Photoisomerization- and photopolymerization-induced phase transitions in mixtures of photoresponsive chromophores and reactive mesogens

Phase diagram of various blend systems containing photochromophores and reactive mesogenic diacrylate monomers has been constructed both experimentally and theoretically. Differential scanning calorimetry was commonly utilized to determine phase transition temperature, and theoretical curve was obtained by solving self-consistently the combined free energy of Flory-Huggins, Maier-Saupe, and phase field theory. Depending on the system, it exhibited various single and coexistence phases including crystal, nematic, crystal + isotropic, nematic + isotropic, crystal + nematic, and crystal + crystal. Guided by the phase diagram, we further investigated (i) self-motion of azobenzene chromophore (AC) in triacrylate (TA) solution, (ii) photoisomerization-induced phase transition of liquid crystalline azobenzene (LCAC) and spiropyran (SP) chromophore in its mixtures with reactive mesogenic diacrylate (RMDA), and (iii) photopolymerization-induced phase separation of RMDA/low molecular weight liquid crystal (LMWLC) blends.;Self-motion of AC single crystal in TA solution was investigated in relation to the solid-liquid phase diagram. Upon thermal quenching from the isotropic melt to the crystal + liquid gap, the concentration gradient that formed as the result of TA expulsion induced convective flows and generated spatial variability of surface tension usually responsible for Marangoni effect. A stationary rhomboidal crystal was also shown to swim upon irradiation with UV light due to mechanical torque generated by trans-cis isomerization.;The trans-cis photoisomerization of LCAC not only exerted a dramatic effect on the mesophase transitions, but also on the stratification on the crystal morphology. The phase diagram of photoisomerized mixtures displayed a significant depression of nematic ordering. Meanwhile, spiropyran-merocyanine molecular shape change induced the I-N phase transition, which was infusible.;Upon photopolymerization of RMDA/LMWLC blends, the homogeneous mixtures underwent phase separation. As the crystalline and nematic ordering of monomeric unit was frozen, LMWLC was dispersed in the continuum of isotropic or anisotropic networks. Phase diagram exhibited the broad immiscible regions.;The phase diagram of annealed hyperbranched polymer/LMWLC mixtures was an upper azeotrope, exhibiting the coexistence of nematic + isotropic phase above the clearing temperature of neat LMWLC. With decreasing temperature a focal-conic fan shaped texture developed in the composition range of 63~93 wt %, suggestive of induced smectic phase in the mixture.