PolymerBulletin9, 382-389 (1983)

Polymer Bulletin 9 Springer-Verlag 1983

Rheo-Optical Fourier Transform IR Spectroscopy of Polyurethane Elastomers 1. Principle of the Method and Measurements at Ambient Temperature H.W. Siesler Bayer AG, Werk Dormagen, Forschung und Entwicklung, Posffach 1140, D-4047 Dormagen, Federal Republic of Germany In memoriam Pro~ D~ Offo Bayer SUMMARY R h e o - o p t i c a l FTIR s p e c t r o s c o p y has emerged as an e x t r e m e l y valuable tool to study d e f o r m a t i o n p h e n o m e n a in p o l y m e r i c solids. With the aid of a s p e c i a l l y d e s i g n e d stretching apparatus short-time spectroscopic and m e c h a n i c a l data can be o b t a i n e d s i m u l t a n e o u s l y during the d e f o r m a t i o n and r e l a x a t i o n of polymers. P o l y u r e t h a n e s are p a r t i c u l a r l y suited to such investigations because they contain functional groups with c h a r a c t e r i s t i c IR a b s o r p t i o n s which can be a s s i g n e d to specific domain locations of the polymer. Apart from a general i n t r o d u c t i o n to the principle of the technique and its a p p l i c a t i o n to p o l y u r e t h a n e s the data obtained with a series of three model p o l y e s t e r urethanes of d i f f e r e n t hard and soft segment content at a m b i e n t t e m p e r a t u r e are d i s c u s s e d in terms of the segmental o r i e n t a t i o n i n d u c e d during uniaxial e l o n g a t i o n and recovery. INTRODUCTION The m e c h a n i c a l p r o p e r t i e s of p o l y m e r i c m a t e r i a l s are of c o n s i d e r a b l e i m p o r t a n c e for their e n g i n e e r i n g applications. In this respect the unders t a n d i n g of the m o l e c u l a r m e c h a n i s m s involved in p o l y m e r d e f o r m a t i o n is a n e c e s s a r y p r e r e q u i s i t e for a r e a s o n a b l e s t r u c t u r e - m o r p h o l o g y - p r o p e r t y correlation. An e x t r e m e l y powerful m e t h o d for the study of transient p h e n o m e n a in p o l y m e r d e f o r m a t i o n and r e l a x a t i o n is rheo-optics which describes the relation b e t w e e n stress, strain and an optical quantity (for example birefringence, IR absorption, light s c a t t e r i n g or X-ray diffraction) m e a s u r e d s i m u l t a n e o u s l y with stress and strain as a function of time (READ 1962, S T E I N 1966, ONOGI and A S A D A 1971). The advent of r a p i d - s c a n n i n g FTIR systems has t r e m e n d o u s l y expanded the a p p l i c a t i o n of vibrational spectroscopy in the field of rheo-optics (SIESLER and H O L L A N D - M O R I T Z 1980) and will c e r t a i n l y stimulate further p r o g r e s s in this research area. EXPERIMENTAL The r h e o - o p t i c a l spectra were obtained on a N i c o l e t 7199 FTIR spectrom e t e r w i t h a N i c o l e t 1280 64K computer. The e l e c t r o m e c h a n i c a l apparatus c o n s t r u c t e d for the simultaneous m e a s u r e m e n t of F T I R spectra and s t r e s s - s t r a i n diagrams during elongation, r e c o v e r y and stress r e l a x a t i o n of p o l y m e r films is shown s c h e m a t i c a l l y in Fig. I. By e x p l o i t i n g the a u t o m a t e d information p r o c e s s i n g c a p a b i l i t y of the d e d i c a t e d computer in the F T I R system m u c h of the routine analysis of spectra series as they are p r o d u c e d in rheo-optical m e a s u r e m e n t s has been a l l e v i a t e d w i t h the aid of s p e c i f i c a l l y d e v e l o p e d BASIC software (SCHLEMMER

383

"~-,- stres--~s ~-~ '-~7 ~-...... ~- strain F I G U R E i F i l m s t r e t c h i n g machine: (i) F T I R detector, (2) p n e u m a t i c polarizer unit, (3) clamp, (4) p o l y m e r film sample, (5) stress transducer, (6) d i s p l a c e m e n t transducer, (7) d r i v i n g motor, (8) h e a t i n g accessory, (9) cartridge heater, (iO) t e m p e r a t u r e control, (ii) KBr window, (12) specimen p r e p a r a t i o n and transfer device. and S I E S L E R 1981). Thus, for the e v a l u a t i o n of the p o l a r i z a t i o n m e a s u r e ments where a series of spectra are recorded a l t e r n a t e l y with light p o l a rized p a r a l l e l and p e r p e n d i c u l a r to the d i r e c t i o n of e l o n g a t i o n a p r o g r a m is a p p l i e d which, based on the p e a k m a x i m u m or i n t e g r a t e d intensity, autom a t i c a l l y c a l c u l a t e s the d i c h r o i c ratio or o r i e n t a t i o n function (in the case of w e l l - d e f i n e d t r a n s i t i o n m o m e n t directions) of s p e c i f i e d a b s o r p t i o n bands for each s p e c t r u m by a p p r o p r i a t e l y c o r r e l a t i n g the s u c c e s s i v e l y m e a s u r e d i n t e n s i t y values. Upon data p r o c e s s i n g with this routine the individual v a l u e s can s u b s e q u e n t l y be p l o t t e d as a function of strain. The r e s u l t s have b e e n o b t a i n e d from a series of three p o l y e s t e r urethanes s y n t h e s i z e d from d i p h e n y l m e t h a n e - 4 , 4 ' - d i i s o c y a n a t e , a d i h y d r o x y t e r m i n a t e d adipic a c i d / b u t a n e d i o l / e t h y l e n e glycol p o l y e s t e r (molecular w e i g h t 2000) and butane diol as chain extender w i t h p o l y e s t e r : c h a i n ext e n d e r : d i i s o c y a n a t e m o l a r ratios of 1.O:2.2:3.4 (a), 1.O:5.4:6.6 (b) and 1.O:7.5:8.7 (c), respectively. The soft segments of this type of polyu r e t h a n e b a s i c a l l y c o n s i s t of the r e a c t i o n p r o d u c t s of the d i i s o c y a n a t e c o m p o n e n t and the m a c r o g l y c o l , w h e r e a s the hard segments c o n t a i n l a r g e l y aromatic and butane diol m o i e t i e s linked t o g e t h e r by u r e t h a n e groups. Films of the d i f f e r e n t p o l y e s t e r u r e t h a n e s were p r e p a r e d under identical c o n d i t i o n s w i t h a t h i c k n e s s of a p p r o x i m a t e l y O . O 1 0 m m by c a s t i n g from 2% w / v DMF solutions on s u r f a c e - r o u g h e n e d glass p l a t e s and d r y i n g at 323 K in v a c u u m for 6 h. The film samples were then r e m o v e d from the glass plates in hot water, b o i l e d for 0.5 h and finally dried in v a c u u m at 323 K for 0.5 h. The d e n s i t i e s of the d i f f e r e n t p o l y e s t e r u r e t h a n e s as m e a s u r e d in a gradient column (carbontetrachloride/chlorobenzene) were 1.236 gcm -3 (a), 1.251 gcm -3 (b) and 1.257 gcm -3 (c), respectively. In the m e c h a n i c a l t r e a t m e n t film specimens of 12 mm length and IO mm w i d t h were e l o n g a t e d at a d r a w rate of 1.2% strain/s to 220% strain and then u n l o a d e d at the same rate to zero stress and 12-scan spectra were

384

8

(a)

wovenurnber (crn-I)

6oo

sbo

e00

56o

~oo

~o

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~

~

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~

z~oo

10C 8r 6c o

(c)

~4c

4ooo

wavenumber

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F I G U R E 2 IR spectra of the i n v e s t i g a t e d p o l y e s t e r urethane films based on d i f f e r e n t p o l y e s t e r : c h a i n e x t e n d e r : d i i s o c y a n a t e molar ratios: (a) 1.0:2.2:3.4, (b) 1.0:5.4:6.6, (e) 1.O:7.5:8.7. taken

in 14-seconds intervals at a r e s o l u t i o n of 2 cm -I. F r o m DSC m e a s u r e m e n t s the hard segments were found to m e l t in dependence of the c o m p o s i t i o n in the temperature range from 400 K to 490 K and the c o r r e s p o n d i n g heats of m e l t i n g increased w i t h hard segment content from 4.8 jg-i (a) to 7.9 jg-i (b) and 13.7 jg-1 (c). In the w i d e - a n g l e X-ray diagrams only for the p o l y e s t e r urethane (c) with the largest hard segment content the Bragg r e f l e c t i o n at about 0.75 nm (BONART et al. 1974, B L A C K W E L L et al. 1981) could be d e t e c t e d apart from the intense amorphous halo at about 0.45 nm. Furthermore, no s t r a i n - i n d u c e d c r y s t a l l i n i t y of the soft segments was o b s e r v e d in the 200% elongated samples.

385

RESULTS A N D D I S C U S S I O N The IR spectra of the i n v e s t i g a t e d p o l y e s t e r u r e t h a n e s are shown in Fig. 2. On the basis of the e s t a b l i s h e d f r e q u e n c y c o r r e l a t i o n s for the functional groups of p o l y u r e t h a n e s (ISHIHARA et al. 1974) the extent of hard and soft segment o r i e n t a t i o n can be m o n i t o r e d by means of the polar i z a t i o n p r o p e r t i e s of the V(NH) (3331 cm -I) and ~(CH2) (2959 cm -I) absorption bands, respectively. The t r a n s i t i o n m o m e n t anglos for b o t h v i b r a tions have b e e n taken as 90 ~ a l t h o u g h it is r e c o g n i z e d that some d e v i a t i o n of this value may occur for the ~(CH2) s t r e t c h i n g v i b r a t i o n due to superp o s i t i o n w i t h the c o r r e s p o n d i n g w a g g i n g mode. Under these a s s u m p t i o n s the o r i e n t a t i o n functions for the hard and soft segments (FRASER 1956) read: f =

(R-l) -2. - (R+2)

(1)

where R = AII/A& is the e x p e r i m e n t a l l y d e t e r m i n e d dichroic ratio of the v(NH) and M(CH2) a b s o r p t i o n bands, respectively. Generally, for p e r f e c t p a r a l l e l chain a l i g n m e n t f is i, for p e r p e n d i c u l a r a l i g n m e n t f is -i/2 and for random o r i e n t a t i o n f b e c o m e s zero. In v i e w of the hard s e g m e n t crystal structure p r o p o s e d by B L A C K W E L L et al. (1981), however, the m a x i m u m value to be e x p e c t e d for the M(NH) o r i e n t a t i o n function of the hard segments is only about 0.65. F u r t h e r m o r e it should be kept in m i n d that the i n t e n s i t y c o n t r i b u t i o n s of the M(CH2) a b s o r p t i o n bands of the chain extender and the d i p h e n y l m e t h a n e f u n c t i o n a l i t y i n c r e a s i n g l y c o n t r i b u t e to the M(CH2) absorption i n t e n s i t y of the soft segments in the sequence p o l y e s t e r u r e t h a n e (a) to (c). N e v e r t h e l e s s , the p r i n c i p a l d i f f e r e n c e s of hard and soft s e g m e n t o r i e n t a t i o n can be r e a d i l y d e r i v e d from the s p e c t r o s c o p i c data. The close r e l a t i o n b e t w e e n the c o m p o s i t i o n and the m e c h a n i c a l p r o p e r ties of these p o l y m e r s is r e f l e c t e d in the s t r e s s - s t r a i n d i a g r a m s m e a s u r e d at 300 K (Fig. 3). Hence, for the specified e x p e r i m e n t a l c o n d i t i o n s a distinct increase of initial m o d u l u s (ii, 45 and 120 MNm-2), s t r e s s - h y s t e r e s i s (ratio of area b o u n d e d by a strain cycle to the total area u n d e r n e a t h the e l o n g a t i o n curve: 60, 80 and 90%), and extension set (30, 65 and 1OO%) can be o b s e r v e d w i t h i n c r e a s i n g hard segment c o n t e n t for the p o l y e s t e r urethanes (a) to (c)

25

300 K

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0

' , 100 150 strain (N)

=

200

250

F I G U R E 3 S t r e s s - s t r a i n d i a g r a m s of the l o a d i n g - u n l o a d i n g cycles of the i n v e s t i g a t e d p o l y e s t e r u r e t h a n e films w i t h d i f f e r e n t h a r d and soft s e g m e n t c o m p o s i t i o n (see text) at 300 K.

386

The segmental structure of p o l y u r e t h a n e s becomes apparent from Fig. 4 w h i c h shows the linear p o l y m e r p r i m a r y chains made up of a l t e r n a t i n g hard and soft segments. However, in the solid p o l y m e r the p r i m a r y chains do not r e a l l y exist s e p a r a t e l y but rather the hard segments tend to associate with each other t h r o u g h h y d r o g e n bonding and aromatic q-electron attraction. As a consequence, the hard segments form domains in the mobile soft segm e n t m a t r i x and a two-phase system results. The separate hard segment domains e f f e c t i v e l y c r o s s l i n k the p r i m a r y chains and produce a network w h i c h accounts for the elastic character of the polymer. These virtual crosslinks can be r e v e r s i b l y overcome by heat or solvation w h e r e u p o n the p o l y m e r p r i m a r y chains are more or less regenerated. Numerous studies are available on the segmental structure of p o l y u r e t h a n e s p a r t i c u l a r l y the hard segment domains (BONART et al. 1974, B L A C K W E L L et al. 1981, BORN et al. 1982) which m i g h t be e x p e c t e d to be crystalline but do not appear to be by c o n v e n t i o n a l c r y s t a l l i n i t y tests. It seems that the strong mutual attraction of the h a r d segments restricts their a b i l i t y to readily organize themselves into a c r y s t a l l i n e lattice. In this respect p r i m a r i l y small-angle X-ray s c a t t e r i n g (WILKES and Y U S E K 1973) and electron m i c r o s c o p y (FRIDMAN and THOMAS 1980) have c o n t r i b u t e d to a better knowledge of the size, order and s e p a r a t i o n of these domains. Recently, B O N A R T and H O F F M A N N (1982) have shown that the extent of o r i e n t a t i o n of the hard segment phase during e l o n g a t i o n depends on the m o r p h o l o g y of the hard segment domains and the i n t e r r e l a t i o n s h i p of two d e f o r m a t i o n m e c h a n i s m s based on a m o r p h o l o g i c a l and a m o l e c u l a r level. As long as the chains of the soft segments are randomly coiled the m a t r i x can be r e g a r d e d as a c o n t i n u u m and the hard segment domains will be oriented by a c o n t i n u u m m e c h a n i c a l transfer of stress with their long axis dimension into the d i r e c t i o n of stretch. Therefore, small, fibrillar hard segments in w h i c h the long axis d i m e n s i o n coincides with the p o l y m e r chain axes (see Fig. 4, F) will take up a p o s i t i v e o r i e n t a t i o n while lamellar d o m a i n s with their long axis d i m e n s i o n p e r p e n d i c u l a r to the p o l y m e r chain axes (see Fig. 4, L) will be n e g a t i v e l y oriented. This d e f o r m a t i o n mechan i s m d o m i n a t e s at low strains up to about 150% strain and the strain value of the m a x i m u m negative o r i e n t a t i o n depends on the stability of the lamellar m o r p h o l o g y and the length of the soft segments. With i n c r e a s i n g exten-

heat or

solvent H FIGURE

4

S

S c h e m a t i c of phase separation in a p o l y u r e t h a n e elastomer: H hard segments, S soft segments; F fibrillar hard segments, L lamellar h a r d segments.

387

II

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0.40

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0.20

3500

3300 3 1 0 0 2900 wavenumbers

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3300 3 1 0 0 2900 wavenumbers

2700

F I G U R E 5 F T I R s p e c t r a of p o l y e s t e r u r e t h a n e (c) in the ~(NH) and ~(CH2) s t r e t c h i n g v i b r a t i o n r e g i o n r e c o r d e d at 300 K d u r i n g u n i a x i a l e l o n g a t i o n to 220% strain and s u b s e q u e n t r e c o v e r y to zero stress w i t h r a d i a t i o n p o l a rized a l t e r n a t e l y p a r a l l e l and p e r p e n d i c u l a r to the d i r e c t i o n of stretch.

388

sion of the soft segments a m o l e c u l a r transfer of stress by individual chains becomes operative. For lamellar hard segment domains this m e c h a n i s m leads to a d i s r u p t i o n of the i n i t i a l l y t r a n s v e r s e l y oriented structural units with a s u b s e q u e n t positive o r i e n t a t i o n of the fragments into the d i r e c t i o n of stretch. In the case of fibrillar hard segments c o n t i n u u m and m o l e c u l a r m e c h a n i c a l stress transfer both contribute to a p o s i t i v e alignm e n t of the p o l y m e r chains. To i l l u s t r a t e the dichroic effects the p o l a r i z a t i o n spectra taken d u r i n g a l o a d i n g - u n l o a d i n g cycle of p o l y e s t e r urethane (c) at 300 K in the 3500 - 2700 cm -I w a v e n u m b e r region are shown in a t h r e e d i m e n s i o n a l repres e n t a t i o n in Fig. 5. In Fig. 6 the c o r r e s p o n d i n g orientation functions of the V(NH) and ~(CH2) a b s o r p t i o n bands have been p l o t t e d for the investigated p o l y e s t e r u r e t h a n e s versus strain. Despite the c o m p a r a t i v e l y low values of these o r i e n t a t i o n functions (see above) distinct differences b e t w e e n the hard and soft segment orientation, respectively, can be derived both during e l o n g a t i o n as well as recovery w i t h i n c r e a s i n g hard segment content. Generally, up to the m a x i m u m e l o n g a t i o n of 220% strain the soft segments

8

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FIGURE 6 O r i e n t a t i o n f u n c t i o n - s t r a i n plot of the hard and soft segments of the p o l y e s t e r u r e t h a n e s (a) - (c) as m o n i t o r e d by the v(NH) (A) and V~H2) (1) a b s o r p t i o n bands, respectively, at 300 K.

389

exhibit a better average alignment in the direction of stretch than the hard segments. Additionally, the soft segments directly respond to the application of stress with an almost linear increase in positive chain orientation as a function of strain. In contrast, an initial strain interval with orientation function values in the vicinity of zero is observed for the hard segments before the onset of significant positive orientation. This difference can be explained in terms of the abovementioned antagonism of lamellar hard segment alignment during elongation. In this region obviously the positive orientation of small fibrillar hard segments is compensated by the negative orientation of lamellar hard segment domains. The shift in the onset of significant positive orientation to higher strain values for increasing soft segment content (polyester urethane (c) to (a)) is an indication of the corresponding increase in soft segment length. Upon recovery to zero stress the orientation is more effectively retained by the hard segments. This phenomenon may be attributed to the entropy-driven relaxation and flow of the soft chain segments during unloading. As the soft segments relax they exert a tension on the hard segments thereby imposing an additional barrier to their recovery. The larger amount of this residual orientation for increasing hard segment proportion is the consequence of a more extensive disruption of the hard segments during elongation and their reorganization during recovery. Further experimental evidence of the aforementioned structural changes occurring during elongation and recovery in this class of polymers will be reported in subsequent papers on rheo-optical FTIR investigations of NHdeuterated specimens and studies at elevated temperature. ACKNOWLEDGEMENTS The author gratefully acknowledges the experimental assistance of H. Devrient, H. P. Schlemmer and W. Schmitt and helpful discussions and the supply of the model polyester urethanes from Dr~ H. Hespe. The author also thanks Bayer AG for the permission to publish the experimental data. REFERENCES B. E. READ:

Polymer 3, 143 (1962). R. S. STEIN: J. Polym. Sci. C15, 185 (1966). S. ONOGI, and T. ASADA: Progr. Polym. Sci. Jap. 2, 261 (1971). H. W. SIESLER, and K. HOLLAND-MORITZ: Infrared and Raman Spectroscopy of Polymers, Marcel Dekker, New York, 1980. H. P. SCHLEMMER, and H. W. SIESLER: unpublished data, 1981. R. J. BONART, L. MORBITZER, and E. H. MULLER: J. Macromol. Sei. Phys. B9, 447 (1974). J. B L A C K W E L L , M. R. NAGARAJAN, and T. B. HOITINK: Polymer 22, 1534 (1981). H. ISHIHARA, I. KIMURA, K. SAITO, and H. ONO: J. Macromol. Sai. Phys. BIO, 591 (1974). L. BORN, H. HESPE, J. CRONE, and K. H. WOLF: Colloid & Polym. Sci. 260, 819 (1982). C. E. WILKES, and C. S. YUSEK: J. Macromol. Sci. Phys. B7, 157 (1973). X. D. FRIDMAN, and E. L. THOMAS: Polymer 21, 388 (1980). R. BONART, and K. HOFFMANN: Colloid & Polym. Sci. 260, 268 (1982).

Received January 7, accepted January 13, 1983

C