Crystallization behavior of PVDF in PVDF‐DMP system via thermally induced phase separation

The crystallization behavior of PVDF (poly (vinylidene) fluoride) in PVDF‐dimethylphthalate(DMP) system was investigated in the liquid–liquid (L–L) phase separation region, solid–liquid (S–L) phase separation region and different quenching conditions via thermally induced phase separation (TIPS). Differential scanning calorimetry (DSC) indicated the crystallinity of PVDF in PVDF‐DMP system increased in the early stage of phase separation and polymer‐rich phase crystallized completely in the late stage of phase separation. The scanning electron microscopy (SEM) showed the different quenching temperatures had effects on the spherulite size of polymer rich phase and the ultimate membrane structure in the different phase separation regions. The wide angle X‐ray diffraction (WAXD) was used to quantify the crystal structure of PVDF in PVDF‐DMP system. The α‐phase PVDF was obtained when the system quenched to different temperatures above 40°C, and the area of diffraction peaks changed when quenching temperatures changed. While the β‐phase PVDF was formed when PVDF‐DMP system was quenched form liquid nitrogen and crystallized for 24 h in 25°C water bath. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3714–3719, 2006     

Formation of microporous polymeric membranes via thermally induced phase separation: A review

A review of recent research related to microporous polymeric membranes formed via thermally induced phase separation (TIPS) and the morphologies of these membranes is presented. A summary of polymers and suitable diluents that can be used to prepare these microporous membranes via TIPS are summarized. The effects of different kinds of polymer materials, diluent types, cooling conditions, extractants and additive agents on the morphology and performance of TIPS membranes are also discussed. Finally new developments in TIPS technology are summarized.

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Effects of mixed diluent compositions on poly(vinylidene fluoride) membrane morphology in a thermally induced phase‐separation process

The effects of a mixed diluent (MD) composition [dibutyl phthalate/dioctyl phthalate (DOP)] on poly (vinylidene fluoride) (PVDF) membrane morphology were investigated with scanning electron microscopy, and a bicontinuous morphology could be obtained with MD in a thermally induced phase‐separation process. The reasons for the morphology formation were explained according to the effect of MD on the phase diagrams. In addition, the effects of the PVDF concentration on the membrane morphology were examined. For the system with less DOP, the large spherulite morphology was obvious under all investigated concentrations, whereas no large spherulite structure existed in the membrane as the DOP content increased to concentrations other than 20%. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Poly(vinylidene fluoride-co-hexafluoro propylene) membranes prepared via thermally induced phase separation and application in direct contact membrane distillation

A non-toxic and environmentally safe diluent, acetyl tributyl citrate, was employed to prepare poly(vinylidene fluoride)-co-hexafluoropropylene membranes via thermally induced phase separation. Effects of the polymer concentration on the phase diagram, membrane morphology, hydrophobicity, pore size, porosity and mechanical properties (tensile stress and elongation at break) were investigated. The results showed that the pore size and porosity tended to decrease with increasing polymer concentration, whereas the contact angle, liquid entry pressure and mechanical properties showed the opposite trend. In direct contact membrane distillation operation with 3.5 wt-% sodium chloride solution as the feed solution, the prepared membranes performed high salt rejection (>99.9%). Furthermore, the prepared membranes retained excellent performance in long-term stability tests regarding the permeate flux and salt rejection.     

Formation of porous poly(ethylene‐co‐vinyl alcohol) membrane via thermally induced phase separation

Crystalline poly(ethylene‐co‐vinyl alcohol) (EVOH) membranes were prepared by a thermally induced phase separation (TIPS) process. The diluents used were 1,3‐propanediol and 1,3‐butanediol. The dynamic crystallization temperature was determined by DSC measurement. No structure was detected by an optical microscope in the temperature region higher than the crystallization temperature. This means that porous membrane structures were formed by solid–liquid phase separation (polymer crystallization) rather than by liquid–liquid phase separation. The EVOH/butanediol system showed a higher dynamic crystallization temperature and equilibrium melting temperature than those of the EVOH/propanediol system. SEM observation showed that the sizes of the crystalline particles in the membranes depended on the polymer concentration, cooling rate, and kinds of diluents. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2449–2455, 2001     

Preparation of polyvinylidene fluoride membrane via a thermally induced phase separation using a mixed diluent

Polyvinylidene fluoride (PVDF) membranes were prepared via a thermally induced phase separation method with a mixed diluent (dibutylphthalate/dioctyl phthalate). The effects of PVDF concentration and cooling bath temperature (CBT) on the structure and properties of the membranes were investigated. Scanning electron microscopy photos showed that the cross‐section of all the membranes, regardless of PVDF concentration and CBT, presented a bi‐continuous structure with the spherulitic pattern; moreover, the spherulitic patterns became clear gradually from the top surface to the bottom surface, and the top surface was denser than the bottom surface. As a result, all the membranes exhibited an asymmetric structure. The membrane property measurement indicated that, as PVDF concentration increased from 25 to 35 wt %, the pure water flux (PWF) decreased from 342 to 80 L m^?2 h^?1, and the porosity decreased slightly, whereas the minimum bubble point pressure (BPP) increased, which indicates maximum pore size decreased. In addition, with the increase in CBT, the PWF increased, but, the minimum BPP and porosity decreased. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Membrane formation of poly(vinylidene fluoride)/poly(methyl methacrylate)/diluents via thermally induced phase separation

The role of the single diluents and mixed diluents on the poly (vinylidene fluoride) (PVDF)/poly(methyl methacrylate) (PMMA) blend membranes via thermally induced phase separation (TIPS) process was investigated. The crystallization behaviors of PVDF in the diluted samples were examined by differential scanning calorimetry. The melting and crystallization temperatures of those diluted PVDF blend were decreased with the enhanced interactions between polymer chains and diluent molecules. The crystallinity of PVDF in the diluent was always higher than that obtained in PVDF blend sample. This can be explained by the dilution effects, which increased the average spatial separation distances between crystallizable chains. Thus, the PVDF crystallization was favored. Additionally, solid‐liquid (S‐L) phase separation occurred in the quenched samples. Illustrated by scanning electron microscopy, inter‐ and intraspherulitic voids were formed in the ultimate membranes, which related to the polymer/diluent interactions, the kinetics of crystallization and diluent rejection from the growing crystal. The porosity of the PVDF blend membranes obtained from the mixed diluents was higher than those obtained from the single diluent samples. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Formation of a bicontinuous structure membrane of polyvinylidene fluoride in diphenyl ketone diluent via thermally induced phase separation

The polyvinylidene fluoride (PVDF)‐diphenyl ketone (DPK) mixture was studied as a new system to prepare PVDF membranes via thermally induced phase separation (TIPS). The phenomena of liquid–liquid phase separation was found in this mixture when the temperature of mixture was decreasing and the PVDF concentration was less than 30 wt %. Using DPK as diluent, PVDF membrane with bicontinuous structure was obtained without necessity to add a nonsolvent or a stretching process further. The phase diagram of PVDF‐DPK system was also constructed to help investigate the effect of PVDF concentration and coarsening temperature on morphology of resulting membrane. The experiments showed that high coarsening temperatures and low PVDF concentrations resulted in the formation of the large pore size membrane. The strength of the wet membrane was decreasing with decreasing PVDF concentration. On condition that the PVDF concentration was larger than 30 wt %, thermally induced solid–liquid separation occurred and bicontinuous structure disappeared. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characteristics of poly(ether ether ketone) microporous membranes prepared via thermally induced phase separation (TIPS)

The morphology and bulk properties of microporous membranes based on poly (ether ether ketone) (PEEK) have been investigated as a function of initial casting composition and thermal and mechanical processing history. Membranes were prepared via solid—liquid phase separation of miscible blends of PEEK and polyetherimide (PEI), with subsequent extraction of the PEI diluent. Scanning electron microscopy studies revealed a microporous morphology with two distinct pore size scales corresponding to diluent extraction from interfibrillar and interspherulitic regions, respectively. The membrane structure was sensitive to both initial blend composition and crystallization temperature, with the resulting pore size distribution reflecting the kinetics of phase separation. For membranes prepared with lower initial diluent content or at lower crystallization temperatures, mercury intrusion porosimetry indicated a relatively narrow distribution of fine interfibrillar pores, with an average pore size of approximately 0.04 microns. Membranes prepared at higher diluent content or at higher crystallization temperatures displayed a broad pore distribution, with a sizeable population of coarse, interspherulitic pores (0.1 to 1 μm in size). Uniaxial drawing led to a fibrillated network structure with markedly higher water flux characteristics compared to the as-cast membranes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2347–2355, 1997     

Preparation of polypropylene microfiltration membranes via thermally induced (solid–liquid or liquid–liquid) phase separation method

Polypropylene (PP) hollow fiber microfiltration membranes with excellent performance were successfully prepared from the PP‐binary diluent system via thermally induced liquid–liquid (L–L) phase separation method. The binary diluent consisted of myristic acid and diphenyl carbonate. The effects of the binary diluent on phase separation and membrane structure were systematically investigated. With the decrease in the weight ratio of myristic acid to diphenyl carbonate, the Flory–Huggins interaction parameter between PP and the binary diluent became more positive, and the mechanism of phase separation changed from solid–liquid (S–L) to L–L. This resulted in the membrane structure changing from spherulitic to bicontinuous. Moreover, as the weight ratio of myristic acid to diphenyl carbonate decreased from 11/9 to 2/3, the L–L phase separation region kept enlarging while the viscosity of the whole system became higher. The pore size of the cross‐section increased due to the longer coarsening time while the surface pore size decreased due to the higher viscosity of the system. The bulk porosity of resultant PP membranes was mostly higher than 70% and pure water flux were generally larger than 650 L m^?2 h^?1. In addition, the PP hollow fiber microfiltration membrane possessed excellent mechanical properties (tensile strength of 3.47 MPa and elongation of 118%) and good separation performance (rejection to PEO (M_w = 1000 kDa) of 94.6%) when the weight ratio of myristic acid to diphenyl carbonate was 2/3. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42490.     

Effect of diluents on the crystallization behavior of poly(4‐methyl‐1‐pentene) and membrane morphology via thermally induced phase separation

The effect of diluents on polymer crystallization and membrane morphology via thermally induced phase separation(TIPS) were studied by changing the composition of the mixed‐diluents systematically, in the system of poly(4‐methyl‐1‐pentene) (TPX)/dibutyl‐phthalate (DBP)/di‐n‐octyl‐phthalate (D‐n‐OP) with TPX concentration of 30 wt %. The TPX crystallization was observed with differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). The membranes were characterized with scanning electron microscopy (SEM), porosity, and pore size measurement. As the content of D‐n‐OP increased in mixed‐diluents, the solubility with TPX increased, inducing the phase separation changing from liquid–liquid phase separation into solid–liquid phase separation, which changed the membrane morphology and structure. When the ratios of DBP to D‐n‐OP were 10 : 0, 7 : 3; 5 : 5, and 3 : 7, membranes were formed with cellular structure and well connected pores, while the ratio was 0 : 10, discernable spherulities were found with not well‐formed pore structure. The effect of composition of the mixed‐diluents on membrane morphology was more remarkable in TPX/dioctyl‐sebacate (DOS)/dimethyl‐phthalate (DMP) system, since good cellular structure was formed when the ratios of DOS to DMP were 10 : 0, 7 : 3, while spherulites were observed when 5 : 5. Dual endotherm peaks behavior on DSC melting curves emerged for all the samples in this study, which was attributed to the special polymer crystallization behavior, primary crystallization, and secondary crystallization occurred when quenching the samples. As the content of D‐n‐OP increased, the secondary crystallization enhanced which induced the first endotherm peak on DSC melting curves moving to a lower temperature and the broadening of the overall melting peak, as well as the increasing of the overall crystallinity. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fabrication of poly(vinylidene fluoride) membrane via thermally induced phase separation using ionic liquid as green diluent

Ionic liquid(IL), 1-butyl-3-methylimidazolium hexafluorophosphate([BMIM]PF6) as a new and environmentally friendly diluent was introduced to prepare poly(vinylidene fluoride)(PVDF) membranes via thermally induced phase separation(TIPS). Phase diagram of PVDF/[BMIM]PF6 was measured. The effects of polymer concentration and quenching temperature on the morphologies, properties, and performances of the PVDF membranes were investigated. When the polymer concentration was 15 wt%, the pure water flux of the fabricated membrane was up to nearly 2000 L·m~(-2)·h~(-1), along with adequate mechanical strength. With the increasing of PVDF concentration and quenching temperature, mean pore size and water permeability of the membrane decreased. SEM results showed that PVDF membranes manufactured by ionic liquid(BMIm PF6) presented spherulite structure. And the PVDF membranes were represented as β phase by XRD and FTIR characterization. It provides a new way to prepare PVDF membranes with piezoelectric properties.     

Preparation and membrane performance of poly(ethylene-co-vinyl alcohol) hollow fiber membrane via thermally induced phase separation

Poly(ethylene-co-vinyl alcohol) (EVOH) hollow fiber membranes with ultrafiltration performance were prepared from EVOH/glycerol systems via thermally induced phase separation (TIPS). The diluent glycerol was used as bore liquid to make a lumen of the hollow fiber for the purpose of prevention of the diluent evaporation and the larger pores formation at the inner surface of the hollow fiber. The obtained hollow fiber membranes showed asymmetric structures with skin layer near the outer surface, the larger pores just below the skin layer and the smaller pores near the inner surface. The formation of the larger pores near the outer surface was due to the enhanced pore growth by the water penetration. Some primary factors affecting the structure and performance of the membranes such as ethylene content (EC) in EVOH, cooling water bath temperature and take-up speed were studied extensively. The water permeability can be improved by increasing the water bath temperature and the take-up speed and by decreasing the EC. Both the pore size at the outer surface and the connectivity between the pores have to be considered together to understand the experimental result of the water permeability and the solute rejection.     

Fabrication of cellulose acetate ultrafiltration membrane with diphenyl ketone via thermally induced phase separation

With diphenyl ketone as diluent, cellulose acetate (CA) ultrafiltration (UF) membrane with a bicontinuous structure was prepared via thermally induced phase separation (TIPS) method. The liquid–liquid phase separation region of CA/diphenyl ketone system was measured and the maximum corresponding polymer concentration was approximately 53 wt %. The effects of polymer concentration, coarsening time and coarsening temperature on the morphologies, and mechanical properties of CA membranes were investigated systematically. As the polymer concentration increased from 15 to 30 wt %, the bicontinuous structure could be obtained and the tensile strength of CA membranes increased from 3.92 to 30.17 MPa. With the increase of coarsening time, the thickness of dense skin layer and the asymmetry of cross‐section reduced. However, excess coarsening rendered the membrane morphology evolved from a bicontinuous structure to a cellular structure. When the coarsening time was 5 min, the bicontinuous structure in cross‐section showed good interconnectivity and the dense skin layer exhibited a thin thickness of 2 μm. The fabricated CA hollow fiber UF membrane exhibited a high tensile strength of 31.00 MPa and rejection of 96.10% for dextran 20 kDa. It is indicated that diphenyl ketone is a competitive diluent to prepare CA membranes with excellent performance via TIPS. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42669.     

Formation of microporous membrane of isotactic polypropylene in dibutyl phthalate‐soybean oil via thermally induced phase separation

Isotactic polypropylene (iPP) microporous membranes were prepared via the thermally induced phase separation (TIPS) process with the diluents being dibutyl phthalate (DBP) and soybean oil mixture. By changing the weight ratio of DBP to soybean oil systematically, it was determined experimentally that the cloud‐point curves were influenced to a great extent, while the crystallization curves showed much less dependence on the diluents composition. Scanning electron microscopy (SEM) showed that the resulting membrane morphologies changed significantly by varying the composition of the diluents, i.e., by changing the interaction parameter and other characteristics of diluents, the interwoven or celluar structure can be fabricated successfully at a fixed polymer concentration under the same cooling conditions. Different growth rates of iPP spherulite were obtained in the diluents with different composition. It is shown that the spherulites growth rates may be also attributed to the great variations of the final microporous morphology to a certain extent. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Membrane formation via phase separation induced by penetration of nonsolvent from vapor phase. I. Phase diagram and mass transfer process

The isothermal phase diagram for poly(vinylidene fluoride)/dimethyl formamide/water system was derived. The binodal and spinodal were calculated based on the Flory–Huggins theory and the calculated binodal was approximately in agreement with the experimental data of the cloud points. The isothermal crystallization line was also obtained according to the theory of melting point depression. Mass transfer of the three components during membrane formation by the precipitation from the vapor phase has been analyzed. During this process, phase separation of the polymer solution is induced by the penetration of water vapor in the solution. The calculated result on the changes of the cast film weights indicated the good agreement with the experimental data. The time‐course of the polymer concentration profile in the film was calculated for various cases of different humidity of the vapor phase and different initial polymer concentration. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 159–170, 1999     

Specially elaborated thermally induced phase separation to fabricate poly(L‐lactic acid) scaffolds with ultra large pores and good interconnectivity

Poly(L ‐lactic acid) (PLLA) scaffolds with pore diameters from several micrometers to ～300 μm were fabricated by a specially elaborated thermally induced phase separation technique. Two different coarsening protocols, i.e., normal coarsening and multi‐step coarsening were compared in consideration of phase separation and domain growth. A normal coarsening route produced scaffolds with pore size from several micrometers to 150 μm depending on the coarsening time after phase separation, accompanying with the emergence of isolated pores at long time coarsening. Scaffolds with large pores with size up to ～300 μm were fabricated by the two‐step coarsening technique, e.g., the PLLA‐solvent (dioxane/water) system was coarsened at a temperature after phase separation for a period, followed by coarsening at a lower temperature for another period. In parallel with formation of the large pores, the interconnectivity between pores was also improved, which was evidenced by scanning electron microscopy, gelatin solution pervasion, and collagen entrapment. The present technique provides the ability to produce scaffolds with high purity, controllable microstructures, and ease of modification, and hence can be widely used in tissue engineering field. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3336–3342, 2006     

Estimation of phase diagrams for copolymer‐diluent systems in thermally induced phase separation

The binary interaction model was introduced to estimate phase diagrams of copolymer‐diluent systems in thermally induced phase separation. The crystallization curves and cloud points of poly(ethylene‐co‐vinyl alcohol) (EVOH) with 1,4‐butanediol, EVOH/1,3‐propanediol, and EVOH/glycerol were calculated and compared with experimental value or literature data. Fair agreement was obtained. To confirm the importance of incorporating intramolecular interactions, calculations with and without the consideration of intramolecular interactions were performed and compared. It was found that better results can be obtained if intramolecular interaction was introduced. The reason for the small differences between the calculated value and the experimental data of the liquid–liquid phase separation is discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

F127对热致相分离法制备的亲水性PVB/F127共混中空纤维膜性能的影响(英文)

Hydrophilic poly(vinyl butyral) (PVB)/Pluronic F127 (F127) blend hollow fiber membranes were prepared via thermally induced phase separation (TIPS), and the effects of blend composition on the performance of hydrophilic PVB/F127 blend hollow fiber membrane were investigated. The addition of F127 to PVB/polyethylene glycol (PEG) system decreases the cloud point temperature, while the cloud point temperature increases slightly with the addition of F127 to 20% (by mass) PVB/F127/PEG200 system when the concentration of F127 is not higher than 5% (by mass). Light scattering results show that the initial inter-phase periodic distance formed from the phase separation of 20% (by mass) PVB/F127/PEG200 system decreases with the addition of F127, so does the growth rate during cooling process. The blend hollow fiber membrane prepared at air-gap 5mm, of which the water permeability increases and the rejection changes little with the increase of F127 concentration. For the membrane prepared at zero air-gap, both water permeability and rejection of the PVB/F127 blend membrane are greater than those of PVB membrane, while the tensile strength changes little. Elementary analysis shows that most F127 in the polymer solution can firmly exist in the polymer matrix, increasing the hydrophilicity of the blend membrane prepared at air-gap of 5mm.     

Effect of posttreatment on morphology and properties of poly(ethylene‐co‐vinyl alcohol) microporous hollow fiber via thermally induced phase separation

Poly(ethylene‐co‐vinyl alcohol) (EVOH) hollow fiber membranes were prepared by thermally induced phase separation (TIPS) process. Water, methanol, and acetone were used to extract the diluents in the fibers, respectively. Bigger shrinkage of fibers during extractant evaporation was observed when water or methanol was used. Their interaction parameters with EVOH were calculated via Hansen solubility, respectively. The mechanism of hollow fiber volume shrinkage was discussed. It was found that affinity of the extractant with polymer was the critical factor except for the surface tension of extractant. Through the X‐ray diffraction analysis during extraction and evaporation, the crystallization behavior of the polymer was studied. From the SEM photos, it was observed that the volume shrinkage was derived from the collapse of porous structure. The fiber sample extracted by acetone had similar morphology with the sample freeze‐dried. The gas and water permeability were also measured and the results were coincident with the morphology of fibers and shrinkage data. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 4106–4112, 2007