DETAILED DESCRIPTION OF THE INVENTION The polyimide used as an alignment film in the liquid crystal display device of the present invention is a polycondensation-imidization reaction product of an aromatic tetracarboxylic dianhydride component with an aromatic diamine component.
The aromatic tetracarboxylic dianhydride component comprises pyromellitic dianhydride as the major dianhydride ingredient.
The aromatic tetracarboxylic dianhydride component may contain, in addition to pyromellitic dianhydride, up to 40 mole %, preferably up to 20 mole %, of an additional tetracarboxylic dianhydride.
Tetracarboxylic dianhydrides which can be used in combination with the pyromellitic dianhydride include, but are not limited to, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride and oxydiphthalic dianhydride.
These tetracarboxylic dianhydrides may be used alone or in combination.
Fluorinated dianhydrides such as 2,2-bis(3,4-dicarboxyphenyl)-hexafluoropropanedianhydride, 9,9-bis(trifluoromethyl)-2,3,6,7-xanthenetetracarboxylic dianhydride, 9-phenyl-9-trifluoromethyl-2,3,6,7-xanthenetetracarboxylic dianhydride, cyclobutanetetracarboxylic dianhydride, 2,3,5-tricarboxycyclopentyl acetic acid dianhydride, and the like, may also be used but in amounts of less than 20 mole % due to the tendency of the fluorinated polyimides to give higher tilt angles.
When the tetracarboxylic dianhydride component contains less than 60 mole % of pyromellitic dianhydride, the tilt angle provided by the polyimide may be too high for use in normally black TN liquid crystal displays.
The stability of the tilt angle may also be adversely affected.
The aromatic diamine component comprises, as the major ingredient, an aromatic diamine having the formula ##STR6## wherein X is a divalent radical selected from the group consisting of ##STR7## The aromatic diamine component may contain, in addition to the major aromatic diamine, an additional aromatic diamine comprising not more than 50 mole %, preferably not more than 20 mole %, of other aromatic diamines
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