UCLA

Stimuli-responsive polymers are materials that undergo physical or chemical properties changes triggered by light, temperature, mechanical force, insertion of small molecules, electric fields, magnetic fields, or pH. Stimuli-responsive materials can be designed for a desired physical response, such as compression, shape change, or variable stiffness and have been used in coatings, sensors, drug delivery, self-healing, and mechanical actuators. Some stimuli-responsive materials utilize several trigger mechanisms to amplify and increase the resulting change in physical properties. For example, cellulose polymer nanocomposites exhibit stiffness changes triggered by both water and temperature to increase the modulus differential of the material. The modulus ranges from GPa range to the low MPa range with the assistance of the dual-stimuli technique. Although this modulus differential is large, for applications in biomaterials, the low-end modulus of the material must be in the kPa range to limit mechanical mismatch of an implant for practical use. Another category of stimuli-responsive materials is dielectric elastomer materials, electric-field responsive materials that expand and contract with an applied voltage. Rather than change stiffness, these materials change shape. When sandwiched between two compliant electrodes and an electric field is applied, the material is compressed by the attraction of the opposite charges formed on the electrodes. With electro-response, these materials are useful in soft robotics applications, however, commercially available dielectric elastomer materials require prestretching for high actuation performance and are incapable of molecular modification. Additionally, dielectric elastomers are difficult to process due to the crosslinked nature, a controlled synthetic approach to more precisely design molecular architectures is desired. Overall, these materials can be precisely tuned to respond to triggers based on the application requirements. Synthesizing and optimizing new stimuli- responsive materials that are precisely tuned opens the door for expanded applications in fields such as biomedicine or soft robotics.

The research outlined in this dissertation focuses on the synthesis and fabrication of novel stimuli- responsive polymer materials to address challenges previously outlined. The main body of this dissertation describes new cellulose polymer composite materials with ultra-wide stiffness range, new dielectric elastomers with high actuation performance without prestretch, and new BAB triblock copolymers with variable stiffness. The first chapter surveys current stimuli-responsive polymer materials technology with a focus on thermo-responsive, photo-responsive, electro-responsive, and dual responsive materials. The second chapter outlines research aimed to increase the modulus differential in cellulose composite materials using a thermo-responsive variable stiffness polymer and cellulose microfibers. The resulting composite utilizes two stimuli, the first is temperature to soften the stiff polymer matrix by melting the crystalline segments to form a soft crosslinked polymer. The second stimulus is the addition of water, to nullify the reinforcing network formed by percolating cellulose fibers and further soften the material. The material exhibits an ultra wide modulus differential from 1 GPa down to 40 kPa stimulated by water and temperature. An ultra wide modulus range allows for further applications development with potential for biomedical devices. The third chapter outlines a new dielectric elastomer (DE) material that exhibits performance similar to commercially available materials in an aim to address the limitations of prestretching and to introduce DEs capable of modification. A bimodal interpenetrating crosslinked network was established by combining a short chain di-functional acrylate monomer with a long-chain high molecular weight di-functional acrylate monomer to form a material with mechanical properties similar to commercially available DEs. Additional mono-functional diluents were added to further tune the electro-mechanical properties and improve performance. The new DE exhibited maximum actuation strains near 200% and rapid response over 100% strain at 2 Hz. The new DE material exhibits performance higher than other synthetic dielectric elastomer and opens the door to optimization of DE materials for a new generation of polymer actuator materials. The fourth and last chapter of the main text presents a comparison study of three different length BAB triblock copolymers in an aim to synthesize a triblock copolymer for use as a bistable electroactive polymer (BSEP). BSEP materials are stiff at room temperature and softened at elevated temperature to actuate as dielectric elastomers. BSEP is typically processed by bulk polymerization making it difficult to modify post-fabrication. In the BAB polymer described, a two-sided RAFT chain transfer agent was synthesized, for symmetrical synthetic processing, using poly (ethylene glycol) for high stiffness at room temperature and increased flexibility at elevated temperature. The poly (stearyl acrylate) B-blocks were then incorporated to add further stiffness at room temperature and control the material microstructure. Of the three BAB copolymers synthesized, two exhibited variable stiffness from 1 GPa to 10 kPa with spherulite microstructural formations confirmed by optical and scanning electron microscopy. By introducing a controlled synthetic pathway using RAFT living polymerization, these materials can be finely tuned for specific properties before and after fabrication.