The formation of supramolecular polymers in water through rational design of a benzene-1,3,5-tricarboxamide (BTA) motif is presented. Intermolecular hydrogen bonding and hydrophobic effects cooperate in the self-assembly into long fibrillar aggregates. Minimal changes in molecular structure significantly affect the internal packing of the aggregates.

A set of chiral, amphiphilic, self-assembling discotic molecules based on the 3,3'-bis(acylamino)-2,2'-bipyridine-substituted benzene-1,3,5-tricarboxamide motif (BiPy-BTA) was prepared. Amphiphilicity was induced into the discotic molecules by an asymmetrical distribution of alkyl and oligo(ethylene oxide) groups in the periphery of the molecules. Small-angle X-ray scattering, cryogenic transmission electron microscopy, and circular dichroism spectroscopy measurements were performed on the discotic amphiphiles in mixtures of water and alcohol at temperatures between 0 °C an 90 °C. The combined results show that these amphiphilic discotic molecules self-assemble into supramolecular fibers consisting of either one or three discotic molecules in the fiber cross-section and that the presence of water induces the bundling of the supramolecular fibers. The rich phase behavior observed for these molecules proves to be intimately connected to the mixing thermodynamics of the water-alcohol mixtures.

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Supramolecular hydrogels formed by decorating benzene-1,3,5-tricarboxamide (BTA) units with amphiphilic ethylene glycol-based side chains are presented; careful selection of the substituents of the BTAs allows for the tuning of the self-assembly behaviour and hence the mechanical properties of the resultant hydrogel.

Cross-link density is an important parameter for the macroscopic mechanical properties of hydrogels. Increasing network density leads to an increase in the storage and loss moduli of the gel and can be accomplished by either increasing the concentration of cross-linkers, or by reducing the fraction of mechanically inactive cross-links. Mechanically inactive crosslinks consist of loops in the network, which do not contribute to the mechanical properties. Suppression of loop formation is demonstrated in a system of semiflexible supramolecular rods of poly(ethylene glycol)−bis(urea) bolaamphiphiles. Use of a cross-linker which, due to self-sorting of its hydrophobic segments, preferentially connects different rods, increases the modulus of a hydrogel by a factor of 15 compared to a system without self-sorting. By using statistical-mechanical calculations, it is shown that this increase can be explained by the increased tendency of the cross-linkers to form bridges between the semiflexible rods and thus increasing the cross-link density in the supramolecular hydrogel. ■ INTRODUCTIONHydrogels are cross-linked materials that absorb a substantial amount of water. They are of enormous economical importance due to their use as food additives, in the oil industry, and for biomedical applications. 1−3 In all applications, their mechanical behavior is of paramount importance, and it is determined to a large extent by cross-link density. In physical hydrogels, the cross-links are reversible, and control over mechanical behavior can be obtained by tuning chemical structure to create well-defined and specific cross-linking interactions. Cross-links are formed by specific parts of the components through aggregation by noncovalent interactions, such as ion complexation and hydrophobic interactions. 4 Specificity and additional strength of aggregation may be obtained by additional physical interactions such as hydrogen bonding. The combination of multiple noncovalent interactions gives highly desirable mechanical properties to natural hydrogelators such as collagen or actin. 5,6 The recent realization that mechanical properties and forces play an important role in the behavior of cells has opened up new markets for materials with tunable mechanical properties, with considerable potential for use in, for instance, tissue engineering. Despite their attractive mechanical properties, the use of natural hydrogelators in biomedical applications is limited by biocompatibility issues. With the aim of gaining full control over properties of biocompatible hydrogels, several synthetic approaches to physical hydrogels have been reported. Amino acids are popular building blocks in synthetic hydrogelators, both in engineered polypeptides 7 and synthetic peptide amphiphiles 8 because the chemical diversity of amino acids allows tuning of hydrophobicity and creates the possibility to engineer specific recognition motifs. Alternatively, hydrogelators have been developed with biocompatible poly(ethylene oxide) (PEO) as hydrophilic component and pep...