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 ForceDistance 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.

Diffusion of a Ring Threaded on a Linear Chain

A mesoscopic simulation is applied to investigate the effects of hydrodynamic interactions and axial chains on the dynamics of threaded rings.The hydrodynamic interactions significantly speed up the diffusion and relaxation of both free and threaded rings.The decoupled diffusion and relaxation dynamics indicate the broken of the Einstein-Stokes relationship.The diffusion of a ring threaded on a flexible chain exhibits a synergism effect compared to that on an axial rod,which originates from the self-diffusion of the ring and the reptation-like motion of the axial chain.Meanwhile,hydrodynamic interactions significantly improve the synergism effect,leading to an enhanced sliding motion of the threaded ring.The faster sliding of threaded rings suggests that the entropic barrier is negligible,which agrees well with the basic assumption of barrier-less confining tube at equilibrium in tube theory.Our results provide a new perspective on analysis of the effects of topology constraints on polymer dynamics.

Shape Accuracy and Residual Stress Distribution of Nano-molded Semicrystalline Polymer:A Simulation Study

The shape accuracy and residual stress distribution of a nano-molded semicrystalline polymer are studied by molecular dynamics simulations.Semicrystalline polyethylene flakes are obtained by continuous cooling inside templates with four shapes.We find that the curvature of contour curve near template corners decreases with corner angle.A simple 2D shape model of minimum surface energy is proposed to understand the shapes for repulsive and attractive templates.The confinement of template induces highly ordered chain packing in surface region.According to the spacial distribution of local stress,we find the contracting stress caused by volume shrinkage during cooling concentrates on the chains perpendicular to the direction of relative stress principal.The distribution of von Mises stress indicates the outer layer of semicrystalline polymer flake has lower distortion.Our results provide a theoretical insight for better nano-molding techniques.