High Strain Shape Memory Polymers

Shape memory polymer (SMP) is a class of smart material that can be programmably fixed into temporary shapes and recover to the permanent shape upon external stimulation. Although polymer shape memory behavior has been known for over half a century, the recent emergence of a variety of high value applications including flexible electronics, biomedical devices, and deployable aerospace structures has attracted increasing attention for both the scientific and industrial communities. The diverse applications require SMPs with different and tunable properties. With this in mind, this dissertation focuses on two classes of SMPs:1. epoxy SMP with high strain capability as stamps for transfer printing of flexible electronics; 2. Biodegradable thermoset polyurethane SMP with an ultra-high maximum recoverable strain while maintaining robust shape memory performances and body temperature actuation capability.Epoxy polymers as recently emerged thermoset SMPs provide superior thermo-mechanical stability, excellent processability, and low curing shrinkage. However, epoxy SMPs typically suffer from low recoverable strains, which severely limit their potential applications. In this dissertation, a set of epoxy-based SMPs were synthesized. These epoxy polymers have tunable Tg (between 40℃ and 80℃) and excellent shape memory properties in terms of shape fixity, shape recovery, and cycling stability. In particular, one epoxy SMP was found to exhibit record high maximum recoverable strain (180%), considerable larger than the previously reported highest value of 75% for epoxy SMPs. Additionally, this epoxy SMP, with its high strain and low rubbery modulus was found to be ideal for use as stamps in transfer printing of flexible electronics with printing efficiency of 100%. We believe that this high strain epoxy system will significantly broaden the opportunities for device applications of SMPs beyond flexible electronics.Relative to epoxy thermoset SMPs, Tm based thermoplastic polyurethanes provide much higher recoverable strains. However, this type of materials often suffer from compromised shape fixity and recovery due to their physically crosslink nature.To overcome this problem, we synthesized thermoset polyurethanes based on the reaction between aliphatic isocyanates and polycaprolactone diol. By reducing the ratio of crosslinker to its limit, polyurethane with record high breaking strain (1150%) and ultra-low rubbery modulus was obtained. Importantly, this polyurethane exhibits excellent shape memory properties and can be actuated by body temperature. Since the polyurethane itself has excellent biocompatibility and the polycaprolactone is biodegradable, we anticipate that this material might bring forth great opportunities in biomedical applications.