CHANGES OF THE HOLE VOLUME IN MODEL EPOXY NETWORKS

Positron annihilation lifetime spectroscopy measurements were conducte d on a series of epoxy specimens, prepared by systematically varying t he network rigidity and crosslink density. The investigations demonstr ate that changes in the molecular structure of the constituent monomer s change the microscopic parameters of the epoxy networks. An increase of chain rigidity increases the size and the number density of the tr apping sites where orthopositronium localizes, which leads to an incre ase of the hole volume fraction at T-g. It was found that the hole vol ume fraction at T-g approximates a linear relationship with the packin g densities of the epoxies under investigation. The results are interp reted as a decrease of the efficiency of molecular packing with increa sing network rigidity. As a result, macroscopic materials properties s uch as glass transition temperature, density and thermal expansion sho w considerable variations.