Influence of molecular weight between crosslinks on the mechanical properties of polymers formed via ring-opening metathesis

Abstract

The apparent molecular weight between crosslinks (M_c,a) in a polymer network plays a fundamental role in the network mechanical response. We systematically varied M_c,a independent of strong noncovalent bonding by using ring-opening metathesis polymerization (ROMP) to co-polymerize dicyclopentadiene (DCPD) with a chain extender that increases M_c,a or a di-functional crosslinker that decreases M_c,a. We compared the ROMP series quasi-static modulus (E), tensile yield stress (σ_y), and fracture toughness (K_IC and G_IC) in the glassy regime with literature data for more polar thermosets. ROMP resins showed high K_IC (>1.5 MPa m^0.5), high G_IC (>1000 J m⁻²), and 4–5 times higher high rate impact resistance than typical polar thermosets with similar T_g values (100 °C to 178 °C). The overall E values were lower for ROMP systems. The σ_y dependence on M_c,a and T–T_g for ROMP resins was qualitatively similar to more polar thermosets, but the overall σ_y values were lower. In contrast to more polar thermosets, the K_IC and G_IC values of the ROMP resins showed strong M_c,a and T–T_g dependence. High rate impact (∼10⁴–10⁵ s⁻¹) trends were similar to the K_IC and G_IC behavior, but were also correlated to σ_y. Overall, a ductile failure mode was observed for quasi-static and high rate results for a linear ROMP polymer (M_c,a = 1506 g mol⁻¹ due to chain entanglement), and this gradually transitioned to a fully brittle failure mode for highly crosslinked ROMP polymers (M_c,a ≤ 270 g mol⁻¹). Molecular dynamics (MD) simulations showed that low M_c,a ROMP resins were more likely to form molecular scale nanovoids. The higher chain stiffness in low M_c,a ROMP resins inhibited stress relaxation in the vicinity of these nanovoids, which correlated with brittle mechanical responses. Overall, these differences in mechanical properties were attributed to the weak non-covalent interactions in ROMP resins.