Mechanochromic Polymers

Abstract

Mechanochromic polymers are macromolecular materials that change their color in response to deformation, either on account of altered absorption or reflection. A broader definition that we apply for this special issue of Macromolecular Rapid Communications includes polymers in which other optical characteristics change upon application of mechanical force, notably photoluminescent materials that change their emission properties. We also include mechanochromic polymer systems, whose mechanoresponsiveness results from the integration of multiple components. Mechanochromic materials are (potentially) useful for a broad range of applications that range from pressure-sensing films to tamper-evidencing packaging films. The wealth of possible applications is certainly one of the reasons why research on such materials is booming. The first examples of (commercially successful) mechanochromic materials go back a century. For example, photoelasticity of celluloid specimens was reported on January 1 in 1921,[1] i.e., one hundred years before online publication of this special issue. The technology to create carbonless copy paper was first invented in the 1950s,[2] and reports on mechanically tunable distributed-Bragg-reflectors based on multilayer polymers date back to the late 1970s.[3] Around the same time, poly(diacetylene)s were reported to show piezochromic[4] and thermochromic[5] properties, and somewhat later demonstrated to show a mechanochromic response when employed as stress-reporters in polyurethanes.[6] “Mechanochromic polymers” became a term in the early 1990s, when Nallichieri and Rubner reported mechanochromic behavior of segmented polyurethanes containing chromogenic poly(diacetylene)s,[7] and Kim and Reneker observed this effect in polyurethane elastomers containing azobenzenes that had been converted to the cis-form.[8] These examples illustrate that mechanically induced color changes in polymers can be achieved by very different mechanisms that include physical effects, chemical transformations, or engineering approaches that may involve combinations of both. In the three decades since, research on mechanochromic polymers has developed into a vibrant interdisciplinary field whose exponential growth reflects the significant scientific interest in, and technological usefulness of, materials that translate mechanical inputs into optical outputs. The range of transduction principles that can be utilized to impart polymers with mechanochromic behavior has been greatly expanded. Tremendous progress has been made in understanding and controlling relationships of between macroscopic forces, molecular, microscopic, and macroscopic changes, and optical changes in materials developed on these schemes. Collectively, the 16 articles that make up this special issue of Macromolecular Rapid Communications attempt to provide a current account of the state of this rapidly emerging, interdisciplinary field. We are delighted that both established researchers who have contributed to the development of the field for a long time, and emerging investigators, who in some cases are just starting their independent careers, have contributed to this issue. It is heartening to see that despite worldwide Covid-19 related campus and workplace closures in this past year, a large number of authors have been able to contribute and we extend our gratitude for the extra efforts that they have made. The three reviews, three feature articles, and ten communications cover relevant aspects that span a wide range of transduction principles and cover effects at all length scales. At the molecular level, spiropyrans have emerged as the most widely used mechanochromic mechanophore type; in these motifs, a (reversible) color change originates from a mechanochemical ring-opening reaction. In his feature article Michael Sommer discusses how substituent effects can be used to control spiropyran-merocyanine equilibria.[9] In their communication, Stephen L. Craig and co-workers report how the strain rate affects the activation of such spiropyrans in silicone elastomers.[10] Guillaume De Bo and co-workers report a new mechanoresponsive fluorescent hydrogen-bonded rotaxane based on a maleimide dye and demonstrate its force-sensing properties in a synthetic model of living tissue.[11] Ester Verde-Sesto, José A. Pomposo and co-workers review on techniques that can be used to manipulate and mechanically activate single-chain molecules and thus allow one to elucidate information on the stress-induced response of individual mechanophores and polymer molecules.[12] Several communications address investigations of structure-property relations of polymer systems based on mechanochromic mechanophores. Stephen L. Craig's group shows that mechanophores based on coumarin dimers exhibit a strength that is comparable to that of sulfur-sulfur bonds that represent the weakest bonds in vulcanized rubbers and allowed them to investigate how macroscopic mechanical stress is transferred at the molecular scale in such polymers.[13] Hideyuki Otsuka's team demonstrate that multicolor mechanochromism in mixtures of two mechanochromic polystyrene samples containing different mechanochromophores allows one to detect the duration mechanical stimulation.[14] In their communication on semi-interpenetrating elastomer network nanocomposites containing Janus nanoparticles and a mechanoluminescent motif, Yulan Chen and co-workers further raise the bar with respect to complexity.[15] They show that the mechanophore is useful to study the stress transfer between the polymer and Janus nanoparticles and thereby the toughening mechanism and the failure process of complex polymer nanocomposites with high spatial and temporal resolution. Finally, Harald Rupp and Wolfgang H. Binder present mechanochromic 3D-printed composites that rely on the compression-activated generation of triazole-based luminophores.[16] The review of Stephen Schrettl and co-workers lays out that chromic effects can also be achieved without covalent bonds scission, but instead exploiting mechanically induced conformational or morphological changes of polymers containing chromogenic moieties.[17] In their feature article, Ben Zhong Tang and co-workers further elaborate on one particular aspect and effect—aggregation-induced emission (AIE)—that can be used to impart polymers with mechanochromic fluorescent behavior.[18] A new mechanophore—8-(2-hydroxyethoxy)pyrene-1,3,6-trisulfonate—whose mechanochromic luminescent behavior is driven by the dissociation of molecular aggregates and the mechanochromic behavior of polyurethanes that carry this motif is reported in the communication by Rint Sijbesma and co-workers.[19] In a similar vein, the communication of Andrea Pucci, Giacomo Ruggeri and their team revolves around the mechanochromic response of blends of polyethylene and a perylene bisimide derivative, whose dichroic absorption is related to the distinct anisotropic polarizability of the chromophores.[20] Moving to another length scale and different operating principle, Jess Clough et al. review the current state of structurally colored polymeric materials.[21] Two communications deal with specific new materials platform based on such photonic structures. Markus Gallei and co-workers report dye-containing mechanically and pH responsive elastomeric opal films,[22] whereas Luyi Sun and co-workers developed stretchable bilayers with mechanical-responsive wrinkled surfaces that exhibit a considerable transparency change.[23] Addressing, finally, the microscopic scale, the feature article by Céline Calvino focuses on capsule-release systems and provides an overview of the different encapsulating approaches that have been employed to prepare mechanochromic polymers, with a focus on the containers and the chromic operating principles used for this purpose.[24] As exemplified by the excellent contributions to this themed issue, the fast progress in the field of mechanochromic polymers will no only offer tremendous possibilities to tackle fundamental and application-oriented questions in smart materials science, but also impact a broad range of fields including catalysis, drug-release technologies, sensors, and many others. We hope that this issue will provide you with new and stimulating insights, encourage fruitful scientific discussions, and introduce new ideas and expertise to this exciting emerging area. Yulan Chen received her bachelor degree at Jilin University in 2005. She obtained her Ph.D. degree in 2010 from the Institute of Chemistry, Chinese Academy of Sciences. During 2010–2014, she did postdocs in Eindhoven University of Technology and Max Planck Institute for Polymer Research. Since September 2014, she was appointed as a professor for polymer chemistry at Tianjin University, China. Her current research interests include functional conjugated polymers and mechano-responsive polymers. Michael Sommer is a professor of polymer chemistry at Chemnitz University of Technology (CUT), where he is currently managing director of the Institute for Chemistry. Michael studied chemistry at the Universidad de Granada, Spain, and the University of Bayreuth, Germany. With a Ph.D. from Bayreuth in macromolecular chemistry, in 2009 he went to Cambridge, UK, for a two year postdoctoral stay with W. T. S. Huck. In 2012 he returned to Freiburg, Germany, to work on his habilitation between 2012–2016. Since 2017 he has been full professor at CUT. His research interest encompass all kinds of conjugated polymers for energy conversion and storage, functional membranes and smart materials. Christoph Weder is director of the Adolphe Merkle Institute (AMI) at the University of Fribourg (Switzerland). Chris was educated at ETH Zurich, where, after a postdoctorate at MIT, he also earned a habilitation. He then assumed a faculty position at Case Western Reserve University before joining the AMI as a professor of polymer chemistry and materials in 2009. His research is focused on bio-inspired, stimuli-responsive polymers and nanomaterials. Chris is a member of the Swiss Academy of Technical Sciences, and a Fellow of the American Chemical Society's Division of Polymer Chemistry. He has mentored about a hundred Ph.D. students and postdocs.

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