Effect of \textit{Trans-Cis} Photoisomerization on Phase Equilibria and Phase Transition of Azobenzene Chromophore and Reactive Mesogenic Diacrylate Mixtures

Photoisomerization-induced phase transition of neat liquid crystalline azobenzene chromophore (LCAC) and its mixtures with reactive mesogenic diacrylate monomer (RMDA) has been investigated experimentally and theoretically. Upon irradiation with UV light, the nematic phase of LCAC transformed to isotropic while the crystal phase showed the undulation on the surface (i.e., ripples). The phase transition temperatures and corresponding morphologies of the blends have been measured by means of differential scanning calorimetry and optical microscopy. Theoretical phase diagram of binary nematic and crystalline system was constructed by self-consistently solving the combined free energies of Flory-Huggins, Maier-Saupe, and phase-field theory. It displayed various coexistence regions such as nematic + isotropic (N$_{1}$ + I$_{2})$, crystal + isotropic (Cr$_{1}$ + I$_{2})$, crystal + nematic (Cr$_{1}$ + N$_{2})$, and crystal + crystal (Cr$_{1}$ + Cr$_{2})$ over the broad range of compositions and the single phase nematic (N$_{1}$, N$_{2})$ at the RMDA and LCAC rich compositions. The calculated liquidus lines were in good accord with the mesophase transition points. Of particular interest is that the \textit{trans-to-cis} isomerization of LCAC has led to suppression of N$_{1}$ + I$_{2}$ coexistence regions and disappearance of the N$_{2}$ phase of LCAC.