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Title: Photoalignment of a Nematic Liquid Crystal Fluid and Glassy-Nematic Oligofluorenes on Coumarin-Containing Polymer Films

Abstract

The orientations of both a nematic liquid crystal fluid and a series of monodisperse glassy-nematic oligofluorenes were investigated on photoalignment films comprising a polymethacrylate backbone with 7-benzoyloxycoumarin pendants. Both classes of liquid crystalline material were found to undergo a transition from a parallel to a perpindicular orientation with reference to the polarization axis of UV-irradiation at a sufficiently high extent of dimerization.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Laboratory for Laser Energetics, University of Rochester
Sponsoring Org.:
USDOE
OSTI Identifier:
901010
Report Number(s):
DOE/SF/19460-697
Journal ID: 6983-6989; 2006-147; 1659; TRN: US200722%%29
DOE Contract Number:
FC52-92SF19460
Resource Type:
Journal Article
Resource Relation:
Journal Name: Macromolecules; Journal Volume: 39
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; DIMERIZATION; LIQUID CRYSTALS; ORIENTATION; POLARIZATION; ORGANIC FLUORINE COMPOUNDS; ALIGNMENT; ULTRAVIOLET RADIATION

Citation Formats

Trajkovska, A., Kim, C., Marshall, K.L., Mourey, T.H., and Chen, S.H. Photoalignment of a Nematic Liquid Crystal Fluid and Glassy-Nematic Oligofluorenes on Coumarin-Containing Polymer Films. United States: N. p., 2007. Web.
Trajkovska, A., Kim, C., Marshall, K.L., Mourey, T.H., & Chen, S.H. Photoalignment of a Nematic Liquid Crystal Fluid and Glassy-Nematic Oligofluorenes on Coumarin-Containing Polymer Films. United States.
Trajkovska, A., Kim, C., Marshall, K.L., Mourey, T.H., and Chen, S.H. Mon . "Photoalignment of a Nematic Liquid Crystal Fluid and Glassy-Nematic Oligofluorenes on Coumarin-Containing Polymer Films". United States. doi:.
@article{osti_901010,
title = {Photoalignment of a Nematic Liquid Crystal Fluid and Glassy-Nematic Oligofluorenes on Coumarin-Containing Polymer Films},
author = {Trajkovska, A. and Kim, C. and Marshall, K.L. and Mourey, T.H. and Chen, S.H.},
abstractNote = {The orientations of both a nematic liquid crystal fluid and a series of monodisperse glassy-nematic oligofluorenes were investigated on photoalignment films comprising a polymethacrylate backbone with 7-benzoyloxycoumarin pendants. Both classes of liquid crystalline material were found to undergo a transition from a parallel to a perpindicular orientation with reference to the polarization axis of UV-irradiation at a sufficiently high extent of dimerization.},
doi = {},
journal = {Macromolecules},
number = ,
volume = 39,
place = {United States},
year = {Mon Mar 19 00:00:00 EDT 2007},
month = {Mon Mar 19 00:00:00 EDT 2007}
}
  • Polymers containing 6- and 7-substituted coumarin moieties were prepared as photoalignment films through linearly polarized UV irradiation to a varying fluence for an investigation of liquid crystal orientation. Model coumarin monomers and dimers were also synthesized and characterized as part of a novel approach to the interpretation of liquid crystal orientation in terms of monomer conversion. The experimental results for monomer conversion as a function of fluence were used to validate the first-order kinetics with an exponentially decaying rate constant as the reaction proceeds. A kinetic model was constructed to describe the evolutions of the orientational order on the partsmore » of the reacted and the unreacted coumarin moieties. The model was instrumental to the visualization of liquid crystal orientation on photoalignment films at the early and late stages of dimerization. Furthermore, the observed crossover in liquid crystal orientation on the polymer film comprising 7-substituted coumarin moieties was successfully interpreted by considering three factors: the relative abundance of the reacted and unreacted coumarin moieties, the degrees of their orientational order predicted by the kinetic model, and the energetics of molecular interaction.« less
  • Orientation of a nematic liquid crystal, E-7, was investigated using coumarin-containing polymethacrylates to elucidate the roles played by the dilution of coumarin and the polarization ratio of irradiation. Dilution of coumarin by inert moieties had adverse effects on a nematic cell’s number density of disclinations and its orientational order parameter in the parallel but not the perpendicular regime. In addition, both dilution of coumarin and a decreasing polarization ratio resulted in a lower extent of coumarin dimerization at crossover, Xc. The significantly reduced Xc in a homopolymer comprising triphenylamine and coumarin was attributed to the dilution of coumarin and themore » diminished polarization ratio caused by competing absorption with simultaneous triplet energy transfer from triphenylamine to coumarin moieties.« less
  • Specular x-ray reflectivity probes morphological changes in a crosslinkable coumarin photoalignment polymer film resulting from ultraviolet irradiation. An ordered surface layer with density oscillations compatible with planar side-chain alignment is obtained before irradiation. The ordering is enhanced in the early stages of crosslinking. This is attributed to the photoinduced increase of mobility of the side-chains resulting from the creation of free volume by the crosslinking process. The expansion of the thin film confirms that free volume is created. The surface ordering decreases with prolonged ultraviolet irradiation because of increased material viscosity resulting from a high crosslinked density. The implications ofmore » surface ordering on liquid crystal photoalignment are discussed.« less
  • The photoalignment of a nematic fluid, E-7, and a glassy-nematic oligofluorene, F(MB)5, was investigated on films of Polymers 1 and 2 in the parallel regime. Polarized absorption spectroscopy and computational chemistry were employed to characterize coumarin monomer's and dimer's molar extinction coefficients and to locate absorption dipoles as parallel to their long molecular axes. Moreover, their orientational order parameters, S_m and S_d, were experimentally determined as functions of the extent of dimerization. Higher S_d and Y_d, coumarin dimer's mole fraction, were achieved in films of Polymer 1 than in Polymer 2 because of the greater coumarin mobility of the former.more » The ability of a coumarin-containing photoalignment film to orient a spin-cast F(MB)5 film was found to improve with increasing Y_d S_d to an extent comparable to that of a rubbed polyimide film. Because of the relatively short lengths of its constituent molecules, E-7 was oriented equally well on both polymer films regardless of the Y_d S_d values.« less
  • The first series of monodisperse glass-forming, nematic oligofluorenes was synthesized following a divergent-convergent approach. Both chain length and pendant structure were found to affect solid morphology and phase transition temperatures. With optimized structural parameters, a Tg close to 150C and a Tc beyond 375C were achieved. This material class is characterized by its ability to form monodomain glassy nematic films via spin-coating, thermal annealing at 10 to 20C above Tg for 15-30 min, and cooling to room temperature without encountering crystallization. The absorption and emission dichroic ratios of 80-90-nm-thick films increase from 7 to 17 with an increasing molecular aspectmore » ratio, while the photoluminescence quantum yield varies erratiexecy from 43 to 60%. Superior stability of emissive color and of emission dichroic ratio and that against thermally activated crystallization were also demonstrated through prolonged heating of a film at 10C above Tg under argon.« less