Polymer–Nematic Liquid Crystal Interface: On the Role of the Liquid Crystalline Molecular Structure and the Phase Sequence in Photoalignment

Abstract

We provide experimental evidence for the influence of the molecular structure of the nematic liquid crystal (NLC) on the photoalignment process in three dimensions at the interface with a polymer layer. In particular, the experimental findings are explained through the presence (or absence) of the π−π aromatic interactions between the NLC and the polymer. The influence of the nematic-to-smectic A phase transition on the photocontrol is also addressed. Furthermore, we demonstrate that the photo-induced reorientation scenarios can be eventually connected to conformational changes in the photosensitive polymer.

Photo-Orientation of Liquid Crystals on Azo Dye-Containing Polymers

“…The LC compounds with a biphenyl core were 4-cyano-4’-penthylbiphenyl (5CB), 4-cyano-4’-n-octylbiphenyl (8CB), 4-cyano-4’-n-octyloxybiphenyl (8OCB), 3,4,5-trifluoro-4’-(4-pentyl-cyclohexyl)biphenyl (5CPUF), as well as mixtures E7 and E63 (both from Merck). We note that the three fluorene atoms at 3,4,5 positions in 5CPUF almost “invert” the molecular electrostatic surface potential (MESP) [ 22 ] of the attached phenyl ring compared to the case of cyano-biphenyls (5CB, E7, E63) (as can be seen in [ 19 ]). LCs with phenylcyclohexane core were 1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) [ 19 ] and some members of the PCH homologous series (PCH3, PCH5 and PCH7) [ 23 ], while LC with bicyclohexane core was the mixture ZLI1695 [ 23 ].…”

“…We note that the three fluorene atoms at 3,4,5 positions in 5CPUF almost “invert” the molecular electrostatic surface potential (MESP) [ 22 ] of the attached phenyl ring compared to the case of cyano-biphenyls (5CB, E7, E63) (as can be seen in [ 19 ]). LCs with phenylcyclohexane core were 1-(trans-4-hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) [ 19 ] and some members of the PCH homologous series (PCH3, PCH5 and PCH7) [ 23 ], while LC with bicyclohexane core was the mixture ZLI1695 [ 23 ].…”

“…The change in the phase-shift between the ordinary and extraordinary components of the probe beam was also measured, giving information of a possible zenithal reorientation during photoalignment. Details of the setup can be found in [ 19 ].…”

“…For the study of the influence of the liquid crystalline molecular structure on the photo-orientation process pDR1 ( polymethyl methacrilate (PMMA) functionalised with azo dye Disperse Red 1 ( Figure 2 )) was used—see [ 19 ] for the molecular structure of the polymer. The synthetic route of pDR1 is presented, e.g., in Ref.…”

“…Indeed, measurements with the light polarisation of the pump beam perpendicular to n (where no azimuthal photo-orientation is expected) have indicated a considerable zenithal photoalignment for nematics with cyano-biphenil rigid core (5CB, E7, E63) in the temperature range where the efficiency of the azimuthal photo-orientation has decreased [ 21 ]. In contrast, for other nematics (including 5CPUF), no such considerable zenithal photoalignment was detected at any temperature of the nematic phase [ 19 , 23 ]. Therefore, the decrease in the efficiency of the azimuthal photoalignment at high temperatures ( C) in these compounds—see, e.g., ZLI1695 in Figure 3 a—remained puzzling.…”