Investigation of Polymer Stabilized Liquid Crystal Formation Using Fluorinated and Aliphatic Monoacrylates

This study focuses on the photo-polymerization of a fluorinated monoacrylate monomer and aliphatic analog within a room temperature smectic liquid crystal (LC) in an effort to understand how factors such as LC order, monomer segregation, and monomer chemical structure affects the polymerization mechanism in polymer stabilized liquid crystalline systems (PSLC). Specifically, a fluorinated monoacrylate exhibits significantly enhanced polymerization rates when compared to an aliphatic monoacrylate. Moreover, this rate enhancement is particularly pronounced in the smectic phase of the LC, where the fluorinated monoacrylate displays a polymerization rate in the smectic phase that is over three times faster than the aliphatic monoacrylate in the smectic phase. Also the fluorinated monoacrylate exhibits enhanced segregation between the smectic layers of the LC both before and after polymerization, whereas the aliphatic monoacrylate phase separates during polymerization. The results of this study demonstrate how changes in the monomer chemical structure (i.e. fluorination) can significantly impact the polymerization mechanism and segregation in polymer stabilized systems. This study also offers the potential to further the understanding of tailoring these unique systems for display applications

Phase Behavior and Polymerization Kinetics of a Semifluorinated Lyotropic Liquid Crystal

Recently, amphiphilic monomers that are capable of forming lyotropic liquid crystalline phases have been utilized in a variety of applications including emulsion polymerization, development of nanocomposites, and the formation of polymeric surfactants. A new class of fluorinated amphiphilic monomers that exhibit lyotropic mesophases has shown great promise in ophthalmic applications. Initial studies indicate that the monomer possesses a lamellar morphology at certain concentrations that when polymerized would yield a material with anisotropic properties ideal for repairing retinal tears. Characterization of the polymerization kinetics of these fluorinated monomers provides a better understanding of conditions such that structure retention can be obtained. The fluorinated amphiphiles exhibit varying phase morphology ranging from an isotropic micellar phase to discontinuous cubic and lamellar liquid crystalline phases with increasing concentration and variation in the percent neutralization of the acid moiety. The lyotropic liquid crystalline order changes significantly with respect to percent neutralization. The polymerization kinetics follow a trend of decreasing order with increasing neutralization in that the fastest rates are seen in samples with higher degrees of order specifically in the lamellar liquid crystalline phase. The polymerization rate decreases to a minimum in samples of cubic morphology with low degrees of overall order. The higher polymerization rates in the lamellar phase are due to a decrease in the termination rate. Additionally, the polymerization behavior and morphology have a tremendous impact on the resulting polymer

Photopolymerization Kinetics of Nanostructured Polymers Templated by Lyotropic Liquid Crystals

Nanostructured materials have been the focus of much attention due to their applicability in nanocomposites, separations media, drug delivery devices, and many other applications requiring a nanometer size scale. Recently, using lyotropic liquid crystalline (LLC) phases to template their unique nanostructure onto organic polymers has been proposed. This work details the photopolymerization of acrylamide in various phases of LLC systems. The photopolymerization kinetics are correlated to monomer organization for different phases at a variety of concentrations and temperatures. The photopolymerization kinetics of acrylamide in the LLC phases depend strongly on the LLC morphology. The polymerization rates are significantly faster in compositions of surfactant resulting in a hexagonal geometry as the acrylamide monomer is preferentially oriented. These results indicate that acrylamide is strongly associated with the LLC interface and the surfactant. Photopolymerization of these templated systems results in structure retention of the parent, LLC phase

Ordering Effects on the Photopolymerization of a Lyotropic Liquid Crystal

The synthesis of polymers bearing the highly ordered nanostructure of lyotropic liquid crystal (LLC) phases has recently been of great interest. This work describes the polymerization behavior and structural evolution of a cationic amphiphile in various LLC phases. The type and degree of LLC phases formed froth this monomer depend strongly on the composition and temperature. By adding a nonpolymerizable surfactant a variety of LLC phases are formed including hexagonal, bicontinuous cubic, and lamellar morphologies while maintaining a constant monomer concentration. The highly ordered lamellar LLC phase exhibits the fastest polymerization rate with the slowest occurring in the hexagonal phase. The polymerization rates of the bicontinuous morphology were intermediate to the lamellar and hexagonal phases. The faster polymerization kinetics is due to diffusional limitations imposed on the propagating polymer by the highly ordered lamellar LLC phase. Also, the order of this LLC system has a strong dependence on temperature. At higher temperatures, the degree of LLC order and correspondingly the polymerization rate decrease. The original LLC phase morphology appears to be retained to the greatest extent in the faster polymerizing lamellar phase. The original nanostructure is also retained in the hexagonal and cubic LLC phases but with some slight changes in structure. This LLC structure is preserved at temperatures well exceeding the thermal phase transitions of the unpolymerized LLC samples. (C) 2002 Elsevier Science Ltd. All rights reserved

Investigation of Polymer Nanostructure Evolution During the Formation of Polymer/Smectic Liquid Crystal Composites

Polymer stabilized liquid crystalline systems (PSLCs) have been of considerable research interest due to their great potential in liquid crystal display applications. Of particular importance in the properties of PSLC systems is the evolution of the polymer nanostructure. The unique characteristics of fluorinated monomers not only may provide unique polymer nanostructure but also may enhance desirable properties of PSLC materials. This study focuses on the polymerization and polymer nanostructure of low surface energy fluorinated materials in a liquid crystalline solvent. Enhanced polymerization rates are observed as the order of the liquid crystalline solvent is increased with particularly pronounced acceleration for a fluorinated monoacrylate. This behavior is primarily due to segregation both before and after polymerization. Fluorinated monomers segregate between the smectic layers of the liquid crystal comparable to segregation behavior of analogous aliphatic monomers. The monomer structure has a significant impact on the polymer segregation behavior in these polymer/liquid crystalline composites as well. Network polymer structures, obtained from both aliphatic and fluorinated diacrylate monomers, phase separate from the liquid crystal while linear fluorinated polymer structures remain segregated between the smectic layers of the liquid crystal. Not only does this linear polymer remain between the smectic layers and retain its segregation behavior throughout the polymerization, but the polymer is also ordered to a much greater degree than the monomer. This ordered structure significantly alters the polymer/LC interaction and leads to birefringence at temperatures well above the liquid crystalline isotropic clearing point

Photopolymerization Kinetics of Pigmented Systems Using a Thin-Film Calorimeter

A thin-film calorimeter was used to evaluate the polymerization rates of pigmented photopolymerizable systems. As a result of a wide linear response range, which is more than an order of magnitude greater than that of the photo-DSC, the thin-film calorimeter is capable of measuring both small and large signals accurately. It was used to measure the polymerization exotherms of thin-films of a photocurable acrylate monomer with added pigment as well as commercial UV curable pigmented ink formulations

Photopolymerization Kinetics and Structure Development of Templated Lyotropic Liquid Crystalline Systems

The templating of the ordered nanostructures of lyotropic liquid crystals (LLC) onto organic polymers has recently been of great interest. This work describes the polymerization and segregation behavior of polar and nonpolar monomers in an LL(: environment. Nonpolar monomers partition to the oil-soluble domains of the LLC phase, whereas mol e polar monomers segregate at the interface of the liquid crystals. The polymerization kinetics in both cases are significantly influenced by the LLC phase morphologies although in different ways. The nonpolar monomers exhibit the fastest polymerization rates in micellar aggregates. This behavior is a result of an increase in the rate of propagation, induced by higher local concentrations of monomer in the micelles as compared to other mesophases. For more polar monomers, the opposite polymerization behavior is observed. The fastest polymerizations for these monomers are observed in the highly ordered lamellar mesophase with the minimum polymerization rate observed in the isotropic micellar phase. In this case, the enhanced polymerization rates are a consequence of depressed termination rates. This decrease in termination rate is due to diffusional limitations induced by the high degree of order in a lamellar mesophase. Initial results also indicate that the original LLC phase morphologies are retained after photopolymerization