【作者】 栾英豪；

【导师】 刘燕刚； 张永明；

【作者基本信息】 上海交通大学 ， 应用化学， 2009， 博士

【摘要】 本文系统研究了全氟磺酸离子聚合物(PFSI)/N,N-二甲基甲酰胺(DMF)体系的成膜过程和机理,取得了创新性研究结果。本文首次对全氟磺酸离子聚合物/DMF溶剂体系中的PFSI分子聚集形态及其变化进行了详细研究。研究发现,PFSI/DMF二元体系在不同的PFSI浓度下表现为溶液、溶液/分散液、分散液、分散液/凝胶、凝胶状态,并详细研究了其转变过程。对于典型交换容量(IEC)值为0.95meq/g的PFSI/DMF二元体系, PFSI浓度高于0.5mg/g后发生聚合物溶液向分散液的转变,PFSI浓度为208-336mg/g时发生分散液向宏观凝胶的逐渐转变,为进一步的研究提供了重要参考依据。研究证明,在PFSI浓度增大的过程中,离子聚合物分子的聚集是通过主链段之间的相互作用进行的,并且PFSI分子在收缩和聚集的过程中会形成类似聚四氟乙烯中的晶体结构。对成膜过程和膜的结构形态研究表明,PFSI/DMF体系制得的流延膜与熔融挤出膜的微观结构具有较大的差异,流延膜体系更接近各向同性,晶粒无规地分布在无定形区中,晶粒之间较少存在长程有序的排列,离子区由磺酸基团形成的较小的离子点和细小的离子通道组成,而不是类似熔融挤出膜中形成的较大的离子团簇,从而解释了熔融挤出膜力学性能较高,而流延膜含水率和电导率较高的原因。在由PFSI分散液向离子膜演变的初期,随着溶剂分子不断逸出体系,PFSI分子收缩形成直径为20nm左右的颗粒,随着溶剂的减少,这些颗粒再形成200-1000nm左右的聚集体。在热、颗粒之间毛细管力和范德华力等的共同作用下,通过具有相似特征的链段和基团之间的作用,体系内部形成结晶区、离子聚集区和无定形区。系统考察了成膜温度和温度变化对离子膜结构和性能的影响,证实温度不但是分子链段的运动能力和溶剂挥发的驱动力,而且是离子聚合物颗粒融合的驱动力。研究发现,不同温度制备的膜的微观结构差异主要是结晶度和晶体尺寸的变化,而对离子簇的影响较小。随着成膜温度的升高,PFSI膜的密度增大、晶体颗粒增大并且更加完善,从而使膜在极性溶剂中的溶解度下降,断裂拉伸强度升高,含水率和甲醇透过率都下降。更多还原

【Abstract】 Solution-casting is an important method to prepare perfluorosulfonic membranes, yet fabrication process and mechanism of solution-cast perfluorosulfonic membranes is not clear. In this paper, we made systematic study on morphological evolution of perfluorosulfonic ionomer (PFSI) molecules in N,N-dimethylformamide (DMF) at different concentrations. Process of membrane fabrication, microstructure and properties of perfluorosulfonic membranes were studied to find out the fabrication mechanism of solution-cast PFSI membranes.Morphological evolution of PFSI molecules in DMF was studied in detail. It was found that PFSI molecules shrink and aggregate in DMF gradually with increasing concentration, accordingly, mixture of PFSI and DMF changes from solutions to dispersions and gels. For the typical perfluorosulfonic ionomer with ion exchange capacity of 0.95meq/g, the prepared PFSI/DMF mixture changes from solutions to dispersions at PFSI concentration of 0.5mg/g, and the mixture changes to be gels from dispersions gradually at PFSI concentrations of 208-336mg/g. Understanding of properties of the PFSI/DMF mixture provided important reference for further study on membrane fabrication mechanism.It was found that aggregation of the PFSI particles in the dispersions and gels is due to the interaction of backbones of the PFSI molecules. With shrinkage and interaction of the PFSI backbones, the segments of the main chains may arrange regularly to form crystals similar with the ones in polytetrafluoroethylene.Morphology of the DMF-based solution-cast PFSI membranes is different from that of thermo-extruded membranes. The solution-cast membranes are more isotropic. Crystallites in the solution-cast membranes seem to be randomly distributed over the amorphous phase. Hydrophilic zone in the membranes is composed of continuous ionic thin channels and nanoparticles that are smaller than the clusters in the thermo-extruded membranes. The different microstructure of the two kinds of membranes is the reason that the thermo-extruded PFSI membranes are stronger and conductivity of the solution-cast membranes is bigger. The solution-cast PFSI membrane is assembled by the PFSI aggregates formed in the dispersion. The PFSI molecules shrink to form about 20nm particles in dilute dispersions, and the particles aggregate to form bigger PFSI aggregates in the concentrated dispersions. The PFSI particles and aggregates fuse into each other with heat, capillary force and Van der Waals’ force to form membranes.Effect of casting temperature, casting time and change of temperature on morphology and properties of PFSI membranes was studied. It is proved that casting temperature has important effect on the membranes. Increasing casting temperature may increase movement ability of the PFSI segments, volatilization of the solvent and driving force of fusion of the aggregates, so morphology and property of the membranes are different when prepared at different temperatures. With increasing casting temperature, the perfluorosulfonic aggregates fuse into each other deeper to form denser membranes, so density of the membranes increases and crystals in the membranes turns bigger and more perfect. Accordingly, proton conductivity, water uptake, methanol permeability and solubility in organic solvents of the membranes decrease with increasing casting temperature, whereas tensile stress at break of the solution-cast membranes increases.更多还原