Preparation of novel low-kappa polyimide fibers with simultaneously excellent mechanical properties, UV-resistance and surface activity using chemically bonded hyperbranched polysiloxane

High-strength and high-modulus fibers with low dielectric constants (low-kappa) play an important role in newly-developed wave-transparent composites. Herein, a method was developed to prepare novel low-kappa polyimide fibers with simultaneously ultrahigh mechanical properties, excellent UV-resistance and surface activity using chemically bonded hyperbranched polysiloxane. The dielectric constant of PI/HBPSi composites conspicuously decreases with increasing HBPSi loadings, namely, from 3.6 of pure PI to 2.6 of the composite containing 20 wt% HBPSi, along with a dielectric loss decreasing from 0.047 to 0.01 at 1 GHz, mainly owing to the dielectric confinement effect of the HBPSi and increased free volume of polymer chains. These functionalized nanoparticles were further proved to effectively enhance the mechanical properties of the PI fibers, leading to a maximum 19% increase in the tensile strength (3.44 GPa, ca. 27.3 g per denier) and a 37% increase in the modulus (115.2 GPa, ca. 914.2 g per denier) relative to the pure PI fiber, which are better than those of typical high strength fiber-Kevlar 49. Attributed to the potential crosslinking reaction between carbonyl groups in polyimide chains and methyl groups in HBPSi under UV irradiation, the PI/HBPSi composite fibers retain (94-96)% tensile strengths after exposure to a UV light source for 168 h, and meanwhile, the moduli of the irradiated fibers increase by (2-23)% compared to un-irradiated fibers, showing excellent UV resistances. Additionally, the increased roughness and introduced amino groups of the fiber surface produce higher interfacial adhesion interactions between the composite fibers and the epoxy matrix. The single-fiber pullout test indicates that the composite fiber containing 20 wt% HBPSi shows a 30% increase in interfacial shear strength. Accordingly, such multiscale reinforced composite fibers show great potential to be likely applied in the new generation of stronger and more durable wave-transparent composites.