8 Surface Analysis of Fibers and Polymers by X-Ray Photoelectron Spectroscopy: Industrial Applications M E R L E M. MILLARD U.S. Department of Agriculture Berkeley, CA 94710
Applications of surface analysis by X-ray photoelectron spectroscopy (XPS) are reviewed, emphasizing analysis of chemically modified fibers and polymers. The review is restricted to materials science applications involving non-metal-metal oxide systems. Results from the surface modification of fibers by low temperature plasmas, including oxidation treatments and application of coatings by deposition and plasma grafting, are reviewed. Examples emphasizing the ability of XPS to obtain results on non-uniform solids encountered in typical materials science applications, such as chemically modified polymer beads and granules, o-ring seals, wood and various intractable polymers, are presented. Recent results from the determination of inelastic mean free path lengths in organic solids are reviewed. In this review the term X-ray photoelectron spectroscopy (XPS) will be used rather than the term adapted by Sieghahn, namely electron spectroscopy for chemical analysis (ESCA), to describe the surface sensitive electron spectroscopy under discussion. The fact that this symposium is taking place on the topic of industrial applications of surface analysis reflects the growth of the application of different areas of surface sciences in practical industrial and technological materials science areas. Initially, scientists working in XPS were concerned with establishing the fundamentals of the physics associated with the processes involved in the
This chapter not subject to U . S . copyright. Published 1982 American Chemical Society.
144
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ANALYSIS
t e c h n i q u e and t h e o r y e n a b l i n g u n d e r s t a n d i n g and p r e d i c t i o n o f b e h a v i o r o f t h e phenomena. I t s o o n became a p p a r e n t t h a t a technique that allows d e t e r m i n a t i o n of c h e m i c a l c o m p o s i t i o n and i s s e n s i t i v e t o d i f f e r e n c e s i n c h e m i c a l b o n d i n g w i t h i n a r e g i o n o f s e v e r a l n a n o m e t e r s o f t h e s u r f a c e has g r e a t p r a c t i c a l u t i l i t y i n a r e a s where s u r f a c e b e h a v i o r was i m p o r t a n t s u c h as c a t a l y s t s and i n t e r f a c i a l p h e n o m e n a . The r e c e n t e m p h a s i s o n new m a t e r i a l s and a d v a n c e d t e c h n o l o g y a s r e v i e w e d i n a r e c e n t v o l u m e o f S c i e n c e (_1) has r e q u i r e d t h e d e v e l o p m e n t o f a n a l y t i c a l t e c h n i q u e s t o c h a r a c t e r i z e new m a t e r i a l s and a i d i n t h e d e v e l o p ment o f f a b r i c a t i o n p r o c e s s e s . C o n s i d e r a t i o n o f some o f t h e s e t e c h n o l o g i e s s u c h as polymer d e v e l o p m e n t , c o n d u c t i v e p o l y m e r s , biopolymers, f i b e r reinforced composites, m e t a l l i c glasses, heterogeneous c a t a l y s t s , p h o t o v o l t a i c m a t e r i a l s , i n t e g r a t e d c i r c u i t p r o c e s s i n g , i n l i g h t o f t h e need t o c o n t r o l and c h a r a c t e r i z e not o n l y b u l k c o m p o s i t i o n but e s p e c i a l l y interfaces makes i t o b v i o u s why s u r f a c e o r i e n t e d a n a l y t i c a l t e c h n i q u e s a r e needed and u s e d e x t e n s i v e l y i n t h e s e a r e a s . XPS h a s a n a d v a n t a g e i n t h a t o f t e n m a t e r i a l s as p r o c e s s e d and u s e d c a n be d i r e c t l y a n a l y z e d w i t h l i t t l e o r no s a m p l e a l t e r a t i o n p r i o r t o analysis. T h i s r e v i e w w i l l c o n c e n t r a t e on n o n m e t a l l i c and r e l a t e d a r e a s s u c h as o r g a n i c f i b e r s and v a r i o u s f o r m s i n w h i c h o r g a n i c m a t e r i a l s a r e u s e d s u c h as f i l m s , g r a n u l e s and s m a l l e x t r u d e d geometries. R e a d e r s w i l l be r e f e r r e d e l s e w h e r e f o r d i s c u s s i o n s o f f u n d a m e n t a l s o f t h e t e c h n i q u e and i n s t r u m e n t a t i o n . Applicat i o n s o f XPS t o p r o b l e m s o f i n d u s t r i a l i n t e r e s t s u c h a s f i b e r m o d i f i c a t i o n and a n a l y s i s c a r r i e d out i n o u r l a b o r a t o r y w i l l be r e v i e w e d . Recent
Reviews
A number o f r e v i e w s c a n be c o n s u l t e d f o r an i n t r o d u c t i o n t o t h e f u n d a m e n t a l s b o t h t h e o r e t i c a l and p r a c t i c a l c o v e r i n g XPS. T h e s e i n c l u d e R i g g s and P a r k e r (2^) and t h e book b y Carlson (3). E l e c t r o n spectroscopy i s reviewed i n a l t e r n a t e y e a r s i n the Fundamental Reviews i s s u e of A n a l y t i c a l C h e m i s t r y . The l a s t l i t e r a t u r e r e v i e w was p u b l i s h e d i n 1980 (40 and t h i s and p r e v i o u s r e v i e w s c a n be c o n s u l t e d f o r a c o v e r a g e o f a l l a s p e c t s o f t h e l i t e r a t u r e o f X P S . A number o f r e c e n t s y m p o s i a have b e e n h e l d on a p p l i c a t i o n s o f s u r f a c e a n a l y t i c a l m e t h o d s i n v a r i o u s a s p e c t s o f m a t e r i a l s s c i e n c e s u c h as t h e symposium on c h a r a c t e r i z a t i o n of m o l e c u l a r s t r u c t u r e s o f polymers by p h o t o n , e l e c t r o n , and i o n p r o b e s a t t h e M a r c h 1980 A m e r i c a n C h e m i c a l S o c i e t y m e e t i n g s i n H o u s t o n (_5) and t h e I n t e r n a t i o n a l Symposium on P h y s i o c h e m i c a l A s p e c t s o f P o l y m e r S u r f a c e s a t t h i s m e e t i n g a s w e l l as t h e symposium on i n d u s t r i a l a p p l i c a t i o n s of s u r f a c e a n a l y s i s of which t h i s a r t i c l e i s a p a r t . Review a r t i c l e s o n v a r i o u s a p p l i c a t i o n s o f XPS i n m a t e r i a l s s c i e n c e are l i s t e d i n Table I .
8.
MILLARD
TABLE Recent
145
Analysis of Fibers and Polymers
Reviews
I
i n Photoelectron
Spectroscopy
XPS o f P o l y m e r S u r f a c e s Analytical Applications R a d i a t i o n Damage Surface Modified Electrodes I n t e r f a c i a l C h e m i s t r y and A d h e s i o n S u r f a c e C h e m i s t r y and C a t a l y s i s M i n e r a l o g y and G e o c h e m i s t r y Microelectronics Processing
(6 (10, (13) (14) (15) (16, (18)
9) 11,
12)
17)
(19)
Ins t rument a t i on The f u n d a m e n t a l s o f i n s t r u m e n t d e s i g n on a n e l e m e n t a r y l e v e l a r e d i s c u s s e d by C a r l s o n ( 3 ) . D i s c u s s i o n s of i n s t r u m e n t a t i o n from a p o i n t of view of m a n u f a c t u r e r s specifications is pres e n t e d i n two o l d e r a r t i c l e s ( 2 0 , 2 1 ) . Among t h e m a n u f a c t u r e r s l i s t e d i n E v a n s * a r t i c l e (_21 ) , H e w l e t t - P a c k a r d and E . I . d u P o n t no l o n g e r m a n u f a c t u r e i n s t r u m e n t s . It i s a d v i s a b l e to c o n s u l t c u r r e n t m a n u f a c t u r e r s f o r up to d a t e s p e c i f i c a t i o n s o f i n s t r u m e n tation. Development of i n s t r u m e n t a t i o n has not kept pace w i t h r e c e n t a c t i v i t y i n t h e f i e l d and h o p e f u l l y i n s t r u m e n t m a n u f a c t u r e r s c u r r e n t l y m a n u f a c t u r i n g i n s t r u m e n t s w i l l d e v o t e more e f f o r t to development i n the f u t u r e . 1
Introduction
to
Core E l e c t r o n
Spectra of
Polymeric Materials
An e x c e l l e n t i n t r o d u c t i o n t o t h e common f e a t u r e s o b s e r v e d i n e l e c t r o n s p e c t r a o f n o n m e t a l l i c s u r f a c e s and a s i m p l e d i s c u s s i o n o f the p h y s i c a l p r o c e s s e s g i v i n g r i s e to these f e a t u r e s is i n c l u d e d i n t h e r e v i e w a r t i c l e b y Holm and S t o r p ( 7 ) . A good d i s c u s s i o n from a p r a c t i c a l i n t r o d u c t o r y v i e w p o i n t i s i n c l u d e d i n a Handbook o f XPS ( 2 2 ) . More e x o t i c e f f e c t s t h a t l e a d t o s e c o n d a r y l i n e s o f l o w e r i n t e n s i t y s u c h as m u l t i c o m p o n e n t s t r u c t u r e a r e more o f t e n e n c o u n t e r e d i n t r a n s i t i o n m e t a l s and m e t a l l i c o x i d e s o r t r a n s i t i o n compounds. These e f f e c t s a r e d i s c u s s e d i n more d e t a i l i n C a r l s o n ( 3 ) and W e r t h e i m ( 2 3 ) . Shake up l i n e s i n a d d i t i o n t o c o r e l i n e s a r e s o m e t i m e s o b s e r v e d f r o m p o l y m e r s and r e s u l t f r o m p r o c e s s e s i n v o l v i n g t r a n s i t i o n s t o s t a t e s w i t h e n e r g y above the ground s t a t e . These l i n e s a r e u s u a l l y l e s s t h a n a t e n t h o r more o f t h e i n t e n s i t y o f t h e prominent core l i n e . When f i r s t i n v e s t i g a t i n g a s a m p l e , i t is u s e f u l to o b t a i n a survey scan at lower r e s o l u t i o n over r e g i o n s t h a t i n c l u d e e l e m e n t s t h o u g h t t o be p r e s e n t . G e n e r a l l y 550 t o 50 ev i s a window t h a t c o n t a i n s s t r o n g l i n e s f r o m most o f t h e elements encountered i n polymer s t u d i e s e x c l u d i n g exceptions s u c h as f l u o r i n e . A survey scan w i l l not only i d e n t i f y the
146
SURFACE
ANALYSIS
e l e m e n t s p r e s e n t b u t w i l l a i d i n p l a n n i n g t h e r e g i o n s to be i n v e s t i g a t e d i n d e t a i l and p r o v i d e some i d e a o f t h e number o f s c a n s t o be c o l l e c t e d f o r l i n e s o f l o w e r s e n s i t i v i t y and i n t e n s i t y . S u r v e y s p e c t r a i n t h e r e g i o n 550 ev t o 50 ev ( u s u a l l y a n a c c u m u l a t i o n o f e i g h t s c a n s ) a r e p r e s e n t e d i n F i g u r e 1 f o r a number of s u r f a c e s i l l u s t r a t i n g the o c c u r r e n c e of v a r i o u s elements of i n t e r e s t i n p o l y m e r s and t h e f a c t t h a t XPS s u r v e y s p e c t r a s e r v e as a f i n g e r p r i n t e n a b l i n g r a p i d i d e n t i f i c a t i o n o f t h e c o m ponents present i n polymer samples. Tabulations of l i n e p o s i t i o n s f o r t h e e l e m e n t s and b i n d i n g e n e r g i e s f o r compounds of the elements a r e a v a i l a b l e ( 2 2 ) . B i n d i n g e n e r g i e s have b e e n p u b l i s h e d f o r a number o f p o l y m e r s o f common i n t e r e s t i n c l u d i n g commonly o c c u r r i n g e l e m e n t s s u c h as o x y g e n , n i t r o g e n halogen s u l f u r and s i l i c o n (24) . Some o f t h e o l d e r l i t e r a t u r e i s s t i l l q u i t e u s e f u l and c a n be c o n s u l t e d f o r b i n d i n g e n e r g i e s o f o x y g e n (25) and n i t r o g e n compounds ( 2 6 ) . A useful t a b l e of carbon b i n d i n g e n e r g i e s f o r carbon s u b s t i t u t e d w i t h f l u o r i n e has been p u b l i s h e d by C l a r k ( 2 7 ) . E l e m e n t a l compos i t i o n c a n be e s t i m a t e d f r o m e l e c t r o n s p e c t r a and t a b l e s o f 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 have been c o m p i l e d f o r v a r i o u s i n s t r u m e n t s and a p r o c e d u r e f o r o b t a i n i n g e l e m e n t a l composition presented (22, 28). The a u t h o r ' s e x p e r i e n c e h a s b e e n w i t h some p r e c a u t i o n s as to t h e n a t u r e o f t h e s a m p l e u n d e r i n v e s t i g a t i o n u s i n g s e n s i t i v i t y f a c t o r s determined i n the instrument f r o m known p u r e compounds o f s i m i l a r n a t u r e , r e a s o n a b l e c o m p o s i t i o n d a t a c a n be o b t a i n e d w i t h a c c u r a c y i n t h e r a n g e o f +3-5%. E l e c t r o n Escape
Depths
A parameter of fundamental i m p o r t a n c e i n e l e c t r o n s p e c t r o s c o p y i s t h e i n e l a s t i c mean f r e e p a t h l e n g t h (IMFP) f o r e l e c t r o n s of a g i v e n k i n e t i c energy o r i g i n a t i n g i n s o l i d s . T h i s p a r a m e t e r h a s t o do w i t h t h e f r a c t i o n o f o b s e r v e d e l e c t r o n s t h a t o r i g i n a t e f r o m a g i v e n d e p t h i n t o t h e sample and i s o f c r i t i c a l i m p o r t a n c e i n u n d e r s t a n d i n g XPS r e s u l t s . The i d e a l i z e d e s c a p e o f e l e c t r o n s f r o m m a t t e r i s i l l u s trated d i a g r a m a t i c a l l y i n Figure 2a. An i d e a l i z e d c u r v e a p p r o x i m a t i n g t h e r e l a t i o n s h i p b e t w e e n t h e IMFP and t h e e l e c t r o n k i n e t i c e n e r g y i s shown i n F i g u r e 2 b . f
C a r l s o n ( 3 ) s u m m a r i z e s t h e e a r l i e r d a t a on I M F P s f o r e l e c t r o n s whose k i n e t i c e n e r g y v a r i e s b e t w e e n 100 and 1500 ev t h r o u g h m a t e r i a l i n the e l e m e n t a l o r o x i d e f o r m . I n e l a s t i c mean f r e e p a t h s c a n be m e a s u r e d b y d e t e r m i n i n g t h e a t t e n u a t i o n o f a s i g n a l f r o m a s u b s t r a t e w i t h o v e r l a y e r s o f v a r y i n g known thickness. E x p e r i m e n t a l l y t h i s seems t o be s l i g h t l y l e s s d i f f i c u l t for inorganic materials. S i l i c o n d i o x i d e grown o r o x i d i z e d o v e r s i l i c o n i s an e x a m p l e . O v e r l a y e r f i l m s must be u n i f o r m and t h e i r t h i c k n e s s d e t e r m i n e d . P r o c e d u r e s have b e e n d e v e l o p e d i n t h e m i c r o e l e c t r o n i c s and t h i n f i l m p h y s i c s a r e a s t o
8.
MILLARD
Analysis
of Fibers and
Polymers
Figure 1. Wide scan spectra over the region 550-50 eV. Key: a, oxygen plasma cleaned glass; b, aluminum foil; c, solvent cleaned rayon fibers; d, solvent cleaned nylon fibers; e, solvent cleaned wool fiber; f, copolymer film (one monomer contains a chlorine substituted side chain); g, sulfonated 8% cross-linked styrene divinylbenzene copolymer beads; h, copolymer containing diphosphine chelating ligand.
148
SURFACE
ANALYSIS
grow o x i d e s o f known t h i c k n e s s o v e r t h e r e s p e c t i v e e l e m e n t . The agreement among d a t a f o r i n o r g a n i c s y s t e m s i s f a i r l y g o o d , h o w e v e r , when one c o n s i d e r s IMFP*s f o r o r g a n i c compounds and p o l y m e r s , the agreement i s f a i r . L i t t l e data existed for e s c a p e t h r o u g h o r g a n i c o v e r l a y e r s u n t i l r e c e n t l y and a number o f r e p o r t s now e x i s t r e p o r t i n g I M F P s f o r e l e c t r o n s w i t h k i n e t i c energies of i n t e r e s t escaping through organic o v e r l a y e r s . In a d d i t i o n , a c o m p i l a t i o n lias r e c e n t l y been p u b l i s h e d o f the d a t a e x i s t i n g i n the l i t e r a t u r e f o r IMFP s f o r v a r i o u s c a t a g o r i e s o f m a t e r i a l s and r e l a t i o n s h i p s b e t w e e n IMFP and k i n e t i c e n e r g y derived (29). E a r l i e r m e a s u r e m e n t s r e p o r t e d by Cadman ( 3 0 ) and C l a r k (31) u s i n g d i f f e r e n t a p p r o a c h e s were n o t i n a g r e e m e n t c o n c e r n i n g IMFP t h r o u g h o r g a n i c m a t e r i a l s . Cadman m e a s u r e d t h e i n t e n s i t y o f c o r e e l e c t r o n s e s c a p i n g t h r o u g h o r g a n i c p o l y m e r s and o r g a n i c compounds and r e l a t e d t h e s e i n t e n s i t i e s to I M F P s m e a s u r e d f o r C Is e l e c t r o n s i n g r a p h i t e a s a s t a n d a r d . Clark measured the a t t e n u a t i o n of e l e c t r o n s e s c a p i n g t h r o u g h p o l y ( p - x y l e n e ) u s i n g the o v e r l a y e r a p p r o a c h . C l a r k ' s I M F P s were s h o r t e r at comparable k i n e t i c e n e r g i e s t h a n were Cadman s v a l u e s . B r u n d l e (32) d e p o s i t e d m u l t i l a y e r s o f cadmium a r a c h i d a t e o v e r metalized glass substrates. The a t t e n u a t i o n o f t h e substrate s i g n a l was m e a s u r e d as a f u n c t i o n o f o v e r l a y e r t h i c k n e s s . The d e r i v e d IMFP s v a r i e d w i t h the angle between the p l a n e of the s a m p l e and a l i n e n o r m a l t o d e t e c t o r . This v a r i a t i o n with g r a z i n g a n g l e was i n t e r p r e t e d i n t e r m s o f i n c o m p l e t e substrate c o v e r a g e and a p a t c h c o r r e c t i o n was i n c l u d e d . For e l e c t r o n k i n e t i c e n e r g i e s b e t w e e n l n 3d KE 1042 eV and A u 4 f 1402 eV t h e I M F P s v a r i e d b e t w e e n 4 . 5 nm and 5 . 1 nm. With the p a t h c o r r e c t i o n t h e s e v a l u e s became 3 . 6 nm t o 4 . 1 nm. H a l l (33) measured I M F P s f r o m m u l t i l a y e r s o f b a r i u m s t e r a t e on g e r m a n i u m and copper. The v a l u e s r a n g e d b e t w e e n 2 . 7 t o 5 nm a t 230 eV t o 6 . 5 + 0 . 5 nm a t 1480 e V . C l a r k (34) u s e d s t e a r i c a c i d , 9 - n - b u t y l - 1 0 - a n t h r y l p r o p i o n i c a c i d and d i a c e t y l e n e f a t t y a c i d s t o p r e p a r e cadmium s a l t m u l t i l a y t e r s . A t 1170 eV I M F P s were 4 . 5 nm, 7 . 0 nm and 5 . 7 nm f o r t h e m u l t i l a y e r s respectively. R o b e r t s (35) c o a t e d s i l i c o n s u b s t r a t e s w i t h f i l m s o f P o l y ( m e t h y l m e t h a c r y l a t e ) ( P M M A ) and m e a s u r e d t h e t h i c k n e s s b y ellipsometry. The I M F P ' s o b t a i n e d were 2 . 9 + 0 . 4 nm f o r 1196 eV e l e c t r o n s and 3 . 3 + 0 . 5 nm f o r 1328 eV e l e c t r o n s . A p l o t of IMFP s v s . e l e c t r o n k i n e t i c energy determined f o r organic m a t e r i a l t a k e n from R o b e r t s (35) i s g i v e n i n F i g u r e 3. 1
1
1
f
1
T
1
T
1
1
Plasma O x i d a t i o n of Wool F i b e r s
F i b e r s and S u r f a c e G r a f t i n g
of
Coatings
to
P l a s m a o x i d a t i o n o f f i b e r s i s an example o f a t r e a t m e n t aimed a t c h e m i c a l l y m o d i f y i n g t h e s u r f a c e t o i m p r o v e a s u r f a c e property. T h e s e t r e a t m e n t s have w i d e a p p l i c a t i o n i n i n d u s t r y and a r e u s e d t o i m p r o v e w e t t a b i l i t y and p r i n t a b i l i t y o f p l a s t i c s , the a d h e s i o n of m a t e r i a l s to s u r f a c e s i n c l u d i n g t i s s u e c u l t u r e c e l l s , and a v a r i e t y o f o t h e r a p p l i c a t i o n s (36).
8.
MILLARD
Analysis
of Fibers and
)I ι 0
Polymers
ι
ι
100
ι
149
u
10,000
ELECTRON ENERGY (eV)
b Figure 2.
Electrons photoejected from a surface (a). Idealized graph of the mean free path length as a function of electron kinetic energy (b).
ELECTRON
KINETIC ENERGY
(EV)
Figure 3. Electron mean free path as a function of electron kinetic energy through organic materials. Key: V , PMMA; • , poly(p-xylene); O , plasma polymerized fluorocarbon; Φ, barium stéarate; 0, cadmium arachidate; and A, carbon. (Reproduced, with permission from Ref. 35.)
150
SURFACE
ANALYSIS
O x i d a t i o n o f t h e s u r f a c e o f w o o l f i b e r s i s known t o r e d u c e f e l t i n g s h r i n k a g e as w e l l as i m p r o v e o t h e r p r o p e r t i e s o f w o o l f i b e r s (37). O x i d a t i o n may be p r o f o r m e d o r e f f e c t e d i n a number o f ways i n c l u d i n g gas p h a s e p l a s m a t r e a t m e n t (38). E a r l y a t t e m p t s were made t o d e t e c t c h a n g e s o n t h e s u r f a c e o f wool f i b e r s u s i n g the magnetic d e f l e c t i o n e l e c t r o n s p e c t r o m e t e r at the U n i v e r s i t y of C a l i f o r n i a at B e r k e l e y . The s i g n a l to n o i s e r a t i o f o r s p e c t r a o b t a i n e d was m o d e s t and r u n t i m e s amounted t o s e v e r a l h o u r s ; h o w e v e r , i n t e r e s t i n g r e s u l t s were o b t a i n e d a l l o w i n g optimum d e v e l o p m e n t o f t h e p r o c e s s . The p l a s m a a p p a r a t u s e m p l o y e d i n t h e s e e x p e r i m e n t s i s shown s c h e m a t i c a l l y i n F i g u r e 4. Wool y a r n i s c o n t i n u o u s l y p a s s e d t h r o u g h t h e c a p i l l a r y i n l e t and o u t l e t w h i l e t h e gas i n w h i c h t h e p l a s m a i s m a i n t a i n e d i s i n t r o d u c e d on e i t h e r s i d e o f t h e r e a c t i o n chamber. E l e c t r o n s p e c t r a i n t h e s u l f u r 2p r e g i o n , o b t a i n e d a f t e r exposure of wool y a r n to an a i r p l a s m a , a r e shown i n F i g u r e 5. The l i n e a t h i g h e r e n e r g y i s due t o o x i d i z e d s u l f u r s p e c i e s s u c h as s u l f a t e i o n o r c y s t e i c a c i d . S u l f u r 2p e l e c t r o n s p e c t r a m shown i n F i g u r e 6 a r e t a k e n f r o m wool y a r n exposed to a plasma m a i n t a i n e d i n n i t r o g e n ( l o w e r s p e c t r u m ) and o x y g e n ( t o p s p e c t r u m ) . As a n t i c i p a t e d a h i g h e r l e v e l of o x i d a t i o n i s o b t a i n e d i n a pure oxygen p l a s m a . The s u l f u r 2p e l e c t r o n s p e c t r u m o b t a i n e d f r o m c o r o n a d i s c h a r g e t r e a t e d w o o l t o p i s shown i n F i g u r e 7a w h i l e t h e s p e c t r u m t a k e n f r o m u n t r e a t e d c o n t r o l i s shown i n F i g u r e 7 b . Coatings o f o r g a n i c s u b s t a n c e s c o u l d be p e r m a n e n t l y a t t a c h e d t o t h e s u r f a c e o f f i b e r s b y a d s o r b i n g t h e s u b s t a n c e on t h e w o o l f i b e r and t h e n e x p o s i n g t h e c o a t e d y a r n t o a d i s c h a r g e i n a i r o r argon (39). E l e c t r o n s p e c t r a were t a k e n a f t e r v a r i o u s c h a n g e s i n t h e p r o c e s s t o f o l l o w t h e amount o f m a t e r i a l a t t a c h e d t o t h e s u r f a c e and i t s l o s s a f t e r s o l v e n t e x t r a c t i o n and c l e a n i n g . The change i n t h e a r e a o f t h e v a r i o u s e l e c t r o n l i n e s c h a r a c t e r i s t i c o f t h e c o a t i n g s was t a k e n as an e s t i m a t i o n o f t h e r e l a t i v e l o s s of c o a t i n g a f t e r solvent e x t r a c t i o n . Figure 8 i l l u s t r a t e s t y p i c a l s p e c t r a t a k e n from d i s c h a r g e g r a f t e d f l u o r o c a r b o n c o a t i n g s and a c o u p l e o f f l a m e r e t a r d a n t c o a t i n g s containing phosphorus and h a l o g e n a r e i l l u s t r a t e d i n F i g u r e 9. T h e s e s p e c t r a were o b t a i n e d f r o m i n d i v i d u a l f i b e r s o r woven w o o l y a r n a t t a c h e d t o d o u b l e b a c k e d t a p e (40). A n a l y s i s of Plasma D e p o s i t e d F i l m s and Wool F i b e r s
F l u r o c a r b o n C o a t i n g s on P o l y m e r
A v a r i e t y o f o r g a n i c monomers may be p o l y m e r i z e d i n a p l a s m a and d e p o s i t e d o n t o s u b s t r a t e s 4 1 , 4 2 . This process was i n v e s t i g a t e d f o r a v a r i e t y o f f l u r o c a r b o n monomers and s u b s t r a t e s i n c l u d i n g p o l y m e r f i l m s and f a b r i c s o f woven w o o l fibers. S p e c t r a were o b t a i n e d u s i n g a V a r i a n I E E - 1 5 s p e c t r o meter. T y p i c a l e l e c t r o n l i n e widths e n c o u n t e r e d from t h e s e
Figure 4.
Plasma reactor for continuous treatment of yarn.
152
SURFACE
ANALYSIS
6500 h 1078.5 I
171
1081.5 I
1084.5
1087.5
KINETIC ENERGY (eV) » « I
168
165
BINDING ENERGY
162
(eV)
Figure 5. Sulfur 2p electron spectrum from plasma treated wool yarn. (Repro duced from Ref. 38. Copyright 1972, American Chemical Society.)
a ζ Ο
ζ Ο υ
BINDING ENERGY (ev)
Figure 6.
Sulfur 2p electron spectrum from wool yarn exposed to oxygen plasma (top) and to a nitrogen plasma (bottom).
8.
Analysis
MILLARD
of Fibers and
Polymers
153
7500
Ζ Q 7000
u
6500 r1 1078.5
1
I
1 171
1 1081.5
I
ι 1084.5
1087.5
KINETIC ENERGY (eV) I I ι
168
165
BINDING ENERGY
162
(eV)
Figure 7a. Sulfur 2 ρ electron spectrum obtained from untreated wool. (Repro duced from Ref. 38. Copyright 1972, American Chemical Society.)
5500
L
1078.5
1081.5
1084.5
1087.5
KINETIC ENERGY (eV) I 171
I
I 168
I
I 165
BINDING ENERGY (eV)
162
Figure 7b. Sulfur 2p electron spectrum obtained from plasma treated wool. (Re produced from Ref. 38. Copyright 1972, American Chemical Society.)
154
SURFACE
ANALYSIS
30000
556.3
I
1
693.3
557.4
I
692.2
558.6
559.6
560.7
561.9
KINETIC ENERGY (eV) ι
691.0
ι
689.9
I
688.8
I
687.7
563.0
I
686.6
564.1
I
685.5
BINDING ENERGY (eV) Figure 8.
Fluorine Is spectrum from fluorocarbon coating plasma grafted to wool yarn.
8.
MILLARD
84
Analysis
of Fibers and
155
Polymers
74
64
BINDING ENERGY (ev) Figure 9a.
Electron spectra from flame retardant coatings plasma grafted to fabric.
156
SURFACE
ANALYSIS
PHOSPHORUS 2p
210
200
190
BINDING ENERGY (ev) Figure 9b.
Electron spectra from flame retardant coatings plasma grafted to fabric.
8.
MILLARD
Analysis
of Fibers and
Polymers
157
s u r f a c e s i n t h i s i n s t r u m e n t were of the o r d e r o f magnitude o f 2.6 + 0.2 e V . At t h i s r e s o l u t i o n , a curve d e c o n v o l u t i o n p r o c e d u r e was n e c e s s a r y t o e x t r a c t t h e p o s i t i o n o f o v e r l a p i n g electron lines. We u s e d a n o n - l i n e a r l e a s t s q u a r e s c u r v e f i t t i n g program w r i t t e n at the U n i v e r s i t y of C a l i f o r n i a at Berkeley (43). T h i s p r o g r a m u s e s L o r e n t z i a n and G a u s s i a n f u n c t i o n s t h a t a r e d e s c r i b e d by f o u r independent parameters and a c o n v e r g e n c e w i t h r e a s o n a b l e v a l u e s f o r t h e p a r a m e t e r s i s u s u a l l y obtained after several t r i a l s . The q u a l i t y o f f i t o f t h e s p e c t r a was j u d g e d i n p a r t b y v a l u e s o f t h e w e i g h t e d v a r iance. A c c e p t a b l e d e c o n v o l u t i o n s u s u a l l y had v a l u e s o f t h e weighted v a r i a n c e below 10. Examples o f d e c o n v o l u t e d s p e c t r a i n the C Is r e g i o n o b t a i n e d from 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 h e x a f l u r o e t h a n e d e p o s i t e d on p l a s t i c substrates a r e shown i n F i g u r e 10a and b t o p and b o t t o m r e s p e c t i v e l y . The c a r b o n Is s p e c t r u m f r o m t h e f l u r o c a r b o n f i l m d e p o s i t e d u s i n g h e x a f l u o r o e t h a n e ( F i g u r e 11a) and t e t r a f l u o r o e t h y l e n e ( F i g u r e l i b ) on wool f a b r i c i s shown. The p a r a m e t e r s d e r i v e d f r o m t h e s e s p e c t r a a r e shown i n T a b l e V I . Examples o f the o x y g e n Is s p e c t r a o b t a i n e d f r o m p o l y m e r i z e d h e x a f l u o r o e t h a n e on p o l y p r o p y l e n e and w o o l t e x t i l e a r e shown i n F i g u r e 12a and b. I n t e r e s t i n g l y , t h e f l u o r i n e Is s p e c t r a o b t a i n e d f r o m f l u o r o c a r b o n f i l m s o n w o o l c o n t a i n e d two c o m p o n e n t s . The h i g h e r b i n d i n g e n e r g y component c o r r e s p o n d e d t o f l u o r i n e c o v a l e n t l y bonded t o c a r b o n w h i l e t h e l o w e r b i n d i n g e n e r g y component was i n the r e g i o n r e p o r t e d f o r i o n i c f l u o r i d e . The l a t t e r were p r e s u m a b l y a s s o c i a t e d w i t h c a t i o n i c s i t e s o n w o o l s u c h as protonated amines. An example o f t h e d e c o n v o l u t e d f l u o r i n e Is s p e c t r a i s shown i n F i g u r e 13 t a k e n f r o m h e x a f l u o r o e t h a n e film on w o o l . P a r a m e t e r s d e r i v e d f r o m t h e o x y g e n and f l u o r i n e spectra are included i n Table I I . P l a s m a T r e a t e d Wool F i b e r s The s p e c t r a f r o m p l a s m a t r e a t e d w o o l f i b e r s was r e - e x a m i n e d u s i n g t h e V a r i a n I E E s p e c t r o m e t e r and c h a n g e s i n t h e c a r b o n , o x y g e n and n i t r o g e n Is l e v e l s c o u l d be a n a l y z e d ( 4 4 ) . Lines at higher b i n d i n g energy r e p r e s e n t i n g oxidized carbon s p e c i e s could be r e a d i l y o b s e r v e d . As e x p e c t e d , t h e i n t e n s i t y o f h i g h e r b i n d i n g e n e r g y l i n e s were maximum u s i n g a n o x y g e n p l a s m a and l e a s t using a nitrogen plasma. Of i n t e r e s t i n t h e d e v e l o p m e n t o f e f f i c i e n t p r o c e s s s c a l e e q u i p m e n t was t h e d i s c o v e r y t h a t active o x i d i z i n g s p e c i e s were p r e s e n t a t c o n s i d e r a b l e d i s t a n c e s b e y o n d the r e g i o n o f plasma a c t i v a t i o n and s p e c t r a o b t a i n e d from wool f i b e r s i n the a i r a f t e r g l o w r e g i o n r e v e a l e d l e v e l s o f o x i d a t i o n o n l y s l i g h t l y below those o b t a i n e d near the source of the plasma. D e c o n v o l u t e d c a r b o n Is e l e c t r o n s p e c t a o b t a i n e d f r o m o x y g e n p l a s m a and a i r p l a s m a a f t e r g l o w t r e a t e d f i b e r s a r e shown i n F i g u r e 14 a and b . T h e s e s p e c t r a were f i t t e d w i t h L o r e n t z i a n f u n c t i o n s w i t h a l e a d i n g c o n s t a n t t a i l whose FWHM v a l u e was
Figure 10.
Carbon Is spectrum from hexafluoroethane plasma polymerized onto polystyrene.
GO
>
as
>
> o w
c!
GO
Ux oo
Figure lia.
Carbon Is spectrum from hexafluoroethane plasma polymerized on wool fabric. (Reproduced, with permission, from Ref. 42. Copyright 1976, Dekker.)
3
s
f
σ
>
g Ρ
9°
Figure lib.
Carbon Is spectrum of tetraftuoroethylene plasma polymerized onto wool. (Repro duced, with permission, from Ref. 42. Copyright 1976, Dekker.)
I—*
00
> >
Ο W
c >
GO
ON Ο
Figure 12a.
Oxygen Is spectra obtained from hexafluorοethane
plasma polymerized on polystyrene.
BINDING ENERGY (EU)
Figure 12b.
Oxygen Is spectra obtained from hexafluoroethane plasma polymerized on wool fiber. Reproduced, with permission, from Ref. 42. Copyright 1976, Dekker.
00
00
>
>
> ο w
GO
to
ON
Figure 13.
Fluorine Is spectrum from hexafluoroethane plasma polymerized on wool fabric. (Re produced, with permission, from Ref. 42. Copyright 1976, Dekker).
Q\
Hi
CO*
g
g Ρ f >
oo
Figure 14 a.
Carbon Is spectra from wool fibers oxidized in oxygen plasma. (Reproduced, with permission, from Ref. 44. Copyright 1977, Academic Press.)
GO
00
> >
ο w
GO
I—* ON
Figure 14b.
ENERGY
(EU)
Carbon Is spectra from wool fibers oxidized beyond the excitation region. (Reproduced, with permission, from Ref. 44. Copyright 1977, Academic Press.)
KINETIC
00
n
)
'>•>>'/./
532.0 (D
2.
3.
532.4 (D
1.
Sample
(o.t>2)
533.9
533.4 (1.2)
534.5 (0.54)
0 Is B.E. eV ( r a t i o )
2.6
2.6
3.49
FWHM (eV)
C 2 F 4 on wool 30B
6.
on wool 31B
7,240
6,830
3,660
Total intensity (counts/sec)
2.06
286.8 (0.85)
287.2 (2.2)
286.4 (2.5)
286.0 (0.37)
287.6 (0.426)
287.6 (0.402)
13,900
13,600
7,000
290.1 (0.105)
292.1 (0.508)
292.1 (0.648)
4.95
27.2
96.3
116.5
292.9 (0.6)
293.5 (1.41)
687.1 (D
686.5 (D
689.5 (0.5)
688.9 (3.9)
689.3
689.2
2.6
2.6
2.67
2.8
FWHM (eV)
3.19
2.66
1.7
2.26
2.72
2.69
1,580
4,905
4,467
7,230
3,730
2,373
0 Is I n t e n s i t y (counts/sec)
0.35
2
7
16.6
F/0
F/0
F/0
F/0
Surface atom r a t i o
FWHM (eV)
Surfaces
290.4 (0.0008)
292.9 0.124)
294.1 (0.316)
F Is B.E. eV ( r a t i o )
291.0 (0.6)
291.6 (2.62)
F ls intensity/10^ (counts/sec)
289.7 (0.40)
289.3 (1.42)
287.7 289.0 (0.0008) (0.505)
288.3 (0.22)
289.8 (0.336)
290.0 (0.415)
294.0 (0.380)
Fluorocarbon
Carbon l s B.E. e V ( r a t i o )
from Plasma-Polymerized
Corrected intensity (counts/sec)
285.0 (D
285.0 (D
C 2 F 4 on polypropylene with argon, C
5.
6
18.231
285.0 (D
4.124
C F
4.
2
285.0 (D
4.779
2
on wool 31C
C F
3.
6
285.0 (D
7.2
C 2 F 6 on polystyrene
2.
285.0 (D
8.0
C2F6 on polypropylene
1.
Sample
C Is I n t e n s i t y 1000 (Counts/Sec)
X-ray Photoelectron S p e c t r a l Data Obtained
TABLE I I .
>
>
> o w
ON ON
8.
MILLARD
Analysis
of Fibers and
167
Polymers
2.6 eV. The oxygen and n i t r o g e n s p e c t r a were deconvoluted i n t o two components. The r e s u l t of plasma treatment was t o i n c r e a s e the r e l a t i v e i n t e n s i t y of the lower b i n d i n g energy l i n e i n each case. The higher b i n d i n g energy oxygen l i n e was i n the r e g i o n normally reported f o r oxygen bound t o carbon and s u l f u r w h i l e the lower b i n d i n g energy l i n e i s i n the r e g i o n reported f o r oxides and may represent reduced oxygen species c o n t a i n i n g a negative charge. The two n i t r o g e n I s l i n e s d i f f e r e d by 1.6 t o 1.9 eV f o r the v a r i o u s plasma treatments. The unique s t r u c t u r a l i n t e r p r e t a t i o n f o r these two l i n e s i s d i f f i c u l t due to u n c e r t a i n t y i n the i n t e r p r e t a t i o n of simple p r o t e i n s p e c t r a . P o s s i b i l i t i e s i n c l u d e polypeptide chain n i t r o g e n and s i d e chain n i t r o g e n and protonated b a s i c n i t r o g e n groups and the unprotonated group. The oxygen and n i t r o g e n s p e c t r a were f i t t e d w i t h gaussian f u n c t i o n s w i t h an exponent i a l t a i l . F u l l width a t h a l f maximum was set a t 2.6 eV f o r oxygen and 2.4 eV f o r n i t r o g e n . Examples of deconvoluted oxygen and n i t r o g e n s p e c t r a from oxygen plasma t r e a t e d wool f i b e r as w e l l as untreated c o n t r o l are shown i n Figures 15 and 16. Parameters d e r i v e d from the v a r i o u s s p e c t r a are i n c l u d e d i n Table I I I .
Table I I I Carbon Is E l e c t r o n Line Parameters Spectra From Plasma Treated Wool F i b e r
Binding energy (eV)
Relative l i n e area
Shift i n (eV)
285.0 286.07 287.77
1.0 0.092 0.043
0.0 1.07 2.77
N i t r o g e n plasma t r e a t e d wool f i b e r
285.0 286.43 288.46
1.0 0.28 0.31
0.0 1.43 3.46
A f t e r g l o w plasma t r e a t e d wool f i b e r
285.0 286.56 288.42
1.0 0.24 0.33
0.0 1.56 3.4
Oxygen plasma t r e a t e d wool f i b e r
285.0 286.55 289.04
1.0 0.23 0.45
0.0 1.55 4.04
Sample
Untreated fiber
Wool
168
SURFACE
Table Oxygen
Is
III
(Continued)
Electron Line
Parameters
S p e c t r a From P l a s m a T r e a t e d Wool
Binding energy (eV)
Sample
ANALYSIS
Fiber
Relative l i n e area
Shift i n (eV)
530.4 532.06
1.0 9.19
0.0 2.02
N i t r o g e n plasma t r e a t e d wool f i b e r
530.73 532.25
1.0 2.89
0.0 1.52
Afterglow
Untreated fiber
Wool
530.87
1.0
0.0
fiber
532.41
3.1
1.53
Oxygen p l a s m a t r e a t e d wool f i b e r
530.61 532.29
1.0 3.62
0.0 1.68
treated
plasma
wool
N i t r o g e n Is E l e c t r o n L i n e P a r a m e t e r s S p e c t r a From P l a s m a T r e a t e d Wool F i b e r 398.03 399.94
1.0 8.9
0.0 1.9
N i t r o g e n plasma t r e a t e d wool f i b e r
398.6 400.3
1.0 3.0
0.0 1.67
Oxygen p l a s m a t r e a t e d wool f i b e r
398.95 400.54
1.0 2.74
0.0 1.59
Untreated fiber
Wool
The e l e c t r o n s p e c t r a d i s c u s s e d were t a k e n o n woven p i e c e s o r f i b e r s a t t a c h e d t o d o u b l e b a c k e d t a p e i n a manner s u c h t h a t m i n i m a l t a p e was e x p o s e d . The e f f e c t o f s u r f a c e u n i f o r m i t y and d e n s i t y was i n v e s t i g a t e d by c o m p a r i n g t h e s p e c t r a t a k e n f r o m w o o l f i b e r s and f r o m w o o l f i b e r s c o m p r e s s e d i n t o t h i n c o n t i n u o u s transparent f i l m s . Wool f i b e r o r woven t e x t i l e c a n be p r e s s e d i n t o t h i n t r a n s p a r e n t f i l m s by a p p l y i n g p r e s s u r e t o m o i s t samples between heated p l a t e s i n a h y d r a u l i c p r e s s (45). The n i t r o g e n and s u l f u r e l e c t r o n s p e c t r a o b t a i n e d f r o m i n d i v i d u a l f i b e r s a t t a c h e d t o d o u b l e b a c k e d t a p e and p r e s s e d i n t o a f i l m a r e shown i n F i g u r e 1 7 . The e l e c t r o n l i n e i n t e n s i t i e s o f t h e s p e c t r a o b t a i n e d f r o m t h e s e two s a m p l e s a r e g i v e n i n Table IV. I n g e n e r a l , t h e e l e c t r o n l i n e s f r o m t h e smooth d e n s e f i l m w e r e t h r e e t i m e s as i n t e n s e and t h i r t y p e r c e n t n a r r o w e r t h a n the e l e c t r o n l i n e s o b t a i n e d from f i b e r s . The p o s i t i o n o f t h e l i n e s were i d e n t i c a l .
Figure 15.
Oxygen Is spectra from control and oxygen plasma treated wool fabric. duced, with permission, from Ref. 44. Copyright 1977, Academic Press.)
(Repro-
170
SURFACE
ANALYSIS
Figure 16a. Nitrogen Is spectra from control and oxygen plasma treated wool yarn. (Reproduced, with permission, from Ref. 44. Copyright 1977, Academic Press.)
8.
MILLARD
Analysis
of Fibers and
Polymers
111
Figure 16b. Nitrogen Is spectra from control and oxygen plasma treated wool yarn. (Reproduced, with permission, from Ref. 44. Copyright 1977, Academic Press.)
172
SURFACE
ANALYSIS
TABLE I V Electron
L i n e I n t e n s i t i e s f r o m I s o l a t e d Wool T o p F i b e r s T r a n s p a r e n t F i l m P r e s s e d f r o m Wool F i b e r s
Electron Sample
C
Unoxidized
wool
t o p ffiibbeerrss fibers
into
a thin
3 cps) l i n e i n t e n s i t i e s (10 S 2p Ν Is 0 Is Is
30.6 pressed
100
12 36.6
5
1.4
15.5
3.2
film
172 170
Figure 17.
and T h i n
Electron lines from wool fibers (a) and wool film (b).
8.
MILLARD
Analysis
of Fibers and Polymers
173
Chemical M o d i f i c a t i o n s of Surface F u n c t i o n a l Groups w i t h Elements Having High P h o t o e l e c t r i c Cross Sections The surface s e n s i t i v i t y of XPS can be enhanced by c h e m i c a l l y modifying s u r f a c e f u n c t i o n a l groups w i t h reagents that c o n t a i n elements w i t h higher d e t e c t i o n s e n s i t i v i t y . I f the d e r i v a t i o n r e a c t i o n i s q u a n t i t a t i v e o r r e p r o d u c i b l e , the enhanced s u r f a c e s i g n a l can be used to determine the surface c o n c e n t r a t i o n of f u n c t i o n a l groups not n o r m a l l y d e t e c t a b l e due t o l a c k of s u f f i c i e n t b i n d i n g energy s h i f t o r low s e n s i t i v i t y ( 4 6 ) . We explored the d e t e c t i o n and q u a n t i f i c a t i o n of s p e c i f i c f u n c t i o n a l groups i n p r o t e i n s using q u a n t i t a t i v e s p e c i f i c chem i c a l m o d i f i c a t i o n s t h a t c o n t a i n elements w i t h h i g h d e t e c t i o n s e n s i t i v i t y . F l u o r i n e was introduced s p e c i f i c a l l y i n t o Bovine Serum Albumin (BSA) by N - t r i f l u o r a c e t y l a t i o n of t h e ε-amino group o f l y s i n e using e t h y l t h i o t r i f l u o r a c e t a t e ( 4 7 ) . The f l u o r i n e e l e c t r o n spectrum from N - t r i f l u o r a c e t y l a t e d BSA i s shown i n Figure 18. Wool f i b e r surfaces could be e f f i c i e n t l y t r i f l u o r o a c e t y l a t e d u s i n g t h i s reagent and s p e c t r a obtained from wool f i b e r s modified i n t h i s way i s shown i n F i g u r e 19. E t h y l v i n y l s u l f o n e was a l s o used t o d e r i v a t i v e l y s i n e i n c a s e i n and BSA (48). The a c t i v e hydrogen i n l y s i n e w i l l add across the double bond r e s u l t i n g i n the i n t r o d u c t i o n of the s u l f o n e group i n t o the p r o t e i n . The p r o t e i n contains no s u l f u r l i n e higher i n b i n d i n g energy than 163 eV and the s u l f o n e s u l f u r l i n e i s r e a d i l y i d e n t i f i a b l e . The s u l f u r 2p spectrum from e t h y l v i n y l s u l f o n e (EVS) modified c a s e i n i s shown i n Figure 20. A s e r i e s of s p e c t r a obtained from EVS modified BSA i s shown i n Figure 21. I n i t i a l l y o n l y a low b i n d i n g energy l i n e i s present i n BSA and as the l e v e l o f s u l f o n e m o d i f i c a t i o n i n c r e a s e s the i n t e n s i t y o f the higher binding energy l i n e i n c r e a s e s . These s p e c t r a were obtained by r a p i d l y scanning the S 2p r e g i o n a t low anode i n t e n s i t y due to the r a p i d l o s s o f the sulfone l i n e i n the s p e c t r a as a r e s u l t o f r a d i a t i o n exposure i n the instrument. A s e r i e s o f s p e c t r a obtained as a f u n c t i o n of time of X-ray exposure i n the instrument revealed the i n t e n s i t y of the sulfone s u l f u r r a p i d l y decreases w i t h time. This example i l l u s t r a t e s the p o t e n t i a l of XPS to d e t e c t and study r a d i a t i o n s e n s i t i v e materials (48). The s u l f h y d r y l group i n papain was modified using mercury c o n t a i n i n g reagents and the spectrum i n the mercury r e g i o n i s i l l u s t r a t e d i n Figure 22a obtained from mercury papain. The spectrum obtained from PCMB modified ovalbumin i s shown i n F i g u r e 22b. The mercury 4f e l e c t r o n l i n e can be detected i n f a i r l y low c o n c e n t r a t i o n and i s an i n t e r e s t i n g case o f d e t e c t i o n of s p e c i f i c a l l y c h e m i c a l l y modified f u n c t i o n a l groups. These p r i n c i p a l s can be a p p l i e d t o study and estimate the presence of f u n c t i o n a l groups on the s u r f a c e of a v a r i e t y of m a t e r i a l s that would not normally be d e t e c t a b l e .
174
SURFACE
ANALYSIS
Σόδομ
α Ζ ο υ
Ζ ζ> ο
6815 696
676
686
BINDING ENERGY (ev) Figure 18. Fluorine Is spectrum from bovine serum albumin trifluoroacetylated with ethyl thiotrifluoracetate. (Reproduced from Ref. 47. Copyright 1974, Ameri can Chemical Society.)
6520 h
ζ Ο υ
Ζ D Ο υ
4430 696
686
676
BINDING ENERGY (ev) Figure 19.
Fluorine Is spectrum of wool trifluoroacetylated with ethyl thiotrifluoracetate.
8.
Analysis
MILLARD
of Fibers and
175
Polymers
700
u LU CO \ CO
»— Ζ Ο υ 440 176
166
156
BINDING ENERGY (ev) Figure 20. Sulfur 2p spectrum from casein modified with ethyl vinyl sulfone. (Reproduced, with permission, from Ref. 48. Copyright 1976, North-Holland Publishing Co.)
Figure 21. Sulfur 2p spectra contained from a series of ethyl vinyl sulfur modified samples of BSA. (Reproduced, with permission, from Ref. 48. Copyright 1976, North-Holland Publishing Co.)
176
SURFACE
2600h
Û Ζ
o u Ζ 3 Ο u
2215h
I
J_ 104.0
110.6
BINDING ENERGY
Figure 22a.
97.3 (ev)
Mercury 4î spectrum from mercury papain.
3290
z O
u
LU to Ζ
Ο u
2600h-
±
Π0.6
JL 104.0
97.3
BINDING ENERGY (ev) Figure 22b.
Mercury 4i spectrum from PCMB modified ovalbumin.
ANALYSIS
8.
MILLARD
Examples of
Analysis Recent
of Fibers and
177
Polymers
Materials Science
A p p l i c a t i o n s o f XPS
XPS i s p a r t i c u l a r l y s u i t e d t o a n a l y z e s o l i d m a t e r i a l s i n v a r i o u s m a t e r i a l s s c i e n c e a p p l i c a t i o n s of polymeric m a t e r i a l s . S e v e r a l e x a m p l e s o f t h e u s e o f XPS t o a n a l y z e t h e s u r f a c e o f s o l i d s i n i r r e g u l a r f o r m s s u c h as f i b e r s , p o w d e r s , f i l m s , b e a d s , and v a r i o u s e x t r u d e d s h a p e s s u c h as o - r i n g s w i l l be presented. XPS c a n p r o v i d e a r a p i d s u r v e y a n a l y s i s as w e l l as q u a n t i t a t i v e a n a l y s i s w i t h i n s e v e r a l p e r c e n t d e p e n d i n g on the s e n s i t i v i t y f o r the element i n q u e s t i o n . Unique s t r u c t u r a l i n f o r m a t i o n c a n o f t e n be o b t a i n e d on s o l i d s t h a t , due t o t h e i r i n t r a c t a b i l i t y and l a c k o f s o l u b i l i t y w o u l d p r e s e n t problems f o r i n v e s t i g a t i o n by o t h e r s p e c t r o s c o p i c methods. Growth R e g u l a t o r s
Encapsulated i n Natural
Polymers
Controlled release of various p h y s i o l o g i c a l l y a c t i v e s u b s t a n c e s i n t o o r g a n i s m s o r the e n v i r o n m e n t has r e c e i v e d considerable attention i n recent years. C o n t r o l l e d r e l e a s e from m a t r i c e s that are n a t u r a l l y b i o d e g r a d a b l e i s d e s i r a b l e from a s t a n d p o i n t of m i n i m i z i n g a d d i t i o n a l c o n t a m i n a t i o n of the environment by m a t e r i a l s t h a t are degraded s l o w l y . The a n a l y s i s of v a r i o u s f o r m u l a t i o n s of p l a n t growth r e g u l a t o r s i n b i o d e g r a d a b l e m a t r i c e s s u c h as c h i t o s a n , f e a t h e r k e r a t i n and c e l l u l o s e r e p r e s e n t s a n i n t e r e s t i n g example o f t h e u s e o f XPS f o r a n a l y t i c a l and s t r u c t u r a l s t u d i e s on s o l i d m a t e r i a l s . A v a r i e t y of approaches to c o n t r o l l e d r e l e a s e of p h y s i c a l l y a c t i v e s u b s t a n c e s f r o m s o l i d f o r m u l a t i o n s has b e e n p r o p o s e d and studied. Some o f t h e s e a p p r o a c h e s a r e s c h e m a t i c a l l y diagrammed i n F i g u r e 23 w h e r e R i s some a c t i v e s u b s t a n c e t o be r e l e a s e d into the environment. C h i t o s a n was c h o s e n as a p o s s i b l e m a t e r i a l t o e n c a p s u l a t e p l a n t g r o w t h r e g u l a t o r s as many r e g u l a t o r s a r e o r g a n i c a c i d s and c h i t o s a n i s r e a d i l y a v a i l a b l e f r o m c h i t i n and h a s a n a m i n o g r o u p t h a t w o u l d be a s i t e f o r a c i d b a s e o r i o n e x c h a n g e type entrapment. The s t r u c t u r a l f o r m u l a o f c h i t i n and t h e d e a c e t y l a t e d product c h i t o s a n i s r e p r e s e n t e d i n F i g u r e 24. C h i t o s a n c a n be d i s s o l v e d i n a c i d s and c a n be c r o s s l i n k e d w i t h g l u t e r a l d e h y d e as w e l l ( 4 9 ) . We e q u i l i b r a t e d c r o s s l i n k e d c h i t o s a n w i t h v a r i o u s o r g a n i c a c i d type growth r e g u l a t o r s c o n t a i n i n g c h l o r i n e s u c h as 2 , 4 - D and p - c h l o r o p h e n o x y a c e t i c acid. A wide s c a n s p e c t r u m o f g l u t e r a l d e h y d e c r o s s l i n k e d c h i t o s a n i n the form of g r a n u l e s i s presented i n F i g u r e 25. The i n s e t c o n t a i n s a c c u m u l a t e d s c a n s o v e r t h e Ν Is and C I 2p region. The Ν Is s p e c t r u m h a s a l i n e a r o u n d 399 eV due t o t h e n e u t r a l amine n i t r o g e n w i t h a s h o u l d e r a t t h e h i g h e r e n e r g y s i d e o f t h e l i n e due t o some p r o t o n a t e d amine g r o u p s . The c h l o r i n e l i n e i s due t o t h e c h l o r i d e i o n r e s u l t i n g f r o m some n e u t r a l i z a t i o n o f t h e amine by H C 1 . F i g u r e 26 c o n t a i n s accumulated s p e c t r a over these regions for c h i t o s a n e q u i l i b r a t e d w i t h 2,4 D. The n i t r o g e n s p e c t r a c o n t a i n s a l o w b i n d i n g e n e r g y
178
SURFACE
Figure 23.
Scheme of biodegradeable controlled release approaches.
\ NH,
CH OH 2
NHC0CH3 Figure 24.
NH.
NHCOCH
3
CH2OH
Structure of chitosan (top) and chitin (bottom).
ANALYSIS
Figure 25.
Electron spectra from glutaraldehyde cross-linked chitosan.
180
SURFACE
BE, EV
Figure 26.
ANALYSIS
BE, EV
Chlorine 2ρ and nitrogen Is region from 2,4-D in chitosan.
8.
MILLARD
Analysis
of Fibers and
Polymers
1
l i n e ^ 399 due t o f r e e amine and a h i g h e r b i n d i n g e n e r g y l i n e a t ^ 401 due t o t h e p r o t o n a t e d amine g r o u p . The c h l o r i n e 2p s p e c t r u m c o n t a i n s two u n r e s o l v e d c h l o r i n e l i n e s , one p r e s u m a b l y due t o t h e c h l o r i d e i o n p r e s e n t i n t h e m a t r i x as w e l l as c h l o r i n e c o v a l e n t l y bound t o c a r b o n f r o m t h e 2 , 4 - D . Spectra obt a i n e d from g r a n u l e s o f c h i t o s a n c o n t a i n i n g p - c h l o r o p h e n o x y a c e t i c a c i d i s shown i n F i g u r e 2 7 . The h i g h e r b i n d i n g e n e r g y n i t r o g e n l i n e p r e d o m i n a t e s due t o p r o t o n a t e d amine and t h e c h l o r i n e 2p r e g i o n c o n t a i n s two u n r e s o l v e d lines due t o i o n i c and c o v a l e n t c h l o r i n e . The p r e s e n c e o f 2 , 3 , 5 t r i i o d i o b e n z o i c a c i d i n c h i t o s a n c o u l d be r e a d i l y d e t e c t e d due to a s t r o n g l i n e due t o i o d i n e . F e a t h e r k e r a t i n was a n i n t e r e s t i n g m a t e r i a l t h a t l e n d e d i t s e l f to c o n t r o l l e d r e l e a s e f o r m u l a t i o n s . F e a t h e r k e r a t i n can be s o l u b i l i z e d b y s o l u t i o n i n t o b o i l i n g e t h a n o l i n t h e presence of b i s u l f i t e to reduce the d i s u l f i d e l i n k a g e . Varying amounts o f t h e g r o w t h r e g u l a t o r i s added a t t h i s s t a g e and t h e s o l u t i o n a l l o w e d to c o o l to a g e l . T h i s g e l c a n be p r e s s e d i n t o a f i l m a t 1 0 0 ° C and 2 0 , 0 0 0 l b s p e r s q u a r e i n c h . This process p a r t i a l l y reforms d i s u l f i d e c r o s s l i n k s r e s u l t i n g i n a c r o s s l i n k e d m a t r i x w i t h entrapped growth r e g u l a t o r . Electron s p e c t r a o b t a i n e d from f e a t h e r k e r a t i n f i l m s c o n t a i n i n g 2 , 4 - D i s shown i n F i g u r e 2 8 . A survey scan i n d i c a t e s the presence of c h l o r i n e and i n d i v i d u a l a c c u m u l a t e d scans i n d i c a t e t h e p r e s e n c e o f o r g a n i c c o v a l e n t l y bound c h l o r i n e due t o t h e p r e s e n c e of 2 , 4 - D . Growth r e g u l a t o r s c o n t a i n i n g f u n c t i o n a l groups c a p a b l e o f c o m p l e x i n g o r i n t e r a c t i n g w i t h m e t a l h a l i d e s were a t t a c h e d t o c e l l u l o s e b y a p r o c e d u r e i n v o l v i n g b r i d g i n g by m e t a l h a l i d e s s u c h as T 1 C I 4 ( 5 0 ) . C e l l u l o s e powder was t r e a t e d w i t h T 1 C I 4 i n an i n d i f f e r e n t s o l v e n t complexing the T 1 C I 4 to the c e l l u l o s e . The e l e c t r o n s p e c t r u m o b t a i n e d f r o m t h e T 1 C I 4 c e l l u l o s e c o m p l e x i s shown i n F i g u r e 2 9 . A wide s c a n s p e c t r u m r e v e a l s t h e p r e s e n c e o f t h e t i t a n i u m 2p l i n e s as w e l l as c h l o r i n e 2p l i n e s . I n d o l e a c e t i c a c i d was a d d e d t o t h e c e l l u l o s e T 1 C I 4 c o m p l e x and t h e s p e c t r u m o b t a i n e d f r o m t h e c e l l u l o s e T 1 C I 4 i n d o l a c e t i c a c i d c o m p l e x i s shown i n F i g u r e 30. I n a d d i t i o n t o l i n e s p r e s e n t b e f o r e , t h e n i t r o g e n Is l i n e i n d i c a t e s the presence of i n d o l e a c e t i c a c i d . The t i t a n i u m p r e s u m a b l y a c t s as a b r i d g e c o m p l e x i n g t h e n i t r o g e n and o x y g e n from c e l l u l o s e to a t t a c h the i n d o l e a c e t i c a c i d to cellulose. Siloxane
Urethane I n t e r p e n e t r a t i n g
Network
T h e r e has been c o n s i d e r a b l e r e c e n t i n t e r e s t i n the d e t e r m i n a t i o n by XPS o f t h e s u r f a c e c o m p o s i t i o n o f c o - p o l y m e r s whose components d i f f e r c o n s i d e r a b l y i n t h e i r s u r f a c e f r e e e n e r g y (51) When t h e i n d i v i d u a l components o f b l o c k c o - p o l y m e r s o r i n t e r p e n e t r a t i n g n e t w o r k s c a n be d e t e c t e d and i n d i v i d u a l l y i d e n t i f i e d by X P S , a f a i r l y r a p i d c o - p o l y m e r c o m p o s i t i o n d e t e r m i n a t i o n i s p o s s i b l e a s w e l l as t h e d e t e r m i n a t i o n o f t h e c o m p o s i t i o n o f
182
SURFACE
BE, EV
Figure 27.
ANALYSIS
BE, EV
Nitrogen Is spectrum and chlorine 2p spectrum from p-chlorophenoxyacetic acid in chitosan.
8.
MILLARD
Analysis
of Fibers and Polymers
183
•δ
I
•S a, Q
v.
I
Ο
g δ ε ο s*. Ο
od
Figure 29.
h
Electron spectra from cellulose power treated with TiCl .
GO
GO
1
> r
>
ο w
>
α
GO
oo
Figure 30.
k
Electron spectra from TiCl
bridged indoleacetic acid complex anchored to cellulose.
OO
t—*
2
Si SX*
J!
f
>
r
g
oo
186
SURFACE
ANALYSIS
t h e c o p o l y m e r s y s t e m a t v a r i o u s i n t e r f a c e s and t h r o u g h o u t t h e bulk. An i n v e s t i g a t i o n o f t h e c o m p o s i t i o n o f a u r e t h a n e f l u o r o a l k y l s i l o x a n e i n t e r p e n e t r a t i n g network i l l u s t r a t e s these p o s s i b i l i t i e s (52) . A f l u o r o a l k y l s i l o x a n e o f modest d e g r e e o f p o l y m e r i z a t i o n was o b t a i n e d b y h y d r o l y s i s u n d e r m i l d c o n d i t i o n s o f 3-(heptafluoroisopropoxy)propyltrichlorosilane [(CF3) CFO(CrI 2)381013]. A c o m m e r c i a l l y a v a i l a b l e u r e t h a n e s u c h a s A d i p r e n e L315 d e s c r i b e d as a f u l l y s a t u r a t e d l i q u i d p o l y m e r w i t h 9 . 4 5 + 0.20 a v a i l a b l e i s o c y a n a t e and t h e f l u o r o a l k y l s i l o x a n e d e s c r i b e d above were combined i n e q u a l p r o p o r t i o n s b y w e i g h t i n t h e p r e s e n c e o f 1 , 4 - d i a z a ( 2 , 2 , 2 ) b i c y c l o o c t a n e ( P a b c o ) and d i b u t y l t i n d i l a u r a t e i n c a t a l y t i c amounts and a l l o w e d t o s t a n d f o r one week. A s o l i d i n s o l u b l e r u b b e r - l i k e m a t e r i a l cream i n c o l o r r e s u l t e d a f t e r t h i s period of time. The p o l y m e r was e x t r a c t e d w i t h t o l u e n e and F r e o n 113 f o r e i g h t h o u r s . T o l u e n e was a s o l v e n t f o r t h e u r e t h a n e and F r e o n 113 f o r t h e s i l o x a n e p r i o r t o treatment with the c a t a l y s t . A 12% w e i g h t l o s s r e s u l t e d a f t e r this extraction. T h i s s o l i d i n t e r p e n e t r a t i n g network polymer was a n a l y z e d b y XPS u s i n g t h i n s e c t i o n s o b t a i n e d f r o m v a r i o u s l o c a t i o n s i n t h e p o l y m e r s a m p l e i n c l u d i n g t h e a i r and r e a c t i o n flask interface. The s p e c t r u m t a k e n f r o m t h e s a m p l e i s i l l u s trated i n Figure 31. A n i t r o g e n Is l i n e c h a r a c t e r i s t i c o f t h e u r e t h a n e and t h e f l u r o i n e I s , s i l i c o n 2p and C Is l i n e s a r e a l l c h a r a c t e r i s t i c of the f l u o r o a l k y l s i l o x a n e . The c o m p o s i t i o n o f the samples o b t a i n e d from l o c a t i o n s t h r o u g h o u t the polymer i s g i v e n i n T a b l e V . The a n a l y s i s was c a l c u l a t e d b y c o r r e c t i n g t h e e l e c t r o n l i n e i n t e n s i t i e s u s i n g Wagner*s s e n s i t i v i t y factors and summing t h e c o r r e c t e d i n t e n s i t i e s . E l e m e n t a l c o m p o s i t i o n was c a l c u l a t e d by d i v i d i n g each e l e m e n t a l e l e c t r o n l i n e i n t e n s i t y by t h e sum o f a l l e l e c t r o n l i n e i n t e n s i t i e s . S e v e r a l samples were a n a l y z e d f o r f l u o r i n e b y c o m b u s t i o n a n a l y s i s and t h e f l u o r i n e c o n t e n t so d e t e r m i n e d a g r e e d w i t h t h a t c a l c u l a t e d b y XPS measurements w i t h i n l e s s t h a n 5%. The v a r i a t i o n i n c o m p o s i t i o n was c o n s i d e r a b l e i n g o i n g f r o m t h e a i r i n t e r f a c e a t t h e t o p t o the g l a s s i n t e r f a c e a t the bottom. The s i l i c o n c o n t e n t w a r i e d b e t w e e n 5 . 3 and 6.7%. The f l u o r i n e c o n t e n t v a r i e d b e t w e e n 3 4 . 5 and 25.3% w h i l e t h e n i t r o g e n c o n t e n t v a r i e d b e t w e e n 1.2% and a trace. 2
P e n e t r a t i o n o f a Vacuum Pump F l u i d I n t o O - R i n g S e a l s The n e x t s u r f a c e a n a l y s i s example h a s t o do w i t h t h e q u e s t i o n o f p e n e t r a t i o n o f a vacuum pump f l u i d i n t o 0 - r i n g s e a l s (53) . A B u n a - N 0 - r i n g was o p e r a t e d i n t h e p r e s e n c e o f F o m b l i n vacuum pump f l u i d . The p o i n t o f i n t e r e s t was t o f i n d i f t h e pump f l u i d had p e n i t r a t e d t o 0 - r i n g m a t e r i a l . A Buna-N 0 - r i n g was o p e r a t e d i n t h e p r e s e n c e of h y d r o c a r b o n n o n f l u o r i n a t e d f l u i d and t h e s p e c t r u m o b t a i n e d f r o m t h e s u r f a c e o f t h i s 0 - r i n g i s shown i n f i g u r e 3 2 . E l e c t r o n l i n e s f r o m f l u o r i n e and s i l i c o n a r e p r e s e n t and a r e p r o b a b l y due t o a f l u o r o s i l o x a n e l u b r i c a n t
8.
MILLARD
Analysis
of Fibers and
300 Figure 31.
187
Polymers
290
BE, EV
Electron spectra from fluoroalkylsiloxane urethane interpenetrating network.
BE, EV
Figure 32.
Electron spectra from buna-N seal exposed to hydrocarbon fluid.
13.6(64)
Center
38.2(5.34)
Glass
Interface
13.6(5.27)
Near G l a s s Interface
Cut
15.9(6.5)
Below A i r Interface
181(25.3)
81(31.4)
73.5(34.5)
85(34.6)
120(31.5)
135.0(17.2)
0 Is
in
186.0(26)
62.3(24.1)
38.4(18)
43.8(17.9)
70.0(18.4)
Interface
25.7(6.7)
Sample
Air
300(38.4)
F Is
the
and P e r c e n t a g e s
of
33.2(28)
48.8(6.24)
S i 2p
Intensities
Regions
from
307.0(43)
100.0(38.7)
86.5(40.6)
100.0(40.7)
165.0(43.3)
77.4(65.4)
298.5(38.1)
C Is
parentheses
Urethane I n t e r p e n e t r a t i n g Network
V
A d i p r e n e L315 urthane
Fluorosiloxane polymer
Sample
Siloxane Various
Corrected
Composition of
TABLE
2(.46)
2.97(.41)
1.
0.95(.45)
0.95(.4)
trace
7.76(6.55)
Ν Is
00
00
*
>
Ο Η
>
!*>
GO
oo oo
8.
Analysis
MILLARD
of Fibers and
189
Polymers
o c c u r s a t a b o u t 5 3 3 . 1 e V . F i g u r e 33 i l l u s t r a t e s t h e e l e c t r o n s p e c t r a t a k e n from the Buna-N 0 - r i n g o p e r a t e d i n the presence of Fomblin. F o m b l i n i s a f l u o r o c a r b o n e t h e r w i t h the f o l l o w i n g formula:
[-CF - C F
2
- 0)
m
-
CF )n 2
I n a d d i t i o n t o f l u o r i n e and e l e c t r o n l i n e s due t o c a r b o n bonded t o t h r e e and two f l u o r i n e a t o m s , a n i n t e n s e o x y g e n e l e c t r o n l i n e a t 5 3 5 . 1 eV i s p r e s e n t . T h i s i s h i g h e r than the v a l u e s r e p o r t e d f o r o x y g e n i n n o n - f l u o r i n a t e d o r g a n i c compounds b u t i s i n t h e r a n g e p l a u s i b l e f o r a f l u o r o c a r b o n e t h e r where o x y g e n i s bonded to carbon w i t h e l e c t r o n w i t h d r a w i n g f l u o r i n e groups a t t a c h e d . T h i s f l u o r o c a r b o n e t h e r oxygen i d e n t i f i e s the m a t e r i a l p r e s e n t on t h e s u r f a c e as F o m b l i n and i n d i c a t e s t h a t t h i s f l u i d h a s p e n e t r a t e d t o t h e e x t e n t o f a t l e a s t 10 nm i n t o t h e surface. S u l f o n a t i o n o f 8% C r o s s l i n k e d S t y r e n e D i v i n y l b e n z e n e C o p o l y m e r Beads The e x t e n t o f s u l f o n a t i o n o f 8% c r o s s l i n k e d s t y r e n e d i v i n y l b e n z e n e c o - p o l y m e r beads ( B i o r a d B i o Bead SX 8) was i n v e s t i g a t e d by c o m p a r i n g t h e s u r f a c e c o m p o s i t i o n a f t e r s u l f o n a t i o n o f t h e beads u s i n g c h l o r o s u l f o n i c a c i d , s u l f u r i c a c i d and f u m i n g s u l furic (54). E a c h r e a g e n t was r e f l u x e d i n m e t h y l e n e c h l o r i d e f o r a s i m i l a r period of t i m e . A wide-scan spectrum i n d i c a t e s the p r e s e n c e o f s u l f u r 2p and 2s e l e c t r o n s , i n d i c a t i v e o f s u l f o nation. The s u r f a c e s u l f u r c o n t e n t was f a i r l y s i m i l a r i n t h e s u r f a c e r e g i o n a n a l y z e d by X P S . The c o m p o s i t i o n d e t e r m i n e d f r o m b e a d s s u l f o n a t e d by t h e t h r e e methods i s i n d i c a t e d i n T a b l e V I . Electron
spectra
taken
from a
sample
of
beads
sulfonated
i n s u l f u r i c a c i d i s i l l u s t r a t e d i n F i g u r e 34. The s u l f u r 2p l i n e i s f a i r l y i n t e n s e and more t h a n a d e q u a t e f o r a n a l y t i c a l purposes. A n o t h e r f e a t u r e o f i n t e r e s t was t h e d e t e r m i n a t i o n o f t h e b a r i u m i o n to s u l f u r atom r a t i o i n t h e r e s i n s when t h e y were c o n v e r t e d t o t h e b a r i u m i o n f o r m . A s i n g l e scan over the b a r i u m 3s r e g i o n i s shown i n F i g u r e 34 f o r a sample i n t h e h y d r o g e n i o n f o r m and a f t e r c o n v e r s i o n t o t h e b a r i u m i o n f o r m . The s u l f u r t o b a r i u m r a t i o v a r i e d b e t w e e n 1.3 t o 2 . 4 f o r a number o f s a m p l e s a n a l y z e d i n d i c a t i n g c o n s i d e r a b l e v a r i a t i o n i n t h e e x t e n t o f s h a r i n g b a r i u m i o n s among t h e s u l f u r a t o m s . The d i s t r i b u t i o n o f s u l f a t e g r o u p s on t h e s u r f a c e and i n t o t h e i n t e r i o r o f b e a d s s u l f o n a t e d u n d e r c o n d i t i o n s to y i e l d s u r a c e m o d i f i c a t i o n and s u r f a c e p l u s i n t e r i o r m o d i f i c a t i o n was i n v e s t i g a t e d by o x y g e n p l a s m a e t c h i n g ( O P E ) ( 5 5 ) o f b e a d s t h a t were s u l f o n a t e d i n r e f l u x i n g s u l f u r i c and i n c o l d s u l f u r i c f o r the same t i m e i n t e r v a l . The c o l d s u l f o n a t i o n y i e l d e d beads s u l f o n a t e d so m i n i m a l l y t h a t t h e s u l f a t e g r o u p c o u l d n o t be d e t e c t e d by t i t r a t i o n w h i l e t h e c a p a c i t y o f t h e beads s u l f o n a t e d h o t c o u l d be t i t r a t e d as u s u a l . The beads s u l f o n a t e d c o l d a r e l a b e l l e d s u r f a c e w h i l e the o t h e r are l a b e l l e d n o r m a l . The
190
SURFACE ANALYSIS
TABLE V I Corrected
I n t e n s i t i e s and C o m p o s i t i o n C a l c u l a t e d f o r S t y r e n e D i v i n y l b e n z e n e C r o s s l i n k e d Beads
Sulfonated
C Is
0 Is
S 2p
Chlorosulfonic* acid %
42,6
12.5
3.5
76.6
21.3
5.9
Fuming S u l f u r i c acid
42.6
14.4
3.5
70.0
24.0
5.7
44.4
12.5
3.0
74.0
20.8
5.0
% Sulfuric
acid
% * I n t e n s i t y i n 1000
counts/sec.
8.
MILLARD
Analysis of Fibers and Polymers
191
i n t e n s i t y of the barium and s u l f u r e l e c t r o n l i n e s were more intense by a f a c t o r of 4 f o r the normal compared to the surface s u l f o n a t e d . The s u r f a c e s u l f u r - t o - b a r i u m r a t e was 2.1 f o r the s u r f a c e compared t o 1.6 f o r the normal. The d i s t r i b u t i o n of the s u l f u r and barium i n t o the i n t e r i o r of the beads was determined by oxygen plasma e t c h i n g the beads f o r v a r y i n g periods of time and determination of the s u r f a c e composition a f t e r e t c h i n g by XPS. Under the e t c h i n g c o n d i t i o n s used, the r a t e of e t c h i n g i n t o the i n t e r i o r of p o l y s t y r e n e spheres has been determined t o be ^ 10.0 nm per minute. When a c o a t i n g i s present o n l y on the s u r f a c e , the c o n c e n t r a t i o n of the elements i n the c o a t i n g s l o w l y decreases as the surface i s o x i d i z e d away. The elements forming n o n v o l a t i l e oxides remain as an ash on the s u r f a c e and become d i l u t e d as e t c h i n g proceeds. I f the m o d i f i c a t i o n penetrates i n t o the i n t e r i o r , the c o n c e n t r a t i o n of the elements i n the c o a t i n g increases u n t i l the unmodified i n t e r i o r m a t e r i a l i s r e a c t e d . This behavior i s i l l u s t r a t e d by the data shown i n Table V I I . The i n t e n s i t y of the barium and s u l f u r l i n e s decreases w i t h OPE w h i l e the i n t e n s i t i e s of these elemental l i n e s i n c r e a s e s up t o f i f t e e n minute e t c h i n g and then begins to decrease i n the case o f s u r f a c e s u l f o n a t i o n . This f u r t h e r s u b s t a n t i a t e s the e x t e r n a l surface nature of the m i l d s u l f o n a t i o n as compared to the presence of s u l f o n i c a c i d s i t e s much deeper i n t o the i n t e r i o r of the bead i n the case of s u l f o n a t i o n under more vigorous c o n d i t i o n s . The s u r f a c e e l e c t r o n l i n e s as a f u n c t i o n of e t c h time are i l l u s t r a t e d i n Figure 35 f o r the two s e t s of samples. Wood Surface Treatment An XPS study of the o x i d a t i v e s u r f a c e treatment of white pine represents and i n t e r e s t i n g example of the a b i l i t y of the technique to o b t a i n surface s t r u c t u r a l i n f o r m a t i o n on a sample t h a t would present problems f o r many a n a l y t i c a l mmethods (56)· A survey and carbon Is spectrum from untreated white pine i s shown i n Figure 36. Survey s p e c t r a and c h l o r i n e 2p as w e l l as carbon Is s p e c t r a are shown i n Figure 37 taken from white pine t r e a t e d with hydrogen peroxide, HCL, and a c e t i c a c i d with h e a t i n g . This o x i d a t i v e treatment has o x i d i z e d carbon as evidenced by shoulders i n the C Is s p e c t r a at higher b i n d i n g energies and introduced carbon c h l o r i n e bonds on the surface as evidenced by the c h l o r i n e 2p l i n e around 200 eV. Polymer Anchored C a t a l y s t s The d e t e r m i n a t i o n of s t r u c t u r e and bonding of polymer anchored c a t a l y s t s i s another area where the i n s o l u b i l i t y of the m a t e r i a l s o f t e n precludes s o l u t i o n s p e c t r o s c o p i c s t u d i e s and one i s l i m i t e d to techniques t h a t can be a p p l i e d to i r r e g u l a r s o l i d s (57). In a d d i t i o n , combining oxygen plasma e t c h i n g and surface a n a l y s i s allows i n v e s t i g a t i o n of the depth of penetrat i o n of the metal i n t o the polymer and allows d e t e c t i o n of components that r e q u i r e c o n c e n t r a t i o n to a l l o w d e t e c t i o n .
192
SURFACE ANALYSIS
Figure 34.
786
Electron spectra taken from sulfonated styrene divinylbenzene beads.
NORMAL
Figure 35.
786
SURFACE
786
NORMAL
Electron spectra as a function of oxygen plasma etching exposure from normally sulfonated beads and surface only sulfonated beads.
MILLARD
Analysis
Figure 37.
of Fibers and
Polymers
Electron spectrum from surface oxidized white pine.
194
SURFACE ANALYSIS TABLE V I I
I n t e n s i t i e s o f E l e c t r o n L i n e s f r o m S u r f a c e S u l f o n a t e d and N o r m a l S u l f o n a t e d B e a d s as a F u n c t i o n o f O x y g e n P l a s m a E t c h i n g Ba 3s I n t e n s i t y S u l f u r 2p I n t e n s i t y Etch
time
1000 c p s (min) Surface
1000 Normal
Surface
cps Normal
0
2.4
11.0
0.35
1.4
5
1.25
23.0
0.30
2.0
15
1.1
20.6
0.25
2.5
25
1.0
16.0
0.20
2.3
An example o f s u c h a n a n a l y s i s o f r h o d i u m p h o s p h i n e l i g a n d anchored c a t a l y s t s i s presented (58). A survey scan of a c o polymer c o n t a i n i n g a d i p h o s p h i n e l i g a n d p r i o r to i n t r o d u c t i o n of t h e m e t a l i s shown i n F i g u r e 1, s p e c t r u m h . The p r e s e n c e o f phosphorus i n the polymer i s e v i d e n t from t h i s s p e c t r u m . F i g u r e 38 i l l u s t r a t e s a c c u m u l a t e d s u r f a c e s c a n s i n t h e r h o d i u m 3d and p h o s p h o r u s 2p r e g i o n t a k e n f r o m g r a n u l e s o f t h e r h o d i u m anchored c a t a l y s t . The s u r f a c e c o n c e n t r a t i o n i s l o w e n o u g h t h a t s c a n a c c u m u l a t i o n was n e c e s s a r y t o d e t e c t t h e s e e l e m e n t s . T h e s e p a r t i c l e s were o x y g e n p l a s m a e t c h e d f o r t h i r t y m i n u t e s and F i g u r e 39 i n c l u d e s a s u r v e y s p e c t r u m a s w e l l as Rh 3d and Ρ 2p s p e c t r a t a k e n f r o m t h e s a m p l e a f t e r O P E . The i n t e n s i t y o f t h e r h o d i u m and p h o s p h o r u s l i n e s i s e n h a n c e d c o n s i d e r a b l y a s a r e s u l t of e t c h i n g . To i n v e s t i g a t e t h e d e p t h o f p e n e t r a t i o n o f the anchored m e t a l i n t o the s u r f a c e of the p a r t i c l e s , s u r f a c e s p e c t r a were o b t a i n e d as a f u n c t i o n o f OPE t i m e s . This data i s g i v e n i n T a b l e V I I I and the p h o s p h o r u s and r h o d i u m s p e c t r a as a f u n c t i o n o f e t c h t i m e i n m i n u t e s i s shown i n F i g u r e 4 0 . The i n t e n s i t y o f t h e r h o d i u m and p h o s p h o r u s l i n e s i n c r e a s e s up t o t w e n t y m i n u t e s o f e t c h i n g o r e q u i v a l e n t t o p e n e t r a t i o n o f 160 nm i n t o the s u r f a c e of the p a r t i c l e s . This analysis indicates that rhodium i s f a i r l y u n i f o r m l y d i s t r i b u t e d i n t o the p a r t i c l e s a t l e a s t 160 nm i n t o t h e i n t e r i o r . TABLE V I I I Intensities OPE E t c h i n g (min.)
f r o m o x y g e n P l a s m a E t c h e d (OPE) P o l y m e r A n c h o r e d Rhodium C a t a l y s t B e a d s Rh 3d I n t e n s i t y Ρ 2p I n t e n s i t y (1000 c o u n t s / s e c . ) (1000 c o u n t s / s e c . )
0
0.40
0.10
5
1.25
0.80
10
1.75
1.00
20
2.40
1.60
MILLARD
Analysis
of Fibers and
Polymers
Rh 3d 16 SCANS P2P 16 SCANS
312
125.5
Figure 38.
Accumulated surface scans from phosphine anchored rhodium catalyst.
Figure 39.
Electron spectrum from phosphine anchored rhodium containing catalyst after 30-min exposure to oxygen plasma etching.
196
SURFACE ANALYSIS
140
135
130
BE, EV
Rh 3d
322 Figure 40.
315
310
BE, EV
Electron lines from phosphine anchored catalyst as a function of oxygen plasma etching exposure.
8.
MILLARD
Analysis
of Fibers and
Polymers
197
Sulfone Polymer R a d i a t i o n Chemistry Often d u r i n g the study of substances by XPS, one observes changes i n the s p e c t r a such as l i n e broadening, changes i n i n t e n s i t y and l e s s f r e q u e n t l y appearance of new l i n e s t h a t are i n d i c a t i v e of chemical changes as a r e s u l t of r a d i a t i o n damage i n the instrument. I n i t i a l l y these observations were u s u a l l y considered t o be annoying and v a r i o u s measures such as r e d u c t i o n of the i n t e n s i t y , lower temperatures o r f a s t e r scan r a t e s were taken i n an attempt to o b t a i n s p e c t r a f r e e from r a d i a t i o n damage a l t e r a t i o n s . More r e c e n t l y , i t has been recognized t h a t XPS i s a s e n s i t i v e and u s e f u l technique w i t h which t o study r a d i a t i o n chemistry of s o l i d s (13). M a t e r i a l s can be subjected t o r a d i a t i o n treatment and these e f f e c t s analyzed by XPS ( 5 9 ) . A l t e r n a t e l y , one can observe changes due to X-ray i r r a d i a t i o n i n XPS spectrometers and i n v e s t i g a t e s t r u c t u r a l changes accompanying t h i s process. Sulfones seem t o be p a r t i c u l a r l y s u c c e p t i b l e t o r a d i a t i o n damage and we observed the decrease i n the i n t e n s i t y of t h e s u l f o n e s u l f u r e l e c t r o n l i n e d u r i n g s t u d i e s on p r o t e i n s c h e m i c a l l y modified w i t h e t h y l v i n y l s u l f o n e (48). This observ a t i o n c o r r e l a t e d w i t h the known r a d i a t i o n chemistry of p o l y s u l f o n e s where r a d i o y s i s leads to carbon s u l f u r bond cleavage and l o s s of SO2 (60). Sulfone polymers are used as p o s i t i v e r e s i s t s as a r e s u l t of t h e i r r a d i a t i o n chemistry ( 6 1 ) . As an example of the type of observations t h a t can be made by observing the s p e c t r a of r a d i a t i o n s e n s i t i v e m a t e r i a l s as a f u n c t i o n of time of i r r a d i a t i o n , s p e c t r a of p o l y s u l f o n e s a f t e r v a r i o u s times of i r r a d i a t i o n i n a Dupont 650B are shown i n F i g u r e 41b. These s p e c t r a are n o v e l i n that i n i t i a l l y a s i n g l e l i n e i s present due to the s u l f o n e s u l f u r i n the S 2p r e g i o n and upon i r r a d i a t i o n i n the instrument a lower b i n d i n g energy l i n e appears a t 166 eV which represents one of the products of the r a d i a t i o n induced decomposition r e a c t i o n . This l i n e increases i n i n t e n s i t y w i t h time. The r e l a t i v e i n t e n s i t y of these l i n e s observed a t i n c r e a s i n g X-ray exposure time i s g i v e n i n Table IX. Another more common type of behavior i s i l l u s t r a t e d i n Figure 41a, spectrum a. X-ray r a d i a t i o n exposure of a p o l y s u l f o n e r e s u l t e d i n the decrease of the s u l f o n e s u l f u r l i n e w i t h no other l i n e s appearing. The decrease i n i n t e n s i t y of t h i s l i n e i s g i v e n as a f u n c t i o n of r a d i a t i o n time i n Table IX.
198
SURFACE ANALYSIS
174
170
BE, EV
175
170
BE, EV
Figure 41. Sulfur 2 p spectrum as a function of irradiation time of a polysulfone illustrating decrease in intensity of one line (left). Sulfur 2p spectrum of a polysulfone as a function of irradiation time illustrating the decrease in intensity of the original sulfone line and the appearance of a sulfur containing product (right).
8.
MILLARD
Analysis
of Fibers and Polymers
199
TABLE I X Sulfur
Intensities
S 2p I n t e n s i t y 1 6 8 . 6 eV 1000 c o u n t s / s e c .
as
a Function of
S 2p I n t e n s i t y 166 eV 1000 c o u n t s / s e c .
R a d i a t i o n Exposure
S 2p I n t e n s i t y 1 6 8 . 7 eV 1000 c o u n t s / s e c .
Irradiation Time min.
1.
15.3
0.0
1.2
0.0
2.
14.1
0.7
0.7
1.0
3.
13.0
1.4
0.6
2.0
4.
11.8
2.0
0.3
6.0
5.
11.2
2.6
6.
10.4
3.1
7.
9.0
8.
8.3
11.
7.6
Surface A n a l y s i s of Modified P o l y e s t e r F i b e r s U t i l i z i n g a V a r i e t y of Spectroscopic Methods. Up t o t h i s p o i n t t h i s r e v i e w h a s c o n c e r n e d t h e u s e o f X ray 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 to a n a l y z e s u r f a c e s of f i b e r s and i r r e g u l a r s o l i d s . T h i s i s so b e c a u s e t h e a u t h o r b e l i e v e s t h a t XPS i s u n i q u e i n i t s s u r f a c e s e n s i t i v i t y and a b i l i t y t o d i f f e r e n t i a t e among v a r i o u s c h e m i c a l f o r m s o f e l e m e n t s o n t h e s u r f a c e as w e l l . Problems can a r i s e as a r e s u l t o f r a d i a t i o n damage t o o r g a n i c s u b s t r a t e s due t o X - r a y r a d i a t i o n . However, t h i s p r o b l e m i s much l e s s s e v e r e t h a n t h e damage p r o b l e m s r e s u l t i n g from o t h e r t e c h n i q u e s u t a l i z i n g e l e c t r o n s o r i o n s s u c h a s A u g e r , SIMS and t h e e l e c t r o n m i c r o p r o b e . A r e c e n t p a p e r has a p p e a r e d u t i l i z i n g a number o f s u r f a c e a n a l y s i s t e c h n i q u e s to s t u d y c h e m i c a l l y m o d i f i e d p o l y e s t e r f i b e r s (63) . The m e t h o d s employed i n c l u d e d 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 (SEM), w e t t a b i l i t y , f o u r i e r transform i n f r a r e d spectroscopy (FTIR) i n combination w i t h attenuated t o t a l reflectance s p e c t r o s c o p y ( A T R ) , raman m i c r o s c o p y ( M O L E ) , l a s e r m i c r o p r o b e mass a n a l y s i s (LAMMA), s e c o n d a r y i o n mass s p e c t r o m e t r y ( S I M S ) and X P S . The e f f e c t o f a s u r f a c e t r e a t m e n t t o i m p r o v e t h e a d h e s i o n of a t i r e cord a d h e s i v e to p o l y ( e t h y l e n e terephthlate) (PET) f i b e r s c o n s i s t i n g of a c o m m e r c i a l f o r m u l a t i o n c o m p r i s i n g a p h e n o l b l o c k e d m e t h y l e n e b i s p h e n y l e n e d i i s o c y a n a t e and a
200
SURFACE ANALYSIS
g l y c e r o l epoxy r e s i n was s t u d i e d . The l i m i t a t i o n s of s e v e r a l of these methods became apparent when a p p l i e d to organic substrates combined w i t h the f a c t that the m o d i f i c a t i o n was confined to the outer few nanometers of the surface and represented much l e s s than one percent of the t o t a l . FTIR and MOLE f a i l e d to d e l e c t s i g n a l s c h a r a c t e r i s t i c of the m o d i f i c a t i o n i n the presence of the f i b e r . Only products from the decomp o s i t i o n of the s u b s t r a t e f i b e r were detected by LAMMA and SIMS. The technique most able to detect the surface to the chemical m o d i f i c a t i o n was XPS. The isocyanate and the g l y c e r o l epoxy component of the m o d i f i c a t i o n c o n t a i n n i t r o g e n and c h l o r i n e which are unique to the m o d i f i c a t i o n . Nitrogen and c h l o r i n e core e l e c t r o n l i n e s were detected and the n i t r o g e n to c h l o r i n e atom r a t i o was s e n s i t i v e to c l e a n i n g treatments a p p l i e d t o the modified yarn.
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Science, 1980, 208, 775-966. Riggs, W.M.; Parker, M.J. i n "Methods of Surface Analysis", A.W. Czauderna, ed. Elsevier Scientific Publishing: New York, 1975, pp 103-157. Carlson, T.A. "Photoelectron and Auger Spectroscopy:, Plenum Press, New York, 1975. Baker, A.D.; Brisk, M.A.; Liotta, D.C. Anal. Chem. 1980, 52, 161R-174R. Dwight, D.W.; Fabish, T . J . ; Thomas, H.R., Photon, Electron, and Ion Probes of Polymer Structure and Properties," ACS Symposium Series 162, American Chemical Society, Washington, D.C., 1981, pp. 1-442. Dilks, A. "Electron Spectroscopy". Vol. 4 Brundle, C.R.; Baker, A.D., Eds. Academic Press, London, 1981, pp 277-359. Holm, R.; Storp, S. Surf, and Interface Anal., 1980, 2,(3), 96-106. Clark, D.T. CRC Crit. Rev. Solid State Matter S c i . , 1978, 8, p 1. Clark, D.T. "Polymer Surfaces". Clark, T.D.; Feast, W.J. eds. John Wiley, New York, 1978, pp 305-358. Briggs, D. "Electron Spectroscopy". Baker, A.D.; Brundle, C.R., eds. Academic Press, London, 1978 Vol. 2, 305-358. Seah, M.P. Surf, and Interface Anal., 1980, 2(6), 222-239. Hoffmann, S. Surf, and Interface Anal., 1980, 2, 148-160. Copperthwaite, R.G. Surf. and Interface Anal., 1980, 2(1), 17-25. Snell, K.D.; Keenan, A.G. Chem. Soc. Quart. Rev., 1979, 8, 259-282. Mittal, K.L. Pure and Appl Chem., 1980, 52, 1295-1305. Roberts, M.W. Chem. Soc. Quart. Rev., 1977, 6(4), 373-391. Barr, T.L. Am. Lab., 1978, 10, 40-51.
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18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41.
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Analysis
of Fibers and Polymers
201
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202
42. 43 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63.
SURFACE
ANALYSIS
Millard, M.M.; Pavlath, A.E. J. Macromol. Sci-Chem., 1976, A10(3), 579-97 Fadley, C.S. "Core and Valence Electronic States Studied With XPS". Lawrence Radiation Laboratory Report, 1970, UCRL 19535. Millard, M.M. "Characterization of Metal and Polymer Surfaces". Vol. 2, Lee, Lieng-Huang, ed., 1977, Academic Press, New York, pp. 85-100. Ward, W.M., 1976, Western Regional Research Center, USDA, Berkeley, CA, unpublished data. Batich, C.D; Wendt, R.C. in ref 5 pg. 221. Millard, M.M.; Masri, M.S. Anal. Chem., 1974, 46, 1820. Millard, M.M.; Friedman. M; Biochem. Biophys. Res. Commun., 1976, 70, 445-51. Masri, M.S.; Randall, V.G.; "Proceedings of the First International Conference on Chitin/Chitosan". Muzzarelli, R.A.A.; Pariser, E.R., ed., 1978, MIT, Cambridge, MA, pp. 277-87 Messing, R.A. Methods in Enzymology, Vol. 29, Imobilized Enzymes, 1976. Mosbach, K., ed., Academic Press, New York, 166-169. Dwight, D.W.; Thomas, H.R. Polymer Preprints Division of Polymer Chemistry, ACS, 1981, 22(1), 300-304. Pittman, A.G.; Millard, M.M. "Interaction of Urethanes with Fluoroalkyl Siloxanes" submitted to J. App. Polym. Sci. Raby, B.A., 1976. UTHE Technology Int., Sunnyvale, CA, unpublished data. Wood, R.D., 1978. Biorad Laboratories, Inc., Richmond, CA, unpublished data. Millard, M.M.; Bartholomew, J.C. Anal. Chem., 1977, 49, 1290-1296. Johns, Wm., 1976. Forest Products Laboratory, Richmond, CA, unpublished data. Kaneko, M.; Tsuchida, E. J. Polym. Sci. Macromol. Rev., 1981, 16, 397-522. Stille, J.K., 1979. Colorado State University Chemistry Dept., Fort Collins, Colo., personal communication. Sasaki, T.; Williams, R.S.; Wong, J.S.; Shirley, D.A. J. Chem. Phys., 1979, 71, 4601-10. Brown, J.R.; O'Donnell, J.H. J. Applied Polym Sci, 1975, 19, 405-417. Bowden, M.J. CRC Critical Reviews in Solid State and Materials Science, 1979, 8(3), 223-65. Wall, L.A. "Fluoropolymers". 1972, John Wiley, New York. Gillberg, G.; Kemp, D. J. App. Polym. Sci., 1981, 26, 2023-51.
RECEIVED
April 19, 1982
