CRYSTALLIZATION STUDIES OF POLYMER BLENDS BY FOURIER TRANSFORM IR PHOTOACOUSTIC SPECTROSCOPY

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CRYSTALLIZATION STUDIES OF POLYMER BLENDS BY FOURIER TRANSFORM IR

PHOTOACOUSTIC SPECTROSCOPY

E. Balizer, H. Talaat

To cite this version:

E. Balizer, H. Talaat. CRYSTALLIZATION STUDIES OF POLYMER BLENDS BY FOURIER

TRANSFORM IR PHOTOACOUSTIC SPECTROSCOPY. Journal de Physique Colloques, 1983, 44

(C6), pp.C6-131-C6-136. �10.1051/jphyscol:1983620�. �jpa-00223179�

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CRYSTALLIZATION STUDIES OF POLYMER BLENDS BY FOURIER TRANSFORM I R PHOTOACOUSTIC SPECTROSCOPY

E . Balizer* and H. Talaat*+

*Acoustics Division, NavaZ Research Laboratory, Washington, D.C., U.S.A.

+Ain Shams University, Cairo, Egypt

REsumE

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La c i n g t i q u e de c r i s t a l l i s a t i o n du PET pur ou mElangE e s t etudiEe par TF PA e t p a r I R .

A b s t r a c t

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We p r e s e n t a s t u d y o f t h e c r y s t a l l i z a t i o n k i n e t i c s of polyethylene t e r e p h t h a l a t e and i t s b l e n d s by F o u r i e r transform p h o t o a c o u s t i c spectroscopy and compare it t o I R spectroscopy.

Introduction

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The c r y s t a l l i n i t y of polymer f i l m s determines t h e mechanical and e l e c - t r i c a l responses of t h e f i l m s . The s t a n d a r d t e c h n i q u e s t o determine t h e f i l m ' s crys- t a l l i n i t y c o n t e n t a r e x-ray d i f f r a c t i o n / I / , I R t r a n s m i s s i o n spectroscopy /1/ and d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) /2/. However, x-ray d e t e r m i n a t i o n of t h e c r y s t a l l i n i t y of t h e polymer f i l m r e q u i r e s t h i c k samples and a minimum s i z e of crys- t a l l i t e s ; IR r e q u i r e s v e r y t h i n samples and DSC r e e q u i r e s d e s t r u c t i v e m e l t i n g of t h e sample. A l t e r n a t i v e l y , p h o t o a c o u s t i c spectroscopy (PAS) p r o v i d e s b o t h a nondestruc- t i v e and a q u a n t i t a t i v e technique t o determine t h e c r y s t a l l i n i t y o f t h e polymer sam- p l e s . In a d d i t i o n PAS r e q u i r e s no o r minimum sample p r e p a r a t i o n and hence i s advantageous i n s t u d y i n g s p e c i a l geometrical s t r u c t u r e s , e.g. polymer c o a t i n g s on o p t i c a l f i b e r s / 3 / . Crystallization s t u d i e s of p o l y e t h y l e n e t e r e p h t h a l a t e (PET) /4/,/5/

have been c a r r i e d o u t p r e v i o u s l y u s i n g v a r i o u s t e c h n i q u e s e s p e c i a l l y c a l o r i m e t r y and both I R and F o u r l e r t r a n s f o r m (FT) I R t r a n s m i s s i o n s p e c t r o s c o p i e s . I n t h i s paper, we r e p o r t t h e use of PAS i n c o n j u n c t i o n w i t h FTIR t o s t u d y t h e c r y s t a l l i z a t i o n of PET and blends of PET and polybutylene t e r e p h t h a l a t e (PBT). A n a c c u r a t e measure of c r y s t a l l i n i t y can be o b t a i n e d by c o r r e l a t i n g t h e s e r e s u l t s with t h e measurements of d e n s i t y determination. More r e c e n t l y , Boerio and Koenig /5/ u s i n g t h e s e n s i t i v e t e c h n i q u e s of FTIR, were a b l e t o i d e n t i f y t h e I R a b s o r p t i o n bands f o r t h e amorphous phase and t h e c r y s t a l l i n e phase of PET. Due t o t h e thermal p r o p e r t i e s of PET, t h i n s e m i c r y s t a l l i n e f i l m s a r e e a s i l y quenchable; t h i s e n a b l e s t h e p r e p a r a t i o n s of PET f i l m s of v a r y i n g c r y s t a l l i n e c o n t e n t . Furthermore, a s t u d y of t h e e f f e c t of mixing o r blending PET w i t h a compatible polymer a s PBT can a l s o be c a r r i e d o u t . The use of FTIR-PAS allows u s t o observe and determine t h e c r y s t a l l i z a t i o n behavior of each component by f o l l o w i n g t h e c r y s t a l l i z a t i o n a b s o r p t i o n peak corresponding t o t h a t component. Though our i n t i t i a l s t u d i e s of PET were conducted u s i n g d i s p e r s i v e IR-PA spectroscopy, t h e low S/N r a t i o and t h e l i r ~ i t a t i o n of t h e range of t h e d i s p e r - i v e element, d i d n o t a l l o w an a c c u r a t e e v a l u a t i o n of t h e c r y s t a l l i z a t i o n . These e f f o r t s were t h e n abandoned i n f a v o r of t h e more s e n s i t i v e t e c h n i q u e s of FTIR-PAS.

I n a d d i t i o n t h e r e l a t i v e l y s h o r t time r e q u i r e d i n FTIR-PAS t o o b t a i n a good c o n t r a s t s p e c t r a of d e s i r a b l e s p e c t r a l i n t e n s i t y and t h e a b i l i t y t o avoid s a t u r a t i o n of t h e s i g n a l by proper c h o i c e of t h e v e l o c i t y of t h e moving m i r r o r of t h e i n t e r f e r o m e t e r , made t h e t e c h n i q u e q u i t e a t t r a c t i v e t o our s t u d i e s .

Sample P r e p a r a t i o n

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The PET and i t s blend of 80/20, by weight, of PET/PBT were o b t a i n e d i n p e l l e t form from t h e P l a s t i c s D i v i s i o n , General E l e c t r i c Company /6/.

These samples were kept i n a d e s s i c a t o r p r i o r t o melting. The p e l l e t s were t h e n p l a c e d between aluminum f o i l s with 250 micrometer b r a s s shims and p r e s s e d on a Carver P r e s s (Model C ) a t a temperature of 340OC. S p e c i a l l y designed a u x i l i a r y

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983620

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p l a t e n s of s t a i n l e s s s t e e l 6" i n diameter and 1 5/8" t h i c k were used a s thermal r e s e v o i r s and s e t i n t o t h e p r e s s . The aluminum f o i l packet was p l a c e d between t h e s e p l a t e n s . Upon opening t h e p r e s s , t h e a u x i l i a r y p l a t e n s k e p t t h e polymer molten s o t h a t it c o u l d t h e n be quenched i n t o an i c e water bath. This technique h a s been shown t o quench both t h e f a s t c r y s t a l l i z i n g PBT and PET /4/. A f t e r t h i s i n i t i a l quenching t h e s e samples were annealed a t llS°C and 1500C i n a Cole Parmer f l u i d i z e d sand b a t h . The c r y s t a l l i z a t i o n t i m e s v a r i e d between 20 s e c t o 1000 s e c and t h e samples were quenched a g a i n i n an i c e water bath. A s i m i l a r s e t of samples was given t h e same thermal h i s t o r y , however t h e t h i c k n e s s of t h e s e samples was of t h e o r d e r of 25 micrometers and were used f o r t h e IR t r a n s m i s s i o n s t u d i e s .

S p e c t r o s c o p i c Methods

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The c r y s t a l l i z a t i o n k i n e t i c s of s p e c i a l l y prepared samples of PET and i t s blends were determined by I R spectroscopy a s a guide f o r t h e PAS stud- i e s . A P e r k i n Elmer 983 i n f r a r e d spectrophotometer i n t h e absorbance mode was used with a 3 an-l r e s o l u t i o n . The FTIR-PA s p e c t r a r e p o r t e d i n t h i s paper were recorded u s i n g a PA c e l l i n a D i g i l a b model FTS-20 F o u r i e r transform i n f r a r e d spectrometer.

The PA c e l l was powered from t h e F T I R i n s t r u m e n t and t h e PA s i g n a l from t h e c e l l i s r o u t e d t o t h e normal s i g n a l p r e c e s s i n g e l e c t r o n i c s of t h e FTIR i n s t r u m e n t . The mod- u l a t i o n frequency f (Hz) of t h e scan i n t e r f e r o m e t r y , which corresponds t o t h e chop- p i n g frequency i n r e g u l a r d i s p e r s i v e spectroscopy, i s determined by t h e v e l o c i t y of t r a n s l a t i o n v (cm/sec) of t h e moving m i r r o r of t h e Michelson i n t e r f e r o m e t e r , f=2vv, where v i s t h e l i g h t frequency. Generally, t h e PA s i g n a l i s l a r g e r f o r s m a l l e r f , and i n o u r work, t h e v e l o c i t y of t h e m i r r o r was 0.16 cm/sec, which r e s u l t e d i n modulation f r e q u e n c i e s ranging from 130 t o 1300 Hz f o r t h e s p e c t r a l r e g i o n 400 t o 4000 cm-l. Residual moisture i n t h e PA c e l l was removed by p l a c i n g a small amount of d r y i n g a g e n t ( e . g . D r i e r i t e ) i n t h e c e l l and s p e c i a l p r e c a u t i o n was t a k e n t o i s o -

l a t e t h e PA c e l l from ambient v i b r a t i o n by p l a c i n g it on s p e c i a l i n s u l a t i o n rubber mounts. Most of t h e PA s p e c t r a were recorded a t a s p e c t r a l r e s o l u t i o n of 4 cm-l and on t h e average r e q u i r e d 4000 s c a n s which were c a r r i e d o u t i n approximately 40 minutes.

R e s u l t s

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The s e t of polymer samples w i t h a t h i c k n e s s on t h e o r d e r of 25 microme- t e r s was used i n t h e IR t r a n s m i s s i o n s t u d i e s ; whereas, t h e s e t of r e l a t i v e l y t h i c k samples on t h e o r d e r of 250 micrometers was used I n t h e FTIR-PA s t u d i e s . Follow- i n g t h e work of Boerio and Koenig /5/, t h e a b s o r p t i o n band a t 848 a n - l , a s s i g n e d t o t h e r o c k i n g mode of t h e t r a n s conformation of t h e e t h y l e n e g l y c o l segments, i s i d e n t i f i e d a s t h e c r y s t a l l i z a t i o n a b s o r p t i o n band. Also, t h e a b s o r p t i o n band a t 632 cm-', a s s i g n e d t o t h e C-C-C bending mode of t h e benzene r i n g i n t h e backbone of PET, i s identified a s t h e s t a n d a r d i n t e r n a l r e f e r e n c e band /4/. The i n t e n s i t i e s of c r y s t a l l i z a t i o n bands a t 848 an-l were r a t i o e d o r normalized t o t h e peak i n t e n s i t i e s of t h e i n t e r n a l r e f e r e n c e bands a t 632 cm-l. I n t h i s way, t h e v a r i a t i o n s both i n t h e t h i c k n e s s of t h e samples and i n t h e d e n s i t i e s were t a k e n i n t o account. A rep- r e s e n t a t i v e spectrum f o r I R a b s o r p t i o n of PET with a n n e a l i n g t i m e s v a r y i n g between 40 s e c t o 600 s e c a r e shown i n F i g u r e 1. I n r e g u l a r t r a n s m i s s i o n spectroscopy, t h e a b s o r p t i o n peak amplitudes f o l l o w t h e Beer-Lambert r e l a t i o n s h i p and t h e ampli- t u d e i n c r e a s e s due t o an i n c r e a s e i n t h e p o p u l a t i o n of t h e modes. The c r y s t a l l i z a t i o n bands I n F i g u r e 1 can be seen t o i n c r e a s e a s t h e a n n e a l i n g t i m e s a r e in- creased. On t h e o t h e r hand, t h e FTIR-PAS i s a s u r f a c e spectroscopy and t h e s i g n a l i s a complex f u n c t i o n of t h e o p t i c a l and thermal p r o p e r t i e s of t h e sample. However,we a r e s t i l l a b l e t o use t h e FTIR-PAS q u a n t i t a t i v e l y by u s i n g an i n t e r n a l s t a n d a r d technique. This t e c h n i q u e h a s been prev5ously a p p l i e d by Rockley e t . a l . /7/

f o r t h e q u a n t i t a t i v e a n a l y s i s of s o l i d mixtures. It i s t h e r e f o r e p a r t i c u l a r l y s u i t ed to our s t u d y of c r y s t a l l i z a t i o n i n PET and i t s blends of PET/PBT. A r e p r e s e n t a - t i v e spectrum by FTIR-PAS f o r t h e 80/20 PET/PBT blend i s shown i n F i g u r e 2 f o r t h e r e g i o n 4 0 0 t o !+OOO an-'. The r e l e v a n t p o r t i o n s of t h e PA spectrum f o r PET t h a t j n c l u d e t h e c r y s t a l l i z a t i o n band and t h e i n t e r n a l s t a n d a r d r e f e r e n c e band, f o r t h e

amorphous s t a t e a s w e l l a s f o r t h e polymer annealed f o r 60 s e c and 300 s e c a r e shown i n F i g u r e 3. Here a l s o , t h e a b s o r p t i o n bands a s w e l l a s t h e c r y s t a l l i n i t y bands show more d e f i n i t i o n and growth a s t h e a n n e a l i n g t i m e s a r e i n c r e a s e d . The r a t i o of t h e i n t e n s i t i e s i n t h i s c a s e c o u l d a c c u r a t e l y b e determined. The c a s e f o r t h e blends

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...

^,A. IR-Xc Bands ^.Lo ^.b of PET ^-⁰⁹ *a"e"Umber, ^-^D^O^.

Figure 1

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^The

^IR

spectra of PET showing growth of the 848 cm-' band with crystallization time.

3000 2000 1000

Wavenumbers FTIR-PAS of 8 0 / 2 0 Blend

Figure

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The FTIR-PA spectrum of the crystallized 80120 PET/PBT polymer blend.

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60 sec. 300 sec.

wavenumbers

FTIR-PAS of PET

Figure 3

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The FTIR-PA spectrum of PET showing the growth of the

848

cm-' band with annealing time.

requires a different procedure. The intensity for the PET and PBT crystalline bands in the blends must be corrected to take into account the contribution to these bands of the other component and its dilution effect. In the case of PBT, the crystallization band for PBT was identified as 917 cm-l /4/. The corrected intensity will then reflect the change in crystallinity of the PET and PBT phases individually based upon the weight of that phase i.e., the corrected intensity for each component in the blend would be the observed intensity if the sample has only one component and has been crystallized at the same rate and manner as the blend. To make this correction,the intensity of the crystallization band of PET in the blend was taken as

Icorr = (Iexp

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^M ^~ ^~ ^~ ^l ^~ ^~ ^~ ⁾ ^/ ^M ^~ ^~ ^~

where Iexp is the experimental value measured in the blend, IpBT is the value of the intensity of the 848 cm-' band for PBT (this value does not change with crystallization), MpBT and MpET are the monomer mole fractions for PBT and PET in the blends.

Discussion

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The variation in the ratio of the IR absorbances of the 848 cm-l crystallization line to the internal reference at 632 an-' is plotted versus the time of annealing in Figure 3. These data points are shown for PET annealed at 150°C and for the blend of PET/PBT annealed at 150°C and 115°C. The top and bottom curves have a sigmoidal shape indicative of a transition. The kinetic curve of the blend at 150°C indicates that this crystallization rate was faster than the least annealing time used (20 sec). A parameter which is characteristic of the kinetic curve is the time it takes for half of the final crystallization to occur (the halftime);

in this case it is estimated (from Figure 4) to be 50 sec for the PET at 150°C and 100 sec for the blend at 115OC.

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i s e a s i l y seen t h a t t h e two k i n e t i c c u r v e s have t h e same c h a r a c t e r i s t i c sigmoidal shape a s t h e c u r v e s o b t a i n e d by t h e I R method. The d i f f e r e n c e i n s p r e a d a l o n g t h e a b s c i s s a i n d i c a t e s t h a t t h e FTIR-PAS method i s l e s s s e n s i t i v e t h a n t h e IR method.

However t h e c r y s t a l l i n i t y h a l f times e s t i m a t e d from t h e s e c u r v e s a g r e e w i t h i n few p e r c e n t w i t h t h o s e determined by I R . Furthermore, we n o t e from both s p e c t r a t h a t t h e r a t e of c r y s t a l l i z a t i o n of t h e blend i s f a s t e r t h a n t h a t of t h e PET of t h e same thermal h i s t o r y . This can be a s c r i b e d t o a lower g l a s s t r a n s i t i o n temperature and t h u s a lower v i s c o s i t y of t h e blend. This e f f e c t was a l s o e v i d e n t i n t h e a n n e a l i n g of PET and t h e PET/PBT blend a t 115OC. I n t h i s c a s e , t h e PET c r y s t a l l i z a t i o n was weak and c o u l d n o t be observed, whereas i n t h e blend it was e a s i l y observable.

Furthermore, i t can be seen from F i g u r e s 4 and 5 t h a t t h e c r y s t a l l i z a t i o n h a l f times a s determined by PAS, although of t h e same o r d e r a s t h o s e determined by IR, appear t o be s l i g h t l y l a r g e r . This d i f f e r e n c e i n t h e e s t i m a t e d h a l f t i m e s should be i n t e r p r e t e d with c a r e , due t o t h e p o s s i b i l i t y of t h e d i f f e r e n c e of t h i c k n e s s of t h e samples used i n t h e two methods and t h e n a t u r e of t h e two measurements.

I

Crystallization Kinetics by IR

I

3.4 PET 150 C

I

Figure 4- C r y s t a l l i z a t i o n k i n e t i c s determined by IR f o r PET and t h e 80120 blend f o r c r y s t a l l i z a t i o n temperatures of 115' .C and 150' C.

C r y s t a l l i z a t i o n K i n e t i c s b y FTIR-PAS 8 0 / 2 0 1 1 5 C

Figure 5- C r y s t a l l i z a t i o n k i n e t i c s determined by FTIR-PAS f o r PET a t 1 5 0 ' ~ and t h e 80120 blend a t 1 1 5 ~ ~ .

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Conclusions

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We have used PA spectroscopy i n c o n j u n c t i o n with FTIR t o be a b l e t o c h a r a c t e r i z e t h e c r y s t a l l i n i t y of PET and PET/PBT b l e n d s . We have a l s o been a b l e t o e s t i m a t e t h e c r y s t a l l i z a t i o n k i n e t i c h a l f t i m e s f o r t h e s e polymer systems. The v a l u e s o b t a i n e d by PA spectroscopy a r e i n agreement w i t h t h e v a l u e s o b t a i n e d by IR t r a n s m i s s i o n spectroscopy. However t h e PAS o f f e r s a t e c h n i q u e of measuring t h e c r y s t a l l i n i t y with minimal sample p r e p a r a t i o n which can prove advantageous i n t h e c a s e of s p e c i a l g e o m e t r i c a l l y shaped samples.

References

1. Methods of Experimental P h y s i c s , Vol. 16 p a r t B, "Polymers-crystal S t r u c t u r e and Morphology", e d i t e d by R. A. Fava (Academic P r e s s , New York, 1980).

2. J. Runt and I. R. Harrison, "Thermal Analysis of Polymers", i n Methods of Experimental Physics, Vol. 16 p a r t B , e d i t e d by R. A. Fava, (Academic P r e s s , 3. L. J. Donalds, W. G. French, W. C. M i t c h e l l , R. M. Swinehard, and T. Wei,

E l e c t r o n i c s L e t t . ,

18,

327 (1982).

4. S. E s c a l a , Ph.D. D i s s e r t a t i o n , Dept. of Polymer Science and Engineering, U n i v e r s i t y of Massachussetts, Amherst, Mass., 1978

5. F. J. Boerio and J. L. Koenig, J. Polym. S c i . A-2, 7, 1517 (1971).

6. These samples a r e c o u r t e s y of D r . Rchard Allen, G.E. Corporation, P i t t s f i e l d , MA.

7. M. G. Rockley, D. M. Davis and H. H. Richardson, Appl. Spec.,35, 185 (1981).