Origin of the γ relaxations in polyethylene and polytetrafluoroethylene

Abstract

AbstractMechanical relaxation has been studied at 0.67 cps in linear polyethylene (LPE) and polytetrafluoroethylene (PTFE) between −190 and −20°C. Specimens were prepared by use of various thermal treatments to produce in LPE a range of crystalline fractions from 0.690 to 0.825 and in PTFE from 0.615 to 0.870. An empirical theory is proposed relating the modulus of the crystalline–amorphous composite solid to the moduli and the volume fractions of the two phases. The empirical theory is shown to be in accord with the bounds of Hill and of Hashin and Shtrikman. The theory is used to determine the magnitudes of the crystalline and amorphous components of the low temperature relaxations in LPE and PTFE from measurements of logarithmic decrement and shear modulus. In PTFE the γ relaxation occurs in the amorphous fraction alone. In LPE the γ relaxation is a composite one, formed from the superposition of a small crystal relaxation and a large amorphous relaxation. For the crystal relaxation in LPE the ratio of relaxed to unrelaxed modulus equals 0.78; for the amorphous relaxation, the ratio equals 0.23. In a specimen of LPE with crystal fraction 0.69 the crystal contribution to the relaxation is 25% of the total. The magnitude of the unrelaxed modulus of the crystal fraction of LPE (modulus of polycrystalline LPE at −190°C) is in reasonable agreement with theoretical calculations of Odajima and Maeda.