The phase-dependent photophysics and photochemistry of side-chain substituted liquid crystalline polyarylcinnamates

The synthesis, characterization and photochemistry of side-chain thermotropic liquid-crystalline (LC) polymers containing UV-sensitive cinnamate ester chromophores incorporated as part of the pendant groups is described. The methacrylate monomer was polymerized by the free radical polymerization method using azo-bis-isobutyronitrile (AIBN) as the free radical initiator. The acrylate monomer was resistant to polymerization under the same reaction conditions. The monomers and the polymer are liquid crystalline in nature, exhibiting the nematic, the Smectic A and the Smectic B mesophases. UV-VIS spectral studies of thin films suggest that aryl cinnamate chromophores tend to aggregate significantly. Degree of aggregation is dependent on the phase of the polymer film. A study of aggregates as a function of temperature concluded that higher temperatures lead to a decrease in the degree of aggregation. Photolysis (313 nm) of solutions and freshly cast films of the polymer yielded two principal photochemical reactions. One was the (2+2) photocycloaddition, leading to cyclobutane formation, and the other was the photo-Fries rearrangement of the aryl cinnamate chromophore, leading to yellowing of the film. The effects of phase type on the photochemistry of the aggregated chromophores in the liquid crystalline phases of the polymer is also described. A dramatic loss of aggregate species with appearance of the absorption of unassociated chromophores occurs during the early stages of irradiation of the Smectic B and the Smectic A phases at 313nm. Fries product formation is essentially eliminated upon 366 nm irradiation where only aggregates are excited, indicative of preferential reaction of aggregates to form the cycloaddition product. The aggregation phenomenon was also seen as a consequence of hydrophobic association in aqueous media and wavelength dependent photochemistry was found prevalent. Generation of oriented crosslinked polymer networks by photopolymerization of liquid crystalline methacrylate monomers is also described. Macroscopic orientation was introduced by surface orientation effect. The order parameter, as evaluated by infrared dichroism, for polymer films obtained from mixtures of liquid crystalline monomers; one with a cyano substituted pendent group as a probe, suggested a very high degree of order of the photocrosslinked polymer network in the direction of the surface induced alignment.