【作者】 陆茵；

【导师】 李伯耿； 陈欢林；

【作者基本信息】 浙江大学 ， 化学工程， 2003， 博士

【摘要】 本文研究了干湿相分离法制备聚偏氟乙烯(polyvinylidene fluoride)微孔膜时的成膜机理和各种因素对膜结构的影响。 在实验研究的基础上，提出了动态分相、浓相固化浓度线、预凝胶结构等新概念，对湿法相转换成膜机理和制膜规律作了研究。认为在制膜时，任一铸膜液微分区内分相是和传质过程同时进行，互相影响的一个动态过程；动态分相时由成核-生长机理、或旋节分相机理形成的浓稀相结构仅仅是膜的预凝胶结构，该预凝胶结构在界面张力、膨胀应力等外力作用下，聚合物浓相和聚合物稀相的形态会发生长大、聚并、变形等粗化过程；只有当浓相组成越过浓相固化浓度线后，当时当地的凝胶结构才固定下来，形成最终的膜结构。 研究了PVDF溶液在不同状态下的力学性能和相分离特性。实验表明成膜过程中，使聚合物浓相固化的原因是由结晶导致的凝胶，而非玻璃态化；降低温度、提高非溶剂浓度或聚合物浓度是聚合物溶液发生结晶性液固相分离的三个关键因素；实验发现结晶性液固分相按照成核-长大机理发生，当溶液状态由于上述三个参数的变化进入液固相分离区后，聚合物结晶首先以分散相形式产生，然后以这些结晶核为中心，聚合物结晶逐渐长大，最后互相联接形成网络状支撑结构，溶液转换成凝胶而发生固化；在凝胶点处从流动态转换成凝胶的过程大致需要3～4分钟，具体的凝胶点与将发生凝胶固化的溶液量有关，溶液量越多，用于形成网络状支撑结构的固态聚合物结晶需要量越多，要形成凝胶需要更低的温度、更高的非溶剂浓度或更高的聚合物浓度。 研究了添加剂对相分离过程的影响，浊点法测定了各添加剂时不同温度下PVDF溶液体系的相图，采用自制光透射仪测定了成膜时的相分离过程，发现添加PVP、PEG、LiCl的PVDF铸膜液体系在成膜过程中都存在凝胶分相和液液分相两种相变方式，在30～60℃区间凝胶分相在较低的非溶剂浓度下先于液液分相发生，升高温度使凝胶分相趋向在较高的非溶剂浓度下发生，LiCl作为添加剂较PEG、PVP对铸膜液有较强的致凝胶作用；凝胶分相段时间依PVP、PEG、LiCl的顺序延长，这导致液液分相初始分相点处聚合物浓度增大，添加LiCl溶液的强凝胶性能加剧了初生态膜的固化，阻止了大孔结构的充分发展。因此，添加PVP的膜大孔发展充摘要分，终止于膜底部的海绵状结构，而添加PEG 1 000或LICI的膜大孔终止于中部的海绵状结构，并进而在底部成为瘤节状结构，制得的膜依PvP、PEG、LICI的顺序有效孔隙率和通量降低，结晶度升高。 以L iCI为添加剂制得的膜几乎不改变PVDF膜的.疏水性，而以四P或即G为添加剂的膜隔水压差降低约0.Zbar(以0.1微米孔径的膜孔为例)。膜反面的疏水性能优于正面。 研究进一步发现对PVDF一DMAc一PvP一H必铸膜液体系，较低的铸膜液温度和凝胶浴温度有利于增加膜的有效孔隙率，同时减小了平均孔径;在铸膜液中加入非溶剂是提高膜性能的一种手段，根据本文中关于相图的测定和制膜实验结果，在铸膜液中加入6.swt%的非溶剂水能得到有较好性能的膜;未加非溶剂的铸膜液随溶剂的蒸发不会发生凝胶分相，所以膜性能受蒸发时间的影响较小，而加入非溶剂的铸膜液受蒸发时间的影响较显著;随蒸发时间的增加，平均孔径增加，有效孔隙率减小，所以制膜时较短的蒸发时间有利于得到高性能的膜。随聚合物浓度的减小，膜有效孔隙率和平均孔径逐渐增加，在制平板膜时可以选择较低聚合物浓度的铸膜液以得到高性能的膜。 在对制膜规律研究的基础上，制得PVDF微孔膜，泡点法测定孔径，最大孔径为0.63。。，平均孔径0.3。。，在lat。下水通量为7.7ml/mi。·。mZ，NZ通量为1 .12/min .emZ。 本文最后建立了PvDF中空纤维膜湿法相转换成膜过程的传质模型，模拟成膜过程中湿态膜内组成随时间和位置的变化情况。并定量地提出了初始分柑点处溶剂浓度分界线的概念，认为大孔发展的条件是:当初生态膜中非溶剂浓度达到初始分相点时，相应位置上的溶剂浓度应大于一定的分界值，此时大孔能够发展，几当初始分相点处溶剂浓度小于此分界值时，大孔停止发展，形成海绵状结构。在不同工艺条件下用PvDF(l8%)一PEG10oo(5%)一DMAc(77%踌膜液纺制了中空纤维膜，并模拟了相应条件下湿态膜内组成变化情况。模拟结果表明成膜时的工艺参数如凝胶浴组成、膜厚等会影响液态膜中各个位置上初始分相点处的溶剂浓度，相应条件下制得的膜电镜照片显示在初始分相点处溶剂浓度大于分界线的位置上为大孔结构，初始分相点浓度小于分界线的位置上为海绵状结构，模拟结果与膜电镜照片有很好的对应关系，完满地解释了不同制膜条件下膜结构的变化规律和浙江大学博士学位论文原因，证明了上述大孔形成机理的正确性。 本文的主要特色与创新:1.提出了预凝胶结构的概念，指出液液分相仅形成膜预凝胶结构，只有当聚合物 浓相固化后膜结构才定形。研究了使聚合物浓相固化的原因，测定了不同组成 的聚合物溶液的固化速度，证明在PVDF成膜过程中，结晶性液固相分离所致 凝胶导致聚合物浓相固化和膜更多还原

【Abstract】 In this paper the polyvinylidene difluoride (PVDF) membrane formation mechanism during dry-wet phase separation process was studied, and the effect of techanical parameters on membrane morphological structure and property was discussed.Some new concepts such as ’dynamic phase separation’, ’sodification line of polymer-rich phase’, ’pre-gelation morphological structure’ were proposed. The membrane formation mechanism stands on these concepts. It was considered that the mass transfer process and the phase separation process went along simultaneously in any differential area of casting solution and changed its composition and mechanical property. The morphological structure of polymer-rich and polymer-lean phase produced by this dynamic phase separation was an unsteady pre-gelation structure. The polymer-rich and polymer-lean phase would grow, aggregate or deform on the effect of interfacial tension or swell stress of polymer-lean phase. Only when the composition of polymer-rich phase crossed over the solidification line the pre-gelation sructure changed no more. The membrane morphological structure was then formed.The mechanical and phase separation property of PVDF solution were reserached. It was crystalline solid-liquid phase separation gelation, not vitrification, caused the polymer-rich phase solidification. Temperature decreasing, non-solvent or polymer concentration increasing are the three factors to trigger off gelation. The crystalline solid-liquid phase separation happened under nucleation and growth mechanism. The dispersed polymer crystalline nuclei grown and connected with each other to form the supported network and the polymer solution transformed to gelation and solidified. At the gelation point the transformed process from fluid to solid arised in about 3 to 4 minute. The gelation point of polymer solution is influenced by its amount. The greater the polymer solution amount, the lower the temperature and the higher the nonsolvent and polymer concentration at gelation point.The influence of additive on phase separation 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. It was found that 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-liquiddemixing in the range of 30~60℃. Increasing temperature would increasing thenonsolvent concentration at gelation point. LiCl as additive leads to a strong action for PVDF solution to form gel than PEG or PVP. The gelation period 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 section 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 and 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 0.2bar. the hydrophobicity of membrane reverse side exceled than that of obverse.. For the PVDF-DMAc-PVP-H2O casting solution system, low temperature of casting solution and coagulation bath tend to increase the effective porosity of membra更多还原