Influence of chain topology and bond potential on the glass transition of polymer chainssimulated with the bond fluctuation model

Abstract

The bond fluctuation model with a bond potential has been applied to investigation of the glasstransition of linear chains and chains with a regular disposition of small branches. Cooling andsubsequent heating curves are obtained for the chain energies and also for the mean acceptanceprobability of a bead jump. In order to mimic different trends to vitrification, a factorB gauging the strength of the bond potential with respect to the long-rangepotential (i.e. the intramolecular or intermolecular potential betweenindirectly bonded beads) has been introduced. (A higher value ofB leads to a preference for the highest bond lengths and a higher total energy, implyinga greater tendency to vitrify.) Different cases have been considered for linearchains: no long-range potential, no bond potential and several choices forB. Furthermore, twodistinct values of B have been considered for alternate bonds in linear chains. In thecase of the branched chains, mixed models with different values ofB for bonds in the main chain and in the branches have also been investigated. The possiblepresence of ordering or crystallization has been characterized by calculating thecollective light scattering function of the different samples after annealing ata convenient temperature below the onset of the abrupt change in the curvesassociated with a thermodynamic transition. It is concluded that ordering is inheritedmore efficiently in the systems with branched chains and also for higher values ofB. The branched moleculeswith the highest B values in the main chain bonds exhibit two distinct transitions in the heatingcurves, which may be associated with two glass transitions. This behavior hasbeen detected experimentally for chains with relatively long flexible branches.