Polymer Bulletin 9, 557-562 (1983)

Polymer Bulletin 9

Rheo-Optical Fourier Transform IR (FTIR) Spectroscopy of Polyurethane Elastomers 3. Investigation of NH-Deuterated Specimens H.W. Siesler Bayer AG, Werk Dormagen, Forschung und Entwicklung, Postfach 1140, D-4047 Dormagen, Federal Republic of Germany In memoriam Prof. Dr. Otto Bayer

SUMMARY In the preceding papers of this study (SIESLER 1983, 1983a) the results of rheo-optical FTIR investigations at ambient and elevated temperature obtained from a series of model polyester urethanes with varying composition have been discussed in terms of the phase separation and segmental orientation of these polymers. The present work demonstrates the potential of this technique applied to NH-deuterated specimens of these polyester urethanes with reference to a more detailed insight into phase separation and the orientation mechanism of the phase-separated hard segments during uniaxial elongation and recovery. INTRODUCTION As has been pointed out already polyurethanes are particularly suited to rheo-optical FTIR spectroscopic investigations because they contain functional groups which are predominantly located in specific domains of these polymers. Therefore, the polarization properties of the corresponding characteristic IR absorption bands can be utilized to monitor the orientation of the individual domains during mechanical treatment. In this respect more detailed information becomes available from investigations of NH-deuterated specimens due to the fact that isotope exchange offers a means to differentiate the hard segments into phase-separated species and moieties dispersed in the soft segments or located at domain interfaces. Among isotope exchange techniques direct deuteration with liquid or gaseous D20 in combination with IR spectroscopic investigations has been widely applied in studies of polymeric structure (LIANG 1964, DECHANT 1972, SIESLER and HOLLAND-MORITZ 1980). Owing to the mass dependence of the vibrational frequency isotopic substitution reaction will result in frequency shifts of those absorption bands which belong to vibrations of the involved isotopes. The technique, however, is limited to polymers containing loosely bonded protons in NH or OH functional groups (e.g cellulose, polyamides, polyurethanes) and its applicability to the characterization of molecular order in polymers is based on the fact that hydroxyl and amido groups residing in chain segments of different degree of order are differently exposed to isotopic substitution. Thus, the rate and extent of the exchange reaction will strongly depend on the mechanical and thermal history of the polymer under examination. Given the applicability of Beer's law and assuming that the reason for a proton not to exchange is due to it being inaccessible to D20 the percentage of accessible regions (Z) at any stage of deuteration can be determined from the relationship:

558

Z

=

1

A (X-H) I 100(%) A (X-H) i

(1)

where A(X_H) is the absorbance of an absorption band which can be assigned to a primarily uncoupled X-H vibration and the subscript i refers to the undeuterated sample. Previous investigations of aliphatic and aromatic polyamides of different degree of crystallinity (SIESLER and HOLLAND-MORITZ 1980) have established a direct correlation between the extent of readily deuterated NHprotons and the percentage of amorphous regions determined by independent methods. In polyurethanes, however, the readily exchanged NH-protons reflect the percentage of small hard segments dispersed in the accessible soft segment phase while the successive region of reduced substitution rate corresponds to the slow penetration of the deuteration agent into larger phaseseparated hard segment domains. Therefore, a combination of the deuteration technique and rheo-optical FTIR investigations should provide a more quantitative picture of phase separation and a better differentiation of the individual domain contributions to the overall orientation of the polymer.

100

8O

V V urethane

(a)

i~ 2O

~

0

3~0 wovenumber, crn 1

100 8o

(b)

(deuterated)

i4~ 2O

04000

FIGURE I polyester

~DO

1,500 wovenumber cr~1

~oo

IR spectra of an undeuterated urethane film.

10(90

500

(a) and partially deuterated

(b)

559

EXPERIMENTAL The r h e o - o p t i c a l F T I R spectra were o b t a i n e d at 300 K on a Nicolet 7199 F T I R s p e c t r o m e t e r 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 synthesized from diphenylmethane-4,4'-diisocyanate, 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/ ethylene glycol p o l y e s t e r (molecular w e i g h t 2000) and butane diol as chain extender with 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), 1.0:5.4:6.6 (b) and 1.O:7.5:8.7 (c). The experimental and instrumental details of the rheo-optical m e a s u r e ments and the p o l y m e r sample p r e p a r a t i o n have been d e s c r i b e d in a p r e c e d i n g paper ISIESLER 1983). The d e u t e r a t i o n p r o c e d u r e and the c o n s t r u c t i o n of the d e u t e r a t i o n cell have been reported p r e v i o u s l y (HOLLAND-MORITZ and S I E S L E R 1976). Basically, the p o l y m e r film is m o u n t e d in a sealed cell with I R - t r a n s p a r e n t w i n d o w s w h i c h is kept at a constant temperature of 323 K. D e u t e r i u m exchange is induced by saturating d r y n i t r o g e n with liquid D20 prior to flushing the d e u t e r a t i o n cell. After five days of d e u t e r i u m exchange the p o l y m e r film is finally t r a n s f e r r e d to the sample chamber of the stretching machine. RESULTS

AND D I S C U S S I O N

The IR spectra of an u n d e u t e r a t e d and p a r t i a l l y d e u t e r a t e d p o l y e s t e r urethane (c) film are shown in Fig. i. The most obvious spectral changes upon d e u t e r a t i o n d i r e c t l y r e f l e c t those a b s o r p t i o n bands which b e l o n g to v i b r a t i o n s involving the N H g r o u p s . Thus, the H-D isotope exchange results in partial r e p l a c e m e n t of the ~(NH) absorption band at 3331 cm -I by a band complex at 2480 cm -I (GARTON and PHIBBS 1982). While the a b s o r p t i o n bands at 1703 and 1733 cm -I, essentially a t t r i b u t e d to the 9(C=O) absorptions of the h y d r o g e n b o n d e d urethane and n o n b o n d e d ester carbonyl groups, are almost u n a f f e c t e d by d e u t e r a t i o n the bands at 1590 and 1531 cm -I w h i c h have been i n t e r p r e t e d as ~ ( N H ) + ~(CN) modes (ISHIHARA et al. 1974) decrease in intensity upon s u b s t i t u t i o n of H by D.

~Q60

I

0.40

deuteration -h/\ \ \1\ //,~V~ t \ \ ~ time(.) ;/~ ~ ~ \,~v~ l \ \, .~'~-~"~-~'~'~'01 start

0.20- _~-

3500

;48.37.7 243

32'00 L;:K~O ~ wavenumbers

2:~30

FIGURE 2 FTIR spectra of the p o l y e s t e r urethane (a) film in the 3500 2300 cm -I region taken in d i f f e r e n t time intervals during deuteration.

560

70,

I1-~~>----II--~-

60,

>,

50

__~

~---tl-o-

.o 4 0

8o

30

20

lb

2'0 30 4'0 5'0 40 ~0 8o ~

lOO111~5

time (h)

F I G U R E 3 A c c e s s i b i l i t y of the investigated polyester urethane (b) ([3), (c) (O)] as a function of deuteration time (see text).

films

[ (a)(Z~,

/ 25 J

300 K

20

i 0

|

|

100 150 strain (%)

i

200

250

F I G U R E 4 S t r e s s - s t r a i n curves of loading-unloading cycles of the original and d e u t e r a t e d p o l y e s t e r urethane films with different hard and soft segm e n t composition (see text) at 300 K.

561

The time d e p e n d e n c e of the isotope exchange reaction can be m o n i t o r e d q u a n t i t a t i v e l y by taking IR spectra of the p o l y e s t e r urethane film in regular time intervals d u r i n g the d e u t e r a t i o n p r o g r e s s (Fig. 2) and e v a l u a t i n g the i n t e g r a t e d a b s o r b a n c e of the ~(N1]) a b s o r p t i o n band with the aid of Eq. (i). The a c c e s s i b i l i t y curves d e r i v e d in this manner for the three d i f f e r ent p o l y e s t e r u r e t h a n e s are shown in Fig. 3. Principally, the curves can be separated into two portions. Thus, in the region of rapid exchange p r o g r e s s a p p r o x i m a t e l y 60% (a), 52% (b) and 42% (c) of the available N H - p r o t o n s have been s u b s t i t u t e d by d e u t e r i u m within 25 hours. At the slightly elevated d e u t e r a t i o n temperature of 323 K no significant e n h a n c e m e n t of a c c e s s i b i l i ty due to phase m i x i n g has to be taken into account. In a n a l o g y to the DSC data of these p o l y e s t e r urethanes (SIESLER 1983a) the d i f f e r e n t accessibi-~ lity levels in Fig. 3 clearly d e m o n s t r a t e the improvement of phase s e p a r a tion w i t h increasing hard segment content. F u r t h e r experimental support of

0.10"

~ (105(a)

"'- "'" "-

0.05 ~ ~"'A'*~ strain

-0.05 :

40

8"0

120

~

~ ~

i

strain

,5 (b)

0.10.

=,o/ .....99

-0.05.

.-=

~ "" "

,.as"~

~ 260 I~,J

m%l

recover

160

1~)

80

120

BO

4'0

.m,

,,,,6

.! 0.OOJ strain

-(105,

~

:

--~

recover

i

4~0

80

120

160

260 200 strain (%)

160

0.15' ._~ 0.t0

(c)

.ii

0.05.

"~ 0,00, -(105,

.~

.A~

.~f/"

,40

.E.-<"

~

strain

~

1L~O 1~0

~

[ -*

i

~

~ strain (%)

recover 1E~O 1L~3

FIGURE 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 d e u t e r a t e d p o l y e s t e r urethanes (a) - (c) as m o n i t o r e d by the ~(NH) ( ~ and ~(CH2) (B) absorption bands, respectively, at 300 K (see text).

562

the above results can be drawn from deuteration experiments of the same p o l y m e r s in the 200% drawn state which yield about 5-10% higher accessibility values. This increase of a c c e s s i b i l i t y can be interpreted in terms of the d i s r u p t i o n of large hard segment domains into smaller subunits during elongation. The stress-strain diagrams of the d e u t e r a t e d samples measured at 300 K did not deviate significantly from the corresponding mechanical data of the original u n d e u t e r a t e d specimens (Fig. 4). The o r i e n t a t i o n functions of the hard and soft segments derived from the d i c h r o i s m of the v(NH) and ~(CH2) absorption bands, respectively, in the p o l a r i z a t i o n spectra m o n i t o r e d during the l o a d i n g - u n l o a d i n g cycles of the d e u t e r a t e d p o l y e s t e r urethanes at 300 K are shown in Fig. 5. Here, the ~(NH) o r i e n t a t i o n function represents the orientation of the inaccessible hard segment domains. A c c o r d i n g to BONART'S model of segmental orientation in p o l y u r e t h a n e s (BONART and HOFFMANN 1982) the p r o n o u n c e d transverse hard segment o r i e n t a t i o n observable in Fig. 5 is therefore fully consistent with the a s s i g n m e n t of the involved urethane groups to p h a s e - s e p a r a t e d lamellar domains. As can be readily derived from Fig. 5 the negative orientation of the hard segments does not immediately start with the application of stress. This supports a structural model p r o p o s e d by VAN BOGART et al. (1979) in w h i c h the hard segment domains are p a r t l y interconnected. Thus, the interc o n n e c t i o n s of such a hard segment network have to break up first before any transverse a l i g n m e n t can take place. The shift of the m a x i m u m negative o r i e n t a t i o n to higher strain values in the polymers with lower hard segment c o n t e n t [110% (a), 80% (b) and 70% (c)] is indicative of the delayed onset of the m o l e c u l a r - m e c h a n i c a l stress transfer owing to longer soft segments (BONART and M U L L E R - R I E D E R E R 1981). Generally, the orientation of the inaccessible hard segments induced during elongation to the m a x i m u m experim e n t a l strain of 220% has been found m u c h smaller than the total average hard segment o r i e n t a t i o n of the u n d e u t e r a t e d samples (SIESLER 1983). In fact, in the p o l y e s t e r urethane (a) with the lowest hard segment content the lamellar hard segments do not take on a positive orientation at all (Fig. 5). The deuteration technique, therefore, offers the p o s s i b i l i t y to d i f f e r e n t i a t e the individual contributions to the average hard segment orientation. 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 experimental assistance of H. Devrient, H. P. Schlemmer and W. Schmitt and thanks Bayer AG for the p e r m i s s i o n to p u b l i s h the experimental data. REFERENCES SIESLER: Polymer Bulletin 9, 382(1983) SIESLER: Polymer Bulletin 9, 471(1983a) LIANG: in Newer Methods o~ Polymer C h a r a c t e r i z a t i o n (BACON KE ed.) Interscience, N e w York, p33, 1964. J. DECHANT: U l t r a r o t s p e k t r o s k o p i s c h e U n t e r s u c h u n g e n an Polymeren, Akademie Verlag, Berlin, 1972. H. W. SIESLER, and K. HOLLAND-MORITZ: Infrared and Raman Spectroscopy of Polymers, Marcel Dekker, N e w York, 1980. A. GARTON, and M. PHIBBS: Makromol. Chem. Rapid Commun. 3, 569 (1982). K. HOLLAND-MORITZ, and H. W. SIESLER: Appl. Spectroscopy Revs. ii, 1 (1976). H. ISHIHARA, I. KIMURA, K. SAITO, and H. ONO: J. Macromol. Sci. Phys. BIo, 591 (1974). R. BONART, and K. HOFFMANN: Colloid & Polym. Sci. 260, 268 (1982). J~ W. C. V A N BOGART, A. L I L A O N I T K U L , S. L. COOPER: Adv. Chem. 176, 3 (1979). R. BONART, and G. MULLER-RIEDERER: Colloid & Polym. Sci. 259, 926 (1981). H. W. H. W. C. Y.

Accepted January 13, 1983

C