Enthalpy relaxation in glassy polystyrenes. Part 3: Stress and enthalpy relaxation in poly(4-methylstyrene) and poly(4-chlorostyrene)

Structural recovery in poly(4-chlorostyrene) and poly(4-methylstyrene) at temperatures below their glass transitions has been examined by determining the time dependent stress relaxation modulus, G(t), as a function of the ageing time. Time-ageing time superposition of the modulus curves gives a set of ageing time shift factors which characterise the structural state of the glassy polymer in terms of its mechanical response. The mechanical ageing rate of poly(4-methylstyrene) decreases with increasing temperature, like that of polystyrene, but that of poly(4-chlorostyrene) increases with decreasing temperature. Free volume arguments cannot adequately explain this behaviour and it is proposed that dipole interactions, which appear not to affect enthalpic ageing behaviour in these polymers, become active under the mechanical probe. The ratios of enthalpic and mechanical ageing rates are compared and found to be constant over the range of ageing times examined.