Filler-induced heterogeneous nucleation of polymer crystals investigated by molecular dynamics simulations

Filler particles are known to act as nucleants for polymer crystallisation yet the connection between the filler surface properties and polymer crystallisation are not well understood. In this work, molecular dynamics simulations were used to investigate homogeneous and heterogeneous polymer nucleation and crystallisation using a generic linear bead-spring polymer model with a bond bending potential. The polymer systems were equilibrated at high temperature and then cooled at a constant rate. Without a surface present, polymers with stiff chains were found to crystallise more readily than more flexible polymers. The degree of crystallinity was estimated based on the mass fraction of straight chain segments which we equate to stem mass fraction. At a temperature Tc a sharp increase in density, radius of gyration and stem mass fraction occurred. After cooling, the systems were reheated and some systems showed hysteresis with a sharp decrease in these properties occurring upon melting at Tm > Tc. For slower heating rates, crystal growth occurred during heating from between the glass transition temperature, Tg, and Tc until just before melting at Tm. The presence of an isotropic surface was found to promote crystallisation in flexible systems that did not crystallise in the bulk, where the stem mass fraction and Tc increased with the interaction strength between the surface and the polymer beads. Changes in Tc and degree of crystallinity with cooling rate are consistent with experimental observations. This model captures polymer crystallisation phenomena and provides insight into heterogeneous nucleation, demonstrating that strong interfacial interactions promote crystallisation, thus aiding the choice or design of nucleants for control of polymer crystallisation and microstructure.