Possible explanation of anomalous ductility in thermoset/thermoplastic polymer alloys

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 linear chain mass fraction $\Gamma_l$, for different chain lengths $N_l$, and strain rates $\dot{\varepsilon}$. Our results show that tensile strain (i.e; strain to break) decreases with increasing mass fraction $\Gamma_l$, up to a threshold value $\Gamma_{l}^*$, beyond which it increases with $\Gamma_l$. This non-monotonic behavior, which we call ``anomalous ductility", is qualitatively independent of $\dot{\varepsilon}$ and $N_l$, so long as fracture occurs in bulk. $\Gamma_{l}^*$ decreases with increasing chain length and we observe microscopic evidence that this threshold value signifies the onset of interchain interactions. A simple scaling argument suggests that $\Gamma_{l}^*$ is related to the overlap concentration of the thermoplastic homopolymer in the cured thermoset matrix.