Low frequency modes in the Raman spectra of polyethylene and paraffins. I. Lattice vibrations and their pressure dependence

Abstract

Raman spectra of crystalline polyethylene n-C23H43 and n-C44H90 at pressures as high as 40 kbar have been studied at 77 and 298°K, and five bands characteristic of lattice modes have been observed at various pressures. Each of these bands has been assigned to a lattice mode through symmetry arguments and vibrational analysis of unit cell modes, and the pressure dependence of these five lattice frequencies have been calculated and compared with the measured values. Effects of chain folding on the polyethylene spectrum, of end groups on the spectra of n - paraffins, and of chain length on the lattice frequencies are discussed quantitatively. For a test of the present method for lattice frequency calculations, which is based upon Williams' nonbonded atom-atom pair potentials, the lattice vibrational spectrum of naphthalene was reproduced. This method with Williams' V potential also reproduces the measured lattice frequencies of n-C23H43 at 1 atm, but could not explain the pressure dependence of these lattice frequencies equally as well.