*2.1. Materials*

PI fibers in double yarns (29.2 tex) were provided by Aoshen New Material Inc. (Jiangsu, China). C.I. Disperse Red 153 (DR 153), C.I. Disperse Blue 60 (DB 60), C.I. Basic Red 46 (BR 46) and C.I. Basic Blue 41 (BB 41) were used for dyeing of PI fibers, and provided by DyStar (Shanghai, China). MFA, PIP, and AP were used as swelling agents, and provided by Sigma-Aldrich (Shanghai, China). Figure 1 shows the chemical structures of PI fibers, dyes and swelling agents. Soaping agent, hydrosulphite (Na2S2O4) and sodium hydroxide (NaOH) were used as auxiliaries for soaping and reduction clearing of the dyed PI fibers.

**Figure 1.** Chemical structures of Polyimide (PI) fibers, dyes and swelling agents.
