Molecular hydrogels from bile acid analogues with neutral side chains: network architectures and viscoelastic properties. Junction zones, spherulites, and crystallites: phenomenological aspects of the gel metastability

Abstract

Structural and rheological properties of hydrogels made up of neutral bile acid derivatives are studied. Complementary scattering, diffraction, and microscopy techniques provide a precise structural description of the network architecture and its variation as a function of concentration, aging time, composition of the solvent, and type of gelator. Two derivatives (TH and PH) are considered as presenting favorable scattering features to approach the issue of the competition between gelation versus crystallization. PH and TH fibers are semirigid cylinders with monodisperse cross-sections (R₀ = 92 and 80 Å, respectively) involving 25 or 12 molecules per cross-sectional repeating unit along the fiber axis. Bundles are cross-links in the networks, and a scattering protocol is developed to determine the nodal and fibrillar fractions in the networks. The effects of alcoholic mixtures, dimethylsulfoxide, and temperature on the network properties are analyzed in terms of the bending modulus of the fibers, the degree of nonaffine character of the regime of deformation, and the dispersion degree of the nodal heterogeneities. It is shown that fibers are semirigid and the scaling laws of the elasticity of the gels with the concentration (exponent 5/2) also support the theoretical context. Head-to-tail molecular arrangements are shown to be similar in the solid and gel phases. Birefringent textures show that spherulitic microdomains coexist in the network texture and are the seeds for a slow crystallization process. The whole pattern might be more general for numerous other self-assembled fibrillar networks found in molecular gels.