Modelling of Glassy State Relaxations in Polymers:
the rotation of the methoxy Group in PVME
J.C. Berthet and G.R. Davies
IRC in Polymer Science and Technology, University of Leeds, Leeds LS2 9JT
Glassy state relaxations in polymers occur too slowly to be modelled by conventional molecular dynamics. We are therefore investigating alternative ways of obtaining information about such processes in solid, amorphous polymers.
Saelee and Davies previously showed that the distribution of energy barriers for the rotation of the CH3 group in amorphous PVME could be obtained by the quasi-static method. In this method, the dihedral angle of each CH3 group in an amorphous cell of PVME was forced to a series of different values while all other co-ordinates were minimised. Good agreement was obtained with experimental energy barrier data from quasi-elastic neutron scattering.
We have now investigated the application of this method to the rotation of the larger and asymmetric OCH3 group to predict dielectric relaxation data. We find that the relaxation strength is dominated by those few groups which have two energy minima of similar energy with 4% of the groups contributing more than 90% of the relaxation strength. Prediction of the relaxation times required the effective moment of inertia of the group. This was obtained from an analysis of the librations in a short dynamics run at 150 K. The complete frequency and temperature dependence of the dielectric permittivity could then be predicted. Good agreement with experiment is found.