Polymer Engineering Faculty Research


Holographic photo-polymerization induced phase separation in reference to the phase diagram of a mixture of photo-curable monomer and nematic liquid crystal

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Theoretical investigation on pattern photopolymerization has been undertaken in various coexistence regions of a mixture of photocurable monomer and nematic liquid crystal. The phase diagram of the mixtures is constructed in accordance with a combined Flory−Huggins (FH) free energy of isotropic mixing and the Maier−Saupe (MS) free energy of nematic ordering. The pattern formation dynamics of holographic photopolymerization has been explored in conjunction with the phase diagram of the starting mixture of monomer and nematic liquid crystal. To mimic the evolution of holographic structure, the combined FH and MS free energy is incorporated into the time-dependent Ginzburg−Landau (model C) equations coupled with the photopolymerization kinetic equation. The morphology evolution and mechanisms have been investigated in relation to the situations whether the system should remain in the single phase after photopolymerization or be thrust into the two-phase region. Of particular significance is that the diffraction efficiency (DE) of gratings, a key property in electrooptical applications, can be monitored in-situ during photopatterning. In the gratings of inhomogeneous structures, scattering occurs due to the interparticle interference, which needs to be accounted for in the evaluation of the DE. A fast Fourier transformation (FFT) technique of the emerging morphological domains of the holographic polymer dispersed liquid crystals uniquely provides the simultaneous determination of the diffraction peaks from the holographic layers as well as scattering from the internal LC domains; thereby, the DE correction is feasible. The simulated patterns and the trends of diffraction efficiency evolution are compared with experimental observations.

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