Molecular Orientation at Rubbed Polyimide Surfaces Determined with X-ray Absorption Spectroscopy:  Relevance for Liquid Crystal Alignment

Abstract

The molecular orientation at the surface of polyimide (PI) alignment layers used for liquid crystal displays has been determined using X-ray absorption spectroscopy (NEXAFS). The second moments of the orientation distribution function obtained from a detailed analysis reveal that, upon rubbing, the PI substrates develop biaxiality and an inclination of a few degrees. For a nematic liquid crystal (LC) in contact with these alignment layers, the pretilt angle was determined and revealed a weak correlation with buffing strength.

Liquid crystal alignment by rubbed polymer surfaces: a microscopic bond orientation model

“…All of them showed the same asymmetries with a larger p intensity along y than x and a larger intensity for negative than positive values of u . This asymmetry is also consistent with measurements on polyimides by others [16,18]. In contrast, for polystyrene the p intensity is larger along x than y and it is larger for positive than negative values of u.…”

“…These studies also showed the preferential near-surface alignment of polyimide chain segments along the rubbing direction [17] and the decay of the alignment from the surface toward the bulk of the film. NEXAFS studies also gave clear evidence for a preferential outof-plane tilt of phenyl rings at polyimide surfaces [15,16,18,19]. This tilt was linked with the pretilt angle of the LC on the surface.…”

“…In this model the LC rods are envisioned to align parallel to the polymer chain segments in the crystalline regions. Surface sensitive studies have also been carried out using the near edge X-ray absorption fine structure (NEXAFS) technique [15][16][17][18][19]. These studies also showed the preferential near-surface alignment of polyimide chain segments along the rubbing direction [17] and the decay of the alignment from the surface toward the bulk of the film.…”

“…This tilt was linked with the pretilt angle of the LC on the surface. Some of the NEXAFS studies utilized partially ordered (semi-crystalline) polyimides like BPDA-PDA [17] and PMDA-ODA [18] and the suggested LC alignment models implicitly assumed the presence of ordered surface regions giving rise to epitaxial effects. These studies did not address the known fact that LC alignment also occurs on surfaces of disordered polymers.…”

Solvent Effects on Chain Orientation and Interchain π‐Interaction in Conjugated Polymer Thin Films: Direct Measurements of the Air and Substrate Interfaces by Near‐Edge X‐ray Absorption Spectroscopy

“…Although X-ray diffraction techniques have been used extensively as a structural probe, they are not particularly sensitive to these interfaces. Near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), on the other hand, is sensitive to surface orientation and has been applied to investigate polymer structures, for example, in polyimide, [1] polyfluorene, [2] and polythiophene.[3] Here we show that NEXAFS band shapes and intensities are in fact also sensitive to the intermolecular p-interaction in regioregular poly(3-hexylthiophene) (rreg-P3HT) films. NEXAFS spectra of the same material deposited from different solvents and processing conditions are substantially different.…”