Preparation, Structure And Pervaporation Properties Of Copolymer Membranes For Removal Of Organic From Water

To improve the overall performance of pervaporation (PV) membranes, including separation performance, membrane formation property, heat stability and mechanical strengh and so on, copolymer membrane materials have been paid attention now. Compared with homopolymer, copolymers possess more complicated chain structure and aggregated structure, which determine the separation performance and other properties together. The main aim of this paper was to discuss the aggregated structure of different kinds of copolymers as well as their pervaporation performance and separation mechanism for recovery of trace aroma compounds from their aqueous solutions. So, three kinds of typical copolymers, ie, rubbery copolymer HTPB-PU and HTPB-DVB-PU, semi-crystalline copolymer EVA and glassy copolymer PTET were synthesized or selected as pervaporation membranes respectively.1 .Rubbery copolymer Hydroxyterminated polybutadiene (HTPB)-based polyurethaneurea (PU),HTPB-PU, was synthesized and firstly used as membrane materials to recover aroma compound, ethyl acetate (EA), from aqueous solution by PV. The effects of the number average molecular weight (M_n) of HTPB, EA in feed, operating temperature and membrane thickness on the PV performance of HTPB-PU membranes were investigated. The membranes demonstrated high EA permselectivity as well as high EA flux. Two transition temperatures (T_g) were observed from DSC curve and distinct micro phase domain existed in transmission electron micrographs (TEM) of cross sections, which means HTPB-PU membrane had micro-phase separation structure From the results of contact angle measurements, we concluded that the air-side surface is more hydrophobic than that of the glass-sides surface, which were inducedby glass plate and air respectively due to movement of the soft hydrophobic polybutadiene (PB) segments in HTPB-PU chains. Furthermore, the PV performance of the HTPB-PU membrane with the hydrophobic surface facing the feed was much better than that with the hydrophilic surface.A new PV membrane material, HTPB-DVB-PU, was synthesized and the effect of cross-linker DVB content on the pervaporation performance for recovery of EA from its aqueous solution was investigated. When an aqueous solution of 2.5 wt% EA was permeated through the cross-linked HTPB-DVB-PU membranes, they showed a high EA permselectivity and permeability of these membranes was enhanced with increasing DVB content significantly. The best permeation rate and separation factor of HTPB-DVB-PU membrane were 253 g/m²h and 656 respectively with membrane thickness of about 100 μm. Only one T_g can be observed from DSC curve, which means the hard segments did not form bigger micro domain in HTPB-DVB-PU. Furthermore, with the increasing DVB content the differnce of PV performance between the two sides of the membrane disspeared, that is, the membrane became more homogeneous. In conclusion, the smaller dimension of hard segments micro domain led to a better PV performance. Compared with HTPB-PU membrane, the separation process was mainly governed by sorption process.and the density of HTPB-DVB-PU membrane was smaller, so the degree of swelling (DS) and the total flux were larger. HTPB-DVB-PU membrane had low heat sensitivity than HTPB-PU membrane. 2.Crystalline copolymerAggregated behavior and pervaporation characterization of ethylene-vinyl acetate (EVA) copolymer membranes with different vinyl acetate (VA) cotent from 26 (EVA26) to 100 (EVA100) wt% for recovery of ethyl acetate (EA) from water were investigated. The pervaporation (PV) characterization, such as separation factor, EAflux, diffusion selectivity, and the swelling behavior of EVA membranes with VA content displayed a turning phenomena at 38 wt% VA content. Aggregated behavior of the crystallization degree, the bulk density and contact angle of EVA membranes with VA content were also turned at 38 wt% VA content. The PV characterization and the swelling behavior of EVA membranes with different VA contents could be explained in terms of the aggregated behavior of the EVA copolymer.The solution property [η] and K_H of EVA copolymer with 38 wt% VA content in selective solvents of chloroform (CF), 1, 2-dichloroethane (DCE) and cyclohexane (CYH) and the bulk properties the swelling degree at equlibrium, sorption selectivity, contant angle and crystalinity of EVA membranes cast in selective solvents above were measured by an Ubbelohode viscometer, swelling, sorption, contact angle and wide-angle X-ray diffraction (WAXD) experiments respectively. It was drawn that the solution structures of the EVA and the aggregated structures of its membrane were quite different, i.e., displaying a random homogeneous structure, a core-shell structure with the shell of VA sequences and a core-shell structure with the shell of ethylene sequences, due to different selective solvents, CF, DCE and CYH used respectively. The pervaporation performance of the EVA membrane with 38 wt% VA content increased with the aggregated structures of its membrane, the core-shell structure with the shell of ethylene sequences, the core-shell structure with the shell of VA sequences and the random homogeneous structure in order. These aggregated structures disappeared basically after the EVA membranes annealed for 12 hours at 80℃. 3.Glassy copolymerAromatic copolyester of poly(trimethylene-co-ethylene terephthalate) (PTET) with different composition was synthesized and the PTET sample with 60 percent weight fraction of polytrimethylene (PTET-60) was amorphous. The compatibility of PTET-60/cellose acetate (CA) blends and the pervaporation of their membranes forseparation of EA/water mixtures were investigated. It was found that PTET-60 is compatible with CA when the weight fraction of PTET-60 (W_PTET-60) in PTET-60/CA blends is lower than 0.35 and more than 0.5. In the region of compatibility (W_PTET-60≤ 0.35), the PTET-60/CA blend membrane showed good film-forming property. So , we prepared PTET-60/CA blend membrane with W_PTET-60≤0.35 for separation EA from water. The pure CA membrane showed water selective, while EA could preferentially permeat through PTET-60/CA blend membrane and both the maximum value of total flux and minimum value of the separation factor existed at W_PTET-60= 0.25.As a whole, in our study the PV performance of the rubbery copolymers for recovery of EA from aqueous solution is better than that of semi-crystilline copolymer and glassy copolymer, ie, HTPB-DVB-PU>HTPB-PU>EVA > PTET-60/CA, and the separation factor and the permeability of HTPB-DVB-PU membrane reached 656 and 30300 gμm/m²h, respectively...