Effect of quenching temperatures on the morphological and crystalline properties of PVDF and PVDF-TiO₂ hybrid membranes

Effect of quenching temperature on the morphological and crystalline properties of PVDF and PVDF-TiO₂ hybrid membranes was studied. PVDF and PVDF-TiO₂ hybrid membranes were prepared via thermally induced phase separation (TIPS). The membranes were characterized by scanning electron microscopy, differential scanning calorimetry and X-ray diffraction. Results showed the blends underwent solid-liquid (S-L) phase separation when quenching temperature was below onset crystallization temperature and the size of spherulities and interspherulite microvoids increased with the increase of the quenching temperature. Spherulities became more regular at higher quenching temperature. The crystallinity (X_c) of neat PVDF membranes decreased with the increase of quenching temperatures while the X_c of hybrid membranes decreased first, then increased. Spherulities in hybrid PVDF membranes were more uniform and smaller than those in neat PVDF membrane quenched at 0°C because of heterogeneous nucleation of TiO₂ particles. And spherulities in hybrid PVDF membranes were larger than those in neat PVDF membrane quenched at the same temperature when the quenching temperature was higher than 60°C. TiO₂ particles were easy to be separated from PVDF at high quenching temperature. The added TiO₂ and the increase of quenching temperature restrained the growth of (020) plane of PVDF. The crystal type (α) was not affected by the change of quenching temperatures and the addition of TiO₂. The non-isothermal crystallization process was studied by Ozawa and Jeziorny approach. The results showed the crystallization rate of PVDF increased with the increase of cooling rate. The addition of TiO₂ could accelerate the crystallization rate.