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Structure of High Modulus Fibers of Poly-pPhenylene Benzbisthiazole 2
ERIC J. ROCHE1, TOSHISADA TAKAHASHI , and EDWIN L. THOMAS Department of Polymer Science and Engineering, The University of Massachusetts, Amherst, M A 01003
Poly-p-phenylene b e n z b i s t h i a z o l e , with repeat u n i t has r e c e n t l y been synthesized as part o f the " A i r Force Ordered Polymer Research Program" ( 1 ) . This program, d i r e c t e d towards the preparation o f very high s t r e n g t h , high temperature r e s i s t a n t polymers, has l e d to the synthesis o f various rigid rod polymers, among which PBT shows the most promising p r o p e r t i e s . PBT f i b e r s spun from a nematic dope o f the polymer can e x h i b i t a higher modulus than the well-known poly-p-phenylene terephthamide (PPT) fibers. In previous work, the x-ray d i f f r a c t i o n pattern o f PBT f i b e r s was i n t e r p r e t e d as a r i s i n g from a nematic arrangement o f PBT molecules ( 2 ) ; t h i s arrangement was derived on the assumption that PBT molecules can be considered as cylindrically symmetric, which in t u r n , suggested a hexagonal packing o f such rods. Although the c h a r a c t e r i s t i c features o f the x-ray p a t t e r n , i.e., broad e q u a t o r i a l r e f l e c t i o n s , meridional streaks and the absence o f other ( h k l ) r e f l e c t i o n s were fully e x p l a i n e d , t h i s model does not agree with the observed d e n s i t y o f the fibers. E l e c t r o n microscopic data presented i n t h i s paper suggest another, more ordered model. This study a l s o illustrates the c a p a b i l i t i e s o f e l e c t r o n microscopy techniques, p a r t i c u l a r l y e l e c t r o n d i f f r a c t i o n coupled with dark field imaging, f o r the c h a r a c t e r i z a t i o n o f the s t r u c ture o f f i b r o u s m a t e r i a l . Experimental Material. A 9.85 weight percent nematic s o l u t i o n o f PBT i n methane s u l f o n i c a c i d was dry j e t - w e t spun using a tapered glass c a p i l l a r y o f 96 ym e x i t r a d i u s . The f i b e r was coagulated i n a Permanent Address: CERMAV-CNRS, 53X, 38041 Grenoble-cedex,France Permanent Address: F a c u l t y o f E n g i n e e r i n g , Fukui U n i v e r s i t y , Bunkyo 3-9-1, Fukui 910, Japan
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0-8412-0589-2/80/47-141-303$05.00/0 © 1980 American Chemical Society French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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50/50 methanol/sulfolane bath at room temperature. E l e c t r o n Microscopy. A JEOL 100 CX e l e c t r o n microscope, operated at 100 kV, was used throughout t h i s work. To prepare t h i n specimens, f i b e r s immersed i n water were repeatedly peeled i n t o small fragments with the a i d of sharp needles. The f i b r i l l a r fragments were then d i r e c t l y picked up on carbon coated g r i d s . In some c a s e s , the suspension was m i l d l y sonicated to a i d in d i s persal of the fragments. D a r k f i e l d (DF) imaging was performed with the t i l t e d beam technique, the r e f l e c t i o n s e l e c t e d by a 6 x 1 0 " r a d . o b j e c t i v e aperture. A l l p i c t u r e s were recorded on Kodak SO-163 f i l m s , with maximum m a g n i f i c a t i o n (DF) of 10,000 X. 3
Results and Discussion Electron D i f f r a c t i o n . Preparation o f the f i b e r s f o r e l e c tron microscopy i s i l l u s t r a t e d in Figure 1 which shows the image (scanning e l e c t r o n microscopy) of a p a r t i a l l y peeled f i b e r . The i n t e r n a l f i b r i l l a r s t r u c t u r e i s q u i t e apparent. Repeated s p l i t t i n g gives fragments s u i t a b l e f o r transmission e l e c t r o n microscopy (TEM), as shown in Figure 2. Small f i b r i l s o f v a r i a b l e w i d t h , as small as a cross s e c t i o n of about 70 A , are observed in some regions. An e l e c t r o n d i f f r a c t i o n p a t t e r n , o r i e n t e d as i n d i c a t e d i n the F i g u r e , could be e a s i l y recorded from the h i g h l y o r i e n t e d f i b r i l l a r bundles. Three types of patterns were o b t a i n e d , as shown i n Figures 2, 3a and 3b r e s p e c t i v e l y , i n d i c a t i n g d i f f e r i n g degrees of order f o r the same s t r u c t u r e , encountered in d i f f e r e n t l o c a t i o n s o f a given f i b e r . A n a l y s i s of the most ordered patterns was most i n s t r u c t i v e . In a d d i t i o n to the very high number o f e q u i d i s t a n t meridional streaks (up to 20 orders being observable on the n e g a t i v e s ) , which correspond to a f i b e r repeat o f 12.35 A, the e q u a t o r i a l r e f l e c t i o n s are well r e s o l v e d , and i n d i c a t e the f o l l o w i n g spacings (the l e t t e r s f o l l o w i n g each distance have t h e i r usual meaning f o r the r e l a t i v e observed i n t e n s i t i e s ) : 5.83 S 3.54 VS 3.16 M (2.96) VW
1.82 M-W (1.75) VW 1.71 W
F a i n t , smeared (hkl) r e f l e c t i o n s are a l s o observed, but are not resolved enough to be used f o r u n i t c e l l determination. Equat o r i a l and meridional spacings i n d i c a t e the f o l l o w i n g monoclinic eel 1s: Unit C e l l I: a = 5.83, b = 3.54, c = 12.35 Y = 96°, z = 1 Unit C e l l I I : a = 7.10, b = 6.65, c = 12.35 Y = 63°, z = 2
French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Figure 1. Scanning electron micrograph of a partially peeled PBT fiber
French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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Figure 2. Bright field electron micrograph of a fibrillar fragment of a PBT fiber and corresponding electron diffraction pattern
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Figure 3. Electron diffraction patterns fromfibrillarfragments of a PBT fiber exhibiting different degrees of order. The arrow points toward the reflection used for DF imaging. Note the splitting of this reflection, indicating a higher degree of order as compared to the same reflection in the pattern of Figure 2.
C = 12.35
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Unit c e l l I corresponds to a very simple arrangement of p a r a l l e l sheets, whereas Unit c e l l II (Figure 4) would allow more p o s s i b i l i t i e s , i . e . , t r a n s l a t i o n or r o t a t i o n o f the center chain with r e s pect to the c o m e r c h a i n . The u n i t c e l l s have a c a l c u l a t e d dens i t y of 1.69, which c l o s e l y matches the observed density of approximately 1.6 f o r a f i b e r without macroscopic v o i d s . The value of the f i b e r repeat corresponds e x a c t l y to the length of the r e peat u n i t (3J. Confirmation of the monoclinic packing, s e t t i n g angle and choice between the two c e l l s may be p o s s i b l e with more h i g h l y ordered f i b e r s . Nevertheless, the present packing already allows a very reasonable i n t e r p r e t a t i o n of the d i f f e r e n t e l e c t r o n d i f f r a c t i o n patterns. I f we assume a nearly planar molecular conformation ( i n the c r y s t a l s t r u c t u r e o f the model compound [ 3 ] , the angle between the two moieties o f the repeat u n i t i s 2 3 ° ) , the PBT " c r y s t a l l i t e " may be s c h e m a t i c a l l y shown as in Figure 5 (an a l l - p a r a l l e l packing i s taken f o r s i m p l i c i t y ) . The schematic emphasizes the twodimensional c h a r a c t e r of t h i s o r d e r i n g , which e x p l a i n s e s s e n t i a l features of the d i f f r a c t i o n p a t t e r n s . Small random t r a n s l a t i o n s of the chains along t h e i r axes causes the l o s s of a l l non-equat o r i a l r e f l e c t i o n s and the appearance of r e c i p r o c a l l a t t i c e d i s c s as observed, f o r example, i n pattern 3b. Such d i s o r d e r i s t y p i cal of r i g i d rod systems. It has already been observed from nematic f i b e r s of p o l y - y - b e n z y l - l - g l u t a m a t e (4) and c e r t a i n a r a mid f i b e r s ( 5 j . In comparison to these c a s e s , the t r a n s l a t i o n a l freedom of PBT molecules i s enhanced by the absence of hydrogen bonding. D i f f r a c t i o n patterns as in Figure 3a and Figure 2 are much more f r e q u e n t l y observed than pattern 3b. These patterns can be considered to a r i s e from c r y s t a l l i t e s of smaller l a t e r a l extent with more complete t r a n s l a t i o n a l freedom along the chain axes. The number of meridional streaks i s unaffected by the extent of t h i s d i s o r d e r , as i t derives from the r i g i d i t y o f the molecules. D a r k f i e l d E l e c t r o n Microscopy. A l l d a r k f i e l d images below have been obtained from the strongest e q u a t o r i a l r e f l e c t i o n , arrowed in Figure 3b. D a r k f i e l d images obtained from patterns s i m i l a r to the one shown i n Figure 2 do not e x h i b i t high c o n t r a s t . Figure 6 i s such a DF image obtained from patterns s i m i l a r to the pattern in Figure 3. Small c r y s t a l l i t e s are r e g u l a r l y d i s t r i b uted throughout the fragment. Such features are comparable to other observations on c e r t a i n PPT f i b e r s 05) or PE f i b e r s (6). The strongest d i f f r a c t i n g c r y s t a l l i t e s are elongated in shape with t h e i r average length being about 5 times l a r g e r than t h e i r w i d t h , which ranges from approximately 60 to 80 A. The more numerous, smaller c r y s t a l l i t e s are d i f f i c u l t to d i s t i n g u i s h from the background, due to i n e l a s t i c s c a t t e r i n g . Figure 7 shows c o r responding b r i g h t f i e l d (BF) and DF images of a peeled fragment. Although no p e c u l i a r c o n t r a s t i s observed in the BF image, a very marked banding transverse to the f i b e r d i r e c t i o n appears in the
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Figure 6. Equatorial dark field image of a fragment of a PBT fiber
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DF image. This o b s e r v a t i o n , a g a i n , i s comparable to what has been seen i n PPT f i b e r s (j5). In the t h i n n e r part of the fragment the banding i s r e g u l a r , and each transverse dark band, ~ 200 A in w i d t h , makes an angle of about 70° with the f i b e r d i r e c t i o n . The p e r i o d i c i t y of the banding i s of about 1200 A, as compared to 5,000 A f o r PPT f i b e r s . The r e s o l u t i o n i s not s u f f i c i e n t i n t h i s image to p r e c i s e l y describe the c h a r a c t e r i s t i c s of the c r y s t a l lites. When fragments of the same f i b e r s are m i l d l y s o n i c a t e d , t h i n n e r r i b b o n l i k e f i b r i l s are observable, as shown i n Figure 8 (left). Each o f these f l a t ribbons appears to c o n s i s t of smaller " m i c r o f i b r i l s " of l a t e r a l dimensions varying from 50 to 80 A as p r e v i o u s l y mentioned. These ribbons e x h i b i t a wavy t e x t u r e , each m i c r o f i b r i l changing i t s d i r e c t i o n i n r e g i s t e r with i t s neighbors. The corresponding DF image (Figure 8, r i g h t ) shows a c h a r a c t e r i s t i c banding a s s o c i a t e d with the "waves". The s i z e of well defined c r y s t a l l i t e s in the DF image i s about 40 x 150 A but a great number of smaller c r y s t a l l i t e s i s a l s o noted. T h e i r long d i r e c t i o n makes a s l i g h t angle (10° to 20°) with the f i b e r d i r e c t i o n , s i m i l a r to that made by the m i c r o f i b r i l s in the corresponding parts o f the BF image. The c o n t r a s t between dark and b r i g h t zones depends on the sharpness of the k i n k s . As i n e l a s t i c s c a t t e r i n g i s small because of the thinness of the r i b b o n s , the grey background i s mainly due, here, to d i f f u s e s c a t t e r i n g from l e s s ordered r e g i o n s . This l a s t observation suggests that o r i e n t a t i o n c o n d i t i o n s , and not n o n c r y s t a l l i n e zones, are r e s p o n s i b l e f o r the observation of the dark zones. T h i s i s f u r t h e r demonstrated i n Figure 9, which shows an "s-shaped" f i b e r . As the f i b e r p r o g r e s s i v e l y changes i t s o r i e n t a t i o n , the small zones (A) reverse t h e i r cont r a s t from dark to b r i g h t , and conversely f o r the l a r g e zones (B). T h e r e f o r e , textured ordered regions are present a l l along the r i b b o n , with neighboring c r y s t a l l i t e s i n approximately the same o r i e n t a t i o n w i t h i n a band. The banding period here v a r i e s from 1,000 to 2,000 A. A double system o f banding i s a l s o noted (see region C in Figure 9 ) . From the above o b s e r v a t i o n s , a schematic o f the texture of these kinked ribbons i s proposed in Figure 10. The ribbon i s b u i l t up of c l o s e l y packed m i c r o f i b r i l s , well apparent i n the kink zones. Each m i c r o f i b r i l c o n s i s t s o f a succession o f narrow c r y s t a l l i t e s embedded i n a somewhat l e s s ordered m a t r i x . The l e f t hand p o r t i o n o f the schematic i l l u s t r a t e s the banding observed i n the DF image. Whether the bands appear due to the f r a g mentation of the f i b e r s during sample p r e p a r a t i o n , or are charact e r i s t i c of the as-spun f i b e r s , i s not known at present. The n o n l i n e a r s t r e s s - s t r a i n and the elongation*at break (3%) suggest the bands may be shear bands. Whatever t h e i r o r i g i n , the bands r e f l e c t the s u s c e p t i b i l i t y o f the f i b r i l s to transverse kinking or b u c k l i n g . Development of a s k i n - c o r e morphology i n the coagulation process may e x p l a i n the
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Figure 7. BF (right) and DF (left) images of a fragment of a PBT fiber showing the band structure which appears in DF
Figure 8. BF (left) and DF (right; images of ribbonlike fragments of a PBT fiber showing the fibrillar texture of the ribbons
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Figure 9. Electron micrograph showing orientation effects on the banded structure
Figure 10.
Schematic of the fibrillar structure of the ribbonlike fragments obtained after peeling and mold sonication of PBT fibers
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discrepancy between d a r k f i e l d images (banded or n o t ) , so they may a l s o e x p l a i n the d i f f e r e n t degrees o f order encountered. Conclusion As already i l l u s t r a t e d by a previous paper {$) on the s t r u c ture o f high modulus f i b e r s , e l e c t r o n microscopy can be very succ e s s f u l when a p p l i e d to these beam-resistant m a t e r i a l s , and so c o n s t i t u t e s an e s s e n t i a l complement to x-ray s t u d i e s . In the present work, e l e c t r o n d i f f r a c t i o n coupled with BF and DF imaging has allowed d e t e c t i o n of the best ordered zones w i t h i n PBT f i b e r s which i l l u s t r a t e s the s t r u c t u r e p o s s i b l y obtainable by f i b e r proc e s s i n g refinement. The well ordered s t r u c t u r e s observed thus f a r compare r a t h e r w e l l , with the exception o f t h e i r f i b r i l l a r t e x t u r e , to the s t r u c t u r e o f PPT high-modulus f i b e r s . The two dimensional c h a r a c t e r o f the c r y s t a l l i t e s i s l i k e l y due to the freedom o f a x i a l t r a n s l a t i o n o f the molecules. Future work should determine i f t h i s f e a t u r e i s a d i r e c t consequence o f the chemical s t r u c t u r e o f the PBT molecule o r i s simply the r e s u l t o f non-optimized processing c o n d i t i o n s . Acknowledgements The f i b e r s t u d i e d was k i n d l y provided by P r o f e s s o r G. Berry of Carnegie-Mellon U n i v e r s i t y . The authors thank Mr. S. A l l e n f o r f u r n i s h i n g the SEM p i c t u r e o f Figure 1. We a l s o thank Dr. A. Kulshreshtha and Mr. W. Adams f o r h e l p f u l d i s c u s s i o n s throughout t h i s work. F i n a n c i a l support was r e c e i v e d from the U.S. A i r Force through c o n t r a c t #F33615-78-C-5175 and the M a t e r i a l s Research Laboratory o f the U n i v e r s i t y o f Massachusetts. One o f us (EJR) i s indebted to the CNRS f o r f a v o r i n g h i s stay a t the U n i v e r s i t y o f Massachusetts.
Literature Cited 1. 2. 3. 4. 5. 6.
Helminiak, T.E. 177th ACS Meeting, Hawaii, 1979, 675. Adams, W.W.; Azaroff, L.V.; Kulshreshtha, A.K. Z. Kristal., in press. Fratini, A.V.; Wiff, D.R.; Wellman, M.W.; Adams, W.W., to be published. Samulski, E.T.; Tobolsky, A.V. Biopolym., 1971, 10, 1013. Dobb, M.G.; Johnson, D.J.; Saville, B.P. J . Polym. S c i . , 1977, 58, 217. Gohil, R.M.; Petermann, J . J . Polym. S c i . , Polym. Phys. Ed., 1979, 17, 525.
RECEIVED May 21,
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French and Gardner; Fiber Diffraction Methods ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
