**4. Conclusions**

PI fibers were shown to be difficult to dye with disperse and basic dyes. When treated with swelling agents such as MFA, PIP, and AP, the dyeability of the PI fibers could be significantly enhanced. It was found that swelling agents could penetrate into the PI fibers in amounts greater than 10% (pertaining to the weight of PI fibers). The swelling agen<sup>t</sup> penetrating into the PI fibers did not really swell the PI fibers, but broke the interaction forces between the PI macromolecules. When the swelling agents were used for dyeing of PI fibers, they preferentially penetrated into PI fibers and expanded the pore apertures of PI fibers so that the dye molecules readily diffused into the PI fibers. The swelling agents exhibited different performances in enhancing the dyeablity of PI fibers, and AP was most effective, followed by MFA and PIP. The PI fibers dyed with the assistance of the swelling agents exhibited good colorfastness to washing, tested by color change. The dyed PI fibers' colorfastness to washing tested by color stain was moderate, which was mainly dependent on the dyes but not the swelling agents. Therefore, using a swelling agen<sup>t</sup> would be an applicable approach for enhancing the dyeability of PI fibers.

**Author Contributions:** Conceptualization, D.S.; validation, J.D. and H.W.; formal analysis, D.S. and C.X.; investigation, D.S.; resources, J.D. and H.W.; writing—original draft preparation, D.S.; writing—review and editing, D.S. and C.X.; funding acquisition, C.X., J.D, and H.W.

**Funding:** This work was supported by the National First-class Discipline Program of Light Industry Technology and Engineering under Grant LITE2018-21 (China) and the 111 Project under Grant B17021 (China).

**Conflicts of Interest:** The authors declare no conflict of interest.
