POLYMER ADDITIVE COMPRISING ZWITTERIONIC MOIETIES FOR VINYLIDENE FLUORIDEPOLYMER BASED MEMBRANES

Abstract

The present invention pertains to composition suitable for manufacturing membranes based on vinylidene fluoride (VDF) polymers, to porous membranes thereof, to methods for their manufacture and to uses thereof, especially for the filtration of water phases. Said composition comprising vinylidene fluoride (VDF) polymers and polymer additives comprising zwitterionic moieties delivers outstanding hydrophilization performances of manufactured membranes.

Claims

1. A composition (composition (C)) comprising: at least one vinylidene fluoride (VDF) polymer (polymer (VDF)), and at least one polymer (polymer (N—ZW)) comprising zwitterionic recurring units derived from at least one ethylenically unsaturated monomer having at least two ionic groups, at least one of them being a cationic group (group (C+)) and at least one of them being an anionic group (group (A−)), and further comprising at least one hydroxyl group (units (R.sub.ZW)).

2. The composition (C) according to claim 1, wherein polymer (VDF) is selected from the group consisting of 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 selected from the group consisting of: (a) C.sub.2-C.sub.8 perfluoroolefins; (b) hydrogen-containing C.sub.2-C.sub.8 fluoroolefins different from VDF, perfluoroalkyl ethylenes of formula CH.sub.2═CH—R.sub.f1, wherein R.sub.f1 is a C.sub.1-C.sub.6 perfluoroalkyl group; (c) C.sub.2-C.sub.8 chloro- and/or bromo-containing fluoroolefins; (d) perfluoroalkylvinylethers (PAVE) of formula CF.sub.2═CFOR.sub.f1, wherein R.sub.f1 is a C.sub.1-C.sub.6 perfluoroalkyl group; (e) perfluorooxyalkylvinylethers of formula CF.sub.2═CFOX.sub.0, wherein X.sub.0 is a C.sub.1-C.sub.12 perfluorooxyalkyl group comprising one or more than one ethereal oxygen atom; and (f) (per)fluorodioxoles of formula: ##STR00010## wherein each of R.sub.f3, R.sub.f4, R.sub.f5 and R.sub.f6, equal to or different from each other, is independently a fluorine atom, a C.sub.1-C.sub.6 perfluoro(oxy)alkyl group, optionally comprising one or more oxygen atoms.

3. The composition (C) of claim 2, wherein polymer (VDF) is a polymer comprising: (a′) at least 60% by moles of recurring units derived from vinylidene fluoride (VDF); (b′) optionally from 0.1 to 30% by moles of recurring units derived from a fluorinated monomer different from VDF; and (c′) optionally from 0.1 to 10%, by moles of recurring units derived from one or more hydrogenated monomer(s), all aforementioned % by moles being referred to total moles of recurring units of the polymer (VDF).

4. The composition (C) according to claim 1, wherein units (R.sub.ZW) are derived from at least one monomer selected from the list consisting of a) hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl acrylates or methacrylates, acrylamido or methacrylamido; b) heterocyclic betaine monomers comprising at least one hydroxyl group, and c) hydroxyalkyl sulfonates or phosphonates of dialkylammonium alkyl styrenes.

5. The composition (C) according to claim 1, wherein polymer (N—ZW) further comprises recurring units different from units (R.sub.ZW), derived from at least one ethylenically unsaturated monomer deprived of ionisable groups (units (R.sub.N)).

6. The composition (C) according to claim 5, wherein units (R.sub.N) are derived from at least one monomer selected from a list consisting of methyl methacrylate, ethyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, vinyl acetate and N,N-dimethylacrylamide (units (R.sub.N-1)); 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, poly(ethylene 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 (R.sub.N-2)); 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 (R.sub.N-1)) and 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, poly(ethylene 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 (R.sub.N-2)).

7. The composition (C) according to claim 5, wherein polymer (N—ZW) comprises from 0.1 to 30% by moles of units (R.sub.ZW), with respect to total moles of recurring units of polymer (N—ZW).

8. The composition (C) according to claim 5, wherein polymer (N—ZW) comprises 70% or more by moles of units (R.sub.N), with respect to total moles of recurring units of polymer (N—ZW).

9. The composition (C) according to claim 6, wherein polymer (N—ZW) comprises recurring units (R.sub.N-1) and comprises from 0.1 to 50% by moles of recurring units (R.sub.ZW) and (R.sub.N-2), with respect to total moles of recurring units of polymer (N—ZW).

10. The composition (C) according to claim 1, wherein polymer (VDF) is present in predominant amount over polymer (N—ZW) in composition (C), and weight ratio polymer (N—ZW)/polymer (VDF) is of at least 1/99 wt./wt., and/or it is less than 50/50 wt./wt.

11. The composition (C) according to claim 1, which further comprises at least one liquid medium (medium (L)) comprising at least one organic solvent (composition (C.sup.L)).

12. The composition (C) according to claim 11, which composition comprises an overall amount of polymer (N—ZW) and polymer (VDF) of at least 1 wt. % based on total weight of medium (L), polymer (N—ZW) and polymer (VDF), and/or composition (C.sup.L) comprises an overall amount of polymer (N—ZW) and polymer (VDF) of at most 60 wt. % based on the total weight of medium (L), polymer (N—ZW) and polymer (VDF) and/or composition (C.sup.L).

13. A method for manufacturing a porous membrane, said method comprising: step (i): preparing a composition (C) according to claim 1; step (ii): processing the composition provided in step (i) thereby providing a film; and, step (iii): processing the film provided in step (ii), thereby providing a porous membrane.

14. A porous membrane comprising: at least one vinylidene fluoride polymer (polymer (VDF)), and at least one polymer (polymer (N—ZW)) comprising zwitterionic recurring units derived from at least one ethylenically unsaturated monomer having at least two ionic groups, at least one of them being a cationic group (group (C+)) and at least one of them being an anionic group (group (A−)), and further comprising at least one hydroxyl group (units (R.sub.ZW)).

15. A method of separating an aqueous medium, said method comprising contacting said aqueous medium with a porous membrane according to claim 14.

16. A copolymer (N—ZW) comprising zwitterionic recurring units (R.sub.ZW) derived from 3-((3-acrylamidopropyl)dimethylammonio)-2-hydroxypropane-1-sulfonate (AHPS) and recurring units (R.sub.N) derived from at least one monomer selected from the list consisting of alkyl (meth)acrylates, vinyl acetate and N, N-dimethyl acrylamide.

17. The method of claim 13, wherein step (iii) comprises contacting the film with a non-solvent medium (medium (NS)).

Description

EXAMPLES

Synthesis of 3-((3-acrylamidopropyl)dimethylammonio)-2-hydroxypropane-1-sulfonate (AHPS)

[0220] AHPS was synthesized according to the following scheme

##STR00009##

[0221] The synthesis was performed by reacting N-[3-(dimethylamino) propyl] acrylamide (DMAPA) and 3-chloro-2-hydroxy-1-propane sulfonic acid, sodium salt (CHPSNa) in 50% water in the presence of hydroquinone monomethyl ether (MEHQ) to inhibit polymerization.

[0222] In a four necked round bottom flask equipped with mechanical stirring, temperature control and reflux condenser were added 60 g of water and 0.02 g (mmol) of MEHQ under stirring. Then 43.53 g (221 mmol) of crystal solid CHPSNa were added via a powder funnel and the temperature was raised up to 60° C. Liquid DMAPA was then added in a steady stream over 20 min. resulting in a maximum temperature of 76° C. The reaction mixture was finally heated to 90° C. and maintained at this temperature during 4 hours while keeping the pH to 10 by adding sodium hydroxide solution 50 wt % in water when necessary (typically 0.14 g). Conversion was followed by HPLC and the product structure was confirmed by .sup.1H and .sup.13C NMR.

Synthesis of Poly(MMA-Stat-AHPS) 95/5 Mol/Mol

[0223] Statistical copolymer poly(methyl methacrylate-stat-3-((3-acrylamidopropyl)dimethylammonio)-2-hydroxypropane-1-sulfonate was prepared by free radical polymerization using 2,2′-Azobis(2-methylbutyronitrile) (AMBN) as the initiator.−MMA=95 mol %−AHPS=5 mol %)

[0224] In a 500 mL kettle reactor equipped with a water condenser and a mechanical agitation, were introduced, at room temperature (22° C.), 7.5 g (18.73 mmol) of a methyl methacrylate (MMA) solution (25 wt % in DMSO), 88.3 g of dimethyl sulfoxide (DMSO, at 99% purity) and 5.80 g (9.86 mmol) of a solution of 3-((3-acrylamidopropyl)dimethylammonio)-2-hydroxypropane-1-sulfonate in water (AHPS content 50.0 wt %). The mixture was degassed by nitrogen bubbling for 50 minutes while the temperature of the reaction medium was raised up to 70° C. 15.16 g (1.5 mmol) of an AMBN solution (2% in DMSO) were further introduced under a nitrogen blanket. Then, 67.5 g (168.57 mmol) of MMA solution were added within 4 hours (flow rate of 0.28 g/min) and the reaction medium was let stirred for further 8 hours at 70° C.

[0225] Afterwards, a sample was taken for .sup.1H NMR analysis to determine the MMA and AHPS conversions.

[0226] Results: MMA monomer conversion=99.9%; AHPS monomer conversion=84.4%.

[0227] M.sub.W=37200 g/mol

Synthesis of Poly(MMA-Stat-SPE) 95/5 Mol/Mol

[0228] 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-1-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.

[0229] Afterwards, a sample was taken for .sup.1H NMR analysis to determine the MMA and SPE conversions. Results: MMA monomer conversion=98.1%; SPE monomer conversion=94.1%.

M.sub.W=69000 g/mol

Preparation of Membranes Containing Zwitterionic Additive

[0230] Membranes were cast from dope solutions containing blends of PVDF SOLEF® 1015 and of the synthesized zwitterionic p(MMA-s-SPE) or (MMA-s-AHPS) copolymers in dimethyl sulfoxide (DMSO) or N-methyl-2-pyrrolidone (NMP) and immersed in a coagulation bath in order to induce phase separation (NIPS for non-solvent induced phase separation).

General Method for Preparing Dope Solutions

[0231] To prepare the dope solutions, the zwitterionic additive was dissolved in NMP at approximately 65° C. and PVDF was added. The resulting mixture was then stirred overnight at 65° C. Several zwitterionic copolymer:PVDF ratios were fixed at 5/95, 10/90, and 20/80 wt./wt., totaling a 0.5 g total polymer in 4.5 g of solvent.

[0232] The dope solutions were degassed in a vacuum oven set at 40° C. for 24 h. The dope solutions were casted on a glass plate using an adjustable film applicator set to a 200 μm 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 membranes were moved to a fresh DI water bath and stored at least overnight before use. As a control, additive-free PVDF membrane was manufactured by dissolving 0.5 g PVDF in 4.5 g NMP and following the NIPS procedure explained above.

Hydrophilicity Evaluation by Contact Angle Measurement

[0233] 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. 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 and, as the membranes are already wet, swelling and absorption are suppressed.

[0234] Theoretically the Air Contact Angle (ACA) and WCA are complementary, meaning that increasing ACA corresponds to increasing hydrophilicity.

WCA(°)=180−ACA(°).

[0235] The principle of the CAB method is illustrated in the FIG. 1.

[0236] Air Contact Angle (ACA) measurements were carried out at room temperature, using an adapted environment controlled chamber filled with de-ionised water (1) (DI water). Prior to analysis, the wet samples (2) were wrapped on a 15×15 mm glass substrate, fixed on a sample holder (3) with double-sided tape. Samples were then immersed in DI water, and a 2 μL air bubble (4) was dropped on the sample surface using a J-shaped syringe (5).

[0237] 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 (1984×1264) 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°.

[0238] 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.

Chemical Aging of Membranes

With Sodium Hydroxide (NaOH)

[0239] Membranes (sample size around 2×2 cm) were soaked in 20 mL of a sodium hydroxide (NaOH) solution at pH=11.5 (0.003 mol/L) for one week at room temperature. No stirring was applied.

With Sodium Hypochlorite (NaOCl)

[0240] Membranes (sample size around 2×2 cm) were soaked in 20 mL of a sodium hypochlorite (NaOCl) solution at a concentration of 5000 ppm and pH=8 for one week, at room temperature. No stirring was applied. The solution of NaOCl was prepared by dilution of a 5% active chlorine commercial solution, and the pH was adjusted to 8 by the addition of hydrochloric acid HCl. Aging was performed in the dark and the aging solution was replaced at least every 2 days.

Results

[0241] As previously mentioned, an increase of air contact angle (ACA) corresponds to an increase of hydrophilicity for given membranes.

[0242] In the tables below are compiled the values of ACA measured for PVDF membranes containing or not copolymer additive and for PVDF membranes having or not being aged in NaOH or NaOCl. Table 1 contains results about membranes casted from NMP containing dope solutions.

TABLE-US-00001 TABLE 1 air contact angle (ACA °) measured from membranes casted from NMP dope solutions Membrane Membrane Membrane Additive Non aged aged in aged in composition composition membrane NaOH NaOCl Membrane (wt./wt.) (mol/mol) ACA (°) ACA (°) ACA (°) 0 Solef ® 1015 — 124 ± 4 113 ± 7  113 ± 6 (100/0) 1 Solef ® 1015 MMA/SPE n.d. n.d.  135 ± 15 p(MMA-s-SPE) (95/5) (95/5) 2 Solef ® 1015 MMA/SPE  142 ± 10 140 ± 12  129 ± 12 p(MMA-s-SPE) (95/5) (90/10) 3 Solef ® 1015 MMA/SPE 160 ± 7 156 ± 7  160 ± 5 p(MMA-s-SPE) (95/5) (80/20) 4 Solef ® 1015 MMA/AHPS 163 ± 5 149 ± 11  140 ± 12 p(MMA-s-AHPS) (95/5) (95/5) 5 Solef ® 1015 MMA/AHPS 157 ± 7 149 ± 13 156 ± 8 p(MMA-s-AHPS) (95/5) (90/10) 6 Solef ® 1015 MMA/AHPS 154 ± 6 160 ± 7  159 ± 6 p(MMA-s-AHPS) (95/5) (80/20)

[0243] It can be seen in table 1 that, the influence of the additive on the hydrophilization of the PVDF membrane is clearly demonstrated when comparing the ACA value measured on membrane free of any additive (membrane 0) which is lower than the ACA value of any membrane containing any additive (membranes 1 to 6).

[0244] Moreover, results of table 1 show that aging of the membranes in NaOH or NaOCl is not detrimental to the hydrophilization of the PVDF membranes containing additives. Indeed, ACA value measured on any membrane containing any additive beforehand aged in NaOH or NaOCl is still higher than ACA value measured on membrane free of additive after aging in similar conditions.

[0245] For a composition PVDF/additive of low additive content i.e. 95/5, the presence of AHPS in the additive allows surprisingly to reach ACA value of 163° (see membrane 4). For obtaining a similar ACA value i.e. 160° with the additive comprising SPE, a composition PVDF/additive of higher additive content i.e. 80/20 is required (compare membrane 3 with membrane 4).

[0246] From the results of table 1, it is clear that hydrophilization capability of the the resulting copolymer additive is enhanced when changing the zwitterionic monomer involved in the copolymerization with MMA from SPE to AHPS.

[0247] In other terms, high hydrophilization of PVDF based membrane can be obtain with less additive when adding an additive comprising zwitterionic recurring units further comprising an hydroxyl group than when adding similar additive comprising zwitterionic recurring units which does not comprise any hydroxyl group.

[0248] A good hydrophilization of PVDF based membranes can be reached with less additive, thus avoiding detrimental effect du to the presence in too large amount of said additive on the mechanical, thermal and chemical resistance of the porous PVDF membrane.