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.
Materials that change their physical properties on mechanical agitation are known as thixotropic materials. In the human body, for example, the functions of protoplasm, red blood cells, synovial fluid, and muscular activities are regulated by thixotropy. [1][2][3] Similarly, the properties of materials such as paints, bentonite clay, cosmetics, and foodstuffs arise from thixotropy. Despite the significant potential of this dynamic phenomenon, there is a lack of materials that can act as a model system to investigate vital natural processes such as muscle thixotropy and nerve fiber regeneration. In this respect, low-molecular-weight organogels [4] (LMOGs), which generally consists of 1D fibrous molecular assemblies, are envisioned to be attractive candidates when provided with thixotropic properties. However, not only are examples of LMOGs that exhibit thixotropy rare, [5][6][7][8][9] but there is also little information related to their aggregation and stability. These aspects circumvent the use of LMOGs for the study of this unique phenomenon.Of the hundreds of functional LMOGs reported to date, only very few exhibit the unique property of thixotropy. In general, LMOGs are extremely sensitive to mechanical stress and these materials irreversibly expel solvent molecules from their network when subjected to flow. On removal of the external force, these materials behave as a solid suspension that loses its original elastic properties. On the other hand, thixotropic gels can disintegrate in solution under an external mechanical stress and can regain their elastic properties upon removal of the stress. This operation can normally be carried out for an infinite number of cycles.Thixotropic LMOGs represent an intriguing and unique class of truly dynamic self-assembled supramolecular systems. Discernible visual insights into the process would be required in order to understand how such self-assembled entities evolve under mechanical stress followed by a resting time. To obtain such "snapshots", the thixotropic gel has to operate under a strict real concentration domain. However, the LMOGs reported to date show thixotopic properties only at concentrations of 10 wt % or more. At these high concentrations, the molecules are closely associated and therefore the reaggregation to the gel state from the disaggregated solution state occurs within seconds, thus rendering these systems unviable for real-time imaging. Hence, from the perspective of a supramolecular chemist, a successful thixotropic LMOG candidate has to fulfill three stringent requirements: firstly, the LMOG has to be an extremely efficient gelator (close to a super-gelator); secondly, it must exhibit the thixotropic property at such low concentrations; and thirdly, the gel has to maintain its original state without undergoing aging or forming crystalline domains during the cycles of breaking and regeneration processes. It is therefore difficult to obtain a supramolecular system that comprises all of these favorable functional features.Herein we report the discove...
Viscosity measurements are made on aqueous borax solutions of poly(vinyl alcohol) (PVA) samples over a wide range of PVA concentration, C, to examine the hydrodynamic properties of associated polymers compared to those of neutral polymers. The dynamic viscoelastic data are reanalyzed using a statistical−mechanical theory for elastically effective chains in transient gels developed by Tanaka and Ishida. The viscosity data are not superposed in the plot of viscosity against C[η] using the Huggins relationship, especially around and over a critical concentration, C N, at which the dynamical behaviors drastically change and an inflection point of a curve in the plot of the viscosity against C is located. On the other hand, when we assume that the chain dimension around C N is the unperturbed one, the viscosity data around C N is well-superposed on a curve in the plot of viscosity against C/C*up, where C*up is the overlapping concentration of the unperturbed chain. The inflection point of this composite curve is located at C/C*up = 1, which infers that C N corresponds to the overlapping concentration. All data of the density of elastically effective chains, νeff, are superposed on a curve in the plot of νeff/ν against (C − C N)/C N where ν is the density of polymer chains, which implies that C N should be regarded as a kind of gel point. Some discrepancies between the experimental data and the theoretical curve were observed, some of which cannot be explained using the pairwise association of PVA chains.
A review of the fabrication of polysaccharide ion gels with ionic liquids is presented. From various polysaccharides, the corresponding ion gels were fabricated through the dissolution with ionic liquids. As ionic liquids, in the most cases, 1-butyl-3-methylimidazolium chloride has been used, whereas 1-allyl-3methylimidazolium acetate was specifically used for chitin. The resulting ion gels have been characterized by suitable analytical measurements. Characterization of a pregel state by viscoelastic measurement provided the molecular weight information. Furthermore, the polysaccharide ion gels have been converted into value-added sustainable materials by appropriate procedures, such as exchange with other disperse media and regeneration.
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