Role of Entanglement\nin Polymer Crystal Growth and\nMelting: Molecular Dynamics Simulations

The influence of entanglement on the growth and melting\nof polymer\ncrystals is investigated by using molecular dynamics simulations across\nsystems with different entanglement densities. Reducing entanglement\ndensity leads to a slight decrease of crystal thickness <i>L</i> but an increase of melting temperature <i>T</i><sub>m</sub>. To elucidate the entanglement-related inverse relation between <i>T</i><sub>m</sub> and <i>L</i>, we introduce the fold\nsurface free energy σ<sub>f</sub> predominantly governed by\nthe conformational entropy loss of amorphous segments. Reducing the\nentanglement density results in a decrease of σ<sub>f</sub>.\nCombining the contributions of σ<sub>f</sub> and <i>L</i>, a linear relation between <i>T</i><sub>m</sub> and σ<sub>f</sub>/<i>L</i> is obtained, which is consistent with\nthe prediction of the Gibbs–Thompson equation. Crystallization\ncommonly accompanies with disentanglement, while crystal melting precedes\nbefore entanglement reconstruction. Considering the thermodynamic\ncontribution of entanglement, polymer crystallization and melting\nare not two mutually reversible phase transitions, not only kinetically\nbut also thermodynamically.