Investigating Tube Dynamics of Highly Entangled Poly(dimethyl-co-diphenyl) Siloxane Melts via Coarse-Grained Molecular Dynamics Simulation

Polydimethylsiloxane is widely used in different industrial applications and scientific research. The incorporation of phenyl siloxane components, e.g., as in a poly(dimethyl-co-diphenyl)siloxane (PDMS-co-PDPS) random copolymer has been an important approach to improve their mechanical and thermal properties. Previously, we developed a coarse-grained (CG) molecular dynamics (MD) model based on the all-atomistic (AA) counterpart to overcome the limitation of temporal and spatial scales of AAMD simulations. We also proposed a lever rule to generate φ-dependent non-bonded potentials for CG systems without AA reference. We showed that the molar ratio of the diphenyl content φ significantly changes the microscopic structures and dynamics of unentangled and weakly entangled systems. In this work, the CG model is further validated by the fact that it predicts values of the viscosity of the copolymer melts that are quantitively consistent with the experimentally measured results. We then focus on the dynamics of the entangled systems (with above 10 entanglements per linear chain and with different φ ranging from 0 to 0.4). We systematically characterize the tube-like entanglement structures using Z1 algorithm and probe their evolution that dictates the viscoelastic properties of the copolymer materials. Specifically, we analyze the relaxation of the Rouse normal modes and the structural factors. While relaxation at the chain level qualitatively follows the classic reptation-like model, the order-of-magnitude slowdown in the dynamics is also observed in the highly entangled systems. More importantly, the sub-chain level relaxation is highly correlated with the diphenyl component. This work sheds light on the φ-dependent chain relaxation and thus viscoelastic properties of the copolymer which is valuable in understanding random copolymer systems.