Abstract. Alkali-treated sisal fibres were used as novel reinforcement to obtain composites with self-synthesized ureaformaldehyde resin as matrix phase. The composites were prepared by means of compression molding, and then the effects of sisal loading on mechanical properties such as impact strength, flexural strength, and wear resistance were investigated. In addition, water uptake was studied and structural features were revealed by the scanning electron microscopy (SEM). The composite with 30 wt% sisal fibres gives excellent flexural strength, water absorption, and especially the wear resistance showing that it has the most superior bonding and adhesion of all the composites. In particular, the highest value 9.42 kJ/m 2 of charpy impact strength is observed in the composite with 50 wt% sisal fibre. SEM micrographs of impact fractured and worn surfaces clearly demonstrate the interfacial adhesion between fibre and matrix. This work shows the potential of sisal fibre (SF) to improve the composite wear resistance and to be used in fibreboard.

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Polyimide (PI) films with simultaneously low dielectric loss (Df) values at high frequencies and excellent general properties are urgently needed with the rapid development of high frequency communication technology. Herein, we proposed that dielectric loss can be decreased by reducing intermolecular friction work. Inspired by liquid crystals with low intermolecular frictions, novel liquid-crystal-like PI films were designed by simultaneously introducing mesogen units and promoting groups (−CF3) and then being thermally imidized via a unique solvent-assisted continuous ramping process without any stretching or shearing process. The liquid-crystal-like PI films exhibited ultralow Df values such as 0.00365 at 1 MHz, 0.00184 at 10 GHz, and 0.0027 at 0.75THz, which were 28.7%, 45.4%, and 97.0% lower, respectively, than those of PI films without liquid-crystal-like structures. The results of molecule simulations further demonstrated that liquid-crystal-like structures can reduce intermolecular friction work by slowing dipole rotation, leading to ultralow Df values of PI films. The liquid-crystal-like PI films also exhibited thermal expansion coefficients of 11.4 ppm/K, tensile strengths of 228.7 MPa, Young’s moduli of 6.15 GPa, and elongations at break of 16.4%.

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