Porous membranes modified by hyperbranched polymers: I. Preparation and characterization of PVDF membrane using hyperbranched polyglycerol as additive

Porous membranes were prepared via phase inversion process from casting solution composed of poly(vinylidene fluoride) (PVDF), N,N-dimethylacetamide (DMAc), and hyperbranched polyglycerol (HPG). The membranes were characterized in terms of surface and bulk chemical compositions, morphology, water contact angle, porosity, and water flux. The effects of HPG content on membrane structures and properties were investigated. The effect of HPG addition on the hydrophilicity was discussed as well when the compositions of coagulation bath were changed. To better understand the special effects of HPG on the structures and properties of the membranes, PVDF membranes prepared using HPG as the additive were compared with those prepared using polyethylene glycol (PEG) as the additive.     

Effect of the preparation conditions on the formation of asymmetric poly(vinylidene fluoride) hollow fibre membranes with a dense skin

Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes were prepared and characterized. The effects of phase inversion methods (dry-wet or wet) and spinning conditions, such as the type of solvent (NMP, DMAc), the concentration of polymer in dope solution, temperature of the external coagulation bath and the composition of the inner coagulant on the morphology and on the formation of a dense skin layer were investigated. The structure of the membranes was analyzed by scanning electron microscopy and the gas permeation properties with six different gases (He, H₂, N₂, O₂, CH₄ and CO₂) were measured at 25 °C to confirm the integrity of the selective skin layer. Under the proper conditions highly selective and permeable PVDF hollow fibre membranes were thus obtained by dry-wet spinning of a 30 wt.% PVDF solution in DMAc, using hot water (50 °C) as the external coagulant and a bore fluid of pure water as the internal coagulant. The best membrane had a selective outer skin with an effective thickness of approximately 0.2 μm. The ideal selectivity of the hollow fibres approached or even exceeded the intrinsic ideal selectivity of a dense PVDF film, for instance the selectivity for He over N₂ was 86.2 for the hollow fibre, whereas it was 83.5 for a dense PVDF reference film. DSC and FT-IR/ATR analysis indicated a higher fraction of the β-crystal phase in the selective skin and a high overall crystallinity than in the melt-processed film. The latter explains the relatively high selectivity and low permeability of the membranes. Intrinsic polymer properties make the membranes also suitable for vapour transport than for gas separation.     

Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (T_m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.     

Morphology Evolution of Poly（vinylidene fluoride） Membranes during Supercritical CO2 Assisted Phase Inversion

A supercritical carbon dioxide（Sc CO2） assisted phase inversion was developed to produce microporous poly（vinylidene fluoride）（PVDF） membranes whose morphology characteristics arise from both liquid-liquid demixing and solid-liquid demixing（crystallization）. This result was confirmed by Fourier transform infrared spectroscopy（FTIR）, from which both α and β crystals were found. As revealed by contact angle experiment, the PVDF membranes prepared via Sc CO2 assisted phase inversion were more hydrophobic compared with the control membrane produced via conventional immersionprecipitation technique. In particular, the sample with 15 wt% PVDF prepared at 45 °C and 13 MPa exhibited a contact angle of 142°, which was mainly caused by the multilevel micro- and nano- structure. The effects of polyethylene glycol（PEG）, polyvinyl pyrrolidone（PVP） and lithium chloride（Li Cl） on the structures and crystal form were investigated. PVP promoted the formation of β phase crystal form, while PEG boosts the evolution of α phase. Li Cl restrained the crystallization degree of PVDF membrane under Sc CO2.     

Viscometric behaviour of polymer blends based on poly (vinylidene fluoride)

The viscosity behaviour of dilute dimethylformamide solutions of poly(vinylidene fluoride)-poly (methyl methacrylate) and poly(vinylidene fluoride)-polystyrene has been studied at 25°C. The polymer concentration ranges are such that neither phase separation nor microgel formation occurs, although we are very close to theta conditions. The intrinsic viscosity and viscosity interaction parameter of the ternary mixtures have been calculated. The estimation of the compatibility of the above polymer pairs has been studied based on: a) specific viscosities; b) viscosity interaction parameters, according to Krigbaum and Wall formalism, and c) viscosity interaction parameters of a system formed by a dilute probe polymer in the presence of a matrix polymer and a small molecule solvent.     

聚偏氟乙烯微孔膜的亲水化改性及功能化研究进展

聚偏氟乙烯(PVDF)微孔膜的亲水化改性方法有物理共混、化学共聚、表面涂覆、表面化学处理、表面接枝等几种。其中物理共混和表面涂覆法比较成熟且已获得应用，而PVDF微孔膜的表面化学处理、等离子体或光引发改性技术以及环境敏感性等将成为PVDF微孔膜的改性和功能化研究的主要方向。     

Poly(vinylidene fluoride) membranes by phase inversion: the role the casting and coagulation conditions play in their morphology, crystalline structure and properties

Flat membranes with controlled morphology, pore dimensions, mechanical properties and crystal structure were prepared by wet and dry wet phase inversion from polyvinylidene fluoride (PVDF). The effects of several parameters such as precipitation temperature, composition of the polymer solution (concentration, type of solvent), exposure time before immersion in the coagulation bath, type of coagulant on the sequence and the extent of the two phase separation processes, i.e. liquid-liquid and liquid-solid demixing (crystallization), were studied.Using solvent/nonsolvent pairs with different mutual affinity (DMA/water, DMA/C1-C8 alcohols), different morphologies were obtained. High casting solution temperature plays important role to increase the rate of the liquid-liquid demixing on the crystallization, i.e. the type of crystallites formed (α-type) also by using a soft coagulation bath. Exposure time before immersion favours the first type of phase separation and therefore once again crystallites of α type were observed. At room temperature, using C1-C8 alcohols as nonsolvents, the presence of crystallites of α type can be related to molar volume of the coagulant.     

Crystallization kinetics and morphology of poly(vinylidene fluoride)/poly(ethylene adipate) blends

The miscibility, isothermal crystallization kinetics and morphology of the poly(vinylidene fluoride)(PVDF)/poly(ethylene adipate)(PEA) blends have been studied by differential scanning calorimetry(DSC), optical microscopy(OM) and scanning electron microscopy(SEM). A depression of the equilibrium melting point of PVDF was observed. From the melting point data of PVDF, a negative but quite small value of the interaction parameter ?PVDF-PEA is derived using the Flory-Huggins equation, implying that PVDF shows miscibility with PEA to some extent. Nonisothermal and isothermal crystallization kinetics suggest that the crystallization rate of PVDF decreases with increasing the amount of PEA, and a contrary trend was found when PEA crystallizes with the increase of the amount of PVDF. It was further disclosed that the blend ratio and crystallization temperature affect the texture of PVDF spherulites greatly, which determines the subsequent crystallization of PEA. At high temperatures, e.g. 150 ℃, the band spacing of PVDF spherulites increases with the addition of PEA content and the spherulitic structure becomes more open. In this case, spherulitic crystallization of PEA is not observed for all blend compositions. At low temperatures, e.g. 130 ℃, for the PEA-rich blends, the interpenetrated structures are eventually formed by the penetration of the spherulites of PEA growing within the pre-existing PVDF spherulites.     

Crystallization behavior and hydrophilicity of poly (vinylidene fluoride) (PVDF)/poly (styrene-<Emphasis Type="Italic">co</Emphasis>-acrylonitrile) (SAN) blends

Poly (styrene-co-acrylonitrile) (SAN) is a hydrophilic non-crystalline copolymer, which is initially used in this paper to improve the hydrophilicity of poly (vinylidene fluoride) (PVDF). Investigation of the crystallization behavior of PVDF/SAN blends showed that the samples presented only α phase regardless of SAN content as cooling from the melt. A double-melting phenomenon was related to the perfection or crystal size of PVDF crystals. As the SAN content is increasing, crystallization of PVDF was limited, leading to a decreased crystallinity and lamellar growth. Besides, the hydrophilicity of PVDF was improved by blending with SAN. The sample containing 70 wt.% SAN performed a similar surface property of the neat SAN owing to the besieging of the PVDF phase by SAN. Observed from the cross section of the blends, PVDF/SAN blends were partially miscible with less than 50 wt.% SAN addition. As the SAN content was more than 50 wt.%, the crystalline PVDF particles clearly dispersed in the amorphous matrix.     

Thermal stability of poly(vinylidene fluoride) films pre-annealed at various temperatures

This paper investigates the relationship between the pre-annealing conditions and the thermal stability of uniaxially-drawn poly(vinylidene fluoride) (PVDF) films in order to clarify their technical limits in terms of temperatures that can be used for assembly processes and for practical applications. Specimens that are pre-annealed below their melting temperature apparently shrink in the stretch-direction when they are exposed to elevated temperatures above the pre-annealing temperature. Since the content of β-PVDF in the films decreases simultaneously with the shrinkage, their piezoelectric properties also deteriorate. In addition, there is a suggestion that the level of polarization in the remaining β-phase decreases significantly during annealing above 90-100 °C. However, the dimensions and the piezoelectric coefficients of the films remain stable during annealing below the pre-annealing temperature. Therefore, the thermal stability of PVDF films can be controlled practically by using the appropriate pre-annealing temperature. By contrast, the films were softened at 90-100 °C when the pre-annealing treatment was conducted above the melting temperature. The softening of films that are pre-annealed above the melting temperature is a different phenomenon from that observed in specimens that are pre-annealed below the melting temperature.     

Features and origin of the dynamic and the nonlinear piezoelectricity in poly(vinylidene fluoride)

Some of the piezoelectric coefficients of poly(vinylidene fluoride) are distinctly stress dependent. We report on their dependence on the frequency of an oscillatory stress excitation, on the amount of the remanent polarization in the sample materials, and on the magnitude of an offset stress. In order to explain the observed dynamic and nonlinear piezoelectric properties, a model is developed that attributes them to changes in the local polarization in an interphase between the crystalline and amorphous phases in this semicrystalline polymer.     

Thermoreversible gelation of blend of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) in γ-butyrolactone solution

Thermoreversible gelation behavior of blend of poly(vinylidene fluoride) and poly(vinylidene fluoride-trifluoroethylene) in γ-butyrolactone solution was studied. Sol-gel transition temperature increased with the increase of polymer concentration, but was independent of the blend ratio of two polymers. An equation for gelation rate was derived, assuming that the gelation is a first-order reaction and that the gelation rate obeys an Arrhenius type. According to the equation, the growth index of gelation and supercooling temperature had a dominant effect on gelation rate. The growth index of gelation, which was calculated from the dependence of activation energy on the supercooling temperature in the isothermal gelation, varied with the blend ratio of two polymers. Growth index of gelation larger than 2 was obtained for the blend gels studied in this experiment. It may suggest that the multidimensional growth of gels occurs in such polymer blend solutions. X-ray diffraction and differential scanning calorimetry measurements showed existence of separate crystals due to each component of polymer in the blend gels. © 1996 John Wiley & Sons, Inc.     

The morphology of poly(vinylidene fluoride) crystallized from blends of poly(vinylidene fluoride) and poly(ethyl acrylate)

A combined optical and electron microscopical study has been carried out of the crystallization habits of poly(vinylidene fluoride) (PVF₂) when it is crystallized from blends with noncrystallizable poly(ethyl acrylate) (PEA). The PVF₂/PEA weight ratios were 0.5/99.5,5/95, and 15/85. Isothermal crystallization upon cooling the blends from the single-phase liquid region was carried out in the range 135–155°C, in which the polymer crystallizes in the α-orthorhombic unit cell form. The 0.5/99.5 blend yielded multilayered and planar lamellar crystals. The lamellae formed at low undercoolings were lozenge shaped and bounded laterally by {110} faces. This habit is prototypical of the dendritic lateral habits exhibited by the crystals grown from the same blend at high undercoolings as well as by the constituent lamellae in the incipient spherulitic aggregates and banded spherulites that formed from the 5/95 and the 15/85 blends, respectively. In contrast with the planar crystals grown from the 0.5/99.5 blend, the formation of the aggregates grown from the 5/95 blend is governed by a conformationally complex motif of dendritic lamellar growth and proliferation. The development of these aggregates is characterized by the twisting of the orientation of lamellae about their preferential b-axis direction of growth, coupled with a fan-like splaying or spreading of lamellae about that axis. The radial growth in the banded spherulites formed from the 15/85 blend is governed by a radially periodic repetition of a similar lamellar twisting/fan-like spreading growth motif whose recurrence corresponds to the extinction band spacing. This motif differs in its fan-like splaying component from banding due to just a helicoidal twisting of lamellae about the radial direction. © 1993 John Wiley & Sons, Inc.     

Direct measurement of gas solubility and diffusivity in poly(vinylidene fluoride) with a high-pressure microbalance

We present solubility and diffusion data for the gases methane and carbon dioxide in the polymer poly(vinylidene fluoride). The polymer was cut from extruded piping intended for use in offshore oil and gas applications. Measurements were carried out using a purpose-built high-pressure microbalance. These properties were determined in the temperature range 80-120 °C and in the pressure range 50-150 bar for methane and 20-40 bar for carbon dioxide. In general, good agreement was obtained for similar measurements reported in the literature. Solubility follows a Henry’s law (linear) dependence with pressure. Diffusion coefficients for each of the gases in the polymer were also measured using the balance. Activation energies for diffusion and heats of solution for the two gases in the polymer were also determined.     

Chemical reactivity assay and surface characterization of a poly(vinylidene fluoride) microfiltration membrane (“Durapore DVPP”)

Surface functionalization of a commercially available poly(vinylidene fluoride) (PVDF) filtration membrane (Millipore DVPP) was performed using organic synthesis at the solid–liquid interface. Hydroxyl groups present on this hydrophilic membrane were activated by reaction with several activating and coupling agents prior to the covalent grafting of lysine. Transamination was used as well as a grafting method. The apparent surface of the PVDF membrane was characterized using SEM and XPS techniques before and after the wet chemical treatments. On the other hand, chemical reactivity of the open surface was assayed by the coupling to tritiated lysine and liquid scintillation counting (LSC) of the radioactivity associated to the samples.     

Graft copolymerization of N-vinyl-2-pyrrolidone onto pre-irradiated poly(vinylidene fluoride) powder

Graft copolymerization of N-vinyl-2-pyrrolidone (NVP) onto ⁶⁰Co γ-ray pre-irradiated poly (vinylidene fluoride) (PVDF) powder was investigated to find out the relationship between the degree of grafting (DG) and various factors, including monomer concentration, irradiation dose, reaction time, catalyst and so on. The DG can be calculated by comparing the amount of nitrogen element in the resulting copolymer (PVDF-g-PVP) powder with that in PVP on the basis of element analysis. The presence of PVP in the resulting PVDF powder was confirmed by the comparative studies of pristine PVDF and grafted PVDF powder through Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), thermo gravimetric analyzer (TGA) and differential scanning calorimetry (DSC), respectively. When the reaction was performed at the monomer concentration of 20% (vol.) and the absorbed dose of 40 kGy for 3 h in water, the max. DG of 17.7% was obtained.     

Modification effects of amphiphilic comb-like polysiloxane containing polyether side chains on the PVDF membranes prepared via phase inversion process

Amphiphilic comb-like polysiloxane (ACPS) containing polyether side chains was used as the modification reagent in the preparation of hydrophilic porous poly (vinylidene fluoride) (PVDF) membranes via a phase inversion process. The effects of ACPS on morphology, crystallinity, mechanical properties, reservation of ACPS in the phase inversion process, chemical structure, hydrophilicity and filterability performance of porous PVDF membranes were discussed. It was found that the addition of ACPS would result in the delayed demixing which yields “sponge-like” sublayers and longer crystallization time during the membrane formation process. It was revealed that O/F ratios of the bulk membrane were almost the same as those of the corresponding casting solutions which obviously indicated the high reservation of ACPS in the membrane formation process. The fact that the O/F ratios in the membrane surface layers were much higher than those in the bulk membrane proved the enrichment of ACPS on the surface. The filterability experiments and water contact angle testing proved the hydrophilicity of the blend membranes. Through a schematic model, the mechanism relating the membrane structure and performance was interpreted. From the observed results, it can be concluded that ACPS acts as a potential candidate material for preparing PVDF membranes with extraordinary hydrophilicity and filterability. __________ Translated from Acta Polymerica Sinica, 2007, 12: 1168–1175     

Polymerisation of vinylidene fluoride in supercritical carbon dioxide: Formation of PVDF macroporous beads

Poly(vinylidene fluoride) (PVDF) macroporous beads with diameter in the range of hundreds of micrometers were produced by batch polymerisation of vinylidene fluoride (VDF) in supercritical carbon dioxide (scCO₂) using diethyl peroxydicarbonate (DEPDC) as a free radical initiator. The rate and type of stirring were found to influence strongly the morphology of polymers, and the results indicated that the shear force was the key factor. A low shear force and a suitable monomer concentration range at 6-8 mol/l were needed for the formation of PVDF macroporous beads. Scanning electron microscopy (SEM) was employed to characterize the polymer morphology, and Brunauer-Emmett-Teller (BET) method was used to analyze the surface area of the polymer macroporous beads. In addition, polydimethylsiloxane monomethylacrylate (PDMS-ma) and poly(1H,1H,2H,2H-perfluorooctyl methacrylate) (PFOMA) were found to be able to control the size of PVDF macroporous beads. We propose that the formation of PVDF macroporous beads results from the aggregation of semi-crystalline PVDF primary particles.     

Preparation and properties of dynamically cured poly(vinylidene fluoride)/silicone rubber blends

Blends of poly(vinylidene fluoride) (PVDF) and silicone rubber (SR) were prepared through dynamic vulcanization. The effects of SR content on crystallization behavior, rheology, dynamic mechanical properties and morphology of the blends were investigated. Morphology characterization shows that the crosslinked spherical SR particles with an average diameter of 2-4 μm form a “network” in the PVDF continuous phase. The dynamic mechanical properties indicate the interface adhesion between PVDF and rubber phase is improved by the dynamic vulcanization. The rheology study shows that with the increase of rubber content the blends pseudoplastic nature is retained, while the viscosity increases, and hence the processability is less good. The incorporation of SR phase promotes the nucleation process of PVDF, leading to increased polymer crystallization rate and crystallization temperature. However, a higher content of SR seems to show a negative effect on the crystallinity of the PVDF component.     

Quantitative confirmation of the crystal-amorphous interphase in semicrystalline poly(vinylidene fluoride) and poly (vinylidene fluoride)/poly (ethyl methacrylate) blends

Dielectric and thermal characterizations were performed for poly (vinylidene fluoride) (PVDF)/poly (ethyl methacrylate) (PEMA) blends of different composition. The characteristics of PVDF β relaxation were shown to be little affected in the semicrystalline blends with PEMA. The relaxation strength, however, depends strongly on the PEMA content and a linear relation was found between the intensity of the β relaxation and the weight fraction of the PVDF crystal-amorphous interphase. Phase structures of the PVDF/PEMA blends are also proposed. © 1994 John Wiley & Sons, Inc.