We present an extensive neutron scattering study on the ester methyl dynamics of poly(methyl methacrylate) (PMMA) in the temperature range from 1 to 200 K. The crossover from quantum tunneling to classical hopping has been investigated in an energy window from 0.5 μeV to 2 meV. Inelastic measurements of the librational levels have been performed in the millielectronvolt range. The results have been analyzed in terms of a model that considers a Gaussian distribution of potential barriers for methyl group rotation. A 6-fold correction to the main 3-fold term of the potential has been added to get a good description of the librational peak. This distribution of mixed-symmetry barriers gives an excellent description of the spectra in the whole temperature range.

This work has been supported by the Spanish Ministry of Education (project PB97-0638), the Government of the Basque Country (Projects GV-PI98/20 and EX-1999/11), and the University of the Basque Country (Project 206.215-G20/98). The support of Donostia International Physics Center and Iberdrola S.A. is also acknowledged. A.J.M. acknowledges a grant of the Basque Government.

Peer reviewed