Kinetics of gelation in photoreversible gels

Abstract

Smart materials, or materials that respond to some stimulus by changing their properties, make up an active area of research in many fields. Light can be considered an especially attractive choice of stimulus because it can be applied with precise spatial and temporal control, and is non-invasive. This thesis explores light sensitive gels and colloids, which could be used as valves in microfluidics devices, as tunable templates for the production of nanoparticles, or as devices for capturing pollutants or delivering drugs. At the basis of the sensitivity to light is the azobenzene chemical group, which isomerizes from cis to trans under visible light and the reverse under UV light. When this group is embedded in the hydrophobic tail of a surfactant, the aggregation properties of the surfactant become light-sensitive. The trans form of the azobenzene surfactant is more likely to form micelles than the cis. When mixed with a hydrophobically modified polymer, these micelles can act as crosslinking sites for a gel network. Upon UV irradiation, the crosslinking is disrupted and the gel transitions to a solution state. NMR methods were used to characterize the micelles and gels, and to understand the steps that control the kinetics in these photoreversible systems. The gelation process can be considered to consist of photoreaction, micelle formation, and possibly polymer relaxation. It was found that the photon flux through the material limits the rate of reaction, which then controls the remaining processes in the system. The photoreaction was studied under varying conditions, including concentration, light intensity, and wavelength. Due to their optical thickness, these materials are possibly better suited for use as thin films. NMR experiments were also used to probe the interactions between the polymer and surfactant. In contrast to surfactant-only solutions, trans-dominated and cis-dominated micelles appeared equally likely to form aggregates with an appropriate polymer. The cis-rich aggregates failed to effectively crosslink the polymer and form a gel. This was confirmed by using diffusion measurements to monitor the size of crosslinked polymer clusters. This cluster growth correlated well with previous rheology results, but the high tendency of cis samples to form aggregates had not been anticipated. It is hypothesized that cisdominated aggregates are too small and unstable to act as crosslinking sites. In an effort to create a wider array of tunable colloids, the azobenzene surfactant was then mixed with a traditional surfactant of opposite charge. Solutions consisting of oppositely charged surfactants have been known to result in unilamellar vesicles, when prepared at appropriate concentrations and mixing ratios. The size, type and number density of the aggregates in this work were found to be controllable through the use of light. Depending on the light conditions, either nanodiscs or vesicles could be observed.