The past several years have witnessed significant advances in the field of shape memory polymers (SMPs) with the elucidation of new compositions for property tuning, the discovery of new mechanisms for shape fixing and recovery, and the initiation of phenomenological modeling. We critically review research findings on new shape memory polymers along these lines, emphasizing exciting progress in the areas of composites, novel recovery triggering, and new application developments. 445Annu. Rev. Mater. Res. 2009.39:445-471. Downloaded from www.annualreviews.org by NORTH CAROLINA STATE UNIVERSITY on 09/29/12. For personal use only.

Many applications ranging from biomedical to aerospace have been proposed for the use of shape memory polymers (SMPs). To optimize SMPs properties for appropriately targeting such wide-spreading application requirements, it becomes necessary to understand the structure/property relationships in SMPs. The literature was reviewed and the recent advances made in the development of SMPs were determined to establish guidelines for composition and structure considerations for designing SMPs with targeted chemical, physical, and shape memory (SM) properties. It was concluded that covalently crosslinked glassy thermosets appear to be better SMP candidates because of their intrinsically higher modulus, greater thermal and chemical stability, higher shape fixity and recovery, and possibly their longer cycle life. However, material design allows for reaching comparable or better properties for all classes of SMPs. This emphasizes that optimization of SMPs requires application-specific molecular, structural, and geometrical design. Current techniques for improving stress recovery and cycle time, which compared to shape memory alloys are the two main limitations of SMPs, are extensively discussed. Understanding the relationships between the composition and structure of an SMP and its SM properties as well as its limitations enables one to better define the development areas for high performance SMPs.

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Side-chain liquid crystalline elastomers (LCEs) have been widely studied over the past two decades because of their thermomechanical properties that include large strain reversible actuation and soft elasticity. 1,2 The thermally stimulated actuation behavior is explained by a coupling between liquid crystalline order and rubber elasticity resulting from the underlying cross-linked structure. Yet, higher actuator performance has been long expected 3 and recently shown for main-chain liquid crystalline elastomers (MC-LCEs) due to an enhanced coupling between their intrinsically high, yet labile, ordering and network strain as compared to their side-chain analogues. 4 Challenges exist, however, for main-chain liquid crystalline polymers due to high transition temperatures and processing difficulties so that to date they have not received adequate attention. Network architectures have been varied, but only limited attention has been given to the influence of varying mesophases structure -from nematic to cholesteric to smecticon thermomechanical behavior. 5 We therefore sought in this study to synthesize and investigate the behavior of new smectic-C LCEs as candidates for shape memory purposes. Here, shape memory refers to a thermomechanical phenomenon in which large distortions may be "fixed" and later relaxed to an equilibrium shape under an environmental trigger. 6 Shape memory materials require not only tailored transition temperatures (including, for LCEs, isotropization), but also specific mechanical properties. Such characteristics can be readily achieved using liquid crystalline elastomers with varying composition, thereby enabling specific transition temperatures for facile actuation. Herein, we present the thermomechanical characterization of newly synthesized main-chain smectic-C LCEs that reveals the exceptional capabilities of such materials as shape memory elastomers possessing low transition temperatures.We have designed and prepared MC-LCEs incorporating two distinct benzoate-based mesogenic groups, 1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone and 2-tert-butyl-1,4-bis[4-(4-pentenyloxy)benzoyl]hydroquinone, symbolized 5H and 5tB, respectively, coupled with hydride-terminated poly(dimethylsiloxane) spacers (DP ) 8). Both mesogens, varying in their pendant group, were synthesized in our laboratory following well-known synthetic routes. 4,7 While 5H has been previously synthesized (although incompletely characterized 8,9 ), the preparation of the 5tB structure has not yet been reported. Despite their similar structure, the thermal, physical, and optical behaviors of these LC dienes differ considerably: while 5H melts to a nematic phase at 136.6°C and clears to an isotropic phase at 229.5°C, 5tB exhibits similar transitions at much lower temperatures of 80.6 and 91.4°C, respectively. Therefore, we expected that, via copolymerization of these two mesogens, properties of the final material could be specifically targeted, although we could not predict the observed smecticity from these nematogens. The overa...

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