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  1. Abstract

    High-performance lightweight architectures, such as metallic microlattices with excellent mechanical properties have been 3D printed, but they do not possess shape memory effect (SME), limiting their usages for advanced engineering structures, such as serving as a core in multifunctional lightweight sandwich structures. 3D printable self-healing shape memory polymer (SMP) microlattices could be a solution. However, existing 3D printable thermoset SMPs are limited to either low strength, poor stress memory, or non-recyclability. To address this issue, a new thermoset polymer, integrated with high strength, high recovery stress, perfect shape recovery, good recyclability, and 3D printability using direct light printing, has been developed in this study. Lightweight microlattices with various unit cells and length scales were printed and tested. The results show that the cubic microlattice has mechanical strength comparable to or even greater than that of metallic microlattices, good SME, decent recovery stress, and recyclability, making it the first multifunctional lightweight architecture (MLA) for potential multifunctional lightweight load carrying structural applications.

  2. Benefits of employing graphene nanopletlates (GNPLs) in composite structures include mechanical as well as multifunctional properties. Understanding the impedance behavior of GNPLs reinforced syntactic foams may open new applications for syntactic foam composites. In this work, GNPLs reinforced syntactic foams were fabricated and tested for DC and AC electrical properties. Four sets of syntactic foam samples containing 0, 0.1, 0.3, and 0.5 vol% of GNPLs were fabricated and tested. Significant increase in conductivity of syntactic foams due to the addition of GNPLs was noted. AC impedance measurements indicated that the GNPLs syntactic foams become frequency dependent as the volume fraction of GNPLs increases. With addition of GNPLs, the characteristic of the syntactic foams are also observed to transition from dominant capacitive to dominant resistive behavior.
  3. Engineering applications of current thermoset shape memory polymers are limited by three critical issues: demanding fabrication conditions (from 70 to 300 °C temperatures for hours or days), lack of reprocessability or recyclability, and low recovery stress and energy output. To address these problems simultaneously, a new UV curable and vitrimer-based epoxy thermoset shape memory polymer (VSMP) has been synthesized. A 1.1 mm thick VSMP film can be readily cured at room temperature under UV-irradiation (61 mW cm −2 ) in just 80 s. It possesses 36.7 MPa tensile strength, 230 MPa compressive strength, and 3120 MPa modulus at room temperature. It still has a compressive strength of 187 MPa at 120 °C. The covalent adaptable network (CAN) imparts the VSMP with recyclability, as reflected by two effective recycling cycles (>60% recycling efficiency). In addition, the VSMP exhibits good shape memory properties for multiple shape recovery cycles. With 20% compression programming strain, up to 13.4 MPa stable recovery stress and 1.05 MJ m −3 energy output in the rubbery state are achieved. With good mechanical strength, thermal stability, recyclability, and excellent shape memory properties combined with in situ UV-curing capabilities, the new VSMP is a promising multifunctional thermoset for engineering applications.
  4. Self-healing thermoset epoxy based on dynamic covalent bond chemistry has been developed in the past several years, which warrants the creation of recyclable epoxy. However, the existing systems produce epoxy that has lower strength, stiffness, and glass transition temperature, making them unsuitable for load-bearing structures. In this study, we developed a new recyclable thermoset epoxy through solid form recycling. The epoxy has strength, stiffness, and glass transition temperature similar to those found in conventional thermoset epoxy. The effect of healing temperature, healing time, healing pressure, and powder size on the healing efficiency was experimentally investigated. It was found that the healing efficiency is as high as 88.1%, and the epoxy can be recycled more than one time.