Molecular relaxation and the elastic and dielectric properties of plastics

Abstract

The elastic modulus for various thermoplastics has been calculated by means of Maxwell's relation η=Gτ (G being the modulus of rigidity), using the coefficients of viscosity, η, measured by observations of rate of flow, and the times of relaxation, τ, of their molecules deduced from observations of dielectric behaviour. For the phenolic thermoplastics the calculated values are practically independent of the state of the resin, from the hard to the very soft condition, and they lend some support to the suggestion put forward by W. Kühn that the true elastic modulus has nearly the same value for all materials, and that the wide differences in the elastic behaviour of materials like glass, rubber and plastics generally is due, not to differences in their true elastic modulus, but to differences in the rates at which stresses in these materials decay on account of molecular relaxation. The true modulus for materials with large molecules is considered to represent the total effect of molecular forces of many kinds, each with its own time of relaxation, and it is the distribution of these times of relaxation which determines the general character of the stress-strain relationship observed in ordinary practice. The values of τ deduced from dielectric behaviour refer to one group of forces only, viz., those due to the polar constituents of the molecules. Since Maxwell's simple equation appears to give the true modulus for the thermoplastics, we must suppose that in this case the same values of τ represent approximately all the molecular forces involved in elasticity and viscosity. For the benzyl alcohol resin the equation does not appear to give the true modulus, possibly because in this material there are relatively few polar groups, and therefore the time of relaxation representing the motion of these groups is less representative of the molecular forces as a whole.