UCLA

Smart materials are defined as materials that can change their properties to adapt the versatile surrounding environment. Variable stiffness polymer (VSP) represents a highly interesting class of smart material. VSPs change their stiffness in response to the environmental cues such as temperature, light, moisture, magnetic field, etc. Temperature-stimulated VSPs stand out due to their dry and compact formfactor that doesn’t require bulky external stimulus sources. Typically, the VSPs, which can also be attributed to shape memory polymers, exhibit a modulus change as high as a few hundred folds across glass transition temperature. The polymer could assume specific rigid shapes at working conditions but can be programed to different ones via deforming at an elevated temperature. Moreover, the stiffness tunability enables VSP to adapt in different working conditions. As one representative of the intelligent polymeric systems, VSPs have gained significant interest in wearable electronics, bio-inspired robotics and human-assistive devices in recent years. However, the use of most VSPs is limited by the wide glass transition temperature band, especially in human-contact applications. Moreover, the exploration of VSPs has mainly focused on manipulating the polymer network for multiple temporary shapes transition and large elastic energy storage. Little work has been done to increase the stiffness variation, which is vital for VSP’s adaptiveness.

The phase-changing bistable electroactive polymer (BSEP) is a unique VSP we synthesized. The BSEP can adjust its modulus over 3000-fold within a narrow temperature band. The stiffness tunability is realized by reversibly crystallizing and melting of nanocrystalline in the polymer network. Unlike most glass transition VSPs, which have a broad transition temperature band of over 30 �C, BSEP possesses a sharp phase transition within 10 �C and a tunable inflection point between 30-50 �C. Such narrow transition band and low inflection point reduce thermal energy consumption and ensure the use of BSEP in human-contact and in-body applications.

This dissertation summarizes important research in material properties explorations (one chapter) and novel devices (two chapters) based on BSEPs. Advancements related to highly compliant and efficient Joule heating electrodes --- a crucial component of VSP devices are also covered. The main body of the dissertation comprises three chapters. The first chapter depicts the research in BSEP’s mechanical property engineering. We introduced reversible cross-links and modified the BSEP formulation to achieve high mechanical strain and stress for improved toughness. We also synthesized a composite combining BSEP and bacterial cellulose nanofibrils, resulting in a material with ultrawide tunable stiffness range from tens of kPa to 1 GPa. Such wide stiffness range grants the BSEP more potential applications in various conditions. The second chapter describes a refreshable tactile display using the BSEP and a stretchable serpentine Joule heating electrode. The reported tactile device utilizes the stiffness variation of BSEP to achieve large pixel displacement, high blocking force and compact formfactor with low activation voltage. The work could open a path to building compact, user-friendly and cost-effective tactile devices for variety of important applications. The third chapter presents a highly sensitive capacitive touch sensor based on the BSEP as a self-conformable smart skin. The device combines the properties of variable stiffness BSEP and touch sensor, which grants the sensor the ability of adapting on various surfaces and in different working conditions. The device integrates sensing and adaptiveness to mimic human skin and presents a novel platform in wearable electronics on epidermis or next generation robotics.