Molecular dynamics simulation of cis-1,4-polybutadiene. 2. Chain motion and origin of the fast process

Molecular dynamics (MD) simulations of cis-1,4-polybutadiene in bulk amorph ous state were performed to elucidate the origin of a fast relaxation proce ss observed by quasielastic neutron scattering (QENS) measurements. The det ails of the torsional motion for each dihedral angle were investigated with the torsional auto- and cross-correlation functions for several temperatur es in this study. Temperature dependence of the correlation between the tor sional autocorrelation and cross-correlation functions is also evaluated. T he origin of the fast process of cis-1,4-polybutadiene is found to be mainl y the cooperative conformational transitions of two dihedral angles located at both sides of the CH2-CH2 bond when the bond is in the trans conformati on. The cooperative conformational transitions exhibit even below the glass transition temperature of cis-1,4-polybutadiene. The cooperative motion ap pears at about 50 K below the glass transition temperature, corresponding t o the Vogel-Fulcher temperature. (C) 2001 Elsevier Science Ltd. All rights reserved.