Birefringence change in epoxy glass aged under strain

Structural relaxation in epoxy glass aged under strain (strain aging) was studied by means of birefringence. Two types of samples were prepared from the same epoxy precursor: one has a crosslinked structure and the other has a linear molecular structure. Epoxy glasses were uniaxially elongated and then strain aged for various aging times. Elongation was resumed at the end of strain aging. Birefringence change was measured over strain aging tests. Modified stress-optical rule, MSOR, was applied to experimental results to separate (total) stress into two stress components: glassy stress and rubbery stress. Experimental results indicated that crosslinks cause co-operation between the thermal motions of flexible polymer chains and the rotational orientation of the monomer unit around the chain axis.