J. Phys. Chem. B 1999, 103, 3043-3048
3043
Ion Beam Induced Crosslinking Reactions in Poly(di-n-hexylsilane) Shu Seki,*,† Kensaku Maeda,† Yoshihisa Kunimi,† Seiichi Tagawa,† Yoichi Yoshida,† Hisaaki Kudoh,‡ Masaki Sugimoto,‡ Yosuke Morita,‡ Tadao Seguchi,‡ Takeo Iwai,§ Hiromi Shibata,§ Keisuke Asai,| and Kenkichi Ishigure| The Institute of Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka, Japan 567-0047, Japan Atomic Energy Research Laboratories, Takasaki Radiation Chemistry Institute, 1233 Watanuki-machi, Takasaki, Gunmma, Japan 370-1207, Research Center for Nuclear Science and Technologies, The UniVersity of Tokyo, 2-22 Shirakata-shirane, Tokai, Ibaraki, Japan 319-1106, and Faculty of Engineering, The UniVersity of Tokyo, Japan, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654 ReceiVed: December 10, 1998
Thin solid films of poly(di-n-hexylsilane) were irradiated with a variety of high-energy ion beams, electron beams, and 60Co γ-rays of which linear energy transfer (LET) ranges from 0.2 to 1620 eV/nm. The beams caused nonhomogeneous reactions of crosslinking and main chain scission in the films. The molecular weight of the polymer was traced to give the efficiency of crosslinking reactions: G(x) based on the CharlesbyPinner relationship. The value of G(x) increases from 0.042 to 0.91 with increasing values of LET. We adopt a reaction model in a single ion track to the crosslinking reactions, and the expanding chemical track along an ion trajectory is responsible for the increasing crosslinking G values. The theoretical aspects of the energy distribution in the penumbra area give a good interpretation of the chemical track radii obtained in this study.
Introduction Recent studies of polysilanes have suggested many interesting features of polysilanes, such as semiconductive property,1,2 nonlinear optical property,3 and photoconductivity.4,5 These features of polysilanes have been ascribed to the σ electrons delocalized along the skeleton. The UV light induced photolysis in polysilane derivatives has been widely studied because of their potential use as UV photoresist materials.6,7 Trefonas et al. reported that high molecular weight poly(n-hexylmethylsilane) showed a UV absorption spectral shift and molecular weight reduction upon exposure to 313 nm light.8 Zeigler et al., Hofer et al., and Miller reported that photovolatilization occurred during irradiation by an excimer laser for alkyl substituted polysilanes.9-11 Polysilane derivatives and their reactions induced by electron beam (EB) irradiation are also of interest. Miller et al. reported that alkyl substituted polysilanes could be made to have positive patterns using EB because of the dissociation reactions of polymer main chain.12 Taylor et al. also studied the EB induced reactions in polysilane copolymers but reported the low efficiency of main chain scission reactions.6 They attributed it to the increased efficiency of crosslinking reactions under high vacuum conditions. UV light induced scission reactions, however, become a problem for the use of polysilanes as electrooptical materials making the best use of the electronic properties of the polymers. The crosslinking reactions should play a significant role in the practical use of the polymer materials. Our previous studies reported on reactive intermediates of polysilane derivatives irradiated by ions, electrons, and γ-rays.13-16 Predominant reactive intermediates in polysilanes were assigned to silyl * To whom all correspondence should be sent. E-mail:
[email protected].
radicals showing great stability in comparison with carboncentered alkyl radicals.13,14 The ion beam irradiation effects on polysilanes were also investigated, and the reactions in the polymers changed with the energy deposition rate of incident particle: linear energy transfer (LET) of radiation sources.15,16 Polymers were crosslinked for high LET ion beam irradiation despite the predominant main chain scission reaction for low LET radiation. The difference in radiation-induced reactions was ascribed to a variation of the density of stable reaction intermediates, i.e., silyl radicals generated by radiation. It has been suggested that the spatial distribution of the deposited energy by charged ions has played a significant role in the chemical reactions occurring in the target materials.17 Models of the energy distribution were proposed experimentally and theoretically as “track core” and “penumbra” models by Magee et al.,18 Varma et al.,19,20 Wingate et al.,21 Katz et al.,22 and Wilson.23 Despite the theoretical modeling efforts, there are still unknown factors in the relationship between the ion track structure and the values of track radii that were experimentally obtained by the analysis of irradiation products.24-26 Puglisi et al., Licciardello et al., and Calcagno et al. also reported the effects of ion beam bombardment on polystyrene leading to the aggregation of molecules and crosslinking reactions.27-29 The abnormal change in molecular weight distribution was ascribed to the intratrack reaction; however, the estimated size of ion tracks was also larger than that of the track core. LaVerne et al. reported the considerable decrease in the radiation chemical yield for ferric production in the Fricke dosimeter. They suggested a model of a deposited energy density in an ion track that depends on the LET and the atomic number of an irradiation particle.25,30 We discuss the efficiency of crosslinking and main chain scission reactions induced by ion beams, EB, and γ-rays with
10.1021/jp9847080 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/01/1999
3044 J. Phys. Chem. B, Vol. 103, No. 15, 1999
Seki et al.
TABLE 1: G Values of Crosslinking and Main Chain Scission upon Irradiation of a Variety of Radiation Sources radiation Ar8+
175 MeV 2 MeV N+ 160 MeV O6+ 225 MeV O7+ 2 MeV He+ 220 MeV C5+ 20 MeV He2+ 2 MeV H+ 20 MeV H+ 45 MeV H+ 20 keV e30 keV e60 Co γ-rays
LET (eV/nm)
Dga (MGy)
G(x)b
G(s)b
r + dr (nm)
F (eV/nm3)
1620 1580 300 230 180 110 35 17 2.7 1.4 2.1 1.6 0.25
0.81 0.89 3.1 3.4 3.3 2.9 5.4 6.8d
0.91 0.86 0.69 0.65 0.66 0.81 0.19 0.12 0.059 0.061 0.083 0.078 0.042
0.17 0.17 0.064 0.10 0.33 0.35 0.16 0.11 0.27 0.32 0.28 0.32 0.45
7.0 6.8 6.1 5.9 6.0 6.6 3.2 2.5 1.7e 1.8e
7.0 10 1.6 1.2 1.3 0.48 0.67 0.54 0.14 0.067
typec CL CL CL CL CL CL CL CL CS CS CS CS CS
a Dg, gelation dose. b G(x) and G(s) are G values of crosslinking and main chain scission, respectively. c CL and CS denote predominant reactions as crosslinking and chain scission, respectively. d The values were obtained for PDHS-L. e Estimated by the G values of crosslinking.
a variety of LET ranging from 0.2 to 1620 eV/nm in the present study. The molecular weight of the polymer is traced as a function of absorbed dose, giving the G values (number of reactions per absorbed 100 eV) of the reactions. Both reactions in polysilanes are able to be regarded as reactions occurring in an individual ion track. A model of intratrack reactions reveals the chemical core to have a variety of sizes with changing LET. Experimental Section Poly(di-n-hexylsilane) (PDHS) was synthesized by the reaction of di-n-hexyldichlorosilane with sodium in refluxing toluene. The reaction was carried out under an atmosphere of predried argon. The chlorosilane was purchased from Shinetsu Chemical Inc. and doubly distilled prior to use. The molecular weight distribution in PDHS was measured by gel permeation chromatography (Shimadzu class VP-10) with tetrahydrofuran (THF) as an eluent. The chromatograph equipped with four columns of Shodex KF-805L from Showa Denko Co. Ltd. PDHS has initially a bimodal molecular weight distribution, and the molecular weight was controlled by the reaction time and additives (12-crown-4 and diethyleneglycol dimethyl ether as sodium activators). The high or low molecular weight peak was cut off by separatory precipitation, leading to PDHS having a lower molecular weight (PDHS-L) and a higher molecular weight (PDHS-H) with a monomodal distribution. The molecular weights of the polymers were Mn ) 0.98 ≈ 1.3 × 104 and Mw ) 3.9 ≈ 5.6 × 104 for PDHS-L and Mn ) 5.6 × 105 and Mw ) 1.4 × 106 for PDHS-H, each determined by polystyrene calibration standards. The PDHS polymers were dissolved in toluene and spin-coated on Si wafers (0.5 mm thick), Kapton films (0.03 mm thick), and PEEK films (12 µm thick) for a 1-3 µm thickness. These films were irradiated in a vacuum chamber (
