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12 Radiation-Induced D e g r a d a t i o n o f Poly(2-methyl-1 -pentene sulfone) Kinetics and Mechanism M. J. BOWDEN, D. L. ALLARA, W. I. VROOM, J. FRACKOVIAK, L. C. KELLEY, and D. R. FALCONE Bell Laboratories, Murray Hill, NJ 07974 PMPS degrades by a mechanism involving random chain scission followed by extensive depropagation from the chain ends. Both radical and cationic depropagation pathways may exist. The initial rate of loss of SO is greater than the rate of loss of 2-methyl-1 -pentenealthough at high doses the amount of SO and olefin lost from the film are roughly comparable. The discrepancy between the amounts of SO and olefin lost at low doses is attributed to oligomerization of the olefin (possibly by cationic reactions) as it diffuses through the film. These oligomers can be easily removed by post-exposure baking leaving "pure" PMPS behind. These results are consistent with degradation studies of other poly(olefin sulfones), but are contrary to thefindingsof Pacansky et al. whose results appear to be anomalous. We may also note that the kinetics of radiation-induced depolymerization in purefilmor in mixture with a novolac resin appear to be qualitatively similar although there is a slight inhibition caused by the novolac. It appears though that the contrast of the vapor development process increases with dilution in spite of slight inhibition by the novolac resulting in efficient material removal at high doses. 2
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0097-6156/ 84/ 0242-0135S06.00/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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Poly(2-methyl-l-pentene sulfone) P M P S , is an alternating copolymer of 2 methyl-1-pentene and S 0 produced by free radical polymerization. The radiation-induced degradation of P M P S has been studied extensively using both 1.0 M e V electrons ( 0 (Van de Graaff electrostatic generator) and 10-20 k e V electrons (2—4). In common with other poly (olefin sulfones), P M P S undergoes random scission followed by depropagation from the broken chain ends. In a previous paper, the degradation reaction occurring during irradiation with 1.0 M e V electrons was followed by monitoring changes i n the infrared spectra of thin films as a function of dose. Since depropagation leads to material loss, the spectral absorbance decreases with increasing dose. A priori, one might expect that depropagation of radical chain ends produced by random scission should lead to equal yields of S 0 and the parent olefin. However, it was found that, in common with other poly (olefin sulfones) (5—8), the yield of S 0 was greater than that of the olefin provided the irradiation temperature was < 1 0 0 ° C . Above 1 0 0 C , the peaks i n the infrared spectrum attributable to S 0 and olefin moitiés decreased at equal rates. Bowden (1) suggested that the discrepancy in the rate of removal of S 0 and 2 - m e t h y l - l pentene at lower temperatures might be due to reaction of the olefin, e.g., cationic homopolymerization, as it diffused through the film. Evidence for cationic reactions during radiolysis of poly (olefin sulfones) was presented by Bowmer et al (7). in their studies on the effect of radical and ionic scavengers on the yields of volatile radiolysis products from several poly (olefin sulfones). In particular, they found that in the presence of cation scavengers, the overall product yield was reduced with concurrent reduction of the S 0 / o l e f i n ratio towards unity. They also found that olefin isomerization reactions which frequently accompanied depropagation (8) were eliminated. They accounted for these results in terms of two pathways for depropagation, viz., free radical and cationic, with associated reactions for cationic homopolymerization of olefin and for isomerization in the formation of olefin. Cationic homopolymerization was believed to be initiated at cationic sites on the polymer chain. This overall mechanistic picture has been questioned recently by Pacansky et a l . ( 9 - 1 0 ) They reported virtually total elimination of S 0 from P M P S films at an incident electron dose of 0.4 μ Ο / α η at 25 k V with little or no loss of olefin. Olefin loss took place at much higher doses. Pacansky proposed a two-step mechanism involving S 0 elimination via a chain process leaving hydrocarbon polymer which subsequently decomposed to olefin. The results are totally contrary to those previously reported by Bowden who found that at doses corresponding to 0.4 μ Ο / α η only — 3 5 % of the S 0 was removed while 30% of the olefin was removed. The mechanism proposed by Pacansky et a l . to explain their results is counter to the widely accepted mechanism of polysulfone degradation which is in accord with the thermodynamics of these polymers. It is difficult, for example, to conceive of a chain mechanism for S 0 formation that does not include olefin formation. The "instantaneous" reformation of the 2
Downloaded by UNIV OF ARIZONA on May 26, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch012
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Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by UNIV OF ARIZONA on May 26, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch012
12.
BOWDEN ET AL.
PMPS
Degradation:
Kinetics and
Mechanism
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polyolefin i n the case of P M P S after multiple C - S bond scission as suggested by Pacansky would seem to be highly improbable. The material loss occurring as a result of depolymerization is reflected as a thickness loss which can also be monitored as a function of dose (2—4). This technique was used by Bowden and coworkers to follow the degradation of thin P M P S films (
