In recent years there has been immense interest in studying gels derived from low molecular mass gelators (supramolecular, or simply molecular gels). The motivation for this is not only to understand the fundamental aggregate structures in the gels at different length scales, but also to explore their potential for futuristic technological applications. Gels have been made sensitive to external stimuli like light and chemical entities by incorporating a spectroscopically active or a receptor unit as part of the gelator molecule. This makes them suitable for applications such as sensing and actuating. The diversity of gel structural architectures has allowed them to be utilized as templates to prepare novel inorganic superstructures for possible applications in catalysis and separation. Gels derived from liquid crystals (anisotropy gels) that can act as dynamically functional materials have been prepared, for example, for (re-writable) information recording. Supramolecular gels can be important in controlled release applications, in oil recovery, for gelling cryogenic fuels etc. They can also serve as media for a range of applications. This tutorial review highlights some of the instructive work done by various groups to develop smart and functional gels, and covers a wide spectrum of scientific interest ranging from medicine to materials science.

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Supramolecular gels: Functions and uses

Novel crosslinking methods to design hydrogels

Critical phenomena and renormalization-group theory

Silk spinning by insects and spiders leads to the formation of fibres that exhibit high strength and toughness. The lack of understanding of the protein processing in silk glands has prevented the recapitulation of these properties in vitro from reconstituted or genetically engineered silks. Here we report the identification of emulsion formation and micellar structures from aqueous solutions of reconstituted silkworm silk fibroin as a first step in the process to control water and protein-protein interactions. The sizes (100-200 nm diameter) of these structures could be predicted from hydrophobicity plots of silk protein primary sequence. These micelles subsequently aggregated into larger 'globules' and gel-like states as the concentration of silk fibroin increased, while maintaining solubility owing to the hydrophilic regions of the protein interspersed among the larger hydrophobic regions. Upon physical shearing or stretching structural transitions, increased birefringence and morphological alignment were demonstrated, indicating that this process mimics the behaviour of similar native silk proteins in vivo. Final morphological features of these silk materials are similar to those observed in native silkworm fibres.

Mechanism of silk processing in insects and spiders

The entanglement concept in polymer rheology

A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which is characteristic of cobalt (II) in the tetrahedral (T(d)) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5-30 nm. The observed widths are larger than a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of T(d) coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 degrees C. A pale pink solution is obtained at 0 degrees C, indicating the formation of octahedral (O(h)) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G' and G") shows minima around at 27 degrees C, at which temperature they gave comparable values. On the other hand, G' exceeds G" both in the gel-like phase (temperature above 27 degrees C) and in the solution phase (temperature below 25 degrees C). These observations indicate that T(d) complexes are present as low-molecular weight species around at 25-27 degrees C. They are self-assembled to polymeric T(d) complexes by heating and form gel-like networks. Upon cooling the solution below 25 degrees C, T(d) complexes are converted to O(h) complexes and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed unique thermochromic transition (pink solution --> blue gel-like phase) is accompanied by an exothermic peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.

Heat-set gel-like networks of lipophilic Co(II) triazole complexes in organic media and their thermochromic structural transitions.

Rheology and dynamic light scattering of silk fibroin solution extracted from the middle division of Bombyx mori silkworm.

Dynamic light scattering and sedimentation velocity measurements were performed on dilute benzene solutions of poly(macromonomer) with poly(methyl methacrylate) (PMMA) as the backbone chain and polystyrene (PS) as branches to obtain the diffusion coefficient D and the sedimentation coefficient s, respectively. Dependences of D and s at infinite dilution on the backbone chain length as well as the branch length were found to be quantitatively described by the prolate ellipsoid model with values of the min and the minor axis calculated from the planar zigzag PMMA backbone and the gaussian PS branched chains, respectively. Application of the wormlike cylinder model gave the Kuhn's statistical length λ -1 ∼90 nm, which is substantially larger than λ -1 =3.2 nm of homo-PMMA. The concentration coefficient of D and s also showed unique dependences on the backbone chain length as well as the branch length

Diffusion and sedimentation studies on poly(macromonomer) in dilute solution

Sol-Gel transition behavior of pure iota-carrageenan in both salt-free and added salt states.

Steady viscosity, dynamic viscoelasticity (DVE), and dynamic light scattering (DLS) measurements were made to study the effects of temperature, T, on association behaviors of poly(vinyl alcohol) (PVA) with weight-average degree of polymerization DPw = 350 in aqueous Borax solution. Intrinsic viscosity, [η], monotonically decreased by 23% with a rise in T from 10 to 65 °C. At low T, enhancement in viscosity was observed with increasing polymer concentration, C, due to formation of viscoelastic network with the didiol complex as temporal cross-links, and the time−temperature superposition principle was found to be applicable for construction of a composite curve at each C. Those composite curves were further reduced so as to give one master curve. The system tended to lose its viscoelastic nature, when raising the temperature, toward a viscous solution owing to a decrease in the number of the didiol complexes as well as the shrinkage of the PVA chains. We found from analysis of DVE and DLS data that a clear...

Norio Nemoto

Papers

Heat-set gel-like networks of lipophilic Co(II) triazole complexes in organic media and their thermochromic structural transitions.

Rheology and dynamic light scattering of silk fibroin solution extracted from the middle division of Bombyx mori silkworm.

Diffusion and sedimentation studies on poly(macromonomer) in dilute solution

Sol-Gel transition behavior of pure iota-carrageenan in both salt-free and added salt states.

Dynamic Light Scattering and Dynamic Viscoelasticity of Poly(vinyl alcohol) in Aqueous Borax Solutions. 5. Temperature Effects