Abstract

Polypropylene(PP)polymer has considerable practical usages in various engineering applications,such as food packaging,coating,glass panel for automobiles,and filler for concrete[1-4].During long-term service life,PP-based materials are usually subjected to sustained external loads(e.g.wind and structural loading),causing a time-dependent deformation(i.e.creep)of the polymer material.The continuously increasing deformation during creep leads to damage and fracture of the material even at a loading level much lower than the designed material strength[4-7].Therefore,understanding the creep behavior of PP is crucial to the knowledge of long-term performance of polymer materials.In this work,atomistically informed coarsegrained(CG)technique is employed to model the PP polymer system,which is demonstrated to possess similar structural and mechanical properties as the experimental sample.Afterwards,CG molecular dynamics(MD)simulations are conducted to investigate the creep behavior of the PP molecule model under different stress levels.According to the obtained strain-time curves,it is revealed that there exists a threshold stress,above which the maximum strain of PP within the simulation timespan increases dramatically.Meanwhile,by observing the changes in polymer chains and potential energy,it is learned that the conformational changes of the polymer chains,including chain stretching,unfolding,and sliding accounts for the creep at different stages.Our study provides physical insight into the creep behavior of PP at a fundamental molecular level.