Stimulus-responsive polymers

Peter J. Roth

University of Surrey, UK

Andrew B. Lowe

Curtin University, Australia

Stimulus-responsive, or “smart”, (co)polymers are fundamental to broad, and ever-growing, fields of research that encompass phase behaviour of unimer solutions, smart nanoparticles, and intelligent bulk materials with shape-memory or self-healing abilities. In recent years, (co)polymers have been developed that respond to a fascinating range of chemical, physical, and mechanical stimulation and have been crafted into materials serving a multitude of applications. This special issue of Polymer Chemistry dedicated to stimulus-responsive polymers features articles on the cutting-edge development of novel “smart” materials, as well as up-to-date and in-depth reviews covering a range of specialist areas.

Undoubtedly, the recent advent of reversible deactivation radical polymerization (RDRP) methods has greatly facilitated the synthesis of well-defined polymers with tailored functionality, including stimulus responsive materials. In an extensive review, with nearly 700 references, Graeme Moad reviews the synthesis of stimuli-responsive polymers by the RAFT process (DOI: 10.1039/C6PY01849A).

To date, the most commonly employed stimuli remain temperature and pH. Hoogenboom and Schlaad review recent advances in the emerging field of temperature-responsive amide-based materials, including polypeptoids, polypeptides, and poly(2-oxazolines) (DOI: 10.1039/C6PY01320A). While the prevailing type of temperature-responsiveness in aqueous solution is lower critical solution temperature (LCST) behaviour, upper critical solution temperature (UCST) transitions in water have recently been attracting increasing research attention. Niskanen and Tenhu (DOI: 10.1039/C6PY01612J) address the question of how to manipulate the UCST transition temperature and review multi-responsive polymers in which counter ions, electricity, light, or pH can affect aqueous UCST transitions, while Laschewsky and co-workers (DOI: 10.1039/C6PY01220E) detail the synthesis and structure-dependent UCST behaviour of a range of novel poly(sulfobetaine methacrylate)s. Blackman, Gibson, and O'Reilly probe the causes of thermal hysteresis, the difference in the measured transition temperatures between heating and cooling of solutions of thermoresponsive polymers, by investigating the behaviour of micelles with a tuneable density of chains (DOI: 10.1039/C6PY01191H).

The synthesis and applications of pH-responsive polymers, including their pH-triggered self-assembly into nano-structures, is reviewed by Kocak, Tuncer, and Bütün (DOI: 10.1039/C6PY01872F). Aoyagi and co-workers prepared temperature- and pH-responsive micelles and exploited a photo-caged acid to trigger spatially controlled micelle aggregation through UV irradiation (DOI: 10.1039/C6PY01269H). While reversible pH-triggered assembly/disassembly of nanoparticles is quite common, Armes and co-workers describe nano-objects prepared through polymerization-induced self-assembly (PISA) that carry a tertiary amine end-group and undergo reversible ‘shape-shifting’ between vesicular and worm-like or vesicular and spherical morphologies upon protonation (DOI: 10.1039/C6PY01076H). Patrickios and co-workers detail the preparation of double networks prepared by sequential RAFT polymerization in which one provides pH responsiveness, while another, interpenetrating, network equips the resulting hydrogels with improved mechanical stability (DOI: 10.1039/C6PY01340F).

Apart from temperature and pH, recent years have seen a great deal of development of other stimuli to trigger (reversible) responses of smart (co)polymers, including on a molecular (unimer) scale, of nanoparticles, and on a macroscopic scale. Feng, Theato, and co-workers summarise recent progress in the area of polymer materials responsive to CO₂, an abundant and versatile gaseous trigger (DOI: 10.1039/C6PY01101B). Manouras and Vamvakaki review field-responsive materials, including photo-, electro-, magneto-, and ultrasound-sensitive polymers, and how their remote and spatiotemporally controlled switching can be exploited for applications (DOI: 10.1039/C6PY01455K). Glutathione-responsive polymers and the potential of this biological trigger for drug delivery applications are the topic of a review by Quinn, Whittaker, and Davis (DOI: 10.1039/C6PY01365A). Due to the ease of controlling light, photo-responsive polymers are attracting increasing interdisciplinary attention. In their review, Bertrand and Gohy provide guidelines toward the rational design of photo-responsive block (co)polymers and their application in materials science (DOI: 10.1039/C6PY01082B).

Several articles in this issue address stimulus-responsiveness on a macroscopic scale. Yang and Urban present their development of novel self-healing polymer networks based on the release of primary amines through material damage (DOI: 10.1039/C6PY01221C). Hu and co-workers studied the evolution of phase domain architecture in shape-memory polyurethanes by dissipative particle dynamics simulations, which is relevant to the understanding and further development of smart shape-memory materials (DOI: 10.1039/C6PY01214K). In another paper, Hu's team addresses the question of whether animal hair is a multi-responsive smart material as the authors demonstrate its shape-memory effects in response to various applied stimuli (DOI: 10.1039/C6PY01283C).

One of the largest areas of emerging applications of this multitude of “smart” materials is, without doubt, in the biomedical arena. Applications in this field, including bio-sensing, actuation, and drug delivery, are reviewed by Serpe's group, providing specific examples and the authors’ work on poly(N-isopropylacrylamide)-based microgels and assemblies (DOI: 10.1039/C6PY01585A). Hunter and Moghimi address the interactions between “smart” polymers and living organisms from a strictly biological perspective and review challenges of three different routes for exploiting “smart” polymers for drug delivery (DOI: 10.1039/C6PY00676K).

We are very pleased that this issue provides such an extensive overview of this broad field and includes a large number of up-to-date reviews. We thank all the authors for their contributions and the reviewers for ensuring a high standard of the published work, and we hope that the readers enjoy this special issue of Polymer Chemistry.

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