Primitive chain network study on uncrosslinked and crosslinked cis-polyisoprene polymers

In this paper, the polymer chain packing and primitive path (PP) network of uncrosslinked and crosslinked cis-polyisoprene (PI) polymer are analyzed upon employing coarse-grained molecular dynamics simulation. The crosslinking effect is found to enhance intra-chain packing of PI polymers, while weakening their inter-chain packing. Surprisingly, these effects cancel each other in the global packing behavior of this polymeric system. We systematically study the effects of molecular weight (MW) and crosslink density on the PP. Both the PP contour length and number of entanglements per chain, (Z), are found to increase linearly with MW for uncrosslinked cis-PI. The corresponding entanglement molecular length N_e of cis-PI is estimated to be 76 ± 1, in good agreement with experimental results. The polymer end-to-end distance, the PP contour length as well as (Z) of crosslinked PI are reduced by higher intra-chain packing density, compared with uncrosslinked PI, if the crosslinkers are ignored in the PP analysis. At the same time, the tube diameter of crosslinked PI is enlarged by the sparse inter-chain packing. By dividing the crosslinked cis-PI chain network into subchains through crosslinked or crosslinker beads, the PP networks of these partial systems are treated as well. We obtain scaling laws between MW/crosslinking density and (Z) for crosslinked PI polymers. The simulation results indicate that the random walk assumption, often encountered during the analysis of PPs, can only be applied to the entanglement-dominated (low crosslink density) polymers. For crosslink-dominated (high crosslink density) polymers, whose subchains have a molecular length below 100, this assumption would imply a greatly overestimated entanglement density; we thus avoid the assumption in our analysis. To our best knowledge, this is the first work to uncover the PP of crosslinked polymers.