The unique properties of aligned nanomaterials make them attractive for a number of applications such as in flexible electronics and sensing. When anisotropic nanofillers such as carbon nanotubes, graphene and cellulose are aligned in the final composite and when deposition of nanomaterials is coupled with subsequent processing (thermal, laser annealing) it gives rise to enhanced mechanical strength, thermal and electrical properties. As a first step towards achieving this goal, we report on three different projects which deal with developing anisotropic fillers – depositing carbon nanotubes (CNT) on flexible substrates for electronics, developing graphene composites for thermal management devices and combining cellulose nanocrystals with thermoplastic polyurethane (TPU) to form conducting composites for use as sensors.
Due to the 2-D geometry, sp2 bonding structure, and the large aspect ratio of graphene, formation of electrically percolated networks at low nano-filler loadings is possible. Graphite composed of a few layers of graphene can therefore serve as structural components in polymer composites with multiple functionality. Utilization of CNCs has also gained popularity due to their excellent mechanical properties, high aspect ratios, and reactive surfaces that permit conductive metal particle surface functionalization. We take advantage of these properties through additive manufacturing where alignment is a result of shear forces during printing.
A method was developed to deposit CNT’s on flexible polydimethoxysilane (PDMS) and TPU and the samples were characterized by microscopy and Raman Spectroscopy to identify the CNT’s. Alignment is induced in the nanotubes by printing a concentrated sample onto the flexible susbstrate. For the cellulose project, an ink was successfully developed using Dimethyl Sulfoxide (DMSO) as a solvent and the samples successfully 3D printed using a printer. An interplay of cellulose concentration, nozzle diameter and print speed was used to achieve optimal print resolutions. By maintaining the solids volume fraction at 20% by weight, we are able to extrude CNC inks of varying cellulose concentrations using the 100 micron nozzle for the best 3D printing resolutions. Mechanical testing of the printed structures was initiated to test the effect of alignment of cellulose crystals on mechanical properties. Graphite nanoplatelets and a two-part epoxy resin (EPON 862) were mixed in a planetary centrifugal mixer to form conductive ink dispersions. The rheology of the ink was tuned by varying concentration of graphite, filler materials (carbon fibers) and solvent (acetone) to achieve multi-layer printability and thermal conductivity. Once the samples were thermally cured, the electrical, thermal and mechanical properties were measured.