Chemistry Reference
In-Depth Information
Table 5.
X-ray reflections of polyimides from C
m
-OADA
Middle angle
(a)
Polymer
code
WAXS 2
θ
(degree)
d
(Å)
(b)
2
θ
(degree)
C
4
-PDA
-
-
15.0/18.3
4.35/8.59
(c)
C
8
-PDA
19.6/10.3
13.5/18.8/22.1
C
12
-PDA
3.28
26.9
18.5
C
4
-ODA
8.27
10.7
15.5/broad halo
C
8
-ODA
4.86
18.7
16.4
C
12
-ODA
3.34
24.5
15.5
C
4
-MDA
8.07(broad)
10.9
18.2
C
8
-MDA
4.57
19.3
18.8
C
12
-MDA
3.53
24.7
19.2
C
4
-FDA
8.15
10.8
11.5/13.8/15.7/17.8/22.9/24.8/28.3
4.71/8.26
18.7/10.7
17.8
C
8
-FDA
C
12
-FDA
3.48
25.4
18.4
5.86
15.1
15.0/17.9
C
4
-TOL
C
8
-TOL
5.46
16.6
18.4
C
12
-TOL
3.57
24.7
19.0
(a)
2
<
2
θ<
8,
(b)
Layer distance is calculated via the Bragg equation;
(c)
multiple numbers indicate dis-
tinctive peaks
obtained at various
temperatures. As shown in Figure 10, a broad endothermic peak appears in the
DSC thermogram of
Figure 11 depicts the X-ray diffractograms of
C
8
-FDA
. The diffractograms obtained at 175 and 220°C
show no significant change in their diffraction patterns except that their small an-
gle reflections at 4.71 degree are shifted to slightly smaller angle region. This
means that at higher temperatures, the layer spacing is increased because of the
thermal expansion of the polymer. The discernible X-ray reflections at 8.26 de-
gree and higher angle peaks disappeared above 260°C, indicating that it had
passed the mesophase transition [25]. But the remaining strong small-angle reflec-
tion indicates the layered-structure to be still present. In the cooling scans, the re-
flections were reproduced meaning that its transitions were reversible.
Moreover, a higher crystallinity of
C
8
-FDA
was observed, presumably because
of the strong dipolar interchain interaction of the hexafluoroisopropylidene groups
and this result agrees with its poor solubility.
C
4
-FDA
3.4. Liquid crystal (LC) alignment properties
The polyimides from
coated onto glass substrates were rubbed with
various rubbing densities (L/l) [28-30] using a laboratory rubbing machine
C
n
-PMDA
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