Enthalpy relaxation near the glass transition for comb-like polymer: Power law relaxation revealed by DSC experiment

A series of enthalpy relaxation experiments at different ageing temperatures was conducted on a sample of poly(cyanobiphenyl hexylacrylate) with variation in the molecular weight using a DSC technique. The value of the relaxation function (ϕ) was obtained as a function of the ageing time (tA) from the experimental results using the excess enthalpy in the limit of infinity of tA and that of the given tA. The double logarithmic plot of − lnϕ and tA was constructed to determine the functional form of the relaxation function. Two possible forms are found, the stretched exponential and the power law forms, which suggests that the functional form varies continuously between these two according to the ageing experiment. The activation energy spectrum was derived for the two functional forms, which show the distribution of the activation energy of the molecular process controlling the decrease in enthalpy. It was revealed that, for the spectrum based on the power law relaxation, the overwhelming fraction of the relaxation processes is at the lower energy side and the population of the process decreases rapidly toward the higher energy side.