Journal of Polymer Science Part B: Polymer Physics, Vol.39, No.19, 2252-2262, 2001
Photoviscoelastic behavior of amorphous polymers during transition from the glassy to rubbery state
Tensile stress-relaxation experiments with simultaneous measurements of Young's relaxation modulus, E, and the strain-optical coefficient, C-epsilon were performed on two amorphous polymers-polystyrene (PS) and polycarbonate (PC)-over a wide range of temperatures and times. Master curves of these material functions were obtained via the time-temperature superposition principle. The value of C-epsilon of PS is positive in the glassy state at low temperature and time; then it relaxes and becomes negative and passes through a minimum in the transition zone from the glassy to rubbery state at an intermediate temperature and time and then monotonically increases with time, approaching zero at a large time. The stress-optical coefficient of PS is calculated from the value of C-epsilon. It is positive at low temperature and time, decreases, passes through zero, becomes negative with increasing temperature and time in the transition zone from the glassy to rubbery state, and finally reaches a constant large negative value in the rubbery state. In contrast, the value of C. of PC is always positive being a constant in the glassy state and continuously relaxes to zero at high temperature and time. The value of C-sigma of PC is also positive being a constant in the glassy state and increases to a constant value in the rubbery state. The obtained information on the photoelastic behavior of PS and PC is useful for calculating the residual birefringence and stresses in plastic products.
Keywords:amorphous polymers;polystyrene;polycarbonate;relaxation;linear viscoelasticity;photoviscoelasticity;optical memory;Young's modulus;strain-optical coefficient;stress-optical coefficient;residual stresses;frozen-in birefringence