The Relationship Between Dynamic and Chemical Factors On the Morphology in AOT-Water-Polymer Systems
Photopolymerization of reverse microemulsion demonstrates strong influences from dynamical and chemical factors on morphology of formed polymer systems. Upon polymerization the clear microemulsion with nanoscale structures transforms into opaque films with larger aggregates that scatter light. Studied here were microemulsions based on acrylate and on thiol-ene chemistries. The difference in the two reaction mechanisms results in the phase separation occurring at different stages of the material formation. Consequently, the morphology of the material demonstrates distinct dissimilarities in the two systems. In addition, the chemical structure of the monomers promotes these morphological differences. When the microemulsion is composed of acrylate and diacrylate monomers, the formed polymer has two different structures. Large sphere-shaped structures are formed from the aggregation of the nanometer-sized droplets in the microemulsion. The droplets, which are mainly filled with water, are large enough to scattering visible light and give the films their opacity. However, they are too small to be observable under an optical microscope. Secondly, some portion of the surfactant self-assembles into a bilayer structure connecting the larger pools of water. The structure of the film was evaluated by Small Angle Neutron Scattering and Ultra-Small Angle Neutron Scattering. In the case of microemulsions based on thiol-ene chemistry, the film's morphology is considerably different. Large droplets with size varying from 2 to 100 μm are visible in the films under the magnification of the optical microscope. In addition, the surfactant is more readily excluded from the polymer network than with acrylates; thus, the aggregates are filled not only with water but also with so-called aqueous phase composed of water and surfactant. The morphology of both systems is sensitive to the rate of the polymerization. The thiol-ene microemulsions were able to self-polymerize, which resulted in phase separation of the material. Resulting two layers were analyzed. The top layer contained mainly polymer. The lower stratum was water-rich with a small amount of polymer confined in droplets. This result proved that films formed through light induced reaction are frozen in a far-form-equilibrium state.