Allan Guymon

Photopolymerization of Lyotropic Liquid Crystalline Systems:

A New Route to Nanostructured Materials

Department of Polymer Science

University of Southern Mississippi

 Lyotropic liquid crystalline (LLC) materials are ordered liquids consisting of an amphiphile (soaps, surfactants, etc) and a typically aqueous solvent.  At sufficient amphiphile concentrations a variety of ordered phases are formed with well-defined nanoscopic pore sizes.  LLC materials are extremely important in a number of applications including soaps, surfactant solutions, thickeners, and in materials processing.  Polymerizations of lyotropic liquid crystals (LLCs) and templated polymerizations in LLCs have been of particular interest due to the range of geometries produced by different mesogenic phases.  Applications ranging from biomimetic devices to separation media and nanocomposite synthesis have been proposed and investigated.  The primary reason that these materials have not been used more extensively is the difficulty in retaining and controlling the structure throughout polymerization.  Typically, phase separation of the polymer occurs during polymerization leading to a much different polymer morphology than would be desired.  It is obvious that the processes occurring during polymerization are the driving factors of the ultimate polymer structure, but little information is available about the polymerization behavior itself.  Before materials with controlled lyotropic structure can be developed consistently and controllably, the polymerization process and the implications of this polymerization must be understood.   

The photopolymerization behavior and structural evolution of LLC systems are detailed utilizing two particular routes to prepare material possessing the desired LLC morphology.  One is the polymerization of amphiphiles that are, in and of themselves, polymerizable, whereas; the other utilizes the segregation of monomers in LLC phases of common low molecular mass surfactants.  The polymerization behavior of a semifluorinated methacrylic acid is shown to be heavily dependent on the type and degree of LLC structure.  The lamellar aggregates polymerize faster than either cubic or isotropic morphologies due to diffusional limitations on the growing polymer chain imposed by the highly ordered lamellar phase.  Similar trends are observed in the polymerization of polar nonamphiphillic monomers in the various mesophases of common surfactants.  Once again the fastest polymerization rates are observed in the lamellar phase when compared to other morphologies.  Like the polymerizable semifluorinated surfactant the increased polymerization rate of polar monomers in LLC phases is shown to be caused by increased ordering effects that places diffusional limits on the forming polymer.  The opposite trend is observed with nonpolar monomers that segregate in the LLC phases.  The fastest polymerization rates occur in micellar aggregates and the slowest in lamellar morphologies.  This phenomenon is a result of local monomer concentration increases in the smaller micellar aggregates and with higher surfactant content in the lamellar mesophase.  Initial studies indicate that to a large extent the original LLC order of these systems is being retained upon photopolymerization although in some cases the LLC phases have been shown to change upon cure.  Additionally, polymeric gels synthesized in this fashion have exhibited vastly different physical properties with respect to the pre-polymerization morphology.