2.1 Introduction
Polyimide （PI） fibers refer to a type of polymeric materials containing imide rings in macromolecular chains. The highly conjugated structure has endowed it with outstanding mechanical properties， relatively high thermal stability， good solvent resistance， and excellent light stability， which give it greater advantages compared with some other high-tech polymeric fibers when employed in severe conditions， possessing a wide application prospect in areas of aerospace and environmental conservation ［1-4］.
In the 1960s， the cutting-edge lab at Dupont and the relative research institution from the former Soviet Union began researching PI fibers ［5］. However， no quick popularization and applications of PI fibers were obtained due to the immaturity of PI synthesis and spinning technology， as well as the high cost of the fiber production. Afterwards， Rhône Poulenc France developed a type of poly（amide-imide） fiber based on m-aromatic polyamide， and Kermel France used the brand name Kermel for commercial development ［6］. Nowadays， the Kermel-Tech poly（amide-imide） fiber is designed and developed for the specific requirements on temperature and chemical reactions. The long-term working temperature of this aromatic fiber reaches as high as 220 °C， and the highest acceptable temperature is close to 240 °C. The glass transition temperature （Tg） is 340 °C， at present， and it has been widely used in the high-temperature filtration area for energy production and various manufacturing industries as it can maintain excellent mechanical proper H2O ties at extremely high working temperatures.
In the 1980s， Lenzing AG Austria launched a novel type of PI fiber with the brand name of P84， which was produced from toluene diisocyanate， diphenylmethane-diisocyanate， and 3，3′，4，4′-benzophenonetetracarboxylic dianhydride （BTDA） ［7］. The P84 PI fibers possess an irregular foliaceous cross-section， increasing the surface area by 80% compared with the normal round cross-section. To date， the P84 PI fiber has been extensively used in high-temperature filtration areas ［8］.
In recent years， the production technology of PI fiber in China has been developing rapidly. In the 1960s， a small amount of PI fibers was first produced in the Shanghai Synthetic Fiber Research Institute. The PI fibers were used for the radiation protection cladding of cables， radiation-resistant strings， etc. However， large-scale production of PI fibers was not achieved. Since the 2000s， based on the unique overall performance of PI fiber and requirements in some special areas， many scientists in Chinese universities and research institutions have begun to investigate PI fibers again. Currently， there are several companies in China focusing on the production technology of advanced PI fibers， including Changchun Polyimide Materials Co.， Ltd， Jiangsu Aoshen Hi-tech Materials Co.， Ltd， and Jiangsu Shino New Materials & Technology Co.， Ltd， etc. A series of commercially available PI fibers including the thermal-resistant PI fibers and high-performance PI fibers have been produced using different production techniques. Fig. 2.1 shows photographs of typical commercial PI fibers， which can be used in the areas of environmental protection， aerospace， sophisticated weapons， and personal protection， etc.
FIGURE 2.1 Relative product photos of Superlon polyimide fibers （from left to right： short-cut polyimide fibers， filaments， and color yarns）.