Generating Stress in Strained Hydrogel Fibers During Crystallization

Shape memory polymers store elastic energy by trapping stretched polymer chains in an extended state, which is analogous to a spring. Essentially, the work available by a spring is constrained by the spring constant and displacement of the spring. The work presented here reveals a new method to further increase the useable work in shape memory polymers beyond the stored electric energy in stretched chains. Specifically, a new class of fiber actuators have been reported that contract or rotate when triggered by heat or hydration. The fibers, termed "strain crystallized actuators" (SCAs), are produced by combining solution-phase block copolymer self-assembly and strain-programmed crystallization. The strained fibers consist of highly aligned nanoscale structures with alternating crystalline and amorphous domains, resembling the ordered and striated pattern of mammalian skeletal muscles. The usable work generated by the fibers is a combination of elastically stored energy (i.e., a spring) and energy stored through a phase change (i.e., transitioning between crystalline and amorphous solid states). The presentation highlights new insight into increasing the actuation properties of shape memory polymers.