KINEMATICS OF POLYMER-CHAINS IN DENSE MEDIUM .4. EFFECT OF BACKBONE GEOMETRY AND APPLICATION TO POLYBUTADIENE

The present study is an extension of the cooperative kinematics (CK) a pproach (Bahar, I.; Erman, B.; Monnerie, L. Macromolecules 1992, 25, 6 309), which has been demonstrated to give an efficient and realistic a ccount of the mechanism of local relaxational processes in polyethylen e(PE). In the present work, the effects of(i) departure from tetrahedr al backbone geometry and (ii) differences in torsional potentials of b ackbone bonds are addressed by considering cis- and trans-polybutadien e (PBD) chains. The method is based on the minimization of the atomic displacements of chain units and the torsional energy changes, succeed ing the isomerization of a given bond. In contrast to the highly local ized response of PE to bond rotational jumps, in which the strongest c oupling between rotational motions was observed between second neighbo ring bonds, the coupling in PBD is shown do involve longer chain segme nts, embodying the strongly correlated torsions of third or fourth nei ghboring bonds along the chain. The mechanism of motion is unique for each type of rotating bond for cis and trans structures: In trans-PSD, strong counterrotations are encountered at the second neighboring bon ds separated by a double bond, whereas in cis-PBD, the same pair of bo nds undergoes coupled corotations. Orientational and translational mot ions of chain units located between successive double bonds are signif icantly affected by nonbonded intramolecular interactions in cis-PBD, while this effect is not seen in the trans structure. Changes in envir onmental conditions do not affect the mechanism of localization phenom ena, but rather modulate the amplitudes of motion. Under the same fric tional environment, cis-PBD atomic displacements are larger than those in trans-PBD by a factor of similar to 1.5. The predictions of the th eory are in good agreement with the results from recent molecular dyna mics (MD) simulations of PBD (Kim, E.-G.; Mattice, W. L. J. Chem. Phys . 1994, 101, 6242; Gee, R. Il.; Boyd, R. H. J. Chem. Phys. 1994, 101, 8028). More interestingly, good agreement between the results obtained for cis-PBD and those from the bulk state MD simulations of cis-polyi soprene (Moe, N. E.; Ediger, M. D. Polymer, submitted) is found, confi rming that backbone geometry has the major role in determining the mec hanism of local conformational relaxations. An advantage of the presen t approach is that the computational time required is at least 2 order s of magnitude less than that of conventional MD simulations.