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

Abstract

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.

Thermal oxidation of Poly(dicyclopentadiene)– kinetic modeling of double bond consumption

“…The final kinetic model appears to be suitable at least in the domain of moderate degrees of conversion and allows the extent of PDCPD degradation to be predicted with time, temperature and external oxygen pressure as input parameters. The case of stabilized PDCPD should now be addressed to get closer to practical cases [4,5,6,29].…”

“…Many authors have emphasized the effects of thermal oxidation [4] and crosslinking [5] on physical and mechanical properties through experiments (e.g. T g , rubbery modulus, tensile properties).…”

The temperature‐dependent ballistic performance and the ductile‐to‐brittle transition in polymer networks

“…Greater ductility is associated with a lower T g and a lower modulus, whereas structural composites require high T g and high modulus, which lowers ductility . Materials based on ring‐opening metathesis polymerization (ROMP) show promise as materials to escape this traditional tradeoff . Recent work on ROMP‐synthesized polymers using monomers such as dicyclopentadiene (DCPD) and 5‐ethylidine‐2‐norbornene (ENB) has shown high (ballistic) ballistic impact resistance deep in the glassy state, while still maintaining glass‐transition temperatures in excess of 100 °C and elastic modulus values in excess of 1 GPa .…”

“…Materials based on ring‐opening metathesis polymerization (ROMP) show promise as materials to escape this traditional tradeoff . Recent work on ROMP‐synthesized polymers using monomers such as dicyclopentadiene (DCPD) and 5‐ethylidine‐2‐norbornene (ENB) has shown high (ballistic) ballistic impact resistance deep in the glassy state, while still maintaining glass‐transition temperatures in excess of 100 °C and elastic modulus values in excess of 1 GPa . The ductility of the ROMP materials (DCPD and ENB) is such that they maintain a relatively high ballistic performance across the entire temperature range of interest for Army protection applications, and molecular simulations hypothesize this to be a result of their ability to form nanoscale voids during deformation due to their lack of hydrogen bonds or other strong intermolecular interactions .…”

Synthesis and properties of UV‐curable waterborne urethane modified acrylic coatings with varying vinyl content

“…The UV‐WUA‐1 sample with low crosslinking degree had a very low value of tensile strength and elongation at break because the molecular weight was not large enough and the chains could move easily when being stretched. [ 20,21 ] As the crosslinking degree of the cured film rose, the tensile strength increased but the elongation at break declined. This was because crosslinking points would hinder the movement of molecular chains if being stretched, and the more crosslinking points, the more obvious this kind of effects was.…”