Dynamic, conformational and topological properties of ring-linear poly(ethylene oxide) blends from molecular dynamics simulations

摘要

We present results for the equilibrium conformational and dynamic properties of ring-linear poly(ethylene oxide) (PEO) blends from detailed molecular dynamics (MD) simulations with a thoroughly validated and very accurate forcefield. The simulations have been performed in the isothermal-isobaric (NPT) statistical ensemble with blends where the two types of chains (ring and linear) have the same size. Simulations with two different chain lengths, corresponding to molecular weights equal to 1800 and 5000 g/mol, allowed us to study the dependence of these properties on molecular length. Overall, the presence of linear chains seems to considerably slow down the orientational relaxation of ring molecules and to lower their diffusivity, to a degree that depends strongly on chain length and level of contamination of the melt by linear chains. The longer the size of the molecules the more pronounced the decrease in ring diffusivity is, at a given mass fraction of linear chains. To explain the reduction in the relaxation and mobility of ring molecules when they mix with linear chains to form a blend, selected configurations from the MD simulations were subjected to a detailed topological analysis which revealed significant threading of the cyclic molecules by the linear ones. Our simulation data indicate that, due to threading, ring dynamics in ring-linear PEO melts is strongly heterogeneous. An analysis of the statistics of the lifetimes of ring-linear topological constraints (TCs) reveals a long tail on the long time scale, demonstrating that many of these TCs are long-lived. By inspecting individual ring-linear PEO pairs we found that, in many cases, the lifetimes of these TCs are up to one order of magnitude larger than the typical time characterizing ring relaxation in the pure ring melt. This phenomenon was more pronounced in the blend with the longer molecules (molecular weight - 5000 g/mol).