Understanding the “stuff” of life, such as DNA and proteins – both polymers – was just a fantasy (at best) in the early parts of the 20th century. In this time period, the mere existence of large “macromolecules” was not even fathomable, so the possibility of polymers dictating who we are was simply science fiction. The brave work of Hermann Staudinger (1953 Nobel Prize in Chemistry), Wallace Carothers, and Herman Mark led to the gradual, and eventual global belief that such large molecular weight macromolecules exist, and their work has paved the way for the work of trailblazers such as: Paul J. Flory (1974 Nobel Prize in Chemistry), Jean-Marie Lehn (1987 Nobel Prize in Chemistry), and Pierre-Gilles de Gennes (1991 Nobel Prize in Physics).
Through their combined efforts over many years, polymer science is what it is today, and its development has led to polymers being present in nearly everything people around the world encounter on a daily basis. The ubiquity of polymers is a result of our deep understanding of polymer behavior/physics, and our vast synthetic capabilities, such that polymer chemistry, structure, and ultimately function can be tailored almost at will to have an impact on nearly any application under the sun.
This pair of special issues focuses on a very special class of polymers, typically referred to as “smart,” “intelligent,” “stimuli responsive,” and/or “environmentally responsive” polymers. These polymers, like their “nonresponsive” counterparts are indeed high molecular weight structures, composed of monomers as building blocks, and can be synthesized to have a variety of chemistries and morphologies. Unlike traditional polymers, responsive polymers are able to respond to their environment or a stimulus by undergoing some change, whether physical and/or chemical in nature.
To date, a number of different responsivities have been engineered/synthesized into polymers, including responsivity to: light, temperature, pH, magnetic and electric fields, and analyte concentration. In fact, a number of years ago Dr. Yoseph Bar-Cohen issued a challenge to synthesize a polymer-based device that is capable of beating a human in arm wrestling. Through the use of electroactive polymers, progress is being made in this area, and soon polymer-based devices will be strong enough to compete with humans (see figure).
These two special issues are composed of articles from some of the top researchers operating in the area of responsive polymers. The topics in the special issues can be broken up into three main themes: 1) Synthesis; 2) Fundamental Properties; and 3) Applications.
This special issue of the Journal of Polymer Science, Part B: Polymer Physics highlights aspects of the topics above to achieve various outcomes with a focus on understanding the fundamental properties and behavior of responsive polymer-based systems. In his progress report, Thomas Hellweg describes the synthesis, characterization, and possible applications of core-shell microgels. The paper focuses on coupling inorganic structures and responsive polymers, with the potential use as novel catalysts. Locklin and coworkers discuss various methods to fabricate responsive polymer thin films, focusing on the design constraints that must be considered to make impacts in, e.g., the medical field. Relatedly, Boyes and coworkers highlight work that has been conducted on the use of pH-responsive organic polymers to non-invasively determine pH in vivo. They go on to highlight recent efforts to develop pH-responsive organic polymers as imaging agents.
Of a more fundamental nature, Moya and Irigoyen describe the combined use of quartz crystal microbalance (QCM) and ellipsometric techniques to quantify and describe polyelectrolyte brush hydration and collapse. Kojima and Richtering and colleagues investigate a very interesting phenomenon that occurs in certain responsive polymer-based systems called co-nonsolvency. Co-nonsolvency is a phenomenon where a polymer is soluble in two different pure solvents, but insoluble in certain mixtures of the two pure solvents. In their submission, they study the co-nonsolvency of poly(N-isopropylacrylamide) (pNIPAm)-based microgels, and compare the observed behavior to linear pNIPAm. Kojima and Tanaka discuss how the depression of a polymer's lower critical solution temperature depends on the monomer composition and ultimately polymer structure. A complete understanding of these very fundamental properties is important, and can lead to numerous applications.
In these special issues, we hope it is apparent that while much is known about polymers in general, the research space carved out for responsive polymers and responsive polymer-based systems is, and will continue to be, vibrant. Through the evolution of our understanding of the fundamental properties of this interesting class of polymers, and the synthetic tools and techniques available for our use, the future applications of these polymers is vast and seemingly limitless. We hope you enjoy the special issues.
Michael J. Serpe
University of Alberta