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 dependence 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 the volume of holes seen by positrons. Then, having the values of thermal expansion coefficients of bulk and free volume, it is possible to compute the temperature dependence of fraction of free volume $f(T)$ and the thermal expansion coefficient of occupied volume α₀. It is also possible to compute the Vogel temperature T₀, WLF coefficient c2g, and the ratio c1g/B, where c1g is the WLF coefficient and B is the Doolittle constant. The paper also presents computed values of abovementioned parameters for six different polymers. In the case of poly(isobutylene) and cis-trans 1,4 poly(butadiene) the results are compared with that obtained from viscosity η(T) and temperature shift parameter aT(T) measurements.