Molecular origin of viscoelastic transition in molecular granular materials: insights from molecular dynamics...

Molecular granular materials (MGMs) constructed from the simple packing of molecular clusters (MCs) demonstrate both resilient elasticity and feasible processability; however, an understanding of these properties at the molecular level is still vague. Herein, coarse-grained molecular dynamics (CGMD) simulations are conducted on a polymer brush with 1 nm polyhedral oligomeric silsesquioxanes (POSSs) as side chains, and the lengths of the linkers (L) that bond POSS to the polymer backbone are systematically varied to locate the microscopic key factors that determine the viscoelasticity of MGMs. As suggested from the CGMD snapshots, the incompatibility between POSS and the polymer backbone drives the formation of POSS-enriched zones, which serve as the dynamic physical crosslinkers that strengthen the MGMs. A non-monotonic relationship between shear viscosity (η) and L is observed where η reaches its peak at L = 3. The relaxation dynamics of the backbone, linker and POSS are measured from the mean square displacement and the intermediate scattering function. The short linkers (L < 3) disfavor the close packing of POSSs, and the interpenetration of different polymer brushes is weak, while moderate lengths (L = 3) enhance interpenetration, leading to stronger local constraints and high viscosity. For L > 3, the increased molecular volume weakens the global topological constraints, allowing the fast dynamics of POSSs to resume.