Role of Entanglement in Polymer Crystal Growth and Melting: Molecular Dynamics Simulations

The influence of entanglement on the growth and melting of polymer crystals is investigated by using molecular dynamics simulations across systems with different entanglement densities. Reducing entanglement density leads to a slight decrease of crystal thickness L but an increase of melting temperature T_m. To elucidate the entanglement-related inverse relation between T_m and L, we introduce the fold surface free energy σ_f predominantly governed by the conformational entropy loss of amorphous segments. Reducing the entanglement density results in a decrease of σ_f. Combining the contributions of σ_f and L, a linear relation between T_m and σ_f/L is obtained, which is consistent with the prediction of the Gibbs–Thompson equation. Crystallization commonly accompanies with disentanglement, while crystal melting precedes before entanglement reconstruction. Considering the thermodynamic contribution of entanglement, polymer crystallization and melting are not two mutually reversible phase transitions, not only kinetically but also thermodynamically.