A fully atomistic model of an amorphous polybenzoxazine at bulk density

A recent rotational isomeric state (RIS) model for a polybenzoxazine (PBO) was used for generation of a fully atomistic model for the amorphous material at its bulk density of 1.1 g cm(-3). The initial structure was relaxed by a combination of molecular mechanics and molecular dynamics. There is a decrease in the number of hydrogen bonds during this relaxation. The solubility parameter calculated from the model is 8.3 +/- 0.7 (cal cm(-3))(1/2). The distribution of dihedral angles in the bulk state is broader than the distribution in the RIS model. There is a correlation in the orientation of two phenyl rings if they are separated by less than 5 Angstrom, but no correlation otherwise. The free volume was evaluated using spherical probes of different radii, and its size and shape were evaluated using the probe of radius 1.0 Angstrom. The shape distribution of the voids was characterized in terms of the ratio of the principal moments of the radius of gyration tensor, and was found to be very similar to that of an unperturbed chain. Thus the asymmetry of the large voids can be described by a gaussian distribution. The non-polar side chain methyl groups were enriched around the voids. (C) 1998 Elsevier Science Ltd. All rights reserved.