Radiation Effects on Polymers - American Chemical Society

Chapter 8

Gamma Radiolysis of Styrene-co-Methyl Acrylate Copolymers An Electron Spin Resonance Study 1

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Raymond Kellman , David T. J. Hill , D. S. Hunter , James H. O'Donnell , and Peter J. Pomery 2

Downloaded by UNIV OF ARIZONA on August 10, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch008

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Department of Chemistry, San Jose State University, San Jose, CA 95192-0101 Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, Brisbane, Queensland, 4072 Australia

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Gamma radiolysis of poly(styrene-co-methyl acrylate) was carried out at 77K and 300K. Radical yields were determined by ESR spectroscopy using double integration. At 77K G[R·] values decreased with increasing styrene content and a styryl protective effect was observed. At 300K results were complicated by T effects. Photobleaching at 77K and annealing experiments allowed structural elucidation of the radical anion and neutral radical species formed on radiolysis. g

The r a p i d l y expanding use of polymers as materials i n h o s t i l e environments, i n microlithography and i n medical a p p l i c a t i o n s makes an understanding of the fundamental processes i n v o l v e d i n r a d i a t i o n i n d u c e d polymer degradation an incumbency. The Polymer and R a d i a t i o n Group at the University of Queensland has f o r some time pursued a broad i n v e s t i g a t i o n of t h e fundamental r e l a t i o n s h i p between molecular structure and r a d i a t i o n s e n s i t i v i t y of polymers and copolymers i n the s o l i d state (2,2) . As part of this program we were interested i n the degradation of poly(styrene-co-methyl acrylate) under high-energy r a d i a t i o n (3). In t h i s paper we wish t o report results of an ESR study on the formation and fate of radical species produced on gamma i r r a d i a t i o n of these copolymers. Experimental The styrene-methyl acrylate copolymers as well as the styrene and methyl acrylate homopolymers used i n t h i s 0097-6156/91/0475-0119S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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RADIATION EFFECTS ON POLYMERS

study were p r e p a r e d by b u l k , AIBN initiated polymerization which has been described i n d e t a i l p r e v i o u s l y ( 3 ) . S i x copolymers which v a r i e d i n composition from 12 to 80 mole % styrene were prepared and used (see Table I ) . Composition was determined by elemental microanalysis and HI FT-NMR spectroscopy (Joel GX-400). M u l t i p l e i n t e g r a t i o n of the C H - aromatic s t y r y l s i g n a l and t h e -OCH a c r y l a t e s i g n a l were averaged. 6

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Table I. Composition and Tq's of PolyUtyreneco-methyl acrylatels no.

mole % S T Y

1 2 3 4 5 6 7 8

Tg, C

0.0 12 18 29 43 60 80 100

13 26 42 56 68 77 91 105

a- Determined by DSC; 0 to 125C * 10C/min. Polymer samples subjected to r a d i o l y s i s were prepared by packing the s o l i d polymers to a depth of ca. 3 cm i n Spectrosil-A grade quartz ESR tubes (i.d. 2.87 mm). The samples were evacuated overnight(295 Κ

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CH —C —CHf ^ 2

C0 CH 2

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Figure 10. a) ESR spectra of copolymer X = 0.43 as a function of temperature; b) Difference spectra. g


RADIATION EFFECTS ON POLYMERS

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and 25% of [R«] i n 4 (29 mole % STY). In those copolymers r i c h i n methyl acrylate the carbomethoxy pendant group r a d i c a l anion was present i n s i g n i f i c a n t amounts. The post photolysis ESR spectra at 77K revealed the presence a complex mixtures of f r e e r a d i c a l s which have been p a r t i a l l y elucidated by warm-up experiments and spectral subtraction techniques (See Table IV). The styrene units along the polymer backbone gave r i s e to cyclohexadienyl and 2° backbone r a d i c a l s but not t o 1 or b e n z y l i c r a d i c a l s which would result from chain s c i s s i o n . On the other hand, under r a d i o l y s i s the methyl a c r y l a t e u n i t s d i d s u f f e r chain s c i s s i o n t o give the 2° propagating r a d i c a l . Additionally, homolytic bond cleavage resulted in backbone radicals and pendant group radicals which are capable of crosslinking.


σ

Acknowledgments The authors (especially grantee RK) wish t o thank the Fulbright Foundation, the Australian-American Educational Foundation for financial support of this work. References

1- D.J.T. H i l l , J.H. O'Donnell, and P.J. Pomery, "Fundamental Aspects of Polymer Degradation by HighEnergy Radiation," in Materials for Microlithography, L.F. Thompson, C.G. Wilson and J.M.J. Frechet (Eds.), ACS Symposium Series, 266, 125-150, (1984). 2- D.J.T. H i l l , J.H. O'Donnell, and P.J. Pomery, "Applications of ESR in Polymer Chemistry," in Electron Spin Resonance 1985, 9, 223. 3- W.K. Busfield, J.H. O'Donnell, and C.A. Smith, J. Macro. Sci.- Chem. 1982, A17(8), 1263. 4- J.H. O,Donnell in The Effects of Radiation on High-Technology Polymers, E. Reichmanis and J.H. O'Donnell (Eds.), ACS Symposium Series, 381, 1, 1989. 5- R.W. Garrett, Ph.D. Thesis, University of Queensland, 1985, Brisbane, Australia. RECEIVED

July 11, 1991


Radiation Effects on Polymers - American Chemical Society

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