Aromatic Polyamide-Imides and Aromatic Polyester-Imides
The aromatic polyamide-imides are related to the aromatic polyamides described in the previous section. Aliphatic materials of this type were reported originally in 1947. They were formed by reacting tricarboxylic acids with diamines [83]:
The aliphatic polyamide-imides prepared to date don’t have desirable properties. When aromatic diacids are employed, however, the products exhibit good heat stability and toughness. This led to a development of a number of useful materials. Three general methods are employed to form aromatic polyamide-imides [88]. The first one consists of an initial reaction of a mole of a diacid chloride with two moles of a diamine. The product is then reacted with a dianhydride and after that condensed to an imide:
In the second method, a dianhydride is prereacted with an excess of a diamine. The product is then reacted with a diacid chloride by interfacial polymerization technique. In the third method, an anhydride, like, for instance, trimellitic anhydride, is condensed with a diamine to form a preliminary condensate. An acetylated diamine can be used in this initial condensation [85]:
This is followed by a reaction with the same or a different diamine:
The same techniques are applied to preparations of polyester-imides. A diester can be formed first from trimellitic anhydride:
The product is then condensed with a diamine to form a polyester-imide:
Several polyamide-imides are available commercially. Two are based on trimellitic anhydride and methylene dianiline. They are, however, prepared by two different processes. In the first one, the polymer is formed from the anhydride and a diamine:
In the second one, trimellitic anhydride is reacted with a diisocyanate:
Another polyamide-imide is formed through a reaction of trimellitic anhydride, isophthalic acid, and diisocyanate. It has the following structure:
Most polyamide-imides are not as heat-resistant as are the polyimides discussed in the next section. They are, however, easier to process. The polyester-imides might be considered as “upgraded” polyesters, though properties vary, depending upon chemical structure [86].
