Effect of Crystallization and Entropy Contribution Upon the Mechanical Response of Polymer Nano-fibers: A Steered Molecular Dynamics Study

The mechanical response of polyethylene nano-fibers with the same chain length but different chain numbers are studied by using steered molecular dynamics simulations. The shrinking or stretching forces acted on the chain ends are investigated according to the chain-end distance and temperature under isothermal or continuous warming-cooling conditions, respectively. An inflection point is found in the Force-Distance response when temperature is below 500 K. This inflection point is related to the balance between entropy force and inter-monomer interaction and it reflects the strong effect of crystallization on the mechanical response of the nano-fibers. The force at inflection point is also affected by the buckling effect due to increased stiffness when crystallization occurs. The two stages found in the Force-Temperature response and the difference between the shrinking and stretching forces indicate the hysteresis of crystallization and melting. The forces at different shrinking and stretching rates reveal the entropy contribution upon the mechanical response, indicated by the Ramachandran plot of dihedrals. The chain-conformation entropy is majorly contributed by dihedrals and is quantified by the information entropy of dihedrals, which has a highly similarity to the mechanical Force-Temperature response. The enlarged forces in multiple chains over a single chain are attributed to the enhanced dihedral-conformation entropy. Our study provides a new insight to the dynamically mechanical response of polymer nano-fibers according to the effect of crystallization and entropy contribution.