Optical Monitoring of Supramolecular Interactions in Polymers.

Many stimuli-responsive materials harness the reversible association of supramolecular binding motifs to enable advanced functionalities such as self-healing, switchable adhesion, or mechan-ical adaptation. Despite extensive research into the structure-property relationships of these materials, direct correlations between molecular-level changes in supramolecular binding and macroscopic material behaviors have mostly remained elusive. Here, we show that this challenge can be overcome with supramolecular binding motifs featuring integrated binding indicators. We demonstrate this using a novel motif that combines a hydrogen-bonding ureido-4-pyrimidinone (UPy) with two strategically placed pyrene fluorophores. Dimerization of this motif promotes pyrene excimer formation, facilitating the straightforward optical quantification of supramolecular assembly under various conditions. We exploit the new motif as a supra-molecular cross-linker in poly(methyl acrylate)s to probe the extent of (dis)assembly as a function of cross-linker content, processing history, and applied stimuli. We demonstrate that the stimuli-induced dissociation of supra-molecular linkages strongly depends on the initial cross-link density, which also dictates whether the force-induced dissociation in polymer films correlates with the applied stress or strain. Thus, beyond introducing a robust tool for the in-situ study of dynamic (dis)assembly mechanisms in supramolecular systems, our findings provide new insights into the mechanoresponsive behavior of such materials.

Structure and Properties of Metallosupramolecular Polymers with a Nitrogen-Based Bidentate Ligand.

The solid-state properties of supramolecular polymers that feature metal-ligand (ML) complexes are, in addition to the general nature of the monomer, significantly affected by the choice of ligand and metal salt. Indeed, the variation of these components can be used to alter the structural, thermal, mechanical, and viscoelastic properties over a wide ranges. Moreover, the dynamic nature of certain ML complexes can render the resulting metallosupramolecular polymers (MSPs) stimuli-responsive, enabling functions such as healing, reversible adhesion, and mechanotransduction. We here report MSPs based on the bidentate ligand 6-(1'-methylbenzimidazolyl) pyridine (MBP), which is easily accessible and forms threefold coordination complexes with various transition metal ions. Thus, a poly(ethylene-co-butylene) telechelic was end-functionalized with two MBP ligands and the resulting macromonomer was assembled with the triflate salts of either Zn2+, Fe2+, or Ni2+. All three MSPs microphase separate and adopt, depending on the metal ion and thermal history, lamellar or hexagonal morphologies with crystalline domains formed by the ML complexes. The melting transitions are well below 200 °C, and this permits facile (re)processing. Furthermore, defects can be readily and fully healed upon exposure to UV-light. While the three MSPs display similar moduli in the rubbery regime, their extensibility and tensile strength depend on the nature of the ML complex, which similarly affects the long-range order and dynamic behavior.

Microscopic strain mapping in polymers equipped with non-covalent mechanochromic motifs.

The mechanical failure of polymers remains challenging to understand and predict, as it often involves highly localised phenomena that cannot be probed with bulk characterisation techniques. Here, we present a generalisable protocol based on optical microscopy, tensile testing, and image processing that permits the spatially resolved interrogation of mechanical deformation at the molecular level around defects in mechanophore-containing polymers. The approach can be applied to a broad range of polymeric materials, mechanophores, and deformation scenarios.

Reversible Microscale Assembly of Nanoparticles Driven by the Phase Transition of a Thermotropic Liquid Crystal.

The arrangement of nanoscale building blocks into patterns with microscale periodicity is challenging to achieve via self-assembly processes. Here, we report on the phase-transition-driven collective assembly of gold nanoparticles in a thermotropic liquid crystal. A temperature-induced transition from the isotropic to the nematic phase under anchoring-driven planar alignment leads to the assembly of individual nanometer-sized particles into arrays of micrometer-sized agglomerates, whose size and characteristic spacing can be tuned by varying the cooling rate. Phase field simulations coupling the conserved and nonconserved order parameters exhibit a similar evolution of the morphology as the experimental observations. This fully reversible process offers control over structural order on the microscopic level and is an interesting model system for the programmable and reconfigurable patterning of nanocomposites with access to micrometer-sized periodicities.

Plasticization of a Semicrystalline Metallosupramolecular Polymer Network.

The assembly of ligand-functionalized (macro)monomers with suitable metal ions affords metallosupramolecular polymers (MSPs). On account of the reversible and dynamic nature of the metal-ligand complexes, these materials can be temporarily (dis-)assembled upon exposure to a suitable stimulus, and this effect can be exploited to heal damaged samples, to facilitate processing and recycling, or to enable reversible adhesion. We here report on the plasticization of a semicrystalline, stimuli-responsive MSP network that was assembled by combining a low-molecular-weight building block carrying three 2,6-bis(1'-methylbenzimidazolyl) pyridine (Mebip) ligands and zinc bis(trifluoromethylsulfonyl)imide (Zn(NTf2)2). The pristine material exhibits high melting (T m = 230 °C) and glass transition (T g ≈ 157 °C) temperatures and offers robust mechanical properties between these temperatures. We show that this regime can be substantially extended through plasticization. To achieve this, the MSP network was blended with diisodecyl phthalate. The weight fraction of this plasticizer was systematically varied, and the thermal and mechanical properties of the resulting materials were investigated. We show that the T g can be lowered by more than 60 °C and the toughness above the T g is considerably increased.

Supramolecular Rings as Building Blocks for Stimuli-Responsive Materials.

Stimuli-responsive polymers are of great interest due to their ability to translate changing environmental conditions into responses in defined materials. One possibility to impart such behavior is the incorporation of optically active molecules into a polymer host. Here, we describe how sensor molecules that consist of a π-extended benzothiadiazole emitter and a naphthalene diimide quencher can be exploited in this context. The two optically active entities were connected via different spacers and, thanks to attractive intramolecular interactions between them, the new sensor molecules assembled into cyclic structures in which the fluorescence was quenched by up to 43% when compared to solutions of the individual dyes. Detailed spectroscopic investigations of the sensor molecules in solution show that the extent of donor/acceptor interactions is influenced by various factors, including solvent polarity and ion concentration. The new sensor molecule was covalently incorporated into a polyurethane; the investigation of the optical characteristics in both the solid and solvent-swollen states indicates that a stimulus-induced formation of associated dye pairs is possible in polymeric materials. Indeed, a solvatochromic quenching effect similar to the behavior in solution was observed for solvent-swollen polymer samples, leading to an effective change of the green emission color of the dye to a yellow color.

Metallosupramolecular polymers as precursors for platinum nanocomposites.

Nanocomposites comprising a polymer matrix and metallic nanoparticles (NPs) can merge the structural features of the matrix material with the functional characteristics of the NPs. While such materials are promising for a wide range of applications, their preparation typically requires multi-step processes that can be difficult to control. Alternatively, materials with NPs can be directly accessed in a controlled manner by exploiting zero-valent metallosupramolecular polymer (MSP) precursors. We here report how the nature of the polymer and its molecular weight affect the nanocomposite formation and structure. Poly(tetrahydrofuran)-based macromonomers with suitable ligands are used to prepare MSPs based on bis(η2-alkyne)platinum(0) complexes. Heating these materials causes disassembly of the complexes and, upon the release of Pt0-atoms, Pt-NPs form in the matrix polymer. The Pt content in the MSP influences the NP formation and thereby the characteristics of the nanocomposites. It is also possible to trigger the complex dissociation and NP formation by exposure to UV light. This allows photolithographic processing and thus the preparation of nanocomposites that contain Pt-NPs in a spatially controlled manner.

Mechanically robust supramolecular polymer co-assemblies.

Supramolecular polymers are formed through non-covalent, directional interactions between monomeric building blocks. The assembly of these materials is reversible, which enables functions such as healing, repair, or recycling. However, supramolecular polymers generally fail to match the mechanical properties of conventional commodity plastics. Here we demonstrate how strong, stiff, tough, and healable materials can be accessed through the combination of two metallosupramolecular polymers with complementary mechanical properties that feature the same metal-ligand complex as binding motif. Co-assembly yields materials with micro-phase separated hard and soft domains and the mechanical properties can be tailored by simply varying the ratio of the two constituents. On account of toughening and physical cross-linking effects, this approach affords materials that display higher strength, toughness, or failure strain than either metallosupramolecular polymer alone. The possibility to combine supramolecular building blocks in any ratio further permits access to compositionally graded objects with a spatially modulated mechanical behavior.

Rotaxane-Based Dual Function Mechanophores Exhibiting Reversible and Irreversible Responses.

Mechanochromic mechanophores permit the design of polymers that indicate mechanical events through optical signals. Here we report rotaxane-based supramolecular mechanophores that display both reversible and irreversible fluorescence changes. These responses are triggered by different forces and are achieved by exploiting the molecular shuttling function and force-induced dethreading of rotaxanes. The new rotaxane mechanophores are composed of a ring featuring a luminophore, which is threaded onto an axle with a matching quencher and two stoppers. In the stress-free state, the luminophore is preferentially located in the proximity of the quencher, and the emission is quenched. The luminophore slides away from the quencher when a force is applied and the fluorescence is switched on. This effect is reversible, unless the force is so high that the luminophore-carrying ring slips past the stopper and dethreading occurs. We show that the combination of judiciously selected ring and stopper moieties is crucial to attain interlocked structures that display such a dual response. PU elastomers that contain such doubly responsive rotaxanes exhibit reversible fluorescence changes over multiple loading-unloading cycles due to the shuttling function, whereas permanent changes are observed upon repeated deformations to high strains due to breakage of the mechanical bond upon dethreading of the ring from the axle. This response allows one, at least conceptually, to monitor the actual deformation of polymer materials and examine mechanical damage that was inflicted in the past on the basis of an optical signal.

Tough Bioinspired Composites That Self-Report Damage.

The increasing use of lightweight composite materials in structural applications requires the development of new damage monitoring technologies to ensure their safe use and prevent accidents. Although several molecular strategies have been proposed to report damage in polymers through mechanochromic responses, these approaches have not yet been translated into lightweight bioinspired composites for load-bearing applications. Here, we report on the development of bioinspired laminates of alternating polymer and nacre-like layers that combine optical translucency, high fracture toughness, and damage-reporting capabilities. The composites signal damage via a fluorescence color change that arises from the force activation of mechanophore molecules embedded in the material's polymer phase. A quantitative correlation between the applied strain and the fluorescence intensity was successfully established. We demonstrate that optical imaging of mechanically loaded composites allows for the localized detection of damage prior to fracture. This fluorescence-based self-reporting mechanism offers a promising approach for the early detection of damage in lightweight structural composites and can serve as a useful tool for the analysis of fracture processes in bulk transparent materials.

Folded Perylene Diimide Loops as Mechanoresponsive Motifs.

A supramolecular mechanophore that can be integrated into polymers and indicates deformation by a fluorescence color change is reported. Two perylene diimides (PDIs) were connected by a short spacer and equipped with peripheral atom transfer polymerization initiators. In the idle state, the motif folds into a loop and its emission is excimer dominated. Poly(methyl acrylate) (PMA) chains were grown from the motif and the mechanophore-containing polymer was blended with unmodified PMA to afford materials that display a visually discernible fluorescence color change upon deformation, which causes the loops to unfold. The response is instant, and correlates linearly with the applied strain. Experiments with a reference polymer containing only one PDI moiety show that looped mechanophores that display intramolecular excimer formation offer considerable advantages over intermolecular dye aggregates, including a concentration-independent response, direct signaling of mechanical processes, and a more pronounced optical change.

Dynamics and healing behavior of metallosupramolecular polymers.

Self-healing or healable polymers can recuperate their function after physical damage. This process involves diffusion of macromolecules across severed interfaces until the structure of the interphase matches that of the pristine material. However, monitoring this nanoscale process and relating it to the mechanical recovery remain elusive. We report that studying diffusion across healed interfaces and a correlation of contact time, diffusion depth, and mechanical properties is possible when two metallosupramolecular polymers assembled with different lanthanoid salts are mended. The materials used display similar properties, while the metal ions can be tracked with high spatial resolution by energy-dispersive x-ray spectrum imaging. We find that healing actual defects requires an interphase thickness in excess of 100 nm, 10 times more than previously established for self-adhesion of smooth films of glassy polymers.

Mechanically Responsive Luminescent Polymers Based on Supramolecular Cyclophane Mechanophores.

A new approach to cyclophane-based supramolecular mechanophores is presented. We report a mechanically responsive cyclic motif that contains two fluorescent 1,6-bis(phenylethynyl)pyrene moieties that are capable of forming intramolecular excimers. The emission spectra of dilute solutions of this cyclophane and a polyurethane elastomer into which a small amount of the mechanophore (0.08 wt %) had been covalently integrated are dominated by excimer emission. Films of the cyclophane-containing polyurethane also display a considerable portion of excimer emission, but upon deformation, the fluorescence becomes monomer-dominated and a perceptible change from cyan to blue is observed. The response is instant, reversible, and consistent with a mechanically induced change of the molecular conformation of the mechanophore so that the excimer-promoting interactions between the luminophores are suppressed. In-depth investigations show a correlation between the applied strain and the emission color, which can conveniently be expressed by the ratio of monomer to excimer emission intensity. The current study suggests that cyclophanes can be utilized to develop various supramolecular mechanophores that detect and visualize weak forces occurring in polymeric materials or generated by living tissues.

Mechanochromism in Structurally Colored Polymeric Materials.

Mechanochromic effects in structurally colored materials are the result of deformation-induced changes to their ordered nanostructures. Polymeric materials which respond in this way to deformation offer an attractive combination of characteristics, including continuous strain sensing, high strain resolution, and a wide strain-sensing range. Such materials are potentially useful for a wide range of applications, which extend from pressure-sensing bandages to anti-counterfeiting devices. Focusing on the materials design aspects, recent developments in this field are summarized. The article starts with an overview of different approaches to achieve mechanochromic effects in structurally colored materials, before the physical principles governing the interaction of light with each of these materials types are summarized. Diverse methodologies to prepare these polymers are then discussed in detail, and where applicable, naturally occurring materials that inspired the design of artificial systems are discussed. The capabilities and limitations of structurally colored materials in reporting and visualizing mechanical deformation are examined from a general standpoint and also in more specific technological contexts. To conclude, current trends in the field are highlighted and possible future opportunities are identified.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials.

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Optical gap and fundamental gap of oligoynes and carbyne.

The optoelectronic properties of various carbon allotropes and nanomaterials have been well established, while the purely sp-hybridized carbyne remains synthetically inaccessible. Its properties have therefore frequently been extrapolated from those of defined oligomers. Most analyses have, however, focused on the main optical transitions in UV-Vis spectroscopy, neglecting the frequently observed weaker optical bands at significantly lower energies. Here, we report a systematic photophysical analysis as well as computations on two homologous series of oligoynes that allow us to elucidate the nature of these weaker transitions and the intrinsic photophysical properties of oligoynes. Based on these results, we reassess the estimates for both the optical and fundamental gap of carbyne to below 1.6 eV, significantly lower than previously suggested by experimental studies of oligoynes.

Mechanochromic Polymers Based on Microencapsulated Solvatochromic Dyes.

The development of polymers with built-in sensors that provide readily perceptible optical warning signs of mechanical events has received considerable interest. A simple and versatile concept to bestow polymers with mechanochromic behavior is the incorporation of dye-filled microcapsules. Such capsules release their cargo when their shell is damaged, and the dye is subsequently activated through a chemical or physical change that causes a chromogenic response. Here, we report the preparation of fluorescent poly(urea-formaldehyde) microcapsules containing solutions of a solvatochromic cyanostilbene dye and their integration in different polymers. When objects made from such composites are damaged, the dye solution is released from the containers, diffuses into the matrix, and the solvent evaporates. As a result, the polarity around the dye molecules changes, and this leads to a change of the fluorescence color. Alternatively, the dye is blended into the polymer matrix, microcapsules are loaded with a solvent, and the release of the latter triggers the color change. Both mechanisms afford ratiometric signals because the capsules that remain intact or dye molecules that are not exposed to the solvent can be used as a built-in reference; therefore, a quantitative assessment of the damage inflicted on the material is a priori possible.

Spatially Resolved Production of Platinum Nanoparticles in Metallosupramolecular Polymers.

Nanocomposites consisting of a polymer matrix and metallic nanoparticles can merge the functional, structural, and mechanical properties of the two components and are useful for applications that range from catalysis to soft electronics. Gaining spatial control over the nanoparticle incorporation is useful, for example to confine catalytic sites or create electrically conducting pathways. Here, we show that this is possible by the controlled disassembly of a metallosupramolecular polymer containing zerovalent platinum complexes to form nanoparticles in situ. To achieve this, a telechelic poly(ethylene-co-butylene) was end-functionalized with diphenylacetylene ligands and chain-extended through the formation of bis(η2-alkyne)Pt0 complexes. These complexes are stable at ambient conditions, but they can be dissociated upon heating or exposure to ultraviolet light, which allows producing Pt nanoparticles when and where needed and without auxiliary reagents or formation of byproducts. This approach was exploited to create objects with well-defined catalytically active areas.

Healing of Polymeric Solids by Supramolecular Means.

Equipping a polymeric material with the ability to heal an inflicted damage is a crucial advantage for many applications. The incorporation of reversible and dynamic supramolecular interactions into polymeric systems has proven to be a promising route towards such materials. In this article, recent developments in the field of healable materials are highlighted with a particular focus on the design principles, driving forces, and mechanisms that allow healing to occur.