Fuk Kay Lee

Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films

Tangle-Free Polymer Dynamics The dynamics of polymer chains at interfaces will differ from the dynamics in the bulk. While hard interfaces generally cause chain motions to slow down, at free surfaces the dynamics should generally speed up. A consequence is that for thin polymer films, there should be a reduction in the glass transition temperature (Tg), but a wide range of effects have been seen in different polymeric materials. Now, Yang et al. (p. 1676) show that for short, unentangled polystyrene polymers, there is a direct correlation between the viscosity and the glass transition temperature, and that the reduction in Tg is due to a surface mobile layer that dominates the behavior as the films decrease in thickness. Thin films of short, unentangled polystyrene chains possess a highly mobile surface layer. Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (Tg). There is an ongoing debate on whether the Tg changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the Tg of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

Liquid crystal pretilt angle control using nanotextured surfaces

A scalable and economical method to control the pretilt alignment of nematic liquid crystal (LC) to ∼45° has long been sought as it constitutes the foundation of some key technologies of LC displays. We demonstrate that nanotextured surfaces fabricated by mixing horizontal and vertical polyimides allow complete LC pretilt control from ∼0° to 90°. Devices made with these surfaces show response times four times superior to the state-of-the-art panel.

Continuous liquid crystal pretilt control through textured substrates

Reliable control on the pretilt alignment of nematic liquid crystal (LC) in the 30°–50° range is a well-known challenge. An unconventional approach, involving microtextured surfaces with domains favoring dissimilar LC alignments, has recently demonstrated applicability in bi- and tristable displays. These textured domains realize the so-called frustrated boundary condition in which the LC elastic energy built-up (frustration) can drive the LC alignment into macroscopic uniformity. Here we show that one can harness the frustrated boundary to achieve variable LC pretilt control up to 40°.

Substrate patterning for liquid crystal alignment by optical interference

Inhomogeneous liquid crystal (LC) alignment surfaces comprising a succession of microdomains favoring different LC alignment directions have been demonstrated for a number of optoelectronic applications. However, the prevalent method used to fabricate these surfaces is time consuming and produce functional areas that are too small for practical use. Here, we demonstrate a simple method based on photopatterning of an azodye layer with an interference pattern produced by intercepting two coherent UV beams. This method can produce alignment patterns within seconds with a practical size of ∼(0.5cm)2.

Fabrication of mesoscopic devices using atomic force macroscopic electric field induced oxidation

We demonstrate the fabrication of mesoscopic devices on aluminum, titanium, and silicon-on-insulator thin films by using atomic force microscopic electric field induced oxidation together with selective wet etching. The fabricated device structure is a percolating network consisting of conducting dots (50–100 nm in diameter) randomly distributed within an area of 1×1 μm2. Details on how to fabricate the network structure and the making of electrical contacts to the device will be focused upon. Good agreement between results from transport measurement of an aluminum test sample we made and data from the literature warrants reliability of our sample fabrication technique.

Polarization-independent liquid crystal grating on azo-dye film fabricated through intensity holography

Diffraction phase gratings have found applications in many fields. In this letter the authors described how intensity holography was used to fabricate liquid crystal diffraction phase gratings on azo-dye films. The diffraction efficiency of this liquid crystal phase grating was determined to be independent of the polarization direction of incident light.