RELATIVE CONTRIBUTIONS OF COUPLED ROTATIONS AND SMALL-AMPLITUDE TORSIONS TO CONFORMATIONAL RELAXATION IN POLYMERS

Local dynamics of polyethylene and 1,4-trans-polybutadiene chains are analyzed by the cooperative kinematics approach. The method includes t he effects of environmental friction and the conformational energy cha nges involved in bond rotational motions. A given bond is rotated by 1 20 degrees and the response of the surrounding bonds along the chain, which are necessary for localizing the motion, are analyzed. The confo rmational responses are classified into two sets, one exhibiting a cou pled rotation of another bond, i.e. involving a rotational barrier cro ssing, and the other accommodated by collective small-amplitude torsio ns (librational) of several bonds in the neighborhood of the rotating bond. Coupled rotations are shown to increase significantly with incre asing environmental frictional resistance. Conversely, librational mot ions are dominant for localization in less viscous environments. An im portant finding is that, for polyethylene, the subset of transitions i n which the relaxation is achieved by torsional librations only has th e same mechanism as that of the complete set of transitions. Thus, coo perative torsional librations may localize a motion as efficiently as coupled rotational jumps in polyethylene, although their proportion ma y be low in highly viscous environments. This is in support of the pos tulate advanced recently by Moe and Ediger [Moe, N. E.; Ediger, M. D. Macromolecules 1996, 29, 5484] on local dynamics of polyisoprene, in t he sense that libration alone can dissipate local disturbances in the conformations. However, for 1,4-trans-polybutadiene, the set of motion s including librations only exhibit some departure from that of the ov erall set of transitions. The size of the correlated sequence of bonds responding to the isomerization of the central bond by libration is a bout eight bonds and is symmetrically distributed on bath sides of the central bond for polyethylene and slightly biased for one side in pol y 1,4-trans-polybutadiene. Coupled barrier crossing rotations, on the other hand, take place either on one side of the central bond only in the former or absolutely in one side in the latter and hence involve a shorter correlation length along the chain.