Monte Carlo simulation of polyethylene on the confined geometry and elastomeric polypropylene.

摘要

Polymers confined on the nanoscale geometry have been paid much attention since their properties are significantly different from the bulk system. The crystallization of a free-standing thin film and free-standing nanofiber of short polyethylene chains, and the surface behavior of the polyethylene binary mixture with different chain lengths have been investigated by a Monte Carlo method based on a high coordination lattice, which bridges the coarse-grained and a fully atomistic models of a dense polymer.; The simulation results show that the crystallization of polyethylene on the confined geometry after a deep quench initiates on the surface regions and propagates into the interior part. For the thin film, the repeated simulations find two distinctly different structures. Usually, the independently initiated crystals at the two surfaces of the thin film have different orientations and produce a less ordered grain boundary in the center of the thin film when they impinge on one another. However, occasionally, the independently initiated crystals happen to have a similar orientation and the final configuration has no grain boundary in the interior. A well-defined melting point is observed in the simulation by investigating various parameters. An annealing, beginning from the structure with differently oriented crystals, causes growth of one crystal at the expense of the other. This growth eventually leads to a complete crystal with elimination of the grain boundary. The annealing process finds that one crystalline domain in the thin film becomes partially disordered first then the other crystal propagates into the less ordered region and finally forms a larger uniform crystal. Both similarities and differences exist during the crystallization and annealing processes for the free-standing polyethylene thin film and nanofiber.; The simulation of mixtures of short polyethylene chains with different chain lengths in the free-standing thin film shows that the shorter chains segregate on the surface in the melt. However, the mixing behavior below the melting temperature depends on the history of the mixture. If the quench initiates from a homogeneous mixture in the melt, the two kinds of chains crystallize together. On the other hand if the system is quenched from the equilibrated thin film above the melting point, the shorter chains remain segregated on the surface region, and this segregation is enhanced due to the crystallization.; The above simulation method was also applied to another heterogeneous system, elastomeric polypropylene. This kind of material becomes more heterogeneous at low temperature. Some specific configurations, the extended and helical structures were found at low temperature as well as the heterogeneity in dynamics.; The chain increment method has been applied to the PP blends with different stereochemical sequences and PE free-standing thin films. The chemical potential and free energy on mixing confirmed the miscibility of PP blends. The surface internal energy and surface free energy have been obtained for the free-standing thin film. The validation of the application of this method was discussed.