1 A Review of Recent Advances in Plasma Polymerization MITCHEL SHEN and ALEXIS T. BELL Department of Chemical Engineering, University of California, Berkeley, CA 94720
A plasma i s a p a r t i a l l y ionized gas composed of i o n s , e l e c t r o n s and n e u t r a l species· I t i s a s t a t e o f m a t t e r t h a t c a n be c r e a t e d by such d i v e r s e t e c h n i q u e s as f l a m e s , e l e c t r i c a l d i s c h a r g e s , e l e c t r o n beams, l a s e r s o r n u c l e a r f u s i o n . The t e c h n i q u e o f most i n t e r e s t t o plasma p o l y m e r i z a t i o n i s t h e glow d i s c h a r g e , i n w h i c h f r e e e l e c t r o n s g a i n energy from an imposed e l e c t r i c a l f i e l d , and s u b s e q u e n t l y l o s e s i t t h r o u g h c o l l i s i o n s w i t h n e u t r a l m o l e c u l e s i n t h e gas. The t r a n s f e r o f energy t o gas m o l e c u l e s l e a d s t o t h e forma t i o n o f a h o s t o f c h e m i c a l l y r e a c t i v e s p e c i e s , some o f w h i c h become p r e c u r s o r s t o t h e plasma p o l y m e r i z a t i o n reaction. The plasma c r e a t e d by a glow d i s c h a r g e p o s s e s s e s a v e r a g e e l e c t r o n e n e r g i e s - i n the..range o f 1-10 e v and e l e c t r o n d e n s i t i t e s o f 10 - 10 / c . c . I n a d d i t i o n , t h e e l e c t r o n t e m p e r a t u r e (T ) o f t h e plasma i s n o t e q u a l t o t h e gas t e m p e r a t u r l (T ) b u t has a Τ /Τ r a t i o o f 10 - 100. I ti s therefore possible fof plasma p o l y m e r i z a t i o n t o p r o c e e d a t n e a r ambient tem peratures i n t h e presence o f e l e c t r o n s t o rupture c o v a l e n t bonds i n t h e gas m o l e c u l e s . Thus plasmas p r o duced by t h e glow d i s c h a r g e a r e c a l l e d n o n - e q u i l i b r i u m p l a s m a s , i n c o n t r a d i s t i n c t i o n t o e q u i l i b r i u m plasmas c r e a t e d by a r c s o r plasma j e t s where Τ = Τ . The very h i g h temperatures i n these plasma! (i8 Shousands o f d e g r e e s K e l v i n ) r e n d e r them u n s u i t a b l e f o r plasma p o l y m e r i z a t i o n s , s i n c e p o l y m e r s produced under t h e s e c o n d i t i o n s w i l l be r a p i d l y d e g r a d e d . S i n c e de W i l d e (1) and Thenard (2) f i r s t r e p o r t e d t h e f o r m a t i o n o f s o l i d p r o d u c t s i n a plasma o f o r g a n i c v a p o r more t h a n a c e n t r u r y ago, many w o r k e r s i n t h e f i e l d o f plasma o r g a n i c c h e m i s t r y have o b s e r v e d t h e p r e s e n c e o f h i g h m o l e c u l a r w e i g h t m a t e r i a l s as r e a c t i o n b y - p r o d u c t s . These m a t e r i a l s adhered t i g h t l y g
a
0-8412-0510-8/79/47-108-001$08.25/0 © 1979 American C h e m i c a l Society
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t o t h e w a l l s o f r e a c t i o n v e s s e l s , and were i n s o l u b l e i n most s o l v e n t s . They were c o n s i d e r e d a n u i s a n c e un t i l Goodman ( i ) d e m o n s t r a t e d t h a t a 1 mm t h i c k plasma p o l y m e r i z e d s t y r e n e f i l m d e p o s i t e d on t i t a n i u m f o i l made a s a t i s f a c t o r y d i e l e c t r i c f o r a n u c l e a r b a t t e r y . S i n c e t h a t t i m e p l a s m a - d e r i v e d p o l y m e r s have been s u g g e s t e d f o r numerous a p p l i c a t i o n s because o f t h e i r u n i q u e p h y s i c a l p r o p e r t i e s , ease o f p r e p a r a t i o n , and propensity f o r the formation o f t h i n pinhole-free f i l m s . A w i d e v a r i e t y o f o r g a n i c and o r g a n o m e t l l i c com pounds c a n be p o l y m e r i z e d t o form a t h i n f i l m on a s u b s t r a t e p l a c e d i n a glow d i s c h a r g e . Although the r a t e a t w h i c h a monomer w i l l p o l y m e r i z e depends on a l a r g e number o f p a r a m e t e r s , one o f t h e most u n i q u e f e a t u r e s o f plasma p o l y m e r i z a t i o n i s t h a t monomers need n o t p o s s e s s such r e a c t i v e f u n c t i o n a l groups as d o u b l e bonds t o be p o l y m e r i z a b l e . F o r i n s t a n c e , ethane and benzene have been r e a d i l y p o l y m e r i z e d i n t h e plasma. F u r t h e r m o r e , p o l y m e r s have a l s o been p r e p a r e d from i n o r g a n i c s t a r t i n g m a t e r i a l s . H o l l a h a n and McKeever (4) r e p o r t e d t h a t p o l y m e r s were formed i n an e l e c t r o d e l e s s d i s c h a r g e s u s t a i n e d i n m i x t u r e s o f CO, H / and Ν · The s t r u c t u r e o f t h o s e m a t e r i a l s was found t o r e s e m b l e t h a t o f p r o t e i n s . The purpose o f t h i s paper i s t o d i s c u s s some o f t h e r e c e n t advances i n o u r u n d e r s t a n d i n g o f t h e k i n e t i c s and mechanism o f plasma p o l y m e r i z a t i o n , t h e s t r u c t u r e and p r o p e r t i e s o f plasma p o l y m e r s and some o f t h e i r p o t e n t i a l a p p l i c a t i o n s . I t i s not intended t o be e x h a u s t i v e , as e a r l i e r r e v i e w s (5-10) a r e a l r e a d y available. I n t e r e s t e d readers are r e f e r r e d t o the literature cited f o rfurther details. 2
2
Kinetics The r a t e o f plasma p o l y m e r i z a t i o n depends on t h e n a t u r e o f t h e monomer g a s . I n a d d i t i o n , such p a r a meters as f l o w r a t e , p r e s s u r e , power, f r e q u e n c y , e l e c t r o d e gap and r e a c t o r c o n f i g u r a t i o n a l s o s t r o n g l y i n f l u e n c e t h e p o l y m e r i z a t i o n r a t e f o r a g i v e n monomer. G e n e r a l l y a t low f l o w r a t e s t h e r e i s an abundance o f r e a c t i v e s p e c i e s so t h e p o l y m e r i z a t i o n r a t e i s l i m i t e d o n l y by t h e a v a i l a b i l i t y o f monomer s u p p l y . At high f l o w r a t e s , however, t h e r e i s an overabundance o f monomer c o n c e n t r a t i o n and t h e p o l y m e r i z a t i o n r a t e now depends on t h e r e s i d e n c e t i m e . A t i n t e r m e d i a t e f l o w r a t e s t h e s e two c o m p e t i n g p r o c e s s e s r e s u l t i n a maximum. T h i s b e h a v i o r i s i l l u s t r a t e d i n F i g u r e 1 f o r e t h a n e , e t h y l e n e , and a c e t y l e n e ( 1 1 ) . These d a t a a l s o demonstrate t h e e f f e c t o f i n c r e a s e d u n s a t u r a t i o n i n
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Rates of plasma polymerization of acetylene, a function of monomer flow rate
ethylene,
and ethane as
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monomers on t h e p o l y m e r i z a t i o n r a t e . A t comparable r e a c t i o n c o n d i t i o n s , a c e t y l e n e w i t h a t r i p l e bond p o l y m e r i z e s an o r d e r o f magnitude f a s t e r t h a n e t h y l e n e w h i c h has a d o u b l e bond; w h i l e e t h y l e n e i n t u r n i s much more r e a c t i v e t h a n t h e s a t u r a t e d e t h a n e . The p o l y m e r i z a t i o n r a t e s f o r t h r e e o l e f i n i c h y d r o c a r b o n s (11) a r e shown i n F i g u r e 2. Isobutylene i s seen t o p o l y m e r i z e more s l o w l y t h a n p r o p y l e n e , w h i l e ethylene polymerizes f a s t e r than e i t h e r i s o b u t y l e n e or p r o p y l e n e by an o r d e r o f magnitude. These d a t a i n d i c a t e t h a t i n a g i v e n homologous s e r i e s o f h y d r o c a r b o n s , h i g h e r p o l y m e r i z a t i o n r a t e s a r e f a v o r e d by l o w e r molecular weights. F i g u r e 3 compares t h r e e monomers c o n t a i n i n g f o u r c a r b o n atoms. Here b u t a d i e n e w i t h two d o u b l e bonds p o l y m e r i z e s s u b s t a n t i a l l y f a s t e r t h a n e i t h e r c i s - 2 - b u t e n e o r i s o b u t y l e n e w i t h one d o u b l e bond ( 1 1 ) . Yasuda (12) s u r v e y e d 28 monomers and found t h a t monomers c o n t a i n i n g a r o m a t i c g r o u p s , n i t r o g e n ( e . g . , -NH, -NH , -CN), s i l i c o n and o l e f i n i c d o u b l e bonds a r e more p o l y m e r i z a b l e w h i l e t h o s e c o n t a i n i n g oxygen (e.g. -C=0,-0-,-OH), c h l o r i n e , a l i p h a t i c h y d r o c a r b o n and c y c l i c h y d r o c a r b o n s t e n d t o decompose. Brown (13.) r e p o r t ed i n h i s s t u d i e s o f a s e r i e s o f v i n y l h a l i d e s t h a t the d i h a l o e t h y l e n e s p o l y m e r i z e more r a p i d l y t h a n t h e c o r r e s p o n d i n g m o n o h a l i d e s and t h a t c h l o r i d e s and bromides p o l y m e r i z e more r a p i d l y t h a n t h e f l u o r i d e s . Kobayashi, e t . a l . (14) found t h a t t h e a d d i t o n s o f c e r t a i n h a l o g e n a t e d compounds t o h y d r o c a r b o n monomer streams o f t e n d r a m a t i c a l l y i n c r e a s e s t h e p o l y m e r i z a t i o n r a t e . Thus, t h e s e h a l o g e n a t e d compounds may be c o n s i d e r e d t o a c t as gas phase c a t a l y s t s f o r t h e plasma p o l y m e r i z a t i o n of hydrocarbons. The r a t e o f plasma p o l y m e r i z a t i o n g e n e r a l l y i n c r e a s e s w i t h i n c r e a s i n g power, u n t i l a t h i g h power dens i t i e s when i t becomes n e a r l y i n d e p e n d e n t o f power. This i s i l l u s t r a t e d i n Figure 4 f o r t e t r a f l u o r o e t h y l ene ( 1 4 ) . The i n t e r e l e c t r o d e gap a l s o e x e r t s a s i m i l a r e f f e c t . F i g u r e 5 shows t h a t as t h e gap i s narrowed (higher e l e c t r o n d e n s i t y ) the r a t e i n c r e a s e s . For s m a l l e l e c t r o d e gaps t h e r e i s s i g n i f i c a n t powder f o r m a t i o n , w h i c h i s t y p i c a l o f the p r o d u c t formed a t h i g h p o l y m e r i z a t i o n r a t e s ( 1 5 ) . The e f f e c t o f i n c r e a s e d p r e s s u r e i s t o d e c r e a s e . t h e r a t e o f p o l y m e r i z a t i o n (15,16) as shown i n F i g u r e 6 f o r s t y r e n e ( 1 6 ) . The k i n e t i c s o f plasma p o l y m e r i z a t i o n i s o f t e n a f f e c t e d by t h e r e a c t o r c o n f i g u r a t i o n (9,17,18). K o b a y a s h i , e t . a l . (17) showed t h a t under o t h e r w i s e i d e n t i c a l c o n d i t i o n s , t h e p o l y m e r i z a t i o n r a t e s o f e t h y l e n e a r e n o t t h e same f o r t h o s e u s i n g t u b u l a r t y p e and b e l l - j a r t y p e r e a c t o r s .
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r
Flow Rate (STPcmVmin) Figure
Figure
2.
3.
Rates of plasma polymerization of ethylene, propylene, ene as a function of monomer flow rate (11)
and
isobutyl
Rates of plasma polymerization of butadiene, cis-2-isobutylene, isobutylene as a function of monomer flow rate (11)
and
P L A S M A POLYMERIZATION
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Power (W) Journal of Macromolecular Science, Chemistry Figure
4.
The rate of plasma polymerization of tetrafluoroethylene of power (14)
as a
function
— ι — Ρ «100 W ρ = 2 torr F - 80 cm /min
io 1
3
R · n
g
(Radical Adsorption)
S
k R ·+ M n g
p
q
g
>R · n+l
(Homogeneous propagation)
>R · n+l
(Heterogeneous propagation)
g
k R ·+ M n S
g
p
s
s
where e, M and R r e f e r to electrons, monomers and r a d i c a l s , and s and g designate substrate and gas r e s p e c t i v e l y . The expression for the rate of plasma polymerization was written as: r
ρ
= (| k + k Κ ) [M ] [R ] 2 pg ps R g g
(1)
where d i s the space between the electrodes, and K i s the adsorption c o e f f i c i e n t of the r a d i c a l s on the electrode surface: R
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[R ] s
POLYMERIZATION
= K [R ] R
(2)
g
To compute r , the concentrations of monomer and free r a d i c a l s i n i h e gas phase must be determined. Solution of the appropriate species conservation equations and s u b s t i t u t i o n of the solutions into eq. 1 leads t o : r
p
= [M ]2 2 a c [ l - e g
( a
-
b ) T
]/(a-b)e
( a + b ) T
]
where a=k.[e], b=(2/d)k [ S ] , c=(d/2)k.+k , and ι a ι ps
(3)
T=V/Q.
By adjusting the magnitudes of the rate c o e f f i c i e n t s appearing i n the model, good agreement was obtained between polymerization rates predicted by the model and those measured experimentally for a v a r i e t y of unsaturated monomers (Figure 8). The magnitudes of the f i t t e d rate c o e f f i c i e n t s describing the i n i t i a t i o n of polymerization and gas phase oligomerization were found to be i n good q u a n t i t a t i v e agreement with independently observed rated c o e f f i c i e n t s . Structure Variations i n the plasma parameters often produce s i g n i f i c a n t changes i n the structure and properties of the plasma polymerized materials. For a given monomer polymerized i n a reactor at a fixed frequency, a " c h a r a c t e r i s t i c map" may be constructed (11*15/42). Figure 9 shows that f o r ethylene polymerized at a frequency of 13,56 MHz, both powder and f i l m are formed at low pressure and low monomer flow rates (42). At high pressures and high flow r a t e s , an o i l y product i s produced. Only at low pressure and high flow rate can a s o l i d , pinhole-free f i l m be obtained. I f the pressure i n the reactor i s high and the monomer flow rate i s low, then the discharge becomes unstable. Figure 9 also shows that decreased power renders i t possible to produce a f i l m at lower flow rates. For conditions near the v i c i n i t y of the powder-film border l i n e , the films are not transparent because of the incorporation of very f i n e powder p a r t i c l e s (43,44,45, · Upon further increase i n flow rate, however, a transparent f i l m can be formed i n d i c a t i n g that pow-
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Figure 8. Computed and experimental rates of plasma polymerizations for fins as a function of monomer flow rate (41). (Φ) Ethylene, (^) butadiene, propylene, ([J) isobutylene.
1
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Unstable Discharge \
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Oily Film
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FILM
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SJOOW
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Rigid Film
\ POWDER AND
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ole (O)
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FILM
II 20
\
\50W
Λ
I 30
I 40
I 1 50
I1 60
I 1 70
I 80
Ethylene Flow Rate (STP cm /min) 3
Journal of Macromolecular Science, Chemistry
Figure
9.
Characteristic
map for the plasma
polymerization
of ethylene
(15)
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d e r s t r a p p e d i n t h e f i l m must now be s m a l l e r t h a n t h e wavelength o f the v i s i b l e l i g h t . The f o r m a t i o n o f powder i s a t t r i b u t e d t o t h e homogeneous r e a c t i o n s i n t h e gaseous phase. Here t h e r e a c t i v e s p e c i e s i n t h e plasma p o l y m e r i z e f a s t e r t h a n the r a t e o f d i f f u s i o n o f these s p e c i e s t o t h e s u b s t r a t e . As a consequence m a c r o s c o p i c p a r t i c l e s a r e formed w h i l e suspended i n t h e gaseous phase u n t i l g r a v i t a t i o n a l f o r c e e v e n t u a l l y c a u s e s t h e powder t o p r e c i p i t a t e o n t o t h e s u b s t r a t e s u r f a c e . We r e c a l l t h a t a c e t y l e n e undergoes plasma p o l y m e r i z a t i o n a t a much h i g h e r r a t e t h a n e t h y l e n e . F i g u r e 10 shows i n f a c t t h a t f o r t h i s monomer o n l y powdery p r o d u c t s a r e formed. On the o t h e r hand, ethane p o l y m e r i z e s s l o w e r t h a n e t h y l e n e by an o r d e r o f magnitude under comparable c o n d i t i o n s (11). I n t h i s i n s t a n c e we n o t e t h a t t h e f i l m f o r m a t i o n r e g i o n i s much g r e a t e r t h a n t h o s e f o r powder and o i l f o r m a t i o n ( F i g u r e 11) . D u v a l and T h e o r e t (4J7) e n v i s i o n t h e g r o w i n g c h a i n s t o be i n i t i a t e d i n t h e gas phase and s u b s e q u e n t l y d e p o s i t e d on t h e e l e c t r o d e s u r f a c e as o l i g o m e r s . They s u g g e s t t h a t a t h i g h r e a c t i o n p r e s s u r e (>1 t o r r ) t h e r e i s s u f f i c i e n t monomer on t h e s u r f a c e t o d i s s o l v e t h e r e a c t i v e c h a i n and l i m i t t h e degree o f p o l y m e r i z a t i o n , t h u s e x p l a i n i n g t h e formation o f low molecular weight o i l y products a t h i g h r e a c t i o n p r e s s u r e s . A t low p r e s s u r e s , the s u r f a c e s a r e d e f i c i e n t i n monomer and t h e r e a c t i v e o l i g o mers c o n t i n u e t o grow y i e l d i n g h i g h m o l e c u l a r w e i g h t s o l i d products. These a u t h o r s used g e l p e r m e a t i o n chromatography t o e v a l u a t e g r o s s s t r u c t u r a l changes i n p o l y m e r form r e s u l t i n g from changes i n r e a c t i o n p r e s s u r e and power l e v e l . The m o l e c u l a r w e i g h t d i s t r i b u t i o n s obtained i n t h i s study i n d i c a t e t h a t higher m o l e c u l a r w e i g h t i s f a v o r e d by l o w p r e s s u r e and h i g h power, w h i c h was l a t e r v e r i f i e d by K o b a y a s h i , e t . a l . ( 4 2 ) . D u v a l and T h e o r e t a l s o found t h a t a t a g i v e n p r e s s u r e and power l e v e l , p o l y m e r d e p o s i t e d on t h e e l e c t r o d e s u r f a c e has a h i g h e r m o l e c u l a r w e i g h t t h a n t h a t formed on a s u b s t r a t e p l a c e d i n t h e i n t e r e l e c t r o d e gap. The morphology o f t h e plasma p o l y m e r i z e d f i l m s has been examined by e l e c t r o n m i c r o s c o p y by a number o f w o r k e r s (43,44,4^,46,48). F i g u r e 12 shows t h e r e p l i c a e l e c t r o n m i c r o g r a p h o f plasma p o l y m e r i z e d e t h y l e n e d e p o s i t e d on chromium s u b s t r a t e a t s e v e r a l gas p r e s s u r e s (46>) . The p r e s e n c e o f powder p a r t i c l e s i s c l e a r l y e v i d e n c e d i n F i g u r e s 12a-c. The s i z e and d e n s i t y o f t h e powdery p r o d u c t s d e c r e a s e w i t h i n c r e a s i n g p r e s s u r e u n t i l a t a p r e s s u r e o f 3 t o r r when t h e p o l y m e r i s m a i n l y f i l m and c o n t a i n s v e r y few p a r t i c l e s .
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map for the plasma
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60 80 Monomer Flow Rote
for the plasma
'
of acetylene
1
'
(11)
1
100 120 (STPcc/min)
polymerization
of ethane
(102)
PLASMA
POLYMERIZATION
Figure 12. Transmission electron micrographs of plasma-polymerized ethylene on chromium substrate at 80 mL/min, 100 W , and (a) 0.7 torr, (b) 1.5 torr, (c) 3 torr, and (d) substrate alone (46)
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The e f f e c t o f monomer f l o w r a t e on t h e f i l m appearance i s s i m i l a r . A t low f l o w r a t e s ( h i g h p o l y m e r i z a t i o n r a t e ) , p a r t i c l e s a r e b u r i e d i n s i d e t h e f i l m . As t h e f l o w r a t e i s i n c r e a s e d , b o t h t h e s i z e and t h e d e n s i t y o f t h e p a r t i c l e s d e c r e a s e t o y i e l d a smoother f i l m . F i g u r e s 13 and 14 compare the s u r f a c e s t r u c t u r e s o f t h e plasma p o l y m e r i z e d e t h y l e n e on T e f l o n , and c l e a v e d m i c a (46). Because o f t h e r o u g h e r i n i t i a l s u r f a c e on T e f l o n , t h e r e i s a g r e a t e r c o n c e n t r a t i o n o f powder on t h a t s u r f a c e . The m i c a s u r f a c e i s smooth b e f o r e polymer d e p o s i t i o n . The p o l y m e r f i l m s a r e a l s o v e r y smooth and f e a t u r e l e s s , i n d i c a t i n g t h e s t r o n g dependence o f morphology o f plasma p o l y m e r i z e d f i l m s on t h e s u r f a c e roughness o f the s u b s t r a t e s . Plasma p o l y m e r i z e d f i l m s a r e t h i n (from hundreds o f angstroms t o s e v e r a l m i c r o n s ) and p i n h o l e - f r e e . The absence o f p i n h o l e s was d e t e r m i n e d by Lee and c o - w o r k e r s (49,50) u s i n g hydrogen gas e v o l u t i o n and e l e c t r o p h o r e t i c decoration techniques. X-ray d i f f r a c t i o n s t u d i e s show the c o m p l e t e absence o f c r y s t a l l i n i t y ( 4 2 ) . B o t h f i l m s and powders a r e i n s o l u b l e i n conv e n t i a l o r g a n i c s o l v e n t s , i n d i c a t i n g the h i g h l y c r o s s l i n k e d n a t u r e o f t h e p o l y m e r . O i l y p r o d u c t s , on t h e o t h e r hand, a r e c o m p l e t e l y s o l u b l e . These a r e g e n e r a l l y branched oligomers. G e l p e r m e a t i o n chromatography (27) and v a p o r phase osmometry (JL5) d e t e r m i n a t i o n s show t h a t t h e m o l e c u l a r w e i g h t s o f the o i l s a r e o f t h e o r d e r o f a few t h o u s a n d grams p e r mole. The c h e m i c a l c o m p o s i t i o n o f t h e polymer g e n e r a l l y b e a r no s i m p l e r e l a t i o n t o t h a t o f t h e s t a r t i n g monomer. F o r i n s t a n c e , the c a r b o n t o hydrogen r a t i o i s about 2/3 f o r plasma polymerized ethylene, i n c o n t r a d i s t i n c t i o n t o that o f the c o n v e n t i o n a l p o l y e t h y l e n e w h i c h i s 1/2(15,42,51,52). However, t h e s t o i c h i o m e t r i c r a t i o o f t h e p o l y m e r may v a r y w i t h the c o n d i t i o n under w h i c h t h e p o l y m e r i z a t i o n r e a c t i o n was c a r r i e d o u t ( 1 5 ) . D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y and t h e r m a l g r a v i m e t r i c a n a l y s i s have been used by s e v e r a l a u t h o r s (27*53) t o show t h a t p l a s m a - d e r i v e d p o l y m e r s have no phase t r a n s i t i o n s u n t i l d e c o m p o s i t i o n o c c u r s . The r e markable thermal s t a b i l i t y o f these m a t e r i a l s i s e v i d e n c e d by d a t a w h i c h show t h a t 80 wt.% o f a f i l m p r e p a r e d from m e t h y l c h l o r i d e r e m a i n s a t 800°C and t h a t 40 wt.% o f a s t y r e n e d e r i v e d f i l m r e m a i n s a t 700°C. I n f r a r e d s p e c t r o s c o p y has been e x t e n s i v e l y used i n e l u c i d a t i n g the m i c r o s t r u c t u r e o f plasma-polymerized m a t e r i a l s . E a r l i e r works (54,55) have shown t h a t s h o r t a l k a n e segments and v a r i o u s t y p e s o f v i n y l groups a r e t h e p r e d o m i n a n t s t r u c t u r a l groups o b s e r v e d .
PLASMA
Figure 13. Transmission electron on Teflon substrate. Polymerization
POLYMERIZATION
micrographs of plasma-polymerized conditions are the same as in Figure
ethylene 12 (46).
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Figure 14. Transmission electron micrographs of plasma-polymerized on cleaved mica. Polymerization conditions are the same as in Figure
ethylene 12 (46).
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P L A S M A POLYMERIZATION
There i s no e v i d e n c e o f l o n g a l k a n e segments i n t h e products. These d a t a s u p p o r t t h e h y p o t h e s i s t h a t p l a s m a - d e r i v e d p o l y m e r s have a mesh s t r u c t u r e , w i t h o n l y s h o r t c h a i n segments between b r a n c h and c r o s s l i n k p o i n t s . T h i s s t r u c t u r a l d e s c r i p t i o n has a l s o been s u p p o r t e d by NMR (56.) and mass s p e c t r a l (.27,38) s t u d i e s . More r e c e n t l y T i b b i t t , B e l l and Shen (56,57) c a r r i e d out q u a n t i t a t i v e determinations of the s t r u c t u r e o f plasma p o l y m e r i z e d h y d r o c a r b o n s by i n f r a r e d spectroscopy. T y p i c a l s p e c t r a o f f i l m s d e p o s i t e d on sodium c h l o r i d e s u b s t r a t e s a r e shown i n F i g u r e 15. A b s o r p t i o n s peaks were i d e n t i f i e d t o be a s s o c i a t e d w i t h v a r i o u s group f r e q u e n c i e s , c o r r o b o r a t e d w i t h t h e f i n d i n g s from NMR s p e c t r a o f d i l u t e s o l u t i n s o f t h e o i l y products. From t h e knowledge o f t h e e x t i n c t i o n c o e f f i c i e n t s , c o n c e n t r a t i o n s o f t h e f u n c t i o n a l groups i n t h e plasma p o l y m e r i z e d m a t e r i a l s were d e t e r m i n e d . A p o s t u l a t e d m o l e c u l a r model f o r t h e plasma p o l y m e r i z e d e t h y l e n e i s shown i n F i g u r e 16 (56) . The p o l y m e r does not c o n t a i n r e g u l a r r e p e a t i n g u n i t s of methylene g r o u p s , as i n c o n v e n t i o n a l p o l y e t h y l e n e . R a t h e r i t has numerous u n s a t u r a t e d g r o u p s , a r o m a t i c g r o u p s , and s i d e b r a n c h e s . I n a d d i t i o n , t h e polymer i s v e r y h i g h l y c r o s s l i n k e d , w i t h about one c r o s s l i n k p e r s i x t o t e n c h a i n c a r b o n atoms. These s t r u c t u r a l u n i t s combine t o r e n d e r t h e f i n a l polymer u n c r y s t a l l i z a b l e . Not i n cluded i n the p o s t u l a t e d s t r u c t u r e are the t e t r a f u n c t i o n a l c r o s s l i n k s , w h i c h a r e no doubt p r e s e n t , because t h e s e groups a r e n o t amenable t o d e t e c t i o n by the i n f r a r e d technique. P y r o l y s i s / g a s chromatography (P/GC) has been used a l s o t o i n v e s t i g a t e t h e s t r u c t u r e o f t h e plasma p o l y m e r i z e d m a t e r i a l s (58/59). F i g u r e 17 shows a c o m p a r i s o n o f t h e P/GC pyrograms o f plasma p o l y m e r i z e d e t h y l e n e (PPE) and c o n v e n t i o n a l p o l y e t h y l e n e ( P E ) . I t i s seen t h a t t h e methane (C, ) peak i s v e r y pronounced f o r PPE b u t a p p e a r s o n l y as a s h o u l d e r i n t h e pyrogram o f c o n v e n t i o n a l PE. The i m p l i c a t i o n i s t h a t PPE i s h i g h l y b r a n c h e d and c r o s s l i n k e d compared t o PE. The i s o a l k a n e s / n - a l k a n e s r a t i o i s l o w e r f o r PE, b u t i s r a t h e r h i g h f o r PPE, i n d i c a t i n g t h e i r r e g u l a r i t y o f s t r u c t u r e i n the l a t t e r m a t e r i a l . Table 2 gives the sums o f t h e h e i g h t s o f a l l i s o a l k a n e peaks and t h e "7F peak," the P/GC d a t a f o r plasma p o l y m e r i z e d e t h y l e n e , b u t a d i e n e and benzene. The 7F peak i s c h a r a c t e r i s t i c o f a r o m a t i c f r a g m e n t s i n the s t r u c t u r e , w h i l e the i s o a l k a n e content i s r e l a t e d to the c o n c e n t r a t i o n of t e r t i a r y carbons. A l s o i n c l u d e d i n t h e t a b l e a r e the f u n c t i o n a l group c o n c e n t r a t i o n s o f t e r t i a r y c a r b o n atoms and p h e n y l groups as w e l l as t h e c r o s s l i n k d e n s i t i e s
1.
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P L A S M A POLYMERIZATION
PP Ethylene Oil 6B
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17.
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d e t e r m i n e d from i n f r a r e d s p e c t r a (56,57). There i s q u a l i t a t i v e agreement between t h e d a t a d e t e r m i n e d by t h e s e two t e c h n i q u e s , t h u s r e i n f o r c i n g t h e p o s t u l a t e d s t r u c t u r e o f t h e plasma p o l y m e r i z e d h y d r o c a r b o n s . E l e c t r o n s p e c t r o s c o p y (ESCA) has been found t o be p a r t i c u l a r l y u s e f u l f o r the s t r u c t u r a l a n a l y s i s o f plasma p o l y m e r i z e d f i l m s u r f a c e s . Most o f t h e a p p l i c a t i o n s a r e d i r e c t e d t o f l u o r o c a r b o n p o l y m e r s because of the l a r g e chemical s h i f t s i n the b i n d i n g energies o f C ( l s ) e l e c t r o n s caused by f l u o r i n e s bonded t o c a r b o n . As a r e s u l t CF-, C F , CF and t e t r a f u n c t i o n a l c a r b o n s can be d i s t i n g u i s h e d (60,61,62,63,64). Knowing t h e r e l a t i v e e l e c t r o n l i n e p o s i t i o n s o f the v a r i o u s groups, t h e C ( l s ) s p e c t r a can be d e c o n v o l u t e d and t h e group concentrations determined. The a p p l i c a t i o n o f ESCA i s n o t l i m i t e d t o plasma p o l y m e r s c o n t a i n i n g f l u o r i n e atoms. Oxygen and n i t r o g e n c o n t a i n i n g p o l y m e r s have a l s o been s u c c e s s f u l l y a n a l y z e d (60/65,66.) . Another important c h a r a c t e r i s t i c o f plasmapolymerized f i l m s i s the e x i s t e n c e o f trapped f r e e r a d i c a l s . These a c t i v e s i t e s a r e p r o b a b l y formed b o t h t h r o u g h i n c o r p o r a t i o n o f f r e e r a d i c a l s from t h e gas phase and by t h e impingement o f a c t i v e plasma s p e c i e s and r a d i a t i o n o n t o t h e d e p o s i t i n g f i l m . Due t o t h e h i g h l y c r o s s l i n k e d s t r u c t u r e , t r a p p e d r a d i c a l s have low m o b i l i t y and do n o t recombine r a p i d l y . Upon e x p o s u r e t o t h e atmosphere, t h e s e t r a p p e d r a d i c a l s r e a c t w i t h oxygen and i n f r a r e d measurements (42,67) demons t r a t e t h a t c a r b o n y l and h y d r o x y l groups a r e formed i n t h e polymer. Denaro (16) h a s shown by t i t r a t i o n o f p l a s m a - p o l y m e r i z e d s t y r e n e w i t h DPPH t h a t one i n twenty p o l y m e r m o l e c u l e s i s a r a d i c a l . E l e c t r o n s p i n resonance s p e c t r o s c o p y has been used t o d e t e r m i n e t h e s p i n d e n s i t i e s i n plasma p o l y m e r i z e d m a t e r i a l s (15, 31,32). These r a d i c a l s were found t o p o s s e s s a l o n g h a l f - l i f e , and has been known t o be r e s p o n s i b l e f o r p o l y m e r i z a t i o n t o c o n t i n u e a f t e r t h e plasma has been t u r n e d o f f . The r e d u c t i o n o f r a d i c a l c o n c e n t r a t i o n can be a c h i e v e d e i t h e r by t h e use o f l o w e r power, o r by p u l s e d plasma (23) a s shown i n F i g u r e 18. 2
Properties There has been a g r e a t d e a l o f i n t e r e s t i n t h e s t u d y o f t h e e l e c t r i c a l p r o p e r t i e s o f plasma p o l y m e r i z e d f i l m s . E a r l y d a t a on t h e d i e l e c t r i c and c o n d u c t i v i t y o f t h e f i l m s has been r e v i e w e d by Mearns ( 8 ) . More r e c e n t l y , t h e d i e l e c t r i c p r o p e r t i e s o f plasma p o l y m e r i z e d s t y r e n e (69-71)/ a c r y l o n i t r i l e ( 7 2 ) , h e x a m e t h y l d i s i l o x a n e (73-75), t e t r a f l u o r o e t h y l e n e
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P L A S M A POLYMERIZATION
Table 2.
F u n c t i o n a l Group C h a r a c t e r i z a t i o n s o f PlasmaPolymerized Products Deduced from IR Spectroscopy, NMR and P/GC
Functional Group Concentration (mole/gm polym)
Isoalkane Apparent Relative C r o s s l i n k Peak Density Height
7F Relative Peak Height
backbone carbons crosslink
(% o f C Peak Height)
(% o f C Peak Height)
0.39
6.4
82
10
Butadiene 12.0
1.2
9.6
66
20
Benzene
7.9
6.9
20
83
Monomer
Ethylene
[R CH] 3
18.0
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[φ]
9
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Figure 18. Electron spin concentrations of plasma-polymerized hexamethyldisilane as a function of time after removal from plasma (23)
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TIME (min)
80
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and h y d r o c a r b o n s (76) have been r e p o r t e d . Sawa, e t . a l . (20/71) n o t e d two major peaks i n t h e d i e l e c t r i c l o s s s p e c t r a o f plasma p o l y m e r i z e d s t y r e n e , a low temperature r e l a x a t i o n which i s a s c r i b e d t o the motion o f s h o r t c h a i n segments and a h i g h e r t e m p e r a t u r e p r o c e s s r e l a t e d t o t h e m o t i o n o f pendant p h e n y l g r o u p s . The a u t h o r s o b s e r v e d a s h i f t o f t h e peak maxima t o h i g h e r t e m p e r a t u r e s a s t h e m a t e r i a l was formed a t l o w e r p r e s s u r e s . They e x p l a i n e d t h e peak t e m p e r a t u r e s h i f t a s a r e s u l t o f i n c r e a s e d c r o s s l i n k i n g a t low r e a c t i o n pressure. Mann (73) and Tuzov, e t . a l . (74./75) s t u d i e d t h e d i e l e c t r i c p r o p e r t i e s o f plasma p o l y m e r i z e d s i l i c o n e f i l m s / and found t h a t t h e r e i s an i n c r e a s e i n d i e l e c t r i c c o n s t a n t and l o s s t a n g e n t upon e x p o s u r e t o a i r . These w o r k e r s a t t r i b u t e d t h e changes t o t h e a b s o r p t i o n of t h e w a t e r v a p o r w h i c h i n c r e a s e s t h e p o l a r i t y o f the f i l m s . R e c e n t l y T i b b i t t / e t . a l . (76) c a r r i e d o u t p a r a l l e l i n f r a r e d measurements w i t h d i e l e c t r i c e x p e r i m e n t s on plasma p o l y m e r i z e d h y d r o c a r b o n s and a f l u o r o c a r b o n b e f o r e and a f t e r s i g n i f i c a n t o x i d a t i o n has t a k e n p l a c e . The d i e l e c t r i c s p e c t r a o f plasma p o l y m e r i z e d e t h y l e n e shows t h a t t h e a s - p o l y m e r i z e d f i l m shows l i t t l e change i n l o s s t a n g e n t a s a f u n c t i o n of t e m p e r a t u r e / e x c e p t a t h i g h e r t e m p e r a t u r e s where dc c o n d u c t i v i t y e f f e c t s s e t s i n ( F i g u r e 1 9 ) . Upon o x i d a t i o n i n t h e atmosphere, however, t h e r e i s a r e l a x a t i o n peak c e n t e r e d a t -50°C (100 H z ) , w h i c h i s s h i f t e d t o h i g h e r temperatures w i t h i n c r e a s i n g f r e quency ( A c t i v a t i o n energy i s 13.8 k c a l / m o l e ) . The peak i s a t t r i b u t e d t o t h e l o c a l mode m o t i o n o f s e v e r a l molecu l a r segments. Oxidation o f the trapped free r a d i c a l s i n t r o d u c e s c a r b o n y l groups t h a t a c t a s " t r a c e r s " t o render the m o l e c u l a r motion observable by d i e l e c t r i c e x p e r i m e n t s . The c a r b o n y l c o n c e n t r a t i o n s d e t e r m i n e d by i n f r a r e d s p e c t r o s c o p y a r e shown t o b e a r a l i n e a r r e l a t i o n s h i p w i t h the strengths o f d i e l e c t r i c l o s s (Figure 20). The c o n d u c t i v i t y and c a p a c i t a n c e o f plasma p o l y m e r i z e d f i l m s has a l s o been r e p o r t e d by numerous w o r k e r s (72,77-81). M o r i t a , e t . a l . (81) found t h a t the d c c o n d u c t i o n c u r r e n t d e c r e a s e s when exposed t o oxygen, and a t t r i b u t e d t h e d e c r e a s e t o t h e d i s a p p e a r ance o f space c h a r g e on e x p o s u r e t o oxygen. H a r a i and Nakada (72) s u g g e s t e d t h a t t h e d c c o n d u c t i v i t y e f f e c t s of plasma p o l y m e r i z e d a c r y l o n i t i l e can be a d e q u a t e l y i n t e r p r e t e d by t h e f i e l d a s s i s t e d i o n i z a t i o n o f i m p u r i t y l e v e l s f i r s t e n u n c i a t e d by P o o l e and F r e n k e l (82) . R e c e n t l y H e t z l e r and Kay (8) s t u d i e d t h e a c -
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26
ol
I
-150
-100
!
I
-50 0 Temperature (°C)
!
I
50
100
Journal of Macromolecular Science, Chemistry Figure 19. polymerized
Dielectric loss tangent as a function of temperature for plasmaethylene at four frequencies (76). (O) 10 Hz, (A) 10 Hz, (Q) 10 Hz, 2
3
4
OKPHz.
Journal of Macromolecular Science, Chemistry Figure
20.
Dielectric
loss constant of plasma-polymerized (Φ) PPEVC, (A)PPTFE.
(O) PPE, (A)
PPE
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c o n d u c t i v i t y o f plasma p o l y m e r i z e d t e t r a f l u o r o e t h y l e n e , and showed t h a t a t h i g h r e q u e e n c i e s and low tempera t u r e s t h e mechanism o f h o p p i n g c o n d u c t i v i t y d o m i n a t e s , a t low f r e q u e n c i e s and h i g h t e m p e r a t u r e s , a d i p o l e o r i e n t a t i o n p r o c e s s i s o p e r a t i v e . On t h e b a s i s o f i r and e s r measurements, t h e s e a u t h o r s c o n c l u d e d t h a t t h e c a r b o n y l groups formed by o x i d a t i o n a r e r e s p o n s i b l e f o r the observed e f f e c t . P h o t o c o n d u c t i v e p r o p e r t i e s o f plasma p o l y m e r i z e d f i l m s have been s t u d i e d . B r a d l e y and c o - w o r k e r s (77) observed negative short c i r c u i t c u r r e n t i n the d i r e c t i o n o f i l l u m i n a t i o n f o r s e v e r a l plasma p o l y m e r i z e d f i l m s on aluminum e l e c t r o d e . G u a s t a v i n o , e t . a l . (83) found p o s i t i v e s h o r t c i r c u i t c u r r e n t f o r plasma p o l y m e r i z e d s t y r e n e (PPS) on g o l d e l e c t r o d e s . Morita, e t . a l . (.84) showed t h a t t h e p h o t o c o n d u c t i v e b e h a v i o r o f PPS depends on t h e n a t u r e o f t h e e l e c t r o d e s . I f b o t h e l e c t r o d e s a r e g o l d , the p h o t o c u r r e n t t h r e s h o l d e n e r g y i s around 1.6 ev, and i s a t t r i b u t a b l e t o t h e energy o f h o l e g e n e r a t i o n i n b u l k w i t h r e s i d u a l f r e e r a d i c a l s a c t i n g as a c c e p t o r s . I f t h e gold/aluminum e l e c t r o d e s a r e u s e d , t h e n t h e e l e c t r o n i n j e c t i o n from aluminum i s t h e more l i k e l y mechanism, w i t h a b a r r i e r h e i g h t o f 2.6 e v . Applications F o r q u i t e some t i m e , the p o t e n t i a l use o f plasma p o l y m e r i z e d t h i n f i l m s as membranes, c o a t i n g s and a s i n s u l a t i n g l a y e r s i n m i c r o e l e c t r o n i c components h a s g e n e r a t e d much i n d u s t r i a l i n t e r e s t . I t i s recognized though, t h a t d e s p i t e the s i g n i f i c a n t r e s e a r c h e f f o r t , few plasma p o l y m e r i z a t i o n p r o c e s s e s have r e a c h e d t h e s t a g e o f c o m m e r c i a l development. Y e t plasma p o l y m e r s s t i l l g e n e r a t e a s i g n i f i c a n t amount o f a p p l i c a t i o n o r i e n t e d r e s e a r c h because o f t h e i r u n i q u e p h y s i c a l p r o p e r t i e s and ease o f p r e p a r a t i o n . The a p p l i c a t i o n o f plasma p o l y m e r i z e d t h i n f i l m s as r e v e r s e o s m o s i s membranes has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . Yasuda and h i s c o - w o r k e r s (85-87) have d e m o n s t r a t e d t h a t r e v e r s e o s m o s i s membranes p r e p a r e d from n i t r o g e n c o n t a i n i n g monomers can y i e l d u p t o 98% s a l t r e j e c t i o n w i t h a f l u x o f 6.4 g a l l o n s / f t day ( 8 5 ) . More r e c e n t e x p e r i m e n t s i n d i c a t e t h a t membranes formed from a m i x t u r e o f a c e t y l e n e , c a r b o n monoxide and w a t e r v a p o r e x h i b i t r e v e r s e o s m o s i s p r o p e r t i e s s i m i l a r t o the n i t r o g e n c o n t a i n i n g m a t e r i a l s , y e t have t h e added advantage o f b e i n g c h l o r i n e r e s i s t a n t ( 8 7 ) . R e v e r s e o s m o s i s membranes p r e p a r e d from a l l y l a m i n e by t h e plasma p o l y m e r i z a t i o n 2
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process also gave excellent r e s u l t s (65,66,88,89) . S t a n c e l l et. a l . (£0) reported the possible use of u l t r a t h i n films deposited onto r e l a t i v e l y permeable substrates as permselective membranes. U l t r a t h i n and highly crosslinked coatings e f f e c t i v e l y d i s t i n g u i s h between molecules of d i f f e r e n t sizes and increase the permselectivity of the substrate f i l m . Chang et. a l . (91) demonstrated that the permeability coe f f i c i e n t of s i l i c o n e rubber to oxygen decreased noticeably a f t e r depositing a plasma-polymerized ethylene f i l m on the surface. C o l t e r , e t . a l . (92,93) found s i m i l a r e f f e c t s of plasma polymerized films as d i f f u s i o n b a r r i e r s i n controlled-released drug d e l i v e r y systems. The use of plasma-polymerized films to protect or modify the surface of an e x i s t i n g material has also been explored. The deposition of protective coatings (2J/95) for j u i c e cans and s t e e l panels have been investigated, but the necessity of operating batchwise i n a vacuum has impeded the rapid implementation of such processes. Byrne and Brown (96.) studied the treatment of various f a b r i c s i n a glow discharge of argon containing various v i n y l monomers. They found that changes i n s o i l release, water repellency, and d y e a b i l i t y , were accomplished i n a very short time. The use of plasma g r a f t i n g of synthetic polymers on wool for shrink-proofing has also been extensively investigated (97,98). Plasma-polymerized materials can also be used for t h e i r unique o p t i c a l properties. For example a l a s e r lightguide f i l m has been prepared by plasmapolymerization of hexamethyl disiloxane (9_9) . An exc e l l e n t lightguide must e x h i b i t extremely low s c a t t e r i n g losses. Films produced i n a glow discharge are amorphous, smooth r e l a t i v e to the wavelength of l i g h t , and do not contain inclusions which can scatter l i g h t nor does i t have regions with abrupt difference i n r e f r a c t i v e index. The use of t h i n films derived from c h l o r o t r i fluoroethylene as o p t i c a l device protective coatings has been reported (100,101). Not only do the films protect the moisture s e n s i t i v e substrates from atmosphereic humidity but they also exhibited a n t i r e f l e c t i o n properties. Reis, et. a l . (102) and Hiratsuka, e t . a l . (103) explored the use of plasma polymerized ethane as protective coatings for l a s e r windows. The absorption and a n t i r e f l e c t i o n c h a r a c t e r i s t i c s of these coatings were reported.
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Acknowledgement This work was supported by a grant from the National Science Foundation.
Abstract Polymers can be obtained by subjecting an organic or organometallic vapor to the plasma created in an electrical glow discharge. By varying the plasma conditions such as pressure, monomer flow rate, power etc., not only films but also powder and oil are formed In this paper we shall consider both the mechanism of polymerization and the influence of reaction conditions upon the rates of polymerization. A kinetic model w i l l be presented, which appears to be in good agree ment with experiments. The plasma polymerized materials can be characterized by IR, NMR, ESCA and other analytical techniques. It has been found that the polymer is highly crosslinked, amorphous and often con tains functional groups not originally present in the monomer. The electrical properties of these films w i l l also be discussed. Some of the potential appli cations of plasma polymerized films w i l l be pointed out. Literature Cited 1. DeWilde, P . , Ber., (1874), 7, 352. 2. Thenard, P . , Thenard, Α . , Compt. Rend., (1874), 78, 219. 3. Goodman, J., J. Polymer S c i . , (1960), 44, 551. 4. Hollahan, J. R., McKeever, R. P . , Adv. Chem. Ser., (1969), 80, 272. 5. Allam, D. S., Stoddard, C. Τ. Η., Chem. B e i t . , (1965), 1, 410. 6. Gregor, L. V . , in Thun, R. E., Hass, G . , eds., "Physics of Thin Films," Vol. 3, p. 61, Academic Press, New York, 1966. 7. Kolotyrkin, V. Μ., Gilman, A. B . , Tsapuk, Α. Κ., Russ. Chem. Revs., (1967), 36, 579. 8. Mearns, Α. Μ., Thin Solid Films, (1969), 3, 201. 9. Millard, Μ., in Hollahan, J. R., B e l l , A. T., eds., "Techniques and Applications of Plasma Chemistry," p. 177, Wiley, New York, 1974. 10. Yasuda, Η., in Shen, M . , ed., "Plasma Chemistry of Polymers," p. 15, Dekker, New York, 1976. 11. Kobayashi, H . , B e l l , A. T., Shen, Μ., Macromol., (1974), 7, 277. 12. Yasuda, J., Lamaze, C. E., J. Appl. Polymer S c i . , (1973), 17, 1533.
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64. Pender, M . , Shen, M . , B e l l , A. T . , this issue. 65. B e l l , A. T . , Wydeven, T . , Johnson, C. C . , J. Appl. Polymer S c i . , (1975), 19, 1911. 66. Peric, D., B e l l , A. T . , Shen, M . , J. Appl. Polymer S c i . , (1977), 21, 2661. 67. Neiswender, Adv. Chem. Ser., (1969), 80, 338. 68. Stuart, Μ., Nature, (1963), 199, 59. 69. Bagirov, Μ. Α . , Abasov, S. Α . , Malin, V. P., Angew. Makromol. Chem., (1970), 12, 137. 70. Sawa, G . , Ito, Ο . , Morita, S., Ieda, M . , J. Polymer S c i . , Polymer Phys. E d . , (1974), 12, 1231. 71. Sawa, G. Suzuki, Κ., Morita, S., Ieda, M . , J. Polymer S c i . , Polymer Phys. E d . , (1976), 14, 173. 72. Hirai, T . , Nakada, O., Jap. J. Appl. Phys., (1968), 7, 112. 73. Mann, H. T . , J. Appl. Phys., (1964), 35, 55. 74. Tuzov, L. S., Gilman, A. B., Shchurov, Α. Ν . , Kolotyrkin, V. Μ., Vysokomol. Soed., (1965), 7, 1802. 75. Tuzov., L. S., Kolotyrkin, V. M., Tuniskii, Ν. N. I n t ' l Chem. Eng., (1971), 11, 60. 76. Tibbitt, M. M., B e l l , A. T . , Shen, M . , J. Macromol. Sci.-Chem., (1976), 10, 519. 77. Bradley, Α . , Hammes, J. P . , J. Electrochem. Soc., (1969), 110, 15. 78. Bashara, Ν. Μ., Doty, C. T . , J. Appl. Phys., (1964), 35, 3498. 79. Carchano, Η., Valentin, Ε., Thin Solid Films, (1975), 30, 335. 80. Kay, E., Hetzler, U . , to be published in J. Appl. Phys. 81. Morita, S., Sawa, G . , Ieda, M . , J. Appl. Phys., (1973), 44, 2435. 82. Frenkel, J . , Phys. Rev., (1938), 54, 647. 83. Guastavino, J . , Carchano, Η . , Bui, Α . , Thin Solid Films, (1972), 22, 225. 84. Morita, S., Shen, Μ., Ieda, M . , J. Polymer S c i . , Polymer Phys. E d . , (1977), 15, 981. 85. Yasuda, H . , Lamaze, C. E., J. Appl. Polymer S c i . , (1973), 17, 201. 86. Yasuda, Η., Appl. Polymer Symp., (1973), 22, 241. 87. Yasuda, Η . , Marsh, H. C . , J. Appl. Polymer S c i . , (1975), 19, 2981. 88. Hollohan, J. R., Wydeven, Science, (1973), 179, 500. 89. Hinman, P. V . , B e l l , A. T . , Shen, Μ., to be published in J. Appl. Polymer Sci. 90. Stancell, A. F . , Spencer, A. T . , J. Appl. Polymer S c i . , (1972), 16, 1505.
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91. Chang, F. Y., Shen, M., B e l l , A. T., J. Appl. Polymer S c i . , (1973), 17, 2915. 92. Colter, K. D., B e l l , A. T., Shen, Μ., Biomat. Med. Dev. Art. Org., (1977), 15, 1. 93. Colter, K. D., Shen, M . , B e l l , A. T., Biomat. Med. Dev. Art. Org., (1977), 5, 13. 94. Brick, R. Μ., Knox, J. R., Modern Packaging, (1965), 123. 95. Williams, T., J. O i l Color Chem. Assoc., (1965), 48, 936. 96. Byrne, G. Α . , Brown, K. C . , J. Soc. Dyers Colorists, (1972), 88, 113. 97. Millard, M . , Lee, K. S., Pavlath, Α. Ε., Textile Res. J . , (1972), .42, 307. 98. Pavlath, A. E., Lee, K. S., J. Macromol. S c i . , Chem., (1976), 10, 619. 99. Tien, P. Κ., Smolinsky, G . , Martin, R. J . , Appl. Opt., (1972), 11, 637. 100. Hollahan, J. R., Wydeven, T., Johnson, C. C. Appl. Optics, (1974), 13, 1844. 101. Wydeven, T., Kubadhi, Appl. Optics, (1976), 15, 132. 102. Reis, T. Α . , Hiratsuka, H . , B e l l , A. T., Shen, Μ., NBS Special Publ. No. 462, p. 230, Washington, D.C., 1976. 103. Hiratsuka, H . , Vinzant, J. W., B e l l , A. T., Shen, Μ., "Proc. High Power Laser Opt. Comp. Mtg.," p. 419, Boulder, CO, 1977. Received March 29, 1979.
