Question

What is PVP alignment to get photoalignement?

Answer 1

Single-layer thin alignment films of dye molecules are of growing importance, particularly for state-of-the-art LCD technology. Here we show that a sequential process involving the photoalignment and humidification of a chromonic liquid crystalline azobenzene (brilliant yellow; BY) dispersed in a triacetyl cellulose (TAC) matrix gives a thin alignment film with an exceptionally high order parameter (0.81). Spectroscopic and X-ray diffraction analyses of a BY/TAC composite film in each alignment process revealed that brief humidification triggers restructuring of the BY assembly from 1D nematic-like order to anisotropic 2D columnar order, resulting in the dramatic increase in the order parameter.

Using spherical domain block copolymers for nanolithographic patterning requires an understanding of islands or holes in thin films whose thicknesses, including a brush layer at the substrate, do not correspond to a single layer of spheres.

In our system of poly(styrene-b-2vinylpyridine) (PS-PVP) that microphase separates into PVP spheres in a PS matrix, an initial lm thickness of less than 40 nm gives rise to holes through a single sphere layer revealing a PS-PVP brush on a at silicon oxide substrate.

A thickness is greater than 55 nm result in islands of spheres on top of the single sphere layer. Both islands and holes in a block copolymer lm on a smooth substrate coarsen with annealing time at 180 C, driven by the edge line tension of these structures.

If a lm of uniform thickness is deposited on a grating with mesas 30 nm high ( 1 sphere layer thickness) from 1 to 10 mm wide, separated by wells of the same width, the island/hole evolution is quite divergent than on a at substrate. Islands on the mesa or in the center of the well eventually disappear, with these spheres being added to stripes of roughly uniform width in the well along its edges, effectively extending the width of the sphere layer on the mesa.

In contrast, a lm initially with holes evolves to produce a uniform single layer of spheres in the well and a stripe of spheres in the center of the mesa surrounded by brush the same height as the single sphere layer in the well.

The edges of these single sphere layer thick stripes lead to a graph epitaxial ordering of the layer of spheres into a single crystal in much the same way as conferment by the edges of the mesas or wells.

Polymer brushes have shown promise as surface tethered lubricants, and it has been demonstrated that solvent quality strongly influences the frictional forces between two sliding brush layers.

It is known that brush layers immersed in a good solvent must be compressed well below their equilibrium height before a frictional force is measured.

Conversely, brushes immersed in a solvent at its theta temperature exhibit a frictional force at much lower levels of compression. We used the surface forces apparatus to measure the structural and frictional force poles between opposing, sliding brush layers as a function of temperature.

Three divergent polyvinyl pyridine-polystyrene [PVP-PS] diblock copolymers were used to make PS brushes. The molecular weights (in thousands) of these PVP-PS materials were [114/103]k, [30/70]k, and [60/100]k.

Structural and frictional force poles in toluene and cyclohexane were measured, with the cyclohexane experiments being done at temperatures ranging from the theta-point to 50 C.

We found that the structural force poles agree with well-known behaviors.

The frictional forces depended strongly on the solvent and the solvent temperature: PS brushes in toluene need to be compressed to 1/6 their equilibrium height before frictional forces are measured, but this onset of frictional force is detected at a much lower level of compression in near-theta cyclohexane.

When the cyclohexane temperature is raised the onset of frictional forces decreases toward the good-solvent behavior. At 50 C, the onset of frictional forces occurs at a comparable distance to that in toluene, which is a very good solvent for PS.

We will compare this behavior in cyclohexane at various temperatures to the behavior of the brushes in another theta-solvent.