Synthesizing A Phase Changing Bistable Electroactive Polymer And Silver Nanoparticles Coated Fabric As A Resistive Heating Element
Transducer technologies that convert energy from one form to another (e.g. electrical energy to mechanical energy or thermal energy and vise versa) are considered as the basic building blocks of robots and wearable electronics, two of the rapidly emerging technologies that impact our daily life. With an emphasis on developing the essential smart materials, this dissertation focuses on two specific transducer technologies, bistable large-strain electro-mechanical actuation and resistive Joule heating, in pursuit of refreshable Braille electronic displays and wearable thermal management element, respectively.
Dielectric elastomers (DEs) have been intensively studied for their promising ability to mimic human muscles in providing efficient electro-mechanical actuation. They exhibit a unique combination of properties, including large strain, fast response, high energy density, mechanical compliancy, lightweight, and low cost. However, the softness of the DE materials, which is a prerequisite for electrically induced large actuation strain, has been hindering their application in adaptive structures. In these applications such as braille displays, a certain amount of mechanical support is necessary in addition to large strains for the device or system to function. Bistable electroactive polymers (BSEP) that leverage the electrically induced large-strain actuation of DE actuators and the bi-stable rigid-to-rigid deformation of shape memory polymers are innovated to provide large electrical actuation strain in their rubbery state and fix the deformation by cooling down to room temperature to incorporate mechanical rigidity.
BSEP materials that can suppress electromechanical instability and exhibit stable mechanical properties in the rubbery state are desired. A bimodal BSEP material with a glass transition temperature right above room temperature has been synthesized employing simple UV curing process. The BSEP has a large storage modulus over 1GPa at room temperature that decreases to several MPa at above 70˚C after a rigid-to-rubbery transition via glass transition. The rubbery BSEP possesses a stable storage modulus regardless of temperature fluctuations, which is beneficial to stable electrical actuation performances under an electric field. The bimodal structure creates a framework involving both long chain crosslinkers and small molecular crosslinkers. Due to the limited chain extensibility of this bimodal framework, the rubbery BSEP can self-stiffen at modest strains to suppress electromechanical instability, which is responsible for the premature electrical breakdown of the previous BSEP materials in their rubbery states. A BSEP actuator with a braille dot size exhibits steadily increased actuation height with increasing electric field at 70 ˚C. A stable actuation with a cycle lifetime of over 2000 cycles at a raised dot height of 0.4 mm was demonstrated. A fabrication process for a page-size braille paper using the BSEP has been developed. A selective heating strategy has been investigated based on a 2-cell device to provide a selective actuation strategy of BSEP braille dots.
The preceding BSEP enters its rubbery state via glass transition at above 70 ˚C, where electrical actuation takes place. This actuation temperature has to be lowered in order to shorten the heating or cooling time for the reversible rigid-to-rubbery transition and to reduce thermal energy consumption. A phase changing bistable electroactive polymer based on stearyl acrylate (SA, or octadecyl acrylate) and urethane diacrylate oligomer (UDA, difunctional long chain crosslinker) has been synthesized to provide sharp rigid-to-rubbery transition (narrow temperature range of about 10 ˚C). At room temperature, the stearyl acrylate moieties are crystallized and harden the SA-UDA copolymers with a storage modulus of 10-100MPa. These aggregates melt at an elevated temperature (below 50 ˚C) with dangling octadecyl chains softening the polymer in its rubbery state. The difference of the storage modulus between the rigid state and the rubbery state ensures good shape memory properties of the SA-UDA polymers with both fixation and relaxation rate of 100%. The SA-UDA based BSEP exhibits steadily increased actuation strain with increasing electrical field. The SA-UDA films can be actuated at 50 ˚C up to 70% strain with the maximum actuation energy density comparable to the figure of merits of dielectric elastomers. No electromechanical instability was observed due to its self-stiffening property resulted from a similar bimodal structure as preceding bimodal BSEP.
Wearable thermal management strategy has presented itself recently as a new challenge to offer an optimal thermal experience for the occupant as well as to reduce building energy usage for heating, ventilation and air conditioning (HVAC). Joule heating based on silver nanoparticles (AgNPs) coated non-woven fabric can provide a wearable localized heating element. Silver nanoparticles (AgNPs) can grow in situ onto non-woven fabrics and form a very uniform and conductive media performing as an efficient heating element. A multistep electroless deposition was employed to initially create heterogeneous nuclei on the surface of the fabrics, which subsequently grow into AgNPs in an electroless deposition solution. The electroless deposition was conducted at a low temperature (in a 4 ˚C refrigerator) in an Ag (I) containing deposition solution with selected metastable redox pairs such that homogeneous nucleation of silver nanoparticles is kinetically unfavorable. With an extended deposition time, AgNPs merge with each other on the surface of the polyester fabrics to form a conductive network. A sheet resistance of <0.3 ohm/square can be achieved for AgNPs-coated polyester fabrics upon thermal annealing. Multistep electroless deposition creates chemical bonding between oxygen groups on the fabrics’ surface and AgNPs. As a result, the bonding between the AgNPs layer and the polyester fabrics is strong enough to resist sonication damage. The resistance only increased slightly after an 80minutes of sonication and therefore the AgNPs-polyester fabrics composite are regarded as washable. The AgNPs coated polyester fabrics was employed as a heating element. A voltage as low as 1volt is adequate to heat up the AgNPs-polyester fabrics to 60 ˚C in 2 seconds. The heat can be dissipated away fast after turning off the heating voltage, due to the mesh structure of the AgNPs-polyester fabrics. The strategy of the wearable heater can potentially play influential roles in energy saving and consumer experience in a localized thermal management system.