Abstract
It is known that the high energy irradiation is a convenient method to improve the mechanical property of polymer. Nevertheless, the kinetic mechanism of the gamma rays-irradiated polymer at annealing temperature above the glass temperature is still unknown. The effect of post-irradiation annealing on polyethylene is studied. We propose the theoretical model to analyze the commercial polyethylene which are HDPE(high density polyethylene) and LDPE(low density polyethylene). The variation of the microhardness with time and different irradiation doses reveal significantly. The HDPE and LDPE samples were exposed in Cobalt-60 gamma rays source at a dose rate of 5 kGy/hr and post annealed at 313-393 K in air condition. The 0 kGy HDPE and 0 kGy LDPE are the control specimens. No matter what the samples are treated before or after irradiation, the microhardness of HDPE is greater than that of LDPE due to the crystallinity. The crystallinity was obtained from XRD(X-Ray Diffraction) and DSC(Differential Scanning Calorimetry). Both the microhardness of HDPE and LDPE achieve saturation below 40 hours. The hardness increases significantly with the irradiation dose. The HDPE exposed in 800 kGy has the best performance in microhardness. The hardness are greater when the PE annealed in the elevated temperature below the Vicant softening point. The result is the same as the PC (polycarbonate) whose glass transition temperature is below the aging temperature. However, the phenomena of PE is opposite to those of PHEMA(Poly(2-Hydroxyethyl Methacrylate)) and PMMA(Poly(methyl methacrylate)). Theoretically, the rate constant of the hardening process obeys the Arrhenius equation. The hardness is controlled by the defect induced by the gamma-rays irradiation. The activation energy of higher irradiation dose is lower than the activation energy of lower irradiation dose. The enthalpy change is larger than zero, and it satisfies endothermic process.