Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units II. Thermal stability and degradation behaviour

The influences of controlled structural differences and molecular weight on the thermal stability and degradation behaviour of a series of novel wholly aromatic polyamide-hydrazides have been investigated in nitrogen and in air using differential scanning calorimetry (DSC), thermogravimetric analysis (TG), infrared spectrophotometry (IR) and elemental analysis. The structural variations of the polymers were achieved by varying the content of the meta- and para-substituted phenylene moieties incorporated into their chains. All the polymers were synthesized by a low temperature solution polycondensation reaction of either 4-amino-3-hydroxybenzhydrazide [4A3HBH] or 3-amino-4-hydroxybenzhydrazide [3A4HBH] with an equimolar amount of either terephthaloyl chloride [TCl], isophthaloyl chloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous N,N-dimethylacetamide [DMAc] as a solvent at -10 degreesC. The content of para- and meta-phenylene moieties was varied within this series so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the polymers have the same structural formula except for the way of linking phenylene units in the polymer chain. The results showed that these polymers have high resistance to elevated temperatures. Their weight loss occurred in three steps. The first was small and was attributed to evaporation of adsorbed surface water. The second was considerable and was assigned to cyclodehydration reactions of the polyamide-bydrazides into the corresponding poly(1,3,4-oxadiazolyl-benzoxazoles) with loss of water. This is not a true degradation, but rather a thermochemical transformation reaction. The third was relatively severe and steep, particularly in air, and corresponded to the decomposition of the polymers. The results clearly indicate that substitution of para-phenylene units for meta-phenylene ones within this polymer series leads to improved polymer stability at elevated temperatures in nitrogen as well as in air. This should be associated with regularity of supermolecular packing within the bulk of the investigated polymers wherein the colinear arrangement of the para-phenylene units should allow for establishment of stronger intermolecular bonds which would be more difficult to break and therefore more resistant to high temperatures. Moreover, polyamide-hydrazides having different molecular weights were also examined. The results clearly reveal that at all temperatures used and in both degradation atmospheres all the investigated samples exhibited similar thermal behaviour regardless of their molecular weights, except in the temperature range 160-200 degreesC where the lower molecular weight samples showed significant weight losses which may be attributed to hydrogen bonded DMAc. This indicates that structural building units of these polymers (which contained characteristic groups, such as: aromatic rings and amide and hydrazide linking bonds in the case of polyamide-hydrazides and aromatic nuclei, 1,3,4-oxadiazolyl rings and benzoxazolyl moieties in the case of poly(1,3,4-oxadiazolyl-benzoxazoles) are responsible for their high thermal stability, rather than the longer chain segments. (C) 2002 Elsevier Science Ltd. All rights reserved.