Motional broadening of the quasi-elastic peak in neutrons scattered from polymeric materials

Abstract

Doppler broadening studies of the quasi-elastic peak of cold neutrons scattered from poly-(dimethylsiloxanes) are reported. The Doppler broadening is observed in the low-molecular-weight liquids and persists in high-molecular-weight gums and even in the cross-linked rubbers. The self-diffusion constants determined for the low-molecular-weight liquids are larger than the corresponding value obtained from n.m.r. spin echo measurements. Since the latter technique undoubtedly measures the translational contribution to self-diffusion, it is concluded that the neutron-scattering result includes a major contribution from internal rotational motions of the polymer chain. The activation energy for the neutron diffusional process is 2.0 kcal mol^–1 compared with 4.0 kcal mol^–1 for the translational diffusion process. A similar Doppler broadening phenomenon has been observed in poly(propylene)oxide at elevated temperatures.