Fluorinated copolyimides for microelectronics applications - Polyimides and Other High Temperature Polymers - page 446

bing [8-10] and non-rubbing methods [11-12]. Of these, the mechanical rubbing
has been the commonly accepted pathway in the LCD industry for high volume
production. The pretilt angle of the liquid crystal is one of the key parameters in
an LCD device, which is closely related to the device performance. For instance,
a high pretilt angle, which could prevent the creation of reverse tilt disclination in
LCDs, is usually required to prevent strip domains in super-twisted nematic LCDs
(STN-LCDs) and to properly operate surface-stabilized ferroelectric LCDs
(SSFLCDs) [13]. Hence, the generation and control of high pretilt angles in LCD
devices have been investigated by many researchers [14-17].
Polyimide coatings rubbed mechanically with cotton have been widely em-
ployed as liquid crystal alignment layers in LCD manufacturing due to their uni-
formity and durability. However, conventional polyimides used in LCDs have
some drawbacks, such as poor processability and low pretilt angle. With the rapid
development of advanced LCD devices with high density and high performance,
the improvement in polyimide features as alignment layers becomes a critical fac-
tor. Many attempts have been made to increase the pretilt angle of the nematic
liquid crystals (NLCs) on a polyimide surface. It was found that polyimide with
long alkyl side chains could generate high pretilt angles by mechanical rubbing
[18-21]. However, a polyimide containing alkyl side chains tends to be hydropho-
bic, which reduces the wettability of liquid crystal [22], and its thermal property,
to some extent, is sacrificed. Recently, high pretilt angles of liquid crystals on
rubbed polyimide with helical backbone segments and trifluoromethyl moieties
were reported [23-26]. And polyimides with trifluoromethyl substituted phenyl or
biphenyl groups on the backbone were also reported to show high pretilt angles
for liquid crystals [22].
A series of aromatic diamines with trifluoromethyl substituted phenyl groups
were synthesized, which were employed to polymerize with an aromatic dianhy-
dride, 4,4'-oxydiphthalic anhydride (ODPA), and another aromatic diamine, 4,4'-
oxydianiline (ODA), to afford fluorinated copolyimides. The thermal and me-
chanical properties of the copolyimides were investigated, and the effects of the
chemical structures of polymers on the pretilt angles for liquid crystals are also
discussed.
2. EXPERIMENTAL
2.1. Materials
4,4'-Oxydiphthalic anhydride (ODPA, Shanghai Chemspec. Corp.) was recrystal-
lized from acetic anhydride prior to use. 4,4'-oxydianiline (ODA) (Beijing
Chemical Reagents Corp.) was recrystallized from ethanol prior to use. Commer-
cially available N -methyl- 2 -pyrrolidinone (NMP), m -cresol, N,N' -dimethylfor-
mamide (DMF) and N,N' -dimethylacetamide (DMAc) were purified by vacuum
distillation over CaH 2 prior to use.