Structure And Properties Of Olefin Copolymer As The Membrane Material In Organophilic Pervaporation

Organophilic pervaporation has a large market when it is applied into the wastewater treatment or recovery of organic compounds with high value. Normally rubbery polymers such as PDMS are employed as the membrane material for organophilic pervaporation. In this paper, crystalline polymers such as PP and PVDF were adopted. And different states of aggregation in polymers were obtained by means of copolymerization. Moreover, polymeric membranes with different composition of crystalline phases were gained by means of changing the solvents. The correlation between the structures of the aggregating state and the pervaporative separation performances were studied in this paper, which is significant for the development of the pervaporation membranes.On the one hand, copolymers of propene and 1-octene (POC) and propene and 1-decene (PDE) were synthesized by Ziegler-Natta catalysts. The results from copolymerization show that if the ratios of mole concentration between propene and 1-octene and that between propen and 1-decene are controlled above 0.4985, 0.6125, respectively, amorphous POC and PDE with microcrystals are obtained under the copolymerization condition as 50°C, Al/Ti being 100, and normal pressure. POC and PDE membranes have good thermal stability. The temperatures of weight loss are both over 400℃. POC and PDE membranes also have better mechanical strength than PDMS membrane whose break strength is 0.2~0.3MPa. The break strengths of POC membrane with 20.19mol.% 1-ocetne content and PDE membrane with 21.18 mol.% 1-decene content are 1.7MPa, 3.6MPa, respectively. The study of binary sequence distribution indicates that POC and PDE have special macromolecular chain structure which can be described as 1-octene or 1-decene exists in the macromolecular chain as a single unit. The average sequence lengths of 1-octene and 1-decene are both between 1 and 2, and the activity ratio of propene is 20-80 times as that of 1-octene or 1-decene, which indicates that homopolymerization of 1-octene or 1-decene is impossible in case of copolymerization.In this paper, POC and PDE membranes were firstly used in organophilic pervaporation. The correlations between structures and pervaporative separation properties of POC or PDEmembranes were discussed. With the 1-octene or 1-decene content in copolymer increasing, the crystallinity and glass transition temperature both decrease. The flexibility of macromolecular chains in the amorphous region increases. And the free-volume in membrane is thus enhanced, which is in favor of improving the permeate flux of the membrane. However, diffusivity of water molecule predominates over chloroform molecule because of its smaller size. So the water permeate flux increases more fast than the chloroform permeate flux, resulting in a decrease of separation faction. PDE membrane has higher permeate flux than POC membrane, although the selectivity of PDE membrane is somewhat lower than that of the POC membrane. In general, PDE membrane has higher PSI value than POC membrane, which implies that 1-decene is more suitable as the monomer copolymerized with propene than 1-octene.On the other hand, P(VDF-HFP) was chosen as another new membrane material in organophilic pervaporation considering that the decreased crystallinity and polarity after VDF was copolymerized with HFP. In this paper, the separation properties of P(VDF-HFP) membrane in pervaporation of MeAc or EtAc/water mixture was firstly studied.Results from FTIR and WAXD analysis show that solvent has large effect on the structure of the membrane prepared by solvent evaporation at constant temperature. The crystalline phase of the P(VDF-HFP)-1 membrane with acetone as solvent is dominantly a phase, and that of the P(VDF-HFP)-2 membrane with DM Ac as solvent is mainly y phase.Compared to PVDF sample, the crystallinities of the two solvent-cast P(VDF-HFP) membranes with lOum thickness decrease about 20%, and their melting points also decrease about 20 °C, which suggests that their crystals are more imperfect than that of PVDF sample. The two solvent-cast P(VDF-HFP) membranes have good thermal stability with weight loss temperature at 400-600 °C, and have excellent mechanical strength. The break strengths of the P(VDF-HFP)-1 membrane and the P(VDF-HFP)-2 membrane are 27.28MPa and 24.25MPa, respectively.Results from the evaporation enthalpy through DSC tests between -150~200°C, and swelling degree (SD), diffusion coefficient, pervaporative separation properties tests show that the interactions between permeanants and the two solvent-cast P(VDF-HFP) membranesdominate the separation performances and mechanism in pervaporation of EtAc or MeAc/water mixture. The interaction between MeAc or EtAc and P(VDF-HFP)-2 membrane is weaker than that between MeAc or EtAc and the P(VDF-HFP)-1 membrane. But the interaction between water and P(VDF-HFP)-2 membrane is stronger than that between water and the P(VDF-HFPF)-1 membrane. The absorption and dissolution of MeAc or EtAc takes place both in the original amorphous region and crystalline region in the P(VDF-HFP)-1 membrane, but that takes place mainly in the original amorphous region in the P(VDF-HFP)-2 membrane. So the SD of MeAc or EtAc in the P(VDF-HFP)-1 membrane is obviously larger than that in the P(VDF-HFP)-2 membrane. And the diffusion coefficient (Do) of the P(VDF-HFP)-2 membrane is larger than that of the P(VDF-HFP)-1 membrane.An abnormal phenomena was observed that at constant operating temperature, the separation factor increases with increasing the feed concentration. It was explained based on the interaction between the permeanants and the two solvent-cast P(VDF-HFP) membranes. The separation mechanisms of the two solvent-cast membranes are both solution-diffusion process.The environment of membrane forming affects the structure and the configuration of the obtained membrane. When the humidity increasing and the temperature decreasing, the crystals in the two solvent-cast P(VDF-HFP) membranes become more perfect, and it is more possible that holes are produced in the membrane bulk. So it is very important to control the humidity and temperature of the environment when preparing the solvent-cast membrane to obtain dense pervaporation membrane. Moreover, the choice of supporting layer for preparing the composite membrane with thin skin layer is also very important.