Temperature dependence of nanoholes free volumes in amorphous polymers

Estimation of the free volume fraction in polymers from positron annihilation lifetime data is usually obtained by assuming spherical nanoholes. Although this guess is sometimes well verified, in other cases non-spherical geometries give better agreement with theoretical predictions. Furthermore, the assumption of isotropic expansion of nanoholes (implicit in these models) could be unrealistic owing to constrained movements of the molecules. Comparison of hole-lattice theory with experimental data for some oligomeric structures gives evidence of anisotropic nanoholes expansion versus temperature.