UCLA

Smart materials have been widely investigated to explore new functionalities unavailable to traditional materials or to mimic the multifunctionality of biological systems. Synthetic polymers are particularly attractive as they already possess some of the attributes required for smart materials, and there are vast room to further enhance the existing properties or impart new properties by polymer synthesis or composite formulation. In this work, three types of smart polymer and composites have been investigated with important new applications: (1) healable polymer composites for structural application and healable composite conductor for electronic device application; (2) conducting polymer polypyrrole actuator for implantable medical device application; and (3) ferroelectric polymer and ceramic nanoparticles composites for electrocaloric effect based solid state refrigeration application. These application entail highly challenging materials innovation, and my work has led to significant progress in all three areas.

For the healable polymer composites, well known intrinsically healable polymer 2MEP4F (a diels-alder crosslinked polymer formed from a monomer with four furan groups and another monomer with two maleimide groups) was first chosen as the matrix reinforced with fiber. Glass fibers were successfully functionalized with maleimide functional groups on their surface. Composites from functionalized glass fibers and 2MEP4F healable polymer were made to compare with composites made from commercial carbon fibers and 2MEP4F polymer. Dramatically improved short beam shear strength was obtained from composite of functionalized glass fibers and 2MEP4F polymer.

The high cost of 2MEP4F polymer can potentially limit the large-scale application of the developed healable composite, we further developed a new healable polymer with much lower cost. This new polymer was formed through the Diels-Alder crosslinking of poly(furfuryl alcohol) (PFA) and 1,1'-(Methylenedi-4,1-phenylene)bismaleimide (MDPB). It showed the same healing ability as 2MEP4F while all starting materials are cheaper and commercially available. To further improve the mechanical strength of the PFA-MDPB healable polymer, epoxy as a strengthening component was mixed with PFA-MDPB healable polymer. The PFA, MDPB and epoxy composite polymers were further reinforced by carbon fiber as done with 2MEP4F matrix and the final composites were proved to have higher short beam shear strength than 2MEP4F while exhibiting a similar healing efficiency.

Healable polymer MDPB (a two maleimide groups monomer) - FGEEDR (a four furan groups monomer) was also designed and synthesized for transparent healable polymer. The MDPB-FGEEDR healable polymer was composited with silver nanowires (AgNWs) to afford healable transparent composite conductor. Razer blade cuts in the composite conductor could heal upon heating to recover the mechanical strength and electrical conductivity of the composite. The healing could be repeated for multiple times on the same cut location. The healing process was as fast as 3 minutes for conductivity to recover 97% of the original value.

For electroactive polymer polypyrrole, the fast volume change upon electrical field change due to electrochemical oxidization or reduction was studied for actuation targeting toward a robotic application. The flexibility of polypyrrole was improved via copolymerization with pyrrole derivatives. Actuator devices are fabricated that more suitable for implantable medical device application than pyrrole homopolymer.

The change of dipole re-orientation and thus dielectric constant of ferroelectric polymers and ceramics upon electrical field may be exploited for electrocaloric effect (ECE) and solid state refrigeration. For ferroelectric ceramics, we synthesized a series of Ba1-xSrxTiO3 nanoparticles with diameter ranging from 8-12 nm and characterized their dielectric and ferroelectric properties through hysteresis measurement. It was found that 8 nm BaTiO3 nanocrystals are stable at cubic crystal structure without ferroelectric character. Ba1-xSrxTiO3 nanoparticles with larger crystalline size (40nm) provide near room temperature transition temperature which could be the Curie temperature. We carefully studied the electrocaloric effect of ferroelectric polymers P(VDF-TrFE) and P(VDF-TrFE-CFE). The nanocomposite of Ba1-xSrxTiO3 nanoparticles dispersed in P(VDF-TrFE-CFE) was fabricated and studied by hysteresis measurement to estimate the electrocaloric effect of the composite. The interdigitated electrode samples were successfully infiltrated with terpolymer and the multilayers ECE device showed 0.01 C/m2 displacement at 70MV/m. Free-standing monolayer ECE devices made from the terpolymer gave 3.4⁰C temperature change measured via an infrared camera.