Experimental resolution of the retardation time spectrum in polymeric solids by a new method: Thermostimulated creep

Abstract

We propose a new method for the investigation of molecular motion in polymeric solids. In a mechanical step-function experiment, we thermally stimulate the response to a constant stress. The high resolving power of this technique permits detailed study of the complex retardation modes observed in polymers. By using “fractional” loading programs it is possible to differentiate a discrete and a continuous distribution of retardation time. This technique allows us to predict the complex compliance in a very wide frequency range: 10⁴-10⁻¹² Hz for experiments performed between liquid nitrogen temperature and 500°K. In low-density polyethylene, we have shown the existence of a discrete spectrum of mechanical retardation times which has the same fine structure as the spectrum of dielectric relaxation times obtained from the study of depolarization thermocurrent on the same sample. The predicted variation versus temperature and frequency of the loss compliance is compared with that of the dielectric loss factor.