Shape memory polymers are materials that can memorize temporary shapes and revert to their permanent shape upon exposure to an external stimulus such as heat, light, moisture or magnetic field. Such properties have enabled a variety of applications including deployable space structures, biomedical devices, adaptive optical devices, smart dry adhesives and fasteners. The ultimate potential for a shape memory polymer, however, is limited by the number of temporary shapes it can memorize in each shape memory cycle and the ability to tune the shape memory transition temperature(s) for the targeted applications. Currently known shape memory polymers are capable of memorizing one or two temporary shapes, corresponding to dual- and triple-shape memory effects (also counting the permanent shape), respectively. At the molecular level, the maximum number of temporary shapes a shape memory polymer can memorize correlates directly to the number of discrete reversible phase transitions (shape memory transitions) in the polymer. Intuitively, one might deduce that multi-shape memory effects are achievable simply by introducing additional reversible phase transitions. The task of synthesizing a polymer with more than two distinctive and strongly bonded reversible phases, however, is extremely challenging. Tuning shape memory effects, on the other hand, is often achieved through tailoring the shape memory transition temperatures, which requires alteration in the material composition. Here I show that the perfluorosulphonic acid ionomer (PFSA), which has only one broad reversible phase transition, exhibits dual-, triple-, and at least quadruple-shape memory effects, all highly tunable without any change to the material composition.

Shape memory polymers (SMPs), as a class of programmable stimuliresponsive shape changing polymers, are attracting increasing attention from the standpoint of both fundamental research and technological innovations. Following a brief introduction of the conventional shape memory effect (SME), progress in new shape memory enabling mechanisms and triggering methods, variations of in shape memory forms (shape memory surfaces, hydrogels, and microparticles), new shape memory behavior (multi-SME and two-way-SME), and novel fabrication methods are reviewed. Progress in thermomechanical modeling of SMPs is also presented. Abbreviations:SCPs shape changing polymers; LCEs liquid crystalline elastomers; SMP shape memory polymer; SME shape memory effect; 2W two-way; 1W oneway; T trans transition temperature; T g glass transition temperature; T m melting temperature; T cl liquid crystal cleaning temperature; T d deformation temperature; T f shape fixing temperature; T c crystallization temperature; R f shape stability ratio; R r shape recovery ratio; T sw switching temperature; ε max maximum recoverable strain; σ max maximum recovery stress; SMC shape memory cycle; ε load strain under load; ε fixed strain; T r recovery Page 2 of 106 A c c e p t e d M a n u s c r i p t 2 temperature; ε rec recovered strain; V r strain recovery rate; T σmax temperature corresponding to σ max ; T sw,app apparent switching temperature; PCL poly(ε-caprolactone); SMPU shape memory polyurethane; EMU elemental memory unit; TME temperature memory effect; PU polyurethane; T i liquid-crystal isotropic transition temperature; T v vitrification temperature; CIE crystallization induced elongation; MIC melting induced contraction

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