Grüneisen parameters of crystalline polyethylene

Values of the Grüneisen parameter γ reported in the literature for polyethylene (PE) vary over nearly four orders of magnitude. Part of the reason for this wide variation is the variety of definitions and experimental techniques used in obtaining γ. A simple unified derivation of these equations will be presented in an attempt to clarify the interrelations among them. Underlying assumptions used in arriving at these equations will be pointed out. Infrared spectra of a bulk polyethylene were taken at room temperature up to 40-kbar pressures. Microscopic Grüneisen parameters were obtained from these data for rocking and bending modes of PE. It will be demonstrated that values of the Grüneisen parameters are dependent on lattice frequency, volume, wave vector, and degree of crystallinity for PE.