This application claims priority from EP application Nr. 22182704.1, filed on 01 July 2022, the whole content of this application being incorporated herein by reference for all purposes.

The present invention relates to the use of a composition comprising zwitterionic moieties in membranes based on vinylidene fluoride (VDF) polymers for increasing flux.

Background

Porous membrane is a thin object whose key property is its ability to control the permeation rate of chemical species through itself. This feature is exploited in applications like separation applications (for example liquid, like water, and gas).

Fluorinated polymers are widely used in the preparation of microfiltration and ultrafiltration membranes due to their good mechanical strength, high chemical resistance and thermal stability. Among them, partially fluorinated polymers based on vinylidene fluoride (VDF) are particularly convenient for controlling porosity and morphology of said membranes. Membranes made from vinylidene fluorine polymers [polymer (VDF)] are hydrophobic in nature and therefore endowed with water repellency, low water permeability and subject to fouling of particles, proteins at their surface. Hydrophobicity impedes water to penetrate into the fluoropolymer membrane and therefore water permeability requires higher pressure and consumes more energy. Fouling reduces temporarily or permanently the flux of permeation of water through the membrane e.g. in ultrafiltration or microfiltration processes.

Membranes made from vinylidene fluorine polymers [polymer (VDF)] are also widely used in beer brewing, removal of contaminants and spoilage organisms from food and beverage items and more and more in biopharmaceutical industry as a result of increasing disease prevalence, and therefore a surge in the demand for more purified drugs and vaccines.

Capability of permeating water through porous PVDF membrane is generally improved by making outer surfaces of the inner pores hydrophilic. Besides, it is generally accepted that an increase of the hydrophilicity of PVDF membranes offers better fouling resistance because most of proteins and other foulants are hydrophobic in nature.

Several strategies have been employed to make the porous PVDF membrane hydrophilic and thus rendering said membrane highly water permeable and highly resistant to fouling. Among approaches that have been pursued, one can cite approaches based on grafting hydrophilic species on the surface of membranes, incorporation of hydrophilic comonomers in polymer chain of main vinylidenefluoride polymer, incorporation of hydrophilization additives, etc. . . These approaches are reviewed e.g. in Surface Modifications for Antifouling Membranes, Chemical Reviews, 2010, Vol. 110, No. 4, p.2448- 2471. The use of zwitterionic structures for hydrophilization of PVDF based membranes is part of these approaches and of the greatest interest.

WO 2015/070004 discloses zwitterionic containing membranes wherein a selective layer formed of a statistical copolymer comprising zwitterionic repeat units and hydrophobic repeat units such as p(MMA-s-SBMA) is disposed on a support layer formed of porous PVDF membrane. However, nothing is said neither about durability of the resulting membrane nor about their resistance to chemical aging.

Hydrophilization additives for PVDF based membranes is proposed in US 2018/0001278 which discloses comb-shaped and random zwitterionic copolymers (e.g. p(MMA-r-SBMA)) useful to enhance hydrophilicity of PVDF membranes. Resulting additivated PVDF membranes show good resistance against fouling and improved permeability when compared to PVDF membranes. However, to obtain such results, a relatively high amount of additive, that can impair mechanical, chemical resistance of the PVDF membrane as well as its economical attractiveness, is required.

It is therefore essential to develop a highly permeable porous membrane, with controlled pores size and demonstrating anti-fouling behavior. Said membrane should show high thermal and chemical stabilities which can ensure durable properties. It is also essential to develop additives, having high thermal and chemical stabilities, capable of hydrophilizing PVDF membranes into which they are dispersed. These additives have to be easily and durably incorporated in the vinylidenefluoride polymer membrane in order to enhance their hydrophilicity, water permeability and anti-fouling behavior on the long term without impairing inherent properties of vinylidenefluoride polymers which are, high mechanical, thermal and chemical properties. In addition, the additives have to be very efficient hydrophilization agents in order to be used sparingly, thus avoiding any detrimental effect due to their presence in too large amount on the mechanical, thermal and chemical resistance of the porous PVDF membrane.

In addition to this, there is a continuous need for providing more sustainable solutions for porous membranes, both in terms of durability of their properties over time and increasing their lifetime while reducing the frequency of cleaning washes required.

As mentioned above, a common phenomenon is observed over time, namely fouling resulting in the reduction of the flux of permeation at constant concentration and pressure and which can go as far as complete blockage of the membrane. Thus, the unavoidable external surface fouling leads to increase the required pressure in order to continue the filtration operation. Moreover, this leads to repeating more often washing cycles, both mechanical backwash (which consists of reversing the pressures to return the water produced through the membrane for eliminating fouling), and chemical cleanings and therefore what leads to excessive energy consumption.

Therefore, there is a real need to improve the flux during filtration operation, while maintaining high filtration properties and this without increasing the energy consumption.

The present invention makes it possible to increase the flux of the porous membrane, while maintaining high filtration properties and performances, and reducing energy consumption and cleaning maintenance cycles and thus chemicals consumption for cleaning.

These long-lasting performances of the membranes can position them as a very attractive solution for water and waste water treatment companies and for biopharmaceutical, and food and beverage companies, willing to install high flux filtration systems, able to deal with high solid contents liquid influx, where lower cleaning maintenance is needed, and with an extended lifetime of the overall device.

Brief description of drawings

Figure l is a simplified scheme of the hollow fiber spinning machine used for manufacturing hollow fiber membranes.

Figure 2 is a schematic cut of the spinneret (annular die), through a plane parallel to the fiber extrusion flow.

Figure 3 is a schematic cut of the spinneret (annular die), through a plane perpendicular to the fiber extrusion flow Figure 4 illustrates the Captive Air Bubble (CAB) method for measuging contact angle.

Summary of invention

Thus, the present invention relates to the use of a composition [composition (C)] in a porous membrane for increasing the flux of said membrane wherein the porous membrane comprises the composition (C) comprising:

- at least one vinylidene fluoride (VDF) polymer [polymer (VDF)], and

- at least one copolymer [copolymer (N-ZW)] comprising

(a) recurring units [units (Rzw)] derived from at least one zwitterionic monomer [monomer (A)], and

(b) recurring units[units (RN)] derived from at least one at least one additional monomer [monomer (B)] different from monomer (A), wherein units (Rzw) represent 0.1 to 7 mol %, preferably 0.1 to 5 mol % based on the molar composition of the copolymer (N-ZW), and wherein the molecular weight of the copolymer (N-ZW) measured by gel permeation chromatography ranges from 25000 g/mol to 350000 g/mol, and wherein the weight ratio copolymer (N-ZW) /polymer (VDF) is at least 0.1/99.9 and/or is less than 25/75.

The invention also refers to a method for increasing the flux of a porous membrane comprising at least one vinylidene fluoride (VDF) polymer [polymer (VDF)], in which said porous membrane further comprises at least one copolymer [copolymer (N-ZW)] comprising:

(a) recurring units [units (Rzw)] derived from at least one zwitterionic monomer [monomer (A)], and

(b) recurring units[units (RN)] derived from at least one at least one additional monomer [monomer (B)] different from monomer (A), wherein units (Rzw) represent 0.1 to 7 mol %, preferably 0.1 to 5 mol % based on the molar composition of the copolymer (N-ZW), and wherein the molecular weight of the copolymer (N-ZW) measured by gel permeation chromatography ranges from 25000 g/mol to 350000 g/mol, and wherein the weight ratio copolymer (N-ZW) /polymer (VDF) is at least 0.1/99.9 and/or is less than 25/75.

The Applicant has surprisingly found that the use of at least one copolymer [copolymer (N-ZW)] in composition (C) as detailed above in membranes, is particularly effective for increasing the flux during filtration operation while delivering outstanding permeability performances in aqueous media filtration and separation processes.

The fact to use the copolymer |copolymer (N-ZW)] in addition to vinylidene fluoride (VDF) polymer in the manufacture of membranes makes it possible to the increasing flux of the porous membrane.

The polymer (VDF)

The expression “vinylidene fluoride polymer” and “polymer (VDF)” are used, within the frame of the present invention for designating polymers comprising recurring units derived from vinylidene fluoride, generally as major recurring units components. So, polymer (VDF) is generally a polymer essentially made of recurring units, more that 50 % by moles of said recurring units being derived from vinylidene fluoride (VDF).

Polymer (VDF) may further comprise recurring units derived from at least one fluorinated monomer different from VDF and/or may further comprise recurring units derived from a fluorine-free monomer (also referred to as “hydrogenated monomer”). The term “fluorinated monomer” is hereby intended to denote an ethylenically unsaturated monomer comprising at least one fluorine atom. The fluorinated monomer may further comprise one or more other halogen atoms (Cl, Br, I).

In particular, polymer (VDF) is generally selected among polyaddition polymers comprising recurring units derived from VDF and, optionally, recurring units derived from at least one ethylenically unsaturated monomer comprising fluorine atom(s) different from VDF, which is generally selected from the group consisting of:

(a) C2-C8 perfluoroolefms such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoroisobutylene;

(b) hydrogen-containing C2-C8 fluoroolefms different from VDF, such as vinyl fluoride (VF), trifluoroethylene (TrFE), hexafluoroisobutylene (HFIB), perfluoroalkyl ethylenes of formula CH2=CH-Rn, wherein Rn is a Ci-Ce perfluoroalkyl group;

(c) C2-C8 chloro- and/or bromo-containing fluoroolefms such as chlorotrifluoroethylene (CTFE);

(d) perfluoroalkylvinylethers (PAVE) of formula CF2=CFORn, wherein Rn is a Ci-Ce perfluoroalkyl group, such as CF3 (PMVE), C2F5 or C3F7;

(e) perfluorooxyalkylvinylethers of formula CF2=CFOXQ, wherein Xo is a a Ci- C12 perfluorooxyalkyl group comprising one or more than one ethereal oxygen atom, including notably perfluoromethoxyalkylvinylethers of formula CF2=CFOCF2ORf2, with RB being a C1-C3 perfluoro(oxy)alkyl group, such as - CF2CF3, -CF2CF2-O-CF3 and -CF ₃ ; and

(f) (per)fluorodioxoles of formula: wherein each of RB, Rf4, Rfs and Rf6, equal to or different from each other, is independently a fluorine atom, a Ci-Ce perfluoro(oxy)alkyl group, optionally comprising one or more oxygen atoms, such as -CF3, -C2F5, -C3F7, -OCF3 or - OCF2CF2OCF3

The vinylidene fluoride polymer [polymer (VDF)] is preferably a polymer comprising:

(a’) at least 60 % by moles, preferably at least 75 % by moles, more preferably 85 % by moles of recurring units derived from vinylidene fluoride (VDF);

(b’) optionally from 0.1 to 30%, preferably from 0.1 to 20%, more preferably from 0.1 to 15%, by moles of recurring units derived from a fluorinated monomer different from VDF; and

(c’) optionally from 0.1 to 10 %, by moles, preferably 0.1 to 5 % by moles, more preferably 0.1 to 1% by moles of recurring units derived from one or more hydrogenated monomer(s), all the aforementioned % by moles being referred to the total moles of recurring units of the polymer (VDF).

The said fluorinated monomer is advantageously selected in the group consisting of vinyl fluoride (VFi); trifluoroethylene (VF3); chlorotrifluoroethylene (CTFE); 1,2-difluoroethylene; tetrafluoroethylene (TFE); hexafluoropropylene (HFP); perfluoro(alkyl)vinyl ethers, such as perfluoro(methyl)vinyl ether (PMVE), perfluoro(ethyl) vinyl ether (PEVE) and perfluoro(propyl)vinyl ether (PPVE); perfluoro(l,3-dioxole); perfluoro(2,2- dimethyl-l,3-dioxole) (PDD). Preferably, the possible additional fluorinated monomer is chosen from chlorotrifluoroethylene (CTFE), hexafluor oproylene (HFP), trifluoroethylene (VF3) and tetrafluoroethylene (TFE).

The choice of the said hydrogenated monomer(s) is not particularly limited; alpha-olefins, (meth)acrylic monomers, vinyl ether monomers, styrenic mononomers may be used; nevertheless, to the sake of optimizing chemical resistance, embodiments wherein the polymer (F) is essentially free from recurring units derived from said hydrogenated comonomer(s) are preferred.

Accordingly, the vinylidene fluoride polymer [polymer (VDF)] is more preferably a polymer consisting essentially of

(a’) at least 60 % by moles, preferably at least 75 % by moles, more preferably 85 % by moles of recurring units derived from vinylidene fluoride (VDF);

(b’) optionally from 0.1 to 30%, preferably from 0.1 to 20%, more preferably from 0.1 to 15% by moles of a fluorinated monomer different from VDF; said fluorinated monomer being preferably selected in the group consisting of vinylfluoride (VFi), chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), tetrafluoroethylene (TFE), perfluoromethylvinylether (MVE), trifluoroethylene (TrFE) and mixtures therefrom, all the aforementioned % by moles being referred to the total moles of recurring units of the polymer (VDF).

Defects, end chains, impurities, chains inversions or branchings and the like may be additionally present in the polymer (VDF) in addition to the said recurring units, without these components substantially modifying the behavior and properties of the polymer (VDF).

As non-limitative examples of polymers (VDF) useful in the present invention, mention can be notably made of homopolymers of VDF, VDF/TFE copolymers, VDF/TFE/HFP copolymers, VDF/TFE/CTFE copolymers, VDF/TFE/TrFE copolymers, VDF/CTFE copolymers, VDF/HFP copolymers, VDF/TFE/HFP/CTFE copolymers and the like.

VDF homopolymers are particularly advantageous for being used as polymer (VDF) in the composition (C).

The melt index of the polymer (VDF) is advantageously at least 0.01, preferably at least 0.05, more preferably at least 0.1 g/10 min and advantageously less than 50, preferably less than 30, more preferably less than 20 g/10 min, when measured in accordance with ASTM test No. 1238, run at 230°C, under a piston load of 2.16 kg. The melt index of the polymer (VDF) is advantageously at least 0.1, preferably at least 1, more preferably at least 5 g/10 min and advantageously less than 70, preferably less than 50, more preferably less than 40 g/10 min, when measured in accordance with ASTM test No. 1238, run at 230°C, under a piston load of 5 kg.

The melt index of the polymer (VDF) is advantageously at least 0.1, preferably at least 0.5, more preferably at least 1 g/10 min and advantageously less than 30, preferably less than 20, more preferably less than 10 g/10 min, when measured in accordance with ASTM test No. 1238, run at 230°C, under a piston load of 21.6 kg.

The polymer (VDF) has advantageously a melting point (T _m ) advantageously of at least 120°C, preferably at least 125°C, more preferably at least 130°C and of at most 190°C, preferably at most 185°C, more preferably at most 180°C, when determined by DSC, at a heating rate of 10°C/min, according to ASTM D 3418.

Copolymer (N-ZW) comprising zwitterionic recurring units

Composition (C) used in the present invention for increasing the flux of a porous membrane comprises at least one copolymer [copolymer (N-ZW)] comprising:

(a) recurring units [units (Rzw)] derived from at least one zwitterionic monomer [monomer (A)], and

(b) recurring units[units (RN)] derived from at least one at least one additional monomer [monomer (B)] different from monomer (A).

The term “flux” is the mass flow through a solid divided by the solid surface (measurement of volume per unit area of membrane and time (1/h/m ² = LMH)) and is used herein in its usual meaning that is it indicates the permeation flux of the membrane in its conditions of use.

Generally, zwitterionic recurring units (Rzw) are derived from at least one zwitterionic monomer (A) that is neutral in overall charge but contains a number of group (C+) equal to the number of group (A-). The cationic charge(s) may be contributed by at least one onium or inium cation of nitrogen, such as ammonium, pyridinium and imidazolinium cation; phosphorus, such as phosphonium; and/or sulfur, such as sulfonium. The anionic charge(s) may be contributed by at least one carbonate, sulfonate, phosphate, phosphonate, phosphinate or ethenolate anion, and the like. Suitable zwitterionic monomers include, but are not limited to, betaine monomers, which are zwitterionic and comprise an onium atom that bears no hydrogen atoms and that is not adjacent to the anionic atom.

In some embodiments, units (Rzw) are derived from at least one monomer (A) selected from the list consisting of a) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl acrylates or methacrylates, acrylamido or methacrylamido, typically

- sulfopropyldimethylammonioethyl (meth)acrylate,

- sulfoethyldimethylammonioethyl (meth)acrylate,

- sulfobutyldimethylammonioethyl (meth)acrylate,

- sulfohydroxypropyldimethylammonioethyl (meth)acrylate,

- sulfopropyl dimethyl ammoni opropy 1 aery 1 ami de,

- sulfopropyldimethylammoniopropylmethacrylamide,

- sulfohydroxypropyldimethylammoniopropyl(meth)acrylamide,

- sulfopropyldiethylammonio ethoxyethyl methacrylate. b) heterocyclic betaine monomers, typically

- sulfobetaines derived from piperazine,

- sulfobetaines derived from 2-vinylpyridine and 4-vinylpyridine, more typically 2- vinyl-l-(3-sulfopropyl)pyridinium betaine or 4-vinyl-l-(3- sulfopropyljpyridinium betaine,

- 1 -vinyl-3 -(3 -sulfopropyljimidazolium betaine; c) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl allylics, typically sulfopropylmethyldiallylammonium betaine; d) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl styrenes; e) betaines resulting from ethylenically unsaturated anhydrides and dienes; f) phosphobetaines of formulae and g) betaines resulting from cyclic acetals, typically ((dicyanoethanolate)ethoxy)dimethylammoniopropylmethacrylami de.

In some preferred embodiments, units (Rzw) are derived from at least one monomer (A) selected from the list consisting of

- sulfopropyldimethylammonioethyl acrylate,

- sulfopropyldimethylammonioethyl methacrylate (SPE),

- sulfopropyldimethylammoniopropyl acrylamide,

- sulfopropyldimethylammoniopropyl methacrylamide,

- sulfohydroxypropyldimethylammonioethyl acrylate,

- sulfohydroxypropyldimethylammonioethyl methacrylate (SHPE),

- sulfohydroxypropyldimethylammoniopropyl acrylamide (AHPS),

- sulfohydroxypropyldimethylammoniopropyl methacrylamide (SHPP)

- l-(3-Sulphonatopropyl)-2-vinylpyridinium (2SPV), and

- l-(3-Sulphonatopropyl)-4-vinylpyridinium (4SPV).

In some more preferred embodiments, units (Rzw) are derived from at least one monomer (A) selected from the list consisting of

- sulfopropyldimethylammonioethyl acrylate,

- sulfopropyldimethylammonioethyl methacrylate,

- l-(3-Sulphonatopropyl)-2-vinylpyridinium, and

- l-(3-Sulphonatopropyl)-4-vinylpyridinium.

In some even more preferred embodiments, units (Rzw) are derived from

- sulfopropyldimethylammonioethyl methacrylate (SPE), or

- l-(3-Sulphonatopropyl)-2-vinylpyridinium (2SPV). Copolymer (N-ZW) according to the invention, besides comprising recurring units (Rzw) derived from at least one zwitterionic monomer (A), also comprises recurring units (RN) derived from at least one at least one additional monomer (B) different from monomer (A).

Often, units (RN) are derived from at least one monomer deprived of ionisable groups.

In some embodiments, units (RN) are derived from at least one monomer selected from the list consisting of methyl methacrylate, ethyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, vinyl acetate and N,N-dimethylacrylamide [units (RN-I)]. Preferably, units (RN-I) are derived from methyl methacrylate, ethyl methacrylate or mixture thereof. More preferably, units (RN-I) are derived from methyl methacrylate.

In some other embodiments, units (RN) are derived from at least one monomer selected from the list consisting of 2-hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate, 2-hydroxyethyl acrylate (HEA), hydroxypropyl acrylate, 4-hydroxybutyl acrylate, polyethylene glycol) methacrylate (PEGMA), poly(ethylene glycol) methyl ether methacrylate (mPEGMA), poly(ethylene glycol) ethyl ether methacrylate, poly(ethylene glycol) methyl ether acrylate and poly(ethylene glycol) ethyl ether acrylate [units (RN-2)]. Preferably, units (RN-2) are derived from 2-hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate or mixture thereof. More preferably, units (RN-2) are derived from 2-hydroxyethyl methacrylate (HEMA).

Still in some other embodiment, units (RN) are derived from at least one monomer selected from at least one monomer selected from the list consisting of methyl methacrylate, ethyl methacrylate, butyl acrylate, methyl acrylate, ethyl acrylate, butyl acrylate, vinyl acetate and N,N-dimethylacrylamide [units (RN-I)] and from at least one monomer selected from the list consisting of 2- hydroxy ethyl methacrylate (HEMA), hydroxypropyl methacrylate, 2- hydroxyethyl acrylate (HEA), hydroxypropyl acrylate, 4-hydroxybutyl acrylate, polyethylene glycol) methacrylate (PEGMA), poly(ethylene glycol) methyl ether methacrylate (mPEGMA), poly(ethylene glycol) ethyl ether methacrylate, poly(ethylene glycol) methyl ether acrylate and poly(ethylene glycol) ethyl ether acrylate [units (RN-2)]. Preferably, units (RN-I) are derived from methyl methacrylate, ethyl methacrylate or mixture thereof and units (RN-2) are derived from 2-hydroxyethyl methacrylate (HEMA), hydroxypropyl methacrylate or mixture thereof. More preferably, units (RN-I) are derived from methyl methacrylate and units (RN-2) are derived from 2-hydroxy ethyl methacrylate (HEMA).

In some preferred embodiments, the copolymer (N-ZW) of the composition (C) used in the present invention comprises recurring units (Rzw) derived from sulfopropyldimethylammonioethyl methacrylate (SPE), l-(3- Sulphonatopropyl)-2-vinylpyridinium (2SPV) or mixtures thereof and recurring units (RN-I) derived from methyl methacrylate.

In some more preferred embodiments, the copolymer (N-ZW) of the present disclosure comprises recurring units (Rzw) derived from sulfopropyldimethylammonioethyl methacrylate (SPE) and recurring units (RN-I) derived from methyl methacrylate.

Still in some more preferred embodiments, the copolymer (N-ZW) of the composition (C) used in the present invention comprises recurring units (Rzw) derived from (SPE) or (2SPV), recurring units (RN-I) derived from methyl methacrylate and recurring units (RN-2) derived from 2-hydroxy ethyl methacrylate (HEMA).

The copolymer (N-ZW) of the composition (C) according to the present disclosure generally comprises 80 % or more by moles, preferably 90% or more by moles, more preferably 93% or more by moles and even more preferably 95% or more by moles of units (RN), with respect to the total moles of recurring units of copolymer (N-ZW).

When recurring units (RN-I) and recurring units (RN-2) are present, copolymer (N-ZW) generally comprises from 0.1 to 50 % by moles, preferably from 0.1 to 40 % by moles, more preferably from 0.1 to 30% by moles and even more preferably from 0.1 to 20 % by moles of recurring units (Rzw) and (RN-2), with respect to the total moles of recurring units of copolymer (N-ZW).

Copolymer (N-ZW) of the composition (C) used in the present invention for increasing the flux is a block copolymer, a branched copolymer or a statistical copolymer. Good results were obtained with copolymer (N-ZW) being a statistical copolymer.

Unless otherwise indicated, when molar mass is referred to, the reference will be to the weight-average molar mass, expressed in g/mol. The latter can be determined by gel permeation chromatography (GPC) with light scattering detection (DLS or alternatively MALLS) or refractive index detection, with an aqueous eluent or an organic eluent (for example dimethylacetamide, dimethylformamide, and the like), depending on the copolymer (N-ZW). The weight-average molar mass (Mw) of the copolymer (N-ZW) is in the range of from 25,000 to 350,000 g/mol, typically from about 35,000 to about 300,000, g/mol, more typically from about 70,000 to 250,000 g/mol, even more typically 80,000 to 200,000 g/mol.

The copolymer (N-ZW) of the composition (C) used in the present invention for increasing the flux of the membrane may be obtained by any polymerization process known to those of ordinary skill. For example, the copolymer (N-ZW) may be obtained by radical polymerization or controlled radical polymerization in aqueous solution, in dispersed media, in organic solution or in organic/water solution (miscible phase).

The monomer deprived of ionisable groups from which can be derived units (RN) may be obtained from commercial sources.

The zwitterionic monomer from which are derived units (Rzw) may be obtained from commercial sources or synthesized according to methods known to those of ordinary skill in the art.

Suitable zwitterionic monomers from which can be derived units (Rzw) include, but are not limited to monomers selected from the list consisting of: a) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl acrylates or methacrylates, acrylamido or methacrylamido, typically:

- sulfopropyldimethylammonioethyl methacrylate, sold by Raschig under the name RALU®MER SPE

- sulfoethyldimethylammonioethyl methacrylate,

- sulfobutyldimethylammonioethyl methacrylate: the synthesis of which is described in the paper “Sulfobetaine zwitterionomers based on n-butyl acrylate and 2-ethoxyethyl acrylate: monomer synthesis and copolymerization behavior”, Journal of Polymer Science, 40, 511-523 (2002),

- sulfohydroxypropyldimethylammonioethyl methacrylate, and other hydroxyalkyl sulfonates of dialkylammonium alkyl acrylates or methacrylates, acrylamido or methacrylamido of formulae below

- sulfopropyl dimethyl ammoni opropy 1 aery 1 ami de, the synthesis of which is described in the paper “Synthesis and solubility of the poly(sulfobetaine)s and the corresponding cationic polymers: 1. Synthesis and characterization of sulfobetaines and the corresponding cationic monomers by nuclear magnetic resonance spectra”, Wen-Fu Lee and Chan- Chang Tsai, Polymer, 35 (10), 2210-2217 (1994),

- sulfopropyldimethylammoniopropylmethacrylamide, sold by Raschig under the name SPP:

- sulfopropyldiethylammonio ethoxyethyl methacrylate: the synthesis of which is described in the paper “Poly(sulphopropylbetaines): 1. Synthesis and characterization”, V. M. Monroy Soto and J. C. Galin, Polymer, 1984, Vol. 25, 121-128; b) heterocyclic betaine monomers, typically:

- sulfobetaines derived from piperazine having any one of the following structures

the synthesis of which is described in the paper “Hydrophobically Modified Zwitterionic Polymers: Synthesis, Bulk Properties, and Miscibility with Inorganic Salts”, P. Koberle and A. Laschewsky, Macromolecules, 27, 2165-2173 (1994), and other hydroxyalkyl sulfonates derived from piperazine of formul ae b elo w

- sulfobetaines derived from 2-vinylpyridine and 4vinylpyridine, such as 2-vinyl-l-(3-sulfopropyl)pyridinium betaine (2SPV), sold by Raschig under the name SP V : and 4-vinyl-l-(3-sulfopropyl)pyridinium betaine (4SPV), the synthesis of which is disclosed in the paper “Evidence of ionic aggregates in some ampholytic polymers by transmission electron microscopy”, V. M. Castano and A. E. Gonzalez, J. Cardoso, O. Manero and V. M. Monroy, J. Mater. Res., 5 (3), 654-657 (1990), and other hydroxyalkyl sulfonates derived from 2-vinylpyridine and 4vinylpyridine of formulae below

- 1 -vinyl-3 -(3 -sulfopropyl)imidazolium betaine: the synthesis of which is described in the paper “Aqueous solution properties of a poly(vinyl imidazolium sulphobetaine)”, J. C. Salamone, W. Volkson, A.P. Oison, S.C. Israel, Polymer, 19, 1157-1162 (1978), and corresponding hydroxy alkyl sulfonate of formula below c) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl ally lies, typically sulfopropylmethyldiallylammonium betaine: the synthesis of which is described in the paper “New poly(carbobetaine)s made from zwitterionic diallylammonium monomers”, Favresse, Philippe; Laschewsky, Andre, Macromolecular Chemistry and Physics, 200(4), 887-895 (1999), d) alkyl or hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl styrenes, typically compounds having any one of the following structures: the synthesis of which is described in the paper “Hydrophobically Modified Zwitterionic Polymers: Synthesis, Bulk Properties, and Miscibility with Inorganic Salts”, P. Koberle and A. Laschewsky, Macromolecules, 27, 2165-2173 (1994), and other hydroxyalkyl sulfonates of dialkylammonium alkyl styrenes of formulae below e) betaines resulting from ethylenically unsaturated anhydrides and dienes, typically compounds having any one of the following structures: the synthesis of which is described in the paper “Hydrophobically Modified Zwitterionic Polymers: Synthesis, Bulk Properties, and Miscibility with Inorganic Salts”, P. Koberle and A. Laschewsky, Macromolecules, 27, 2165-2173 (1994), f) phosphobetaines having any one of the following structures: the synthesis of which are disclosed in EP 810 239 Bl (Biocompatibles, Alister et al.); g) betaines resulting from cyclic acetals, typically ((dicyanoethanolate)ethoxy)dimethylammoniumpropylmethacrylam ide: the synthesis of which is described by M-L. Pujol-Fortin et al. in the paper entitled “Poly(ammonium alkoxydicyanatoethenolates) as new hydrophobic and highly dipolar poly(zwitterions). 1. Synthesis”, Macromolecules, 24, 4523-4530 (1991).

Suitable monomers comprising hydroxyalkyl sulfonate moieties from which can be derived units (Rzw) can be obtained by reaction of sodium 3- chloro-2-hydroxypropane-l -sulfonate (CHPSNa) with monomer bearing tertiary amino group, as described in US20080045420 for the synthesis of SHPP, starting from dimethylaminopropylmethacrylamide according to the reaction scheme: Other monomers bearing tertiary amino group may be involved in reaction with CHPSNa to obtain suitable monomers from which are derived units (Rzw) :

Suitable monomers from which are derived units (Rzw) may be also obtained by reaction of sodium 3 -chloro-2-hydroxypropane-l -sulfonate (CHPSNa) with monomer bearing pyridine or imidazole group: The expression “derived from” which puts recurring units (Rzw) in connection with a monomer is intended to define both recurring units (Rzw) directly obtained from polymerizing the said monomer, and the same recurring units (Rzw) obtained by modification of an existing polymer.

Accordingly, recurring units (Rzw) may be obtained by modification of a polymer referred to as a precursor polymer comprising recurring units bearing tertiary amino groups through the reaction with sodium 3-chloro-2- hydroxypropane-1 -sulfonate (CHPSNa). Similar modification was described in WO2008125512 with sodium 3 -chloropropane- 1 -sulfonate in place of CHPSNa:

Finally, recurring units (Rzw) may be obtained by chemical modification of a polymer referred to as a precursor polymer with a sultone, such as propane sultone or butane sultone, a haloalkylsulfonate or any other sulfonated electrophilic compound known to those of ordinary skill in the art. Exemplary synthetic steps are shown below:

Similarly, recurring units (Rzw) may be obtained by modification of a polymer referred to as a precursor polymer comprising recurring units bearing tertiary amino groups, pyridine groups, imidazole group or mixtures thereof through the reaction with sodium 3 -chloro-2-hydroxypropane-l -sulfonate (CHPSNa), a sultone, such as propane sultone or butane sultone, or a haloalkylsulfonate.

As copolymer (N-ZW) is used as an additive for polymer (VDF), the polymer (VDF) is generally present in predominant amount over copolymer (N- ZW) in composition (C). Generally the weight ratio copolymer (N-ZW)/polymer (VDF) is of at least 0.1/99.9 wt/wt, preferably at least 1/99 wt/wt, more preferably at least 3/97 wt/wt and/or it is less than 25/75 wt/wt, preferably less than 20/80 wt/wt, more preferably less than 15/85 wt/wt and even more preferably less than 10/90 wt/wt.

Composition (C) may optionally comprise at least one further ingredient. Said further ingredient can preferably be selected in the group consisting of nonsolvents (water, alcohols...), co-solvents (e.g. ketones), pore forming agents, nucleating agents, fillers, nanoparticles, salts, surfactants.

When used, pore forming agents are typically added to the composition (C) in amounts usually ranging from 1% to 30% by weight, preferably from 2% to 20% by weight, based on the total weight of the composition (C). Suitable pore forming agents are for instance polyvinyl alcohol (PVA), polyvinyl-pyrrolidone (PVP) and polyethylene glycol (PEG).

Liquid medium

In some embodiments, composition (C) further comprises at least one liquid medium [medium (L)] comprising at least one organic solvent [composition (C ^L )].

The term “solvent” is used herein in its usual meaning, that is, it indicates a substance capable of dissolving another substance (solute) to form a uniformly dispersed mixture at the molecular level. In the case of a polymeric solute, it is common practice to refer to a solution of the polymer in a solvent when the resulting mixture is transparent and no phase separation is visible in the system. Phase separation is taken to be the point, often referred to as “cloud point”, at which the solution becomes turbid or cloudy due to the formation of polymer aggregates.

Generally, in composition (C ^L ), medium (L) comprises at least one solvent (S) for polymer (VDF). The medium (L) typically comprises at least one organic solvent selected from the group comprising:

- aliphatic hydrocarbons including, more particularly, the paraffins such as, in particular, pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane or cyclohexane, and naphthalene and aromatic hydrocarbons and more particularly aromatic hydrocarbons such as, in particular, benzene, toluene, xylenes, cumene, petroleum fractions composed of a mixture of alkylbenzenes;

- aliphatic or aromatic halogenated hydrocarbons including more particularly, perchlorinated hydrocarbons such as, in particular, tetrachloroethylene, hexachloroethane;

- partially chlorinated hydrocarbons such as dichloromethane, chloroform, 1,2- dichloroethane, 1,1,1 -trichloroethane, 1 , 1 ,2,2-tetrachloroethane, pentachloroethane, trichloroethylene, 1 -chlorobutane, 1,2-di chlorobutane, monochlorobenzene, 1,2-di chlorobenzene, 1,3 -di chlorobenzene, 1,4- dichlorobenzene, 1,2,4-trichlorobenzene or mixture of different chlorobenzenes;

- aliphatic, cycloaliphatic or aromatic ether oxides, more particularly, diethyl oxide, dipropyl oxide, diisopropyl oxide, dibutyl oxide, methyltertiobutyl ether, dipentyl oxide, diisopentyl oxide, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dibutyl ether benzyl oxide; dioxane, tetrahydrofiiran (THF);

- dimethylsulfoxide (DMSO);

- glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monophenyl ether, ethylene glycol monobenzyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n- butyl ether;

- glycol ether esters such as ethylene glycol methyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate;

- alcohols, including polyhydric alcohols, such as methyl alcohol, ethyl alcohol, diacetone alcohol, ethylene glycol;

- ketones such as acetone, methylethylketone, methylisobutyl ketone, diisobutylketone, cyclohexanone, isophorone;

- linear or cyclic esters such as isopropyl acetate, n-butyl acetate, methyl acetoacetate, dimethyl phthalate, y-butyrolactone;

- linear or cyclic carboxamides such as N,N-dimethylacetamide (DMAc), N,N- diethylacetamide, dimethylformamide (DMF), diethylformamide or N-methyl-2- pyrrolidone (NMP);

- organic carbonates for example dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethylmethyl carbonate, ethylene carbonate, vinylene carbonate;

- phosphoric esters such as trimethyl phosphate, triethyl phosphate (TEP);

- ureas such as tetramethylurea, tetraethylurea;

- methyl-5-dimethylamino-2-methyl-5-oxopentanoate (commercially available under the trademark Rhodialsov Polarclean®).

The following are preferred: linear or cyclic carboxamides such as N,N- dimethylacetamide (DMAc), N,N-diethylacetamide, dimethylformamide (DMF), diethylformamide or N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), methyl-5-dimethylamino-2-methyl-5- oxopentanoate (commercially available under the trademark Rhodialsov Polarclean®) and triethylphosphate (TEP).

Linear or cyclic carboxamides such as N,N-dimethylacetamide (DMAc),

N,N-diethylacetamide, dimethylformamide (DMF), diethylformamide or N- methyl-2-pyrrolidone (NMP) are particularly preferred.

N-methyl-pyrrolidone (NMP) and dimethyl acetamide (DMAc) are even more preferred.

The medium (L) may further comprise at least one additional liquid component different from solvent (S) (or in other terms, a non-solvent).

Said additional liquid component, which does not have ability to dissolve polymer (VDF), may be added to composition (C ^L ), in an amount generally below the level required to reach the cloud point, typically in amount of from

O.1% to 40% by weight, preferably in an amount of from 0.1% to 20% by weight, based on the total weight of medium (L) of the composition (C ^L ). Without being bound by this theory, it is generally understood that the addition of a non-solvent to composition (C ^L ) could be advantageously beneficial in increasing rate of demixing/coagulation in processes for manufacturing porous membranes, and/or for promoting coagulation by removal of solvent (S) by evaporation.

Generally, the composition (C ^L ) comprises an overall amount of copolymer (N-ZW) and polymer (VDF) of at least 1 wt.%, more preferably of at least 3 wt.%, even more preferably of at least 5 wt.%, based on the total weight of medium (L), copolymer (N-ZW) and polymer (VDF), and/or composition (C ^L ) preferably comprises an overall amount of copolymer (N-ZW) and polymer (VDF) of at most 60 wt.%, more preferably of at most 50 wt.%, even more preferably at most 30 wt.%, based on the total weight of medium (L), copolymer (N-ZW) and polymer (VDF) and/or composition (C ^L ).

Conversely, the amount of medium (L) in composition (C ^L ) is of at least 40 wt.%, preferably at least 50 wt.%, even more preferably at least 70 wt.%, based on the total weight of medium (L), copolymer (N-ZW) and polymer (VDF), and/or the amount of medium (L) in composition (C ^L ) is of at most 99 wt.%, preferably at most 97 wt.%, even more preferably at most 95 wt.%, based on the total weight of medium (L), copolymer (N-ZW) and polymer (VDF).

Composition (C ^L ) may optionally comprise at least one further ingredient. Said further ingredient is preferably selected in the group consisting of pore forming agents, nucleating agents, fillers, salts, surfactants.

When used, pore forming agents are typically added to the composition (C ^L ) in amounts usually ranging from 0.1% to 30% by weight, preferably from 0.5% to 20% by weight, based on the total weight of the composition (C ^L ). Suitable pore forming agents are for instance polyvinyl alcohol (PVA), cellulose acetate, polyvinyl-pyrrolidone (PVP) and polyethyleneglycol (PEG).

Porous membranes

According to the present invention, the composition (C) is used in a porous membrane for increasing flux, said porous membrane comprising:

- at least one vinylidene fluoride polymer [polymer (VDF)], and

- at least one copolymer [copolymer (N-ZW)] comprising

(a) recurring units [units (Rzw)] derived from at least one zwitterionic monomer [monomer (A)], and

(b) recurring units [units (RN)] derived from at least one additional monomer [monomer (B)] different from monomer (A), wherein units (Rzw) represent 0.1 to 7 mol %, preferably 0.1 to 5 mol % based on the molar composition of the copolymer (N-ZW), and wherein the molecular weight of the copolymer (N-ZW) measured by gel permeation chromatography (GPC) ranges from 25000 g/mol to 350000 g/mol, and wherein the weight ratio copolymer (N-ZW) /polymer (VDF) is at least 0.1/99.9 and/or is less than 25/75. The expression “porous membrane" is used according to its usual meaning in this technical field, i.e. to denote membrane including pores, i.e. voids or cavities of any shape and size.

As said, the porous membrane used in the present invention is obtainable from the composition (C ^L ) as detailed above and/or can be manufactured using the method as below detailed.

The porous membrane of the invention may be in the form of flat membranes or in the form of tubular membranes.

Flat membranes are generally preferred when high fluxes are required whereas hollow fibers membranes are particularly advantageous in applications wherein compact modules having high surface areas are required.

Flat membranes preferably have a thickness comprised between 10 pm and 200 pm, more preferably between 15 pm and 150 pm.

Tubular membranes typically have an outer diameter greater than 3 mm. Tubular membranes having an outer diameter comprised between 0.5 mm and 3 mm are typically referred to as hollow fibers membranes. Tubular membranes having a diameter of less than 0.5 mm are typically referred to as capillary membranes.

Membranes containing pores homogeneously distributed throughout their thickness are generally known as symmetric (or isotropic) membranes; membranes containing pores which are heterogeneously distributed throughout their thickness are generally known as asymmetric (or anisotropic) membranes.

The porous membrane according to the present invention may be either a symmetric membrane or an asymmetric membrane.

The asymmetric porous membrane typically consists of one or more layers containing pores which are heterogeneously distributed throughout their thickness.

The asymmetric porous membrane typically comprises an outer layer containing pores having an average pore diameter smaller than the average pore diameter of the pores in one or more inner layers.

The porous membrane of the invention preferably has an average pore diameter of at least 0.001 pm, more preferably of at least 0.005 pm, and even more preferably of at least 0.01 pm. The porous membrane of the invention preferably has an average pore diameter of at most 50 pm, more preferably of at most 20 pm and even more preferably of at most 15 pm. Suitable techniques for the determination of the average pore diameter in the porous membranes of the invention are described for instance in the Handbook of Industrial Membrane Technology, edited by PORTER. Mark C. Noyes Publications, 1990. p.70-78.

In the present invention, the porous membrane typically has a gravimetric porosity comprised between 5% and 90%, preferably between 10% and 85% by volume, more preferably between 30% and 90%, based on the total volume of the membrane.

For the purpose of the present invention, the term “gravimetric porosity” is intended to denote the fraction of voids over the total volume of the porous membrane.

Suitable techniques for the determination of the gravimetric porosity in the porous membranes of the invention are described for instance in SMOLDERS K., et al. Terminology for membrane distillation. Desalination. 1989, vol.72, p.249-262.

The porous membrane of the invention may be either a self-standing porous membrane or a porous membrane supported onto a substrate and/or comprising a backing layer.

The porous membrane comprises at least one layer comprising at least one polymer (VDF) and at least one copolymer (N-ZW).

A porous membrane supported onto a substrate is typically obtainable by laminating said substrate and/or backing with a pre-formed porous membrane or by manufacturing the porous membrane directly onto said substrate and/or said backing.

Hence, porous membrane may be composed of one sole layer comprising polymer (VDF) and copolymer (N-ZW) or may comprise additional layers.

In particular, the porous membrane of the invention may further comprise at least one substrate. The substrate may be partially or fully interpenetrated by the porous membrane of the invention.

The nature of the substrate/backing is not particularly limited. The substrate generally consists of materials having a minimal influence on the selectivity of the porous membrane. The substrate layer preferably consists of non-woven materials, polymeric materials such as, for example, polypropylene, glass, glass fibers.

One can also mention the incorporation of tubular braid or threads/fabric reinforcing the substrate layer, such as polyethylene terephthalate (PET) braid, particularly for improving the mechanical properties of polymer (VDF) porous membranes.

In some embodiments, the porous membrane of the invention is a porous composite membrane assembly comprising:

- at least one substrate layer, preferably a non-woven substrate,

- at least one top layer, and

- between said at least one substrate layer and said at least one top layer, at least one layer comprising at least one polymer (VDF) and at least one copolymer (N-ZW).

Typical examples of such porous composite membrane assembly are the so-called Thin Film Composite (TFC) structures which are typically used in reverse osmosis or nanofiltration applications.

Non limiting examples of top layers suitable for use in the porous composite membrane assemblies of the invention include those made of polymers selected from the group consisting of polyamides, polyimides, polyacrylonitriles, polybenzimidazoles, cellulose acetates and polyolefins.

Porous membrane layers comprising polymer (VDF) and copolymer (N- ZW) may additionally comprise one or more than one additional ingredient. Nevertheless, embodiments whereas porous membrane comprises at least one layer consisting essentially of polymer (VDF) and copolymer (N-ZW) are preferred, being understood that additives, and/or residues of pore forming agents may be present, in amounts not exceeding 5 wt.% of the said layer.

In the porous membrane, copolymer (N-ZW) is used as an additive for polymer (VDF), so it is generally understood that polymer (VDF) is present in predominant amount over copolymer (N-ZW). Generally, the weight ratio copolymer (N-ZW)/polymer (VDF) is of at least 0.1/99.9 wt/wt, preferably at least 1/99 wt/wt, more preferably at least 3/97 wt/wt and/or it is less than 50/50 wt/wt, preferably less than 40/60 wt/wt, preferably less than 30/70 wt/wt.

Manufacturing of porous membrane

According to the present invention, the composition (C) is used in a porous membrane for increasing flux.

The porous membrane is generally manufactured by a manufacturing method comprising: step (i): preparing a composition (C ^L ) as defined above; step (ii): processing the composition provided in step (i) thereby providing a film; and, step (iii): processing the film provided in step (ii), generally including contacting the film with a non-solvent medium [medium (NS)], thereby providing a porous membrane.

Under step (i), composition (C ^L ) is manufactured by any conventional techniques. For instance, medium (L) may be added to polymer (VDF) and copolymer (N-ZW), or, preferably, polymer (VDF) and copolymer (N-ZW) are added to medium (L), or even polymer (VDF), copolymer (N-ZW) and medium (L) are simultaneously mixed.

Any suitable mixing equipment may be used. Preferably, the mixing equipment is selected to reduce the amount of air entrapped in composition (C ^L ) which may cause defects in the final membrane. The mixing of polymer (VDF), copolymer (N-ZW) and the medium (L) may be conveniently carried out in a sealed container, optionally held under an inert atmosphere. Inert atmosphere, and more precisely nitrogen atmosphere has been found particularly advantageous for the manufacture of composition (C ^L ).

Under step (i), the mixing time and stirring rate required to obtain a clear homogeneous composition (C ^L ) can vary widely depending upon the rate of dissolution of the components, the temperature, the efficiency of the mixing apparatus, the viscosity of composition (C ^L ) and the like.

Under step (ii) of the manufacturing process, conventional techniques can be used for processing the composition (C ^L ) for providing a film.

The term “film” is used herein to refer to a layer of composition (C ^L ) obtained after processing of the same under step (ii) of the process of the invention. The term “film” is used herein in its usual meaning, that is to say that it refers to a discrete, generally thin, dense layer. Under step (ii), composition (C ^L ) is typically processed by casting thereby providing a film.

Casting generally involves solution casting, wherein typically a casting knife, a draw-down bar or a slot die is used to spread an even film of composition (C ^L ) across a suitable support.

Under step (ii), the temperature at which composition (C ^L ) is processed by casting may be or may be not the same as the temperature at which composition (C ^L ) is mixed under stirring.

Different casting techniques are used depending on the final form of the membrane to be manufactured.

When the final product is a flat membrane, composition (C ^L ) is cast as a film over a flat supporting substrate, typically a plate, a belt or a fabric, or another microporous supporting membrane, typically by means of a casting knife, a draw-down bar or a slot die.

According to a first embodiment of step (ii), composition (C ^L ) is processed by casting onto a flat supporting substrate to provide a flat film.

According to a second embodiment of step (ii), composition (C ^L ) is processed by casting to provide a tubular film.

According to a variant of this second embodiment, the tubular film is manufactured using a spinneret, this technique being otherwise generally referred to as "spinning method". Hollow fibers and capillary membranes may be manufactured according to the spinning method.

The term “spinneret” is hereby understood to mean an annular nozzle comprising at least two concentric capillaries: a first outer capillary for the passage of composition (C ^L ) and a second inner (generally referred to as “lumen”) for the passage of a supporting fluid, also referred to as “bore fluid”.

Figure l is a simplified scheme of the hollow fiber spinning machine (“Effect of spinning conditions on the structure and performance of hydrophobic PVDF hollow fiber membranes for membrane distillation”, Desalination, 287, 326-339 (15 February 2012)) which can be used for manufacturing hollow fiber membranes, wherein 3 is the dope solution tank equipped with a feeding pump 5, 1 is the nitrogen cylinder, 2 is the bore fluid cylinder, 6 is the spinneret or annular die, 7 is the coagulation bath where is depicted the nascent hollow fiber and 8 is the take-up wheel. Dope solution is pushed for the tank to the filter 4 and then pumped with the gear pump 5 through the nozzle 6. Air gap (distance between the nozzle and the coagulation bath) could be varied from 1 to several cm.

Figure 2 is a schematic cut of the spinneret (annular die), through a plane parallel to the fiber extrusion flow, wherein 1 is the bore fluid die, and 2 is the annular die feeding the dope solution.

Figure 3 is a schematic cut of the spinneret (annular die), through a plane perpendicular to the fiber extrusion flow, wherein 1 is the extruded/spinned bore fluid, 2 is the extruded/spinned dope solution, and 3 is the body of the spinneret/annular die.

According to this variant of the second embodiment, composition (C ^L ) is generally pumped through the spinneret, together with at least one supporting fluid (so called “bore fluid”). The supporting fluid acts as the support for the casting of the composition (C ^L ) and maintains the bore of the hollow fiber or capillary precursor open. The supporting fluid may be a gas, or, preferably, a non-solvent medium [medium (NS)] or a mixture of the medium (NS) with a medium (L). The selection of the supporting fluid and its temperature depends on the required characteristics of the final membrane as they may have a significant effect on the size and distribution of the pores in the membrane.

Step (iii) generally includes a step of contacting the film provided in step (ii) with a non-solvent medium [medium (NS)] thereby providing a porous membrane.

Such step of contacting with a medium (NS) is generally effective for precipitating and coagulating the composition (C ^L ) constituting the film of step (ii) into a porous membrane.

The film may be precipitated in said medium (NS) by immersion in a medium (NS) bath, which is often referred to as a coagulation bath.

As an alternative (or usually before immersing in a coagulation bath), contacting the film with the medium (NS) can be accomplished by exposing the said film to a gaseous phase comprising vapors of said medium (NS).

Typically, a gaseous phase is prepared e.g. by at least partial saturation with vapors of medium (NS), and the said film is exposed to said gaseous phase. For instance, air possessing a relative humidity of higher than 10 %, generally higher than 50 % (i.e. comprising water vapor) can be used.

Prior to being contacted with the non-solvent medium (by whichever technique as explained above), the film may be exposed during a given residence time to air and/or to a controlled atmosphere, in substantial absence of said medium (NS). Such an additional step may be beneficial for creating a skin on the exposed surface of the film through alternative mechanisms.

For instance, in the spinning method, this may be accomplished by imposing an air-gap in the path that the spinned hollow tubular precursor follows before being driven into a coagulation bath.

According to certain embodiments, in step (iii), coagulation/precipitation of the composition (C ^L ) may be promoted by cooling. In this case, the cooling of the film provided in step (ii) can be typically using any conventional techniques.

Generally, when the coagulation/precipitation is thermally induced, the solvent (S) of medium (L) of composition (C ^L ) is advantageously a “latent” solvent [solvent (LT)], i.e. a solvent which behaves as an active solvent towards polymer (VDF) only when heated above a certain temperature, and which is not able to solubilize the polymer (VDF) below the said temperature. When medium (L) comprises a latent solvent or solvent (LT), step (i) and step (ii) of the manufacturing method are generally carried out at a temperature high enough to maintain composition (C ^L ) as a homogeneous solution.

For instance, under step (ii), according to this embodiment, the film may be typically processed at a temperature comprised between 60°C and 250°C, preferably between 70°C and 220°C, more preferably between 80°C and 200°C, and under step (iii), the film may be typically precipitated by cooling to a temperature below 100°C, preferably below 60°C, more preferably below 40°C.

Cooling may be achieved by contacting the film provided in step (ii) with a cooling fluid, which may be a gaseous fluid (i.e. cooled air or cooled modified atmosphere) or may be a liquid fluid.

In this latter case, it is usual to make use of a medium (NS) as above detailed, so that the phenomena of non solvent-induced and thermally-induced precipitation may simultaneously occur.

It is nevertheless generally understood that even in circumstances where the precipitation is induced thermally, a further step of contacting with a medium (NS) is carried out, e.g. for finalizing precipitation and facilitating removal of medium (L).

In cases where the medium (L) comprises both a solvent (S) and a nonsolvent for polymer (VDF), at least partially selective evaporation of solvent (S) may be used for promoting coagulation/precipitation of polymer (VDF). In this case, solvent (S) and non-solvent components of medium (L) are typically selected so as to ensure solvent (S) having higher volatility than non-solvent, so that progressive evaporation, generally under controlled conditions, of the solvent (S) leads to polymer (VDF) precipitation, and hence actual contact of the film with the medium (NS).

When present in composition (C ^L ), pore forming agents are generally at least partially, if not completely, removed from the porous membrane in the medium (NS), in step (iii) of the method of the invention.

In all these approaches, it is generally understood that the temperature gradient during steps (ii) and (iii), the nature of medium (NS) and medium (L), including the presence of non-solvent in medium (L) are all parameters known to one of ordinary skills in the art for controlling the morphology of the final porous membrane including its average porosity. The manufacturing method may include additional post treatment steps, for instance steps of rinsing and/or stretching the porous membrane and/or a step of drying the same.

For instance, the porous membrane may be additionally rinsed using a liquid medium miscible with the medium (L).

Further, the porous membrane may be advantageously stretched so as to increase its average porosity.

Generally, the porous membrane is dried at a temperature of advantageously at least 30°C.

Drying can be performed under air or a modified atmosphere, e.g. under an inert gas, typically exempt from moisture (water vapor content of less than 0.001% v/v). Drying can alternatively be performed under vacuum.

For the purpose of the present invention, by the term “non-solvent medium [medium (NS)]” it is meant a medium consisting of one or more liquid substances incapable of dissolving the polymer (VDF) of composition (C) or (C ^L ), and which advantageously promotes the coagulation/precipitation of polymer (VDF) from liquid medium of composition (C ^L ).

The medium (NS) typically comprises water and, optionally, at least one organic solvent selected from alcohols or polyalcohols, preferably aliphatic alcohols having a short chain, for example from 1 to 6 carbon atoms, more preferably methanol, ethanol, isopropanol and ethylene glycol.

The medium (NS) is generally selected among those miscible with the medium (L) used for the preparation of composition (C ^L ).

The medium (NS) may further comprise a solvent (S), as above detailed.

More preferably, the medium (NS) consists of water. Water is the most inexpensive non-solvent medium and can be used in large amounts.

Method for increasing the flux of a porous membrane

A second aspect of the invention relates a method for increasing the flux of a porous membrane comprising at least one vinylidene fluoride (VDF) polymer [polymer (VDF)], in which said porous membrane further comprises at least one copolymer [copolymer (N-ZW)] comprising:

(a) recurring units [units (Rzw)] derived from at least one zwitterionic monomer [monomer (A)], and

(b) recurring units [units (RN)] derived from at least one additional monomer [monomer (B)] different from monomer (A), wherein units (Rzw) represent 0.1 to 7 mol %, preferably 0.1 to 5 mol % based on the molar composition of the copolymer (N-ZW), and wherein the molecular weight of the copolymer (N-ZW) measured by gel permeation chromatography ranges from 25000 g/mol to 350000 g/mol, and wherein the weight ratio copolymer (N-ZW) /polymer (VDF) is at least 0.1/99.9 and/or is less than 25/75.

Thus, as this is the case for the first aspect of the invention relating to the use of the composition (C) in porous membrane, in this method, the composition (C) comprising at least one vinylidene fluoride (VDF) polymer [polymer (VDF)], and at least one copolymer [copolymer (N-ZW)] as detailed above is present.

All features above described in connection with the use of the composition (C) for increasing the flux of the invention and with the porous membrane are applicable in connection to this method hereby described.

Separation of an aqueous medium

The porous membrane comprising composition (C) that makes it possible to increase flux can be used for separating an aqueous medium, by contacting said aqueous medium with the porous membrane as described above.

All features above described in connection with the porous membrane are applicable here.

According to certain embodiments, the aqueous phase may be notably a water-based phase comprising one or more contaminants.

The aqueous phase may be a particulate suspension of contaminants, i.e. a suspension comprising chemical or physical pollutants (e.g. inorganic particles such as sand, grit, metal particles, ceramics; organic solids, such as polymers, paper fibers, plants' and animals’ residues; biological pollutants such as bacteria, viruses, protozoa, parasites).

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

Otherwise, this may be notably a method for filtrating water suspensions from suspended particulates; in this case, the used porous membrane generally possesses an average pore diameter of from 5 pm to 50 pm. The invention will now be described in connection with the following examples, whose scope is merely illustrative and not intended to limit the scope of the invention.

Experimental

Raw Materials

PVDF Solef® 1015 provided by Solvay Specialty Polymers was used as VDF homopolymer.

The following solvents reactants and solvents were obtained from Sigma Aldrich and used as received: 2,2'-Azobis(2-methylbutyronitrile) (AMBN), Azobisisobutyronitrile (AIBN), methyl methacrylate (MMA), 3-((2- (methacryloyloxy)ethyl)dimethylammonio)propane-l -sulfonate also named N,N- Dimethyl- N-(2-methacryloyloxyethyl)-N-(3 -sulfopropyl) ammonium betaine (SPE), dimethylsulfoxide (DMSO), N,N-dimethylacetamide (DMAc), N-methyl- 2-pyrrolidone (NMP), polyethilenglycol 400 (PEG 400), Ethylene Glycol (EG), Polyvinylpyrrolidone (PVP) K 30 and isopropyl alcohol (IP A).

Molecular weight determination

Gel permeation chromatography (GPC) was performed at 40°C using a Jasco PU-2080 Plus HPLC pump equipped with 2 SHODEX KD-804 columns and a Jasco Refractive index-4030 detector. The mobile phase was composed of 1.5 % LiBr in DMF and the flow rate was of 1.0 ml/min. 100 pl samples (concentration of approximatively 5.0 mg/ml) were injected, calibration was obtained with PMMA narrow standards.

Example 1: Synthesis of poIy(MMA-stat-SPE) 95/5 mol/mol (MW = 103300 g/mol) - Pl

A 2 1 jacketed reactor with a five-necked kettle head was set with an overhead teflon pitch blade stirrer in the center port. The second port was used for the addition of chemicals. The third port was connected to a nitrogen inlet, while the fourth port was used to connect the double walled condenser. The fifth port was fixed with a temperature probe.

Methyl methacrylate (MMA) monomer (104.63 g) dissolved in 400 ml DMSO, was added into the reactor by transfer funnel. The quantity of MMA that adhered to the weighing container was transferred by additional 200 ml DMSO. Then, the N,N-Dimethyl- N-(2-methacryloyloxyethyl)-N-(3 -sulfopropyl) ammonium betaine (SPE) monomer (15.36 g) dissolved in a mixture of 70 ml of water and 400 ml of dimethyl sulfoxide (DMSO) (under sonication), was added to the methyl methacrylate (MMA) solution in the reactor under constant stirring. The remaining quantity that adhered to the weighing container was transferred to the reactor by rinsing with 82 ml DMSO. The reaction mixture was stirred using the overhead stirrer (maintained at 250 rpm) and N2 gas was purged for 1 h to remove dissolved oxygen in the solution. Simultaneously, the temperature of the solution (measured via the temperature probe) was increased from room temperature to 70 °C. Once the temperature of the solution reached 70 °C, initiator AIBN (1.806 g) was added to the reaction mixture by dissolving it in 18 ml DMSO. Nitrogen purging was continued for another 15 minutes after which the nitrogen purger was kept above the solution level. Addition of the initiator was considered as starting of polymerization and the reaction was continued for further 12 h. Kinetics of the polymerization was monitored by drawing out samples at every one hour interval and taking NMR spectra of the collected aliquots in DMSO-d ⁶ . Final conversion of the monomers was calculated from NMR spectrum and molecular weight was measured via GPC (DMF as solvent). After the polymerization the reaction mass was collected in a beaker and kept in the refrigerator overnight.

In this case to purify the polymer, crude was precipitated in 6.5 1 of water/isopropanol 90:10 v/v mixture under constant stirring (overhead fitted with half-moon stirrer blade) with the help of a peristaltic pump (feed rate of 10 ml/min). After the precipitation, the stirring continued for an additional 2 h. Subsequently, the precipitate was filtered using a Buchner funnel, under vacuum. The precipitate was then dried at 60°C under vacuum for 3 days.

The obtained precipitate exhibited a DMSO content of 12% (data obtained from GC-FID) after drying. The solid polymer was further crushed using a mortar and pestle and was redispersed in 1.5 1 of water/isopropanol 90:10 v / v mixture and stirred for 2 h. The solid was filtered using a Buchner funnel and dried at 60 °C under vacuum for 3 days. This process was repeated again to completely remove DMSO from the polymer.

After the polymerization, a sample was taken for ^l H NMR analysis to determine the MMA and SPE conversions.

The results were the following: MMA monomer conversion was equal to 97 %, SPE monomer conversion was equal to 94 % and molecular weight was of M PI = 103300 g/mol

Example 2: Synthesis of poIy(MMA-stat-SPE) 95/5 mol/mol (MW = 161000 g/mol) - P2

A 2 1 jacketed reactor with a five-necked kettle head was set with an overhead teflon pitch blade stirrer in the center port. The second port was used for the addition of chemicals. The third port was connected to a nitrogen inlet, while the fourth port was used to connect the double walled condenser. The fifth port was fixed with a temperature probe.

Methyl methacrylate (MMA) monomer (156.94 g) dissolved in 400 ml DMSO, was added into the reactor by transfer funnel. The quantity of MMA that adhered to the weighing container was transferred by additional 100 ml DMSO. Then, the N,N-Dimethyl- N-(2-methacryloyloxyethyl)-N-(3 -sulfopropyl) ammonium betaine (SPE) monomer (23.052 g) dissolved in a mixture of 50 ml of water and 200 ml of dimethyl sulfoxide (DMSO) (under sonication), was added to the methyl methacrylate (MMA) solution in the reactor under constant stirring. The remaining quantity that adhered to the weighing container was transferred to the reactor by rinsing with 81 ml DMSO. The reaction mixture was stirred using the overhead stirrer (maintained at 250 rpm) and N2 gas was purged for 1 h to remove dissolved oxygen in the solution. Simultaneously, the temperature of the solution (measured via the temperature probe) was increased from room temperature to 70 °C. Once the temperature of the solution reached 70 °C, initiator AIBN (1.084 g) was added to the reaction mixture by dissolving it in 19 ml DMSO. Nitrogen purging was continued for another 15 minutes after which the nitrogen purger was kept above the solution level. Addition of the initiator was considered as starting of polymerization and the reaction was continued for further 12 h. Kinetics of the polymerization was monitored by drawing out samples at every one hour interval and taking spectra of the collected aliquots in DMSO-d ⁶ . Final conversion of the monomers was calculated from spectrum and molecular weight was measured via GPC (DMF as solvent).

After the polymerization the reaction mass was collected in a beaker and kept in the refrigerator overnight.

In this case to purify the polymer, crude was precipitated in 6.5 1 of water/isopropanol 90:10 v/v mixture under constant stirring (overhead fitted with half-moon stirrer blade) with the help of a peristaltic pump (feed rate of 10 ml/min). After the precipitation, the stirring continued for an additional 2 h. Subsequently, the precipitate was filtered using a Buchner funnel, under vacuum. The precipitate was then dried at 60°C under vacuum for 3 days.

The obtained precipitate exhibited a DMSO content of 12% (data obtained from GC-FID) after drying. The solid polymer was further crushed using a mortar and pestle and was redispersed in 1.5 1 of water/isopropanol 90: 10 v / v mixture and stirred for 2 h. The solid was filtered using a Buchner funnel and dried at 60 °C under vacuum for 3 days. This process was repeated again to completely remove DMSO from the polymer.

After the polymerization, a sample was taken NMR analysis to determine the MMA and SPE conversions.

The results were the following: MMA monomer conversion was equal to 93 %, SPE monomer conversion was equal to 94 % and molecular weight was of MWPI = 161000 g/mol.

Example 3: Synthesis of poly(MMA-stat-SPE) 95/5 mol/mol (MW = 69000 g/mol) - P3

In a 500 ml kettle reactor equipped with a water condenser and a mechanical agitation, were introduced, at room temperature (22°C), 75 g (187.30 mmol) of a methyl methacrylate (MMA) solution (25 wt % in DMSO), 92.5 g of dimethyl sulfoxide (DMSO, at 99% purity) and 55.1 g (9.5 mmol) of a solution of 3 -((2-(methacryloyloxy)ethyl)dimethylammonio)propane-l -sulfonate (SPE) (5% in DMSO). The mixture was degassed by nitrogen bubbling for 50 minutes while the temperature of the reaction medium was raised up to 70°C. Then 15.16 g (1.5 mmol) of an AMBN solution (2% in DMSO) were introduced under a nitrogen blanket. The reaction was conducted for 10 hours at 70°C under stirring.

Afterwards, a sample was taken for ^X H NMR analysis to determine the MMA and SPE conversions.

The results were the following: MMA monomer conversion was equal to 98.1%; SPE monomer conversion was equal to 94.1 % and molecular weight was of MWP3 = 69000 g/mol.

General method for preparing dope solutions (for fibers spinning)

To prepare the dope solutions, the opportune amount of PVDF, pore forming agents (PVP K30/PEG400/EG) and eventually zwitterionic additive were added in a glass tank of DMAC (equipped with a mechanical anchor) and stirred at approximately 65°C. Dope solution quantity was 2 liters. The stirring lasted for several hours at 65°C. Then solutions were left at rest at 65°C for some hours to remove eventual air bubbles. Dope solutions were always homogeneous, transparent and stable for several days at temperatures equal or higher than 40°C.

Flat sheet membranes preparation

In the aim of hydrophilicity evaluation by contact angle measurement, some dope solutions prepared without any pore formers were degassed in a vacuum oven set at 40°C for 24 h. The dope solutions (PVDF + additive + solvent) were casted on a glass plate using an adjustable film applicator set to a 200 pm gate size and polymer blend precipitated out by immersion into a DI water bath at room temperature for 20 min. After this period, the resulting flat sheet membranes were moved to a fresh DI water bath and stored at least overnight before use. As a control, additive-free flat sheet PVDF membrane was manufactured by dissolving 0.5 g PVDF (Solef® 1015) in 4.5 g NMP and following the NIPS procedure explained above.

General method for preparing hollow fibers membranes containing zwitterionic additive

Membranes were spun from dope solutions containing blends of PVDF Solef® 1015 and of the synthesized zwitterionic p(MMA-s-SPE) copolymers in N,N-dimethylacetamide (DMAc) and immersed in a coagulation bath in order to induce phase separation (NIPS for non-solvent induced phase separation).

Thus, polymeric hollow fibers were manufactured by extruding the dope solutions, as detailed in figures 1 to 3, through an annular aperture. Hollow fibers were prevented from collapsing by coextruding water as bore fluid in the center of the annulus. The coagulation water bath enabled producing coagulation by phase inversion. Take up wheel allowed the collection of the fiber. Dope and spinneret temperatures were maintained at 70°C for one set of trials and 40°C for another set in order to match the viscosities. All the other conditions were the same as illustrated in Table 1 below. The spinneret geometry used in the extrusion part had an internal diameter (ID) of 700 pm, an external one of 1300 pm (OD).

The formulation prepared as reference had 18% wt/wt of Solef® 1015. The formulations (referred as Fl or F2 or F3) according to the invention had 16,2% wt/wt of Solef® 1015 and 1,8% of zwitterionic additive (referred as sample Pl or P2 or P3 respectively), in order to have a ratio of 90/10 PVDF/zwitterionic additive.

Table 1 : Main process conditions of the spinning trials.

Characterization of the hollow fiber membranes

Water flux permeability measurements

Water flux (J) through each membrane at given pressure, is defined as the volume which permeates per unit area and per unit time. The flux is calculated by the following equation: whereas V (1) is the volume of permeate, A (m ² ) is the membrane area, and At (h) is the operation time. J is hence measured in l/(h*m ² ), and this unit is otherwise referred to as LMH.

Water flux measurements as detailed in Table 2, were conducted at room temperature (23 °C) using a cross-flow configuration under a constant pressure of either 0.5 or 1 bar.

Gravimetric porosity and pores size

Gravimetric porosity of the membrane is defined as the volume of the pores divided by the total volume of the membrane. Membrane porosity (E _m ) was determined according to the gravimetric method, as detailed below. The porosities were measured using pure IPA as wetting fluid according to the procedure described, for instance, in the Appendix of SMOLDERS, K., et al. Terminology for membrane distillation. Desalination. 1989, vol.72, p.249-262. Perfectly dry hollow fiber membrane pieces were weighed and impregnated in isopropylic alcohol (IPA) for 24h; after this time, the excess of the liquid was removed with tissue paper, and membranes’ weights were measured again. Finally, from the dry and the wet weight of the sample, it is possible to evaluate the porosity of the membrane using the following formula: where W _w is the weight of the wet membrane,

Wa is the weight of the dry membrane, p _w is the IPA density (0.785 g/cm ³ ) and a is the polymer density (equal to 1.72 g/cm ³ for PVDF).

For all membranes types, at least three measurements were performed; then, average values and corresponding standard deviations were calculated.

Pore size distributions were determined by using a capillary porometer Porolux 1000 by Porometer N.V. Bubble point (largest pore diameter), mean flow pore and smallest pore obtained by following ASTM norm F316 - 03 “Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test”.

Mechanical (Tensile) test

All the tests on the extruded fibers were performed following the ASTM D638 type V method with an initial length L0 of 125 mm and a velocity of 125 mm/min.

All the tested fibers were stored in water without any supplementary treatment. During the tests the fibers were maintained wet: each test involved at least five iterations on several fiber specimens. From these measurements, apparent modulus, stress and strain at break were determined.

Different measures are detailed in Table 1 below, whereas: E is the apparent modulus;

<7b is the stress at break; £b is the elongation at break.

Rejection (with PEO 600 kg/mol)

The selectivity of a membrane for a given substance is generally defined by the rejection rate R (also inversely called retention rate) and depends on its nature and structure, the chemical environment near the membrane and the properties of the substance to be separated.

Rejection rate is defined as below: wherein Cpermeate corresponds to the concentration of the substance in the permeate and C _re tentate corresponds to the concentration of the same substance in the retentate. A rejection rate of 100% means that the solute is perfectly retained by the membrane whereas a rejection rate of 0 corresponds to a solute not at all retained by the membrane.

To determine rejection in the present invention, PEO (polyethylene oxide; Mw = 600 kg/mol)) solution is filtered through the hollow-fiber membrane in total recirculating mode: both retentate and permeate are recirculated in the feed tank. After 30 minutes of filtration, samples are withdrawn at permeate and retentate outlets.

Concentrations are measured by TOC (total organic carbon) in permeate and retentate samples with a Shimadzu TOC-L.

Viscosity measurements

Rotational steady state shear measurements were performed at temperatures of interest using a Rheometric Scientific “RFS III” rheogoniometer in the concentric cylinder configuration (Couette). Flow curves were obtained with a sweep performed from the lowest attainable shear rate (0.02 s' ¹ ) to the highest defined by the maximum torque that the instrument can reach. In all the considered cases, a quite large Newtonian range was observed. Viscosity values in the text represent the Newtonian plateau of the flow curves and are expressed in cpoises (cP).

Results

In the first set of samples, the zwitterionic additive prepared in Example 1 (Pl) is used. Take up speed was always 17 m/min. Table 2 sets out the different properties concerning the hollow fiber membranes and this illustrates the difference of fluxes between membranes comprising or not the zwitterionic additive.

Table 2: Comparison of properties of hollow fibers membranes with zwitterionic additive (Fl with zwitterionic additive Pl) and without additive (Ref. Solef® 1015): Bubble point was measured with pure Ethanol. For permeability measurements (performed at 1 bar), three specimens of hollow fibers with a total area of about 30 cm ² were cut and stored in a water/glycerol 80/20 % w/w solution before drying and placing them in a glass testing holder. Fibers were tested in lumen-shell (In-Out) configuration.

It can be seen in the Table 2 that despite a higher bubble point (i.e. smaller largest pore detected) and higher rejection (to PEO), fibers Fl with zwitterionic additive (Pl) according to the invention show improved fluxes compared to standard Solef® 1015 fibers. The effect of the additive on the flux is clearly demonstrated when comparing pure water flux values measured on membranes free of any additive which is much lower than the pure water flux value of membrane containing additive.

The second set of samples was manufactured. In this case the zwitterionic additive prepared in Example 2 (P2) was used. Note that the take up speed was 17,5 m/min.

The properties of different samples are detailed in Table 4. A certain amount of fibers of the different samples were further washed in NaOCl bath (6000 ppm for 24 hours) in order to fully remove the residual poly vinyl pyrrolidone (PVP) and increase permeability. This procedure generated another series of samples. The use of NaOCl to remove PVP is well known in the art and described either in the academic and patent literature. For these new samples, permeability of fibers (flux/pressure) was measured by using the Convergence Inspector tool (from Convergence Industry B.V.-Enschede-The Netherlands) and working again in cross-flow mode. Modules of 20 fibers with a length of 27 cm each were tested in both configurations In-Out: (typical surfaces 130 cm ² ) and Out-In: (typical surfaces 220 cm ² ), with a feed throughput equal to 500 g/min and with a transmembrane pressure equal to 0,5 bar. An in-out fiber module means that the separation layer is the inner surface of fibers and the feed water flows inside the fibers and the permeate flows outside the fibers. The out-in mode means the separation layer is on the outside surface of the fiber and the feed water flows outside the fibers.

With this automated system permeate flow and retentate flow can be directly measured and collected.

Mechanical properties together with rejection, porosity, dope viscosity and temperature of spinning are summarized in Table 3 below. Trial 1 is a repetition of the standard dope sample (Ref. Solef® 1015) in Table 2 (no zwitterionic additive).

Table 3: Comparison of properties of membranes without additive (Ref.

Solef® 1015) and with zwitterionic additive (F2 with zwitterionic additive P2)

Table 4 below shows pore size and permeability data (both in out /in and in /out configuration) on the same formulations Ref. Solef® 1015, F2(a) and F2(b) and their treated versions.

Table 4: Pore size and permeability of formulations Ref. Solef® 1015, F2(a), F2(b) and the same samples post-treated in NaOCl. Permeability (flux/pressure) expressed in LMHB (l/m ² *h*bar) was measured at 0,5 bar in cross-flow configuration

As in the previous case, it is clearly demonstrated that the use of the zwitterionic additive in membranes according to the invention makes it possible to obtain a much better permeability and therefore increased fluxes, compared to membrane without any additive, in both configuration despite very similar of even tighter pores.

Hydrophilicity evaluation by contact angle measurement

Measurement of contact angle (CA)

One way to characterize surface hydrophilicity is to measure contact angles. Surface hydrophilicity is generally assessed by Water Contact Angle (WCA), i.e. by evaluating the contact angle of a water droplet at a sample’s surface.

The water contact angles were evaluated at 25°C by using the DSA10 (Kriiss GmbH, Germany), according to ASTM D 5725-99 only on the shell side of the fibers. Results shown in Table 5 are average of at least 10 drops of water. Volume of drops was 0,5 pl.

Table 5: contact angle (CA) measured from membranes

In this table 5, it is shown that the use of the zwitterionic additive (P2) in membranes according to the invention makes it possible to obtain a slight improvement in terms of surface hydrophilicity, compared to membranes without any additive.

Measurement of contact angle (CAB method)

As mentioned above, one can measure contact angle in order to characterize surface hydrophilicity. Because of absorption phenomena, this method is poorly suited to measure contact angles of porous hydrophilic samples, consequently contact angles were measured by the Captive Air Bubble (CAB) method. Indeed, this method measures the contact angle of an air bubble at a surface immersed in a liquid, in this case water. The CAB method has several advantages for membrane characterization. As the membranes are already wet, swelling and absorption are suppressed. In addition, this avoids surface contamination, chemical reorganization and drying-induced degradation. Moreover, the sample surface is in contact with a saturated and well-controlled environment, thus improving reproducibility.

Theoretically the Air Contact Angle (AC A) and WCA are complementary, meaning that increasing ACA corresponds to increasing hydrophilicity.

WCA (°) = 180 - ACA(°).

The principle of the CAB method is illustrated in figure 4. Air Contact Angle (AC A) measurements were carried out at room temperature, using an adapted environment controlled chamber filled with deionised water (1) (DI water). Prior to analysis, the wet flat sheet membrane (2) were wrapped on a 15x15mm glass substrate, fixed on a sample holder (3) with double-sided tape. Samples were then immersed in DI water, and a 2 pL air bubble (4) was dropped on the sample surface using a J-shaped syringe (5).

Contact Angle measurements were performed on an optical tensiometer (Attension® Theta Flex provided by BIOLIN) equipped with a high quality monochromatic cold LED (6) and a high resolution (1984x1264) digital camera (7). Image acquisition parameters were set at 5 Frames Per Second (FPS) and a minimum acquisition time of 60 s. The instrument was calibrated using a calibration ball (CA = 143.15°) with an accepted error of 0.03°.

Obtained contact angle values are the average of 5 measurements performed on the same sample. Error bars represent Standard Deviation (Std) between measurements with addition of standard deviation during measurements.

Results

In table 6 below are compiled the values of ACA measured for PVDF membranes casted from DMSO based dope solutions containing or not copolymer additive. For these measurements, the zwitterionic additive prepared in Example 3 (P3) is used. As previously mentioned, an increase of air contact angle (ACA) corresponds to an increase of hydrophilicity for given membranes.

Table 6: air contact angle (ACA °) measured from membranes casted from DMSO dope solutions with additive MMA/SPE

It can be seen in table 6 that the effect of the additive (P3) on the hydrophilization of the PVDF membrane is clearly demonstrated when comparing the ACA value measured on membrane free of any additive (Ref. Solef® 1015) which is lower than the ACA value of membrane containing additive (membrane with formulation F3).

According to the above results, the applicant discloses in the present invention a composition comprising a VDF containing polymer and a zwitterionic additive suitable for manufacturing hydrophilized membranes via solution casting. Without being bound to the theory we believe that the increase of fluxes is due to the concomitant effects of improved hydrophilicity and better pore size distribution and/or better pore interconnectivity.

An overall increase of flux during filtration operation, and thus a reduction of energy consumption, is verified, which position this invention as a much more sustainable solution, especially for waste water filtration companies

Indeed, it has been shown an improved flux of PVDF membrane compared to traditional membranes made from vinylidene fluorine polymers [polymer (VDF)] for which properties are being progressively altered due to progressive permanent obstruction of the membranes pores and of the degradation of membrane mechanical resistance consecutive to several cycles of cleaning. These long-lasting performances of the membranes can position them as a very attractive solution for willing to install high flux filtration systems, able to deal with high solid contents liquid influx, and with an extended lifetime of the overall device. Such invention therefore also allows membrane users to lower their specific energy consumption due to lower pressure needed for the same filtration rate.