A novel supramolecular polymer system, in which the terminal pyrenyl groups of a polyamide intercalate into the chain-folds of a polyimide via electronically-complementary pi-pi stacking, shows both enhanced mechanical properties relative to those of its individual components and facile healing characteristics as a result of the thermoreversibility of non-covalent interactions.

A supramolecular polymer blend, formed via π-π interactions between a π-electron rich pyrenyl end-capped oligomer and a chain-folding oligomer containing pairs of π-electron poor naphthalene-diimide (NDI) units, has been reinforced with cellulose nanocrystals (CNCs) to afford a healable nanocomposite material. Nanocomposites with varying weight percentage of CNCs (from 1.25 to 20.0 wt %) within the healable supramolecular polymeric matrix have been prepared via solvent casting followed by compression molding, and their mechanical properties and healing behavior have been evaluated. It is found that homogeneously dispersed films can be formed with CNCs at less than 10 wt %. Above 10 wt % CNC heterogeneous nanocomposites were obtained. All the nanocomposites formed could be rehealed upon exposure to elevated temperatures although, for the homogeneous films, it was found that the healing rate was reduced with increasing CNC content. The best combination of healing efficiency and mechanical properties was obtained with the 7.5 wt % CNC nanocomposite which exhibited a tensile modulus enhanced by as much as a factor of 20 over the matrix material alone and could be fully rehealed at 85 °C within 30 min. Thus it is demonstrated that supramolecular nanocomposites can afford greatly enhanced mechanical properties relative to the unreinforced polymer, while still allowing efficient thermal healing.

The past few decades have seen a significant growth in the field of supramolecular polymerizations, in which (reversible) noncovalent interactions (e.g., hydrogen bonding, metal-ligand coordination, π-π stacking, etc.) between (macro)monomeric units are utilized to build polymeric assemblies. These polymeric aggregates can exist in an equilibrium state between low and high molecular weight species, which in turn opens the door to a new matrix of properties. For example, such supramolecular polymers are potentially an interesting class of stimuli-or environmentally responsive, "smart" materials. Additionally, the establishment of an equilibrium during the assembly process imparts on the system the ability to "proofread" assemblies and, depending on the nature of the (macro)molecules, can allow efficient access to controlled, well-defined nanostructures. This Perspective will focus on the basic concepts of such supramolecular polymerizations, such as mechanism of assembly growth, and the effects of growth kinetics, phase segregation, and growth on the surface has on the assembly and properties of the resulting supramolecular polymers, highlighting these concepts with selected literature examples.

α- and γ-cyclodextrin in columnar structures with only water molecules included were successfully obtained by appropriate recrystallization from their aqueous solutions. These crystals were found to adopt a channel-type structure similar to the cyclodextrin inclusion compounds formed with guest polymers. Experimental investigations of their inclusion properties demonstrate that only α-cyclodextrin in the columnar structure (α-CDcs) is able to include both small molecules and polymers. Thermal measurements reveal that columnar structure α-CDcs contains three different types of water molecules. The most strongly held water molecules are located outside of the cyclodextrin cavity, likely hydrogen-bonded between the rims of neighboring cyclodextrins in the columnar α-CD stacks. X-ray analyses confirm that the channel structure is preserved in the dehydrated α-CDcs and its inclusion compounds formed with various guests. In contrast, a completely different behavior was observed for γ-CDcs in the columnar structure. It appears that α-CDcs, at least, can function as a nanoscopic filter for separating both small molecules and polymers on the basis of their abilities to be included, or not, in the narrow (∼0.5 nm) channels of the α-CDcs crystals.