Influence of molecular weight between crosslinks on the mechanical properties of polymers formed via ring-opening metathesis. Issue 17 (16th April 2018)

• Record Type: Journal Article
• Authors: Long, Tyler R.; Elder, Robert M.; Bain, Erich D.; Masser, Kevin A.; Sirk, Timothy W.; Yu, Jian H.; Knorr, Daniel B.; Lenhart, Joseph L.
• Is Part Of: Soft matter. Volume 14:Issue 17(2018)
• Page Start: 3344
• Page End: 3360
• Publication Date: 2018-04-16
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/c7sm02407j
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : Changes in M c, a in non-polar ROMP resins correspond to a continuous brittle to ductile transition in contrast to polar thermosets. 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 (∼10 4 –10 5 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.