A promising method of synthesizing polymers with useful property relationships utilizes self-assembling lyotropic liquid crystals (LLCs) as photopolymerization templates to direct polymer structure on the nanometer scale. Unfortunately, thermodynamically driven phase separation of the polymer from the LLC template often occurs during polymerization and prevents control over final polymer nanostructure and properties. In this work, the nanostructure of polyacrylamide is controlled through photopolymerization in LLC templates formed using specific concentrations of polymerizable and nonreactive surfactants. Polymer structure information obtained using electron microscopy, X-ray scattering, and polarized light microscopy indicates that LLC nanostructure is retained during photopolymerization at particular reactive surfactant concentrations. Physical properties including water uptake, diffusivity, and mechanical strength are greater in polyacrylamide systems that exhibit nanostructure as compared to isotropic controls of the same chemical composition. Useful property relationships typically unattainable in traditional hydrogel systems are also observed for nanostructured hydrogels including simultaneous increases in water uptake and mechanical strength. These results demonstrate methods of generating and retaining polymer nanostructure during photopolymerization in systems that otherwise phase separate from the LLC template and may be utilized to synthesize nanostructured polymers with property relationships useful in a growing number of advanced applications.

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogels are widely studied stimuliresponsive systems due to their significant volume changes at biologically relevant temperatures and a potential wide range of applications including drug delivery, cell cultures, chemical sensors, and desalination. The successful performance of PNIPAM gels often requires a rapid response rate with a significant degree of deswelling when heated above the lower critical solution temperature. However, it is often difficult to design PNIPAM hydrogels with appropriate mechanical strength for the gels to remain functional in a working environment. Herein, lyotropic liquid crystals (LLCs) are utilized to generate a hexagonal nanostructure in PNIPAM hydrogels in order to improve material properties and transport characteristics. Cross-linked methacrylated poly(dimethylsiloxane) (PDMS) was incorporated into PNIPAM gels at varying concentrations through photopolymerization in the hexagonal LLC phase in order to modulate mechanical properties. The hexagonal LLC nanostructure dramatically increases the hydrogel deswelling rate compared to traditional isotropic PNIPAM-PDMS hydrogels of the same chemical composition. Additionally, the ordered LLC structure allows for considerable incorporation of PDMS into the hydrogel without significantly decreasing the water content of the gels. Interestingly, the hexagonal nanostructured hydrogels exhibit similar compressive moduli compared to isotropic hydrogel controls despite having considerably higher water content. These results may be utilized to generate stimuli-sensitive hydrogels with an appropriate rate and degree of deswelling while maintaining necessary mechanical integrity of the gel for use in numerous biomedical and industrial applications.

Photopolymerization in and of lyotropic liquid crystal (LLC) template phases shows great promise for generating nanostructure in organic polymers. Interestingly, the order imposed on the polymerization system in LLCs significantly alters polymerization kinetics. The rate of polymerization of hydrophilic monomers increases with increasing LLC order, primarily due to monomer/polymer association with surfactant and the resulting decrease of growing polymer chain diffusion. Conversely, as LLC order increases, hydrophobic monomers become less segregated as nonpolar volume increases, which decreases polymerization rate. The efficiency of initiators is also dependent on LLC template order, further contributing to polymerization rate changes. When reactive surfactants are used, LLC mesophase, location of reactive group, and aliphatic tail length also affect polymerization kinetics. Overall, these photopolymerization kinetics directly relate to the segregation behavior and local order of reactive groups and thus can be used to probe nanostructure evolution, facilitating understanding and control of ultimate polymer nanostructure.

Polymerization 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