Polylactide (PLA), a commercially available thermoplastic derived from plant sugars, finds applications in consumer products, disposable packaging, and textiles, among others. The widespread application of this material is limited by its brittleness, as evidenced by low tensile elongation at break, impact strength, and fracture toughness. Herein, a multifunctional vegetable oil, acrylated epoxidized soybean oil (AESO), was investigated as a biodegradable, renewable additive to improve the toughness of PLA. AESO was found to be a highly reactive oil, providing a dispersed phase with tunable properties in which the acrylate groups underwent cross-linking at the elevated temperatures required for processing the blends. Additionally, the presence of hydroxyl groups on AESO provided two routes for compatibilization of PLA/AESO blends:(1) reactive compatibilization through the transesterification of AESO and PLA and (2) synthesis of a PLA star polymer with an AESO core. The morphological, thermal, and mechanical behaviors of PLA/oil blends were investigated, in which the dispersed oil phase consisted of AESO, soybean oil (SYBO), or a 50/50 mixture of AESO/SYBO. The oil additives were found to toughen the PLA matrix, with significant enhancements in the elongation at break and tensile toughness values, while maintaining the glass transition temperature of neat PLA. In particular, the blend containing PLA, AESO, SYBO, and the PLA star polymer was found to exhibit a uniform oil droplet size distribution with small average droplet size and interparticle distance, resulting in the greatest enhancements of PLA tensile properties with no observable plasticization.
mediated process. In this work, we design functional bottlebrush polymer additives with mixed side-chain chemistries that can deliver unique surface properties. Bottlebrush polymers with poly(dimethylsiloxane) (PDMS) side-chains and bottlebrush copolymers with PDMS and poly(lactic acid) (PLA) side-chains are synthesized using ring opening metathesis polymerization. Contact angle goniometry, X-ray photoelectron spectroscopy (XPS), and microscopy demonstrate a spontaneous accumulation of these additives at the film surface without lateral phase segregation. Bottlebrush polymers were found to enrich the film surface more strongly than linear block copolymers of PDMS-b-PLA, and the surface contact angle was tunable by varying the composition and quantity of added bottlebrush copolymer. Significantly, bottlebrush additives segregate rapidly, during film casting. This work demonstrates that lowsurface energy bottlebrush copolymer additives can be used to introduce new surface properties in polymer films.
The mechanical properties of two chemically distinct and complementary thermoset polymers were manipulated through development of thermoset blends. The thermoset blend system was composed of an anhydride-cured diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin, contributing high tensile strength and modulus, and polydicyclopentadiene (PDCPD), which has a higher toughness and impact strength as compared to other thermoset polymers. Ultra-small-angle and small-angle X-ray scattering analysis explored the morphology of concurrently cured thermoset blends, revealing a macroscopically phase separated system with a surface fractal structure across blended systems of varying composition. The epoxy resin rich and PDCPD rich phases exhibited distinct glass transitions (T g 's): the T g observed at higher temperature was associated with the epoxy resin rich phase and was largely unaffected by the presence of PDCPD, whereas the PDCPD rich phase T g systematically decreased with increasing epoxy resin content due to inhibition of dicyclopentadiene ring-opening metathesis polymerization. The mechanical properties of these phase-separated blends were in reasonable agreement with predictions by the rule of mixtures for the blend tensile strength, modulus, and fracture toughness. Scanning electron microscopy analysis of the tensile and fracture specimen fracture surfaces showed an increase in energy dissipation mechanisms, such as crazing, shear banding, and surface roughness, as the fraction of the more ductile component, PDPCD, increased. These results present a facile method to tune the mechanical properties of a toughened thermoset network, in which the high modulus and tensile strength of the epoxy resin can be largely retained at high epoxy resin content in the blend, while increasing the fracture toughness.
The development of interpenetrating polymer networks provides a route to create tough materials that maintain high strength and stiffness, suitable for meeting the demands of an offshore wind turbine environment. This work has focused on a system composed of diglycidyl ether of bisphenol A (DGEBA)-based epoxy resin, contributing high tensile strength and modulus, and polydicyclopentadiene (polyDCPD), which has a higher toughness and impact strength as compared to other thermoset polymers. In situ Fourier transform infrared spectroscopy was used to explore the reaction kinetics in neat, diluted and sequentially cured mixtures of epoxy resin and polyDCPD. There were significant differences in the rate of network formation in the two neat systems, as the rate of anhydride curing of the epoxy was extremely slow at the temperatures required for reasonably measurable dicyclopentadiene (DCPD) ring-opening metathesis polymerization. The dissimilar kinetics of these two systems were leveraged in the design of a sequential curing protocol in which the DCPD was first cured in the presence of the epoxy resin components, followed by curing of the epoxy resin at an elevated temperature. The curing kinetics of the macroscopically phase separated domains of the epoxy resin components in the presence of the polyDCPD network behaved as expected for a diluted epoxy resin. These results provide the kinetic basis for future studies to prepare interpenetrating polymer networks which employ thermodynamic control of phase separation such as through the addition of compatibilizing molecules.
Plant-derived phenolic acids are attractive substitutes for petroleum sources for the derivation of polymers, due to their rigid aromatic rings and chemical groups amenable to functionalization. Difunctional phenolic acids were investigated as replacements for the diglycidyl ether of bisphenol A (DGEBA) in anhydride-cured epoxy resins. Functionalization of each phenolic acid was carried out through allylation, followed by epoxidation. Epoxy resins were synthesized through reaction of either epoxidized salicylic acid (ESA) or epoxidized 4-hydroxybenzoic acid (E4HBA) with the curing agent methylhexahydrophthalic anhydride (MHHPA) (catalyzed by 1-methyl-imidazole, 1-MI). The MHHPA anhydride curing agent (catalyzed by 1-MI) was chosen due to the resulting high conversion and advantageous high polymer glass transition temperature. ESA and E4HBA had similar curing behavior to that of DGEBA when cured with MHHPA. A two-step protocol was developed to avoid monomer evaporation and polymer vitrification during curing. ESA-and E4HBA-based epoxy resins exhibited comparable tensile moduli and strengths relative to a conventional DGEBA-based epoxy resin. E4HBA-and DGEBA-based epoxy resins (with para placement of functional groups) fractured at comparable elongation at break values, which were higher than that of the ESAbased epoxy resin (with ortho placement of functional groups). Epoxidized difunctional phenolic acids were found to be nontoxic and renewably sourced replacements for DGEBA in epoxy resins, producing epoxy resins of high modulus, high glass transition temperature, and elongation at break (in the case of E4HBA) comparable to a conventional DGEBA-based epoxy resin.
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