添加剂对PVDF相转化过程及膜孔结构的影响

研究了PVP、PEG及LiCl 3种成孔添加剂下PVDF DMAc H2 O 添加剂体系的成膜机理 .无论那种添加剂的铸膜液相转换成膜过程中都存在凝胶分相和液液分相两种相变方式 ,在 30～ 6 0℃时凝胶分相在较低的非溶剂浓度下先于液液分相发生 ,LiCl作为添加剂较PEG、PVP对铸膜液有较强的致凝胶作用 ,成膜过程中凝胶分相段时间依PVP、PEG、LiCl的顺序延长 ,导致液液分相初始分相点处聚合物浓度增大 ,阻止了大孔结构的充分发展 .制得的膜依PVP、PEG、LiCl的顺序有效孔隙率和通量降低 ,结晶度升高 .以LiCl为添加剂制得的膜几乎不改变PVDF膜的疏水性 ,而以PVP或PEG为添加剂的膜隔水压差降低约 2 0kPa .

INFLUENCE OF ADDITIVES ON PHASE SEPARATION PROCESS OF PVDF SOLUTION AND MEMBRANE MORPHOLOGY

The membrane formation mechanism of PVDF/DMAc/H 2O/Additive system was studied. The additive was PVP, PEG or LiCl respectively. The phase diagram for polymer solution of PVDF/DMAc/Water/Additive system was measured using cloud point method. The phase separation phenomena during membrane formation process were recorded using light transmission experiment. The crystallinity of membrane was detected by DSC. The result shown both gelation and liquid liquid demixing existed during phase inversion process of casting solution with different additive. Gelation appeared early at lower nonsolvent concentration than liquid liquid demixing in the range of 30～60℃. LiCl as additive lead to a strong action for PVDF solution to form gel than PEG or PVP. This increased the mass transfer resistance in nascent membrane and the liquid liquid demixing delayed time prolonged along with the sequence of PVP, PEG or LiCl as additive during membrane formation process. The polymer concentration of nascent membrane at the onset of liquid liquid demixing increased and restricted the development of macrovoids. The strong gelation tendency of polymer solution intensified the solidification of nascent membrane and restricted the macrovoids development further. Thus the macrovoids structure extended to the bottom and stopped with the sponge like structure when PVP was used as additive. When the additive was PEG or LiCl, the macrovoids developed insufficiently and terminated in the middle of membrane with the sponge like structure and then passed to nodular structure in bottom.The effective porosity and flux decreased and crystallinity increased along with the sequence of PVP, PEG or LiCl as additive. The hydrophobicity of membrane with LiCl as additive was barely changed, but the water resisting pressure of membrane with PVP or PEG as additive decreased about 20 kPa.