Coarse-Grained Molecular Dynamics Simulation of Poly(dimethyl-co-diphenyl) Siloxane: Chain Dynamics of Unentangled and Entangled Melts

Polydimethylsiloxane (PDMS) is the most widely used silicon-based organic polymer, as its versatility and properties lead to many applications. The incorporation of phenyl siloxane, via copolymerization, for example, has been proposed to improve the mechanical properties of PDMS. However, such copolymerization changes the microscopic structural and dynamic properties of the copolymers significantly. We recently used all-atomistic molecular dynamics (AAMD) simulation to study the properties of unentangled linear poly(dimethyl-co-diphenyl) siloxane random copolymer melts, where it was shown that as the molar ratio of the diphenyl content φ increases the chain dynamics slow down by over two orders of magnitude. On the other hand, understanding the properties of copolymers with a higher degree of polymerization (i.e., above the entanglement length) is extremely important. However, an all-atomistic exploration of such an entangled system becomes very challenging due to slow relaxation, especially in presence of phenyl groups. In this work, we addressed this problem using a coarse-grained molecular dynamics (CGMD) approach to investigate the structural and dynamic properties of long-chain copolymers. First, we optimized the CG potentials by iterative Boltzmann inversion (IBI) to ensure that the CGMD model preserves the configurations and microstructures of the copolymer. We then built and equilibrated systems with different chain lengths and molar ratios of the diphenyl content (φ). Relation between the molar ratio φ and the structural properties of the entangled copolymer, such as tube diameter and entanglement density, were revealed by Z1 analysis. More importantly, the reptation-tube-like dynamics of the copolymer were closely studied. As expected, the chain dynamics of the entangled copolymer was found to slow down significantly with increasing φ. However, we found that such deceleration is due not only to the increased local friction of the phenyl groups but also to the complex structure-dynamics interaction within the copolymer environment.