*2.1. Materials*

Polypropylene (PP), as a polymer matrix, was obtained from Japan Polypropylene Corporation, Tokyo, Japan (Polypropylene FY-6, MFR 2.5 g/10 min, CrI 59.80%). It has melting point of 162 ◦C, crystallization temperature of 104 ◦C and degradation temperature of 461 ◦C. Cellulose nanofibers (5 wt% aqueous dispersion, DP 650) were provided from Sugino Machine Limited, Toyama, Japan. The CNFs were produced by a super high-pressure water jet system. SiP powders, Sylosphere 200 (SS, diameter 3.0 μm) and Sylophobic 200 (SP, diameter 3.9 μm) were supplied from Fuji Silysia Chemical, Aichi, Japan. The ethanol (>99.5%) was purchased from Wako Pure Chemical Industries Ltd., Osaka, Japan, and reverse osmosis (RO) water was used throughout the experiments.
