Chapter 18
Scanning Transmission Electron Microscopy To Observe Ionic Domains in Model Ionomers 1
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C. E. Williams , C. Colliex , J. Horrion , and R. Jèrôme 1
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LURE-Centre National de la Recherche Scientifique, University of Paris-Sud, 91405 Orsay, France Physique du Solide, University of Paris-Sud, 91405 Orsay, France Laboratory of Macromolecular Chemistry and Organic Catalysis, University of Liège, Sart Tilman B6, B-4000 Liège, Belgium
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3
Small domains of high electronic density have been imaged in halato-telechelic ionomers by Scanning Transmission Electron Microscopy (STEM) using the technique of atomic number or Z-contrast. The possibility that these are ionic domains is evaluated and the morphology compared with that derived from recent SAXS experiments.
A l t h o u g h t h e r e is l i t t l e doubt that the unique m e c h a n i c a l and transport properties of i o n o m e r s a r e due to a m i c r o p h a s e s e p a r a t i o n between o r g a n i c monomers and i o n i z a b l e g r o u p s , t h e i r exact morphology is not yet u n d e r s t o o d . Direct o b s e r v a t i o n of the ionic domains by e l e c t r o n m i c r o s c o p y has y i e l d e d c o n t r o v e r s i a l r e s u l t s : l a r g e v a r i a t i o n s in s i z e and shape of what was assumed were the domains have been found, and the o b s e r v e d s t r u c t u r e has seldom a g r e e d with that d e r i v e d from X ray s c a t t e r i n g s t u d i e s . A r e c e n t c r i t i c a l e v a l u a t i o n of e l e c t r o n m i c r o s c o p y i n v e s t i g a t i o n s of i o n o m e r s (JJ c l a i m s that the images a r e e i t h e r plagued with a r t i f a c t s r e l a t e d to the p r e p a r a t i o n ( e . g . l a r g e s c a l e p h a s e s e p a r a t i o n in s o l v e n t c a s t f i l m s , s u r f a c e f e a t u r e s , m i c r o c r y s t a l s of n e u t r a l i z i n g agent) or a r e too thick to allow a d e t e r m i n a t i o n of s i z e and s h a p e of the d o m a i n s . The c o n j u n c t i o n of the r e c e n t d e v e l o p m e n t of s o p h i s t i c a t e d imaging techniques using scanning t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y (STEM) and a b e t t e r u n d e r s t a n d i n g and c o n t r o l of the p r o c e s s of ion a g g r e g a t i o n in model i o n o m e r s p r o m p t e d us to attempt again to image d i r e c t l y the ionic domains in s o l v e n t - c a s t f i l m s of b a r i u m n e u t r a l i z e d c a r b o x y t e l e c h e l i c s (£). 0097-6156/89/0395-0439$06.00A) ο 1989 American Chemical Society
Utracki and Weiss; Multiphase Polymers: Blends and Ionomers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
440
MULTIPHASE POLYMERS: BLENDS AND IONOMERS Indeed r e c e n t s t r u c t u r a l
studies
of a , ω - c a r b o x y l a t o -
polybutadiene and p o l y i s o p r e n e by small angle X - r a y scattering (SAXS) have shown that p r o v i d e d the chains between ionic groups have a narrow distribution in length, t h e r e is m i c r o p h a s e separation of the ionic m o n o m e r s into s m a l l domains with s h a r p i n t e r f a c i a l b o u n d a r i e s . The domains a r e d i s t r i b u t e d in the bulk of the polymer at an a v e r a g e distance on the o r d e r of the e n d - t o - e n d distance of the chain between ionic g r o u p s . Few ionic groups a r e i n c o r p o r a t e d in the organic m a t r i x . So each small ionic a g g r e g a t e can be viewed as being s u r r o u n d e d by a l a r g e r volume on the o r d e r of the chain length from which all other domains a r e e x c l u d e d . Hence, for a B a p o l y b u t a d i e n e , it was found that the domains have a r a d i u s of g y r a t i o n of 7 ± 1 A , assuming s p h e r i c a l shape, and a r e s e p a r a t e d by an a v e r a g e distance of 75 A for a chain of M~ = 4600. Of importance n
for our investigation are the facts that (i) t h e r e is l a r g e e l e c t r o n i c density d i f f e r e n c e between s h a r p l y l o c a l i z e d i o n i c domains (multiplets) and the o r g a n i c m a t r i x p r o v i d i n g good amplitude c o n t r a s t and (ii) the distance between the domains can be i n c r e a s e d by v a r y i n g the m o l e c u l a r weight of the p o l y m e r . Experimental For
this
electron
microscopy
examination,
we
used
c a r b o x y l i c p o l y s t y r e n e n e u t r a l i z e d with Ba, (PS-Ba) of FT
W
and F T / F T w
n
= 1.24.
a di-
= 60 000
The e n d - t o - e n d distance is about 198 A . Note
that no SAXS a n a l y s i s is p o s s i b l e f o r such a m o l e c u l a r weight because the "ionomer peak" is v e r y weak (low ionic content) and its p o s i t i o n is such that it is washed out by the s m a l l angle upturn o b s e r v e d in all i o n o m e r s . The s a m p l e s were cast d i r e c t l y on a 250 A - thick c a r b o n film s u p p o r t e d on a copper g r i d . A known amount of a dilute solution of PS-Ba in t o l u e n e containing 0.1 % methanol was d e p o s i t e d on the g r i d , the s o l v e n t was s l o w l y e v a p o r a t e d under dry n i t r o g e n , then the f i l m s were dried at 2 5 C in v a c u u m . A s s u m i n g uniform s p r e a d of the f i l m s o v e r the entire g r i d , the nominal film t h i c k n e s s should be between 250 A and 500 A . T h r e e f i l m s were then o b s e r v e d without staining with a VG-HB 501 STEM m i c r o s c o p e operating at 100 kV. We used the s o - c a l l e d atomic number or Z-contrast method in which the a n n u l a r dark f i e l d image intensity p r o d u c e d by the e l a s t i c a l l y s c a t t e r e d e l e c t r o n s is divided by the i n e l a s t i c component of the bright field image intensity, simultaneously r e c o r d e d (£). With this e
Utracki and Weiss; Multiphase Polymers: Blends and Ionomers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
18. WILLIAMS ET AL.
STEMTo Observe Ionic Domains
441
t e c h n i q u e , f i r s t i n t r o d u c e d by Crewe and d e v e l o p e d r e c e n t l y
for
biological applications (Z), the ratio signal is p r i m a r i l y dependent on the only
atomic number of the weakly
elements in the
dependent on t h i c k n e s s and
sample (I rati ο ~ ^ z
density ( £ ) . The
a
n
d
spatial
r e s o l u t i o n was on the o r d e r of 5 to 6 Â. It is mostly g o v e r n e d by the size of the f i e l d emission probe which was near optimal size with a s e m i - a n g l e of emission of 7.5 m r a d (2).
Result? an 100 Â) o b s e r v e d on some m i c r o g r a p h s c o u l d be due to i m p u r i t i e s p r e s e n t in the s o l u t i o n ( s i l i c o n was d e t e c t e d by X - r a y a n a l y s i s ) . A s l i g h t e x c e s s of n e u t r a l i z i n g agent could a l s o be present, although it has not been in evidence otherwise in the bulk s a m p l e s . The f a v o r a b l e experimental data on s a m p l e s p r e p a r e d in s i m i l a r conditions l e a d us to believe that ionic domains have been v i s u a l i z e d in the t e l e c h e l i c ionomers w h e r e m i c r o p h a s e s e p a r a t i o n is known to o c c u r . However m o r e e x p e r i m e n t s with films of different nature, t h i c k n e s s , and p o l y m e r m o l e c u l a r weight a r e n e c e s s a r y to a s c e r t a i n the nature of these domains. Rather than a detailed study of the morphology of h a l a t o t e l e c h e l i c s , this note is i n t e n d e d to show that with the i m p r o v e d r e s o l u t i o n and c a p a b i l i t i e s of e l e c t r o n m i c r o s c o p y and with the b e t t e r c o n t r o l of the m o l e c u l a r c h a r a c t e r i s t i c s of ionomers, more r e s e a r c h should be d e v o t e d to a d i r e c t c h a r a c t e r i z a t i o n of the morphology of these m a t e r i a l s . Techniques
Utracki and Weiss; Multiphase Polymers: Blends and Ionomers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
444
MULTIPHASE POLYMERS: BLENDS AND IONOMERS
developed f o r other f i e l d s such as biology could be advantageously utilized.
Acknowledgments Thanks a r e due to M a r c e l Tencé and P a s c a l Delzenne f o r help with the experiment and to E . L . Thomas and D.L. Handlin, J r . f o r their comments.
Literature cited 1 - Handlin, D.L. ; Macknight, W.J. ; Thomas, E.L. Macromolecules, 1981, 14, 795 2 - Broze, G. ; Jerome, R. ; Teyssie, Ph. ; Marco, C. Macromolecules, 1985 15, 1376 , and other references therein. 3 - Williams, C . E . ; Russell, T.P. ; Jerome, R. ; Horrion, J . Macromolecules, 1986 19, 287 4 - Ledent, J . ; Fontaine, F. ; Reynars, H. ; Jerome, R.Polym. Bull. 1985 14, 461 5 - Williams, C.E. ; "Contemporary Topics in Polymer Science", Vol. 6, W.H. Culbertson, ed., Plenum Publ. 1989 6 - It is worth noting here that in catalysis studies, small metallic clusters, of dimension down to 10 Å, have been clearly visualized by annular dark field imaging and ratio contrast. See for instance Treacy, M. ; Howie, A. ; Wilson, C . J . ; Phil. Mag. 1978 A38, 5698 and Delcourt, Μ. O. ; Belloni, J. ; Marigner, J . L. Mory, C. ; Colliex, C. ; Rad. Phys. Chem. 1984 23, 4785 7 - Crewe, A.V. ; Langmore E, J.P. ; Isaacson, M.S. "Physical Aspects of Electron Microscopy and Analysis", Ed. Siegel B.M. and Beaman D.R., Wiley 1975 8 - Colliex, C. ; Jeanguillaume, C. ; Mory, C., J. Ultrastructure Res. 1984 88, 177 9 - Mory, C. ; Ph. D Thesis, University of Paris-Sud, Orsay, 1985 RECEIVED February 22, 1989
Utracki and Weiss; Multiphase Polymers: Blends and Ionomers ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
