Polybenzobisthiazoles – Critical issues in their performance and properties - Polyimides and Other High Temperature Polymers - page 228

raised from room temperature to 200°C. Finally the reaction mixture was poured
into water and neutralized by ammonium hydroxide and washed thoroughly by
hot water. The polymer obtained was dried in vacuum at 110°C for 24 hr. The co-
polymers of PBZT using cyclohexane 1,4 dicarboxylic acid and terephthalic acid
were also obtained under similar conditions. PBZTs containing halogens (fluoro
and bromo groups) were synthesized by polycondensation of 2,5-diamino-l, 4-
benzenedithiol dihydrochloride with tetrafluorophthalic acid and 4-bromo-
isophthalic acid [22]. As reported in the literature, halogen containing aromatic
condensation polymers such as polycarbonates [23], polyformals [24], polyke-
tones [25], polyazomethines [26] and their copolymers have some unique proper-
ties such as good solubility in organic solvents, toughness and flexibility at low
temperature, water and oil repellency, low refractive index, and high thermal sta-
bility in addition to the prominent characterstics inherent in the corresponding
polymers containing no fluorine atom. Thus the introduction of fluoro and bromo
groups into the existing aromatic condensation polymers makes them soluble,
processable and gives them several additional characteristics. Solubility in some
solvents and thermal properties of these polymers have been studied.
Benzothiazoles have been prepared from isophthalic acid and 5-hydroxy
isophthalic acid ( with 2,5-diamino-1, 4-benzenedithiol dihydrochloride

[27]. It is reported that a high compressive strength could be achieved with lateral
chain supports such as an inter-digitation network formed by suitable side chains
at regular intervals along the rigid polymer backbone. An obvious approach to
rendering such lateral support at the molecular level is to cross-link the rigid-rod
polymer chains. Research along this line, using potentially cross-linkable units
such as methyl [28-31], fluorine [32] and benzocyclobutene [33] groups has been
reported. Cross-linking of PBZT fibers via high-energy radiation has also been
explored [34]. PBZT with a hydroxy functional group was selected because of its
ability to form hydrogen bonds, coordination bonds and to undergo some nucleo-
philic substitution reactions. It was later found to be relatively inert under the po-
lymerisation conditions required to form high molecular weight PBZT. As re-
ported by Tan et al . [35] the PBZT synthesized using 3,6- dihydroxy1, 4-
phenylene dicarboxylic acid (

VIII)

) had propensity to form a ladder structure, driven
by strong intramolecular hydrogen bonds between the hydroxyl moiety and the ni-
trogen atoms of the benzothiazole units. The ladder structure was similar to the
classical ladder polymer poly [(7-oxo-7, 1 OH benz [ de ] imidazo [4

IX

′ ,5 ′ :5, 6] ben-

zimidazo [2, 1-a] isoquinoline-3, 4:10, 11]-tetrayl)-10-carbonyl] (BBL) (X) .

3. SPECTROSCOPIC STUDIES

The FT-IR spectrum of monohydroxy PBZT did not show any strong and broad
band around 3300-3200 cm -1 where normally intermolecularly bonded -OH group
of phenol appears [36]. Instead, it showed weak and broad absorption bands in the
region 2700-3000 cm -1 that is not attributed to intermolecular hydrogen bonding