Structure Control And Properties Research Of Chlorinated Polyvinyl Chloride Microporous Membranes Via Thermally Induced Phase Separation

As an important derivative of polyvinyl chloride(PVC), chlorinated polyvinyl chloride(CPVC) is produced by the further chlorination of PVC. CPVC not only shares most of the excellent features of PVC, but also its thermal stability and mechanical properties are improved obviously, which make it a potential candidate for the formation of microporous membranes. As present, CPVC membranes were mainly prepared by traditional nonsolvent induced phase separation(NIPS), which has limited its further development. As a simple and novel membrane preparation technology, thermally induced phase separation(TIPS) is another alternative for the preparation of microporous membranes by controlling phase separation process. In this work, TIPS and thermal-nonsolvent induced phase separation methods were employed to produce CPVC microporous membranes. The formation mechanism of CPVC membranes was investigated and the influences of membrane-forming condition on phase separation behaviors and membrane structures were studied. The details of this work were summarized as follows.Microporous CPVC membranes were prepared via TIPS process using diphenyl ether(DPE) as diluent. The thermodynamic state and membrane-forming kinetics of CPVC/DPE system were studied via the cloud point determination, droplet growth and gelation kinetics experiments. The effects of main factors including polymer concentration and cooling rate on membrane structure and performance were investigated. Upon cooling, the homogeneous solution of CPVC/DPE separated into a polymer-rich matrix phase and a diluent-rich droplet phase. As the system was further cooled, the L-L phase separation was arrested by the gelation of CPVC/DPE mixture. Therefore, the resulting CPVC membranes presented symmetric morphology with uniformly distributed cellular pores. The decrease of polymer concentration and cooling rate facilitated the growth of droplet phase and increased the size of droplets. The results matched with the pore size of the obtained CPVC membranes. Correspondingly, these membranes possessed higher water flux and worse mechanical property when lower polymer concentration and higher cooling bath temperature were submitted to experiment. Moreover, it can be indicated form the results of membrane stability tests that the obtained CPVC membranes possessed excellent acid/alkali resistance.In order to improve membrane structure and performance, polyethylene glycol(PEG) was chosen as a nonsolvent polymer additive. The phase behavior of CPVC/DPE/PEG ternary system was systemically studied by changing the molecular weight and the content of PEG-400. Based on the solubility parameter theory, PEG reduced the interaction strength and miscibility between CPVC and DPE. This ternary system underwent liquid-liquid phase separation followed by the formation of CPVC/DPE/PEG gels during cooling process. The cloud points shifted to high-temperature region by increasing the molecular weight of PEG or the content of PEG-400, leading to an increase in the size of diluent-rich phase droplet. Correspondingly, the pore size and water flux of CPVC membranes increased as the molecular weight of PEG or the content of PEG-400 increased.CPVC membranes was fabricated via a combined process of TIPS and NIPS using DPE and N, N-dimethyl formamide(DMF) as a mixed diluent. As the increasing content of DPE, the cross-section structure of CPVC membranes changed from finger-like pores to uniform cellularlike pores. The membrane-forming mechanism of phase separation also can be changed by the variation of cooling bath. When water was used as nonsolvent of cooling bath, TIPS and NIPS occurred serially. When ethanol was used as nonsolvent of cooling bath, TIPS and NIPS occurred simultaneously.