Molecular Dynamics Simulation Of Glass Transition Of Cis-polyisoprene

In the past decades,scientists have been studying the glass transition of polymers,and its essential mechanism has not been well understood in thermodynamics or kinetics.Glass transition is the macroscopic transition of polymer movement form.Near the glass transition,small temperature changes can greatly change the physical properties of natural rubber,thus seriously affecting the properties of natural rubber.Macroscopic property has great influence on the static and dynamic behavior of cis-1,4-polyisoprene during glass transition,while macroscopic property is significantly affected by external pressure and filler size.Therefore,here an all-atom molecular dynamics simulation is performed to systemically investigate the effect of the pressure(P=1-2000 atm)and the packing sizes(d=4?、d=6?、d=8?、d=10?)on the glass transition temperature(T_g)for cis-1,4-polyisoprene,which is dependent on the static and dynamic properties.(1)For the static property,the polymer density and the energy are considered.It is found that the polymer density,the non-bond energy and the torsional energy play an important role during the glass transition process.Meanwhile,the T_gobtained from the non-bond energy and the torsional energy is larger than that from the polymer density.(2)For the dynamic property,the translational mobility,the bond reorientation mobility,the torsional dynamics and the dynamic heterogeneity on the chain backbone are analyzed.The mobility on the chain backbone decreases with increasing the pressure or the packing size,which gradually increases the T_g.In addition,the T_gobtained from the dynamic heterogeneity is largest compared to those from the bond reorientation mobility,the torsional dynamics or the atom translational mobility.(3)For the dynamic heterogeneity.Especially,according to the mean-square fluctuations of the backbone atoms,the immobile atoms are distinguished.Then,the percolation probability of the immobile domain is analyzed,which is further characterized by the size of the largest immobile domain and the number of the immobile domains.By observing the snapshot of the mean-square fluctuations of atoms,the percolation transition of the immobile domains is clearly observed,which can help to understand the glass transition process under the different pressures or the different packing sizes.