Yang-Ming Zhu

Atomic force microscopic study of rubbed polyimide films

Rubbed polyimide films have been studied by atomic force microscopy from micrometer to nanometer scales. On a large scale, oriented scratches and microstructures due to rubbing have been observed, while on a nanometer scale, oriented polyimide aggregates are visible. The alignment of liquid crystals on these films is thus discussed.

Liquid-crystal alignment of rubbed polyimide films: A microscopic investigation

Rubbed polyimide films were investigated by Atomic Force Microscopy (AFM). On a large scale, microgrooves due to rubbing treatment were observed, whilst on a small scale, polyimide chain molecules were found to be non-uniformly oriented. Liquid-crystal mono-layers were adopted in this study as visualizing media for AFM observation.

Liquid crystal alignment on polyimide Langmuir–Blodgett films prepared on a newly designed Langmuir trough using a flowing subphase

Nematic and smectic liquid crystals are homogeneously aligned on polyimide Langmuir–Blodgett films prepared by horizontal lifting on a novel Langmuir trough using a flowing subphase. Atomic force microscopy observation of the polymer monolayer deposited on graphite reveals that the polymer chains are elongated and aligned in the flow direction. Thus the alignment may act through the orientation of polyimide molecules.

Liquid crystal alignment on polymer liquid crystal Langmuir-Blodgett films

Abstract Stable polymer liquid crystal Langmuir-Blodgett films are prepared to align nematic liquid crystals homeotropically. The surface-pressure-independent alignment property is studied, and the alignment mechanism is also discussed.

Liquid-crystal alignment of helical polymer chains

Abstract Polymethylmethacrylate (PMMA) langmuir-Blodgett (LB) films can homogeneously align the nematic liquid crystal. Atomic force microscopy of PMMA LB films, reveals a helical chain structure, based on which the alignment mechanism is discussed. However, no capability of alignment could be observed on rubbed PMMA films, which is probably due to the easy fracture of PMMA chains under rubbing.

Characterization of the monolayer transition of a double-armed crown ether liquid crystal by liquid crystal alignment

Abstract A double-armed crown ether liquid crystal is spread at the air-water interface to form a Langmuir monolayer. The surface pressure-molecular area isotherms indicate that the monolayer undergoes a phase transition when the surface pressure increases, which, by a molecular model, can be ascribed to a change of molecular conformation. The monolayers are deposited on solid substrates to align liquid crystals before and after the transition, and homogeneous and homeotropic alignments can be produced respectively. This further confirms the phase diagram.

Liquid crystal alignment on polymer Langmuir-Blodgett films: the side chain effects

Abstract By virtue of the flexibility of the Langmuir-Blodgett (LB) technique we prepare LB films of polyamic acid alkylamine salt with different structures and use them as alignment layers for liquid crystals. The alignment ability depends upon the controllability of the structure. Also the side chain effects are discussed.

An automatic Langmuir-Blodgett system based on flowing subphase compression

Abstract An automatic system for continuously fabricating Langmuir-Blodgett (LB) films will be described, in which a monolayer on an air-water interface is compressed distributively underneath by a steady laminar flow subphase. The main features of this system are the following. (1) Homogeneous compression to monolayers with a very high surface viscosity, such as polymeric monolayers, can be obtained. (2) Continuous deposition of LB films on a substrate with a large area can be made. The experimental result with the atomic force microscope shows that polymeric chains in a monolayer can be straightened on the flowing water surface. The system is particularly useful in industrial applications for the fabrication of a large quantity of LB films.

Investigation of the Alignment Mechanism of Polyimide Langmuir-Blodgett Films by Monte Carlo Simulation

Abstract Ordered chain orientation of polyimide in Langmuir-Blodgett films is necessary for liquid crystal alignment. In this paper the influence of dipping process on the polyimide chain direction is studied by Monte Carlo method. It is found that the oriented polyimide chain is caused by dipping process and its orientation is almost parallel to the dipping direction. The linearity of the chain depends upon the dipping velocity, which is reinforced by fractal dimension analysis.

Observation of elongated and aligned polymer chains in a two-dimensional system by atomic force microscopy

Abstract Elongational effects on polymer chains in the polyamic acid alkylanine salt monolayer on the air-water interface are studied. Polymer chains elongated by a flow transferred onto graphite are observed by atomic force microscopy. We found that when the polymers are compressed to a condensed state by the elongational flow their chains are aligned in the flow direction.