ABSTRACTPolymerization in highly ordered lyotropic liquid‐crystalline (LLC) media enables controllable synthesis of polymers possessing interesting nanostructure and physical properties. This study investigates the radical polymerization rate and molecular weight (MW) development of monoacrylates of different aliphatic tail length in a range of LLC phases. Polymerization rate data were acquired using photodifferential scanning calorimetry, and linear polymer MW was determined with gel permeation chromatography. Polymerization occurs much more rapidly, and higher MW is attained in the ordered LLC phases relative to isotropic solutions and neat polymerization. These properties change significantly as a function of LLC phase and monomer structure. A direct relationship is observed between polymer MW formation and the polymerization rate. Definitive changes in rate and MW were observed at phase boundaries, indicating the important role of solvent order. This study demonstrates how solvent ordering effects can be used to control polymer MW and rate of polymerization. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 144–154

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 mu m. 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.

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.

Photopolymerization in lyotropic liquid crystalline media is a promising method for the synthesis of polymers with nanoscale architectural control. Through this method materials with monodisperse, nanometer sized pores can be obtained, with the specific pore size and other aspects of polymer structure adjustable through simple variations in the concentration and chemical structure of surfactant. Recently polymers obtained through photopolymerization have been synthesized with indications of complete retention of the LLC mesophase structure in the polymerized material. However, the number of monomer/LLC systems that can be successfully templated and the applications currently benefiting from this technology are limited. Applications ranging from tissue scaffolds, size exclusion barriers, and catalytic supports could be greatly enhanced from the structural control afforded by this method if polymers with the desired chemistry could be templated. A greater understanding of the influence of monomer structure on mesophase retention and ultimate polymer nanostructure could not only help screen potential candidate monomers but also enable the application of LLC templating to a wider range of materials.

Nanostructured polymers have recently received considerable attention because of their potential to advance many diverse applications. Significant research has focused on synthesizing these polymers using lyotropic liquid crystalline (LLC) templates. The goal of this method is to produce polymeric gels with structures templated from the original surfactant system. Extensive synthesis of inorganic polymers has relied on this method. Recently liquid crystalline nanostructure has also been templated onto organic polymers with full retention of structure. The structures formed through this method of synthesis are highly dependent on polymerization kinetics. Significant understanding of the unique polymerization mechanism in LLC material has resulted from monitoring the polymerization kinetics of many monomer/LLC systems. These studies have revealed the roles of the monomer and liquid crystal components in the structure development of the resulting polymers. Monomers of different polarity display significantly different polymerization kinetics and also result in drastically different polymer structures due to their segregation in different domains within the liquid crystalline system.