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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An overview of the development and utilization of organic electro-optic materials is presented with emphasis on the role played by quantum and statistical mechanical calculations in understanding critical structure/function relationships that have guided the improvement of such materials over the past two decades. This review concentrates largely on three classes of organic electro-optic materials prepared by electric field poling of materials near their glass transition temperature: (1) chromophore/ polymer composite materials, (2) dendrimers and polymers containing covalently incorporated chromophores, and (3) matrix-assisted-poling (MAP) materials where specific spatially anisotropic interactions enhance poling efficiency. In particular, the role of chromophore shape, restrictions on chromophore motion associated with covalent bonds, and lattice dimensionality effects are reviewed. The role of device design and auxiliary properties (optical loss, thermal stability, photochemical stability, processability) in influencing the utilization of organic electro-optic materials is also briefly reviewed.

Photoredox-mediated metal-free ring-opening metathesis polymerization (MF-ROMP) is an alternative to traditional metal-mediated ROMP that avoids the use of transition metal initiators while also enabling temporal control over the polymerization. Herein, we explore the effect of various additives on the success of the polymerization in order to optimize reaction protocols and identify new functionalized monomers that can be utilized in MF-ROMP. The use of protected alcohol monomers allows for homo- and copolymers to be prepared that contain functionality beyond simple alkyl groups. Several other functional groups are also tolerated to varying degrees and offer insight into future directions for expansion of monomer scope.

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a b s t r a c tBallistic performance, at effective strain rates of (10 4 -10 5 s À1 ), for polymeric dicyclopentadiene (pDCPD)was compared with two epoxy resin/diamine systems with comparable glass transition temperatures. The high rate response was characterized in terms of a projectile penetration kinetic energy, KE 50 , which describes the projectile kinetic energy at a velocity with a 50% probability of sample penetration. pDCPD showed superior penetration resistance, with a 300-400% improvement in ballistic energy dissipation, when compared with the structural epoxy resins. In addition, unlike typical highly crosslinked networks that become brittle at low temperatures, the improved pDCPD performance occurred over a very broad temperature range (À55 to 75°C), despite exhibiting a glass transition temperature characteristic of structural resins ($142°C). In addition to the high T g , pDCPD exhibited a room temperature glassy storage modulus of 1.7 GPa, offering the potential to circumvent the common structural versus energy dissipation trade-off encountered with conventional crosslinked polymers. Quasi-static measurements suggested that the performance of pDCPD is phenomenologically related to higher fracture toughness and lower yield stress relative to typical epoxies, while molecular dynamics simulations suggest the origin is the lack of strong non-covalent interactions and the facile formation of nanoscale voids to accommodate strain in pDCPD.Published by Elsevier Ltd.

Polydopamine coatings are of interest due to the fact that they can promote adhesion to a broad range of materials and can enable a variety of applications. However, the polydopamine-substrate interaction is often noncovalent. To broaden the potential applications of polydopamine, we show the incorporation of 3-aminopropyltriethoxysilane (APTES), a traditional coupling agent capable of covalent bonding to a broad range of organic and inorganic surfaces, into polydopamine coatings. High energy X-ray photoelectron spectroscopy (HE-XPS), conventional XPS, near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR), and ellipsometry measurements were used to investigate changes in coating chemistry and thickness, which suggest covalent incorporation of APTES into polydopamine. These coatings can be deposited either in Tris buffer or by using an aqueous APTES solution as a buffer without Tris. APTES-dopamine hydrochloride deposition from solutions with molar ratios between 0:1 and 10:1 allowed us to control the coating composition across a broad range.