Shaping shape‐memory (2016)

Establishing active control of material shape has been a long standing theme in materials science. Polymers occupy a special niche in this effort. Unlike shape-changing alloys and piezoelectrics, polymers are lightweight, relatively inexpensive, and can be deformed to several hundred-percent strain. Examples of such actively moving polymers include one-way and two-way shape memory polymers, liquid crystalline elastomers, ion polymer metal composites, ionic gels, conducting polymers, polymer electrets, ferroelectric polymers, dielectric elastomers, and magnetic elastomers. Conventional shape-memory polymers (SMPs) can be elastically stretched and cooled, whereby crystallization or glass formation fixes the material into a non-equilibrium shape. When triggered by heating above the material's glass transition or melting point, stored elastic energy is released, maximizing chain entropy. If cooled to a low enough temperature, virtually all rubbery polymers and elastomers reveal shape-memory. However, SMPs can be engineered to exhibit convenient triggering temperatures, high shape-fixity and shape-recovery, and the ability to release significant amount of stored elastic energy. Over the past decade, new shape-memory triggers (e.g. light, magnetic fields, pH) have been demonstrated, and multi-stage shape recovery and two-way shape-memory have been reported. Shape-memory polymers (SMPs) are now being vigorously applied to the biomedical, aerospace, and textile fields. However, the research itself is morphing into new shapes (see figure). Exciting possibilities include, for example, adhesion control through shape-change of surface features, smarter textiles by integrating memory into knitted fabrics, and new metamaterials by coupling shape change to optical or photonic behavior at sub-micron length scales. These advances are supported by the ever expanding range of shape memory polymer systems through the development of new polymer chemistries, novel blends and composites, and innovative processing techniques, such as additive manufacturing. This special issue offers a combination of forward looking review articles as well new experimental contributions to this rapidly evolving field. The special issue begins by addressing the molecular structure – material property relationship that forms the basis for SMP design. Lewis and Dell review how polymers containing supramolecular and other reversibly binding groups can impart self-healing characteristics as well as alternative shape recovery triggers. Cavicchi reviews the recent use of ionomers in SMPs and their utility for advanced properties, such as high temperature operation, novel formulation, and multiple shape-memory programming. Zhou and Sheiko then highlight recent developments in reversible shape-shifting materials that can reversibly transform between two previously encoded shapes or that allow programming complex shape transformation upon demand. Katzenberg and Tiller examine lightly crosslinked natural rubber and demonstrate shape-memory cycles to extreme strains. Natural rubber is capable of cold storage and control of trigger temperature by application of transverse stresses or solvent exposure as athermal triggers. Hasan, Nash, and Maitland review the fabrication, characterization, modes of actuation, and applications of porous SMPs. These lightweight materials offer a mechanical scaffold that can accommodate mass transport to address needs in drug delivery and tissue engineering. The path dependency of shape-memory is complex. The shape change and amount of stress generated depends on deformation temperature and fixing conditions. Li and Wang propose a new taxonomy of glassy shape-memory polymers whereby the type of shape programming is classified according to the deformation temperature of a glassy SMP. In a related effort, Zhang, Tang, and Guo offer a perspective on easy-to-program “reversible plasticity” SMPs that can be easily programmed, and they examine the role of deformation temperature. Xiao, Yakacki, Guo, Frick and Nguyen investigate the behavior of poly(para-phenylene) as a strong, aromatic thermoplastic, and they fit experimental data to a viscoelastic model that involves multiple relaxation processes. Developing accurate thermomechanical models that capture path dependency will be a critical step toward realizing applications. Two more experimental studies highlight how shape-memory polymers can be precisely tailored to meet specific application needs. Nejad, Garrison and Mather report on novel polymer processing approaches to create self-healing coatings that can close cracks for improved corrosion resistance. Meng, Jiang, and Anthamatten report a shape-memory material that can store large amounts of elastic energy and can be triggered at temperatures near the body temperature. These characteristics are attractive for applications, including sutures, artificial skin, body-heat assisted medical dispensers, and self-fitting apparel. In closing, we would first like to thank the authors and reviewers for their contributions, as well as the editorial staff at the Journal of Polymer Science Part B: Polymer Physics for their effort in assembling this issue. The field of shape-memory polymers is rapidly evolving. We hope that this special issue not only provides a snapshot of the state of this field, but also inspires further developments and research directions in SMPs. Mitchell Anthamatten University of Rochester Department of Chemical Engineering Rochester, NY 14627 Kevin Cavicchi University of Akron Department of Polymer Engineering Akron, OH 44325-0301