Chemistry Reference
In-Depth Information
Zhou and Lenz (1983) and Zhou
et al
. (1985) discussed the substitution
effect from the point of view of steric and polar effects. However, from above
discussions one sees a more intricate picture. A full understanding of the
effect demands further study. Nevertheless, substitution is so effective that
a lot of studies have used this concept in the molecular engineering of liquid
crystalline polymers in order to have desired phase properties.
Back to copolymerization. Even though such monomers as
4-hydroxybenzoic acid (HBA), monomer pairs as 1,4-hydroquinone (HQ)
and terephthalic acid (TA) or 4,4
-dihydroxybiphenyl (BP) and terephthalic
acid can result in only unmeltable rigid polymers, their copolymerizations
have resulted in meltable polymers. The melting points of the HBA-HQ-
TA copolymers were reported to be around 400 (Inai
et al
., 1982), that of
the HBA-BP-TA was also only 380
◦
C-420
◦
C. Based on the latter together
with isophthalic acid, the products “Xydar” and “Ekonol” have been com-
mercialized as high strength high modulus and flame resistant materials.
The heat deflection temperature (HDT) of these materials is in the range of
317
◦
C-355
◦
C (at 1.82 MPa). The excellent mechanical and thermal proper-
ties have qualified these thermotropic liquid crystalline aromatic polyesters
for advanced engineering materials (Suenaga, 1990).
Another famous type of liquid crystalline aromatic polyesters are
those copolymers based on 6-hydroxy-2-naphthoic acid (HNA) and 4-
hydroxybenzoic acid (HBA). These polymers have been commercialized
by Hoechst-Celanese as “Vectra”. The incorporation of 2,6-naphthalylene
moieties brings into the polymer chains with “side-steps” and with the
crank-shaft movements (Figures 3.2 and 3.3). As mentioned in Section 3.2.2,
this type of structural modification will result in less chain rigidity. Calun-
dann (1979) had a systematic study on the relationship of property and the
ratio of HNA/HBA in the copolymers prepared from the melt polyconden-
sations of 4-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid. The result
is shown in Table 3.7. Although the homopolymers of these two monomers
are both crystalline with melting points over 400
◦
C, the copolyesters all
have a much lower melting point and can be processed by melt spinning at
about 310
◦
C.
Table 3.7.
Melting point of HNA/HBA copolyesters
∗
.
HNA/HBA
0/100
25/75
30/70
40/60
50/50
60/40
100/0
m.p.
◦
C
∼
600
302
275
245
260
263
∼
440
∗
Source: Calundann (1979).
