can occur easily in the continuous segregated graphene phase. Since most multi-functional materials are required to provide excellent transport properties while maintaining sufficient mechanical strength, alternative methods of distributing graphene need to be developed. Despite recent progresses on the electrical characterization of graphene-based segregated composites, no results have been published yet regarding the combined electro-mechanical behavior of these highly conductive materials. In this work, a novel capillary-driven, particle-level templating technique was utilized to distribute graphite nanoplatelets into specially constructed architectures throughout a polystyrene (PS) matrix to form multi-functional composites with tailored electromechanical properties. By precisely controlling the temperature and pressure during a melt compression process, highly conductive composites were formed using very low loadings of graphene particles. To improve the mechanical properties, a new processing technique was developed that uses rotary shear during the compression molding process to gradually evolve the honeycomb graphene network into a concentric band structure. Two types of composites, organized and shear-modified, were produced to demonstrate the electro-mechanical tailoring of the composite material. An experimental investigation was conducted to understand the effect of graphene content as well as shearing on the mechanical strength and electrical conductivity of the composites. 24.2 Material and Specimen 24.2.1 Material The graphite nanoplatelets used in this study were xGnP™Nanoplatelets (XG Sciences, USA). These nanoparticles consist of short stacks of graphene layers having a lateral dimension of ~25 μm and a thickness of ~6 nm. The polymeric material chosen for this study was polystyrene (Crystal PS 1300, average molecular weight of 121,000 g/mol) purchased from Styrolution, USA. The PS pellets (~2 mm) used were elliptical prisms with a total surface area of 1.03 0.01 cm2. 24.2.2 Specimen Two types of composites, organized and shear-modified, were produced to demonstrate the electro-mechanical tailoring of the composite material. A two-step process was utilized to produce the GNP/PS segregated composites [31]. For composites consisting of less than 0.2 v/v %, the desired amount of graphene platelets were measured and added directly to 7 g of dry PS pellets. The GNP spontaneously adheres to the dry polymer particles by physical forces, which may be van der Waals forces or electrostatic attraction associated with surface charges. This coating process works well for GNP loadings below 0.2 v/v %. However, at higher GNP loadings, this dry method leaves behind excess GNP because the charge on the pellets is neutralized after the initial coating. To provide a means of temporarily attaching larger quantities of the GNP to the surface of the PS, an additional step is implemented during the fabrication procedure as shown in Fig. 24.1. For GNP loadings greater than 0.2 v/v %, the PS is first soaked in a methanol bath. The excess methanol is drained from the PS pellets. GNP is added, and the mixture is then shaken vigorously, creating a dense coating of graphene on each PS pellet. The methanol temporarily moistens the polymer pellets forming small liquid bridges. The capillary pressure created through these bridges allows the GNP to stick easily to the surface of the pellets. During the subsequent hot melt pressing, the temperature and mold pressure are precisely controlled allowing the pellets to be consolidated into a monolith while maintaining boundaries. In our experiments, a stainless steel mold consisting of a lower base and a plunger was heated to 125 ºC. The graphene coated PS was placed inside the cavity of the lower base and the plunger was placed on top. The temperature of both the plunger and the base mold was maintained for 20 min at which point it was hot-pressed at 45 kN using a hydraulic press. By precisely controlling the temperature and pressure during the melt compression process, highly conductive composites were formed using very low loadings of graphene particles. Modified particle templated composites were fabricated by incorporating a shearing technique during the melt compression process. Following the same coating process as discussed earlier, the graphene coated pellets were placed inside a modified steel mold, which was equipped with guide pins to ensure that the base remained stationary. The plunger was then 194 N. Heeder et al.
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