Polymer Bulletin 9, 471-478 (1983)

Polymer Bulletin 9 Springer-Verlag 1983

Rheo-Optical Fourier Transform IR (FTIR) Spectroscopy of Polyurethane Elastomers* 2. Measurements at Elevated Temperature H.W. Siesler Bayer AG, Werk Dormagen, Forschung und Entwicklung, Posffach 1140, D-4047 Dormagen, Federal Republic of Germany * In memoriam Prof. Dr. Otto Bayer SUMMARY In a p r e c e d i n g p a p e r of this study (SIESLER 1983) the a p p l i c a t i o n of r h e o - o p t i c a l F T I R s p e c t r o s c o p y to a series of m o d e l p o l y e s t e r u r e t h a n e s at a m b i e n t t e m p e r a t u r e has b e e n d i s c u s s e d w i t h r e f e r e n c e to the phase sep a r a t i o n and segmental o r i e n t a t i o n of these p o l y m e r s d u r i n g uniaxial elong a t i o n and recovery. Here, the results of v a r i a b l e - t e m p e r a t u r e F T I R measurements and r h e o - o p t i c a l F T I R i n v e s t i g a t i o n s at e l e v a t e d t e m p e r a t u r e are presented and i n t e r p r e t e d in terms of the t e m p e r a t u r e d e p e n d e n c e of h y d r o g e n b o n d i n g and s t r u c t u r a l o r g a n i z a t i o n of the hard segments. INTRODUCTION W i t h the i n t r o d u c t i o n of a v a r i a b l e t e m p e r a t u r e cell for r h e o - o p t i c a l F T I R s p e c t r o s c o p y (SIESLER 1982) the p o t e n t i a l of this technique has b e e n further e x p a n d e d to study in m o r e detail the t e m p e r a t u r e d e p e n d e n c e of the m e c h a n i c a l p r o p e r t i e s of polymers. V i b r a t i o n a l s p e c t r o s c o p i c studies at v a r i a b l e t e m p e r a t u r e have b e c o m e an i m p o r t a n t tool for the c h a r a c t e r i z a t i o n of the p h y s i c a l s t r u c t u r e of polymers. E s p e c i a l l y in c o m b i n a t i o n w i t h DSC m e a s u r e m e n t s s h o r t - t i m e spectroscopic F T I R i n v e s t i g a t i o n s in c o n t r o l l e d h e a t i n g and c o o l i n g e x p e r i m e n t s p r o v i d e a d e t a i l e d p i c t u r e of structural c h a n g e s as a function of temperature. A n y v a r i a t i o n s of s p e c t r o s c o p i c p a r a m e t e r s such as intensity, w a v e number p o s i t i o n and band shape d i r e c t l y r e f l e c t the t e m p e r a t u r e d e p e n d e n c e of the v i b r a t i o n a l b e h a v i o u r of the i n v e s t i g a t e d p o l y m e r as a c o n s e q u e n c e of c h a n g e s in the inter- and i n t r a m o l e c u l a r i n t e r a c t i o n s and the state of order. Thus, the v i b r a t i o n a l spectra of p o l y m e r s r e c o r d e d in selected temp e r a t u r e intervals are of special value in studies of m e l t i n g and r e c r y s t a l l i z a t i o n processes, thermal d e g r a d a t i o n and h y d r o g e n bonding. A l t h o u g h the energies of h y d r o g e n bonds are weak (20 - 50 kJ/mol) in c o m p a r i s o n to c o v a l e n t bonds (about 400 kJ/mol) this type of m o l e c u l a r i n t e r a c t i o n is large enough to p r o d u c e a p p r e c i a b l e f r e q u e n c y and i n t e n s i t y changes in the v i b r a t i o n a l s p e c t r a of polymers. In fact, the d i s t u r b a n c e s are so s i g n i f i c a n t that IR and Raman s p e c t r o s c o p y p r o v i d e the m o s t informative source of c r i t e r i a for the p r e s e n c e of h y d r o g e n bonds (MURTHY and R A O 1968). Generally, h y d r o g e n b o n d i n g involves the i n t e r a c t i o n b e t w e e n a p r o t o n d o n a t i n g group (RI-X-H) and a p r o t o n a c c e p t o r (Y-R2) and may be d e s c r i b e d s c h e m a t i c a l l y by: RI-X- H. 9 .Y-R2 As a c o n s e q u e n c e

of the h y d r o g e n b o n d i n g

forces the ~(XH)

and ~(YR2)

472

s t r e t c h i n g f r e q u e n c i e s will be lowered, whereas the d e f o r m a t i o n frequencies a s s o c i a t e d w i t h the m o t i o n s of the H and Y atoms p e r p e n d i c u l a r to their X-H and Y-R 2 bonds, respectively, will be increased. M o s t of the i n v e s t i g a t i o n s so far reported deal w i t h the o b s e r v e d f r e q u e n c y shift and i n t e n s i t y increase of the v(XH) stretching v i b r a t i o n upon h y d r o g e n bonding. P r i m a r i l y the frequency shift Av(XH) has been correlated w i t h various chemical and p h y s i c a l p r o p e r t i e s of the h y d r o g e n bond. W i t h i n c r e a s i n g h y d r o g e n bond strength, for example, the RX... Y distance d e c r e a s e s and this decrease is a c c o m p a n i e d by an increase in the d i f f e r e n c e b e t w e e n the a s s o c i a t e d v(XH) and the n o n a s s o c i a t e d v(XH) stretching frequency. In fact, for a number of d i f f e r e n t types of h y d r o g e n bonds relationships b e t w e e n Av(XH) and RX... Y have been e s t a b l i s h e d (PIMENTEL and SEDERH O L M 1956) and in the case of N H . . . O bonds this r e l a t i o n s h i p is e x p r e s s e d by the f o l l o w i n g equation: AT = 0.548 " 103 (3.21 - R) where A~ is given in cm -I and R in ~. the t e m p e r a t u r e d e p e n d e n c e of h y d r o g e n 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 s m a y m e a s u r e m e n t s and r h e o - o p t i c a l studies

(I)

Thus, valuable information r e g a r d i n g b o n d i n g and its influence on the be d e r i v e d from v a r i a b l e t e m p e r a t u r e at elevated temperature, respectively.

EXPERIMENTAL The spectra were o b t a i n e d on a N i c o l e t 7199 FTIR spectrometer equipped w i t h a N i c o l e t 1280 64K computer. 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 were 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 diol/ e t h y l e n e glycol p o l y e s t e r (molecular weight 2000) and butane diol as chain e x t e n d e r w i t h 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 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 e x p e r i m e n t a l and i n s t r u m e n t a l d e t a i l s of the rheo-optical investig a t i o n s and the sample p r e p a r a t i o n have b e e n d e s c r i b e d in the p r e c e d i n g p a p e r (SIESLER 1983). The r h e o - o p t i c a l m e a s u r e m e n t s of this study were p e r f o r m e d at 348 • 0.5 K in the v a r i a b l e - t e m p e r a t u r e cell of the stretching machine. A separate v a r i a b l e - t e m p e r a t u r e cell was used to record the FTIR spectra of the i n d i v i d u a l p o l y e s t e r u r e t h a n e s w i t h o u t a p p l i c a t i o n of stress d u r i n g h e a t i n g above the m e l t i n g point of the hard segments to 500 K. The h e a t i n g rate was 9 K/min and 25-scan spectra were taken in 30-second intervals w i t h a r e s o l u t i o n of 2 cm -I. For the e v a l u a t i o n of these v a r i a b l e temperature m e a s u r e m e n t s in terms of w a v e n u m b e r p o s i t i o n a peak search routine was d e v e l o p e d w h i c h a u t o m a t i c a l l y d e t e r m i n e s and s u b s e q u e n t l y plots the p e a k m a x i m a of selected a b s o r p t i o n bands in a series of spectra taken as a function of temperature. DSC d i a g r a m s of the p o l y e s t e r u r e t h a n e s were recorded on a Perkin Elmer DSC II i n s t r u m e n t w i t h a h e a t i n g rate of IO K/min. R E S U L T S AND D I S C U S S I O N The DSC d i a g r a m s of the three p o l y e s t e r u r e t h a n e s b e t w e e n 380 and 500 K are shown in Fig. i. The o b s e r v e d e n d o t h e r m s w i t h multiple peaks r e f l e c t the s e q u e n t i a l m e l t i n g and d i s r u p t i o n of the hard segment domains with different d e g r e e s of structural organization. The heats of fusion which may be r e g a r d e d as a m e a s u r e of the strength of the hard segment c r o s s l i n k s have been d e t e r m i n e d from the areas under the entire m e l t i n g endotherms. W i t h i n the c o m p o s i t i o n range studied, i n c r e a s i n g the hard segment content r e s u l t e d in h i g h e r AH values of 4.8 jg-1 (a), 7.9 jg-i (b) and 13.7 jg-1 (c). The h i g h e s t state of order in the hard segment phase of p o l y e s t e r u r e t h a n e

473

,

e(D

T 4.

b

l 2

380

410

440

470

500

temperature (K) FIGURE

i

DSC diagrams

of the p o l y e s t e r

urethanes

(a),

(b) and

(c).

(c) is also in a g r e e m e n t w i t h the o b s e r v a t i o n of the 0.75 nm B r a g g reflection (BLACKWELL et al. 1981) in the w i d e - a n g l e X-ray d i a g r a m of this polymer. However, the AH values r e p r e s e n t i n g only about 15%(a), 17% (b) and 25% (c), respectively, of that expected from a model hard segment (CAMBERLIN et al. 1982) d e m o n s t r a t e the s i g n i f i c a n t l y reduced hard segment c r y s t a l l i n i ty of the i n v e s t i g a t e d p o l y e s t e r urethanes. The FTIR spectra of the p o l y e s t e r u r e t h a n e s recorded during heating exhibited c h a r a c t e r i s t i c t e m p e r a t u r e - d e p e n d e n t features for the ~(NH), ~(C=O) and 6(NH)+~(CN) a b s o r p t i o n bands as a consequence of changes in h y d r o g e n bonding. As an example the spectra of p o l y e s t e r urethane (c) taken in the 300 - 507 K interval are shown in Fig. 2. While the intense absorption band at 3331 cm -I can be assigned to the v(NH) stretching v i b r a t i o n of the N H - g r o u p s a s s o c i a t e d through h y d r o g e n bonds [~ass(NH) ], the small shoulder at about 3440 cm -I is characteristic of the v(NH) v i b r a t i o n of the n o n h y d r o g e n b o n d e d N H - g r o u p s [Vfree(NH) ]. Similarly, the 9(C=O) band complex can be separated in a ~ass(C=O) band at 1703 cm -I (primarily contributed from h y d r o g e n b o n d e d urethane carbonyl groups) and a Vfree(C=O) b a n d at 1733 cm -I which can be p r e d o m i n a n t l y assigned to n o n b o n d e d ester carbonyl groups. W i t h increasing temperature the following spectral changes are o b s e r v e d (SRICHATRAPIMUK and C O O P E R 1978): i. The intensity of the 9free(NH) band increases at the cost of the ~ass(NH) band. 2. The peak m a x i m u m of the ~ass(NH) band is shifted toward larger wavenumbers.

474

T(K)

T(K)

L

L

3OO

300

150"

345

345

390

~ 1.00-

4as

~

390

o e0

r

435 0.50-

3400 3 1 0 0 2800 wavenumbers

480

480

507

507 i

1800

I

I

1700 1600 wavenumbers

I

1500

F I G U R E 2 F T I R spectra of p o l y e s t e r urethane film (c) recorded in the 2800 - 3500 cm -I and 14OO - 1800 cm -I w a v e n u m b e r region during heating at 9 K/min from 300 K to 507 K.

3. The half width of the ~ass(NH) band increases c o n s i d e r a b l y with increasing temperature. 4. The ~ass(C=O) band is shifted toward larger w a v e n u m b e r s and e v e n t u a l l y c o a l e s c e s with the Vfree(C=O) band. 5. The 6(NH) + m ( C N ) band is shifted toward smaller wavenumbers. The i n t e n s i t y d e c r e a s e and increase of the ~ass(NH) and mfree(NH) absorptions, respectively, are indicative of the shift in e q u i l i b r i u m conc e n t r a t i o n of the h y d r o g e n b o n d e d and n o n h y d r o g e n b o n d e d NH-groups. The wavenumber shift and increase in band w i d t h of the Vass(NH) band at higher temp e r a t u r e s are the r e s u l t of a g e n e r a l w e a k e n i n g of the hydrogen bonds and a concomitant b r o a d e r d i s t r i b u t i o n of their energies. Similar t e m p e r a t u r e d e p e n d e n t spectral c h a n g e s are also o b s e r v e d for the mass(C=O) a b s o r p t i o n band. The f r e q u e n c y shift in the v i b r a t i o n of the bonded carbonyl groups, however, is less p r o n o u n c e d than that of the N H - g r o u p s because the acceptor atom is c e r t a i n l y less d i s p l a c e d than the h y d r o g e n atom of the donor group. The shift of the 6(NH) + 9(CN) band toward smaller w a v e n u m b e r s reflects the p r e d i c t e d inverse effect of h y d r o g e n b o n d i n g on stretching and d e f o r m a t i o n vibrations. In Fig. 3 the p e a k m a x i m u m w a v e n u m b e r s of the ~(NH) and ~(CH2) absorption bands have been p l o t t e d as a function of temperature with the aid of

475

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T ~-~ 3360-

-2970 i I

E 3350-

e,-

c

-2960 ~ 3340iiil glRin

.2950i~-"

mmmmmmmmmm

3330 - I w l 300

3,50

460 temperat u re ( K )

FIGURE 3 Shift of the peak m a x i m u m wavenumber (A) absorption bands of the p o l y e s t e r urethane temperature.

i

4~0

560

of the ~(NH) (m) and 9(CH 2) film (c) as a function of

348 K

10" I

04

8-

'E

C

~4"

,

50

e

,

100 150 strain (%)

,

w

200

250

FIGURE 4 S t r e s s - s t r a i n diagrams of l o a d i n g - u n l o a d i n g ester urethane films (a), (b) and (c) at 348 K.

cycles

of the poly-

476

the a u t o m a t i c p e a k search and plot routine. Drastic frequency shifts can be o b s e r v e d for the ~(NH) band in the 430 K - 490 K melting t e m p e r a t u r e interval of the hard segments (see also Fig. ic). W i t h Eq. (i) a c o n c o m i t a n t increase of R N . . . O from 3.O1 ~ to 3.07 ~ can be d e r i v e d b e t w e e n 300 K and 500 K. In v i e w of the u n c e r t a i n t y of the exact w a v e n u m b e r p o s i t i o n of the ~free(NH) a b s o r p t i o n band at about 3440 cm -I the value of 3.O1 ~ for 300 K is in r e a s o n a b l e a g r e e m e n t with the value of 2.98 ~ obtained by X-ray analysis on c h e m i c a l l y e q u i v a l e n t model u r e t h a n e s (BORN et al. 1982). The c o r r e s p o n d i n g p e a k m a x i m a plots of the p o l y e s t e r urethanes (a) and (b) d i f f e r b a s i c a l l y only in that the region of the p r o n o u n c e d w a v e n u m b e r shift is o b s e r v e d in the t e m p e r a t u r e interval 390 K - 450 K (a) and 420 K - 490 K (b), respectively. A d d i t i o n a l l y , b e t w e e n 300 K and 348 K, the e x p e r i m e n t a l

5 0.101

(a)

=:::--...... :---:--.:..=:'--'- .............

o o.o5t .-.,t"~""-*" _-- : - - - - 9 , . 9 , L

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40

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strain ,

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recover

160 200

260

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strain (7) 025. ,,,f/,A

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(b)

A.,,,. A 9 9 9 ~ "A

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(c)

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F I G U R E 5 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 ~(NH) (A) and 9(CH2) (m) a b s o r p t i o n bands, respectively, at 348 K.

477

t e m p e r a t u r e of the r h e o - o p t i c a l m e a s u r e m e n t s in the p r e s e n t study, RN... O length i n c r e m e n t s of a p p r o x i m a t e l y O.O1 ~ have been d e t e r m i n e d for the three 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 w i t h Eq. (i). The s t r e s s - s t r a i n diagrams of the three e l a s t o m e r s m e a s u r e d at 348 K are shown in Fig. 4. The c o n s e q u e n c e of t e m p e r a t u r e e l e v a t i o n is a d r a s t i c d e c r e a s e of stress level and initial m o d u l u s [5 MNm -2 Ca), 18 MNm -2 (b) and 24 MNm -2 (c)]. Furthermore, an increase of e x t e n s i o n set [65% (a) and 85% (b)] and an increase and constancy, respectively, of stress h y s t e r e s i s [68% (a) and 80% (b) ] have b e e n e v a l u a t e d for the low and i n t e r m e d i a t e hard segment p o l y m e r s (a) and (b) while a decrease of these p a r a m e t e r s was o b s e r v e d for p o l y e s t e r u r e t h a n e (c) (extension set: 85%, stress hysteresis: 80%) in comp a r i s o n to the m e a s u r e m e n t s at a m b i e n t t e m p e r a t u r e (SIESLER 1983). Thus, at 348 K e x t e n s i o n set and stress h y s t e r e s i s reach their m a x i m u m values already for the p o l y e s t e r u r e t h a n e (b) w i t h i n t e r m e d i a t e hard segment c o n t e n t (FERGUSON and K U M A R 1981). The o r i e n t a t i o n functions of the hard and soft segments d e r i v e d from the d i c h r o i s m of the V(NH) and v(CH2) a b s o r p t i o n bands, r e s p e c t i v e l y , in the p o l a r i z a t i o n spectra m o n i t o r e d d u r i n g the l o a d i n g - u n l o a d i n g cycles 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 at 348 K are shown in Fig. 5. Basically, the f o l l o w i n g s t r u c t u r a l c o n s e q u e n c e s of the m e c h a n i c a l t r e a t m e n t at 348 K are o b s e r v e d w i t h r e s p e c t to the m e a s u r e m e n t s at a m b i e n t temperature: a) b) c) d)

earlier drastic slight larger

o n s e t of p o s i t i v e hard s e g m e n t o r i e n t a t i o n e n h a n c e m e n t of hard segment o r i e n t a t i o n d e t e r i o r a t i o n of soft s e g m e n t o r i e n t a t i o n r e t e n t i o n of hard segment a l i g n m e n t upon r e c o v e r y to zero stress.

These effects can be p r e d o m i n a n t l y a t t r i b u t e d to the t e m p e r a t u r e dep e n d e n c e of the domain structure in the p o l y m e r s u n d e r examination. Thus, it was i n d i c a t e d (WILKES et al. 1975) that upon h e a t i n g p o l y u r e t h a n e s above about 343 K the d e g r e e of domain f o r m a t i o n g r a d u a l l y d e c r e a s e s such that m o r e m i x i n g occurs b e t w e e n hard and soft segments. The a b o v e m e n t i o n e d weakening of the h y d r o g e n bonds b e t w e e n 300 K and 348 K will also c o n t r i b u t e to an i n c r e a s e d d i s r u p t i o n t e n d e n c y of the hard segments upon a p p l i c a t i o n of stress. While this e n h a n c e d d i s r u p t i o n t e n d e n c y of the h a r d segments leads to an e a r l i e r o n s e t of their m o r e p r o n o u n c e d p o s i t i v e o r i e n t a t i o n at elevated t e m p e r a t u r e on the other hand a somewhat lower chain a l i g n m e n t of the soft segments is e f f e c t e d d u r i n g elongation. Owing to the low c r y s t a l l i z a tion t e n d e n c y Of the i n v o l v e d soft segments s t r a i n - i n d u c e d c r y s t a l l i z a t i o n does not a c c o u n t for the m e c h a n i c a l b e h a v i o u r of p o l y e s t e r u r e t h a n e (c). Therefore, it could b e a s s u m e d that due to the higher state of order in the hard segments of p o l y e s t e r u r e t h a n e (c) (see also Fig. i) the smaller domains are p r e f e r e n t i a l l y d i s r u p t e d d u r i n g e l o n g a t i o n at e l e v a t e d t e m p e r a ture (BONART and M U L L E R - R I E D E R E R 1981) r e s u l t i n g in a smaller p o s i t i v e hard s e g m e n t o r i e n t a t i o n at m a x i m u m e l o n g a t i o n than p o l y e s t e r u r e t h a n e (b) (see Fig. 5). The smaller e x t e n s i o n set and stress h y s t e r e s i s of p o l y e s t e r urethane (c) in c o m p a r i s o n to the a m b i e n t t e m p e r a t u r e m e a s u r e m e n t s m a y be int e r p r e t e d in terms of a more h o m o g e n e o u s stress d i s t r i b u t i o n in the elongated sample and the improved r e t r a c t i v e force of the soft segments at elev a t e d temperature. ACKNOWLEDGEMENTS The author g r a t e f u l l y a c k n o w l e d g e s the e x p e r i m e n t a l a s s i s t a n c e H. Devrient, H. P. S c h l e m m e r and W. S c h m i t t and h e l p f u l d i s c u s s i o n s Dr. G. Spilgies. The author also thanks Bayer AG for the p e r m i s s i o n p u b l i s h the e x p e r i m e n t a l data.

of with to

478

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Received January 7, accepted January 13, 1983

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