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14 Methylene Chloride-Hexafluoroisopropyl Alcohol (70/30) Use in High-Performance Gel Permeation Chromatography of Poly(ethylene terephthalate)

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JAMES R. OVERTON and HORACE L. BROWNING, JR. Research Laboratories, Eastman Chemicals Division, Eastman Kodak Company, Kingsport, TN 37662

The solvent system 70/30 methylene chloride/ hexafluoroisopropanol has been in use in our laboratory since 1977 as a solvent for poly(ethylene terephthalate) (PET) and other semicrystalline polar polymers. Some advantages of this solvent are: i t provides rapid room temperature solubilization; i t is transparent at 254 nm (U.V.); i t is a solvent for

polystyrene; and i t is a minimum boiling azeotrope. Disadvantages are its low boiling point (36°C) and the potential safety hazard i t represents. The combination of appropriate HPGPC equipment and this solvent system reveals heretofore unrecognized features of the molecular weight distributions of polyesters. 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) has been analyzed by g e l permeation chromatography (GPC) r o u t i n e l y f o r many years.(1-7) During t h i s time, s a t i s f a c t o r y r e s u l t s have been obtained w i t h s e v e r a l s o l v e n t systems, the most common being m-cresol. The h i g h v i s c o s i t y of m-cresol r e q u i r e s t h a t i t be used a t e l e v a t e d temperatures, and the a s s o c i a t e d h a n d l i n g d i f f i c u l t y i s s u f f i c i e n t reason f o r f i n d i n g a replacement. T h i s paper w i l l present some o f our experience w i t h the s o l v e n t system 70/30 (v/v) methylene c h l o r i d e (MeC^)/hexafluoro­ i s o p r o p a n o l (HFIP). Some comments regarding the use of m-cresol are included.

0097-6156/84/ 0245-0219506.00/0 © 1984 American Chemical Society

In Size Exclusion Chromatography; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Solvent System P r o p e r t i e s The r a t i o of 70/30 (v/v) MeCl /HFIP was chosen because i t i s a minimum-boiling (37°C) a z e o t r o p i c m i x t u r e . The exact composition can be reproduced by d i s t i l l a t i o n from a mixture of approximately the c o r r e c t r a t i o , and one can e a s i l y r e c l a i m >90% of the s o l v e n t used by simple d i s t i l l a t i o n . In view of the c o s t of HFIP the a b i l i t y t o r e c l a i m s o l v e n t i s an important consideration. In a k i n e t i c sense, the system i s a b e t t e r s o l v e n t than HFIP a l o n e . We p o s t u l a t e t h a t MeCl2 s w e l l s the amorphous regions of PET thereby p r o v i d i n g HFIP w i t h an easy access to the c r y s t a l l i n e r e g i o n s . T h i s s w e l l i n g a c t i o n does not occur w i t h HFIP a l o n e , and the d i s s o l u t i o n process takes much l o n g e r . At room temperature, amorphous PET i s i n s t a n t a n e o u s l y s o l u b i l i z e d by t h i s s o l v e n t system. PET t h a t has been annealed f o r >24 h r a t 220°C t o y i e l d maximum c r y s t a l l i n i t y d i s s o l v e s i n 2.0. The t h e o r e t i c a l value o f M /M and the often-quoted experimental value o f 2.0 a r e o n l y f o r l i n e a r s p e c i e s . ( 9 ) Consider the e f f e c t o f 1.5% c y c l i c t r i m e r ( i g n o r i n g the low c o n c e n t r a t i o n o f other c y c l i c s ) on the value o f For M^ = 40,000, ^ ( l i n e a r ) = 20,000. The presence of 1.5% c y c l i c t r i m e r (M=576) lowers M t o 13,000 w i t h e s s e n t i a l l y no e f f e c t on M^ w

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: March 30, 1984 | doi: 10.1021/bk-1984-0245.ch014

High-Performance

OVERTON AND BROWNING

n

n

and .*. M /M ~ 3. Because o f t h e compact s t r u c t u r e of the c y c l i c t r i m e r i t e l u t e s l a t e r than the l i n e a r species o f e q u i v a l e n t mass. The perceived mass o f c y c l i c t r i m e r by the GPC column i s a c t u a l l y ^ 275. In the example c i t e d , t h e presence of 1.5% o f mass 275 lowers M t o about 10,000 and . · . M^M^ Ζ 4. w

n

n

Other workers have suggested t h a t i n a p o l a r s o l v e n t such as m-cresol o r h e x a f l u o r o i s o p r o p a n o l , PET w i l l undergo r a p i d e s t e r interchange l e a d i n g t o t h e " e q u i l i b r i u m d i s t r i b u t i o n " having a r a t i o o f M /M= 2.0.(6, 7) These workers f a i l e d t o recognize t h a t the e q u i l i b r i u m d i s t r i b u t i o n i n a d i l u t e s o l u t i o n i s not the same as e q u i l i b r i u m d i s t r i b u t i o n i n the absence o f a d i l u e n t . ( 1 0 , 11, 12) In d i l u t e s o l u t i o n , i n t r a m o l e c u l a r e s t e r interchange dominates, and the e q u i l i b r i u m d i s t r i b u t i o n c o n s i s t s mostly o f c y c l i c s p e c i e s . I n our l a b o r a t o r y , we have been able to show under c o n d i t i o n s where e s t e r interchange does occur i n s o l u t i o n t h a t a t a c o n c e n t r a t i o n of 1% polymer (w/v) the e q u i l i b r i u m d i s t r i b u t i o n c o n t a i n s >75% c y c l i c t r i m e r . The r e s u l t o f e s t e r interchange i n s o l u t i o n i s , t h e r e f o r e , t o broaden the d i s t r i b u t i o n by the g e n e r a t i o n o f c y c l i c s p e c i e s . w

n

Co n c l u s i o n s The azeotrope 70/30 MeCl /HFIP i s an e x c e l l e n t s o l v e n t f o r PET and s i m i l a r polymers, as w e l l as f o r p o l y s t y r e n e . This combination, along w i t h i t s UV transparency, makes i t a n e x c e l l e n t GPC s o l v e n t . The Du Pont Product Information and M a t e r i a l Safety Data Sheet on HFIP should be c o n s u l t e d before u s i n g t h i s system. 2

In Size Exclusion Chromatography; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

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Literature Cited 1. J. R. Overton, J. Rash, and L. D. Moore, Jr., Sixth International GPC Seminar Proceedings, Miami Beach, Florida October 7-8, 1968, p. 422. 2. G. Shaw, Seventh International GPC Seminar Proceedings, Monte Carlo, 1969, p. 309. 3. L. D. Moore, Jr., and J. R. Overton, J . Chromatogr., 55, 137 (1971). 4. Y. Ishida and K. Kawai, Shirnadzu Hyoron, 29112, 89 (1972). 5. J. R. Overton and S. K. Haynes J. Polym. Sci. Part C, 43 9 (1973). 6. Ε. E. Paschke, B. A. Bidlingmeyer, and J. G. Bergmann, J. Polym. Sci. Polym. Chem., 15 983 (1977). 7. M. Sang, N. Jin, and E. F. Jiang, J. Liq. Chromatog., 5 (9), 1665 (1982). 8. S. Jabarin and D. C. Balduff, J. Liq. Chromatog., 5 (10), 1825 (1982) 9. P. J. Flory, J . Chem. Phys., 12, 425 (1944). 10. H. L. Browning, Jr. and J. R. Overton, Polymer Prepr., 18 237 (1977). 11. H. Jacobson and W. H. Stockmayer, J. Chem. Phys. 87, 931, (1965). 12. H. Jacobson, C. D. Beckmann, and W. H. Stockmayer, J. Chem. Phys., 18, 1607 (1950). RECEIVED October

20, 1983

In Size Exclusion Chromatography; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.