To date, the main method in improving UV stability of aramid fibers is to coat ultraviolet screening agent on the surface of fibers. However, this method has a disadvantage that the coating tends to fall off when the fibers are exposed to an external force. In the present research, A diamine monomer 2-(4-aminophenyl)-5-aminobenzimidazole (PABZ) was introduced to modify poly (p-phenylene terephthalamide) by copolymerization, and corresponding modified aramid fibers (PBIA fibers) were prepared. we found that PBIA fibers can form complex with hydrogen chloride (HCl) which would not lead to the obvious decrease in mechanical properties, and the decomplexation of HCl can only be achieved at high temperature (higher than 280 °C). At the same time, PBIA/HCl complex molecule showed intense fluorescence emission, and which can provide an effective way to dissipate the harmful UV energy, thus, the UV resistance of PBIA fibers can be improved. In addition, the HCl have the ability to quench the photoactive triplet state of PBIA fibers, which can also contribute to improve the UV resistance of PBIA fibers. After 48 h UV irradiation, the PBIA/HCl complex fibers almost remained their tensile performances (only about 3% loss in tensile strength), and UV irradiation did not bring the obvious deterioration in structure and performance of the complex fibers. On the contrary, for PBIA fibers, its chemical structure and physical properties all suffered to obvious damages (about 15% loss in tensile strength). The UV resistance of PBIA/HCl complex fibers is approximately 5 times higher than that of PBIA fibers.

A novel vulcanization method for crude fluoroelastomer by direct fluorination with fluorine/nitrogen gas has been investigated. The results show that the vulcanization reaction of fluoroelastomer is closely related with fluorination temperature, fluorination time and fluorine gas partial pressure. The maximum crosslink degree can be up to 97%, and the fluorine content of fluoroelastomer increased from 48.2% to 60% during the fluorination. The static friction coefficient of fluoroelastomer is decreased from 0.91 to 0.55, which is about 39.6% reduction after fluorination. The ATR-FTIR spectra indicate the crosslink reaction process of fluoroelastomer by direct fluorination, which arises from three reaction stages and successively goes through four elementary reactions: substitution reaction; elimination reaction; addition reaction; crosslink reaction. The increase of fluorine content takes place mainly in the first stage, and the crosslink reaction takes place mainly in the second stage and third stage.

Nonspherical polymer particles have attracted increasing attention recently. In this paper, micron-scale hemispherical polyimide (PI) particles were fabricated using water-soluble poly(amic acid) ammonium salts (PAAS) by a novel inverse emulsion technique. In the process, liquid paraffin was used as a continuous phase, the mixed solution of PAAS and water as a dispersed phase and sorbitan monooleate (Span80) as a surfactant. The research suggested that water as a stabilizing agent played an important role in forming stable emulsion. As the amount of water increased, stability of the emulsion increased gradually and morphology of PI particles transformed from sphere to ellipsoid, and finally to hemisphere. The concentration of PAAS solution and Span80 both affected the shape of particles, which changed from spherical to hemispherical by increasing the PAAS/Span80 concentration. The mechanism of forming hemispherical PI particles was discussed based on interfacial tension and interfacial free energy changes. Via adjusting the composition of the system to change the corresponding interfacial tension, we could get the particles with different morphologies. Furthermore, the change in structure characterized by FT-IR spectroscopy demonstrated that PAAS had been converted to PI after adding the dehydrating agent to the emulsion. And TGA results showed that the obtained PI particles had excellent thermal stability.

In this paper, series of polyimide films with different aggregation structures were prepared through a partially pre-imidization process, and the properties were investigated in detail. The pre-imidization degree (pre-ID) was successfully controlled by adjusting the amount of dehydrating reagents, which was confirmed by FT-IR spectra measurement. PI films exhibited a quite different degree of crystallinity, which increased with pre-ID increasing. DSC curves showed obvious melt endothermic peaks at about 450 °C. The tensile strength, tensile moduli and elongation at break of the PI films increased obviously with pre-ID increasing. The corresponding values of PI-100 increased by 37.9%, 11.7% and 336%, respectively, compared with those of PI-0. TMA measurement showed that the coefficient of thermal expansion of PI films decreased with pre-ID increasing, indicating that the PI molecular chains were restricted by the crystalline structure. Meanwhile, those PI films obtained by the pre-imidization process still possess excellent thermal stability. The present work provided the theoretic indicators for controllable preparing of high-performance polyimide films.Graphical abstract