Inelastic & Quasielastic Neutron Scattering Studies of Polymethyl Methacrylate

Abstract

Neutron scattering experiments have been useful in studying the vibrational and rotational motion of the two methyl sidegroups in poly(methyl methacrylate). It is shown that detailed information about torsional frequencies and barrier heights to rotation for these groups may be obtained from inelastic scattering measurements.

Quasielastic scattering measurements allow the calculation of a function known as the Elastic Incoherent Structure Factor (EISF), which is a direct measure of the spatial distribution of the moving protons, and as such, characterises completely the actual geometry of rotation. Experimental determination of the EISF for both the α-methyl and ester methyl group is discussed in some detail. In addition, measurements relating rotational frequencies (obtained directly from the rotational quasielastic scattering laws) to temperature in order to calculate activation energies are presented, and compared to values of the barrier height obtained from inelastic measurements. A series of supplementary experiments concerned with the measurement of coherent scattering from poly(methyl methacrylate) is mentioned briefly, and it is indicated how these results may be interpreted to give details about local structure in the amorphous material.