Surface Studies on Multicomponent Polymer Systems by X-Ray

19

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Surface Studies on Multicomponent Polymer Systems by X-Ray Photoelectron Spectroscopy 1

H. R. THOMAS and J. J. O'MALLEY

Xerox Webster Research Center, Xerox Square W-114, Rochester NY 14644

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological prop erties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. Although a number of techniques have been devised to investigate the bulk domain structure of multicomponent polymer systems the detailed structure of the surface, i.e., the outer most few tens of angstroms, has been studied in much less detail. Since many of the important properties of a polymeric solid are dependent upon the surface structure and since the surface can differ considerably from the bulk a technique which can differen tiate the surface from bulk properties is likely to be of consid erable importance. In this paper we describe the application of X-ray photo electron spectroscopy (XPS) (2) to the quantitative and qualita tive investigation of the surface properties of several multicom ponent polymer systems in the solid state. We have divided the polymer systems into three categories, 1) regular block copoly mers, 2) random block copolymers, and 3) physical blends. In each category representative examples are presented of polymeric sys tems studied by XPS to delineate the surface topography, morphol ogy, structure and bonding. In all these studies we have used angular dependent XPS XPS( θ ) (3) to depth profile the composi tional variation within the outermost few tens of angstroms of the polymer-air-interface. EXPERIMENTAL In Table 1 are shown the systems reported in this paper. The synthesis and characterization of the polymer samples are 0097-6156/81 /0162-0319$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.

320

PHOTON,

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

r e p o r t e d e l s e w h e r e and t h e r e f e r e n c e s f o r e a c h s y s t e m a r e f o u n d on t h e r i g h t hand s i d e o f T a b l e I . The s a m p l e s were a l l s t u d i e d as t h i n ( ^ 1-10 u ) f i l m s c a s t from t h e a p p r o p r i a t e s o l v e n t s f o r each system, a l s o found i n T a b l e I . ( I n t h i s p a p e r we d e s c r i b e o n l y s y s t e m s c a s t from s o l v e n t s where b o t h p o l y m e r components w e r e s o l u b l e , more d e t a i l e d s t u d i e s on s e l e c t i v e s o l v e n t s c a n be found i n the papers r e f e r e n c e d ) . A l l s o l v e n t s were s p e c t r o s c o p i c g r a d e and t h e s a m p l e s w e r e s t u d i e d as f i l m s c a s t o n t o flat aluminum s u b s t r a t e s by d i p c o a t i n g . A l l f i l m s w e r e d r i e d i n an a r g o n a t m o s p h e r e t o r e d u c e s u r f a c e c o n t a m i n a t i o n and were s u f f i c i e n t l y t h i c k ( > 100 A ) t o mask t h e A l core l e v e l s i g n a l from t h e s u b s t r a t e . ^

Table

I - B u l k C o m p o s i t i o n s , S o l v e n t s and S y n t h e s i s R e f e r e n c e s f o r P o l y m e r Systems S t u d i e d

System

Solvent

% PDMS

%PS wt.

mole

Synthesis Reference

mole

a)

PS-PEO d i b l o c k

19.6 39.3 70.0

9.6 21.4 49.5

CHC1

0

19

b)

PEO-PS-PEO triblock

23.5 38.5 70.3

11.4 21.0 49.8

CHC1

0

19

c)

HMS-DMS Random block

d)

PS-PEO p h y s i c a l blend

27.0 57.2 72.5 20.0 40.0 70.0

* C o m m e r c i a l homopolymers S c i e n t i f i c

58.0 84.0 91.0

CHCl

19

9.8 22.0 49.5

Polymer Products

S p e c t r a w e r e r e c o r d e d w i t h an A E I ES 200 B s p e c t r o m e t e r b y using g ^ 2 exciting radiation. Typical operating conditions w e r g X - r a y g u h , 12 K v , 15mA; p r e s s u r e i n t h e s a m p l e chamber c a . 10 torr. Under the e x p e r i m e n t a l c o n d i t i o n s employed, the g o l d ^jl2 * l 84eV u s e d f o r c a l i b r a t i o n h a d a f u l l w i d t h a t h a l f maximum (FWHM) o f 1.2 e V . No e v i d e n c e was o b t a i n e d f o r r a d i a t i o n damage t o t h e s a m p l e s from l o n g - t e r m e x p o s u r e t o t h e X - r a y beam. O v e r l a p p i n g peaks were r e s o l v e d i n t o t h e i r individual components b y use o f a DuPont 310 c u r v e r e s o l v e r ( a n a n a l o g c o m p u t e r ) . The d e t a i l e d d e c o n v o l u t i o n s w e r e b a s e d on a k n o w l e d g e o f l i n e w i d t h s d e t e r m i n e d from t h e h o m o p o l y m e r s and m o d e l compounds s t u d i e d p r e v i o u s l y . ( 4 ) S t u d i e s h a v e shown t h a t f o r i n d i M

e

K

a

v

e

a

t


19.

THOMAS A N D O ' M A L L E Y

Multicomponent Polymer Systems

321

v i d u a l components o f t h e c o r e - l e v e l s p e c t r a f o r t h e C and 0 ^ l e v e l s , the l i n e shapes approximate f i a r l y c l o s e l y t o G a u s s i a n .

g

R e s u l t s and D i s c u s s i o n A) R e g u l a r B l o c k C o p o l y m e r s . 1) PolyCethylene o x i d e ) / P o l y ( s t y r e n e ) D i b l o c k Copolymer. P r e v i o u s s t u d i e s (5 6) on b u l k m o r p h o l o g y o f t h e PEO/PS c o p o l y mers i n d i c a t e d t h a t t h e i n d i v i d u a l components a r e i n c o m p a t i b l e and t h a t t h e y u n d e r g o m i c r o p h a s e s e p a r a t i o n and domain f o r m a t i o n . We c a n a n t i c i p a t e b a s e d upon t h e d i f f e r e n c e s i n s o l i d state s u r f a c e t e n s i o n b e t w e e n t h e two p o l y m e r s ( 7 ) t h a t t h e s u r f a c e o f t h e s o l i d c o p o l y m e r may w e l l v a r y from t h e b u l k . In order to e s t a b l i s h a f i r m b a s i s f o r the i n t e r p r e t a t i o n o f the d i b l o c k c o p o l y m e r d a t a , i t i s n e c e s s a r y t o s t u d y t h e component h o m o p o l y m e r s , p o l y s t y r e n e ( P S ) and p o l y ( e t h y l e n e o x i d e ) ( P E O ) , and d e t e r m i n e t h e i r a b s o l u t e and r e l a t i v e b i n d i n g e n e r g i e s and r e l a t i v e peak i n t e n s i t i e s . The XPS c o r e l e v e l s p e c t r a f o r PS and PEO a r e shown i n F i g u r e 1 and t h e e x p e r i m e n t a l b i n d i n g e n e r g i e s and peak i n t e n s i t y r a t i o s are t a b u l a t e d i n Table I I . The PS s p e c t r u m has a s t r o n g peak c e n t e r e d a t 285 eV a s s o c i a t e d w i t h t h e d i r e c t p h o t o i o n i z a t i o n o f t h e C^ c o r e l e v e l s and a l o w - e n e r g y s a t e l l i t e peak a t 2 9 1 . 6 eV a r i s i n g from s h a k e - u p t r a n s i t i o n s ( I T * + IT ) a c c o m p a n y i n g c o r e i o n i z a t i o n . These low-energy shake-up t r a n s i t i o n s are understood both t h e o r e t i c a l l y and e x p e r i m e n t a l l y and t h e s p e c t r u m o b s e r v e d i n F i g u r e 1 i s e n t i r e l y consistent with previous work.(8-13) The s p e c t r a f o r PEO show a s i n g l e peak f o r t h e C l e v e l s at 286.5 eV, r e f e r e n c e d t o h y d r o c a r b o n a t 285 e V , and a s i n g l e peak f o r t h e 0 ^ core l e v e l s a t 5 3 3 . 3 e V . The peak a r e a r a t i o o f t h e C ^ t o 0 , core l e v e l s i s 0 . 7 3 ; h o w e v e r , when t h e s e peak a r e a s a r e c o r r e c t e d f o r the d i f f e r e n t t h e o r e t i c a l p h o t o i o n i z a t i o n c r o s s s e c t i o n s ( 1 4 ) , e l e c t r o n mean f r e e p a t h s ( L 5 ) r e l a t i v e t o t h e C ^ and 0 ^ core level electrons, and instrumental factors the correct s t o i c h i o m e t r y f o r PEO i s o b t a i n e d . The c h e m i c a l s h i f t o f ^ 1.5 eV f o r t h e C c o r e l e v e l peak i n P E O , r e l a t i v e t o P S , c a n be a t t r i b u t e d t o e a c h c a r b o n b e i n g a t t a c h e d t o an o x y g e n atom and i s c o n s i s t e n t w i t h t h e o r e t i c a l p r e d i c t i o n s and e x p e r i m e n t a l r e s u l t s on r e l a t e d l o w - m o l e c u l a r w e i g h t m o d e l compounds. ( 4 , _16) As e x p e c t e d , t h e r e i s no s h a k e - u p peak i n The C ^ c o r e l e v e l s p e c t r u m f o r PEO s i n c e t h e p o l y m e r i s fully saturated. (17) These s i g n i f i c a n t d i f f e r e n c e s i n XPS s p e c t r a o f PS and P E O , i . e . , t h e 1.5 eV c h e m i c a l s h i f t i n t h e C^ c o r e l e v e l s , the uniqueness o f the TT * « - TT s h a k e - u p peatc a s s o c i a t e d w i t h t h e PS c o m p o n e n t , c o u p l e d w i t h t h e peak i n t e n s i t y r a t i o s from T a b l e I I , e n a b l e a u n i q u e a n a l y s i s 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 the PS/PEO s y s t e m . 9

l

g

g

s

g

l

g

g

g


322

PHOTON,

I *f^i i I II I I 536 533 530 B.E.(ev)

i

i

i

i I I *n i

293

I

289 BE(ev)

i

i

ELECTRON,

A N D ION PROBES

I I 285

Figure 1. The C-ls and O-ls core-level spectra for polystyrene and poly (ethylene oxide) homopolymers


19.


323


Table I I Experimental Binding Energies and Peak Area Ratios for the Reference Homopolymers, Polystyrene and Poly(ethylene oxide) binding energy, C

polystyrene

285.0 286.5

peak area ratios

ls

( T * * poly(ethylene oxide)

8

V)

291.6

C^ (PS/C^ (PEO) = 1.60 • 0.1 s

533.3

s

C (PEO)/0 (PEO) - 0.73 + 0.05 ls

lg

C (7r * * n )(PS)/C (PS)=0.081 lg

ls

+ 0.005

Referenced to hydrocarbon at 285.0 eV.

F i l m s o f t h e t h r e e b l o c k c o p o l y m e r s w e r e c a s t from c h l o r o f o r m , a m u t u a l s o l v e n t f o r PS and PEO, (6^) and t h e m e a s u r e d and 0c o r e l e v e l s p e c t r a a r e shown i n F i g u r e 2 . The s p e c t r a show t h e c h a r a c t e r i s t i c 0^ peak o f P E O , t h e s h a k e - u p s a t e l l i t e o f P S , and an e a s i l y d e c o n v o i u t e d d o u b l e t f o r t h e c o r e l e v e l s i n PS and PEO. I t i s a p p a r e n t from t h e s p e c t r a t h a t t h e PS c o n c e n t r a t i o n a t t h e c o p o l y m e r s u r f a c e i n c r e a s e s as t h e PS i n t h e c o p o l y m e r i n c r e a s es. More i m p o r t a n t l y , h o w e v e r , a n a n a l y s i s o f t h e s p e c t r a l d a t a c l e a r l y shows t h a t t h e s u r f a c e c o m p o s i t i o n s a r e s i g n i f i c a n t l y r i c h e r i n PS t h a n w o u l d be p r e d i c t e d b a s e d on a k n o w l e d g e o f t h e b u l k c o m p o s i t i o n s o f t h e b l o c k c o p o l y m e r s . I n F i g u r e 3 i s shown a p l o t o f the s u r f a c e - v s - b u l k compositions f o r the d i b l o c k c o p o l y mers . Up t o t h i s p o i n t a l l t h e XPS measurements h a v e b e e n made b y a n a l y z i n g the photoemitted e l e c t r o n s normal t o the surface o f the sample under i n v e s t i g a t i o n . T h i s e x p e r i m e n t a l arrangement i s the most commonly u s e d i n XPS s t u d i e s , and w i t h i t t h e e f f e c t i v e sampling depth i s maximized. F o r s y s t e m s s u c h as P S / P E O , t h e e f f e c t i v e s a m p l i n g d e p t h i s a b o u t 50 A ; i . e . , a b o u t 95% o f t h e s i g n a l comes from t h e o u t e r m o s t ^ 5 0 A , b a s e d upon a p r e v i o u s k n o w l e d g e o f t h e e l e c t r o n mean f r e e p a t h s a p p r o p r i a t e t o p h o t o e m i t t e d e l e c t r o n s from t h e C ( ^ 960 e V ) and 0 ( ^ 720 e V ) c o r e l e v e l s . C5) T h e r e f o r e , t h e s u r f a c e c o m p o s i t i o n d a t a shown i n F i g u r e 3 a r e tl^e a v e r a g e c o m p o s i t i o n on t h i s e f f e c t i v e s a m p l i n g d e p t h o f ^ 50 A . T h e s e measurements c a n be f u r t h e r r e f i n e d b y c o n t r o l l a b l y decreasing the e f f e c t i v e sampling depth. This w i l l , i n effect, a l l o w f o r d e p t h p r o f i l i n g t h e c o m p o s i t i o n o f t h e u p p e r 50 A o f t h e s u r f a c e and p r o v i d e us w i t h a means t o e x p l o r e t h e m o l e c u l a r o r g a n i z a t i o n o f t h e s u r f a c e m a c r o m o l e c u l e s . The method f o r v a r y i n g t h e e f f e c t i v e s a m p l i n g d e p t h 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. The s a m p l e i s r o t a t e d r e l a t i v e t o t h e f i x e d p o s i t i o n e n e r g y a n a l y z e r by a n g l e 0 , d e s i g n a t e d as t h e a n g l e b e t w e e n t h e n o r m a l t o t h e sample and t h e s l i t s i n t h e a n a l y z e r . I t i s r e a d i l y seen that e l e c t r o n s c o l l e c t e d at g r a z i n g e x i t angles r e l a t i v e t o the surface ( 6 9 0 ° ) w i l l enhance s u r f a c e f e a t u r e s r e l a t i v e t o e l e c t r o n s c o l l e c t e d normal to the s u r f a c e . (15,17,18) l

g

l

g

Q


324

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

Figure 2.

PROBES

The C-ls and O-ls core-level spectra for the three PS/PEO diblock copolymers cast from chloroform


19.



325

100

20 40 60 Bulk composition mole

Figure 3.

80 PS

100

Surface vs. bulk compositions for PS/PEO diblock (%) and triblock(A) copolymers (XPS (6) = 0°)

EFFECTIVE ~J SAMPLING d DEPTH JL

T EFFECTIVE > SAMPLING £ DEPTH d/COS0

X(X) 10 15 20 30

TYPICAL MEAN FREE PATHS IN POLYMERS

C

|s

9 5 % SIGNAL (&) 30 45 60 90

(960eV)«l5A

Figure 4. Angularly dependent studies for polymer samples in which spectra are studied as a function of electron take-off angle 6 with respect to the sample surface


326

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

I n F i g u r e 5 i s shown t h e C and 0 ^ c o r e l e v e l s p e c t r a from t h e a n g u l a r d e p e n d e n t XPS s t u d i e l , XPS? 0 ) , on c o p o l y m e r B c a s t from c h l o r o f o r m . S p e c t r a were r e c o r d e d a t t h r e e d i f f e r e n t angles, 0 = 0 , 4 5 , and 8 0 ° , i n o r d e r t o a c h i e v e effective sampling depths of ^50, ^ 2 5 , and ^ 10 A , r e s p e c t i v e l y . Both the and 0 ^ c o r e l e v e l s p e c t r a c l e a r l y show t h a t e a c h component of the copolymer i s present at a l l sampling depths. Furthermore, from t h e r e l a t i v e i n t e n s i t y r a t i o s o f t h e d e c o n v o l u t e d C ^ peaks a s s o c i a t e d w i t h t h e PS and PEO c o m p o n e n t s , we f i n d t h a t i n p a r t i c u l a r c a s e s , t h e r e i s a c o n c e n t r a t i o n g r a d i e n t i n t h e t o p 50 A o f t h e s u r f a c e and t h a t t h e r e l a t i v e c o n c e n t r a t i o n o f PS and PEO i n c r e a s e s as t h e a i r - s o l i d p o l y m e r i n t e r f a c e i s a p p r o a c h e d . A l t h o u g h t h e f i n d i n g on t h e c o m p o s i t i o n a l v a r i a t i o n b e t w e e n t h e s u r f a c e and t h e b u l k i s i n t e r e s t i n g t h e XPS( 0 ) s t u d i e s r e v e a l a much more i n t e r e s t i n g phenomena i n t h e t o p o g r a p h y . Through the use o f d i f f e r e n c e s i n t h e e l e c t r o n mean f r e e paths(15^) f o r p h o t o e m i t t e d e l e c t r o n s from Cand 0 , c o r e l e v e l s and t h e r e l a t i v e Is LS c o r e l e v e l i n t e n s i t y r a t i o s f o r C ^ g / O ^ g a s s o c i a t e d w i t h t h e PEO component a u n i q u e s u r f a c e s t r u c t u r e emerges f o r t h e t o p o g r a p h y , and t h e m o d e l s a r e i l l u s t r a t e d i n F i g u r e 6 . The m o d e l s a - c f o r t h e s u r f a c e t o p o g r a p h y c a n be e l i m i n a t e d from a c a r e f u l e x a m i n a t i o n o f the e x p e r i m e n t a l r e s u l t s . However t h e e x p e r i m e n t a l d a t a s u p p o r t s m o d e l d and f o r t h e s e d i b l o c k c o p o l y m e r s c a s t from c h l o r o f o r m , a m u t u a l s o l v e n t f o r PS and P E O , t h e r e a p p e a r s t o be a t e n d e n c y t o form v e r t i c a l i s o l a t e d domain s t r u c t u r e s at a l l the copolymer c o m p o s i t i o n s s t u d i e d , w i t h a n o n - p l a n a r s u r f a c e o f PS p r o t r u d i n g above t h e PEO d o m a i n . 2) P o l y ( e t h y l e n e o x i d e ) / P o l y ( s t y r e n e ) T r i b l o c k ( P E 0 / P S / P E 0 ) C o p o l y m e r s . A p p l y i n g t h e XPS ( 0 ) t e c h n i q u e t o t h e s e samples c a s t from c h l o r o f o r m r e s u l t s i n t h e s u r f a c e - v s - b u l k c o m p o s i t i o n f o r t h e t r i b l o c k copolymers i l l u s t r a t e d i n F i g u r e 3, a l o n g w i t h the d i b l o c k c o p o l y m e r s d i s c u s s e d p r e v i o u s l y . The s t r i k i n g s i m i l a r i t y i n t h e d a t a f o r t h e d i b l o c k and t r i b l o c k c o p o l y m e r s i n d i c a t e t h a t t h e surface topography i s s i m i l a r i n both cases. The e x p e r i m e n t a l e v i d e n c e i n d i c a t e s t h a t t h e PS and PEO components i n t h e c o p o l y m e r s a r e b o t h e x p o s e d a t t h e s u r f a c e and t h a t t h e y a r e o r g a n i z e d i n t o domains w h i c h a r e t h i c k compared t o t h e XPS s a m p l i n g d e p t h . (3,15) A s l i g h t a n g u l a r dependence i s o b s e r v e d as 0 i s v a r i e d and t h i s a g a i n p o i n t s t o a n o n - p l a n a r s u r f a c e t o p o g r a p h y i n w h i c h PS domains a r e e l e v a t e d s l i g h t l y above t h e PEO d o m a i n s . T h e r e h a v e b e e n numerous s t u d i e s e m p l o y i n g c a l o r i m e t r i c ( 1 9 ) , dynamic m e c h a n i c a l , ( 2 0 ) d i e l e c t r i c , ( 2 1 ) and m o r p h o l o g i c a l ( 2 3 , 2 4 ) techniques to e l u c i d a t e the s o l i d - s t a t e b e h a v i o r o f s t y r e n e - e t h y l ene o x i d e b l o c k c o p o l y m e r s . These measurements h a v e f o c u s e d on t r a n s i t i o n - t e m p e r a t u r e phenomena, and t h e y h a v e p r o v i d e d r e f e r e n c e d a t a on t h e b u l k p r o p e r t i e s o f t h e c o p o l y m e r s . The e v i d e n c e a c c u m u l a t e d t o d a t e i n d i c a t e s t h a t PS and PEO a r e i n c o m p a t i b l e i n t h e b u l k . W h i l e t h i s a p p e a r s t r u e , i n g e n e r a l , one c a n n o t r u l e o u t t h e p o s s i b i l i t y t h a t PS and PEO h a v e some l i m i t e d d e g r e e o f m i s c i b i l i t y i n the c o p o l y m e r s . I t i s a l s o unknown, a t t h i s t i m e , what i n f l u e n c e an i n t e r f a c e ( e . g . , t h e a i r - p o l y m e r i n t e r f a c e ) has l

g

g

g


19.



327

Figure 5. The C-ls and O-ls core-level spectra for XPS(O) studies on PS/PEO diblock copolymer (B) cast from chloroform. The experimental intensity ratios are corrected for absolute signal from Table II to obtain the molar ratios.


328

PHOTON,

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

on p o l y m e r - p o l y m e r c o m p a t i b i l i t y and w h e t h e r t h e d e g r e e o f p h a s e s e p a r a t i o n i s t h e same o r d i f f e r e n t i n t h e b u l k and i n t h e s u r f a c e . The p r e s e n t s t u d y , i n p a r t , a d d r e s s e s t h e s e i s s u e s by u n i q u e l y f o c u s i n g on t h e s u r f a c e p r o p e r t i e s r a t h e r t h a n on t h e b u l k p r o p e r t i e s o f the copolymers. Our d a t a s u g g e s t t h a t , i n d e e d , t h e r e i s some c o m p a t i b i l i z a t i o n o f PS and P E O . I n t h i s s e c t i o n , we w i l l d i s c u s s t h e i n t e r e s t i n g r e s u l t s on the i /°i intensity ratios for t h e PEO component and the C ^ ( TT *•«- TT ) / C - ^ i n t e n s i t y r a t i o s f o r t h e PS component i n t h e t r i b l o c k copolymers. T a b l e I I I c o n t a i n s the measured i n t e n s i t y r a t i o s f o r t h e two homopolymers and t h e t h r e e t r i b l o c k c o p o l y m e r s . The t r i b l o c k s d e v i a t e from t h e homopolymers i n two w a y s : First, they have u n u s u a l l y h i g h ^ / 0 - ^ r a t i o s f o r t h e PEO component compared t o t h e PEO h o m o p o l y m e r , and s e c o n d , t h e y have u n u s u a l l y l o w C ( TT *

Surface Studies on Multicomponent Polymer Systems by X-Ray

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