Releasing silica-confined macromolecular crystallization to enhance mechanical properties of polyimide/silica hybrid fibers

Polyimide (PI)/silica hybrid fibers containing 1% silica were prepared by the sol–gel method. Under tension-free thermal treatment at 300 °C, PI fibers and PI/silica hybrid fibers possess 0.98 GPa and 1.38 GPa respectively. However, it is found that the silica-reinforced effect is almost disappeared after the temperature of tension-free thermal treatment increasing to 420 °C. Wide angel X-ray diffraction (WAXD) patterns indicated that, by 420 °C tension-free treatment, PI fibers are reinforced due to crystallization of PI; PI/silica hybrid fibers are still amorphous since dispersed silica inhibits the crystallization of PI, which counteracts the silica reinforced effect. Through a hot-drawing process at 420 °C, PI fibers cannot obtain additional reinforcement, while tensile strength of hybrid fibers increases from 1.83 GPa to 2.34 GPa for crystallization. Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) tests revealed that the dispersed silica transforms into larger particles after hot-drawing, so the confinement of macromolecular crystallization is released for further reinforcing. As a result, the advantages of silica reinforcing in amorphous domain and macromolecules crystallizing are combined together, resulting in excellent mechanical properties of fibers.