Three-dimensional (3D) macroporous graphene foam based multifunctional epoxy composites are developed in this study. Facile dip-coating and mold-casting techniques are employed to engineer microstructures with tailorable thermal, mechanical, and electrical properties. These processing techniques allow capillarity-induced equilibrium filling of graphene foam branches, creating epoxy/graphene interfaces with minimal separation. Addition of 2 wt % graphene foam enhances the glass transition temperature of epoxy from 106 to 162 °C, improving the thermal stability of the polymer composite. Graphene foam aids in load-bearing, increasing the ultimate tensile strength by 12% by merely 0.13 wt % graphene foam in an epoxy matrix. Digital image correlation (DIC) analysis revealed that the graphene foam cells restrict and confine the deformation of the polymer matrix, thereby enhancing the load-bearing capability of the composite. Addition of 0.6 wt % graphene foam also enhances the flexural strength of the pure epoxy by 10%. A 3D network of graphene branches is found to suppress and deflect the cracks, arresting mechanical failure. Dynamic mechanical analysis (DMA) of the composites demonstrated their vibration damping capability, as the loss tangent (tan δ) jumps from 0.1 for the pure epoxy to 0.24 for ∼2 wt % graphene foam-epoxy composite. Graphene foam branches also provide seamless pathways for electron transfer, which induces electrical conductivity exceeding 450 S/m in an otherwise insulator epoxy matrix. The epoxy-graphene foam composite exhibits a gauge factor as high as 4.1, which is twice the typical gauge factor for the most common metals. Simultaneous improvement in thermal, mechanical, and electrical properties of epoxy due to 3D graphene foam makes epoxy-graphene foam composite a promising lightweight and multifunctional material for aiding load-bearing, electrical transport, and motion sensing in aerospace, automotive, robotics, and smart device structures.
A cryo-system assisted extrusion 3D printing technique has been developed to fabricate Shape Memory Polymer Epoxy (SMPE) thermoset into a near-net shape. With a unique cryogenic sprayer assisted printing system, it is possible to 3D print "pure" thermoset polymer, otherwise which was limited to thermoplastic or highly functionalized co-polymeric thermoset systems. The temperature was maintained below 10oC during 3D printing, which enabled dimensionally accurate structures. Graphene Nanoplatelet (GNP) reinforced SMPE ink was synthesized to be compatible with cryo-assisted extrusion 3D printing of shape memory composites. The addition of a mere 0.1 wt.% GNP exhibited 15% faster shape recovery due to improved thermal conductivity. Also, GNP addition imparted a 30% improvement in the ultimate tensile strength and a 17% increase in 3D printed composites' elastic modulus. Dynamic mechanical testing showed that the addition of 0.1 wt.% GNP in SMPE increased the composite's storage modulus by 365% and loss tangent by 66% compared to pure 3D printed SMPE, indicating enhanced damping characteristics. This study is the first to demonstrate a non-existent 3D printing technology for “pure thermoset” polymers with and without conductive graphene, that has the potential to develop an energy-efficient actuation system, passive thermal sensors, and self-deployable communication system structures.
An electrostatic spraying technique was used to synthesize 3D graphene foam (GrF)‐reinforced polyurethane (PU) composites. Glass transition temperature (Tg) of PU increased from 120 to 148 °C after 3.3 vol% GrF addition. Strong interfacial bonding accounts for the improvement in Tg of PU‐GrF composites. Energy dissipation behavior is probed at multiple load scales to examine the intrinsic and structural damping characteristic. Micro‐scale damping at 1 mN and 1,000 Hz exhibited loss tangent value (tan δ), which is 200% higher and 3 times faster than PU. Nano dynamic mechanical analysis of PU‐3.3 vol% GrF composite demonstrated up to 383% tan δ improvement than of PU at 5 μN dynamic load. Physical mechanisms including rippling effect and weak van der Waals forces in graphene account for the high energy dissipation of PU‐GrF composite. This study suggests that PU‐GrF nanocomposites have the potential to serve as a good damping material in vibrating systems. POLYM. COMPOS., 40:E1862–E1870, 2019. © 2018 Society of Plastics Engineers
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