Chemistry Reference
In-Depth Information
though a rod-like shape has persisted. The formability of a liquid crystal
phase and the phase transition temperatures of the homologous compounds
of poly(1,4-phenylene) are characterized and shown in Table 3.1.
Two conclusions may be drawn from the data in Table 3.1. First, only
the homologues with su
ciently large values of length-to-diameter ratio
can form a liquid crystal phase. Secondly, both the melting point and the
clearing temperature of the liquid crystal phase increase very fast with
an increasing degree of polymerization (n + 2). The compound with only
six 1,4-phenylene units and an axial ratio of 5.5 has a melting point of
438
◦
C and a clearing temperature of 565
◦
C, which is already too high a
temperature for organic molecules to survive. It is therefore not practical
to synthesize liquid crystal forming polymers by the simple linking-together
of rigid structures into rod-like molecules, even if certain flexible elements
are present.
By inserting in between every two phenylene rings a linking unit with a
certain flexibility, such as an ester group, the flexibility of the molecules will
be increased. For example, if an ester group is used as the linking unit, it
will not only allow the crankshaft type movements in the molecule, but also
serve as a separation between the phenylene rings and thus reduce the hin-
drance for a free rotation of the rings. As a result, poly(4-hydroxybenzoic
acid), PHA, with the following formula does have much lower transition
temperatures than poly-phenylenes as shown in Table 3.2. Specifically, the
compound with n = 3 has a melting point of 220
◦
C and a clearing temper-
ature of 464
◦
C. The calculated axial ratio
l/d
is 6.4. In comparison, this
compound has slightly higher axial ratio but much lower transition tem-
peratures than the poly(1,4-phenylene) with n + 2 = 6 (
l/d
=5
.
5, m.p.
Table 3.1. The Axial ratio and Phase Transition Temperature of
Poly(1,4-phenylene)s
∗
.
Melt. Point,
◦
C
Clearing Point,
◦
C
n + 2
Axial Ratio,
l/d
1
1.0
5.4
non-liquid-crystalline
2
2.0
70.5
non-liquid-crystalline
3
3.0
213
non-liquid-crystalline
4
3.9
320
non-liquid-crystalline
5
4.8
388
418
6
5.5
438
565
∗
Source: Irvine and Flory, 1984.
