Photon, Electron, and Ion Probes of Polymer Structure and Properties

24

Downloaded by CALIFORNIA INST OF TECHNOLOGY on November 20, 2017 | http://pubs.acs.org Publication Date: August 13, 1981 | doi: 10.1021/bk-1981-0162.ch024

Oxidation of Polystyrene and Pyrolytic Carbon Surfaces by Radiofrequency Glow Discharge G. K. IWAMOTO, R. N. KING, and J. D. ANDRADE Surface Analysis Laboratory, College of Engineering, University of Utah, Salt Lake City, UT 84112

Plasma treatment is widely used commercially for polymer surface modification. Plasma discharge treatments are used to improve adhesiveness and printing properties, to improve cell adhesion to tissue culture substrates (1) and to etch or clean the surfaces of materials (removal of photoresist materials on semiconductors, for example (2). The surface characterization of plasma-modified surfaces is important in order to provide greater insight into how the properties are changed. Plasma treatment involves the production of chemically active species and ultra-violet radiation. Conventional methods of surface modification are often limited by the temperature needed for surface treatment, the leaching or toxicity of chemical agents used, and the spectral and geometric limitation of UV treatments. Plasma treatment also provides a means of selectively modifying the surface while the bulk properties remain generally unaffected (1). In this study polystyrene and pyrolytic carbon (3) were used to investigate the nature of plasma surface modification. Polystyrene is widely used as a material for bacteriological cell culture and, in a surface-treated, oxidized form, is widely used as a solid substrate for in vitro cell culture. It is generally assumed that commercial polystyrene cell culture substrates are surface-treated by a corona or radio frequency glow discharge (RFGD) process. Although these materials are extensively used, no general surface characterization is available. Radio frequency glow discharge (RFGD) plasmas were used in this study. Glow discharge plasmas are characterized by average eljgtron energies of 1 to 10 eV and electron densities of 10 to 10 cm . Glow discharges, also called cold plasmas, are characterized by a lack of thermal equilibrium between electron temperature (Te) and gas temperature (Tg). Typical ratios are on the order of Te/Tg = 10 to 10 . Thus, the Tg of a glow discharge remains near ambient temperatures while the electrons are suffi-3

0097-6156/81/0162-0405 $05.00/ 0 © 1981 American Chemical Society Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


406

P H O T O N , E L E C T R O N , A N D ION PROBES

c i e n t l y e n e r g e t i c to r u p t u r e b o n d s . T h i s makes t h i s t y p e o f plasma q u i t e u s e f u l i n a p p l i c a t i o n s i n v o l v i n g t h e r m a l l y s e n s i t i v e materials (4). S u r f a c e m o d i f i c a t i o n by a p l a s m a u s u a l l y r e s u l t s i n changes i n s u r f a c e w e t t a b i l i t y , m o l e c u l a r w e i g h t , and o t h e r c h e m i c a l changes. M o l e c u l a r w e i g h t changes o c c u r from c h a i n s c i s s i o n and crosslinking. C h e m i c a l changes o c c u r from t h e a d d i t i o n o r a b s t r a c t i o n o f g r o u p s on the s u r f a c e , w h i c h i n t u r n i n f l u e n c e the w e t t a b i l i t y o f t h e s u r f a c e . A l m o s t a l l changes p r o d u c e d by p l a s m a m o d i f i c a t i o n a r e c o n f i n e d t o the t o p 1 t o 10 u m o f the s u r f a c e (1). The r e a c t i o n s w h i c h o c c u r a r e c o n t r o l l e d by the p r e s s u r e o f g a s , e l e c t r i c f i e l d s t r e n g t h , r e a c t i o n chamber d i m e n s i o n s and t h e gas f l o w r a t e . The e l e c t r i c f i e l d s t r e n g t h d e t e r m i n e s the amount o f energy imparted to the e l e c t r o n s . The gas p r e s s u r e and t u b e d i m e n s i o n s a f f e c t the d e g r e e o f i o n i z a t i o n , a t o m i c l i f e t i m e s , mean f r e e p a t h and gas t e m p e r a t u r e s . The gas f l o w w i l l a f f e c t t h e r a t e t h a t new r e a c t i o n m a t e r i a l c a n r e a c h the s o l i d s u r f a c e (_5). X-ray photoelectron spectroscopy (XPS), scanning e l e c t r o n m i c r o s c o p y ( S E M ) , and a i r / o c t a n e u n d e r w a t e r c o n t a c t a n g l e s were u s e d t o c h a r a c t e r i z e the s u r f a c e s . XPS c a n p r o v i d e b o t h the a t o m i c c o m p o s i t i o n and c h e m i c a l b o n d i n g i n f o r m a t i o n from a p p r o x i m a t e l y the t o p 70& o r l e s s o f t h e sample s u r f a c e s (6). Additional i n f o r m a t i o n may be g a i n e d from the XPS s p e c t r u m by o b s e r v i n g t h e presence of s a t e l l i t e l i n e s . The e j e c t i o n o f a c o r e level e l e c t r o n from an atom changes t h e s h i e l d i n g o f t h e n u c l e a r c h a r g e and i s f e l t by the o u t e r s h e l l e l e c t r o n s . This p e r t u r b a t i o n i n the p o t e n t i a l o f the v a l e n c e e l e c t r o n s i s o f s u f f i c i e n t e n e r g y t h a t an e l e c t r o n c a n be e x c i t e d t o a h i g h e r e n e r g y l e v e l ( s h a k e up) o r be e j e c t e d ( s h a k e - o f f ) . For the C - l s l i n e , satellite s t r u c t u r e i s seen up t o ~ 1 2 eV above the major p e a k s ; any o t h e r f e a t u r e s w o u l d be l o s t i n t h e i n e l a s t i c t a i l w h i c h o c c u r s ~ 1 5 - 2 0 eV above the m a j o r p h o t o i o n i z a t i o n p e a k . C l a r k ' s s t u d i e s on p o l y m e r s y s t e m s have shown t h a t p o l y m e r s must have an u n s a t u r a t e d b a c k b o n e o r u n s a t u r a t e d p e n d a n t g r o u p s t o have o b s e r v a b l e s a t e l l i t e 6 . 6 eV above t h e C - l s l i n e ( 7_). The s a t e l l i t e i s a t t r i b u t e d t 0 7r-#~7r* t r a n s i t i o n s . S t u d i e s o f a l k a n e - s t y r e n e c o p o l y m e r s show t h a t t h e i n t e n s i t y o f t h e s a t e l l i t e peak i s r e l a t e d t o t h e number o f s t y r e n e g r o u p s i n the c h a i n (8), and t o t h e s u b s t i t u e n t s on the pendant p h e n y l group (8). C o n t a c t a n g l e measurements p r o v i d e i n f o r m a t i o n on t h e w e t t a b i l i t y o f t h e s a m p l e , the s u r f a c e e n e r g e t i c s o f t h e s o l i d , and the i n t e r f a c i a l p r o p e r t i e s o f the s o l i d - l i q u i d i n t e r f a c e . The samples were immersed i n w a t e r and c a p t i v e a i r and o c t a n e b u b b l e s were d e t e r m i n e d by m e a s u r i n g the b u b b l e d i m e n s i o n s . By m e a s u r e ment o f b o t h a i r and o c t a n e c o n t a c t a n g l e s t h e s u r f a c e f r e e e n e r g y (7) o f the s o l i d - v a p o r ( ^ o y ) i n t e r f a c e may be c a l c u l a t e d by use o f Y o u n g ' s e q u a t i o n and t h e n a r m o n i c mean h y p o t h e s i s f o r s e p a r a t i o n o f the d i s p e r s i v e and p o l a r components o f the work o f a d h e s i o n . T h i s method f o r d e t e r m i n a t i o n o f s u r f a c e and i n t e r f a c i a l p r o p e r -

Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


24.

IWAMOTO E T A L .

Polystyrene and Pyrolytic Carbon Surfaces

407

t i e s h a s b e e n d i s c u s s e d i n d e t a i l (9^ 10). Because the measurement i s made u n d e r w a t e r , i t i s b a s i c a l l y a r e c e d i n g a n g l e m e a s u r e ment i n t h e c a s e o f t h e a i r / w a t e r / s o l i d measurement, t h u s 7 v a l u e s o b t a i n e d a r e l a r g e r t h a n t h o s e commonly r e p o r t e d , w h i c h a r e g e n e r a l l y a d v a n c i n g a n g l e measurements ( s e e ( 1 0 ) f o r a complete discussion). S c a n n i n g e l e c t r o n m i c r o s c o p y was u s e d t o d e t e c t changes i n s u r f a c e t o p o g r a p h y due t o t h e p l a s m a t r e a t m e n t . Preferential e t c h i n g o f t h e m a t e r i a l w i l l change t h e s u r f a c e t o p o g r a p h y . The f o r m a t i o n o f v o l a t i l e , low m o l e c u l a r weight s p e c i e s under the s u r f a c e o f t h e m a t e r i a l c a n p r o d u c e b u b b l e s COMeasurement o f s u b s t r a t e s u r f a c e c h a r g e was n o t p e r f o r m e d i n t h i s study. A change i n s u r f a c e c h a r g e m i g h t be e x p e c t e d due t o t h e p l a s m a t r e a t m e n t e i t h e r by i o n i m p l a n t a t i o n o r by f o r m a t i o n o f i o n i z a b l e f u n c t i o n a l i t i e s on t h e s u b s t r a t e s u r f a c e . Change i n c o n t a c t a n g l e s as a f u n c t i o n o f s t o r a g e t i m e was a l s o not s t u d i e d . S t u d i e s on p o l y s t y r e n e i n d i c a t e t h a t t h e c o n t a c t a n g l e does change w i t h t i m e a f t e r p l a s m a t r e a t m e n t ( 1 1 ) . Experimental Samples o f p o l y s t y r e n e were c u t from P e t r i d i s h e s ( F a l c o n 1008, F a l c o n P l a s t i c s , Oxnard, C a l i f o r n i a ) . U n a l l o y e d l o w temp e r a t u r e i s o t r o p i c ( L T I ) c a r b o n s a m p l e s were o b t a i n e d from t h e G e n e r a l A t o m i c Company ( P y r o l i t e - r e g i s t e r e d trademark o f G e n e r a l A t o m i c Company, now C a r b o - M e d i c s , I n c . ) . The p o l y s t y r e n e s a m p l e s were u s e d as r e c e i v e d . Examination by XPS showed o n l y c a r b o n on t h e s u r f a c e ( X P S does n o t d e t e c t hydrogen). The p y r o l y t i c c a r b o n samples were p r e p a r e d by a " s t e a d y s t a t e " f l u i d i z e d b e d p r o c e s s (_12). The c a r b o n s a m p l e s were p o l i s h e d by t h e m a n u f a c t u r e r u s i n g 7 - a l u m i n a . B e f o r e use i n t h i s s t u d y t h e p y r o l y t i c c a r b o n samples were u l t r a s o n i c a l l y c l e a n e d i n reagent grade methanol f o r f i v e m i n u t e s . Examination by XPS o f b o t h t h e as r e c e i v e d and u l t r a s o n i c m e t h a n o l c l e a n e d s a m p l e s showed r e m o v a l o f s m a l l amounts o f c h l o r i n e , m a g n e s i u m , s i l i c o n , and s u l f u r by t h e c l e a n i n g p r o c e d u r e . The s a m p l e s were o x i d i z e d u s i n g a c o m m e r c i a l p l a s m a d i s charge u n i t (Plasmod r e g i s t e r e d trademark o f Tegal C o r p o r a t i o n , R i c h m o n d , C a l i f o r n i a ) , w h i c h o p e r a t e s a t 1 3 . 5 6 MHz and h a s a v a r i a b l e power o u t p u t from 0 t o 100 w a t t s . A v a r i a b l e leak valve (Granville-Phillips Company, B o u l d e r , Colorado), a three-way v a l v e and o t h e r m o d i f i c a t i o n s were added t o p r o v i d e a b e t t e r vacuum, t o c o n t r o l t h e gas f l o w r a t e and t o c o n t r o l t h e gas pressure. The samples were i n s e r t e d i n t o t h e RFGD u n i t i n a i r and p l a c e d on t h e b o t t o m - c e n t e r r e g i o n o f t h e r e a c t i o n c h a m b e r . In a l l e x p e r i m e n t s t h e t o p s u r f a c e was t h e a n a l y s i s s u r f a c e . The r e a c t i o n chamber was e v a c u a t e d t o 10 t o r r p r e s s u r e and t h e n b a c k f i l l e d w i t h t h e r e a c t i o n gas t o above 2000mmHg p r e s s u r e and then r e - e v a c u a t e d . T h i s p r o c e s s was r e p e a t e d t h r e e t i m e s . The



408

P H O T O N , E L E C T R O N , A N D ION PROBES

g a s e s u s e d i n t h i s s t u d y were h e l i u m and o x y g e n . The h e l i u m was l i q u i d n i t r o g e n c o l d t r a p p e d d u r i n g b a c k f i l l t o remove c o n d e n s ^ able i m p u r i t i e s . A f t e r the t h i r d b a c k f i l l and pumpdown t o 10 t o r r p r e s s u r e , t h e gas f l o w r a t e was a d j u s t e d t o c o r r e s p o n d t o 0 . 4 t o r r p r e s s u r e , w h i c h was m a i n t a i n e d d u r i n g d i s c h a r g e . 0.4 t o r r p r e s s u r e was d e t e r m i n e d t o g i v e m i n i m a l d e p o s i t i o n o f s i l i c o n on the sa*ole s u r f a c e . The s i l i c o n s p u t t e r i n g o r i g i n a t e d from t h e Pyrex w a l l s o f the r e a c t i o n chamber d u r i n g p l a s m a t r e a t m e n t . Lower p r e s s u r e s gave a p p r e c i a b l e amounts o f s i l i c o n d e p o s i t i o n on the sample as d e t e r m i n e d by XPS e x a m i n a t i o n . The samples were e x p o s e d t o t h e h e l i u m gas f o r v a r i o u s amounts o f t i m e a t 50 w a t t s o f power o u t p u t . The samples were t h e n e x p o s e d t o a h e l i u m gas purge f o r v a r i o u s amounts o f t i m e a t above 4 t o r r p r e s s u r e and t h e n e x p o s e d t o o x y g e n a t above 4 t o r r p r e s s u r e f o r f i v e m i n u t e s . The samples were s t o r e d i n P e t r i d i s h e s i n a i r u n t i l surface c h a r a c t e r i z a t i o n s were p e r f o r m e d . I t was found i f t h e s a m p l e s were e x p o s e d t o a i r i n s t e a d o f o x y g e n a f t e r the i n e r t gas p u r g e t h a t b o t h o x y g e n and n i t r o g e n f u n c t i o n a l i t i e s were o b s e r v e d by XPS e x a m i n a t i o n . Oxygen p l a s m a s were u s e d on the p y r o l y t i c c a r b o n . The p r o c e d u r e was t h e same, e x c e p t t h a t the samples were e x p o s e d t o an o x y g e n p u r g e f o r 5 m i n u t e s a f t e r the d i s c h a r g e . XPS s p e c t r a were o b t a i n e d w i t h a H e w l e t t - P a c k a r d 5950 B i n s t r u m e n t u t i l i z i n g monochromatic A l Ka^ r a d i a t i o n a t 1487 e V . The s a m p l e s were mounted i n a i r , i n s e r t e d ' i n t o ^ the s p e c t r o m e t e r , and a n a l y z e d a t a m b i e n t t e m p e r a t u r e s i n a 10 t o r r v a c u u m . Power a t t h e X - r a y s o u r c e was 800 w a t t s . I n s t r u m e n t r e s o l u t i o n was n o m i n a l l y 0 . 8 eV or l e s s as measured by the f u l l w i d t h a t h a l f maximum o f t h e C - l s l i n e from s p e c t r o s c o p i c grade g r a p h i t e . An e l e c t r o n f l o o d gun o p e r a t i n g a t 0 . 3 mA and 5 . 0 eV s u p p l i e d a f l u x o f low e n e r g y e l e c t r o n s t o m i n i m i z e c h a r g i n g a r t i f a c t s i n the r e s u l t i n g s p e c t r a . Wide s c a n s (0 t o 1000 eV) were p e r f o r m e d f o r s u r f a c e e l e mental analyses. The w i d e s c a n s were c a r e f u l l y i n s p e c t e d f o r t r a c e element c o n t a m i n a t i o n . D e t a i l e d 20 eV s c a n s o f t h e C - l s (275 t o 295 e V ) , 0 - l s ( 5 2 0 - 5 4 0 eV) and A l - 2 s (105 t o 125 eV) r e g i o n s f o r t h e p y r o l y t i c c a r b o n and o f the C - l s and O - l s f o r t h e p o l y s t y r e n e were r u n to d e t e r m i n e b o t h e l e m e n t a l s t o i c h i o m e t r y and c h e m i c a l s h i f t s . S t a n d a r d s were a v a i l a b l e t o g i v e a c c u r a t e chemical s h i f t data for v a r i o u s carbon-oxygen f u n c t i o n a l groups. These i n c l u d e d p o l y ( e t h y l e n e t e r e p h t h a l a t e ) , p o l y ( e t h y l e n e o x i d e ) and a n t h r a q u i n o n e (17^). The l a t t e r was r u n a t - 5 0 C i n o r d e r t o m i n i m i z e v o l a t i l i t y u n d e r o u r h i g h vacuum c o n d i t i o n s . Table I s u m m a r i z e s t h e s e r e s u l t s . A l l s p e c t r a were c h a r g e - r e f e r e n c e d t o a C - l s l i n e f o r an a l k y l - l i k e c a r b o n a t 2 8 4 . 0 ev*. The S c o f i e l d t h e o r e t i c a l XPS p h o t o e l e c t r i c c r o s s s e c t i o n s (1_3) were u s e d f o r elemental quantitation. S c a n n i n g e l e c t r o n m i c r o g r a p h s were o b t a i n e d on a C a m b r i d e Mark I I S t e r e o s c a n SEM. The samples were mounted on t h e s p e c i m e n mounts w i t h d o u b l e - s i d e d t a p e . S i l v e r p a i n t a l o n g the edge o f t h e sample p r o v i d e d e l e c t r i c a l c o n t a c t b e t w e e n the s p e c i m e n mount and 2


24.

IWAMOTO E T A L .


409

Table I Chemical s h i f t s i n the C - l s l i n e f o r p o l y ( e t h y l e n e terepht h a l a t e ) , p o l y e t h y l e n e o x i d e , and a n t h r a q u i n o n e . (See a l s o Reference 1 7 ) . Observed B i n d i n g Energy ( e V ) *

ABE**

F u n c t i o n a l Group

284.0

0.0

a l k y l and/or aromatic

285.6

+1.6

ether

288.0

+4.0

ester

Polyethylene oxide

285.8

+1.8

ether

Anthraquinone

284.0


Material

Polyethylene terephthalate

286.5

a l k y l or aromatic +2.5

quinone

* C h a r g e - r e f e r e n c e d t o C - l s a t 2 8 4 . 0 eV * * B i n d i n g E n e r g y - 2 8 4 . 0 eV = ABE the sample. The samples were c o a t e d w i t h carbon_^and g o l d t o reduce c h a r g i n g . The a n a l y z i n g vacuum was a b o u t 10 torr. C o n t a c t a n g l e s were d e t e r m i n e d by i m m e r s i n g t h e sample i n d o u b l y - d i s t i l l e d w a t e r and m e a s u r i n g t h e h e i g h t and d i a m e t e r o f b o t h a i r and o c t a n e b u b b l e s i n w a t e r (9). The b u b b l e s were i n t r o d u c e d on t h e sample s u r f a c e u s i n g a m i c r o l i t e r s y r i n g e . The h e i g h t and d i a m e t e r were measured by use o f m i c r o m e t e r s w h i c h m a n i p u l a t e d a s t a g e h o l d i n g t h e sample immersed i n w a t e r . The b u b b l e s were o b s e r v e d t h r o u g h a m i c r o s c o p e u s i n g a 20X l o n g w o r k i n g d i s t a n c e o b j e c t i v e and a 15X e y e - p i e c e e q u i p p e d w i t h a crosshair r e t i c l e . The b u b b l e was m a n i p u l a t e d a c r o s s t h e c r o s s h a i r and t h e d i m e n s i o n s were r e a d d i r e c t l y f r o m t h e m i c r o m e t e r s . The sample b o x was b a c k i l l u m i n a t e d by a v a r i a b l e l i g h t s o u r c e . The b u b b l e v o l u m e was a p p r o x i m a t e l y 0 . 1 t o 0 . 2 u l and t h e b u b b l e s were a p p l i e d t o t h e s u r f a c e by f o r m i n g a b u b b l e a t t h e t i p o f a m i c r o s y r i n g e and t h e n s n a p p i n g t h e t i p t o a l l o w t h e b u b b l e t o f l o a t up t o t h e w a t e r - s a m p l e i n t e r f a c e . The b u b b l e v o l u m e was m i n i m i z e d i n o r d e r to a v o i d buoyancy e f f e c t s . The o c t a n e was 99.99% p u r e n - o c t a n e ( A l d r i c h Chemicals -Gold Label Octane). T e m p e r a t u r e o f t h e i m m e r s i o n b a t h was 26 C . F i g u r e 1 s c h e m a t i c a l l y i l l u s t r a t e s t h e g e o m e t r y o f t h e c o n t a c t a n g l e measurement. The c o n t a c t a n g l e s were c a l c u l a t e d u s i n g t h e e q u a t i o n



410

PHOTON, ELECTRON,

CASE I : 0 < 9 O °

e=cos'(^ - i j

Figure 1.

A N D ION

PROBES

CASE 11:9 > 90°

0 = I8O"-2JTAN"'^-

Schematic of both cases for contact angle calculation


24.

IWAMOTO E T A L .


cj) = 180 - 2 t a n

(^)

411

Polystyrene and Pyrolytic Carbon Surfaces f o r < ^ 9 0 ° and = c o s

_i)

f

o

r

CO

00

413

00

BINDING ENERGY,eV

Figure 2. XPS spectra of the C-ls region of PS. Flood gun conditions, 0.3 ma, 5 eV. The C-ls alkyl line was approximately 279.0 eV; the spectra above are charge-referenced to 284.0 eV for the alkyl carbon line. Spectrum A is the as-received material; note the presence of the aromatic satellite at 6.7 eV from the main carbon line at 284.0. Note also the absence of any carbon-oxygen functionalities as evidenced by the lack of structure between 284 and 290: (a) as-received material; (b) methanol-cleaned; (c) oxygen plasma-treated material. Note the decrease in the satellite line at 297 eV; it has disappeared to nearly background level. Also note the presence now of two carbon-oxygen functionalities as evidenced by apparent peaks at about 288.6 and 286.4 eV, characteristic of ester or car boxy lie acid, and ether or hydroxyl carbon, respectively.


414

P H O T O N , E L E C T R O N , A N D ION


C-ls

O-ls

PROBES

Al-2s

BINDING ENERGY.eV Figure 3.

XPS spectra of the (a) C-ls region; (b) O-ls region, and (c) Al-2s regions of the oxygen plasma-treated pyrolytic carbon. The upper spectra are with the electronfloodgun on for charge compensation, the bottom spectra are with the electronfloodgun off. Note in the upper spectrum that the main carbon peak appears at 284.0 eV, as expected for a conducting material such as pyrolytic carbon. Note also the presence of the weak apparent doublet in the vicinity of 280.5 eV. The Al-2s top far right appears at about 115.2 eV or charge shifted down stream from its apparent normal position. Comparing these upper spectra with the floo gun off spectra and looking at the relative peak positions, one can deduce (see text) that there is an insulating component in the surface region of the material that charge shifts to higher binding energies in the absence of thefloodgun and is pushed to lower binding energies in the presence of thefloodgun. Note also that there are a number of major lines that are not affected by thefloodgun conditions. These, of course, are those intrinsic to the conductive pyrolytic carbon structure. The insulating material which is influenced significantly by thefloodgun conditions is attributed to Al O particles embedded in the carbon during the polishing process. 2

s


24.

IWAMOTO E T A L .

415

Polystyrene and Pyrolytic Carbon Surfaces Table

III

Carbon: Oxygen (C/O) r a t i o and a t o m i c p e r c e n t a g e s as d e t e r m i n e d by X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y f o r p y r o l y t i c c a r b o n .


Treatment Type

C/0 Ratios

As Received Methanol Cleaned (ultrasonic)

6

Helium Plasma Treatment

2-4

Oxygen Plasma Treatment

1.2-1.4

* Trace less

than 1 atomic

C

Atomic

Percent

0

Al

Trace* C I , Mg,

85.5

10.4

1.4

85

13.4

1.6

60-70

34-27

5-3

56

36.5

7.5

percent.

s a m p l e s the f l o o d gun w i l l n o t a f f e c t t h e b i n d i n g e n e r g y s i n c e the sample i s i n e l e c t r i c a l c o n t a c t w i t h the g r o u n d e d sample p r o b e . G r u n t h a n e r has r e p o r t e d the e f f e c t o f a f l o o d gun on n o n - c o n d u c t i n g samples and has u s e d i t t o i n v e s t i g a t e t h e c h e m i c a l c o m p o s i t i o n o f n o n - c o n d u c t i n g o x i d e s formed on m e t a l c a t a l y s t systems (20). I n the p y r o l y t i c c a r b o n , a l u m i n u m , o x y g e n , and c a r b o n p e a k s were o b s e r v e d t o move i n b i n d i n g e n e r g y as a f u n c t i o n o f t h e f l o o d gun c o n d i t i o n s . A l s o a c a r b o n peak and an o x y g e n peak r e m a i n e d u n a f f e c t e d by the f l o o d gun ( s e e F i g u r e 3 ) . The m a j o r i t y o f t h e c a r b o n and o x y g e n a r e u n a f f e c t e d by the f l o o d gun and a l l the a l u m i n u m moves w i t h the f l o o d g u n . From t h e s e r e s u l t s i t i s c o n c l u d e d t h a t the m a j o r p o r t i o n o f the sample i s c o n d u c t i v e p y r o l y t i c c a r b o n ; i s l a n d s o f n o n - c o n d u c t i n g A l ^ O ^ , w i t h some c a r b o n and o x y g e n a s s o c i a t e d w i t h i t , a r e b e i n g e x p o s e d on t h e s u r f a c e due t o p l a s m a e t c h i n g . SEM o f the s a m p l e s shows a change i n the s u r f a c e roughness a f t e r glow d i s c h a r g e t r e a t m e n t . The change i s due t o e t c h i n g o f t h e s u r f a c e by the p l a s m a t r e a t m e n t and s u p p o r t s t h e n o t i c e d i n c r e a s e i n amount o f a l u m i n u m d e t e c t e d by X P S . E n e r g y d i s p e r s i v e a n a l y s i s o f X - r a y s o f the sample c o u l d n o t d i s t i n g u i s h b e t w e e n a l u m i n u m on t h e s u r f a c e and a l u m i n u m embedded b e l o w t h e s u r f a c e


PHOTON,

416

ELECTRON,

AND

ION

PROBES

due t o h i g h e r a n a l y s i s d e p t h s . The s u r f a c e a p p e a r e d r o u g h e r a f t e r e t c h i n g . The change i n t o p o g r a p h y i s p r o b a b l y due t o p r e f e r e n t i a l e t c h i n g o f t h e amorphous p o r t i o n o f t h e p y r o l y t i c c a r b o n . As t h e samples were h i g h l y o x i d i z e d from t h e b e g i n n i n g , due t o t h e p o l i s h i n g p r o c e s s (_17), no r e a l change i n c o n t a c t a n g l e was observed.


Summary P y r o l y t i c c a r b o n and p o l y s t y r e n e s u r f a c e s were s t u d i e d by X ray p h o t o e l e c t r o n spectroscopy (XPS), contact a n g l e s , scanning e l e c t r o n m i c r o s c o p y ( S E M ) , and e n e r g y d i s p e r s i v e a n a l y s i s o f X rays (EDAX). The m a t e r i a l s were r a d i o f r e q u e n c y g l o w d i s c h a r g e d (RFGD) i n h e l i u m and o x y g e n p l a s m a s . RFGD o f t h e p o l i s h e d c a r b o n s i n c r e a s e d t h e d e g r e e o f o x i d a t i o n and t h e A l c o n t e n t . The i n c r e a s e d A l c o n t e n t i s i n t e r p r e t e d as due t o e x p o s u r e o f A l ^ O ^ p a r t i c l e s embedded i n t h e p o l i s h i n g p r o c e s s . T h i s i s c o n f i r m e d by flood gun-charging r e s u l t s . RFGD p o l y s t y r e n e was o x i d i z e d , w e t t a b l e , and o f a d e c r e a s e d a r o m a t i c c h a r a c t e r as d e t e r m i n e d by a n a l y s i s o f XPS C - l s s a t e l l i t e spectra. The RFGD o x i d a t i o n p r o c e s s e t c h e s t h e s u r f a c e as w i t n e s s e d by s c a n n i n g e l e c t r o n m i c r o g r a p h s . These d a t a a r e o f i n t e r e s t i n u n d e r s t a n d i n g t h e b e h a v i o r o f p o l i s h e d c a r b o n and o x i d i z e d p o l y s t y r e n e i n b i o m e d i c a l a p p l i c a t i o n s , i n c l u d i n g s u r g i c a l i m p l a n t s and s o l i d s u b s t r a t e s f o r i n vitro cell cultures. Acknowledgements P o r t i o n s o f t h i s work were s u p p o r t e d by NIH G r a n t # H L 1 6 9 2 1 04 and t h e U n i v e r s i t y o f U t a h F a c u l t y R e s e a r c h C o m m i t t e e .

Abstract Surface characterization of RF plasma modified polymers is necessary in order to understand and improve certain properties including cell adhesion to tissue culture substrates. Radio frequency glow discharge (RFGD) plasmas were used to modify the surfaces of polystyrene and pyrolytic carbon. Surface character ization by X-ray photoelectron spectroscopy (XPS), scanning elec tron microscopy (SEM), and air and octane contact angles were performed on the as received and plasma modified samples. Plasma modification of polystyrene produced a number of carbon-oxygen functional groups and decreased both air and octane contact angles. Plasma modification of pyrolytic carbon showed an i n crease in aluminum and oxygen on the surface, probably due to preferential etching of surface organics, exposing the inorganic component of the sample. The aluminum and oxygen are probably from γ-alumina, used in the polishing of pyrolytic carbon. SEM also showed a change in topography indicating preferential etch ing.


24. IWAMOTO ET AL.



Literature Cited 1.

Hudis, M., in "Techniques and Applications of Plasma Chemistry," (J.R. Hollahan and A.T. Bell, Eds.), Chap. 3. John Wiley and Sons, New York, 1974.

2.

Kirk, Ralph W., in "Techniques and Applications of Plasma Chemistry," (J.R. Hollahan and A.T. Bell, Eds.), Chap. 9. John Wiley and Sons, New York, 1974.

3.

Bokros, J.D., Carbon, 1977, 15, 355.

4.

B e l l , A.T., in "Techniques and Applications of Plasma Chemistry," (J.R. Hollahan and A.T. Bell, Eds.), Chap. 1. John Wiley and Sons, New York, 1974.

5.

Hollahan, J.R., J . Chem. Ed., 1966, 43, A401.

6. Hall, S.M., Andrade, J.D., Ma, S.M., King, R.N., J. Electron Spectrosc., 1979, 17, 181-189. 7.

Clark, D.T., and Dilks, A., J . Poly. S c i . , 1976, 14, 533.

8.

Clark, D.T., Adams, D.B., Dilks, A., Peeling, J . , and Thomas, H.R., J . Elect. Spect., 1976, 8, 51.

9.

Andrade, J.D., King, R.N., Gregonis, D.E., Coleman, E . L . , J . Poly. Sci. Symp C., 1978, 66, 313.

10.

Andrade, J.D., Ma, S.M., King, R.N., Gregonis, D.E., J . Coll. Interface S c i . , 1979, 72, 488.

11.

Triolo, P., Thesis, Department of Bioengineering, University of Utah, June, 1980.

12.

Akins, R.J. and Bokros, J.C., Carbon, 1979, 12, 439.

13.

Scofield, J.H., J . Electron Spect., 1976, 8, 129.

14.

Hansen, R.H., Pascale, J.V., DeBenedictus, Rentzepis, P.M., J. Poly. S c i . , 1965, A3, 2205.

15.

Westerdahl, C.A.L., Hall, J.R., Schramm, E.C., and Levi, D.W., J . Colloid Interface S c i . , 1974, 47, 610.

16.

Biscoe, J., and Warren, B.E., J. Appl. Phys., 1942, 13, 364.

17.

King, R.N., Andrade, J.D., Haubold, A.D., and Shim, H.,

T.,


and

All

418

PHOTON, ELECTRON, AND ION PROBES

"Surface Analysis of Silicon-Alloyed and Unalloyed LTI Pyrolytic Carbon," in this volume. 18.

Evans, R. and Kuwana, T., Anal. Chem., 1977, 49, 1632.


19. Millard, M.M., "Surface Characterization of Biological Materials by X-ray Photoelectron Spectroscopy," in D.M. Hercules, et a l . , Eds., Cont. Topics in Anal, and Clin. Chem., 1978, 3, 1. 20.

Grunthaner, F., Ph.D. Dissertation, California Institute of Technology, 1974.

RECEIVED

March 23, 1981.


Photon, Electron, and Ion Probes of Polymer Structure and Properties

Recommend Documents