Chemistry Reference
In-Depth Information
temperatures below the glass transition are roughly the same is an indication that
the modulus of the rubbery material (i.e., for T > T
g
) increases with increasing hy-
droxyl content. The origin of this behavior in unclear, but suggests that some in-
terchain hydrogen bonding begins to play a distinct role above T
g
.
3.3. Dielectric analysis
The dielectric constants for all the polymers at 100 Hz, 10 kHz and 1 MHz are
listed in Table 2, with values at 1 MHz ranging from 2.09 to 2.95. The frequency
dependencies are typical for all of the polymers, i.e., a small monotonic decrease
in
ε
as a function of increasing frequency. The dielectric constants for polymers
5
and
are exceptionally low for non-foamed cast polymers, making them desirable
candidates to be evaluated for the high temperature processing of high-speed inte-
grated circuits.
Other constituents being equal, the polymers or copolymers with the largest
fraction of 6F units should have the lowest dielectric constant. This was observed
to hold true by comparing the dielectric constants of polymers
6
1
and
2
. For poly-
mers
). The correct trend is pre-
sent overall, but there is some deviation from the perfect order, probably due to
small measurement errors or variations among sample properties (e.g., density).
The dielectric constant of polymer
3-6
, the order should be
ε
(
6
)
<
ε
(
5
)
<
ε
(
4
)
<
ε
(
3
9
would be expected to be smaller than poly-
mer
, but it is not. Again this may be caused by small measurement errors or
sample variability. However, another factor which changes with composition is
the interchain conformation whose effect upon dielectric constant is complex, a
subject to be discussed in a future publication.
The effect of the OH groups upon dielectric constant is most clearly ascertained
by comparing polymers
7
. Polymers with two OH
units have a higher dielectric constant than those with one OH unit. On the other
hand, the polymers with one OH unit have a lower dielectric constant than those
having no OH units. The possession of two OH units produces a planar segment
with seven consecutive fused rings, while the single OH-containing segment
would have four fused rings. The greater planarity of the seven-ring system would
allow closer packing of the polymer chains, reducing the free volume and increas-
ing the dielectric constant. The reason for the lower dielectric constants for poly-
mers containing one OH group compared to those with no OH groups is not un-
derstood at this point.
The optical frequency dependence and anisotropy of the dielectric constant of
polymer
2
,
6
, and
9
and polymers
3
and
7
were investigated by prism coupling, and the results are presented in
Table 3. The out-of-plane dielectric constant at the 632.8 nm wavelength is larger
than the dielectric constant at 1152 nm, and both are larger than the out-of-plane
impedance results. The wavelength dependence is to be expected because of the
dispersion associated with the HOMO-LUMO absorption in the UV. The dielectric
anisotropy is common to very thin cast polymer films and is caused by preferential
alignment to the first deposited molecules parallel to the casting dish surface [3].
6
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