Anchoring properties of a nematic liquid crystal on anisotropic hydroxypropylcellulose films

Abstract

Thin solid films of hydroxypropylcellulose (∼15–30 µm) prepared from liquid crystalline and isotropic aqueous solutions are used as liquid crystal alignment layers. Using the standard nematic liquid crystal 5CB we measured the interface properties of these solid films as a function of the polymer concentration in the aqueous precursor solution, expressed in terms of zenithal and azimuthal anchoring orientations and extrapolation lengths. The hydroxypropylcellulose thin films are found to induce a planar orientation of 5CB independently of the polymer concentration, with the alignment along the polymer backbone. The zenithal anchoring strength is found to be strong and essentially independent of the temperature far from the nematic–isotropic transition, with an extrapolation length ξ_θ≈50 nm. The zenithal anchoring becomes weaker near the nematic–isotropic transition, as expected. The azimuthal anchoring strength is found to be intermediately weak and strongly dependent on the polymer concentration, with an extrapolation length varying from ξ_θ≈250 nm to ξ_ϕ≈500 nm. These films are particularly interesting since their surface topography and morphology may be tuned by varying a few parameters in the film preparation process, such as the polymer concentration in the aqueous solution.

Acknowledgments

This work was supported in part by projects POCTI/33453/CTM/2000, POCTI/CTM/56382/2004 – FEDER. J. G. F. thanks the Portuguese Science Foundation for the Scholarship PRAXIS XXI BPD/5633/2001.