Dimensional dependence of photomechanical response in carbon nanostructure composites: a case for carbon-based mixed-dimensional systems

This paper reports dimensional dependence of the mechanical response in carbon nanostructure composites to near-infrared (NIR) light. Using polydimethylsiloxane, a common silicone elastomer, composites were fabricated with one-dimensional multi-wall carbon nanotubes (MWNTs), two-dimensional single-layer graphene, two-and-a-half-dimensional graphene nanoplatelets and three-dimensional highly ordered pyrolytic graphite. An evaporative mixing technique was utilized to achieve homogeneous dispersions of carbon in the polymer composites, and their photomechanical responses to NIR illumination were studied. For a given carbon concentration, both steady-state photomechanical stress response and energy conversion efficiency were found to be directly related to the dimensional state of the carbon nanostructure additive. A maximum observed stress change of ∼60 kPa and ∼5 × 10−3% efficiency were obtained with just 1 wt% MWNT loading. Actuation and relaxation kinetic responses were found to be related not to dimensionality, but to the percolation threshold of the carbon nanostructure additive in the polymer. Establishing a connective network of the carbon nanostructure additive allowed for energy transduction responsible for the photomechanical effect to activate carbon beyond the NIR illumination point, resulting in enhanced actuation. For samples greater than percolation threshold, photoconductivity of the nanocomposite structure as a function of applied pre-strain was measured. Photoconductive response was found to be inversely proportional to applied pre-strain, demonstrating mechanical coupling. Mechanical response dependence to the carbon nanostructure dimensional state could have significance in developing new types of carbon-based mixed-dimensional composites for sensor and actuator systems.