Effect of ionizing radiation on polysilane

Radiat. Phys. Chem. Vol. 47, No. 4, pp. 631-636, 1996

Pergamon

0969-g~6x(gs)00066-6

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0969-806X/96 $15.00 + 0.00

EFFECT OF IONIZING RADIATION ON POLYSILANE JUN K U M A G A I , KEN-ICHI OYAMA, HIROSHI YOSHIDA and TSUNEKI I C H I K A W A t Graduate School of Engineering, Hokkaido University, Sapporo, 060 Japan (Received 12 December 1994; accepted 27 February 1995)

Abstract--ESR and gel permeation chromatographic measurements of poly(diethylsilane) and poly(cyclohexylmethylsilane) v-irradiated at temperatures between 77 K and 300 K were carried out to elucidate the mechanism of radiation-induced degradation of polysilanes. Radical species observed after the irradiation are not silyl-type radicals generated by the cleavage of the Si--Si bond but alkyl radicals generated by the scission of the C--H bond. The G value of the main-chain scission depends strongly on the irradiation temperature and dose. It is proposed that or-conjugation along the main chain allows the migration of the radiation energy to the weakest bond in the main chain where the direct scission of the bond is induced.

INTRODUCTION

Effect of ionizing radiation on polymers is generally divided into main-chain scission (degradation) and crosslinking. Although both kinds of effect occur in parallel in many polymers (Charlesby, 1987), a polymer is termed degrading polymer if the scission surpasses the crosslinking. The degrading polymer such as poly(methyl methacrylate) is used as highresolution positive-type resist material in X-ray and electron beam lithography (Harris, 1973; Hiraoka et al., 1977). The molecular structure of degrading-type polymers with C - - C main chains is generally expressed as [---CH2---CRR'--]n (Chapiro, 1962, Guillet, 1985). The function of the alkyl group is considered to introduce strain into the main chain. It has been believed that the strain prevents the recombination of primary alkyl radicals generated by direct rupture of a main chain, or accelerates conversion of primary polymer radicals into scission-type radicals accompanying the #-scission of the main chain. We have recently studied the mechanism of radiation-induced degradation of poly(methyl methacrylate) and revealed that the scission of the main chain does not arise from the direct effect of radiation but it is induced by the intra-molecular radical conversion of the side-group radical, ---COOHC'~, to the tertiary - - C H 2 C ' C H 3 - - radical followed by the main-chain #-scission (lchikawa and Yoshida 1990, 1991; Ichikawa et al., 1994; Tanaka et al., 1990). The degradation occurs at above 200 K, because the radical conversion takes place above 200 K. The residual monomer in the polymer reacts with the sidegroup radical below 200 K to transform it into the stable propagating-type radical, and to suppress

the degradation occurring at high temperature, We believe such a mechanism is operative generally in the degradation of the other polymers with the C - - C main chain. Recently another type of radiation-degrading polymers, called polysilanes, was demonstrated to be applicable to electron-beam lithography (Miller et al., 1988). Polysilanes are a-conjugated polymers with Si--Si main chain and organic side groups. Recently they have attracted much attention because of their potential utility not only as radiation-resist and photo-resist but also as one-dimensional conductors, nonlinear optical materials, and high-density optical data storage materials (for recent review, see Miller and Michl, 1989; Michl, 1992). Although the G values for the scission of several polysilanes have been reported (Miller and Michl, 1989), the mechanism of the degradation has been still unknown. The present study is aimed at obtaining detailed information on the effect of ionizing radiations on polysilane by means of gel-permeation chromatography (GPC) and electron spin resonance spectroscopy (ESR), and at understanding the mechanism of radiation-induced degradation. EXPERIMENTAL Poly(cyclohexylmethylsilane) was synthesized from corresponding dichlorodialkylsilane by Wurtz type reaction in toluene with Na metal under the reflux condition (Miller et al., 1991). The average molecular weight was A ) n = 33,500. Poly(diethysilane) was kindly donated from Dr N. Kushibiki. The molecular weight of poly(diethylsilane) could not be measured, because it was insoluble in any solvent. The deaerated polysilanes sealed in quartz tubes were irradiated with 6°Co v-rays in darkness with the dose rate 19 kGy/h. The irradiation temperature was controlled by immers-

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--~i--,CH,--CHa

ing a sample in the slurry of an organic solvent (pentane = 143 K, methanol = 175 K, chloroform = 210 K). The irradiated samples were then stored in liquid nitrogen. The ESR spectra of y-irradiated samples were measured at 77 K on a Varian E9 X-band spectrometer. The modulation width and the incident microwave power were carefully chosen to minimize the spectral distortion. For observing the effect of annealing temperature on the ESR spectra, the irradiated samples were immersed in the slurry for 30 min and then stored in liquid nitrogen. The molecular weight of poly(cyclohexylmethylsilane)was measured at room temperature by GPC using polystyrene standards and Sis(CH3)I8 for calibration with appropriate corrections. RESULTS

Poly(diethylsilane)

The ESR spectrum of poly(diethylsilane) 7irradiated at 77 K is shown in Fig. 1. The spectrum at 7 7 K is composed of a broad quintet with a splitting of about 2.3 mT. This splitting value is a characteristic hyperfine coupling of protons in alkyl radicals. The hyperfine structure becomes better resolved by annealing the sample above 143 K, due probably to the conformational relaxation of the alkyl radical to a single geometrical arrangement. The quintet can be assigned as the side-group silylethyl radical, ~ S i C ' H C H 3 with the binomial intensity ratio of 1: 4: 6: 4:1 and the hyperfine splitting constant of 2.3 mT. Annealing of the sample above 143 K also causes the appearance of a sharp quartet with the splitting about 2.1 mT and the binomial intensity ratio of 1: 3 : 3 : 1, which can be assigned to the methyl radical. Increase of the annealing temperature above 210 K causes the increase of the methyl radical at the expense of the silylethyl radical. Annealing of the sample at 300 K causes complete disappearance of the silylethyl radical. The quartet spectrum due to the methyl radical and a broad spectrum due to unidentified radical species remain. The unusual stability of methyl radical at room temperature, together with the smaller hyperfine splitting of 2.1 mT (2.3roT for regular methyl radicals) indicates that the methyl radical is strongly trapped in the polymer (Kubota et al., 1971). In irradiated poly(diethylsilane), no conversion occurs from the primary side-group radical to the

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Fig. 1. ESR spectra at 77 K of ?:-irradiated poly(diethylsilane), (A) immediately after the irradiation, and after thermal annealing for 30 rain (B) at 143 K, (C) at 210 K and (D) at 300 K. Irradiating was made to a dose of 40 kOy. scission-type "Si(C2Hs)2-radical, which is expected to give a seven-line ESR spectrum with the splitting of about 0.45mT and the intensity ratio of 1 : 2 : 3 : 4 : 3 : 2 : 1 due to two sets of methylene fl protons with different hyperfine splitting constants of approximately 0.9 mT and 0.45 mT, respectively (McKinley et aL, 1991). This contrasts with the radical conversion observed in irradiated poly(methyl methacrylate), where side-group radical is the precursor of the scission-type radical and is responsible for the main chain scission (Ichikawa et aL, 1994). It is concluded that, for poly(diethylsilane), the radical reaction is not the main process of the main-chain scission. The side-group radical generated by the dil'ect action of ionizing radiation disappears by radical recombination through the migration of radical sites by hydrogen atom transfer or by intramolecular radical conversion to the methyl radical probably through the reaction.

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Fig. 2. ESR spectra at 77 K of y-irradiated poly(cyclohexylmethylsilane), (A) immediately after the irradiation, and after thermal annealing for 30 min at 143 K, (B) at 175 K, (C) at 210 K and (D) at 300 K. Poly(cyclohexylmethylsilane) ESR analysis. The ESR spectra for 7-irradiated poly(cyclohexylmethylsilane) did not depend on the radiation dose within the range examined (4.8-40 kGy) except for linear increase of the spectral intensity. Figure 2 shows the ESR spectra for poly(cyciohexylmethylsilane) after 7-irradiation at 77 K. The spectrum immediately after the irradiation is interpreted as being due to the overlapping component spectra corresponding to four different cyclohexyl-type radicals. The dominant component spectrum is due to radicals (3) and (4) which give the six-lines spectrum similar to the cyclohexyl radical due to two sets of two fl protons with the hypenqne coupling constants of 4. l mT and ca 0 mT, respectively and one ~ proton with the hyperfine coupling constant of 2.2 mT (Ogawa and Fessenden, 1964). The other component spectra are due to radical (1) and (2) which give narrower spectral patterns due to smaller number of fl protons. As shown in Fig. 2, annealing of the sample at 175 K or 210 K causes the appearance of

the triplet spectrum with a splitting of 3.6 mT at the expense of the six lines due to radical (3) and (4). The triplet spectrum is attributable to radical (1), which has two sets of methylene protons showing different hyperiine coupling constants, 3.6 mT and ca 0 mT, respectively. Annealing of the sample at 300 K causes the decrease in the total concentration of the radical species and the change of the ESR spectrum. The latter is probably due to the conformational change of radical (1). The total yield of the radical species per 100 eV energy absorbed, G, at 77 K was about 1. Annealing of the sample at 300 K for 30 min reduced the radical concentration to 25% of the initial value. The ESR spectrum of poly(cyclohexylmethylsilane) irradiated at 300 K was the same as that of the sample irradiated at 77 K and annealed at 300 K. Observation of no silyl radical indicates that the radical species generated by the direct action of ionizing radiation are the sidegroup alkyl radicals of radical (3) or (4), which converts to the stable tertiary alkyl radical, radical (1) by hydrogen-atom transfer. These results suggest that the radical species are not responsible for the degradation of polysilanes. Scission of the main chain takes place by the direct action of ionizing radiations. GPC analysis. Figure 3 shows the relationship between radiation dose and reciprocal of the numberaveraged molecular weight JQ'n for poly(cyclohexy|methylsilane) y-irradiated at 300 K. The dose D vs l//~tn relationship does not follow the well-known equation of I / , Q n ( D ) + 1/Mn(0) + GsD/(9.65 × 109), where Gs is the number of scissions per 100 eV energy absorbed and D is the dose in Gy unit (Charlesby and Pinner, 1959). It deviates from a straight line below 20 kGy. Assuming that the crosslinking is negligible 12O I

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irradiation temperature below 175 K and is almost zero at about 77 K. Inhibition of the main-chain scission at 77 K indicates that thermal energy is necessary for the main-chain scission to occur, though the energy of ionizing radiation is much larger than thermal energy.

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Fig. 4. Comparison of the gel permeation chromatogram of l~ly(cyclohexylmethylsilane) before (upper curve) and after (lower curve) y-irradiation at 300 K to a dose of 4.8 kGy. (Miller and Michl, 1989), the initial Gs value of the scission is as large as 17.4. It decreases quickly with increasing radiation dose up to 20 kGy where the molecular weight of the polymer is decreased to a half of the initial value. The Gs value beyond 20 kGy is 1.8 and is kept constant up to 400 kGy. The above result indicates that the efficiency of the main-chain scission per polymer unit chain decreases to one-tenth after the first scission of the polymer chain. Figure 4 compares the distribution of molecular weight before and after the irradiation of the polymer to 4.8 kGy. Although the Gs value of the scission is abnormally high at the dose of 4.8 kGy, no abnormal change of the distribution pattern is observed on the GPC pattern. The scission takes place rather randomly. Figure 5 shows the effect of irradiation temperature on the main-chain scission. The scission efficiency is kept constant between 175 K and 300 K. The efficiency, however, rapidly decreases by decreasing the

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DISCUSSION

The ESR and GPC studies indicate that the scission of the Si--Si bond is induced directly by ionizing radiation together with the assistance of thermal energy. It is also indicated that the Gs value of the scission is as high as 17.4 at low radiation dose but it drops down to 1/10 of the initial value, once the polymer chain suffers, on average, one scission. Explanation of these results, especially the decrease in the Gs value, necessitates a particular model which has not been considered for the degradation of vinyl polymers with C---C chain. Decrease in the Gs value with radiation dose might be explained either by the formation of molecular products inhibiting the main-chain scission or the disappearance of some special Si--Si bonds which are easily broken by irradiation. However, the former explanation is ruled out, since the Gs value of the scission is constant after the exposure of 20 kGy. The Gs value would decrease continuously with radiation dose, if the radiation caused the formation and accumulation of the scission-inhibitor. The remaining explanation is therefore the scissile Si--Si bond. Disappearance of the scissile bond after each decrease of the molecular weight to a half of the initial value suggests that each polymer molecule has one scissile bond which may be generated chemically during synthetic process of the polymer, or physically by thermal motion of atoms composing the main chain. Because of entanglement of polymer chains, the polymer chains can not move freely. The thermal motion therefore generates tensile stress to the chains. The tensile stress is especially strong if the polymer chain is fixed at two points, where the chain segments might be easy to be broken. The observed high Gs value and its rapid decrease with the dose can not be explained only by the scissile bond. The initial Gs value would not be affected by the scissile bond if the radiation energy is not selectively deposited on the scissile bond. We therefore need one more assumption, that is, the migration of excitation energy along the main chain through the cr-conjugation of the Si--Si bonds. We propose the following scission mechanism consistent with the observed results, though the structure of the scissile bond is not clear. (1) The scission of the Si--Si bond is not accompanied by the formation of radical species. A possible reaction mechanism for the bond scission is,

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(2) Direct scission of the polymer chain needs quick rearrangement of the original bonds. The rearrangement is slower at lower temperature, so that the scission is prohibited at low temperature. (3) The electronic excitation energy given by ionizing radiations moves along the polymer chain due to Si--Si a-conjugation to find a scissile Si--Si bond, and is dissipated to break the bond. (4) Assuming that one polymer molecule has initially one scissile bond, the G value of the scission decreases tremendously once the scissile bond is broken and the average molecular weight of the polymer decreases to a half of the initial value. The proposed mechanism should be further substantiated by examining for example, the effects of initial molecular weight, pre-irradiation, thermal treatment, etc. on Gs. The further study is now in progress. Acknowledgement--This work was supported by Grant-inAid for ScientificResearch from the Ministry of Education, Science and Culture, Japan.

REFERENCES

Chapiro A. (1962) In Radiation Chemistry of Polymeric Systems, p. 352. Interscience, New York.

Charlesby A. (1987) In Radiation Chemistry: Principles and Applications (Edited by Farhataziz and Rodgers M. A. J.), p. 451. VCH Publishers, New York. Charlesby A. and Pinner S. H. (1959) Proc. R. Soc. London Ser..4 249, 367. Guillet J. (1985) In Polymer Photophysics and Photochemistry, p. 353. Cambridge UniversityPress, Cambridge. Harris R. A. (1973) J. Electrochem. Soc. 120, 270. Hiraoka H., Gipstein E., Bargon J. and Welsh L. W. (1977) J. Appl. Polym. Sci. 22, 3397. Ichikawa T. and Yoshida H. (1990) J. Polym Sci. A 28, 1185. Ichikawa T. and Yoshida H. (1991) Radiat. Phys. Chem. 37, 367. Ichikawa T., Oyama K., Kondoh T. and Yoshida H. (1994) J. Polym. Sci. A 32, 2487. Kubota S., Iwaizumi M. and Isobe T. (1971) Bull. Chem. Soc. Jpn 44, 2684. McKinley A. J., Karatzu T., Wallraff G. M., Thompson D. P., Miller R. D. and Michl J. (1991) J. Am. Chem. Soc. 113, 2003. Michl J. (1992) Synth. Met. 49-50, 367. Miller R. D. and Michl J. (1989) Chem Rev. 89, 1359. Miller R. D., Thompson D., Sooriyakumaran R. and Fickes G. N. (1991) J. Polym. Sci: Part A: Polym. Chem. 29, 813. Miller R. D., Rabolt J. F., Sooriyakumaran R., Fleming W., Fickes G. N. and Guilett J. E. (1988) In Polymers for High Technology: Electronic and Photonic. ACS Symposium Series 360, Chap. 4. American Chemical Society, Washington, DC. Ogawa S. and Fessenden R. W. (1964) J. Chem. Phys. 41, 994. Tanaka M., Yoshida H. and Ichikawa T. (1990) Polym. J. 22, 835.