Effects of Short Chain on Entanglement and Dynamics of Long Chain in Bidisperse Polymer Films

Structure-property and structure-function relationships for macromolecular thin films have attracted intensive research efforts. When tailored film thickness and the size of enwrapped macromolecules fall in the same order, nano-scale structural features are more prominent, which leads to significant change in macro-scale thermodynamical properties. The conformational distortions of macromolecules resulted from these nano-scale structural confinements, and the factor of chain entanglements within the polymer films are found in close causation with the dynamics of chains. The nature of macromolecule polydispersity has significantly complicated the situation. Nevertheless, there is few reported academic studies on corresponding mechanisms in the literature. In this paper, by virtue of Monte Carlo simulation jointed with primitive path analysis, we investigated the effect of polydispersity on the entanglement and dynamics of chains in thin macromolecular films. Specifically, a coarse. grained model of a bidispersed polymer melt under confined conditions was considered, and the focus was on the influence of weight fraction of relatively short chains on the entanglement and dynamics of long chains in polymer films. Our simulation indicated that, for higher weight fraction of short chain, the chain entanglements with low correlations could be easily released; and thus the confinement of neighboring particles dominated long-chain dynamics and caused it to decrease with decreased film thickness. In contrast, for lower weight fraction of short chains, the entanglement with high correlations was relatively hard to be released, the number and correlations of entanglements led to a nonmonotonic variation in long-chain dynamics with the film thickness. The usage of Monte Carlo simulation enables topological ergodicity, which leads to an efficient capture of macromolecular conformation. With the primitive path analysis of polymers under energy optimization, a physical picture demonstrating the relationship between polydispersity and chain entanglements/dynamics could be expected.