Lyotropic liquid crystals (LLCs) have recently been employed as polymerization templates to yield highly ordered nanostructured hydrogels, which have shown promise in biological separations and tissue engineering. To allow greater control of hydrogel structure using LLC templates, this study focuses on the influence of polymerization kinetics and temperature on ultimate polymer morphology and properties. A more in-depth understanding of these phenomena has been obtained through extensive examination of polymer structure with small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM). The impact of different polymerization rate and temperature regimes on polymer morphology was determined using isotropic, cubic, and hexagonal phases as polymerization templates. Polymer morphology varies dramatically depending on LLC phase template, while within a given phase changes in polymerization time scale and temperature greatly impact polymer structure. SAXS reveals a higher degree of retained liquid crystalline order using rapid polymerization at relatively low temperatures. SEM images demonstrate that rapid polymerization yields polymers with highly ordered network structures and nanoscale morphology. Less ordered, thermodynamically driven polymer features result from relatively slow polymerization. The degree of swelling in aqueous solution increases dramatically in ordered LLC phases relative to bulk polymerization, with lower swelling resulting from faster polymerization. Similarly, higher surface area results from polymerization in LLC media relative to isotropic systems, and surface area increases with increasing polymerization rate.
Lyotropic liquid crystals (LLC's) have recently been employed as polymerization templates to yield highly ordered nanostructured materials with potential applications in regulated transport, ultrafiltration, and catalysis. With the goal of understanding the reaction mechanisms ultimately determining polymer morphology in these highly ordered systems, this study focuses on the influence of LLC order on photoinitiation efficiency and monomer segregation behavior. These phenomena have been elucidated through extensive examination of the polymerization kinetics utilizing photodifferential scanning calorimetry. The polymerization kinetics in these ordered liquids are highly dependent on phase morphology because liquid crystalline alignment has a profound effect on monomer segregation, and photoinitiation efficiency is a function of viscosity, polarity, and the diffusional constraints inherent in LLC phases. Through studies of the polymerization kinetics in different LLC phases using a free radical inhibitor, the relative initiation efficiencies of a number of photoinitiators were determined. By incorporating relative efficiency information in the analysis of propagation and termination rate parameters, the individual contributions of monomer segregation and photoinitiation behavior to the polymerization kinetics were differentiated. While segregation of polar monomers in the continuous phase leads to increased polymerization rate in more ordered LLC phases, nonpolar monomers exhibit the opposite trend due to segregation in the discontinuous region of the phases studied. Changes in photoinitiation efficiency either compound or negate the effect of monomer segregation on the polymerization rate depending on initiator structure and polarity. The initiation efficiency with relatively bulky, hydrophobic initiators increases substantially in more ordered systems while the efficiency of less hydrophobic initiators decreases slightly in more ordered phases.
Polymerization in lyotropic liquid crystalline (LLC) media is a promising method enabling synthesis of nanostructured organic polymers. To understand polymer structure development in LLC systems, this study investigates the polymerization of acrylate monomers of differing polarity and size within the hexagonal and lamellar phase of dodecyltrimethylammonium bromide and water. LLC structure, polymer morphology, and polymerization behavior were characterized to understand the connection between polymerization environment and formation of polymer structure. While the order of the template phase highly influences final polymer order, variations in monomer chemistry can lead to significantly different polymer structures even from the same liquid crystalline phase. More rapid polymerization, which leads to higher conversion and higher cross-link density of the relatively low molecular weight monomers, yields highly anisotropic polymer structure exhibiting alignment of 800 nm channels extending for lengths greater than 150 μm. Less ordered polymer structure results with analogous monomers of higher molecular weight. While polymer structure appears to result from a phase separation process in these systems, the structure directing influence of the liquid crystalline media is exhibited in several systems including the formation of highly oriented cylindrical structures with diameter of 200 nm from polymerization of poly(ethylene glycol) dimethacrylate in the hexagonal phase. Significant control of polymer structure has been demonstrated using LLC templates through proper selection of monomer chemistry, concentration, and LLC template structure.
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