Abstract
The subject of the title was investigated. The higher crystalline fibers hot-drawn at temperature above 400°C had poor long-term thermal stability (290°C), regardless of imidization methods of the precursor fibers, presumably as a result of the physical deterioration which was accompanied by increase in crystallinity caused by the heat treatment. The lower crystalline fibers which were chemically imidized and drawn 1.1-1.2 times at 290°C had moderate tensile properties and excellent heat durability; i.e., percents of both tenacity and elongation retained after 65h at 290°C in air were about 100%.
