Anomalous ductility in thermoset/thermoplastic polymer alloys

Abstract

Mechanical properties of highly cross-linked polymer (HCP) networks, e.g., thermosets, can be significantly modified by adding linear polymer chains, e.g., thermoplastics. In this work, we study thermoset/thermoplastic polymer alloys by means of large scale molecular dynamics simulations (MD) of a coarse-grained model. We focus here on the effect of the linear chain fraction, Γ_l, on the mechanical properties of HCP network for a fixed chain length. Our MD simulations show that the ductility (measured by the strain-to-fracture) of an alloy decreases with increasing Γ_l up to a threshold fraction, Γ_l*, beyond which it increases with Γ_l. We find that for Γ_l < Γ_l* the fracture is predominantly cohesive, while for Γ_l > Γ_l* adhesive failure occurs. We suggest that the possible origin of this unexpected non-monotonic behavior is due to a competition between (a) growth of microvoids which stores mechanical energy and is compromised as Γ_l increases, and (b) reduction of cross-linker density with increasing Γ_l.