Phase behavior and orientational ordering in silicate-surfactant liquid crystals.

摘要

We have investigated a new class of hybrid inorganic-organic materials, which are synthesized from surfactant species acting as structure-directing agents for inorganic frameworks. Among the important consequences are the opportunities to design inorganic-organic mesophases, as well as inorganic mesoporous materials with novel morphologies, including lamellar, hexagonal, and cubic structures. For example, addition of an anionic silicate solution to a cationic surfactant solution induces assembly of a silicate-surfactant mesophase. By using stable silicate species at high pH and low temperatures, the silicate-surfactant assembly process is decoupled from the kinetics of interfacial silicate polymerization. The objectives for utilizing this decoupling strategy are many-fold, including the possibility of a systematic study of the phase-behavior, assembly mechanism, and macroscopic alignment of these materials. Several of these closely related topics are investigated, including: (1) The phase behavior of the mesophases using complementary experimental techniques, such as NMR spectroscopy, X-ray diffraction, and polarized optical microscopy, to obtain information on the molecular, mesoscopic, and macroscopic length scales. (2) The use of such corroborative methods in establishing the effects of different molecular parameters, as well as intra- and inter-aggregate interactions, on the mechanism of self-assembly and structure-direction in these hybrid inorganic-organic composites. (3) The molecular origins of potential reversible phase transformations among various silicate-surfactant liquid crystal morphologies. (4) The exploitation of the liquid crystalline properties of silicate-surfactant mesophases in producing macroscopically aligned samples, with considerable control on the direction of macroscopic alignment, in the presence of an external magnetic field. (5) The production of macroscopically aligned mesoporous materials, by the initial alignment of the silicate-surfactant mesophase, condensation of silicate species by lowering the pH and/or increasing the temperature, and the subsequent removal of the organic components by thermal oxidation at high temperatures.