2,2‘-Bis(trifluoromethyl)-4,4‘,5,5‘-biphenyltetracarboxylic
dianhydride (6FBPDA) was synthesized from 1-iodo-4,5-dimethyl-2-nitrobenzene via a six-step
synthetic route. The dianhydride was
polymerized with eight different substituted 4,4‘-diaminobiphenyls in
refluxing m-cresol containing
isoquinoline to afford a series of fluorinated aromatic polyimides.
The polyimides were soluble in polar
aprotic, ether and ketone solvents. They had intrinsic viscosities
that ranged from 1.70 to 6.72 dL/g in
N-methyl-2-pyrrolidinone or m-cresol at 30 °C.
The polymers underwent 5% weight losses when
subjected
to thermal gravimetric analysis between 440 and 570 °C in air and
nitrogen atmospheres. The polymers
could be solution cast into water-white, flexible, tough films which
exhibited “in-plane” structural
anisotropy. Their glass transition temperatures along the
directions parallel to the film surface (in-plane) ranged from 327 to 345 °C (thermal mechanical analysis).
Multiple relaxation processes associated
with segmental and subsegmental motions were also observed with dynamic
mechanical analysis. The
films had in-plane coefficients of thermal expansion (CTEs) that ranged
from 1.58 × 10-5 to 2.50 ×
10-5
°C-1. These films also showed linear
optical anisotropy, which is characterized by the presence of
larger,
isotropic refractive index (n
∥) in-plane, and
a smaller refractive index (n
⊥) perpendicular
to the film surface
(out-of-plane). The optical symmetry axis of the films is along
the out-of-plane direction. This optical
anisotropy is defined as uniaxial negative birefingence (NUB =
n
⊥ − n
∥).
Films having a thickness of 5
μm were transparent above 330 nm, and their in-plane refractive
indices (n
∥) were 0.060 to 0.074
larger
than their out-of-plane refractive indices
(n
⊥). This UNB make the films candidates
for use as retardation
layers in liquid crystal displays.