Grüneisen constants of polymers

Abstract

One result of recent interest in Grüneisen constants, γ = − d lnν/d lnV, of polymers is a considerable spread of reported γ's for solids like polyethylene. From elasticity data (bulk modulus or sound velocities for example) one finds γ ≈ 6 for linear polymer solids. Values of γ from thermal data are much lower than 6 because the relationship usually employed, γ = αB V/C_V, is not valid for polyatomic solids. Measurements of the shifts in lattice frequencies with volume strains are the most direct way of measuring γ. However, the results for pressure‐induced strains differ from those for temperature‐induced strains, and both differ from the results from elasticity data. In this paper we consider vibrations in a simple anharmonic well and show how the apparent shifts in vibrational frequency with pressure and temperature can be derived from changes in force constants.