MOLECULAR-DYNAMICS OF RIGID-ROD POLYMERS

Molecular dynamics (MD) calculations have been used to study the behav iour of isolated rigid rod molecules of poly(p-phenylene), poly(p-phen ylene benzobisthiazole) and poly(p-phenylene benzobisoxazole). The mol ecular mechanics force field was initially modified to improve agreeme nt between minimized structural geometries and available X-ray data, a s well as results from semiempirical molecular orbital calculations. T he MD simulations show the molecules to be surprisingly flexible, with changes in end-to-end distances as large as 16%. An examination of th e energies (calculated by various methods) associated with out-of-plan e bending deformation, suggests that the rigid rod polymers may in fac t be even more flexible than the simulations indicate. The results pro vide rationalizations for the relatively short persistence lengths mea sured in solution and for bending observed in high-resolution electron micrographs of these materials.