Molecular Origin of the Viscoelastic Transition in Molecular Granular Materials: Insights from Molecular Dynamics Simulations

Abstract

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