Fluorinated copolyimides for microelectronics applications - Polyimides and Other High Temperature Polymers - page 451

350 nm, and the transmittance at 450 nm is >80% for most of the fluorine-containing
copolyimides. Obviously, the high transparency is attributed to the presence of fluo-
rine-containing groups in the polymer backbone. The copolyimides exhibited low
water uptake values in the range of 0.39-0.61%, which might be attributed to the
presence of hydrophobic trifluoromethyl substituents in the polymer backbone. PI-1
showed the lowest value, whereas PI-6 had the highest one.

3.2. Thermal properties
Table 1 shows the thermal properties of the copolyimides including onset decom-
position temperature (T d ), temperature at 5% weight loss (T 5% ) and at 10% weight
loss (T 10% ), and the glass transition temperature (T g ). The copolyimides showed
different thermal decomposition behaviors (Figure 3). A two-stage decomposition
is observed for PI-2 to PI-5 and PI-7, while a one-stage behavior is observed for
PI-1, PI-6 and PI-8. In general, the first stage thermal decomposition started at
441.1-481.0°C and the second stage at 596.1-651.4°C for PI-2 to PI-5 and PI-7.
The two-stage decomposition behavior might be explained by the presence of
weak bonding (-N=N-, -NH- or -CONH-) between the side chain substituents and
polymer backbone. The weakly bonded side subsituents would decompose first in
the heating process. The glass transition temperatures are located between 232.4-
288.9°C as determined by DSC (Figure 4) and 215.1-273.6°C as determined by
DMA (Figure 5). For instance, PI-5 has a Tg value of 264.6°C as determined by
DSC and 258.5°C as determined by DMA. The decomposition temperatures at
5% and 10% original weight losses are in the ranges of 476.9-579.9°C and 523.1-
621.0°C, respectively. And the residual weights of the copolyimides determined
by TGA at 750°C in nitrogen are in the range of 46%-61%. Clearly, the copoly-
imides exhibited high thermal stability.

3.3. Mechanical and electrical properties
Table 2 shows the mechanical and electrical properties of the copolyimides. Thin
films (6.35 mm x 127 mm x 22-24 m) were tested for tensile properties in an In-

stron universal testing machine (UTM) at 25°C according to GB10410-79. The
copolyimide films have tensile strengths of 98.6-137.8 MPa, elongation at break
of 8-16%, and tensile modulus of 0.94-1.29 GPa. The surface resistance (R s ) and
volume resistance (R v ) are 1.8-7.2 x 10 14 Ω and 1.2-7.2 x 10 15 Ω·cm, respectively,
indicating that the polyimide films have good electrical insulation properties.

µ

3.4. Pretilt angles, θ p

Usually, nematic liquid crystal molecules can be aligned on a mechanically
rubbed polyimide coating surface along the rubbing direction. TFT-AM-LCDs
require that the liquid crystals should have a low value of pretilt angle (1º-5º),
however, the liquid crystals in super-twisted nematic LCD (STN-LCD) devices
must have higher pretilt angles (4-30º), preferable between 5 and 15° [34]. Table 3