The apparent molecular weight between crosslinks (Mc,a) in a polymer network plays a fundamental role in the network mechanical response. We systematically varied Mc,a independent of strong noncovalent bonding by using ring-opening metathesis polymerization (ROMP) to co-polymerize dicyclopentadiene (DCPD) with a chain extender that increases Mc,a or a di-functional crosslinker that decreases Mc,a. We compared the ROMP series quasi-static modulus (E), tensile yield stress (σy), and fracture toughness (KIC and GIC) in the glassy regime with literature data for more polar thermosets. ROMP resins showed high KIC (>1.5 MPa m0.5), high GIC (>1000 J m-2), and 4-5 times higher high rate impact resistance than typical polar thermosets with similar Tg values (100 °C to 178 °C). The overall E values were lower for ROMP systems. The σy dependence on Mc,a and T-Tg for ROMP resins was qualitatively similar to more polar thermosets, but the overall σy values were lower. In contrast to more polar thermosets, the KIC and GIC values of the ROMP resins showed strong Mc,a and T-Tg dependence. High rate impact (∼104-105 s-1) trends were similar to the KIC and GIC behavior, but were also correlated to σy. Overall, a ductile failure mode was observed for quasi-static and high rate results for a linear ROMP polymer (Mc,a = 1506 g mol-1 due to chain entanglement), and this gradually transitioned to a fully brittle failure mode for highly crosslinked ROMP polymers (Mc,a ≤ 270 g mol-1). Molecular dynamics (MD) simulations showed that low Mc,a ROMP resins were more likely to form molecular scale nanovoids. The higher chain stiffness in low Mc,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.

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Morphological and mechanical properties of semicrystalline polymers are strongly influenced by flow-induced crystallization during processing. We perform extensive molecular dynamics simulations with more than 1 million atoms to describe orientation, drawability, and crystallization of entangled polyethylene melts under uniaxial tensions at three different strain rates and after a subsequent cooling. During tensile deformation at the lowest strain rate of 10 7 s −1 , the polyethylene melt experiences entanglement loss and crystal nucleation. At higher strain rates of 10 8 and 10 9 s −1 , we observe a higher degree of chain alignment and void formation in addition to disentanglement and crystal nucleation. Chain segments make sharp turns relative to the neighboring chain orientations at the entanglement points, which manifests as a bimodal distribution of the local order parameter. Upon cooling below the melting temperature, semicrystalline polyethylene with a crystallinity close to 50% is formed. The entanglements are located in the amorphous regions of the semicrystalline polyethylene, with some located in the crystal/amorphous interface region. The chain ends of the semicrystalline polyethylene are preferentially localized at the crystal/amorphous interface, which is in agreement with recent experimental results.