Time–temperature superposition of the dynamic birefringence of polyethylene

The dynamic strain‐optical coefficient of low‐density polyethylene is found to decrease with increasing frequency and increase with increasing temperature (at temperatures up to 60°C.). It is found that a series of curves representing the real, K ′, and imaginary, K ″, parts of the frequency variation of the strain‐optical coefficient obtained at various temperatures up to 60°C. may be superposed by translation along the frequency axis. The temperature dependence of the shift factor corresponds to an Arrhenius type equation with an activation energy of the order of 25 kcal./mole. This may be compared with the similar value obtained by Takayanagi for the α c ′ mechanical loss and with the dynamic x‐ray diffraction values reported by Kawaguchi et al., but is somewhat lower than the value of 34 kcal./mole reported by Onogi et al. from birefringence relaxation studies. Takayanagi has ascribed the α c ′ peak to cooperative motion of crystalline and amorphous regions of the sort that might be involved in the suggested sliding of lamellae over each other. At temperatures higher than 80°C., horizontal superposition is not possible.