric substrates has been

extensively reviewed in section 2.2. Depending on the

nature of the gas molecules, plasma reactions can be divided

into two categories: 1) plasma polymerization in which a

cross-linked thin polymeric film is deposited on the

substrate surface and 2) plasma treatment in which intensive

oxidation or cross-linking is introduced on the substrate

surface [11]. In addition, several investigators have

utilized the free radicals or peroxides generated by plasma

treatment to initiate graft copolymerization of a vinyl

monomer onto a polymeric substrate [93-96]. Thus, plasma-

induced graft copolymerization was performed by exposing the

substrate to a glow discharge plasma of inert gases followed

by contact with monomers, with or without allowing the

plasma treated polymers to be first exposed to air or

oxygen.

In this laboratory, Yahiaoui [6] investigated a novel

technique, a "Plasma/Gamma method" for graft

copolymerization. This method combines the two most

powerful surface modification techniques. At first,

polymeric substrates are treated with a glow discharge water

plasma. After plasma treatment, substrates are exposed to

air and then transferred to an aqueous monomer solution.

Finally, the plasma treated substrates in a monomer solution

are irradiated in a cobalt-60 gamma source. The purpose of









the water plasma pretreatment is to introduce polar oxygen-

containing functional groups on the polymer surface in order

to enhance the interfacial thermodynamic compatibility

between a hydrophobic surface and a hydrophilic monomer

solution. Results using the "Plasma/Gamma method"

demonstrated that it can improve graft efficiency and

produce a very thin, high quality, and stable graft on a

polymer surface. This method was therefore also

investigated further in this research.














CHAPTER 3
MATERIALS AND METHODS


3.1 Materials



3.1.1 Substrates


Polymeric substrates used for this study include PMMA

(Perspex acrylic sheet, from ICI), PDMS (KE-1935, from

Shin-Etsu Silicones of America), PP (from Himont), PC

(Makrofol De, from Bayer), PVDF (Kynar 730, from Pennwalt),

and FEP (Teflon, from Dupont). Their chemical structure

and physical properties are shown in Figure 3-1 and Table 3-

1, respectively. Substrates were cut into rectangular

strips with an approximate dimension of 1 cm x 2.5 cm. For

XPS analysis, substrates were cut into 1 cm x 1 cm square

slabs.


3.1.2 Monomers


Hydrophilic vinyl monomers used for graft

polymerization included 2-N-vinylpyrrolidone (NVP, from

Kodak) and dimethylacrylamide (DMA, from Polyscience).

Monomers were purified by distillation under reduced

pressure (1-2 mmHg at 55-60 oC) and stored at 4 C until









CH3
-+-C --C
C=O
OCH3

PMMA


CH3
---Si-O--
CH3

PDMS


-- CH-CF --

PVDF


CF3
--+-CF-C F-FCF--C2
FE P
FEP


Figure 3-1. Chemical structures of polymeric substrates.


CH3
--CH- H --t
PP

PP


CH3 0
-o-< -C -O-C--^
CH3

PC








Table 3-1. Physical and mechanical properties of polymeric
substrates. [97]



Property PUII L PDMS PP PC PVDF FEP

Density (g/cc) 1.18 1.14 0.91 1.2 1.75 2.12

Tg (OC) 105 -123 -19 145 -39

Modulus (Kpsi) 325 1.16 165 345 220 50

Tensile strength 8 0.85 4 9 5.5 2.7

(Kpsi)

Contact angle (0) 70 90 90 83 90 110

(with water)

Refractive index 1.49 1.43 1.49 1.58 1.42 1.34

Optical clear clear trans- clear opaque trans-

transparency lucent lucent









used. The structures of the monomers are shown in Figure 3-

2. Poly(vinyl pyrrolidone) (PVP, Plasdone K-90, from GAF

chemicals), a part of monomer in the PVP/NVP system, was

used without further purification. Plasdone K-90 has a

weight average molecular weight of 1.2x106 and a number

average molecular weight of 3.6x105.


H
I 0
HC=C




NVP


H

I
HC=C
H Ch
N(CH3),

DMA


Figure 3-2. Chemical structures of monomers.




3.2 Methods



3.2.1 Substrates Preparation


Substrates of PMMA, PP, PC, PVDF, and FEP were precut

into strips, individually sonicated in a 0.1% Triton X-100

(Fisher) aqueous solution for 30 minutes, rinsed in

Ultrapure': water, and repeatedly sonicated in Ultrapure

water three times for ten minutes each. PDMS slabs were









sonicated two times for 20 minutes each in a 1:1

acetone/ethanol mixture. After cleaning, all samples were

dried under vacuum for 6 hours at 600 C, then stored in a

desiccator until further use.


3.2.2 Plasma/Gamma Induced Surface Modification


3.2.2.1 Radio Frequency Water Plasma Treatment

The schematic diagram of the RF plasma system is shown

in Figure 3-3. The RF plasma system consists of a vertical

"bell-jar" reaction chamber, a monomer/gas inlet system, a

vacuum system with nitrogen cold trap, a RF power generator,

and a matching network. The reaction chamber was

inductively coupled by an eight-turn copper tubing to the RF

power generator (RF plasma products, Inc., model HFS 401 S),

which operates at a fixed frequency of 13.56 MHz with a

maximum output of 500 watts. The matching network was used

to match the impedance of plasma discharge to the RF power

generator. The flow rate of gas/monomer vapor was

controlled by a micro-metering valve (Nupro). The vacuum

pressure of the reaction chamber was monitored by a

thermocouple vacuum gauge (Adap Torr, Vacuum General Inc.,

model ACR-26) located underneath the chamber stage. The

reaction chamber was evacuated by a mechanical pump through

a liquid nitrogen cold trap.

Before plasma treatment, the reaction chamber was

cleaned with 2-propanol/KOH solution, then treated with 50





























Coaxial Cable --


'- Fine Metering Valve


Vacuum Gauge

\ _---


Liquid Nitrogen Cold Trap





Figure 3-3. Schematic diagram of the RF plasma system.









watts RF water plasma at 100 mTorr for 30 minutes to ensure

no contamination. Polymeric substrates to be treated were

placed on a 250 ml Pyrex beaker directly under the plasma

generation region (plasma was generated inside the portion

of the tube that was surrounded by the copper coil). After

mounting the samples, the pressure of the reaction chamber

was pumped down to 4 mTorr for 10 minutes, then brought up

to 100 mTorr by introducing water vapor. When a constant

gas flow was reached, plasma power was turned on to generate

the gas plasma. Yahiaoui [6] showed that 50 watts of plasma

power is the minimum power level needed to oxidize the

polymeric surface. In this study, 50 watts of power level

was selected for all samples, and plasma treatment time

varies from 0 to 25 minutes. The glow discharged throughout

the whole reaction chamber. After plasma treatment, samples

were retrieved by raising the pressure to one atmosphere

with air.

3.2.2.2 Degassing of Monomer Solution

Immediately after plasma treatment, samples were

transferred to borosilicate test tubes (Fisher Scientific,

size: 16x125 mm) which contained 6 ml aqueous monomer

solution. Degassing was done before gamma irradiation.

Monomer solutions with substrates were degassed under

reduced pressure (100-125 mmHg) in combination with 2 to 3

times sonication (5 seconds each). Usually 5 to 10 minutes

were required per sample depending on the type of monomer

and the substrate. The pressure was in the test tubes









brought up to one atmosphere by introducing argon gas. The

test tubes were then sealed with polyethylene snap caps.

3.2.2.3 Gamma Induced Graft Polymerization

Following the degassing, the samples in monomer

solutions were irradiated in a 600 Curie "Co gamma source

at room temperature. The schematic diagram of the gamma

source is shown in Figure 3-4. Samples were mounted on

polypropylene holders and placed at 4" from the source. The

corresponding dose rate was determined by Fricke dosimetry

[65,98,99], based on the oxidation of ferrous sulfate in

acidic solution. Aerated 10- M solutions of ferrous

sulfate 11.hr salt) in 0.8 N sulfuric acid are summitted to

dose increments of approximately 103 rad. The ferric ion

concentration is directly determined with a

spectrophotometer at 304 nm.

3.2.2.4 Washing

After gamma irradiation, samples were removed from the

polymerized solutions and rinsed with 10 ml Ultrapure

water, then soaked in 10 ml of Ultrapure water for one week

at room temperature with three changes of water per day.

After washing, samples were dried under vacuum at 40 "C for

12 hours, then kept in a desiccator until further use.


3.2.3 Two-step Gamma Induced Graft Polymerization


In addition to the plasma/gamma technique, a two-step

gamma irradiation method was also developed in which RF
































Door opening mechanism


Source support rod







Turret (Source Housing)



- L__


Figure 3-4. Schematic diagram of the gamma source.









plasma was not used. The procedure for two-step irradiation

is described as follows:

Step I:

1. Place PMMA slabs into borosilicate test tubes (Fisher

Scientific, size: 16x125 mm) which contain 6 ml aqueous

monomer solutions.

2. Degas, then irradiate the samples to a desired dose

(< 0.15 Mrad) by placing them at 4" from the 60Co gamma

source.

3. Retrieve the samples from the test tubes, rinse with 10

ml of Ultrapure water, then transfer them to clean test

tubes for the second step irradiation.

Step II:

1. The procedure for step II is the same as in step I except

all samples were irradiated to a dose of 0.15 Mrad.

2. After gamma irradiation, samples were retrieved and

rinsed with 10 ml Ultrapure water, then washed and dried

as in the procedures described in section 2.2.4.


3.3 Characterization



3.3.1 Gravimetric analysis


Gravimetric analysis is a simple and convenient

technique to monitor the extent of graft yield. In this

study, a Sartorius Research electronic balance having a

precision of + 0.02 mg was used to weigh the samples









before and after grafting. Graft yield was determined as

follows:

% weight gain = (wg wo / wo ) x 100 (3-1)

where wo and Wg are the dry weight of initial and grafted

substrates, respectively. However, depending on the

sample's geometry (surface to volume ratio), weight gains of

less than 0.1-0.2% were considered insignificant.


3.3.2 Viscosity Measurement


There are several ways to measure the viscosity of

polymers. These methods include capillary rheometry,

parallel plate viscometry, cone and plate viscometry, and

concentric cylinder viscometry. A Wells-Brookfield

cone/plate viscometer (model DV-II, Brookfield) was used to

measure the viscosity of polymer solutions in this study. A

water bath with temperature control was used to maintain the

sample temperature during measurements.

The schematic diagram of the cone and plate viscometer

is shown in Figure 3-5. The cone is rotated with respect to

the plate about the perpendicular axis at an angular

velocity of w. The rate of movement of any point on either

surface is proportional to its distance from the axis and

the separation of the surface at that point is equivalently

proportional to the same radius. The shear rate is defined

by the ratio of the rate of movement of the surface (at any

point) to the distance of separation.

















r ---


U-


Figure 3-5. Schematic drawing of the cone and plate
geometry.


Cone
Sample
Plate








This ratio (shear rate) is fixed for any speed of rotation,

and constant over the entire surface. The viscosity q for

Newtonian fluid is defined mathematically by the equation

[100]:

T = x/S = shear stress/shear rate (3-2)

The equations for the Wells-Brookfield cone/plate viscometer

are listed as follows [101]:

shear rate (sec-1) S = )/sin(8) _= o0/ (3-3)


M
shear stress T= 2/3 (dynes/cm2) (3-4)
2/37r3


apparent viscosity (poise) T = T/S (3-5)



where 0 is the cone angle, r is the cone radius, and M is

the torque input by the instrument. Polymer solutions are

typically shear thinking or pseudoplastic. For pseudoplastic

fluids, a logarithmic plot of T vs. S is found to be linear

over a relatively wide shear rate range and hence may be

described by a power law expression (known as the Ostwald-

de Waele model)[102]:

T = Ks" (3-6)

where K and n are constants. For pseudoplastic polymer

systems, n is less than unity. By analogy to the Newton's

law of equation 3-2, the apparent viscosity of a power law

fluid is expressed as

n = Ksn-1 (3-7)









The limiting apparent viscosity is defined as the intercept

of a linearized line which is the tangent of the viscosity

vs. shear rate curve at 100 sec-'. Therefore, the slope of

this linearized line is expressed as



Slope = K(n-1)100"-- (3-8)



This linearized line equation can then be derived by the

following steps:



K(n-l)100"- = (q-i0o )/(S-100) (3-9)



l = [rT0oo-100K(n-l)100 -2] +

K(n-1)100 -S (3-10)



where rio, is the apparent viscosity at 100 sec-1. The

limiting apparent viscosity no is expressed as



,o = limiting apparent viscosity

= [rl o-100K(n-l)100"-2] (3-11)



A typical example of limiting apparent viscosity (LAV) is

illustrated in the Figure 3-6.

Also, the viscosity index (VI) is defined as the ratio

of apparent viscosity at 3.75 sec-1 to apparent viscosity at

100 sec-.









12000


10000 A Limiting apparent viscosity



= 1-0.918 100 1/sec
R2= 0.9918
S6000


4000 *
y-4713-9x

2000--


0 ,
0 50 100 150

Shear rate (1/sec)

Figure 3-6. Typical example of limiting apparent viscosity
(LAV) .








VI = viscosity index

= 13.75 sec-1/1 00 sec-1 (3-12)



A typical procedure for viscosity measurements includes

1) turn on temperature bath and allow sufficient time for

sample cup to reach the desired temperature; 2) swing sample

cup clip to one side and remove sample cup; 3) using wrench

supplied, hold viscometer lower shaft and screw on cone

spindle; 4) place sample cup against adjusting ring, being

sure to position the notch on the side of cup around the

sample clip; 5) run the Viscometer at 10 rpm by setting the

speed select knod and turning the motor switch on; 6) turn

the adjusting ring to the right in small increments (one or

two minor divisions on the ring) while watching the digital

display until fluctuation of the display reading indicates

that the pins have made contact; 7) once contact has been

made, back off the adjusting ring in small increments until

stabilization of the display reading indicates that the pins

are not contacting; 8) turn the adjusting ring to the right

in very small increments (about 1/64") until the display

reading fluctuates regularly by a small amount; 9) make a

pencil mark on the adjusting ring directly under the index

mark on the pivot housing and turn the adjusting ring to the

left exactly the width of one minor division; 10) remove the

sample cup and place 0.5 ml sample in cup, being sure that

the sample is bubble-free and spread evenly over the surface

of the cup; 11) allow sufficient time for the sample fluid








to reach the desired temperature; 12) press the SPDL key and

enter the spindle number; 13) turn the motor switch on and

allow time for the display reading to stabilize; 14) record

the rpm, viscosity, shear stress, and torque from the

display reading; and 15) switch to higher rpm and record the

data from the display reading if the % mode (torque) is still

less than 100%. If the value of torque is over 100%, the

viscometer will stop reading automatically.


3.3.3 Contact angle measurement


Contact angle measurement is a simple and convenient

technique for surface analysis. In the area of biomaterial

research, contact angle measurements are routinely used to

measure the hydrophilicity and surface energy of

biomaterials. The contact angle (0) is related to the

solid-vapor (Ysv), solid-liquid (ysl), and liquid-vapor (Ylv)

interfacial energies via the Young-Dupree equation [103-

105]:



cos 0 = ( Ysv Ysl )/ Ylv (3-13)



If the contact angle is 0 degree, the liquid is completely

spreading or completely wets the substrate. An extensive

information on contact angles and their variation with

liquid and solid constitution has been reported by Zisman's

group at the Naval Research Laboratory [106,107].









In this study, a Rame-Hart contact angle goniometer

(Mountain Lakes, NJ) with an acrylic water tank (3" X 2" X

2.5" in size) was employed for the contact angle

measurements. The samples were allowed to equilibrate in

Ultrapure water for at least 12 hours prior to measuring.

The contact angle of polymeric substrates was determined by

the underwater captive air bubble technique. The

configuration of the captive air bubble is shown in Figure

3-7. Samples were attached to a microscope slide and placed

on the top of water tank with the sample face down in the

water. Typically, four air bubbles were introduced onto the

substrate surface by a microsyringe. The contact angle

measurements were taken on both sides of each bubble for all

four bubbles. The reported contact angle is an average of

all eight measurements.


3.3.4 Fourier transform infrared/attenuated total reflection
(FTIR/ATR)


Fourier transform infrared spectra has been recognized

as one of the most powerful techniques in chemical analysis.

In the area of polymer surface analysis, Fourier transform

infrared with attenuated total reflectance (FT-IR/ATR) was

used for identifying surface composition and chemical

structure. Figure 3-8 shows the optical configuration of

attenuated total reflection spectroscopy. The ATR crystals

usually have a high refractive index (2.3 to 4.0).

















Sample


Air bubble


Figure 3-7. Schematic diagram of contact angle measurement.



















To detector

S/: ------ Sample (nl)

Crystal (n2)----

Sample (nl)----->

From IR light source


Figure 3-8. Optical configuration of FT-IR/ATR.








When the IR beam passes from a high refractive index medium

(crystal) to a lower refractive index medium (polymer), the

IR beam can be totally reflected. The depth of penetration

(dp) is a function of IR wavelength (k), refractive index of

crystal (nl) and sample (n2), and incident angle (0). Their

relationships are described by the following equation.




dp = (3-14)
2r nI [sin28 n2/n1/2



Therefore, the depth of penetration of IR to the polymeric

surface is between 0.5 to 3 unm. Penetration depth depends

on IR wavelength, the two refractive indices, and the

incident angle. A depth profile can be obtained by

collecting sample spectra at various angles of incidence of

the IR beam.

In this study, a Nicolet 60 SX spectrometer with an MCT

(mercury cadmium telluride) detector was used to identify

the chemical structure of the polymeric grafts. The MCT

detector is cooled with liquid nitrogen. A Wilks model 50

attenuated total reflection (ATR) stage was used with a KRS-

5 crystal. Two samples were pressed against the IRE crystal

which had an entrance and exit face angle of 45.

Typically, 100 scans at a 4 cm-1 resolution were averaged

for each measurement. All spectra were processed with

standard Nicolet software.








3.3.5 X-ray photoelectron spectroscopy (XPS)


X-ray Photoelectron Spectroscopy (XPS) is another

powerful analytical technique for determining the chemical

composition of polymer surfaces. It can provide information

on the top 5 to 50 A which dominate interface properties of

biomaterials. The principle of this technique is based on

the photoelectric effect. As the X-ray beams (usually Mg or

Al) bombard the surface of a specimen in an ultra-high

vacuum environment, the X-ray photons can knock out the

inner shell electrons. Only the emitted electrons at or

near the surface (& 20 A) can escape the solid to be

detected by the hemisphere analyzer. The kinetic energy of

the emitted electron is measured by the detector and is

related to the binding energy by the following equation:



Eb = hv Ek 0 (3-15)



where Eb is the electron binding energy, hv is the photon

energy of X-ray, Ek is the electron kinetic energy measured

by the instrument, and 0 is the work function. The binding

energy measured by the instrument is related to the element

and its chemical environment (bonding). Figure 3-9 is a

typical representation of the photoionization event. The

depth of penetration by XPS analysis varies from 10 A for a

take-off angle of 10" to 50 A for a take-off angle of 90.





















Ejected photoelectron

i -


Figure 3-9. A typical representation of photoionization
event.









The XPS samples were 1 cm in size for all cases. A Kratos

XSAM-800 spectrometer with a Mg Ka X-ray source was used to

acquire the spectra. The pressure in the analyzer

chamber was 10-7 to 10-8 Torr and the Mg KaX-ray gun was

operated at 13 kV and 18 mA. All samples were examined with

a low resolution survey scan and a high resolution element

scan. The spectra were processed and quantified using a

Kratos software (DS800) provided with the instrument. The

binding energy scale was calibrated to hydrocarbon Cls

defined as 285.0 eV.


3.3.6 Ellipsometry


When a light wave is reflected or refracted at the

interface between two optically dissimilar media, the state

of polarization is changed abruptly. Ellipsometric

measurements involve illuminating the surface of a sample

with a monochromatic light of known wavelength and

polarization and then analyzing the polarization state of

the reflected light [108]. Ellipsometry can determine the

properties of the surface and the properties of a partly

transparent film on a known substrate.

In this research, a Gaertner (Chicago,IL) ellipsometer

model L117 was used to measure the refractive index and the

thickness of the graft. The assumptions for this technique

include 1) the graft surface has parallel-plane boundaries;

and 2) the ambient, the graft, and the substrate are all








homogeneous and optically isotropic [108]. Figure 3-10

shows the schematic diagram of an ellipsometer. The angle

of reflection is set equal to the angle of incidence. The

light source is a helium-neon laser having a wavelength of

632.8 nm. The beam is circularly polarized at the laser

output. When the beam passes through the polarizer, it is

converted from circular to linear. The linearly polarized

beam is then converted to elliptically polarized beam by a

quarter-wave compensator. Upon illuminating the surface of

the sample, the reflected light passes through the analyzer

and an optical interference filter. The amount of light

passing by the filter is sensed by a photodetector and is

indicated on an extinction meter. Certain azimuth settings

(Pl,and P2) cause the reflected light to become completely

linearly polarized. The analyzer can then be rotated to a

corresponding position (Al and A2) where no light reaches

the photodetector. These analyzer and polarizer readings

are recorded. The assumption is made that the graft is a

thin film, as shown in Figure 3-11, and is sandwiched

between semi-infinite ambient and substrate media. The

Fresnel equations are shown as follows [108]:


N, cos o) No cos 0,
r = --P (3-16)


N, cos q, N, cos (,
2p = N2 cos N, cos2 (3-17
















%Laser


SCircularly polarized light

Polarizer prism Polarizer drum (P)
Polarizer prism
Linearly polarized
light


Compensator
Elliptically polarized light ,
Angle of incidence

Sample


Photodetector

Filter
Analyzer prism

Analyzer drum (A)


Figure 3-10. Schematic diagram of an ellipsometer.




























SUBSTRATE (2)


Figure 3-11. Oblique reflection and transmission of a plane
wave by an ambient (0)-film (1)-substrate (2)
system with parallel-plane bounaries. [108]








No cos0 N, cos (3
r = O (3-18)
= NO cos o-+N cos(3
N, cos o, N, cos O
cos ,-NCOS2 (3-19)
cos + cos + N, cos 02


where rolp, ro 1, r12p, and rl2s are the interface Fresnel

reflection coefficients. No, N1, and N2 are the refractive

indices of the incident medium (usually air), the unknown

film, and the known substrate, respectively. In general, Ni

is a complex number; but for a non-absorbing medium it is

real number ni. The three angles mo, i1, and 42 are

interrelated by Snell's law.



nosino =n nisin = n2sin4 (3-20)



The overall complex-amplitude reflection coefficients (Rp,

Rs) for the ambient-film-substrate system in terms of 1) the

interface Fresnel reflection coefficients (roi, r12), and 2)

the phase change 0 experienced by the multiply-reflected

wave inside the film on a single transversal between its

boundaries are given by [108]


r01p +r2p-'2
Rp =, +rprpe- (3-21)
S1 +r01pr12p e-2f



ro, + r2. e2
R, 1 r.1+ rr2e' (3-22)
S+ 1+osrl2.p-f2 i








and

P8 = 2r( n cosi, (3-23)



where dl is the film thickness, and X is the wavelength of

the incident light.

To determine the change in amplitude and phase

separately, the overall complex-amplitude reflection (Rp,

Rs) coefficients are written in terms of their absolute

values and angles:


R, RR e'A (3-24)



R = Re' A (3-25)



From measurements of the incident and reflected

polarization, the ratio




RP
P=- (3-26)




of overall complex-amplitude reflection coefficients is

determined. The ratio p can be expressed in terms of the

ellipsometric angles Y and A,


R(
p = tane' = (3-27)
R,








where Y and A are ellipsometric angles. The amplitude

ratio change can be expressed as


R
tanYT=- (3-28)




and the change in phase difference as


A=Ap-A, (3-29)



From two sets of polarizer and analyzer readings at

this condition, ellipsometric parameters PSI (T) and DELTA

(A) can be derived using the following relations:




180 -(A A,)
T= (3-30)
2



A =360-(P + P,) (3-31)



Parameter Psi (Y) varies from 0 to 90, while Delta (A) can

assume values from 0 to 360 [109].

Equations (3-16)~(3-31) can not be solved analytically

for nl and dl, but numerical methods can be applied. A

computer program developed by R. Ochoa of Dr. Simmons'

laboratory was available to calculate the graft thickness








(di) and refractive index (nl). The calculated thickness

can be determined by the following equation [110,111]:



d, =do+ (n -n sin2 0o) mA m= 1,2,3... (3-32)



where dl is the thickness of graft, and do is the graft's

minimum thickness.

A typical procedure for ellipsometric measurement

includes 1) set the polarizer and analyzer angle of

incidence; 2) turn on, warm up, and align sample stage; 3)

place sample on the sample table; 4) set polarizer drum to

read 85 and the analyzer to read 450; 5) adjust gain

control until meter reads midway between 3/4 and full scale

(150 to 200); 6) rotate analyzer drum slowly within the red-

numbered segment (0 to 90) and set this drum to yield the

lowest reading on the extinction meter; 7) rotate polarizer

drum slowly within the red-numbered segment (315 to 1350)

and set this drum to yield a new and even lower meter

reading; 8) work back and forth between analyzer and

polarizer drum setting until the lowest possible meter

reading is obtained; 9) record the first analyzer drum

reading (Al) and then the first polarizer drum reading (P1)

at extension; 10) add 90 to the first polarizer drum

reading (P1) and rotate the polarizer drum to this sum

(P1+900); 11) from 180, substrate the first analyzer drum

reading (Al) and rotate the analyzer drum to this difference









(180-A1); 12) slowly rotate the polarizer drum to obtain

the lowest reading on the meter; 13) slowly rotate the

analyzer drum to obtain a still lower meter reading; 14)

work back and forth between polarizer and analyzer drum

setting to obtain final lowest reading on meter; and 15)

record analyzer and polarizer reading (A2 and P2). These

polarizer readings (P1, P2) and analyzer readings (Al, A2)

were used to determine the minimum graft thickness and

refractive index of graft.


3.3.7 Optical microscopy


An optical microscope (Nikon OPTIPHOT, Japan) was used

to study rabbit lens epithelial cell adhesion and spreading.

The analysis was conducted by Paul Martin in this

laboratory.


3.3.8 Scanning electron microscopy (SEM)


A low voltage scanning electron microscope (JEOL JSM-

6400 SEM) was used to examine the surface morphology of

polymers. Sample preparations are much simpler than with

high voltage SEM, since no gold-palladium coating is

required. Typically, an accelerating voltage of 0.5 kV was

used. SEM operations were performed by Paul Martin. This

new low voltage technology is very useful in studying the

surface morphology of polymers, biomaterials, biological

tissues and coating sensitive surfaces.







3.3.9 Rabbit lens epithelial cell adhesion and spreading


In vitro studies are an important step to understanding

the biocompatibility of polymers. During cataract surgery

and intraocular lens implantation, the adhesion of lens

epithelial cells is deemed to be an important factor in

post-surgery inflammation. Therefore, Hofmeister [112] of

this laboratory, developed a rabbit lens epithelial cell

adhesion and spreading model to investigate the

biocompatibility of polymeric materials. The lens

epithelial test was done by Paul Martin in this laboratory.

Rabbit lens capsule epithelial cells were cultured from

New Zealand rabbits. Cells were trypsinized and suspended

in Medium 199, 15% fetal bovine serum at a concentration of

10,000 cells/ml. Samples were placed in the wells of

tissue-culture polystyrene plates, and equilibrated with the

medium without cells for ten minutes. Two ml of cell

suspension were then applied to each well over the samples.

The incubation conditions for the samples were 37 "C in a

CO2 atmosphere for 24 hours. After incubation, samples were

removed and rinsed with balanced saline solution, and then

placed into 10% neutral buffered formalin. Samples were

stained with crystal violet solution (2%) after 24 hours

fixation. A Nikon optical microscopy was used to determine

the number of cell adhesions and spreading. Two photographs

were taken from different portions of each sample.














CHAPTER 4
RESULTS AND DISCUSSION


4.1 Initial Studies of the PVP/NVP System


Radiation-induced grafting of N-vinylpyrrolidone (NVP)

onto polymeric substrates has been extensively studied in

this laboratory [5,6]. Possible gelation of NVP during

graft polymerization can be one of the major drawbacks of

the grafting process. On the sample handling aspects,

gelation makes sample's post-washing very difficult. The

presoakk method" [6] enhances the grafting efficiency by

soaking the substrate in a highly concentrated monomer

solution before performing the radiation induced graft

polymerization. This technique provides a monomer-rich

environment at the interface between the substrate and

monomer solution. However, gelation could still occur at

high radiation doses. Presoaking may also affect the

dimensional stability of certain polymers. In the present

research, a new PVP/NVP monomer system is investigated and

evaluated for surface modification of polymers. The

incentive for this study is to develop a new monomer system

that would exhibit minimal gelation during radiation-induced

graft polymerization.









Commercial polyvinylpyrrolidone (PVP), Plasdone K-90

from GAF chemicals, was used in this research without

further purification. Plasdone K-90 is currently used as a

granulation binder, controlled release matrix, gel former,

bioadhesive, and tablet coating [113]. Plasdone K-90 has a
6
weight average molecular weight of 1.2x10 and a number
5
average molecular weight of 3.6x10 [113]. The glass

transition temperature for Plasdone K-90 is 174 C.

Plasdone K-90 can be dissolved in various solvents such as

water, alcohol, ketone-alcohol, acids, ether-alcohols,

lactone, chlorinated hydrocarbons etc. [113]. PVP also is a

good wetting agent for most polymers. Mentak [92], in this

research group, has discussed the PVP/NVP system with an

emphasis on graft thickness and graft stability of PVP-g-

PDMS in his dissertation. This present research focuses on

the polymerization solution viscosity, XPS analysis, and

contact angle measurements. A PVP/NVP solution producing

low solution viscosity, high surface nitrogen concentration,

and low contact angle on graft will be selected as a monomer

solution for the plasma/gamma surface modification studies.


4.1.1 Solution Viscosity Measurement


4.1.1.1 Viscosity of PVP/NVP system

Since the PVP/NVP system uses a polymer/monomer aqueous

solution, the effect of gamma-rays is more complex because

of the interaction between PVP and NVP. The interactions









are not easy to study experimentally. On the other hand,

the effect of gamma-rays on separate aqueous solutions of

NVP or PVP can be experimentally determined.

The results of viscosity measurements for the PVP/NVP,

PVP, and NVP solutions are listed in Table 4-1. Viscosity

data was expressed in terms of apparent viscosity (AV),

limiting apparent viscosity (LAV), and viscosity index (VI).

The PVP/NVP solutions were composed of 10 parts of solute

with 90 parts of water by weight. The PVP K-90 solutions

and NVP solutions were composed of 0 to 10 parts of solute

with 90 parts of water by weight. PMMA substrates were

placed in the solutions to simulate the real situation of

gamma-induced graft polymerization. Before gamma

irradiation, all monomer solutions were degassed and filled

with argon gas. Samples were then irradiated in a "Co

gamma source with a dose rate of 510 (rad/min.) up to 0.15

Mrad. Viscosities were taken at 25 C soon after gamma

irradiation.

For the PVP/NVP solutions, the viscosities increase

significantly after gamma irradiation. This implies that

the gamma irradiation has promoted monomer polymerization

and cross-reaction in the aqueous PVP/NVP solutions. The

viscosities of the PVP/NVP solutions after gamma irradiation

decrease as the ratio of PVP K-90 to NVP increases. This

suggests that PVP-NVP cross-reaction and NVP polymerization

is inhibited and that NVP concentration dependence dominates

results.










Table 4-1. Apparent viscosity (AV), limiting apparent
viscosity (LAV), and viscosity index (VI) of
PVP/NVP, PVP, and NVP solutions.



1ON 1P 9N 2P 8N 3P 7N 4P 6N 5P 5N 6P 5N 7P 3N 8P 2N 9P IN 10P
Av' 2 4 10 16 26 33 46 72 92 137 150
(cps)
Av* 8530 1675 1350 1060 810 740 582 674 720 706 720
(cps)
LAV 4713 1186 1074 837 668 532 422 494 505 562 566
(cps)
VI 2.17 1.62 1.38 1.33 1.31 1.39 1.38 1.36 1.43 1.37 1.40




1P 2P 3P 4P 5P 6P 7P 8P 9P 10p
Av" firm soft liquid liquid 2400 262 190 262 360 720
(cps) gel gel gel gel
LAV 635 197 150 200 281 566
(cps)
VI 5.7 1.46 1.39 1.44 1.40 1.40




10N 9N 8N 7N 6N 5N 4N 3N 2N IN -
Ava 8530 2822 1220 530 210 92 39 13 5 2
(cps)
LAV 4713 1956 998 466 187 92 39 13 5
(cps)
VI 2.17 1.66 1.33 1.20 1.16 1 1 1 1 1


Note: 1. Each solution contains


90 parts


of water and 1


10 parts of solute, where P=PVP K-90 and N=NVP.
2. The total radiation dose is 0.15 Mrad.
Viscosities were measured in a Brookfield c/p #40
rheometer at temperature of 250C.
3. a indicates the apparent viscosity at 3.75 1/sec
after gamma irradiation.
4. b indicates the apparent viscosity at 3.75 1/sec
before gamma irradiation.
5. LAV is the limiting apparent viscosity.
6. VI is the viscosity index for the ratio of

113.75/11100









For the PVP solutions, the results vary dramatically

with the concentration of PVP K-90. The viscosity decreases

somewhat at 9% PVP, 8% PVP, 7% PVP, and 6% PVP, probably

because radiation chain scission is dominating but then

begins to increase at 5% PVP where branching and cross-

linking begin to dominate and gelation occurs at 4% PVP, 3%

PVP, 2% PVP, and 1% PVP. The degree of gelation, which was

determined by visual inspection, increases directly with

decreasing PVP concentration. This concentration dependent

relationship between radiation induced degradation and

gelation for high molecular weight PVP is a very interesting

phenomenon.

The effect of gamma radiation on PVP in aqueous

solution was first studied by Alexander and Charlesby

[114,115]. They also found that PVP, when irradiated at

concentrations as low as 1%, were not degraded by

irradiation but rather were crosslinked. The critical dose

for incipient gel formation in PVP solution increased with

the PVP concentration. This is consistent with the gelation

of lower concentrations of PVP in this study shown in Table

4-1. However, below 0.3% PVP, degradation was observed.

The mechanism for the radiation-induced cross-linking of

polymers in solution was discussed by Alexander and

Charlesby [114,115] and by Henglein [116]. In an aqueous

solution, once a polymeric free radical P* is produced, it

can only dimerize by reaction with another P* radical,

leading to branching and eventual cross-linking, or combine








with primary H* or OH* radicals formed by the radiolysis of

the solvent ("direct action" mechanism). On the other hand,

both H* and OH* radicals are very efficient for abstracting

hydrogen from the polymer molecule, leading to the formation

of polymeric free radicals ("indirect action" mechanism).

The radiation-induced cross-linking of PVP in an aqueous

environment most likely follows the "indirect action" rather

than the "direct action". If the cross-linking of polymers

in aqueous solution follows the "direct action" mechanism,

the critical dose for incipient gel formation will decrease

with the concentration of polymers. This is inconsistent

with the experimental results. The "indirect action"

mechanism is favored in aqueous solutions. The experimental

results obtained by Henglein [116] have supported this

hypothesis. When the PVP polymer is dissolved in various

solvents, the rate of cross-linking was found to be much

higher in water than in methanol. In isobutanol, aniline,

and chloroform, no network formation was found [116].

In the case of the very dilute PVP solution (<0.3%), if

no adjacent polymer molecule presents itself in a suitable

orientation for cross-reaction, the activated polymer

molecule would instead suffer main chain cleavage and

degradation [114,115]. Both "indirect action" and "direct

action" would result in polymer degradation.

For the NVP solutions, the viscosity decreases as the

NVP concentration drops. Below 6% NVP, the viscosity

becomes very low. At low dose rates, the rate of









polymerization, Rp, is proportional to the square root of

the rate of initiation, Ri, and to the first order of

monomer concentration if the reaction follows the

conventional free radical mechanism [117]. Therefore, as

the monomer concentration drops, the degree of

polymerization would be reduced as observed.

Obviously, the results in Table 4-1 have shown that the

viscosities of the PVP/NVP solutions are much different from

the PVP or NVP solutions alone. When the PVP in a PVP/NVP

solution is less than 5%, the viscosity of PVP/NVP is lower

than PVP alone and higher than NVP alone. On the other

hand, when the PVP content in PVP/NVP solution is higher

than 5%, the viscosity of PVP/NVP solution is higher than

either the PVP or NVP alone. Below 5% PVP, the cross-

linking of PVP in aqueous solution was inhibited by the

presence of NVP monomer. The activated PVP molecule may not

be able to efficiently cross-react with an adjacent PVP

molecule due to the competitive reaction with the

surrounding NVP monomers. If the PVP content is greater

than 5%, the PVP molecule concentration becomes high enough

to enable reaction with both surrounding NVP molecules or

with an adjacent PVP molecule. Polymerizing NVP may also

serve to cross-react two PVP molecules. In general, it

appears that the NVP monomer only cross-reacts readily with

PVP when the PVP content is greater than the NVP content.








4.1.1.2 The effect oxygen

The presence oxygen during the gamma irradiation has a

significant effect on polymer degradation. Figure 4-1 shows

the apparent viscosity (at 3.75 1/sec) of the PVP/NVP system

irradiated in air and irradiated in argon. The PVP/NVP

system irradiated in air exhibited a lower apparent

viscosity than the same system irradiated in argon. Most

likely, when irradiation is carried out in air, some of the

polymeric radicals react with oxygen to form peroxidic

structures which eventually decompose and lead to a chain

scission [9]. Cross-linking by gamma irradiation in the

presence of air produces hydrogels with lower cross-linking

density than hydrogels irradiated in vacuum [4]. In the

case of a PVP/NVP ratio of 0/10 (NVP only solution), the

solution viscosity was dramatically reduced when oxygen was

present during gamma irradiation. Also, in the PVP only

solution, the cross reaction process between PVP molecules

was retarded by the presence oxygen. Usually, at low dose

rates, oxygen can diffuse into a solution fast enough to

provide sufficient oxygen for peroxide formation. However,

at high dose rates (such as e-beam), oxygen is rapidly used

up and cannot be replenished in a very short time period

thereby leading to less apparent oxygen sensitivity.

Therefore, e-beam source is another useful source for

radiation-induced graft polymerization.












Apparent Viscosity of PVP/NVP System


9000
8000 -
7000 -
6000
5000
4000-
3000 -
2000 -
1000 -
0


-I$ .i la l I I I I I n, rrn.,rl
0/10 1/9 2/8 3/7 4/6 5/5 6/4 7/3 8/2 9/1 10/0
PVP K-90/NVP

0/10 1/9 2/8 3/7 4/6 5/5 6/4 7/3 8/2 9/1 10/0

8530 1675 1350 1060 810 740 582 674 720 706 720

1450 896 602 426 412 380 426 346 386 288 282


Total conc.--10 wt%, Total dose--0.15 Mrad
Shear rate--3.75 1/sec, Temp.--25 deg. C



Figure 4-1. Apparent viscosity of PVP/NVP system irradiated
in argon and irradiated in air.


1 Irradiated in argon
M8 Irradiated in air







4.1.2 XPS Analysis of the Grafting of PVP/NVP noto PMMA


In addition to the viscosity measurements, the grafting

of PVP/NVP system onto PMMA by the gamma method (gamma-

induced graft polymerization) was examined using XPS

analysis. The results are presented in Figure 4-2. XPS

analysis provides much information about the top surface

layer (ca. 20-50 A), such as atomic concentrations and

interpolated chemical structures. The theoretical atomic

composition for PVP is 75% carbon, 12.5% oxygen, and 12.5%

nitrogen. The surface nitrogen concentration of PVP/NVP-g-

PMMA prepared from the PVP/NVP ratio of 2/8, 4/6, 5/5, 6/4,

and 8/2 is 4.92%, 4.08%, 4.24%, 4.85%, and 4.91%

respectively. These values are close to or even higher than

4.28% for the 10% NVP solution. Radiation-induced graft

polymerization using simultaneous irradiation method

involves a hetergeneous polymer-monomer reaction system

where the rate of polymerization is usually diffusion-

controlled [6]. These grafting reactions are mainly

controlled by the rate of production of free radicals on the

substrate surface and their accessibility to the monomer

[92]. In the PVP/NVP system, large PVP molecules might

physically adsorb on the substrate surface before the

grafting. Some of the PVP molecules might be grafted

through the recombination of the PVP polymeric radicals with

substrate surface radicals therefore enhancing the grafting

reaction. In the case of 10% PVP only solution, a









XPS Analysis of PVP/NVP-g-PIVIA
(Irradiated in argon vs. Irradiated in air)
6
5


043
0


zo

PVP K-90/NVP 0/10 2/8 4/6 5/5 6/4 8/2 10/0
Irradiated in araonm 4.28 4.92 4.08 4.24 4.85 4.91 2.53


Irradiated in air


-1]3.62 4.03 3.90 3.19 4.38 3.58 2.43


Total conc.--10 wt%, Total dose--0.15 Mrad.


Figure 4-2. XPS analysis of gamma induced graft of PVP/NVP
system onto PMMA.








significant amount of nitrogen (ca. 2.53%) was found on the

PMMA surface. This indicates that the PVP may be chemically

grafted or strongly physically adsorbed on the PMMA surface.

Chemical grafting might also occur if PVP polymer radicals

can recombine with radicals on the PMMA surface.

The possible strong physisorption of Plasdone K-90 onto

PMMA was examined by XPS analysis. Table 4-2 shows the

results of physical adsorption of Plasdone K-90 onto PMMA.

The polymer solutions for samples #1 to #4 were 10 wt%

aqueous Plasdone K-90, but #5 was a gamma irradiated

Plasdone K-90 (0.15 Mrad). PMMA samples were soaked in the

solutions for various times. The results indicate that one

million MW PVP is adsorbed on PMMA surfaces and yields

hydrophilic surfaces, but the extent of adsorption was less

(ca. 50%) than when the solution is irradiated with the

substrate. This implies that some chemical grafting of

Plasdone K-90 onto PMMA does occur. For physisorption, the

nitrogen content did not increase with increasing soaking

time beyond 2 hours.

The results summarized in Figure 4-2 also indicate that

the extent of grafting for the PVP/NVP system irradiated in

argon are higher than when irradiated in air. These results

are consistent with viscosity measurements.

The PVP/NVP system was also examined by the

"plasma/gamma" method. PMMA substrates were exposed to a

H20 RF-plasma at 100 mTorr and 50 Watts for 15 minutes

before gamma radiation grafting.








Table 4-2. XPS analysis of physical adsorption of Plasdone
K-90 onto PMMA surface.



Sample Type of Soaking Yield Contact Nitrogen

# Solution Time (gravimetri-) Angle Content

(hours) (%) (0) (%)
PMMA #1 10 wt% pure 2 0.1 30 1.21
PVP K-90

PMMA #2 10 wt% pure 3 0.1 24 1.08
PVP K-90

PMMA #3 10 wt% pure 4 0.1 24 1.19
PVP K-90

PMMA #4 10 wt% pure 5 0.2 25 1.23
PVP K-90

PMMA #5 10 wt% gamma 5 0.4 24 1.40
irradiated
PVP K-90

(0.15 Mrad)


Note: The nitrogen content is 2.53% and the yield of graft
is 1.1% for PVP-g-PMMA prepared by gamma-induced graft
polymerization.









The results are given in Figure 4-3. Again, the

PVP/NVP ratio of 2/8 solution shows the highest nitrogen

content in the system. Measurements presented in Figure 4-1

also indicate that at the PVP/NVP ratio of 2/8, the solution

viscosity is six times lower than for NVP monomer alone.

Mentak's studies [92] in this research group have further

shown that for PVP/NVP surface modification of PDMS, less

gelation occurs during gamma graft polymerization which

benefits sample handling and washing.


4.1.3 Contact Angle Measurements


Contact angle is a simple technique for surface

analysis. Since all contact angles for the PVP/NVP system

grafted onto FIII substrates were highly hydrophilic, ca.

20, PDMS substrates were used to investigate changes in

contact angle. PDMS surfaces are more hydrophobic than PI-.L-A

surfaces and are difficult to wet with Plasdone K-90, thus

avoiding the physical adsorption of Plasdone K-90 or gamma-

polymerized PVP. The same preparation conditions as for the

PMMA substrates were used for the PVP/NVP-g-PDMS. The

contact angles for the PVP/NVP system grafted onto PDMS

surfaces are listed in Table 4-3. The contact angle for the

PDMS control is about 90 degrees. The results in Table 4-3

indicate that the PVP/NVP ratio of 2/8 solution is the only

one that could lower the contact angle of PDMS surface to

near 20 degrees.














XPS Analysis of PVP/NVP-g-PMMA
(Plasma/Gamma Method)
10


a6
08

4-4
102


PVP K-90/NVP 0/10 2/8 4/6 5/5 6/4 8/2 10/0

Nitrogen content (%) E 7.09 7.88 5.35 7.14 4.45 4.10 3.65
Total conc.--10 wt%, Total dose-- 0.15 Mrad.


Figure 4-3. XPS analysis of PVP/NVP-g-PilLA using
"plasma/gamma" method









Table 4-3. Contact angle measurements of PVP/NVP-g-PDMS



PVP K-90/NVP I Contact Angle (+5)

Control PDMS 90

0/10 50

1/9 38

2/8 20

5/5 42

8/2 35

9/1 42

10/0 44

(Total conc.-- 10 wt%, Total dose--0.15 Mrad)


This result has further confirmed that the PVP/NVP ratio of

2/8 solution is the best composition for gamma-induced graft

polymerization in terms of viscosity, contact angle and the

extent of graft.


4.1.3 Summary of Initial Studies of the PVP/NVP System


A polymer/monomer mixture solution, PVP/NVP ratio of

2/8, not only gives a lower viscosity than NVP alone, but

also demonstrates a comparable result as NVP alone for the

PVP-g-FtPrA. Thus, the formation of PVP graft on PMMA

surface is attributed to the recombination of surface free

radicals with PVP polymer radicals or polymerizing NVP

radicals. Since the solution viscosity of PVP/NVP is much









lower than the NVP alone, the PVP graft on PMMA surface must

be mainly dominated by the high MW PVP instead of

polymerizing NVP.

The PVP/NVP solution is the first system in the current

research that combines high MW PVP with unsaturated NVP for

the grafting of PVP onto polymeric substrates. A very

encouraging aspect from this study is that high MW polymers

or biomolecules such as proteins or polysaccharides can be

immobilized onto a polymeric substrate through gamma

irradiation.

The mechanism due to the effect of gamma radiation on

PVP/NVP in aqueous solution are very complex. The presence

of NVP monomers or polymerizing NVP may affect the cross-

reaction between two PVP polymers which then inhibits the

gelation during gamma irradiation. The primary H* or OH*

radicals formed by the radiolysis of the solvent also plays

an important role in the "indirect action" mechanisms. The

present research does not intend to reveal the detail of the

mechanism.

However, the results in this study indicate that the

PVP/NVP solution has considerably lower solution viscosity

than NVP alone, which then can shorten the sample's post-

washing cycle during surface treatment. Also, the use of

PVP/NVP solution for gamma-induced graft polymerization is

an alternate way to reduce the gelation during gamma

irradiation without adding unwanted chemicals.








4.2 RF Plasma Treatment of PMMA, PDMS, PP, PC, PVDF, and FEP


Radio frequency glow discharge (RFGD) plasma can

activate or oxidize the polymer surface to a depth of 50-500

A without altering the bulk polymer properties [45]. After

plasma treatment, most polymer surfaces are reactive and

polar. In this study, PMMA, PDMS, PP, PC, PVDF, and FEP

substrates were treated with water vapor RFGD plasma. The

vacuum and the power for the RFGD plasma were 1005 mTorr

and 50 watts respectively. The plasma treatment times were

varied form 1 to 25 minutes for each substrate. After

plasma treatment, samples were characterized by contact

angle measurements, FT-IR/ATR, and XPS analysis.


4.2.1 Contact Angle Measurements


The contact angle of each sample was taken right after

plasma treatment by the captive bubble method. The contact

angle results are summarized in Table 4-4. The PMMA

substrate has a contact angle of 70 before treatment, but

is reduced to 50' after one minute treatment, and finally

decreases to 34 at 15 minutes treatment. The contact angle

for the PDMS substrate dramatically decreases from 90 to

230 after a 1 minute treatment and then reaches 190 after 5

minutes of treatment. The PP control has a contact angle of

900, but changes to 330 after a 1 minute treatment, and

finally to 180 after 10 minutes of treatment. The PC









Table 4-4. Contact angle measurements for water RFGD plasma
treated PMMA, PDMS, PP, PC, FEP, and PVDF




Water RF-plasma Treatment


20 -


PMMA
SPDMS
P PP
P CC
SFEP
0 PVDF


10 15
Plasma Treatment Time (m nutes)


Plasma Treatment Time (minutes)


C.A. (5) 0 1 3 5 8 10 15 20 25

PMMA 70 50 44 42 40 37 34 34 34

PDMS 90 23 20 19 19 19 19 19 19

PP
P 90 33 22 20 20 18 18 18 18

PC 83 25 25 25 20 20 19 19 19

FEP
FEP 110 72 70 60 60 58 55 55 55

PVDF90 34 32 32 30 30 24 24 24
90 34 32 32 30 30 24 24 24


(power--50 watts, pressure--100 mTorr)









substrate follows a similar trend as PDMS, but reaches 190

after 15 minutes. FEP also changes contact angle with the

plasma treatment, and reaches 550 after 15 minutes. In the

case of PVDF, the contact angle changes from 90 to 24'

after 15 minutes.

The effect of RFGD plasma treatment on wettability of

polymeric substrates is easily noticed using contact angle

measurements. Contact angle reveals the interfacial

characteristics of polymer surface. A low contact angle

indicates that the major functional groups on the polymer

surfaces are polar groups, such as hydroxyl (-OH) or

carboxyl (-COOH), which can be introduced by RFGD plasma

treatment.

The results also indicate that the substrate's surface

contact angle is decreasing with increasing the plasma

treatment time, and then exhibits a constant value for

prolonged treatment after reaching a minimum. Yahiaoui [6]

and Iwata et al. [118] reported that the Ols/Cls ratio,

which expresses the extent of oxidation, increases with the

plasma treatment time, but exhibits a tendency to decline to

lower value after reaching a maximum value. This finding

suggests that polymer segments in the surface region are

highly oxidized at the time of reaching its minimum contact

angle. After reaching the minimum, the polymer segments in

the surface region might be broken into fragments during the

prolonged treatment. These weak oxidized fragments may be

ablated from the surface. This leads to a small decrease in









Ols/Cls value which can be observed by XPS analysis, but is

not detected by the contact angle measurement.


4.2.2 XPS Analysis


XPS analysis of unmodified substrates and plasma

treated substrates are listed in Table 4-5. The results in

Table 4-5 after 15 min RF plasma indicate that the oxygen

atomic concentration increases in all cases but is rather

small for some substrates. The result implies that water

RFGD plasma oxidizes the polymer surface and introduces

polar functional groups to the polymer chains. The increase

in oxygen concentration can be accounted for by the

formation of hydroxyl, carboxyl, or hydroperoxide groups.

Figure 4-4 shows the XPS of unmodified PMMA. The

elements presented on the PMMA survey scan are carbon and

oxygen. Figure 4-5 presents the overlapped Cls peaks for

unmodified and plasma modified PMMA. The major peak at 285

eV indicates the -CHx group. The secondary peak at 288.5 eV

indicating the chemical shift for the -CO-O- group,

increases slightly after plasma treatment.

The XPS of an unmodified PDMS and the overlapped Cls

peaks of a control PDMS and a plasma treated PDMS are shown

in Figure 4-6 and 4-7 respectively. The elements presented

for unmodified FPIlI3 are carbon, oxygen, and silicon. In

Figure 4-7, the difference seen between two overlapped Cls

peaks is insignificant, but the right hand side of Cls peak








Table 4-5. XPS data for untreated substrates and after 15
minutes RFGD plasma treatment



Atomic Concentration (%)

Cls Ols Fls Si2p Ols/Cls

Unmodified PMMA 79.90 20.10 0.25

Plasma treated PMMA 75.55 24.45 0.32

Unmodified PDMS 48.91 22.59 28.49 0.46

Plasma Treated PDMS 31. 75 34. 99 33.26 1.10

Unmodified PP 100.0 0.0 0.00

Plasma Treated PP 92. 66 7.34 0.08

Unmodified PC 86.11 13.89 0.16

Plasma Treated PC 80.81 19.19 0.23

Unmodified FEP 41.58 0.50 57.92 0.01

Plasma Treated FEP 39.10 0. 98 59. 92 0.02

Unmodified PVDF 55.67 1.29 43.05 0.02

Plasma Treated PVDF 60.24 4.13 35.63 0.07

(power--50 watts, pressure--100 mTorr, plasma time--15 min.)















20000o


1000 800


Cls

Ols

















600 400 200
Bi ndi no Enerou (ev)


Figure 4-4. XPS spectrum of an unmodified PMMA


a control PMMA
b plasma treated PMMA


ass 290 2as 20B 275
Bi nding Enerou (< U)


Figure 4-5. Overlapped Cls peaks for a control PMMA and a
plasma treated PMMA


C
c
o 10000
U



C
U
C 5000