Effect of free volume and temperature on the structural relaxation in polymethylphenylsiloxane: A positron lifetime and pressure-volume-temperature study Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps ͑ortho-positronium͒ formation in two amorphous polymers. We performed positron lifetime measurements on poly͑vinyl acetate͒ ͑PVAc͒ in the temperature range of 84-414 K and on poly͑methyl methacrylate͒ ͑PMMA͒ in the temperature range of 84-454 K and as a function of time (Ͻ200 h) at four temperatures ͑Tϭ84, 149, 224, and 249 K͒. The glass transition temperature T g and secondary transition temperature were determined from the average o-Ps lifetime 3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method ͑MELT͒. Analysis by the MELT program on the spectra shows that ͑i͒ the standard deviation ( 3 ) of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature T g ; ͑ii͒ the ( 3 ) in PMMA has no observable variation as a function of positron irradiation time, which suggests that the measured free-volume distribution is not influenced by the positron irradiation. The o-Ps formation probability shows large variations with temperature, thermal history, and measuring time. These effects are discussed in the framework of the spur model.

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Hydrogen behavior and hydrogen-enhanced lattice defect formation under elastic stress of tempered martensitic steel were clarified with respect to dislocations and vacancies by thermal desorption analysis (TDA) using hydrogen as a probe of defects and a positron probe microanalyzer (PPMA). The relationship between hydrogen embrittlement and lattice defects associated with hydrogen was also investigated. The amount of lattice defects increased gradually with increasing time of applied stress during hydrogen charging. The specimen fractured under elastic stress in the presence of hydrogen macroscopically showed brittle fracture without necking. Whereas fracture surface was attributed to localized plastic deformation, since the morphology of the microscopic fracture surface was mostly quasi-cleavage fracture. The increased lattice defects in the near-fracture area were subsequently removed by annealing at 200°C. The mean positron annihilation lifetime measured with the PPMA for a fractured specimen was longer in the near-fracture area than in other areas. Thus, the most probable reason for the increase in the amount of lattice defects can be ascribed to an increase in the amount of vacancies or vacancy clusters. Regarding hydrogen embrittlement involving microscopic plastic deformation, the localized enhanced vacancies due to interactions between dislocations and hydrogen under elastic stress directly caused ductility loss, because ductility loss occurred even though hydrogen was completely removed by degassing before the tensile test. Besides hydrogen content and applied stress, the time of formation and accumulation of vacancies are also concluded to be important factors causing hydrogen embrittlement.

Interlaboratory comparison of positron annihilation lifetime measurements using synthetic fused silica and polycarbonate was conducted with the participation of 12 laboratories. By regulating procedures for the measurement and data analysis the uncertainties of the positron lifetimes obtained at different laboratories were significantly reduced in comparison with those reported in the past.

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