Temperature Dependent Photonic Bandgap in a Self-Assembled Hydrogen Bonded Liquid Crystalline Diblock Copolymer
self assembly, liquid crystal, photonic bandgap
A self-assembled hybrid material demonstrating a photonic bandgap in the visible wavelength regime can be tuned thermally or electrically. Hydrogen bonds between a host polymer and liquid-crystal-mesophase-forming moieties guest units to form a self-assembled 1D lamellar structure with a 175nm periodicity. It provides a stop-band in the green wavelength regime.
Results and Discussion
Hydrogen bonds induces the formation of a liquid-crystal mesophase in a block of a coil-coil diblock copoloymer host. Anionic synthetization of polysterene-block-poly(methylacrylicacid) (PS-b-MAA, Fig. 2) block-copolymer self-assembles. Changing the temperature by 50 degree C can shift the stop band by 10nm. The crystal consists of alternating layers of polystyrene (isotropic, n=1.59 @ VIS) and poly(methyacrylic acid-LC) (PMMA-LC, isotropic, n=1.45 @ VIS) while the PMMA-LC layers have asmectic structure. Imidazole mesogen is uniaxial with n=1.8 @ VIS. The indices and layer thicknesses are temperature dependent, but the index change will be larger if an electric field is applied (for the same values for the mesogen birefringence and volume fraction): Either the ordinary or extraordinary components of the mesogen's refractive index can be reversibly aligned parallel to the polarization of the incoming light (TE polarized light is not affected by this rotation) (Fig 1).PS-b-MAA forms hexagonally packed cylinders of poly(methylacrylic acid) embedded in the polystyrene layer. The PS-b-MAA films are homogeneous and no mesogen macrophase separated from the film as indicated by DSC and SAXS measurements (Fig.3), which also showed the liquid crystal nature of the block-copolymer over a certain temperature range.
A layering order of at least 6 was confirmed using SAXS (Fig. 6). Transmission spectra of incident white light filtered by a polarizer were taken for increasing temperature starting from room temperature onwards. A reversible redshift of 9nm was observed resulting in a higher reflectance. An irreversible redshift of up to 40nm was also observed for even higher temperatures (heating beyond the smectic-isotropic transition) as shown in Fig. 7. Simulation results on the angle dependence of the reflectance for both TE and TM polarized light are shown in Fig. 8. If the PC and the extraordinary index are matched the film acts like a polarizing filter (TE modes are stopped while TM modes may propagate).