High-resolution incoherent inelastic neutron scattering spectra of polyisobutylene and polyisoprene

High-resolution (ΔE/E ∼ 2−3%) incoherent inelastic neutron scattering spectra of polyisobutylene and cis-1,4-polyisoprene have been obtained on the TFXA spectrometer at the ISIS pulsed neutron scattering facility. The observed bands are assigned with reference to available infrared and Raman data, and the differences observed are discussed. Particular emphasis is placed on the methyl group torsions which are not observable with the optical techniques. These bands are significantly broadened compared to the instrumental resolution, and this is attributed to a Gaussian distribution of local environment potentials for the methyl group, in agreement with present theoretical models. For polyisoprene, this assignment is confirmed by independent molecular dynamics simulations.