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Chemical Labels to Distinguish Surface Functional Groups Using X-Ray Photoelectron Spectroscopy (ESCA) C. D. BATICH Central Research and Development Department, E . I. du Pont de Nemours and Co., Experimental Station, Wilmington D E 19898 R. C. WENDT Polymer Products Department,E. I. du Pont de Nemours and Co., Experimental Station, Wilmington, D E 19898
Polymer surface chemistry is frequently dominated by the number and types of functional groups present on the surface. Because different groups can have the same elemental composition, a knowledge of atomic ratios is generally inadequate. Much of the ESCA work with polymers attempts to distinguish functional groups by looking at small chemical shifts and deconvoluting overlapping peaks. This approach works best for systems containing fluorine, where chemical shifts are large. Some researchers have tried to avoid this problem by specifically tagging or derivatizing certain functional groups and analyzing for a novel element which has been introduced. This paper w i l l review what has been done using the second approach and w i l l outline what is desirable for such a study. Our results for carboxylic acid group analysis w i l l also be discussed. Some common functional groups which are difficult to distinguish are shown in Figure 1. Carboxylic acids, esters, ketones, alcohols and aldehydes have overlapping oxygen 1s spectra. While i t is generally possible to distinguish between singly and doubly-bonded oxygen, e.g. between alcohols and ketones, combinations of these moieties may require meticulous curve resolution and an expert eye. Many organic nitrogen species are overlapping, including amide, amine and nitrogen in certain heterocyclic structures. Carbon-carbon unsaturation, as well as phenyl groups, are difficult to distinguish from saturated species. Clark (1) has made progress using shake-up structures as distinguishing features, but the amount of shake-up structure present seems to be a variable, depending on the matrix in certain cases, according to Thomas et al.(2). An early attempt to identify a functional group by derivtization and ESCA study, was published by Riggs and Dwight in 1974 (Fig. 2) (3). Treatment of polytetrafluoroethylene (PTFE) with sodium in ammonia produced a surface depleted in fluorine. To 0097-6156/81/0162-0221 $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.
222
PHOTON,
ELECTRON,
A N DION
OXYGEN -C0 H
ACID
-C0 R
ESTER
-C=0 \ R
KETONE
-CH-OH I
ALCOHOL
2
2
R
APPROXIMATE E 532-534
B
ALDEHYDE
-CHO NITROGEN 0 II -C-NR
AMIDE 2
RN
AMINE
400-401
3
R-CN
HETEROCYCLE NITRILE
CARBON DOUBLE BOND PHENYL GROUP -CH,—
Figure 1.
285
SATURATED
Poorly resolved functional groups
Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
PROBES
15.
BATICH
223
Surface Functional Groups
A N D W E N D T
REACTION
SUBSTRATE
Br Br I I • C-C
Br
2
No TREATED PTFE
c=c-
Riggs 8 Dwight (1974)
H 0
BOVINE ALBUMIN
PLASMA INITIALED PAA GRAFTS ON PP
Et-S-C(0)-CF
®-NH
BaClo
2
— C0 ") Ba 2
H0
++
A. Bradley 8 M. Czuha, Jr. (1975)
2
2
Br Br I I ->C-C-
Br
2
C=C-
| -CH
MMA / HYDROXYPROPYL MA
3
3
->-C0 "Ag 2
2
2
CORONA TREATED LDPE
-C0 H-
NaOH
C0 "Na 2
2
0 II -CH -C-
2
r
C=0- —
Spell 8 Christienson (1978) Briggs 8 Kendall Polymer (1979)
+
Br 0 I II ->-C-C I Br
Br
2
F
+
Br Br I I ->-C-C
Br -C=C-
J. Hammond et al (1978)
2
AgN0
-C0 ~Na"
Briggs et al (1977)
O-C
(CF C0) 0 —
H
2
CORONA TREATED PE
Figure 2.
M. Millard 8 Masri (1974)
2
-C0 H
MELTING PE ON Al
EPOXY / ESTER PRIMER
^N-C-CFx
3
r
_ H _
N
H
H
NH
2
F
F
>C=N-NH-^^-F
XPS studies using polymer surface derivatization
Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
224
PHOTON,
ELECTRON,
A N D
ION
PROBES
d e t e r m i n e w h e t h e r u n s a t u r a t i o n was p r e s e n t , t h e s u r f a c e was e x p o s e d to b r o m i n e and t h e n a n a l y z e d f o r the p r e s e n c e o f this element. The N a - t r e a t e d PTFE s u r f a c e d i d show i n c r e a s e d b r o m i n e , w h e r e a s the u n t r e a t e d PTFE s u r f a c e d i d n o t . A study of b o v i n e serum-albumin l a b e l i n g w i t h e t h y l t h i o l t r i f l u o r o a c e t a t e at about the same t i m e showed a r o u g h r e l a t i o n s h i p to known a m i n o - g r o u p concentration (4). P o l y a c r y l i c a c i d g r a f t s on p o l y p r o p y l e n e were s t u d i e d u s i n g an i o n - e x c h a n g e i n t e r a c t i o n w i t h a B a s a l t (_5). B r i g g s i n i t i a l l y s t u d i e d t h e e f f e c t s o f m e l t i n g p o l y e t h y l e n e on a l u m i n u m m e t a l u s i n g the b r o m i n e t e s t f o r u n s a t u r a t i o n ( 6 ) and h a s s i n c e been e x a m i n i n g c o r o n a - t r e a t e d p o l y e t h y l e n e w i t h a v a r i e t y of labels (7). T h i s s y s t e m has a l s o been s t u d i e d by S p e l l and Christensen (8). Hammond et a l . s t u d i e d a m e t h y l m e t h y a c r y l a t e and h y d r o x y p r o p y 1 m e t h a c r y l a t e copolymer s e r i e s reacted with t r i f l u o r o a c e t i c a n h y d r i d e (9^). They a l s o t r e a t e d an e p o x y - e s t e r p r i m e r w i t h s i l v e r s a l t s and found r e a s o n a b l e labeling (10). These f i r s t e x p e r i m e n t s a p p e a r t o have b e e n done on w e l l - d e f i n e d s u b s t r a t e s and a f u l l e r d e s c r i p t i o n o f the h y d r o x y 1 work has now been p r e s e n t e d . B r i g g s has u s e d two methods t o c h a r a c t e r i z e h i s system: ( 1 ) an i o n exchange method u s i n g c a r e f u l w a s h i n g , and ( 2 ) r e a c t i o n o f the c a r b o n y l f u n c t i o n o b t a i n e d , f o r m i n g a p h e n y l hydrazone. A l m o s t a l l o f the work w h i c h has been done t o d a t e has i n v o l v e d a s u b s t r a t e of l o o s e l y defined s t o i c h i o m e t r y , that i s to s a y , an e x p e r i m e n t a l l y t r e a t e d p o l y m e r s u r f a c e o f unknown comp o s i t i o n . In s e v e r a l cases i n d i c a t e d , chemical r e a c t i o n produced a d d i t i o n o f an oxygen s p e c i e s , w h i c h was n o t e x p e c t e d . For i n s t a n c e , no o x y g e n - c o n t a i n i n g r e a g e n t was u s e d i n the s o d i u m t r e a t e d PTFE o r i n some o f or A r c o r o n a - t r e a t e d low-density p o l y e t h y l e n e , y e t c o p i o u s amounts o f o x y g e n a t e d s p e c i e s were formed. What i s needed f o r a method t h a t c a n be s a i d to q u a n t i t a t i v e l y l a b e l the s u r f a c e o f a p o l y m e r i s o u t l i n e d b e l o w . Method Requirements for q u a n t i t a t i v e d e r i v a t i z a t i o n o f p o l y mers: Primary 1.
Q u a n t i t a t i o n o f major p e a k s on the c l e a n , p r e - l a b e l e d , mer .
2.
Q u a n t i t a t i v e l a b e l o f standard polymer s u r f a c e s c o n t a i n i n g s e v e r a l l e v e l s of independently determined functionality, i n c l u d i n g c o n t r o l s w i t h zero l e v e l s .
3.
S e l e c t i v i t y when o t h e r
4.
Well defined laboratories.
5.
S u r v e y s c a n r u n and a l l m a j o r (10% o f l a b e l ) p e a k s q u a n t i f i e d - e s p e c i a l l y the common c o n t a m i n a n t s C, 0 , Pb and S i .
known f u n c t i o n a l g r o u p s
experimental
conditions
are
reproducible
poly-
present. between
Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
15.
BATICH
A N D
W E N D T
Surface Functional Groups
225
Secondary 6.
S e n s i t i v i t y (High
7.
Stability.
cross-section).
8.
C o n s i s t e n c y w i t h o t h e r o b s e r v a t i o n s on " r e a l - w o r l d " s a m p l e s ; e . g . , adhesion, contact angle, etc. These c o n d i t i o n s , as f a r as we c a n d e t e r m i n e , h a v e so f a r n o t a l l b e e n f u l f i l l e d f o r any d e r i v a t i z a t i o n s y s t e m . I n a d d i t i o n to the r e a c t i o n s w h i c h have been e x a m i n e d , a number have a l s o b e e n s u g g e s t e d or t r i e d u n d e r n o n - o p t i m u m c o n ditions. I n 1 9 7 7 , a t a N o r t h e a s t ESCA U s e r s Group (NEUG) M e e t i n g , Wendt d i s c u s s e d p o s s i b l e l a b e l i n g r e a c t i o n s u s i n g N a , Ag o r Pb as labels f o r c a r b o x y l i c a c i d s and a l s o the b r o m i n e i n c a r b o n t e t r a c h l o r i d e method o r b r o m i n e gas f o r u n s a t u r a t i o n . Hydrochlor i c a c i d was p r o p o s e d as a good l a b e l f o r f r e e amino g r o u p s , f o r m a t i o n o f d i n i t r o p h e n y l h y d r a z o n e f o r c a r b o n y l s and a l s o c h l o r ide l a b e l for a l c o h o l s . D w i g h t W i l l i a m s , a t a s u b s e q u e n t NEUG m e e t i n g i n May o f 1979 a l s o p r e s e n t e d a number o f p o s s i b l e reactions: m e r c u r i c a c e t a t e , b r o m i n e , i o d i n e and IC1 r e a c t i o n w i t h u n s a t u r a t i o n , HC1 l a b e l i n g o f e x p o x i d e s , H I f o r e t h e r s , c a r b o n d i s u l f i d e w i t h amines and Ag w i t h t h i o - g r o u p s . I n most s t u d i e s , few e x p e r i m e n t a l v a r i a b l e s a r e s p e c i f i e d . Some o f the more i m p o r t a n t c o m p l i c a t i n g f a c t o r s a r e i n d i c a t e d i n F i g u r e 3. The d e f i n i t i o n o f a s u r f a c e i s a l s o a v a r i a b l e ( F i g u r e 4). A zone o f r e a c t i o n b e t w e e n aqueous s o l u t i o n s and h y d r o c a r b o n ( e . g . P E ) s u b s t r a t e has b e e n s u g g e s t e d by W h i t e s i d e s (_11), b u t t h i s w o u l d p r o b a b l y encompass s e v e r a l m i c r o n s d e p t h f o r r e l a t i v e ly non-polar solutes. F o r s i m p l i c i t y , we d e f i n e a s u r f a c e i n terms o f t h e d e p t h b e i n g measured and w i l l a t t e m p t t o m a i n t a i n a homogeneous l a y e r to t h a t d e p t h . F o r any m a t e r i a l , t h e f r a c t i o n o f s i g n a l a t t a i n e d f o r a g i v e n d e p t h i s assumed t o be an e x p o n e n t i a l function. To see 90% o f the f u n c t i o n a l g r o u p s p r e s e n t , one must q u a n t i t a t i v e l y d e r i v a t i z e t o a d e p t h a b o u t 2 X , and f o r 99% l a b e l , one must r e a c t to a b o u t 5 X , i n o t h e r words a b o u t 100& ( F i g u r e 5). A d e q u a t e d i f f u s i o n and r e a c t i o n may be a s i g n i f i c a n t p r o b l e m w i t h aqueous s o l u t i o n s o f c e r t a i n i o n s . F o r f l a t s a m p l e s , t h i s e f f e c t c a n be m i t i g a t e d to some e x t e n t by d o i n g a v a r i a b l e angle experiment. F i g u r e 6 shows the d e p t h f u n c t i o n f o r 99 and 90% l a b e l a t d i f f e r e n t t a k e - o f f a n g l e s . C a r b o x y l i c A c i d Group M o d e l
System
A s a c a r b o x y l a t e s t a n d a r d s y s t e m , we i n i t i a l l y u s e d a c o p o l y m e r c o n s i s t i n g o f 10% m e t h a c r y l i c a c i d (MAA) r a n d o m l y d i s t r i buted i n ethylene. A s e x t r u d e d , v a r i a b l e amounts o f o x y g e n r e l a t i v e t o c a r b o n were f o u n d , and i t was d e c i d e d t h a t c o n t a m i n a n t s or p r e d o m i n a n t l y h y d r o c a r b o n segments were b e i n g s e g r e g a t e d to the s u r f a c e . As shown i n F i g u r e 7 , t h e t h e o r e t i c a l o x y g e n - t o c a r b o n r a t i o i s 0 . 0 3 4 , w h i l e we o b s e r v e d s a m p l e s c o n t a i n i n g l e s s t h a n and a l s o more t h a n t h i s amount o f o x y g e n . By s c r a p i n g t h e
Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
226
PHOTON,
ELECTRON,
A N DION PROBES
XPS RELATED 1. ROUGHNESS 2. SURFACE SEGREGATION 3. VOLATILES LOST IN VACUUM
REACTION 1. CONTAMINATION 2. DIFFUSION OF REACTANTS POLYMER 1. STRESS 2. RADICALS GENERATED DURING PREPARATION, i.e. CUTTING 3. THERMAL HISTORY 4. UNSUSPECTED ADDITIVES, e.g. La-Pb IN POLYESTERS 5. MOLECULAR WEIGHT DISTRIBUTION (END GROUP CONCENTRATION) 6. CONFORMATION
Figure 3.
Figure 4.
Variables in a polymer labeling study
Three idealized views of a polymer surface ((%) C0 H; (^J) (CH ) ) 2
Dwight et al.; Photon, Electron, and Ion Probes of Polymer Structure and Properties ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2
n
15.
BATICH
A N D
W E N D T
Surface Functional Groups
1
1
1
1
227
i
rV
