Conformational Transition Behavior of Amorphous Polyethylene across the Glass Transition Temperature

Abstract

Molecular dynamics simulations have been used to investigate the conformational transition behavior in amorphous polyethylene with different chain lengths across the glass transition temperature (T(g)). In the present study, we examined the barrier height of conformational transition rates in different states. It was found that two lines of the logarithmic rates versus inverse temperature in the melt state and in the glass state are evidently different. The two lines have an intersection, which indicates T(g) well. The barrier height in the glass state was unexpectedly observed lower than that in the melt state. For gaining better understanding of the transition barrier reduction, we analyzed motion heterogeneity of the systems and found the torsional transition rate distribution becoming gradually heterogeneous when the temperature went down to the glass state. The result indicates that the motion heterogeneity was caused by the torsion transition being "frozen". The frozen torsions made the system into a nonequilibrium state and possess a novel transition behavior, which accounted for most of the transitions that started at a location close to top of barrier, and also the enhancement of a small magnitude of transition jumps.

Temperature Dependence of Polypropylene Configurations

“…In view of gratifying success of MD in many research fields and a high degree of similarity of the temperature dependence of the specific volume between experimental observations and simulation calculations, it has reason to believe that we can get insight into microscopic details about the microstructure and molecular motion by MD. In order to illustrate the justifiability of our simulation calculations, Figure gives the running average of specific volume obtained during dynamics simulations against simulation time at the several specified temperatures for a‐PP of this work, which are reasonable consistent with the reports in literature . Figure is the corresponding relationship between the average specific volume and temperature in the equilibrium state, when the fluctuation of the average values is negligible small.…”

The extent of the glass transition from molecular simulation revealing an overcrank effect

“…Sumpter et al consider the occurrence of a dihedral jump when it is greater than 90 deg . Moreover, Wu made a distinction between two types of jumps (transitions), a “shallow jump” and a “deep jump.” What we propose in this study is an hybrid method between the two latter methods. We actually introduced a lag time (the time interval needed for a trajectory to lose memory of how the system entered a state) usually ranges from 0.1 to 1 ps .…”

Effect of chain structure on the glass transition temperature and viscoelastic property of cis‐1,4‐polybutadiene via molecular simulation

“…However, the time range accessible to MD methods is very short from pico to nano‐seconds, which limits the direct comparison of the T g both through experiment and via computer simulation methods. Through MD methods, we can identify T g by monitoring changes of certain macroscopic or microscopic properties during cooling, such as specific volume, heat capacity, coefficient of thermal expansion, diffusion coefficient, conformational transition rate, and the percolation of immobile atoms . It is noted that many factors can influence the relaxation time and the T g of polymer chains.…”

“…This is due to the increase of free volume near the surface region of thin films . In addition, the T g of amorphous poly(L‐lactide) and amorphous polyethylene is found to have a linear relationship with the reciprocal of the molecular weight dependence. Furthermore, the molecular‐weight polydispersity is found to have very little influence on the T g by adopting the coarse‐grained polymer model .…”

“…Furthermore, the molecular‐weight polydispersity is found to have very little influence on the T g by adopting the coarse‐grained polymer model . Especially, Wu et al find that below T g the conformational transition barrier is lower than that above T g . This possibly can be explained by that the larger transition angles (50–100 ) are inhibited .…”