Isothermal crystallization kinetics of syndiotactic polystyrene exposed to gamma radiation in vacuum
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Julie P. Harmon Chemistry Department, University of South Florida, Tampa, Florida 33620, USA
Sanboh Leea) Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan (Received 27 August 2014; accepted 17 November 2014)
The effect of gamma radiation in vacuum on the isothermal crystallization kinetics of syndiotactic polystyrene (sPS) was investigated via differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction (XRD). Amorphous sPS samples were irradiated in vacuum, heated to 310 °C, cooled down to crystallization temperatures (Tcs) from 220 to 260 °C, and annealed for different times. Upon reheating, overlapping endothermic melting peaks depicted the various crystallization forms, a, b, and b9. The endotherms were resolved using Gaussian functions relating enthalpy changes to the endothermic envelope. Isothermal crystallization kinetic data were analyzed using Avrami’s model with Gaussian functions. The extent of crystallization of b and b9 forms increased with increasing crystallization time and temperature, while that of a form decreased. Crystallization half-time followed a modified Arrhenius equation. Crystallization activation energies for the b and b9 forms of sPS increased with increasing radiation doses. The results are compared to those of air irradiated sPS reported in the literature.
I. INTRODUCTION
Atactic polystyrene (aPS) exhibits moderate mechanical, thermal, and chemical stability and has been used extensively in optical applications. There is a window of transparency from the ultraviolet–visible region of the electromagnetic spectrum from the absorption peak at 250–300 nm assigned to the s0 to s1 transition of the benzene ring through the visible region.1 This particular window of transparency attracted those interested in aPS scintillators.2–4 During scintillation, the phenyl rings in the aPS backbone absorb ionizing radiation and there is a nonradiative energy transfer from the polymer to a fluorescent dye which emits light from 350 to 450 nm. aPS scintillator materials have been used to detect ionizing radiation via fluorescence measurements in a variety of applications. This spurred interest in developing scintillators with properties that are stable when exposed to ionizing radiation. Wallace et al.5 investigated the effect of gamma radiation on color center formation in aPS irradiated in vacuum and in air to doses up to 100 kGy. They found that the absorption at 400 nm is greater in vacuum annealed than in air annealed samples.
Contributing Editor: Franz Faupel a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.402 592
J. Mater. Res., Vol. 30, No. 4, Feb 28, 2015
http://journals.cambridge.org
Downloaded: 03 Mar 2015
Chiang et al. 6 analyzed postradiation annealing data of Wallace et al. 5 and determined that annealing induced color center annihilation data followed the first order kinetics for samples irradiated in air and the second order kinetics f
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