Orientational ordering of 4-pentyl-4′-cyanobiphenyl molecules evaporated on multi-layered polyimide film Dai Taguchi, Takashi Kawate, Ryo Miyazawa, Martin Weis, Takaaki Manaka, Mitsumasa Iwamoto ⁎ Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, Japan abstract article info Available online 24 September 2008 Keywords: Orientational ordering Interface Langmuir–Blodgett film Using optical second-harmonic (SH) generation and polarized absorption (PA) measurements, the orientational ordering process of 4-pentyl-4′-cyanobiphenyl (5CB) was studied during evaporation onto the alignment layer of polyimide (PI) Langmuir–Blodgett films. The intensity of SH signal increased in the beginning of the deposition, but it saturated after the first 5CB monolayer was formed. The PA measurement suggested that a layer-by-layer growth was induced after the formation of 5CB monolayer with accom- panying the orientational ordering. The orientational order of the first 5CB monolayer was studied in terms of the orientational order parameters, S 1 = 〈cosθ〉 and S 2 = 〈(3cos 2 θ − 1)/2〉, using the SHG and PA measurements. It was found that S 1 decreased as the thickness of PI alignment layer increased from 0.4 to 4.4 nm, while S 2 was nearly constant. These results indicate that the Coulomb interaction between the permanent dipole of 5CB and its image dipole makes a significant contribution to the orientational ordering of deposited 5CBs. © 2008 Elsevier B.V. All rights reserved. 1. Introduction It is well known that orientational distribution of director n ˆ in liquid crystal (LC) cells is controlled by using the alignment layer. Homogeneous and homeotropic alignments are yielded using so- called rubbing technique. As reviewed by Jérôme [1], a variety of alignment mechanisms that account for the orientational director distribution, e.g., microgroove theory, have been known so far. On the other hand, many anchoring energy expressions have been proposed to describe the boundary condition, assuming nematic order n ˆ = − n ˆ is established at the interface. Rapini and Papoular expressed the anchoring energy in a quadratic form − A∕2cos 2 Δθ where A is the anchoring energy coefficient, and Δθ is the director deviation from the easy axis. However, the nematic order is broken in the region near the alignment layer in actual LC systems. The enhancement of optical second-harmonic generation signals from this region clearly shows this fact [2]. The polar head of LC molecules is attracted to the align- ment layer, suggesting the presence of polar ordering of LCs at the interface. Nevertheless, this effect is overlooked in the expression of anchoring energies. In order to understand anchoring of LC molecules with taking into account the broken of nematic order at the interface, we studied the orientational ordering process of LCs at the interface by using an evaporation technique. We used orientational order parameter (OP) defined as S n = 〈P n (cosθ)〉 [3,4] where P n is the nth Legendre polynomial, θ is the tilt angle of molecular long-axis from surface normal direction, and 〈〉 is the thermodynamic average. A set of OPs precisely describes ordering of LCs at the interface. S 1 and S 2 represent the polar and nematic orders of LCs at the interface, respectively. Using the in-situ observation of S 1 and S 2 , we could show that the S 1 of cyanobiphenyl LC decreased as the polyimide alignment layer thickness increased from 0.4 to 4.4 nm [5]. We then analyzed the S 1 dependence, with taking into consideration the Coulomb interaction between polar LC molecules and its induced image dipole (μ–μ i interaction). However we are still not confident whether this model is acceptable or not. In the present study, for further understanding of orientational ordering process of LC molecules at the interface, S 1 and S 2 of 5CB molecules deposited on the PILB alignment layer is determined using the optical second-harmonic generation (SHG)and the polarized absorption (PA) measurements. S 2 was nearly constant while S 1 de- creased as the PILB thickness increased. It was found the results well supported our previous proposed model [5] concerning the ordering process of LCs at the interface. 2. Experimental A Kapton-type polyimide alignment layer was yielded on a silica plate in the manner same as described in Ref. [6]. A monolayer of polyamic acid (PAA) on the water surface, compressed up to a surface pressure of π = 30 mN/m, was deposited onto a synthetic silica plate by the LB technique. Here the silica plate was used after washed with aceton, ethyl alcohol, distilled water, and cleaned with a UV/ozone cleaning apparatus for removing organic residuals. N-layer PAA LB Thin Solid Films 517 (2008) 1407–1410 ⁎ Corresponding author. Tel./fax: +81 3 5734 2191. E-mail address: iwamoto@pe.titech.ac.jp (M. Iwamoto). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.054 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf