The thermal-response and mechanical properties of poly(N-isopropylacrylamide) (PNIPAm) are improved by controlling the polymer nanostructure through photopolymerization in a bicontinuous cubic lyotropic liquid crystal (LLC) template. The bicontinuous cubic nanostructure increases the rate and amount of water expelled from PNIPAm for heating above the lower critical solution temperature (LCST) relative to an isotropic PNIPAm hydrogel while maintaining the mechanical integrity of the polymer. These results could allow development of PNIPAm hydrogels with proper water uptake, deswelling kinetics, volume transition, and mechanical properties required for successful performance in a growing number of advanced biological and industrial applications.

The ability to direct polymer structure on the nanometer scale has provided access to enhanced material properties that may be tailored to accommodate a growing number of advanced biological and industrial applications. A promising method of generating nanostructure in organic polymers uses self-assembling lyotropic liquid crystalline (LLC) mesophases as templates for polymerization. Unfortunately, thermodynamically driven phase separation often prevents polymer morphology from being precisely controlled. As the demand for polymers with accurately engineered properties increases, a detailed understanding of the phase separation process is needed to control the nanostructure and properties of LLC templated polymers. In this study, photopolymerization kinetics are utilized to identify phase separation events occurring during the photopolymerization of poly(ethylene glycol) diacrylate and hexanediol diacrylate monomers templated in the normal hexagonal or lamellar LLC mesophases. Discontinuities are found in the polymerization rate of anisotropic polymers at several double bond conversions that are not present in the rate profiles of isotropic controls. The polymer morphology was subsequently characterized at particular conversions using small angle X-ray scattering. Changes in polymer nanostructure occur at double bond conversions that coincide with the observed rate discontinuities. These results demonstrate that photopolymerization kinetics can be used as a probe to monitor the evolution of polymer nanostructure during polymerization and optimize the conditions governing the control of polymer morphology to enhance properties dependent on nanostructure.

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.