CHAIN CONFORMATION AND FLEXIBILITY OF THORNY ROD POLYMERS

Molecular dynamics simulations and rotational isomeric state (RIS) the ory are used to systematically examine the effect of the regioregulari ty of monomer linkages and the size of the pendant on the rotational b arriers, flexibility and chain conformation of substituted polyparaphe nylenes including hydrogen, methyl, tert-butyl, phenyl, methoyl and be nzoyl pendants (R=H, CH3, C(CH3)(3), C6H5, COCH3, and COC6H5, respecti vely). The influence of the substituents on chain structure and proper ties can be broadly distinguished based on the degree of cooperativity for rotation about the paraphenylene linkage necessitated by the ster ic demands of the pendant. For some substituents (R=C6H5, COCH3, and C OC6H5), backbone and pendant motion are intimately coupled leading to thorny rod polymers. In general, the conformation of these polymers de pends on the relative ratio of regiospecific linkages and on the disto rtions of the backbone bond angles necessary to accommodate the pendan ts. For example, the persistence length of the chain decreases by almo st an order-of-magnitude when the ortho-hydrogen of paraphenylene is r eplaced by a benzoyl pendant. Furthermore, chain flexibility is restri cted to specific regioregular linkages and dynamic distortions of the backbone phenyl linkages. The relatively large intramolecular barriers to rotational motion will result in long relaxation times, frustratin g chain packing and potentially leading to kinetically limited metasta ble states in the solid and melt. (C) 1998 Elsevier Science Ltd. All r ights reserved.