Direct computation of the free volume fraction in amorphous polymers from positron lifetime measurements

A model for computation of the free volume fraction from the temperature de pendence of positron lifetime is presented. The model is based on the idea that the total free volume of the sample is expanding in the same rate as t he volume of holes seen by positrons. Then, having the values of thermal ex pansion coefficients of bulk and free volume, it is possible to compute the temperature dependence of fraction of free volume f(T) and the thermal exp ansion coefficient of occupied volume alpha(0). It is also possible to comp ute the Vogel temperature T-0, WLF coefficient c(2g), and the ratio C-lg/B, where c(lg) is the WLF coefficient and B is the Doolittle constant. The pa per also presents computed values of abovementioned parameters for six diff erent polymers. Tn the case of poly(isobutylene) and cia-trans 1,4 poly(but adiene) the results are compared with that obtained from viscosity eta(T) a nd temperature shift parameter a(T)(T) measurements.