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A novel surface morphology of linear pore pattern on poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF–HFP) flat-sheet porous membrane surfaces prepared by phase inversion technique has been observed. This interesting morphology was repeatedly captured from various casting systems, such as PVDF–HFP/N-methyl pyrrolidone(NMP)/glycol, PVDF–HFP/NMP/diethylene glycol, PVDF–HFP/NMP/glycerol and PVDF–HFP/NMP/glucose, with distilled water as a coagulant. The formation of the linear pore pattern was thought to be associated with the orientation of the macromolecular conglomerations in the casting solution, which occurred during the casting process. Thus, relevant experiments were designed and performed to support the hypothesis. The clearly observed linear pore pattern formed with less than 30 s of exposure time before immersing into water bath became faded in 300 s and completely disappeared in 600 s of exposure time, which was in good accordance with the relaxation degree of orientated macromolecular conglomeration. In contrast, the membrane without experiencing the casting process showed a conventional surface morphology, which also demonstrated the effect of the casting process on the membrane surface morphology. In addition, a clearer image of the linear pore pattern was captured when lower concentration polymer dopes were used. In contrast, this phenomenon was invisible for the membrane formed by a higher concentration dope, which is in agreement with the reduction of orientation degree with increasing polymer concentration. Moreover, the systems containing additives with lower viscosity, such as glycerol, glycol, diethylene glycol and glucose, seem to be easier to form a linear pore pattern than the one containing distilled water, the latter produced a nearly elliptical pore pattern on the membrane surface instead. The investigation on this feature may provide more information for a complete description of the initiation and growth of the porous membrane.

In the present work, poly(vinylidene fluoride- co -hexafluropropylene) (PVDF-HFP) asymmetric microporous hollow fiber membranes were successfully fabricated using nonsolvent-induced phase inversion method which has not been reported previously. Analyses of thermodynamic characteristics of the PVDF-HFP/NMP/water system without/with PVP as an additive and the rheological property of the spinning dopes were carried out as a first step. Subsequently, the effects of spinning conditions (wet spinning, air gap, bore fluid composition), dope concentration and PVP addition on the resultant hollow fiber membranes were studied in terms of membrane morphology, structure, pure water flux and molecular weight cut-off (MWCO) as well as their hydrophobicity and mechanical properties. It was found that the wet spinning process produced hollow fibers with irregular inner contour under present experimental conditions. The strong viscoelastic property of PVDF-HFP material was thought to be associated with this phenomenon. A partial release of accumulated back stress in the nascent fibers might have caused the macromolecules to move inward in the transverse direction. Increasing the air gap could eliminate this inner contour deformation, as the outer surface of the nascent fiber was not vitrified prior to entering the water bath, thus allowing the orientated macromolecules to be relaxed. In addition, the nascent fiber is generally expected to experience an elongational stretch over the distance of the air gap, which might help to smooth the inner surface. Membranes made from the PVDF-HFP/NMP dopes were found to have small pure water fluxes no matter what the polymer concentration, the bore fluid composition and the air gap were used in the spinning processes. The pure water flux can be increased by adding PVP into the dope solution, up to 5 wt.% PVP in the dope can promote the formation of macrovoids in the membrane. When the PVP content was increased to 10 wt.%, the macrovoids were suppressed, but the membrane still presented a higher porosity, which was reflected from the significant increase in the pure water flux from 60 to 107 L/(h m 2 atm). In addition, PDVF-HFP hollow fiber membranes were more hydrophobic than those membranes made from pure PVDF, the former also exhibited pronounced ductile behavior when compared with the latter.

In order to gain a better understanding of the membrane's role in the whole process of mass-transfer in membrane gas–liquid contacting systems, PVDF microporous hollow fiber membranes have been fabricated using three different dope solutions containing N -methyl-2-pyrrolidone (NMP) and different additives. The resultant hollow fibers with different structures were used to make membrane modules, which were then applied as gas–liquid membrane contactors for CO 2 absorption in water. The membranes were characterized and the effect of membrane structure on the mass-transfer was analysed accordingly. It was found that the additives used imposed a significant effect on the final membrane structure under the same spinning conditions. Compared with distilled water, phosphorous acid and glycerol showed a stronger pore-inducing ability. As a result, the membrane #1 or #2 made by the dope with phosphorous acid or glycerol as an additive had a larger pore size and a higher value of MWCO. The finger-like pores also occupied a larger portion on the membranes #1 and #2 than on the membrane #3, which used distilled water as an additive. Moreover, the membranes #1 and #2 presented a much wider pore size distribution than the membrane #3. The different membrane structures turned out to affect CO 2 absorption performance when used as contactors. Since the ratio of the membrane resistance over the overall mass-transfer resistance was increased in the order of #1 2 absorption performance was in the sequence of #1 > #2 > #3. In addition, it was noticed that all the membranes exhibited considerable membrane resistances from 22% to 36% though the model system used was pure CO 2 absorption in distilled water. The partial wetting was probably caused by the capillary condensation of water vapour in the membrane pores instead of water penetration, as the experiment shows that the CO 2 flux of three PVDF membranes was kept almost unchanged over 15 days of operation. In comparison with a commercial double skin layer PVDF membrane, it was found that the membrane with an inner skin-free structure plus a porous substrate is favourable to be used in the membrane gas–liquid contacting process.

Preparation of poly(vinylidene fluoride-co-hexafluoro propylene) (F2.6) flat-sheet asymmetric porous membrane has been studied for the first time. Factors affecting F2.6 membrane pore structure and permeate performance, such as macromolecule pore formers (polyethylene glycol-400, 1000, 1540, 2000 and 6000), the small molecule former (glycerol), swelling agent (trimethyl phosphate) in casting solution, precipitating bath component and temperature, exposure time and ambient humidity, were investigated in detail. Average pore radius and porosity were used to characterize F2.6 membrane structure, and respectively, determined by ultrafiltration and gravimetric method for the wet membrane. Morphology of the resultant membranes was observed by scanning electronic microscopy (SEM). Final test on permeate performance of F2.6 porous membrane was carried out by a direct contact membrane distillation (DCMD) setup. The experimental F2.6 membrane exhibits a higher distilled flux than PVDF membrane under the same operational situations. The determination of contact angle to distilled water also reveals higher hydrophobic nature than that of PVDF membrane.

A new method of measuring the mean size of solvent clusters in swollen polymer membrane is presented in this paper. This method is based on a combination of inverse gas chromatography (IGC) and equilibrium swelling. The mechanism is that weight fraction activity coefficient of solvent in swollen polymer is influenced by its clusters size. The mean clusters size of solvent in swollen polymer can be calculated as the quotient of the weight fraction activity coefficient of clustering system dividing the weigh fraction activity coefficient of non-clustering system. In this experiment, the weigh fraction activity coefficient of non-clustering system was measured with IGC. Methanol, ethanol and polyimide systems were tested with the new method at three temperatures, 20, 40, and 60 °C. The mean clusters size of methanol in polyimide was five, four, and three at each temperature condition, respectively. Ethanol did not form clusters (the mean clusters size was one). In contrast to the inherent narrow temperature range in DSC, XRD, and FTIR methods, the temperature range in IGC and equilibrium swelling is broad. Compared with DSC, XRD, and FTIR, this new method can detect the clusters of solvent–polymer system at higher temperature.

Flat-sheet microporous membranes from F2.4 for membrane distillation (MD) were prepared by phase inversion process. Dimethylacetamide (DMAC) and LiClO4·3H2O/trimethyl phosphate (TMP) were, respectively, used as solvent and pore-forming additives. The effects of casting solution composition, exposure time prior to coagulation and temperature of precipitation bath on F2.4 membrane structure were investigated. The morphology of resultant porous membrane was observed by scanning electron microcopy. Some natures of F2.4 porous membrane after drying in air, such as mechanical properties and hydrophobicity, were exhibited and compared with poly(vinylidene fluoride) (PVDF) membrane prepared by the same ways. Stress-at-break and strength stress of F2.4 microporous membrane are higher than that of PVDF membrane, and elongation percentage of F2.4 membrane at break is about eight-fold as great as that of PVDF membrane. Contact angle of F2.4 microporous membrane to water (86.6±0.51°) was also larger than that of PVDF membrane (80.0±0.78°). MD experiment was carried out using a direct contact membrane distillation (DCMD) configuration as final test to permeate performance of resultant microporous membrane. The permeate flux of F2.4 experimental membrane was higher than that of PVDF membrane under the same operational conditions.