An elastomeric, healable, supramolecular polymer blend comprising a chain-folding polyimide and a telechelic polyurethane with pyrenyl end groups is compatibilized by aromatic pi-pi stacking between the pi-electron-deficient diimide groups and the pi-electron-rich pyrenyl units. This interpolymer interaction is the key to forming a tough, healable, elastomeric material. Variable-temperature FTIR analysis of the bulk material also conclusively demonstrates the presence of hydrogen bonding, which complements the pi-pi stacking interactions. Variable-temperature SAXS analysis shows that the healable polymeric blend has a nanophase-separated morphology and that the X-ray contrast between the two types of domain increases with increasing temperature, a feature that is repeatable over several heating and cooling cycles. A fractured sample of this material reproducibly regains more than 95% of the tensile modulus, 91% of the elongation to break, and 77% of the modulus of toughness of the pristine material.

Utilizing metal-ligand binding as the driving force for self-assembly of a ditopic ligand, which consists of a 2,6-bis-(1'-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn(II)) and a lanthanide metal ion (La(III)), we have achieved formation of stimuli-responsive metallo-supramolecular gels. We describe herein a series of experimental studies, including optical and confocal microscopy, dynamic light scattering, wide-angle X-ray diffraction, and rheology, to explore the properties of such gels, as well as the nature of the gelation mechanism. Morphological and X-ray diffraction observations suggest gelation occurs via the flocculation of semicrystalline colloidal particles, which results in the gels exhibiting pronounced yielding and thixotropic behavior. Application of mechanical stress results in a decrease in the particle size, which is accompanied by an increase in gel strength after removal of the stress. Moreover, studies show that the presence of lanthanide(III) perchlorate increases the mechano-responsiveness of the gels, as a consequence of reduced crystallinity of the colloidal particles, presumably due to the different coordination ability of lanthanide(III) and zinc(II), which changes the nature of the self-assembly in these materials.

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Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.

Mechanochemistry offers exciting opportunities for molecular-level engineering of stress-responsive properties of polymers. Reactive sites, sometimes called mechanophores, have been reported to increase the material toughness, to make the material mechanochromic or optically healable. Here we show that macrocyclic cinnamate dimers combine these productive stress-responsive modes. The highly thermally stable dimers dissociate on the sub-second timescale when subject to a stretching force of 1–2 nN (depending on isomer). Stretching a polymer of the dimers above this force more than doubles its contour length and increases the strain energy that the chain absorbs before fragmenting by at least 600 kcal per mole of monomer. The dissociation produces a chromophore and dimers are reformed upon irradiation, thus allowing optical healing of mechanically degraded parts of the material. The mechanochemical kinetics, single-chain extensibility, toughness and potentially optical properties of the dissociation products are tunable by synthetic modifications.

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