The effect of fluorine-substituted monomers on the electrooptical and morphological properties of polymer-dispersed liquid crystals

Polymer dispersed liquid crystals (PDLCs) with semi-fluorinated host matrices have been investigated. Films were formed by photopolymerization-induced phase separation using floodlit and holographic techniques from an initially isotropic solution of monomer and liquid crystal. The electro-optical and morphological properties that resulted from the systematic addition of fluorinated additives to a pentaacrylate-based prepolymer solution are reported. The rational for pursuing partially fluorinated systems is based on the hypothesis that these materials may decrease the chemical compatibility and liquid crystal (LC) interfacial anchoring strength, thereby improving electro-optical and morphological characteristics. Real-time transmittance monitoring and polarized optical microscopy (POM) revealed that the incorporation of additives generally resulted in a delay in the appearance of a LC phase with increasing additive concentration. This was due to an accompanying decrease in average system functionality. However, this delay was less pronounced as the degree of additive fluorination increased. This was interpreted as an indication of lower LC solubility in semi-fluorinated polymer matrices. An improvement in contrast ratio was observed with partial matrix fluorination while chemically-similar, non-fluorinated systems resulted in no appreciable change in contrast ratio and an increase in relaxation time. General increases in polymer network phase separation were observed using low-voltage, high-resolution scanning electron microscopy (SEM) in fluorinated floodlit and holographic films. Film morphologies ranged from highly interconnected, porous networks to spherical bead-like structures dispersed throughout a semi-continuous liquid crystal medium. In order to determine the effect of additive functionality and chemical structure on PDLC properties, difunctional additives were investigated. The synthesis and characterization of several novel fluorinated difunctional vinyl monomers is also reported. These materials are fluorinated variations of a classical liquid crystal architecture featuring a rigid biphenyl backbone with vinyl terminated alkane spacer groups. Synthesis routes included acid chloride and N,N′-dicyclohexylcarbodiimide (DCCI) activated esterification reactions. Holographic polymer dispersed liquid crystal (HPDLC) films were also prepared using fluorinated acrylate and methacrylate additives. In these films, the inclusion of fluorine-substituted monomers resulted in decreased chemical compatibility. Consequently, the diffraction efficiency improved due to an increase in LC volume fraction.