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The kinetics and mode of nucleation and growth of fibers by 5alpha-cholestan-3beta-yl N-(2-naphthyl)carbamate (CNC), a low-molecular-mass organogelator (LMOG), in n-octane and n-dodecane have been investigated as their sols were transformed isothermally to organogels. The kinetics has been followed in detail by circular dichroism, fluorescence, small-angle neutron scattering, and rheological methods. When treated according to Avrami theory, kinetic data from the four methods are self-consistent and describe a gelation process involving one-dimensional growth and "instantaneous nucleation". As expected from this growth model, polarized optical micrographs of the self-assembled fibrillar networks (SAFINs) show fibrous aggregates. However, their size and appearance change abruptly from spherulitic to rodlike as temperature is increased. This morphological change is attended by corresponding excursions in static and kinetic CD, fluorescence and rheological data. Furthermore, the rheological measurements reveal an unusual linear increase in viscoelastic moduli in the initial stages of self-assembly. Each of the methods employed becomes sensitive to changes of the system at different stages of the transformation from single molecules of the LMOG to their eventual SAFINs. This study also provides a methodology for investigating aggregation phenomena of some other self-assembling systems, including those of biological and physiological importance.

Organogels of 12-hydroxystearic acid (HSA) have been investigated to emphasize solvent-dependent relationships between rheological properties and nanostructures in this class of physical gels. Different length scales are considered:  macroscopic with optical opacity, rheology, phase diagram data; nanoscopic with small-angle neutron scattering experiments and molecular with 1H NMR data. HSA networks are shown to be rigid fibrillar networks with rigid junction zones and behave as elastoplastic materials. Shear elasticities, yield stresses, and deformation behaviors appear connected to the crystallinity of networks through the cross-sectional shapes of fibers. Kinetics of molecular aggregation and flowing properties are also used to compare three classes of HSA gels (toluene, dodecane, nitrobenzene). The melting transitions as well as the concentration dependence of yield stresses exhibit remarkable behaviors analyzed in a context of molecular nanomaterials.

The kinetics of the isothermal transformation of sols, comprised of a low molecular-mass organogelator (LMOG) and an organic liquid, to their organogel phases have been followed by circular dichroism (CD), fluorescence, small angle neutron scattering (SANS), and rheological methods. The thixotropic properties (in the sense that severe shearing followed by rest lead to reestablishment of viscoelasticity) of the gels have been examined as well by rheological measurements. The compositions of the samples were either 5alpha-cholestan-3beta-yl N-(2-naphthyl) carbamate (CNC) in an n-alkane (n-octane or n-dodecane) or 3beta-cholesteryl N-(2-naphthyl) carbamate (CeNC) in ethyl acetate. Values of Df, the mass fractal dimension of the microcrystalline self-assembled fibrillar networks (SAFINs) in the gels, have been extracted from the kinetic data using a model developed by Dickinson (J. Chem. Soc., Faraday Trans. 1997, 93, 111). The Df values, 1.1-1.3 for the CeNC gels and 1.3-1.4 or 1.6-1.8 (depending on the temperature of incubation of the sol phase) for CNC gels, are consistent with the gel network structures observed by optical microscopy. In addition, comparison of the temperature dependence of both n (the Avrami component) and K (the Avrami "rate constant") for CeNC/ethyl acetate gelation with those reported previously for gelation of CNC/n-alkane sols demonstrate that the very small change of a single bond in CNC to a double bond in CeNC causes significant differences in their gelation abilities and gel properties. The rheological measurements on CNC/n-alkane gels with spherulitic SAFIN units, formed by incubation of their sols at < or =28 degrees C, indicate that they are thixotropic. Gels with the same chemical composition but formed by incubation of their sols at > or =30 degrees C, leading to fiberlike SAFIN units, remain liquidlike after shearing regardless of the periods they are at rest. The time-dependent viscoelastic properties of the gel networks are treated according to a stretched exponential model. The observations from these studies provide detailed insights into the mechanisms of formation of molecular organogel phases and demonstrate the extreme sensitivity of the SAFINs and viscoelastic properties of such organogels to slight modifications in LMOG structure or sample history.

A low molecular weight bisurea in nonpolar solvents is shown to self-assemble by hydrogen bonding into two distinct high molecular weight structures. At low temperature and high concentration, the most stable structure is a thick cylindrical assembly, responsible for the very high viscosity of the solution. At higher temperature or lower concentration, the thick filaments disappear in favor of thinner filaments, leading to a lower viscosity. The reversible transition occurs over a temperature range of 5 degrees C only, showing that it is highly cooperative. The structural switch can also be triggered by changing the nature of the solvent or the composition in the case of a mixture of two bisureas. The high cooperativity and the tunability of this transition are useful for the design of responsive materials.

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The structure of a bis-urea based reversible polymer is investigated using capillary viscosimetry, infrared spectroscopy, small-angle neutron scattering, and rheology. The highly viscoelastic solutions obtained in toluene are due to the formation of long and rigid fibrillar species. The cross section of these wires is measured and is shown to likely contain two or three molecules per axial repetition unit.

Organogels of cholesteryl 4-(2-anthryloxy)butanoate (CAB) in decane and butanol have been studied by scattering techniques. The neutron and X-ray scattering curves of these gels consist of a mix of the form factor of the aggregates and the structure factor of their organized interacting domains. The results demonstrate that the aggregates are long and rigid fibers, the diameter (d) of which is slightly sensitive to the solvent type: d = 160 Á in decane and d = 192 Á in butanol (assuming a homogeneous circular cross-sectional symmetry hypothesis). The fibers are interconnected by "junction zones" in a random three-dimensional network. In decane, the structures of the aggregates in the junction zones are lyotropic organizations obtained through a transformation from the hexagonal packing of the crystalline state. The fibrils with a diameter of about 75.6 Á, corresponding to approximately twice the molecular length, are "swollen" to ca. 102 Á. In alcohols, the structural organization is closer to that of the solid state and exhibits sharp interfaces with the solvent. The results obtained in the present studies are discussed in the context of previous electron microscopy and fluorescence studies of CAB gels and compared to other structurally related gelators.