| In general, polymers orient the nematic director parallel
to the substrate but do not induce uniform, reproducible
LC alignment. Polyimides became popular in the early
1980?s as LCD alignment layers, and their ease of use,
stability, and reproducible results have made them the
industry standard. Most polyimides are not easily dissolved
(gamma-butyrolactone is a common solvent) and do not
spincoat well. Because of this, they are usually supplied
in a precursor solution of polyamic acid and an organic
solvent such as NMP. These solutions are highly hygroscopic,
able to absorb several percent of their weight in water
within several hours; for this reason, they are typically
stored in tightly sealed containers in a freezer. After
spincoating, the films are imidized (cured) at 200-300·ãC
for one hour. In production settings, an offset printer
is often used for polyimide coatings. The advantages
of this are better materials utilization and ability
to mask off the gasket seal area. Film thicknesses are
typically 500-800 angstroms.
The cured films at this point have no preferred alignment
direction--rubbing the surface of the polyimide gives
it this direction. The cured films are rubbed with a
velvet cloth, which is usually wrapped around a rotating
drum. Alternatively, in a research or prototyping setting,
load rubbing can be used. In load rubbing, a flat weight
covered with velvet is drawn accross the substrate at
a uniform speed. The advantage of load rubbing is its
reproducibility and quantitative nature. Hard rubbing
is not necessary, and can lead to visible scratches
(due to scattering) in the completed cell. Rub strength
can be characterized by a pressure, but when a rub wheel
is used, it is often useful to discuss rub strength
in terms of millimeters of pile contact length. Nissan
provides data using this method.
Rubbing is still one of the least understood aspects
of LCD fabrication. It seems clear that there is both
a mechanical and a chemical component to rubbed alignment
in polyimides. Rubbing causes some grooving of the
surface, which is visible by AFM, and Uchida has shown
that a stamped, grooved epoxy will align liquid crystal.
However, purely mechanical models do not accurately
predict pretilts for rubbed alignment. John West has
investigated purely chemical alignment by irradiation
of cured polyimide films with polarized UV light (250
nm). This clearly gives a chemical anisotropy to the
film and results in good alignment.
Early polyimides gave planar alignment with a small
(1-3·ã) pretilt, but materials are now available that
give pretilts up to 40·ã, as well as homeotropic alignment.
However, obtaining reliable, reproducible pretilts
in the 10-40·ã range is not yet possible--these materials
are still in a preliminary stage. Nissan Chemical
is essentially the sole supplier of these specialty
polyimides, and the materials are very expensive (NOTE:
although the Nissan materials are the best available,
they are impractical to spincoat in a production setting--they
must be printed to be cost effective). Low pretilt
materials are available from numerous sources, including
DuPont, and are accordingly more affordable.
Polyimide chemical structures are not always available,
but some insight can be gained from pretilt studies.
Some crude classifications of the alignment mechanism
can be made based on the behavior of the pretilt with
increased rubbing strength. If pretilts are small,
but increase with rubbing strength, the main chain
structure is thought to be responsible for alignment.
If pretilts are larger, and decrease with rub strength,
side chains probably contribute to the pretilt. Japan
Synthetic Rubber (JSR) has published numerous papers
in which polyimides with known chemical structures
are investigated, but little is known about most commercially
available materials, especially those from Nissan.
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