“One-way” shape memory materials feature an ability to transform shape from a temporary, frozen, shape to a permanent shape when triggered by an environmental stimulus, such as heat, light, or vapor. Used creatively, these phenomena can be exploited for a wide range of applications. While both shape memory alloys (SMAs, e.g., nickel-titanium alloys) and shape memory polymers (SMPs) show similar thermo-stimulated shape memory properties, their mechanisms of action are quite distinct. Advantages of SMAs include rapid strain recovery (within 1 second), the potential training for two-way reversible memory, and an apparent superelasticity within the austenite phase at temperatures near but above the martensite-austenite transition temperature. In contrast, polymers intrinsically exhibit shape memory effects derived from their highly coiled constituent chains that are collectively extensible via mechanical work, and this energy may be stored indefinitely (“shape fixed”) by cooling below the glass transition temperature, Tg, of an amorphous polymer, or the melting point, Tm, of a crystalline or semicrystalline polymer. After shape fixing, the polymeric sample can later perform mechanical work and return to a stress-free state when heated above the critical temperature, mobilizing the frozen chains to regain the entropy of their coiled state. In comparison to SMAs, thermally stimulated SMPs have the advantages of large recoverable deformations in excess of several hundred percent strain, facile tuning of transition temperatures through variation of the polymer chemistry, and processing ease at low cost.
So-called “two-way” shape memory polymers, implemented in a common example as nematic elastomers, have been prepared and studied. Tensile specimens of two-way shape memory polymers switch reversibly between short and long sample lengths, whether above or below a first order nematic-isotropic phase transition, respectively. Known “two-way” shape memory polymers are rubbery at room temperature, with different materials exhibiting different levels of strain and force capability.
While “one-way” and “two-way” shape memory polymers have been separately developed, no single polymer or metal has yet been prepared to exhibit both phenomena in the same material. Such a material would be desirable for its ability to exhibit three distinct shapes as a function of temperature.