This EArly-concept Grant for Exploratory Research (EAGER) award provides funding to evaluate the feasibility of producing graded nanocomposites with controlled linear nanofiller concentration profiles. Experimental research aims focus on a custom processing technique where dual ramped-flowrate pumps, a static mixer, and rapid curing are used to create nanofiller-loaded polymer composites with a constant gradient in reinforcement. The concentration profile will be measured by light absorbance in samples taken from the composite precursor prior to curing. The conductivity profile will be tested pointwise using a four-point probe after curing. The desired nanofiller is pristine graphene, a nanomaterial prized for its outstanding combination of transport properties. Polymer matrices to be explored include Polydimethylsiloxane and epoxy. Because the graphene sheets are conductive, it is hypothesized that the axial conductivity profile mimics percolation scaling laws such that gradient composites may be used to investigate how percolation threshold and critical scaling exponent are affected by varying dispersion quality, nanofiller geometry, and nanofiller size as a function of sonication.
If successful, this project will provide nanoscale insight (into percolating network architecture) and contribute to advanced material functionality (in sensing applications in the aerospace and energy industry). The composites produced by this technique will allow for high-accuracy, high-repeatability comparative studies of nanofiller percolation within polymer matrices. Such composites have immediate application to a range of engineering needs, particularly in next-generation piezoresistive smart materials. Broad scientific communities affected by this exploratory work include those interested in nanomaterial exfoliation and dispersion, composite manufacture, percolation theory, and nanomaterial electrical contacts and networks.
The research objective of this EArly-concept Grant for Exploratory Research (EAGER) proposal was to establish the feasibility of producing polymer nanocomposites with a gradient in nanofiller content. We developed an automated system to produce nanofilled thermoset composites using syringe pumps with graded flowrates to produce nanocomposites with near-linear concentration profiles. Such composites display percolation scaling laws along their length. We demonstrated that this basic concept is feasible for multiple nanofillers (carbon nanotubes, graphene, carbon black) and polymeric matrices (epoxy, crosslinked PDMS). A simple mathematical model was also developed to describe the predicted concentration profile. One unexpected but interesting facet of this work is the gap between the electrical percolation threshold (which determines electrical conductivity) and the rheological percolation threshold (which determines the jump in storage modulus). Rheologically percolating samples tend to suffer from poor mixing, but we find that rheological percolation thresholds are higher than electrical percolation thresholds if the nanofiller-nanofiller contact can be made. Both of these thresholds scale inversely with the aspect ratio of the effective nanofiller. This was explored in detail for graphene/epoxy systems by varying the processing techniques to artificially alter dispersion quality for the same nanofiller choice and concentration. These composites can be used to probe percolating scaling differences between nanofillers of varying aspect ratio and dispersion quality in a single sample; this moves percolation resolution from a many-sample experimental setup to a simple issue of conductivity measurement spatial resolution. Such composites also have potential applications in strain sensing given that the conductivity may vary at the percolation threshold upon deformation. In the course of this work, both graduate and undergraduate researchers were trained in composite processing science and basic numerical modeling.
