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     

聚偏氟乙烯微孔膜制备方法研究进展

介绍了浸没沉淀法、热致相分离法以及蒸发助热致相分离法制备聚偏氟乙烯(PVDF)微孔膜的研究进展.重点介绍了溶剂、凝固浴组成、凝固浴温度、添加剂和蒸发时间等因素对浸没沉淀法制备PVDF膜的影响,并对制备PVDF膜的发展提出建议和展望.同时简单介绍了PVDF微孔膜的亲水性改性.     

稀释剂对TIPS法等规聚丙烯中空纤维微孔膜孔径及其连通性的影响

热致相分离（TIPS）法制备等规聚丙烯（iPP）中空纤维微孔膜,邻苯二甲酸二丁酯（DBP）与邻苯二甲酸二辛酯（DOP）的混合溶剂作为制膜稀释剂。干/湿氮气流量法测定了α（稀释剂中DBP的质量分数）和β（铸膜液中聚合物的质量分数）对膜样品的平均孔径及其分布的影响,并采用膜孔曲折因子定量表达膜孔连通性。发现全部膜样品均体现窄孔径分布特征。对于相同的β, α增加导致平均孔径及膜孔连通性下降。α=0.20时,β增加,膜的平均孔径先增加后降低,膜孔曲折因子稍下降; α=0.35或0.50时,β增加,膜的平均孔径降低,膜孔曲折因子下降。膜孔连通性体现了膜内部的拓扑结构,共溶剂组成和铸膜液固含量能够调节iPP中空纤维微孔膜的孔径及其连通性。     

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     

混合稀释剂对热致相分离法制备聚偏氟乙烯膜性质及结构的影响

采用邻苯二甲酸二丁酯(DBP)为稀释剂,通过加入部分N,N-二甲基乙酰胺(DMAc)或邻苯二甲酸二辛酯(DOP),分别在150℃和220℃下,采用热致相分离法制备了聚偏氟乙烯(PVDF)平板膜,研究了在较低温度下或高温下制得膜的性能和结构。结果表明,加入质量分数10%的DMAc在150℃下制膜,比采用纯的DBP为稀释剂时膜的通量提高了28%,过多DMAc的加入反而会使膜的通量降低;加入质量分数10%的DOP在220℃下制膜,膜的通量则下降了62%。同时采用扫描电镜观察了膜的截面结构,解释了采用不同混合稀释剂时膜形成不同结构的原因。     

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 membrane formation via thermally induced phase separation

The effect of diluents on isotactic polypropylene (iPP) membrane formation via thermally induced phase separation was investigated. The diluents were methyl salicylate (MS), diphenyl ether (DPE), and diphenylmethane (DPM). The cloud-point curve was shifted to a lower temperature in the order iPP–MS, iPP–DPE, and iPP–DPM, whereas the crystallization temperature was not influenced so much by diluent type. Droplet-growth processes were investigated under two conditions: quenching the polymer solution at the desired temperature and cooling at a constant rate. Although droplet sizes were in the order iPP–MS, iPP–DPE, and iPP–DPM in both cases, the difference was more pronounced with the constant cooling rate condition. Scanning electron microscopy indicated that interconnected structures were obtained when the polymer solution was quenched in ice water. The effect of the diluents on these structures was observed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 169–177, 2001     

超疏水聚偏氟乙烯复合微孔膜的制备及性能

提出了一种超疏水聚偏氟乙烯（PVDF）复合微孔膜的制备方法。以相转化法制备的PVDF膜为基膜,通过恒压过滤将多壁碳纳米管（MWCNTs）沉积到PVDF基膜表面,再经聚二甲基硅氧烷（PDMS）溶液修饰,可制得接触角达162°、滚动角约10°的PVDF复合微孔膜。用原子力显微镜和扫描电镜对膜表面进行结构分析,并测试了膜的接触角、气通量和机械强度等性能,考察了MWCNTs及PDMS浓度对膜结构和性能的影响。研究表明,CNTs在具有微米级粗糙度的基膜上强化了纳米结构,提高了膜的粗糙度,PDMS降低了膜的表面能,二者协同作用使复合膜的接触角大幅提高,滚动角显著下降。与高度疏水的PVDF基膜相比,PVDF复合膜的疏水性大幅提高,断裂伸长率加倍,在模拟海水真空膜蒸馏过程中,保持了较高的传质通量和截留率,具有更好的操作稳定性和抗污染性能。     

Preparation of PVDF hollow‐fiber membranes via immersion precipitation

Porous polyvinylidene fluoride (PVDF) hollow‐fiber membranes with high porosity were fabricated using the immersion precipitation method. Dimethylacetamide (DMAc) and N‐methyl‐2‐pyrrolidone (NMP) were used as solvent, respectively. In addition, polyvinylpyrrolidone (PVP), lithium chloride, and organic acids were employed as nonsolvent additives. The effects of the internal and external coagulation mediums on the resulting membrane properties were also investigated. The resulting hollow‐fiber membranes were characterized in terms of maximum pore radius, mean pore radius, effective surface porosity as well as wetting pressure. The structures of the prepared hollow fibers were examined using a scanning electron microscope. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1643–1653, 2001     

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 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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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.     

聚丙烯微孔膜研究进展

聚丙烯（PP）是一种价格低廉的塑料，是目前最主要的制备高分子微孔膜的材料之一。笔者综述了近年来PP微孔膜制备方法的研究进展，重点介绍了熔纺拉伸法（MSCS）、热致相分离法（TIPS）、共混拉伸法、熔融烧结法等PP微孔膜的常用制备方法，并对相关的影响因素进行了讨论。     

热致相分离法制备聚乙烯/纳米二氧化钛共混膜

采用相对分子质量为130 000的聚乙烯(PE)为膜材、DIDP为稀释剂、平均直径为180 nm的TiO2粉末作添加剂,用热致相分离(TIPS)法制备了PE膜和PE/TiO2共混膜。研究了共混组成和冷却速率对膜结构的影响。所有制备的膜均是海绵状孔,冷却速率越大,膜孔越小。对PE质量分数为15%,当PE与TiO2的质量比大于1时,TiO2在膜内的分布不均匀,当PE与TiO2的质量比为0.5时,TiO2能比较均匀的分布于膜内。共混膜浸于水中经过紫外照射后,其亲水性显著增加。     

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     

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.     

Formation of an interconnected lamellar structure in PVDF membranes with nanoparticles addition via solid‐liquid thermally induced phase separation

Novel microporous membranes were prepared via thermally induced solid‐liquid (S‐L) phase separation of mixtures containing poly(vinylidene fluoride) (PVDF)/diphenyl ketone (DPK)/nanoparticles [such as montmorillonite (MMT) and polytetrafluoroethylene (PTFE)] in diluted systems with a mass ratio of 29.7/70/0.3 wt %. The crystallization and melting characteristics of these diluted systems were investigated by polarizing optical microscopy (POM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and wide angle X‐ray diffraction (WAXD). The nanoparticle structure and the interaction between PVDF chains and nanoparticle surfaces determined the crystallization behavior and morphology of the PVDF membrane. The addition of MMT and PTFE had a significant nucleation enhancement on the crystallization of PVDF accompanied by S‐L phase separation during the thermally induced phase separation (TIPS) process. It was observed that an interconnected lamellar structure was formed in these two membranes, leading to a higher tensile strength compared with that of the reference membrane without nanoparticles addition. Additionally, addition of MMT facilitates the fiber‐like β phase crystal formation, resulting in the highest elongation at break. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Morphology of polyvinylidene fluoride and its blend in thermally induced phase separation process

Polyvinylidene fluoride (PVDF) and its blend films were prepared by melt‐blending the binary mixture of PVDF/dibutyl phthalate (DBP) and the ternary mixture of PVDF/CaCO₃/DBP in combination of TIPS. Their morphologies were characterized by scanning electron microscope depended on preparation condition, such as the diluent and CaCO₃ weight fraction, and the cooling rate, and the crystallization information was also investigated by DSC. The results showed that CaCO₃ influenced the morphology of PVDF in the TIPS process regardless of the cooling conditions, and the formation of the spherulitic morphology can be disturbed by CaCO₃ at the quenching condition. The effect of CaCO₃ weight fraction on the tensile strength of PVDF was measured, which indicated CaCO₃ had a negative effect on PVDF tensile strength. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2944–2952, 2006     

Effects of mixed solvents and PVDF types on performances of PVDF microporous membranes

Polyvinylidene fluoride (PVDF) microporous flat membranes were cast with different kinds of PVDFs and four mixed solvents [trimethyl phosphate (TMP)–N,N‐dimethylacetamide (DMAc), triethyl phosphate (TEP)–DMAc, tricresyl phosphate (TCP)–DMAc, and tri‐n‐butyl phosphate (TBP)–DMAc]. The effects of different commercial PVDFs (Solef® 1015, FR 904, Kynar 761, Kynar 741, Kynar 2801) on membrane morphologies and membrane performances of PVDF/TEP–DMAc/PEG200 system were investigated. The membrane morphologies were examined by scanning electron microscopy (SEM). The membrane performances in terms of pure water flux, rejection, porosity, and mean pore radius were measured. The membrane had the high flux of 143.0 ± 0.9 L m^?2 h^?1 when the content of TMP in the TMP–DMAc mixed solvent reached 60 wt %, which was 2.89 times that of the membrane cast with DMAc as single solvent and was 3.36 times that of the membrane cast with TMP as single solvent. Using mixed solvent with different solvent solubility parameters, different morphologies of PVDF microporous membranes were obtained. TMP–DMAc mixed solvent and TEP–DMAc mixed solvent indicated the stronger solvent power to PVDF due to the lower solubility parameter difference of 1.45 MPa^1/2 and the prepared membranes showed the faster precipitation rate and the higher flux. The less macrovoids of the membrane prepared with TEP (60 wt %)–DMAc (40 wt %) as mixed solvent contributed to the higher elongation ratio of 96.61% ± 0.41%. Therefore, using TEP(60 wt %)–DMAc (40 wt %) as mixed solvent, the casting solution had the better solvent power to PVDF, and the membrane possessed the excellent mechanical property. The microporous membranes prepared from casting solutions with different commercial PVDFs exhibited similar morphology, but the water flux increased with the increment of polymer solution viscosity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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