of Lyotropic Liquid Crystalline Systems:
A New Route to Nanostructured Materials
of Polymer Science
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