Date on Master's Thesis/Doctoral Dissertation

5-2013

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Mechanical Engineering

Committee Chair

Panchapakesan, Balaji

Author's Keywords

Photomechanical; Actuators; Nanocomposite; Graphene; Stimuli-responsive; Carbon nanostructure

Subject

Carbon; Graphene; Nanostructured materials; Photomechanical processes

Abstract

Materials that change shape or dimensions in response to external stimuli are widely used in actuation devices. While plenty of systems respond to heat, light, electricity, and magnetism, there is an emerging class of light-driven actuators based on carbon nanostructure/elastomer composites. The addition of nanomaterials to elastomeric polymers results not only in significant material property improvements such as mechanical strength, but also assists in creating entirely new composite functionalities as with photo-mechanical actuation. Efficient photon absorption by nanocarbons and subsequent energy transduction to the polymeric chains can be used to controllably produce significant amounts of pre-strain dependent motion. Photo-mechanical actuation offers a variety of advantages over traditional devices, including wireless actuation, electro-mechanical decoupling (and therefore low noise), electrical circuit elimination at point of use, massive parallel actuation of device arrays from single light source, and complementary metal–oxide–semiconductor / micro-electro-mechanical (CMOS/MEMS) compatible processing. Applications of photo-responsive materials encompass robotics, plastic motors, photonic switches, micro-grippers, and adaptive micro-mirrors. The magnitude and direction of photo-mechanical actuation responses generated in carbon nanostructure/elastomer composites depend on applied pre-strains. At low levels of pre-strains (3–9%), actuators show reversible photo-induced expansion while at high levels (15–40%), actuators exhibit reversible contraction. Large, light-induced reversible and elastic responses of graphene nanoplatelet (GNP) polymer composites were demonstrated for the first time, with an extraordinary optical-to-mechanical energy conversion factor (?M) of 7–9 MPa/W. Following this demonstration, similar elastomeric composite were fabricated with a variety of carbon nanostructures. Investigation into photo-actuation properties of these composites revealed both layer-dependent, as well as dimensionally-dependent responses. For a given carbon concentration, both steady-state photo-mechanical stress response and energy conversion efficiency were found to be directly related to dimensional state of carbon nanostructure additive, with one-dimensional (1D) carbon nanotubes demonstrating the highest responses (~60 kPa stress and ~5 × 10-3% efficiency at just 1 wt% loading) and three-dimensional (3D) highly ordered pyrolytic graphite demonstrating the lowest responses. Furthermore, development of an advanced dispersion technique (evaporative mixing) resulted in the ability to fabricate conductive composites. Actuation and relaxation kinetics responses were investigated and found to be related not to dimensionality, but rather the percolation threshold of carbon nanostructure additive in the polymer. Establishing a connective network of carbon nanostructure additive allowed for energy transduction responsible for photo-mechanical effect to activate carbon beyond the infrared (IR) illumination point, resulting in enhanced actuation. Additionally, in the conductive samples photoconductivity as a function of applied pre-strain was also measured. Photo-conductive response was found to be inversely proportional to applied pre-strain, demonstrating mechanical coupling. Following investigation into photo-mechanical actuation responses between the various carbon forms, use of these composite actuators to achieve both macroscopic as well as microscopic movement in practical applications was evaluated. Using dual GNP/elastomer actuators, a two-axis sub-micron translation stage was developed, and allowed for two-axis photo-thermal positioning (~100 µm per axis) with 120 nm resolution (limitation of the feedback sensor) and ~5 µm/s actuation speeds. A proportional-integral-derivative control loop automatically stabilizes the stage against thermal drift, as well as random thermal-induced position fluctuations (up to the bandwidth of the feedback and position sensor). Nanopositioner performance characteristics were found to be on par with other commercial systems, with resolution limited only by the feedback system used. A mathematical model was developed to describe the elastomeric composite actuators as a series of n springs, with each spring element having its own independent IR-tunable spring constant. Effects of illumination intensity, position, and amount of the composite actuator illuminated are discussed. This model provided several additional insights, such as demonstrating the ability to place not just one, but multiple stages on a single polymer composite strip and position them independently from one another, a benefit not seen in any other type of positioning system. Further investigation yielded interesting and novel photo-mechanical properties with actuation visible on macroscopic scales. Addition of a third component (thermally expanding microspheres), produced a new class of stimuli-responsive expanding polymer composites with ability to unidirectionally transform physical dimensions, elastic modulus, density, and electrical resistance. Carbon nanotubes and core-shell acrylic microspheres were dispersed in polydimethylsiloxane, resulting in composites that exhibit a binary set of material properties. Upon thermal or IR stimuli, liquid cores encapsulated within the microspheres vaporize, expanding the surrounding shells and stretching the matrix. Microsphere expansion results in visible dimensional changes, regions of reduced polymeric chain mobility, nanotube tensioning, and overall elastic to plastic-like transformation of the composite. Transformations include macroscopic volume expansion (>500%), density reduction (>80%), and elastic modulus increase (>675%). Additionally, conductive nanotubes allow for remote expansion monitoring and exhibit distinct loading-dependent electrical responses. Compared to well-established actuation technologies, research into photo-mechanical properties of carbon-based polymer composites is still in its infancy. Results in this dissertation demonstrate some of the enormous potential of light-driven carbon-based composites for actuation and energy scavenging applications. Furthermore, mechanical response dependence to carbon nanostructure dimensional state could have significance in developing new types of carbon-based mixed-dimensional composites for sensor and actuator systems. As the fabrication processes used here are compatible with CMOS and MEMS processing, carbon-based polymer composites allow for not only scaling actuation systems, but also ability to pattern regions of tailorable expansion, strength, and electrical resistance into a single polymer skin, making these composites ideal for structural and electrical building blocks in smart systems. Continued development of carbon-based polymer composites will extend the promising potential of light-driven actuation technologies and will serve as a catalyst to inspire continued research into energy conversion devices and systems.

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