Polybenzobisthiazoles – Critical issues in their performance and properties - Polyimides and Other High Temperature Polymers - page 231

4.2. Thermal properties
Among all the high temperature, high performance polymers, the polybenzobis-
thiazole rigid-rod polymers are one of the most thermally stable systems known.
A variety of thermal techniques have been used to investigate their thermal sta-
bility such as thermogravimetric analysis (TGA), thermogravimetric mass spec-
trometry (TG-MS) and isothermal aging studies. Rigid rod polymers have shown
good thermal stability because of their aromatic backbone, rigid molecular
chains and high order. PBZT fibres, in particular, have excellent thermal behav-
iour. Figure 1 shows the thermogravimetric analysis (TGA) of PBZT, PBZI, and
PBZO polymers [41]. The early weight loss exhibited by these materials from
ambient temperature to approx 200°C is the result of water loss. The degradation
temperature of PBZT in air is around 620°C. PBZT fibres do not melt below the
decomposi-tion temperature and do not support combustion but will char at high
temperature. They generate only low quantities of smoke on burning [42]. The
thermogravimetric mass spectra in Figure 2 show the evolution of different gases
as PBZT was heated to high temperature [43]. The main gases evolved are H 2 S,
HCN and CS 2 , which are the characteristic degradation products of polymer
backbone. It was reported that the total weight loss of 28% at 1000°C was con-
sistent with the loss of 1 mol of H 2 S, 1 mol of HCN and 0.25 mol of CS 2 per mol
of PBZT repeat unit [44].
The thermal stability of various PBZT derivatives synthesized in our laboratory
is comparable to that of parent PBZT containing para phenylene unit. The TGA
analysis was performed in a nitrogen atmosphere at 10°C/min. The thermal stabil-
ity of various PBZT derivatives synthesised in our laboratory is summarized in
Table 2.
Figure 1. TGA of PBZT, PBZO and PBZI.