Role of Short Chain Branching in Crystalline Model Polyethylenes

The role of short chain branches (SCBs) (C₄H₉) on the melt and crystalline properties of monodisperse polyethylene systems (C₄₀₀H₈₀₂) is investigated, using molecular dynamics simulations of a coarse-grained united-monomer model that represents a chemical monomer as one particle. A method is introduced, whereby SCBs are grown out of the linear backbone to minimize computational expense. Here, this concept is proven by introducing differing numbers (N_b = 0, 1, 2, 4, 10, and 20) of regularly spaced SCBs along the chain backbones and studying their influence on the melt and crystalline properties. By growing SCBs into the melt phase, it is demonstrated that they marginally perturb the original topology, justifying a relatively short equilibration time after growth. Upon crystallization, however, each system’s behavior differs considerably. Cooling and heating cycles are performed to study crystallization and melting at progressively slower rates. The crystalline morphology is observed to depend strongly on both cooling rate and number of branches along the linear backbone. In particular, the lamella thickness decreases systematically with both faster cooling and increasing SCB content. At the highest branch content, of one per 20 backbone carbons (N_b = 20), crystallization is almost entirely suppressed, whereas a small number of branches allows control over the average lamellar thickness. This observation, combined with a prudent method for equilibrating systems with SCBs, opens up opportunities to study more complex chain architectures and mimic industrial polyethylene morphologies.