Molecular Dynamics Study Of Crystalline Linear Polymer During Tensile Deformation

Semi-crystalline polymer materials pose long standing puzzles in its structure/property relations,mainly due to the hierarchical structures of crystalline polymer and the coexistence of amorphous and crystalline domains.Moreover,various thermal processes through practices such as extrusion,injection,compression,or annealing,may introduce significant differences in morphology evolution of this amorphous-crystalline binary system.Compared to traditional experiment methods,such as X-ray synchrotron,infrared spectroscopy,differential scanning calorimetry,and scanning electron microscopy,molecular dynamics(MD)simulation has some superior property.The MD simulation provides a new route to reveal the details in structure/property relations in small scales.With MD simulation,in situ study can be readily conducted in chain configuration/conformation as well as mesoscale structures.In this study a coarse-grained molecular dynamics model of linear polyethylene-like polymer chain system was built to investigate the responds of structure and mechanical properties during uniaxial deformation.The influence of chain length,temperature,and strain rate were studied.The molecular dynamic tests showed that yielding may governed by different mechanisms at temperatures above and below T_g.Melt-recrystallization was observed at higher temperature,and destruction of crystal structures was observed at lower temperatures beyond yield point.While the higher temperature and lower strain rate have similar effects on mechanical properties.The correlated influences of time and temperature in the microscopic structures are more complicated.The evolution of microscopic characteristics such as the orientation parameter,the bond length,and the content of trans-trans conformation were calculated from the simulation.The results showed that the temperature have double effects on polymer chains.Higher temperature on one hand makes the chains more flexible,while on the other hand shortens the relaxation time of polymers.During deformation,the trans-trans conformation has experienced a rising process at relatively low temperature while a initial drop process at high temperature.And these microscopic structure parameters exhibit critical transaction,which are closely related to the yield point.In this study,the change of the potential was also investigated.A hypothetical model was thus proposed to describe the micro-structure and property relations based the investigations of this study.