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The growing demand for non-toxic solvents for membrane preparation has motivated the studies for green and sustainable altatives of solvents. The effect of droplet isothermal growth within the liquid-liquid phase separation region on isothermal spherulitic growth rate of isotactic polypropylene (iPP) was investigated. The results showed that the droplets grew up at a rate of 0.0172 μm·s?1. The larger droplets slowed down the isothermal spherulitic growth rate ofiPP. Higher mass ratio of cauba wax (Cwax)/soybean oil (SO) resulted in faster droplet growth due to weak interaction with polymers. The isothermal crystallization behaviors ofiPP in environment-friendly binary diluents consisting of Cwax and SO mixture were further investigated experimentally using polarized optical microscopy. It was demonstrated that the isothermal spherulitic growth rate ofiPP in diluents decreased nonlinearly with the increasing crystallization temperature. Compared with virginiPP, isothermal spherulitic growth rate ofiPP in SO diluent was significantly slowed down. The spherulitic growth was further retarded after the addition of Cwax in mixed diluents, resulting in a lower crystallization rate than that in SO. Moreover, the crystal form ofiPP membranes was found to beα type through the characterization of small angle X-ray scattering and wide angle X-ray diffraction.