Polyimide(PI) fiber as a popular material with considerable development potential is widely used in the preparation of advanced composite materials. However, low surface energy and smooth surface of PI fiber lead to poor interfacial adhesion between the fiber and matrix, which seriously affects the performance of the composites. To overcome the above problems, alkali treatment was often used for surface modification of PI fiber. In this paper, PI fiber was modified by aqueous sodium hydroxide(NaOH) solutions. PI fibers were treated at temperature (30 ±1°C) with four different concentrations of NaOH solutions: 5, 10, 15, and 20% w/w for 2 min. The treated PI fibers were repeatedly washed with deionized water, dried in an oven at 60 °C for 12 hours, and placed in a sealed bag for use. The effects of alkali treatment on the mechanical properties, linear density, and thermal properties of PI fiber were analyzed. At the same time, the influence of modification on the interfacial shear strength of PI fiber was investigated by the single fiber fragmentation test(SFFT). After the alkali treatment, the mechanical properties and fineness of the PI fiber decreased slightly, but the thermal stability was almost unchanged. SEM observation showed that the impurities on the surface of PI fibers were removed after NaOH treatment, and there were corrosion spots or even pits on the surface of PI fibers with different degrees and random distribution. The best treatment effect was 10% concentration. The chemical structure of PI fibers before and after NaOH modification was analyzed by FT-IR. It was found that the modification only occurred on the surface of the fibers due to the small degree of hydrolysis of the imide ring.
And the interfacial shear strength of the PI fiber treated with 10% NaOH solution increased from 20.14 MPa to
23.74 MPa. Excessive concentration will decrease the interfacial adhesion performance of PI fiber, so attention
should be paid to the control of treatment conditions.