Solvent evaporation-induced self-assembly

Zhao[132] demonstrated a simple solvothermal post-treatment method to prepare ordered large-pore (>7nm) FDU-15 laid phase) via phase transformation from 2-D hexagonal mesophases (an SBA-15 prepared by the evaporation-induced self-assembly process). This synthetic strategy of solvothermal post-treatment can be simply performed in many organic solvents such as hexane at 60-100 °C, and be extended to the syntheses of other silica-based mesoporous materials. 

Use of Dilute Solutions, The reactivity of the inorganic precursors can also be controlled by using low concentrations to lower the reaction rates. Consequent evaporation of solvent results in the formation of organized structure. This approach, introduced by Brinker et al. and termed as evaporation induced self-assembly. (EISA), allows the formation of powders, films, gels, and monoliths. 

Evaporation induced self-assembly has proved to be a versatile method for the con-stmction of many different film stmctures (Fig. 24-2), including functionalized films and ordered mesoporous films of materials other than silica. Solutions rich in a volatile solvent, usually ethanol, can be applied by either spin or dip coating, and many papers report using... 

In these syntheses the films with mesophase order form spontaneously at an interface from a dilute solution ofsurfactant micelles and sihca precursor via a co-assembly route. The surfactant concentration is higher than that used for evaporation induced self-assembly, and the concentration of volatile solvents is lower. Films will self-assemble from these solutions in closed containers and on submerged substrates, so the development ofmesoscale order does not rely on evaporation as in the work described above. These films form after an extended induction period, the length of which is dependent on solution concentration, ionic strength, inorganic surfactant ratio, pH and surface to volume ratio. Most of the work on films grown at the solution/air interface concerns the formation mechanisms of these films, and since these are likely to be similar for solid/solution interface films, film growth at the solution/air interface will be treated first. 

For the solvent inversion method the whole block copolymer has to be completely dissolved in a solvent before polymersome formation is initiated. Once the solvent containing the dissolved polymer is poured into an excess of water, the hydrophobic block becomes insoluble and polymersome formation is induced. Here, the created vesicles are typically between 100 and 200nm in diameter. Besides solvent inversion, film rehydration also relies on dissolving the amphiphilic block copolymer in a solvent other than water, hi contrast to solvent inversion, the solvent is slowly evaporated during this method to produce a thin film of precipitated polymer at the wall of the jar used. Once the film is created, the jar is filled with water and the self-assembly starts from the precipitated polymer film. Eventually, polymersomes are formed and the film is totally removed. If the jar surface is chemically altered, vesicles of up to 20 pm can be achieved. Otherwise, film rehydration yields the same vesicle sizes as solvent inversioa... 

An important distinction to be made is that instead of being induced by spontaneous evaporation of solvent, the movement of the contact line will be controlled by the motion of the lower substrate, resulting in a programmable stick-slip motion in FESA. Therefore, ordered patterns (e.g., thin films, snipes and spokes) can be readily produced in a controllable manner. Notably, the transition between the different patterns may be observed by gradually tuning each variable individually (e.g., velocity of lower substrate). The influence of each variable on the evaporative self-assembly process can also be scrutinized separately. 

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