Rubbed polymer surfaces

In this Section we will introduce the principles and experimental details of NEXAFS spectroscopy. The focus of the discussion will be on showing that this technique has all the capabilities required to address the origin of liquid crystal alignment on rubbed polymer surfaces. 

The third material, MES, behaves differently. In this case the adsorbed LC film does show an anisotropy (Fig. 5.4b) the 5CB molecules follow the rubbing direction. The surface alignment is characterized by Qa 0.3 and 0 w 71°. The value of the surface in-plane order parameter is in the lowest range of the usual values for rubbed polymer surfaces Qa 0.3 — 0.5 [29,35,36]. The observed tilt angle is the same as for the other silanes and comparable with alignment angles of cyano-biphenyl monolayers usually found on substrates that induce a planar macroscopic alignment [15,22]. 

For a rubbed polymer surface we note that there are two principal, orthogonal planes perpendicular to the film surface. These are the planes oriented parallel and perpendicular to the rubbing direction (see Fig. 6.3A and B). The first one, which we will refer to as parallel plane, clearly possesses mirror symmetry, while the mirror symmetry of the plane perpendicular to the rubbing direction, referred to as perpendicular plane, may be broken by the directional nature of the rubbing process. Hence, the most general expression for the polarization dependence on rotation of the electric field vector within... 

Rubbed polymer surfaces have been investigated also by others using NEXAFS spectroscopy. See, e.g., references [4,14-16]. 

To quantitatively analyze the molecular distribution within the rubbed polymer surfaces we have recorded absorption spectra for a series of electric field vector orientations within the two principal planes perpendicular to the film surface. These are the previously introduced planes parallel x-z) and perpendicular (y-z) to the rubbing direction. The polarization dependence is summarized by the intensity of the tt resonance in the normalized AEY spectra, which are plotted versus the photon incidence angle a as soUd and open symbols in Fig. 6.7. The solid lines are the result of a fit to the data using (6.3). 

Molecular Anisotropy and Liquid Crystal Alignment on Rubbed Polymer Surfaces... 

Fig. 6.9. The origin of liquid crystal alignment on rubbed polymer surfaces (A) Liquid crystal molecules have highly anisotropic charge distributions with their tt system preferentially oriented perpendicular to their long axis, which gives an ensemble of oriented (nematic) liquid crystals an asymmetric charge distribution (B). This can be characterized by molecular orientation factors (fa,fb,fc) describing the preferential orientation of the tt system. For the experimentally observed alignment directions the anisotropic charge distribution of the liquid crystal is oriented parallel to the one of the rubbed polyimide (C) and polystyrene (D) surface, which optimizes their interaction energy [3].

Summarizing the AFM observations, fine grooves similar to the rubbing fibre structure, and nanoscale undulations in the background texture are observed on the rubbed polymer surfaces. 

It is well known that the main reason for liquid crystalline alignment on rubbed polymer surfaces is the anisotropic characteristics of the stretched polymer surfaces, although it is also due to microgroove structures [1]. 

To avoid the formation of surface walls and to provide a fast switching ofF time, the anchoring energy should be sufficiently high, e.g. > 10 " J/m, comparable with that of the rubbed polymer surface. This is not really observed for polyvinyl-cinnamate (PVCN) derivatives [27-29]. The recent measurements also confirm that 10 to 10 J/m, i.e. about one to two orders of mapitude smaller than the required value [1]. The in-plane sliding mode (IPSL) was proposed in a... 

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