Free-volume distribution and positronium formation in amorphous polymers: Temperature and positron-irradiation-time dependence

Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84–414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84–454 K and as a function of time (<200 h) at four temperatures (T=84, 149, 224, and 249 K).

The glass transition temperature Tg and secondary transition temperature were determined from the average o-Ps lifetime τ3 versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation σ(τ3) 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 Tg; (ii) the σ(τ3) 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.