Enrique Folgado

Matthias Mayor

Didier Cot

Michel Ramonda

Franck Godiard

Vincent Ladmiral

Mona Semsarilar

Preparation of well-defined 2D-lenticular aggregates by selfassembly of PNIPAM-b-PVDF amphiphilic diblock copolymers in solution

PNIPAM-b-PVDF (poly(N-isopropylacrylamide)-b-poly(vinylidene fluoride) amphiphilic block copolymers (BCPs) were synthesized via RAFT polymerization from PNIPAM macromolecular chain transfer agents (macro-CTA). PNIPAM was used as it is one of the few hydrophilic polymers soluble in dimethyl carbonate (DMC) and which can be synthesized using xanthate CTA with relatively good control. The polymerizations were conducted at 73 °C in DMC using two PNIPAM macro-CTAs of different molar masses and targeting various DPs for the PVDF block. The RAFT polymerization of VDF resulted in relatively well-defined BCPs (Đ ≤ 1.50). These amphiphilic BCPs were able to self-assemble into various morphologies such as spherical, crumpled, lamellar and lenticular 2D aggregates by changing the common solvent or the self-assembly protocol. The size of the aggregates could be controlled by varying the DP of the PVDF block. The polymers were characterized by 1 H and 19 F NMR, SEC, TGA, DSC, and the assembled structures were studied by TEM, SEM and AFM. The thermosensitive behavior of the 2D lenticular aggregates was also examined.

Introduction

Poly(vinylidene fluoride) (PVDF), despite its remarkable properties such as piezoelectricity, ferroelectricity, chemical inertness, and biocompatibility 1,2 has not received as much attention as other polymers in fields such as macromolecular engineering and polymer self-assembly. This is likely due to the synthesis constraints inherent to the peculiar reactivity of VDF. In addition, since VDF is a gaseous monomer, its polymerization usually requires the use of high-pressure vessel, not commonly available in every laboratories. However, in recent years the use of reversible-deactivation radical polymerization techniques to prepare PVDF and PVDF-based BCP have been investigated with some success. [3][4][5][6] RAFT polymerization and iodine-transfer polymerization of VDF lead to the accumulation of VDF tail-terminated chains (-CF2CF2CH2-X, X = xanthate or iodine). 7 These chains are not easily reactivated which hinders the preparation of PVDF-based block copolymers. So far, only PVDF-b-PVAc block copolymers (VAc = vinyl acetate) could be synthesized by chain extension of CF2-CH2-XA-terminated-PVDF (XA = xanthate) prepared by RAFT polymerization. 8 Cobalt-mediated radical polymerization was shown to largely mitigate this end-group reactivity issue, and is arguably the best RDRP technique for VDF. 4,9 However, it requires the use of air-sensitive organometallic compounds and is thus not easy to implement. Although, coupling strategies using click chemistry (CuAAC, or thia-Michael addition for example) [10][11][12][13] are efficient, they may require the preparation of functional RAFT agents, and often lead to mixtures of block copolymers and homopolymers which are not easy to separate. Perhaps the most straightforward solution is the chain extension of RAFT macromolecular chain transfer agents (macro-CTAs) with PVDF.

To date, only four reports describe the preparation of PVDFbased block copolymers from RAFT macro-CTAs using sequential addition of VDF. Kostov et al. described the synthesis of PVAc-b-P(VDF-co-TFP) block copolymers (TFP = trifluoropropene). 14 Girard et al. reported the preparation of PDMA-b-PVDF and PDMA-b-P(VDF-co-PMVE) by chain extension of a PDMA macro-CTA (DMA = dimethylacrylamide, PMVE = perfluorinated methyl vinyl ether). 15 Guerre et al. reported the preparation of PEVE-b-PVDF (EVE = ethyl vinyl ether) block copolymers via the sequential combination of cationic RAFT polymerization of vinyl ethers and radical RAFT polymerization of VDF. 16 Guerre et al. also reported the polymerization-induced self-assembly (PISA) of PVAc-b-PVDF block copolymers. 17 VAc units were then hydrolyzed to prepare PVA-b-PVDF amphiphilic BCP (VA = vinyl alcohol). 18 Only PDMAand PVA-based PVDF-containing amphiphilic block copolymers have been prepared by sequential addition of VDF.

PVDF is a semi-crystalline fluorinated polymer, which as such, is appealing for the preparation of self-assembled block copolymer morphologies through crystallization-driven selfassembly (CDSA) in selective solvents. The self-assembly of noncrystalline (coil-coil) BCPs in solution is well-established. 19 However, the complexity of the self-assembly process increases when one block of the BCP can crystallize. The formation of semicrystalline BCP self-assembled structures can be viewed as a twostep process. The structures will first form by minimizing the unfavourable contacts between the solvophobic block and the solvent, and the semi-crystalline block will then start to crystallize in a second step, leading to the final structure. As crystallization takes place in the insoluble core, the initial morphology is either preserved or a morphological transformation into a novel structure is triggered. 20 Diverse polymeric architectures have been obtained by the crystallization-driven self-assembly (CDSA) approach. Arno et al. reported recently the preparation of PCL-b-PMMA-b-PDMA (CL = ε-caprolactone, MMA= methyl methacrylate) biocompatible and biodegradable 1D cylindrical and 2D platelet micelles via CDSA. Interestingly, they were able to control the dimensions and dispersity of the self-assembled nanostructures. 21 Li et al. have reported a poly(L-lactide)-based diblock glycopolymer that assembled into 1D cylinders and 2D diamond-shaped platelets. 22 Qiu and Gao et al. have reported the preparation of rectangular and hollow structures from polymer blends. 23 To date, most reported coil-crystalline self-assembled aggregates with control over their size were prepared via CDSA, requiring heating and aging steps. The most common crystalline blocks in these assembled structures are PEO (poly(ethylene oxide)), 24 PCL, 21,25 PE (polyethylene), 26 and PFS (poly(ferrocenyldimethylsilane)). 21,[27][28][29][30] Only a few studies describe the self-assembly in solution of BCPs containing a fluoropolymer block, 10,12,18,31 and even fewer are dealing with the CDSA behavior of these fluorinated BCPs. 10,17 Dimethyl carbonate (DMC) was shown to be the solvent of choice for the RAFT polymerization of VDF as it allows relatively fast polymerization and acceptable level of transfer reactions. 3,6 However, one major limitation of the chain extension with PVDF approach to prepare amphiphilic PVDF-based BCPs is the rather low solubility of hydrophilic macro-CTAs in DMC.

PNIPAM is hydrophilic and soluble in DMC at the required temperature for VDF polymerization (ca. 73 °C). PNIPAM features a near body lower critical solution temperature (LCST = 32 °C), and biocompatibility, and is thus very appealing for biomedical applications. [32][33][34][35][36] So far the reports mentioning both PVDF and PNIPAM only describe polymer blends for the preparation of electrospun fibers or flat membranes, or the grafting of PNIPAM on PVDF membranes. 37 To date, the preparation of PNIPAM-b-PVDF block copolymers (BCPs) and the study of their selfassembly in selective solvents have not been reported.

The present study recounts the synthesis via RAFT sequential addition of VDF, characterization and self-assembly in solution of amphiphilic PNIPAM-b-PVDF diblock copolymers. The selfassembly of these BCPs in water led to a wide range of morphologies such as spherical, crumpled-spherical, lamellar and well-defined 2D lenticular aggregates without the need for thermal annealing. The formation and molecular arrangement of these self-assembled structures are discussed, and the thermoresponsive behaviour of the PNIPAM-containing aggregates in water was also examined.

Experimental section

Materials

All reagents were used as received unless otherwise stated. 1,1-Difluoroethylene (vinylidene fluoride, VDF) was supplied by Arkema (Pierre-Bénite, France). O-Ethyl-S-(1-methoxycarbonyl) ethyldithiocarbonate (CTAXA) was prepared according to the method described by Liu et al. 38 tert-Amyl peroxy-2ethylhexanoate (Trigonox 121, purity 95%) was purchased from AkzoNobel (Chalons-en-Champagne, France). Ethanol (EtOH), dimethyl carbonate (DMC), diethyl ether, toluene, laboratory reagent grade hexane (purity >95%), N-isopropylacrylamide (NIPAM, purity 97%) were purchased from Sigma Aldrich. All deuterated solvents were purchased from Eurisotop. NIPAM was recrystallized twice from hexane/toluene (10/1, v/v).

Measurements

Nuclear Magnetic Resonance (NMR). The NMR spectra were recorded on a Bruker AV III HD Spectrometer (400 MHz for 1 H and 376 MHz for 19 F).

Coupling constants and chemical shifts are given in hertz (Hz) and parts per million (ppm), respectively. The experimental conditions for recording 1 H and 19 F NMR spectra were as follows: flip angle, 30°; acquisition time, 4 s (2 s for 19 F NMR); pulse delay, 1 s (2 s for 19 F NMR); number of scans, 16; and pulse widths of 9.25 and 11.4 μs for 1 H and 19 F NMR, respectively.

Size-Exclusion Chromatography (SEC).

Size exclusion chromatograms were recorded using a Triple detection GPC system from Agilent Technologies with its corresponding Agilent software, dedicated to multi-detector GPC calculation. The system used two ResiPore 3 µm 300 x 7.5 mm columns with DMF as the eluent with a flow rate of 1 mL/min and toluene as flow rate marker. The detectors used were a PL0390-06034 capillary viscometer and a 390-LC PL0390-0601 refractive index detector. The entire SEC-HPLC system was thermostated at 35 °C. Low dispersity PMMA standards were used for the calibration. Typical sample concentration was 10 mg/mL.

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Please do not adjust margins Differential Scanning Calorimetry (DSC). DSC measurements were performed on 2-3 mg samples on a TA Instruments DSC Q20 equipped with an RCS90 cooling system. For all measurements, the following heating / cooling cycle was employed: cooling from 40 °C to -73°C, isotherm at -73 °C for 5 min, first heating ramp from -73 °C to 200 °C at 10 °C/min, isotherm at 200 °C for 5 min, cooling stage from 200 °C to -73 °C at 10 °C/min, isotherm plateau at -73 °C for 1 min, second heating ramp from -73 °C to 200 °C at 10 °C/min, isotherm at 200 °C for 1 min, and last cooling stage from 200 °C to 40 °C. Calibration of the instrument was performed with noble metals and checked before analysis with an indium sample. Melting points were determined at the maximum of the enthalpy peaks.

Thermogravimetric analysis (TGA). TGA analyses were carried out with a TA Instruments TGA G500 from 20 °C to 800 °C. A heating rate of 10 °C min-1 was used under air atmosphere with a flow rate of 60 mL min-1. Dry sample weight of 3 mg was used.

Dynamic light scattering (DLS)

. DLS measurements of polymer solutions were carried out in a Malvern ZEN1600 using a quartz cuvette. Refractive indices of solvent mixtures were determined using the following equation:

n m 2 -1 n m 2 + 2 = y 1 n 1 2 -1 n 1 2 + 2 + y 2 n 2 2 -1 n 2 2 + 2 (Equation 1)
Where n 1 , n 2 , n m are the solvent 1, solvent 2 and, mixture refractive indices at a certain temperature, and y 1 , y 2 are solvent 1 and solvent 2 volume fractions. Viscosities of solvent mixtures where extracted from scientific publications 39,40 and online resource. [START_REF] Gmehling | Acetone-Water Mixture Viscosity: Datasheet from ``Dortmund Data Bank (DDB) --Thermophysical Properties Edition[END_REF] Transmission electron microscopy (TEM). TEM studies were conducted using a JEOL 1400+ instrument equipped with a numerical camera, operating with a 120 kV acceleration voltage at 25 °C. To prepare TEM samples, a drop (10.0 μL) of micellar solution was placed onto a Formvar/carbon coated copper grid for 60 s, blotted with filter paper and dried under ambient conditions. All TEM grids were prepared from self-assembly solutions without further dilution.

Scanning electron microscopy (SEM)

. SEM analyses were conducted using a Hitachi S-4500 instrument operating at spatial resolution of 1.50 nm at 15 kV energy. The samples were folded on a 45° SEM Mount after being coated with an ultrathin layer of electrically conducting Platinum deposited by high-vacuum evaporation.

Atomic force microscopy (AFM). AFM images were obtained using a Nanoman V from Bruker Instrument employing repulsive-Tapping mode , and a tip Nanosensors PPP NCL freq = 158kHz Q factor = 350, k ~14N/m, Rtip ~5nm. A typical polymerization (Entry 1 in Table 1) of NIPAM was performed as follows: NIPAM (6 g, 53.02 10 -3 mmol), AIBN (25 mg, 0.15 10 -3 mmol) and CTAXA (315 mg 1.51 10 -3 mmol) were dissolved in ethanol (9 mL) in a round bottom flask, under magnetic stirring at 20 °C. The solution was degassed by N2 bubbling for 20 min. Then, the flask was placed in an oil bath at 70 °C for 14h. The reaction was followed by 1 H NMR and was stopped at NIPAM conversion ˃ 99 %. The polymer was isolated by precipitation in chilled ether and centrifugation and was dried overnight under vacuum at 30°C. Typical polymerization of PNIPAM-b-PVDF block copolymers was performed as follow: A thick 8 mL Carius tube containing PNIPAM-XA macroCTA (Table 1, entry 3) (658 mg, 2.34 10 -4 mmol), DMC (5 mL) and the initiator (Trigonox-121) (11 mg, 4.68 10 -5 mmol) were sonicated for 5 min or until complete dissolution of PNIPAM-XA. Then, the tubes were degassed with three freeze-pump-thaw cycles to remove oxygen. The gaseous VDF monomer (1.5 g, 23.42 10 -3 mmol) was transferred into the Carius tubes and condensed in the tube using a liquid nitrogen bath. The tubes were then sealed, before being placed horizontally in a shaking water bath thermostated at 73 °C (see Scheme S1). After 20 hours, the tube was placed into a liquid nitrogen bath and the opened. After return to room temperature, the crude sample was precipitated twice in a This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins tenfold excess of chilled pentane. The PNIPAM-b-PVDF block copolymers were recovered by centrifugation at 4000 rpm for 5 min in 50 mL conical centrifuge tubes. The polymers were dried overnight under vacuum at 25 °C. Polymerization yields were determined gravimetrically (mass of dried precipitated polymers / mass of monomer introduced in the Carius tube).

1 H NMR (400 MHz, (CD3)2CO, δ (ppm), Figure S2): 0.90 -1.26 (m, -NH-CH(CH3)2-), 1.28 -1.90 (m, -CH2-CH-NIPAM), 1.90 -2.50 (m, -CH2-CH-NIPAM), 2.16 -2.37 (t, -CF2-CH2-CH2-CF2-, VDF-VDF HH reverse addition), 2.66 -3.01 (t, -CF2-CH2-CF2-, VDF-VDF HT regular addition), 3.8 (s, CH3-O-(C=O)-(CH3)CH-), 3.95 -4.25 (m, -NH-CH(CH3)2), 4.40 (t, CH2-CF2-CF2-CH2-S-), 4.60 -4.78 (q, -S(C=S)OCH2-CH3, 3 JHH = 7.1 Hz), 6.09 -6.50 (tt, -CH2-CF2-H, 2 JHF = 55.6 Hz , 3 JHH = 4.7 Hz), 6.50 -8.00 (m, -NH-CH(CH3)2). 19 F NMR (376 MHz, (CD3)2CO, δ (ppm), Figure S3): -115.64 (-CH2-CF2-CF2-CH2-CH2-, VDF-VDF HH reverse addition), -114.45-(-CH2-CF2-H), -113.36 (-CH2-CF2-CF2-CH2-CH2-, HH reverse addition), -113.09 (CH2-CF2-CF2-CH2-S-), -112.69 (-CH2-CF2-CF2-CH2-S-), -107.40 (-CF2-CH3) -94.81 (-CH2-CH2-CF2-CH2-, TT reverse addition), -93.00 (CH3-O-(C=O)-O-CH2-CH2-CF2-, DMCinitiated PVDF), -92.50 (PNIPAM-CH2-CF2-), 92.06 (-CH2-CF2-CH2-CF2H), -91.43 (-CH2-CH2-CF2-CH2-CF2-CH2-CF2-, regular VDF-VDF HT addition), -91.00 (-CH2-CF2-CH2-, regular VDF-VDF HT addition).

The degree of polymerization (DP) of the PNIPAM block were calculated from the 1 H NMR spectrum of the purified BCP using the equation found in the supporting information (See S2, Eq. 2). DP values were then used for the determination of the experimental Mn (S2, Eq. 4). The degree of polymerization (DP) of the PVDF block was determined using equation 5 from the integral of the signals (at 0.9 -1.28 ppm) corresponding to the methyl groups (-CH3) of the NIPAM units, used as reference, and the integral of the signals of the -CH2-group of the normal (HT) VDF additions (at 2.70-3.19 ppm). The signal of the -CH3 of the NIPAM unit is the only signal visible for BCP with a DP higher than 150. Regarding the reverse (TT) VDF additions, the average number of monomer additions occurring per chain between two degenerative transfers increases with increasing [VDF]0/[CTA]0. However, the total amount of TT (or HH) VDF additions (intra-chain + chain-end) stabilizes to identical proportion (ca. 4.1%) for PVDF homopolymerization as previously reported by our group. 7 As the signals assigned to these inversions overlap with signals of the PNIPAM macro-CTA, a 1.041 multiplying factor was employed for the determination of the DP of PVDF.

𝐷𝑃 𝑃𝑉𝐷𝐹 = 1 2 ∫
̵ 𝑪𝑯 𝟐 (HT VDF additions) 5) Molar masses were then calculated using equation (6) (with Mn,PNIPAM-XA calculated using eqn (4) and ( 2)):

𝑀 𝑛,𝑁𝑀𝑅 = 𝑀 𝑛,𝑃𝑁𝐼𝑃𝐴𝑀-𝑋𝐴 + 𝐷𝑃 𝑃𝑉𝐷𝐹 × 𝑀 𝑛,𝑉𝐷𝐹 (Equation 6)

Theoretical molar masses were calculated using equation ( 6) with yield = conversion and the [VDF]0/[PNIPAM-XA]0 ratios listed in Table 1. 7)

𝑀 𝑛,𝑡ℎ𝑒𝑜 = [𝑉𝐷𝐹] 0 [𝑃𝑁𝐼𝑃𝐴𝑀-𝑋𝐴] 0 × 𝑌𝑖𝑒𝑙𝑑 × 𝑀 𝑛,𝑉𝐷𝐹 + 𝑀 𝑛,𝑃𝑁𝐼𝑃𝐴𝐴𝑚-𝑋𝐴 (Equation
With Mn VDF = 64.03 g mol -1 .

Self-assembly

Preparation of block copolymer solutions Stock solutions of 2 mg mL -1 of block copolymer were prepared in DMF, acetone or THF at room temperature using magnetic stirring until full solubilisation.

Nanoprecipitation

Glass vials containing 2 mL of non-solvent and a magnetic bar were placed on a stirring plate. To each vial 0.1 mL of block copolymer solution (2 mg mL -1 ) in DMF were added dropwise. After 1h of stirring, samples were analysed by DLS and TEM. Final concentration of the block copolymer was 0.1 mg mL -1 in DMF: water (1:20).

Note: Difference between precipitation and nanoprecipitation.

-Polymer precipitation refers to a polymer purification technique in which a solution of the polymer is cast dropwise into a non solvent. Upon contact between the two miscible solvents, the polymer crashes out of solution while the impurities remained in the solvent phase. In the case of block copolymers, for an efficient precipitation the different blocks must be insoluble in the non-solvent.

-In contrast, polymer nanoprecipitation refers to a BCP selfassembly technique in which a polymer solution is cast dropwise into a solvent selective for one of the blocks. The insoluble block forms the core of the self-assembled structure while the soluble block act as a steric stabilizer of the polymer morphology.

Solvent switch

Vials containing 0.5 mL of the stock solution (2 mg mL -1 ) in different solvents (THF, DMF and acetone) were placed on a stirring plate. Water (2, 3 or 4 mL) was added dropwise using a syringe pump at a fixed rate of (4 mL h -1 ). 10 µL were taken to prepare TEM samples at 1:4, 1:6 and 1:8 solvent:non-solvent ratios.

Thermally-induced Crystallisation-Driven Self-assembly (TI-CDSA)

A 5 mg mL -1 PNIPAM25-b-PVDF35 solution in DMF was selfassembled by adding water to a 1:1 solvent: non-solvent ratio, and this solution was then heated at 90°C for 30 min and slowly cooled down to room temperature.

Results and discussion

PNIPAM-XA macro-CTAs were synthesized by RAFT polymerization using CTAXA following protocols described previously by Sistach et al. [START_REF] Sistach | [END_REF] The reactions were stopped when the NIPAM conversion Please do not adjust margins Please do not adjust margins reached at least 99%. After purification by precipitation, 1 H NMR spectroscopy of the resulting PNIPAM-XA macro-CTAs was employed to determine their molar masses and degrees of polymerization (DP). Amphiphilic PVDF-based block copolymers with PVDF of different DPs were prepared from these PNIPAM macro-CTAs (see Table 1) by chain extension with VDF in DMC (a common solvent for PNIPAM and PVDF allowing relatively high VDF polymerization rate) using Trigonox 121 as the radical initiator. 19 F NMR spectroscopy of the resulting polymers showed the successful chain extension of PVDF from the PNIPAM macro-CTAs. The presence of the signals of -CF2-CF2-CH2-XA at δ = -113.09 ppm and -112.69 ppm and of -CH2-XA at δ = 4.45 ppm and -CH2-SC(S)-O-CH2-at δ = 4.72 ppm in the 19 F NMR and 1 H NMR spectra respectively (Figure S4 and S3b), indicates the formation of the diblock copolymers. As expected, the polymerization of VDF was accompanied by a nonnegligible amount of transfer to DMC. The characteristic signals of these transfer reactions can be observed in the 1 H NMR spectrum (Figure S3a and S3b) as a triplet of triplets at 6.3 ppm corresponding to the -CF2H chain-end, and in the 19 F NMR spectrum (Figure S4) as a doublet of triplet at -114.45 ppm (-CF2H) and a multiplet at -107.3 ppm corresponding to the CF2CH3 chain end. The 1 H NMR spectrum also shows: a singlet at 5.77 ppm assigned to the DMC-xanthate adduct (CH3O(C=O)O-CH2-XA, Figure S3a), eliminated by the precipitation step (Figure S3b); and a well-defined triplet at 4.35 ppm and a singlet at 3.73 ppm assigned to the -CH2-and -CH3 groups of the DMC moieties of the DMC-initiated-PVDF chains, respectively. The RAFT polymerization of VDF is accompanied by a progressive loss of chain-end functionality (loss of xanthate group). [6][7] This phenomenon has previously been reported for the synthesis of PVAc-b-PVDF BCPs. 17 This loss of functionality can be very extensive.

Here the remaining molar fraction of xanthate-functionalized chain end derived from the 19 F NMR spectra (Figure S4 Despite these transfer reactions and loss of functionality, the chain extension of PNIPAM macro-CTAs with PVDF produced relatively well-defined BCP with dispersity below 1.50 and monomodal SEC traces without shoulders or significant tailing (Figure 1). These SEC traces also show a clear shift towards higher molar masses with the increasing DP of PVDF. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins These amphiphilic block copolymers were then used to prepare self-assembled morphologies in different solvents. Note on solvents: in the following self-assembly study, three solvents were used as good solvents for both the PNIPAM and the PVDF segments: Acetone, THF and DMF. All of these solvents are good solvents of PNIPAM. Bottino et al. 43 investigated the solubility of a 450,000 g mol -1 PVDF in 46 liquids and concluded that DMF is a solvent of PVDF, while acetone and THF are good swelling agents. The PVDF blocks synthesized here have much lower molar mass (a PVDF with a DP = 450 would have a Mn of 28,800 g mol -1 ), it is thus not surprising that THF and acetone behave as good solvents especially considering the low concentrations used (2 mg mL -1 , whereas Bottino et al. aimed at preparing 100 mg mL -1 solutions). In addition, the self-assembly described and discussed below concerns PNIPAM-b-PVDF block copolymers which solubility would be enhanced by the PNIPAM block. These copolymers indeed readily dissolved in acetone, and in THF although gentle heating of the solution was necessary for the latter.

Self-assembly of amphiphilic BCP using rapid solvent exchange, usually lead to the formation of colloidal objects via microphase separation. The final structure of these block copolymer colloids is primarily dictated by the volume fraction of the blocks and by the interfacial surface tensions. A feature of the nanoprecipitation process (which consists in the dropwise addition of the BCP solution into a selective solvent under vigourous stirring) is the ability to access kinetically trapped morphologies in nonequilibrium states due to short mixing time. This kinetic trapping is even more pronounced in the case of semicrystalline polymer such as PVDF. In such case, the PVDF segments often crystallize before the polymer chains can reach the equilibrium morphology during phase separation.

The self-assembly of the PNIPAM-b-PVDF block copolymers was achieved via nanoprecipitation in water from a dilute DMF BCP solution. After stirring for 1h, samples were analyzed by DLS and TEM. This approach led to the formation of spherical aggregates of roughly 20 to 60 nm in diameter (Figure 2). The particle sizes measured by TEM were in good agreement with the average hydrodynamic diameters measured by DLS (Figure S6) ranging between 30 and 80 nm and increasing with the DP of the core-forming PVDF block.

Solvent switch experiments (slow addition of water into a solution of PNIPAM35-b-PVDFX in DMF, THF or acetone) were also performed using three (1:4, 1:6 and 1:8) solvent: non-solvent ratios.

When DMF was employed as common solvent and water as selective solvent for the PNIPAM block, irregular crumpled spherical aggregates were obtained with all the solvent: non-solvent ratios tested (Figure 3). CDSA self-assembled polymers are thought to favor the formation of aggregates with low interfacial curvature. 44 These morphologies may be due to a co-nonsolvency of PNIPAM blocks when DMF molar fraction changes in the solvent mixture, provoking the collapse of the metastable aggregates before attaining the spherical morphology obtained by nanoprecipitation. A reversible collapse-swelling behavior has already been observed in PNIPAM latexes in water-DMF mixtures. 45 When DMF molar fraction was between 0.02 and 0.25, the latex volume was smaller from that observed in pure water.

When THF was used as the good solvent for both blocks, solvent switch protocols led to formation of flat sheet morphologies, presumably 2D bilayer aggregates (Figure 4).

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Please do not adjust margins When acetone was used as the good solvent, well-defined flat 2D lenticular morphologies with relatively low dispersity and good dimensional control were obtained (Figures 567). In the case of the PNIPAM25-b-PVDF35 BCP these lenticular objects had an average length and width of 600 nm and 250 nm respectively, and ragged edges that seemed to be formed by aggregation of smaller 1D rod-like aggregates (Figure 5b andS7). Crystallizationinduced aggregation of micelles has already been observed for PB-b-PEO BCPs. 46 These short 1D micelles have an average length and width of 98 and 19 nm, respectively.

In contrast, the PNIPAM-b-PVDF BCP prepared from the PNIPAM35 macro-CTA self-assembled into lenticular objects with much smoother-looking edges with average lengths of 200, 600, 1000 and 2300 nm and average widths of 90, 250, 400 and 850 nm for PVDF DPs of 60, 100, 150 and 450 (Figure 6).

The length and width of the lenticular 2D micelles showed a linear dependence to the degree of polymerization of PVDF up to 150 (Figure S7). The loss of linearity observed for the PVDF450 BCP may be due to the poor accuracy on the DP value calculated from weak NMR signals. The real DP of this PVDF block might be closer to 300 than the calculated 450.

Due to their relatively large size (in the micrometric range), these morphologies were not colloidally stable and sedimented over time (ca. 14h). However, they were easily redispersed by simple shaking. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins The shape and size of these morphologies were not affected by the concentration of the initial BCP solutions over the concentration range studied (Figure S9). In addition, once the self-assembled structures were formed in acetone: water mixture (1:4) they did not evolve within the 2-week observation time, they thus seem to be kinetically trapped. Increasing the water content of the selfassembly media or decreasing the acetone content (by evaporation) of the solvent mixture only affected the aggregation rate. However, the lenticular shape of these assemblies was strongly affected by the ratio of the length of the hydrophobic and hydrophilic blocks. The morphologies were rather patchy with sharp edges for the PNIPAM25-b-PVDF35 (Figure 5), while better-defined objects were found for PNIPAM35-b-PVDFn BCPs with n > 60 (Figure 6). Better defined lenticular objects were obtained as higher DPs of the PVDF were targeted.

The PVDF fraction of these BCP was crystalline as determined from DSC thermograms (Figure S10). However, the PVDF crystallinity could not be observed on these self-assembled structures by electron diffraction during TEM analysis. This was ascribed tothe thinness of these lenticular objects, undergoing fast amorphisation under the electron beam. Nevertheless, the XRD diffraction pattern (Figure 8) obtained from freeze-dried 2D aggregates formed via self-assembly of a 2 mg mL -1 solution of PNIPAM35-b-PVDF100 BCP in acetone:water (1:4) suggests that the PVDF phase of these aggregates was indeed crystalline and in the α-form with characteristic 2ɵ peaks at 17.6 (100), 18.5 (020), 19.8 (110), 26.5 (021), 36.1 (200) and 39 (002). 47 The α-crystal phase is hexagonal, with aligned polymer chains anti-parallel to each other, in the conformation of trans-gauche-trans-gauche' (TGTG'). α-form is the most common and spontaneously forming crystalline form of PVDF. 48,49 SEM images of the lenticular objects (Figure 7) suggest that they can fold and twist to an extent, and are often stacked on top of each other, making their characterization quite difficult. AFM images (Figure 9) confirmed the stacking of the objects but also revealed the increase of their thickness with the increase of the DP of the PVDF block. The presence of short cylindrical 1D micelles (Figure 6b) suggests that the lenticular morphologies may form by aggregation these smaller structures. In addition, the thickness of the lenticular aggregates seems to be related to the length and folding of the PVDF segments. As the degree of polymerization of the PVDF core-forming block increased, the thickness of the aggregates increased. Since the PVDF in these aggregates is crystallized in the α-form, the number of folding of the PVDF chains can be estimated. In α-form Please do not adjust margins Please do not adjust margins crystal, 0.462 nm is the dimensions of 2.5 VDF units in that specific conformation. 50 Crystallized PVDF chains of DP 60, 100, 150 and 450 would thus extend over 11.1, 18.5, 27.7 and 83.2 nm, respectively in α-form. The thicknesses of the lenticular objects derived from the AFM topographic images shown in Figure 9 are in agreement with these calculations for PVDF60, PVDF100 and PVDF150. Figure 9a, corresponding to PNIPAM35-b-PVDF60, shows a thickness of 10-15 nm. Figure 9b and Figure S11, corresponding to PNIPAM35-b-PVDF100 give a thickness of about 40 nm. This value is roughly twice the calculated length of the PVDF100 (18.5 nm). This is because the AFM images in Figure 9b show two aggregates stacked on top of each other. The thickness measured for the PNIPAM35-b-PVDF150 in Figure 9c is 30 nm which also is in agreement with the corresponding calculated value of 27.7 nm. In the case of the PNIPAM35-b-PVDF450 BCP, the calculated thickness of 83.2 nm does not match the thickness of 10 nm measured on Figure 9d. This discrepancy may be explained by considering that in this case the crystallized PVDF chains are folded rather than extended. Indeed, the calculated and measured values can be reconciled if the PVDF450 chains were folded 7 times (or 5 times if the real DP is actually 300 as mentioned above). This explanation is also consistent with the observation of larger aggregates for higher PVDF degree of polymerization. Please do not adjust margins Please do not adjust margins nanoprecipitation and solvent switch protocols was 2 mg mL -1 (5 mg mL -1 in the case of the sample used for TI-CDSA). Rate of selective solvent addition was 4 mL h -1 for the solvent switch protocol and dropwise added in the case of Nanoprecipitation. TI-CDSA sample was placed 30 min in an oil bath at 90°C and slowly cooled down after that time.

As PVDF chains fold more, the repulsion between the PNIPAM hydrophilic chains decrease, allowing the formation of larger but thinner 2D aggregates.

Since the degree of crystallinity can be modified via temperature annealing, a heating and cooling treatment was applied to the crumpled structures (Figure 3 obtained by self-assembly from DMF solution. In addition, at higher temperature the solubility of PVDF in DMF/water mixture increases. A 5 mg mL -1 PNIPAM25-b-PVDF35 BCP solution in DMF was self-assembled by adding water to reach a 1:1 solvent: non-solvent ratio; then this solution was heated at 90°C for 30 min and slowly cooled down to room temperature. Figure S12 shows that this temperature treatment led to a mixture of ill-defined aggregates and spindle-shape morphologies with lengths ranging from 300 nm to 1 μm. The crumpled sphere to spindle morphology transition is thought to proceed via temperature-induced crystallization-driven self-assembly (TI-CDSA) due to the degradation of the solvent quality for the crystallisable block caused by the slow cooling. 20 To sum up, five different morphologies were observed by selfassembly of these PNIPAM-b-PVDF BCPs as depicted in Table 2.

Without surprise, the solvents used and the self-assembly protocol played a crucial role in the resulting BCP structures. The same DMF/water binary mixture of solvents afforded three distinct morphologies depending on the self-assembly process used. Spherical nanoparticles were obtained when nanoprecipitation was used while a solvent switch protocol led to the formation of crumpled spheres. These crumpled spheres evolved into spindle-like aggregates when a TI-CDSA protocol was applied. Solvent switch protocol produced 2D lamellar aggregates in the case of THF/water and well-defined lenticular shape aggregates of increasing length and width with increasing DP of the PVDF block in acetone/water binary mixture.

Thermoresponse of PNIPAM-b-PVDF lenticular nanoparticles

The thermoresponsiveness of the PNIPAM-b-PVDF lenticular aggregates were also investigated. The usual turbidity test could not be used due to the poor colloidal stability of the BCP aggregates even at room temperature. Since the LCST of PNIPAM is affected by the presence of organic solvents, acetone was completely removed from the PNIPAM-b-PVDF BCP suspensions under reduced pressure using a rotary evaporator at room temperature to prepare an aqueous suspension of the self-assembled morphologies. TEM analysis of this suspension (Figure S13) showed that the size and shape of the assemblies were not affected by the removal of acetone.

The samples were heated at 50 °C under gentle stirring for a period of 30 min then cooled down to room temperature.

Figure 10 shows the effect of temperature on the aqueous PNIPAM-b-PVDF BCP morphologies. The lenticular objects partly lost their well-defined shape, crumpled to an extent and also broke into smaller flat sheet aggregates. The change of conformation of the PNIPAM blocks provoked by the decrease of the PNIPAM solubility in water at T > LCSTPNIPAM is thought to be responsible for the aggregates to irreversibly break or evolve into the observed crumpled morphologies.

Conclusions

A series of relatively well-defined PNIPAM-b-PVDF semicrystalline amphiphilic diblock copolymers was successfully synthesized by RAFT polymerization using PNIPAM macro-CTAs. These diblock copolymers were fully characterized by 1 H and 19 F NMR spectroscopy, TGA, DSC and SEC. Despite the loss of xanthate functionality observed when DPs of PVDF higher than 50 were targeted, BCPs with relatively low dispersity (Ɖ<1.50) were achieved. Due to their amphiphilic nature, these BCPs had the ability to selfassemble in aqueous solutions. The self-assembled structures were characterized by TEM, SEM and AFM microscopies. The final structures were strongly affected by the choice of common solvents and self-assembly protocol used. Five types of morphologies were obtained depending on the common solvent and the self-assembly protocol. Surprisingly self-assembly from acetone solutions led to the formation of well-defined 2D lenticular aggregates which dimensions increased with the degree of polymerization of PVDF, reaching lengths of 2.3 µm for the longest PVDF synthesized. These aggregates were morphologically stable over time and accessible even at higher

X-

  Scheme 1. RAFT/MADIX synthesis of PolyNIPAM macro CTA.

  Scheme 2. RAFT/MADIX synthesis of PNIPAM-b-PVDF block copolymers using PNIPAM macro CTA.

  and calculations S5) were 19.2, 5.4, 2.3 and 1.4 % for the PNIPAM35-b-PVDF60, PNIPAM35b-PVDF100, PNIPAM35-b-PVDF150, and PNIPAM35-b-PVDF450 copolymers respectively.

  Reactions conditions: (i) (entry 1 and 3) NIPAM homopolymerization: [I]/[CTAXA] = 0.1 with I = AIBN and CTAXA = O-ethyl-S-(1methoxycarbonyl)ethyldithiocarbonate, T = 70 °C; (ii) (entries 2 and 4-7) chain extension of PNIPAM35-XA : [I]/[CTAXA] = 0.2 with I = Trigonox 121, T = 73 °C.a Determined by 1 H NMR using equation (2). b Determined gravimetrically. c Determined by 1 H NMR using equation (5). d Calculated using yield as conversion and equations (3) for PNIPAM and (7) for the BCP. e Calculated from DPNMR using equations (4) for PNIPAM and (6) for PVDF. f Determined by SEC (RI detector).

Figure 1 .

 1 Figure 1. Normalized SEC traces (viscometric detector) of: PNIPAM35-XA (black trace), PNIPAM35-b-PVDF60 (red trace), PNIPAM35-b-PVDF100 (blue trace), PNIPAM35-b-PVDF150 (green trace) and PNIPAM35-b-PVDF450 (pink trace) after purification by precipitation in chilled ether for PNIPAM or cold pentane for the BCPs.These amphiphilic block copolymers were then used to prepare self-assembled morphologies in different solvents. Note on solvents: in the following self-assembly study, three solvents were used as good solvents for both the PNIPAM and the PVDF segments: Acetone, THF and DMF. All of these solvents are good solvents of PNIPAM. Bottino et al.43 investigated the solubility of a 450,000 g mol -1 PVDF in 46 liquids and concluded that DMF is a solvent of PVDF, while acetone and THF are good swelling agents. The PVDF blocks synthesized here have much lower molar mass (a PVDF with a DP = 450 would have a Mn of 28,800 g mol -1 ), it is thus not surprising that THF and acetone behave as good solvents especially considering the low concentrations used (2 mg mL -1 , whereas Bottino et al. aimed at preparing 100 mg mL -1 solutions). In addition, the self-assembly described and discussed below concerns PNIPAM-b-PVDF block copolymers which solubility would be enhanced by the PNIPAM block. These copolymers indeed readily dissolved in acetone, and in THF although gentle heating of the solution was necessary for the latter.Self-assembly of amphiphilic BCP using rapid solvent exchange, usually lead to the formation of colloidal objects via microphase separation. The final structure of these block copolymer colloids is primarily dictated by the volume fraction of the blocks and by the interfacial surface tensions. A feature of the nanoprecipitation process (which consists in the dropwise addition of the BCP solution into a selective solvent under vigourous stirring) is the ability to access kinetically trapped morphologies in nonequilibrium states due to short mixing time. This kinetic trapping is even more pronounced in the case of semicrystalline polymer such as PVDF. In such case, the PVDF segments often crystallize before the polymer chains can reach the equilibrium morphology during phase separation.The self-assembly of the PNIPAM-b-PVDF block copolymers was achieved via nanoprecipitation in water from a dilute DMF BCP solution. After stirring for 1h, samples were analyzed by DLS and

Figure 2 .

 2 Figure 2. TEM images of spherical aggregates prepared from (a) PNIPAM35-b-PVDF60, (b) PNIPAM35-b-PVDF100, (c) PNIPAM35-b-PVDF150 and (d) PNIPAM35-b-PVDF450. All samples were prepared by nanoprecipitation in water from BCP solutions in DMF at 2 mg mL -1 (final concentration = 0.1 mg mL -1 in DMF: water (1:20)). Scale bars correspond to 200 nm.

Figure 3 .

 3 Figure 3. TEM images of crumpled aggregates obtained by solvent switch of (a and b) PNIPAM25-b-PVDF35, and (b and c) PNIPAM35-b-PVDF450 in DMF at 2 mg mL -1 . Final concentration = 0.4 mg mL -1 in DMF: water (1:4) mixture.

Figure 4 .

 4 Figure 4. TEM images of the bilayer aggregates obtained by solvent switch of PNIPAM35-b-PVDF450. Initial polymer concentration in THF = 2 mg mL -1 . Final concentration = 0.4 mg mL -1 in THF: water (1:4) mixture. Water addition rate was 4 mL h -1 .

Figure 5 .

 5 Figure 5. TEM images of 2D lenticular micelles obtained by selfassembly via solvent switch protocol of PNIPAM25-b-PVDF35 (a -b). Initial BCP concentration in acetone = 2 mg mL -1 . Final concentration = 0.4 mg mL -1 in acetone: water (1:4) mixture. Non-solvent addition rate was 4 mL.h -1 . Arrows indicate the isolated 1D micelles. Schematic representation of the proposed mechanism of formation of PNIPAM25-b-PVDF35 lenticular aggregates from short 1D rod-like micelles (c).

Figure 6 .

 6 Figure 6. TEM images of representative self-assembled 2D lenticular morphologies prepared from (a) PNIPAM35-b-PVDF60, (b) PNIPAM35-b-PVDF100, (c) PNIPAM35-b-PVDF150 and (d) PNIPAM35-b-PVDF450. All samples were prepared via solvent switch in acetone: water mixtures. Initial polymer concentration in acetone = 2 mg mL - 1 . Final concentration = 0.4 mg mL -1 in acetone: water (1:4) mixture. Water addition rate = 4 mL h -1 .

Figure 7 .

 7 Figure 7. SEM images of 2D lenticular objects prepared from: PNIPAM25-b-PVDF35 (a, b), PNIPAM35-b-PVDF450 (c, d).

Figure 8 .

 8 Figure 8. X-Ray diffraction pattern of freeze-dried 2D lenticular aggregates obtained from PNIPAM35-b-PVDF100 BCP. Black dots indicate the characteristic peaks of α-phase of PVDF.

Figure 9 .Table 2 .

 92 Figure 9. AFM images of 2D lenticular aggregates prepared from: (a) PNIPAM35-b-PVDF60, (b) PNIPAM35-b-PVDF100, (c) PNIPAM35-b-PVDF150, and (d) PNIPAM35-b-PVDF450. (e and f) Schematic representation and suggested molecular arrangement of the 2D lenticular aggregates formed by assembly of PNIPAM35-b-PVDF60 and PNIPAM35-b-PVDF450 respectively.

Figure 10 .

 10 Figure 10. TEM images of representative samples of 2D lenticular aggregates prepared from: (a, b) PNIPAM35-b-PVDF150 BCP, (c, d) PNIPAM35-b-PVDF450 BCP in pure water after being heated at 45°C for 30 min.

Table 1 .

 1 Synthesis and characterization of PNIPAM macro-CTA, and PNIPAM-b-PVDF block copolymers prepared by RAFT polymerization of VDF in DMC at 73 °C.

	Entry	CTA	M	[M]0 / [CTA]0	Reaction time (h), Solvent	yield(%)	DP(NMR)(R)	Mn(theo) d (g/mol)	Mn(NMR)(R) e (g/mol)	MnSEC f (g/mol)	Đ f
	1	CTAXA	NIPAM	25	14, EtOH	>99	25 a	3000	3000	3200	1.30
	2	PNIPAM25-XA	VDF	50	20, DMC	60 b	35 c	4900	5300	6700	1.38
	3	CTAXA	NIPAM	35	14, EtOH	>99	35 a	4100	4200	4400	1.19
	4	PNIPAM35-XA	VDF	100	20, DMC	60 b	60 c	8000	8000	7500	1.29
	5	PNIPAM35-XA	VDF	150	20, DMC	61 b	100 c	10000	10600	9200	1.36
	6	PNIPAM35-XA	VDF	200	20, DMC	62 b	150 c	12100	13800	9800	1.43
	7	PNIPAM35-XA	VDF	600	20, DMC	61 b	450 c	27600	33000	25500	1.50

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

Acknowledgements

The authors thank Arkema for providing VDF, and the Institut Carnot Chimie Balard Cirimat, the LabEx CheMISyst (ANR-10-LABX-05-01), IEM and ICGM for funding the PhD of EF.

Notes and references

1

T. Soulestin, V. Ladmiral, F. Domingues, D. Santos and B. Améduri, Prog. Polym. Sci., 2017, 72, 16-60.

ARTICLE

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 11

Please do not adjust margins Please do not adjust margins concentrations. A formation pathway and a possible molecular arrangement of these 2D lenticular aggregates was proposed.

Conflicts of interest

There are no conflicts to declare.


Enrique Folgado

Vincent Ladmiral

Mona Semsarilar

Towards permanent hydrophilic PVDF membranes. Amphiphilic PVDFb-PEG-b-PVDF triblock copolymer as membrane additive

Keywords: RAFT, Poly(vinylidene fluoride), Block copolymer, Poly(ethylene glycol), NIPS, Hydrophilic membrane

Poly(vinylidene fluoride) (PVDF) ultrafiltration membranes were prepared by NIPS using a blend of a new amphiphilic PVDF-based triblock copolymer (PVDF-b-PEG-b-PVDF) and high molar mass PVDF.

During the phase inversion step, the triblock copolymer acts both as a pore forming and surface modifying agent. Thanks to the presence of the short PVDF blocks in the triblock copolymer, the hydrophilic PEG segment is fixed in the PVDF matrix, thus reducing the gradual loss of hydrophilic additive by leaching of the triblock copolymer out of the membrane matrix. The PVDF-b-PEG-b-PVDF triblock copolymer additive improved the surface hydrophilicity and significantly increased the pure water flux and permeability of the PVDF membrane. The membrane composition was optimized in terms of additive concentration and compared to membranes prepared with an equivalent amount of commercial PEG of similar molar mass. Pure water filtration tests and contact angle measurements suggested that addition of small amounts of the triblock copolymer additive (2-5% w/w) has a strong impact on the performance and hydrophilic characteristic of the PVDF membranes. The control tests showed that less than 30% w/w of the additive is lost after 9 months, while most commercial PEG (59 % w/w) leached out of the membrane matrix in only two months.

Introduction:

Membrane technology plays a crucial role in water and energy sustainability. [START_REF] Le | Materials and membrane technologies for water and energy sustainability[END_REF] Access to water is one of the keys for economic, social and cultural development. The main reason why membrane technology has become an important separation technology over the past years is the fact that membranes require relatively low energy [START_REF] Le | Materials and membrane technologies for water and energy sustainability[END_REF][START_REF] Cui | Recent progress in fluoropolymers for membranes[END_REF], are atom-efficient and are nowadays economically viable. [START_REF] Bi | Hydrophilic modification of poly(vinylidene fluoride) membrane with poly(vinyl pyrrolidone) via a cross-linking reaction[END_REF] Polymer membranes lead the membrane separation industry market because they are very competitive in terms of performance, cost and ease of handling. They can also be relatively easy to functionalize if additional properties are desired. Many polymers are available, and the choice of the material is the most important as it dictates the chemical properties and the final performance of the membrane. A polymer must have appropriate characteristics to be used in micro (MF) and ultrafiltration (UF) membrane formulations. It has to tolerate the cleaning conditions (i.e. high pressure backwash with sodium hypochlorite solution for example), the driving forces (i.e. pressure) and has to be compatible with the membrane fabrication method (e.g. phase inversion, stretching of semi-crystalline polymer foils or hollow fibers, interfacial polymerization, temperature-induced phase separation (TIPS), non-solvent-induced phase separation (NIPS)…). Significant effort has been devoted to enhancing the permeation flux, [START_REF] Bi | Hydrophilic modification of poly(vinylidene fluoride) membrane with poly(vinyl pyrrolidone) via a cross-linking reaction[END_REF][START_REF] Wang | Remarkable reduction of irreversible fouling and improvement of the permeation properties of poly(ether sulfone) ultrafiltration membranes by blending with pluronic F127[END_REF][START_REF] Kuila | Multi-functional poly(vinylidene fluoride) graft copolymers[END_REF][START_REF] Du | Modification of poly(vinylidene fluoride) ultrafiltration membranes with poly(vinyl alcohol) for fouling control in drinking water treatment[END_REF][START_REF] Han | Effect of surface charge on hydrophilically modified poly(vinylidene fluoride) membrane for microfiltration[END_REF] fouling resistance, [START_REF] Du | Modification of poly(vinylidene fluoride) ultrafiltration membranes with poly(vinyl alcohol) for fouling control in drinking water treatment[END_REF][START_REF] Dong Kang | Application and modification of poly(vinylidene fluoride) (PVDF) membranes -A review[END_REF][START_REF] Li | Development and characterization of anti-fouling cellulose hollow fiber UF membranes for oil-water separation[END_REF][START_REF] Liu | Acryloylmorpholine-grafted PVDF membrane with improved protein fouling resistance[END_REF][START_REF] Liu | Progress in the production and modification of PVDF membranes[END_REF][START_REF] Revanur | Reactive amphiphilic graft copolymer coatings applied to poly(vinylidene fluoride) ultrafiltration membranes[END_REF] operation stability, [START_REF] Li | Preparation and properties of PVDF/PVA hollow fiber membranes[END_REF] and service life of membranes. [START_REF] Bi | Hydrophilic modification of poly(vinylidene fluoride) membrane with poly(vinyl pyrrolidone) via a cross-linking reaction[END_REF][START_REF] Saffar | Hydrophilic modi fi cation of polypropylene microporous membranes by grafting TiO 2 nanoparticles with acrylic acid groups on the surface[END_REF][START_REF] Liang | A novel ZnO nanoparticle blended polyvinylidene fluoride membrane for anti-irreversible fouling[END_REF] The most common commercial polymers used for fabrication of MF and UF membranes are poly(ether sulfone) (PES), [START_REF] Zhao | Fabrication of antifouling polyethersulfone ultrafiltration membranes using Pluronic F127 as both surface modifier and pore-forming agent[END_REF] polyethylene (PE), [START_REF] Akbari | Analysis of antifouling behavior of high dispersible hydrophilic poly ( ethylene glycol )/ vinyl functionalized SiO 2 nanoparticles embedded polyethylene membrane[END_REF] polypropylene (PP), [START_REF] Saffar | Hydrophilic modi fi cation of polypropylene microporous membranes by grafting TiO 2 nanoparticles with acrylic acid groups on the surface[END_REF] polytetrafluoroethylene (PTFE) [START_REF] Membranes | Membrane Distillation Trial on Textile Wastewater Containing Surfactants Using Hydrophobic and Hydrophilic-Coated Polytetrafluoroethylene[END_REF][START_REF] Khumalo | Separation and Puri fi cation Technology Water recovery from hydrolysed human urine samples via direct contact membrane distillation using PVDF / PTFE membrane[END_REF] and poly(vinylidene fluoride) (PVDF). [START_REF] Dong Kang | Application and modification of poly(vinylidene fluoride) (PVDF) membranes -A review[END_REF][START_REF] Cha | Effect of high-temperature spinning and PVP additive on the properties of PVDF hollow fiber membranes for microfiltration[END_REF][START_REF] Wang | Preparation and characterization of PAA/PVDF membraneimmobilized Pd/Fe nanoparticles for dechlorination of trichloroacetic acid[END_REF][START_REF] Sun | Preparation and Performance of the Novel PVDF Ultra fi ltration Membranes Blending With PVA Modi fi ed SiO 2 Hydrophilic Nanoparticles[END_REF][START_REF] Zhao | Explorations of combined nonsolvent and thermally induced phase separation ( N-TIPS ) method for fabricating novel PVDF hollow fi ber membranes using mixed diluents[END_REF] Ultrafiltration (UF) and microfiltration (MF) membranes are employed when high separation efficiency is required. [START_REF] Li | Development and characterization of anti-fouling cellulose hollow fiber UF membranes for oil-water separation[END_REF][START_REF] Revanur | Reactive amphiphilic graft copolymer coatings applied to poly(vinylidene fluoride) ultrafiltration membranes[END_REF][START_REF] Wang | Preparation and characterization of PAA/PVDF membraneimmobilized Pd/Fe nanoparticles for dechlorination of trichloroacetic acid[END_REF][START_REF] Benhabiles | Preparation and characterization of TiO 2 -PVDF / PMMA blend membranes using an alternative non-toxic solvent for UF / MF and photocatalytic application[END_REF] Such filtration membranes as water treatment devices are susceptible to low fouling resistance. [START_REF] Saffar | Hydrophilic modi fi cation of polypropylene microporous membranes by grafting TiO 2 nanoparticles with acrylic acid groups on the surface[END_REF] Membrane fouling affects productivity, and generates additional operating costs, and the need for regular chemical cleaning procedures that shorten the membrane lifetime. Membrane surface hydrophilicity is generally accepted as the main factor affecting fouling. A hydrophilic membrane surface generally has higher fouling resistance compared to hydrophobic membranes. [START_REF] Higuchi | Chemically modified polysulfone hollow fibers with vinylpyrrolidone having improved blood compatibility[END_REF][START_REF] Ghaemi | Surface Modification of Polysulfone Membranes Using Poly ( Acrylic Acid ) -Decorated Alumina Nanoparticles[END_REF][START_REF] Liu | Improving the hydrophilicity of poly(vinylidene fluoride) porous membranes by electron beam initiated surface grafting of AA/SSS binary monomers[END_REF][START_REF] Wang | Plasma-induced immobilization of poly ( ethylene glycol ) onto poly ( vinylidene fluoride ) microporous membrane[END_REF] To help solve these problems, materials scientists and chemical engineers are working to develop inexpensive, scalable, and sustainable methods to produce hydrophilic membranes for water purification. [START_REF] Dong Kang | Application and modification of poly(vinylidene fluoride) (PVDF) membranes -A review[END_REF] Among all the methods that can be employed for the fabrication of polymer membranes, phase inversion method is the most popular technique. The non-solvent-induced phase separation (NIPS) method is the method of choice in industry. In this method, the polymer is dissolved in a suitable solvent, cast into the desired shape (i.e. flat or hollow fiber) and is then immersed into a non-solvent bath (coagulation bath) where the phase inversion process takes place. For this method to work, the polymers (membrane forming polymer and the additives), need to be insoluble in the non-solvent and solvent and non-solvent should be miscible. Phase inversion can also proceed via thermally induced phase separation (TIPS), [START_REF] Zhao | Explorations of combined nonsolvent and thermally induced phase separation ( N-TIPS ) method for fabricating novel PVDF hollow fi ber membranes using mixed diluents[END_REF][START_REF] Zhang | Study of the dual role mechanism of water-soluble additive in low temperature thermally-induced phase separation[END_REF] vapor induced phase separation (VIPS), [START_REF] Marino | The Formation of Polyvinylidene Fluoride Membranes with Tailored Properties via Vapour / Non-Solvent Induced Phase Separation[END_REF] and evaporation-induced phase separation. [START_REF] Pervin | Tailoring pore distribution in polymer films via evaporation induced phase separation[END_REF] PVDF is one of the most widely used polymers in membrane formulations due to its remarkable properties such as wide chemical compatibility, excellent mechanical properties, relatively easy processing and high temperature resistance. [START_REF] Cui | Recent progress in fluoropolymers for membranes[END_REF][START_REF] Dong Kang | Application and modification of poly(vinylidene fluoride) (PVDF) membranes -A review[END_REF][START_REF] Liu | Progress in the production and modification of PVDF membranes[END_REF] However, PVDF is relatively hydrophobic. This hydrophobicity is a problem for the efficiency and energy consumption of membranes designed for water purification. [START_REF] Wang | Preparation and characterization of PAA/PVDF membraneimmobilized Pd/Fe nanoparticles for dechlorination of trichloroacetic acid[END_REF][START_REF] Benhabiles | Preparation and characterization of TiO 2 -PVDF / PMMA blend membranes using an alternative non-toxic solvent for UF / MF and photocatalytic application[END_REF][START_REF] Liu | Improving the hydrophilicity of poly(vinylidene fluoride) porous membranes by electron beam initiated surface grafting of AA/SSS binary monomers[END_REF] Indeed, ultrafiltration and microfiltration PVDF membranes require high operating pressures resulting in high energy consumption to provide acceptable flux values.

Several studies describe the modification of PVDF membranes with hydrophilic additives such as polymers [START_REF] Bi | Hydrophilic modification of poly(vinylidene fluoride) membrane with poly(vinyl pyrrolidone) via a cross-linking reaction[END_REF][START_REF] Cha | Effect of high-temperature spinning and PVP additive on the properties of PVDF hollow fiber membranes for microfiltration[END_REF] and copolymers [START_REF] Revanur | Reactive amphiphilic graft copolymer coatings applied to poly(vinylidene fluoride) ultrafiltration membranes[END_REF][START_REF] Venault | Low-biofouling membranes prepared by liquid-induced phase separation of the PVDF/polystyrene-b-poly (ethylene glycol) methacrylate blend[END_REF][START_REF] Park | Separation and Puri fi cation Technology E ff ects of methacrylate based amphiphilic block copolymer additives on ultra fi ltration PVDF membrane formation[END_REF] to improve their hydrophilicity. However, most of the added additives (mainly hydrophilic polymers or nanoparticles [START_REF] Saffar | Hydrophilic modi fi cation of polypropylene microporous membranes by grafting TiO 2 nanoparticles with acrylic acid groups on the surface[END_REF][START_REF] Liang | A novel ZnO nanoparticle blended polyvinylidene fluoride membrane for anti-irreversible fouling[END_REF][START_REF] Benhabiles | Preparation and characterization of TiO 2 -PVDF / PMMA blend membranes using an alternative non-toxic solvent for UF / MF and photocatalytic application[END_REF][START_REF] Ghaemi | Surface Modification of Polysulfone Membranes Using Poly ( Acrylic Acid ) -Decorated Alumina Nanoparticles[END_REF]) is lost during the membrane manufacturing process, and through gradual leaching during operation. This is often because there is no specific interaction between the hydrophilic additive and the PVDF since the two components are simply blended with each other.

Post-fabrication treatment via physical surface modification (e.g. coating with a hydrophilic polymer layer) [START_REF] Du | Modification of poly(vinylidene fluoride) ultrafiltration membranes with poly(vinyl alcohol) for fouling control in drinking water treatment[END_REF][START_REF] Revanur | Reactive amphiphilic graft copolymer coatings applied to poly(vinylidene fluoride) ultrafiltration membranes[END_REF][START_REF] Wang | Preparation and characterization of PAA/PVDF membraneimmobilized Pd/Fe nanoparticles for dechlorination of trichloroacetic acid[END_REF] or chemical treatment (e.g. plasma grafting of polar groups) [START_REF] Han | Effect of surface charge on hydrophilically modified poly(vinylidene fluoride) membrane for microfiltration[END_REF][START_REF] Liu | Improving the hydrophilicity of poly(vinylidene fluoride) porous membranes by electron beam initiated surface grafting of AA/SSS binary monomers[END_REF][START_REF] Wang | Plasma-induced immobilization of poly ( ethylene glycol ) onto poly ( vinylidene fluoride ) microporous membrane[END_REF] are some of the techniques used to confer hydrophilicity to PVDF membranes. Finding formulations and preparation methods that optimize fabrication costs, ease of implementation, efficiency, and long-term durability remains a challenge. Blending PVDF with hydrophilic polymers (e.g. polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP)) via phase separation process is the most used method in membrane industry. This is because; preparing membranes from a mixture of polymers is relatively easy, convenient and adaptable to the industrial membrane fabrication set-up.

Numerous studies report the use of additives such as PVP [START_REF] Bi | Hydrophilic modification of poly(vinylidene fluoride) membrane with poly(vinyl pyrrolidone) via a cross-linking reaction[END_REF][START_REF] Cha | Effect of high-temperature spinning and PVP additive on the properties of PVDF hollow fiber membranes for microfiltration[END_REF][START_REF] Higuchi | Chemically modified polysulfone hollow fibers with vinylpyrrolidone having improved blood compatibility[END_REF] and PEG [START_REF] Venault | Low-biofouling membranes prepared by liquid-induced phase separation of the PVDF/polystyrene-b-poly (ethylene glycol) methacrylate blend[END_REF][START_REF] Kim | Effect of PEG Addative on Membrane Formation by Phase Inversion[END_REF]. These polymers act as both pore-formers (increasing porosity and hence membrane permeability) and hydrophilic additives.

However, assessing the real and final impact of these polymer additives is difficult, since the relationship between the amounts of additive added in the casting solution and their final concentration in the membrane matrix after coagulation bath is hard to establish/calculate by regular analysis methods. A systematic study is required each time a new formulation is tested. Likewise, establishing the rate of migration of the additive retained within the polymer matrix is very difficult. [START_REF] Yun | Formation kinetics and characterization of polyphthalazinone ether ketone hollow fiber ultrafiltration membranes[END_REF][START_REF] Liu | Development of high-antifouling PPSU ultrafiltration membrane by using compound additives: Preparation, morphologies, and filtration resistant properties[END_REF] For example, when PEG is blended with PVDF, a large part of it is washed out during the phase inversion step (coagulation bath). The non-solvent is often water, which dissolves most of the PEG, promoting the formation of the pores. In consequence only a small amount of the initial added PEG remains in the PVDF membrane matrix. [START_REF] Venault | Low-biofouling membranes prepared by liquid-induced phase separation of the PVDF/polystyrene-b-poly (ethylene glycol) methacrylate blend[END_REF][START_REF] Liu | Development of high-antifouling PPSU ultrafiltration membrane by using compound additives: Preparation, morphologies, and filtration resistant properties[END_REF] To reach higher hydrophilicity, larger quantities of additive should be added. However larger quantities of additives also results into higher membrane porosity. Finding the right compromise between hydrophilicity and porosity is not straightforward.

Here, we describe the use of an ABA triblock copolymer containing a PEG central block and two PVDF lateral blocks, as a hydrophilic additive for PVDF membranes. This PVDF-b-PEG-b-PVDF amphiphilic triblock copolymer should increase the wettability of the resulting membrane. In addition, leaching of the PEG should be reduced both at the preparation and filtration steps, since the PEG and the PVDF segments would be connected via covalent bond and the two PVDF segments of the triblock cocrystallize with the high molar mass PVDF membrane matrix. Different proportions of triblock copolymer additive (PVDF50-b-PEG136-b-PVDF50, with weight fraction of PEG6000 = 0.47) were used in the casting solution, and the membranes thus prepared via the NIPS process were compared with similar membranes prepared from casting solutions containing an equivalent amount of PEG6000. The prepared membranes were aged over 9 months and their performance as well as the amount of additive left in the membrane matrix was assessed during this period using contact angle, NMR and pure water flux/permeability measurements. The membranes were also characterized using SEM before and after aging.

Experimental

Materials

All reagents were purchased from Sigma-Aldrich and were used as received unless otherwise stated.

High molar mass PVDF (Kynar 761; Mw: 441,000 g/mol) was kindly donated by ARKEMA. Deuterium oxide (D2O) and dimethylsulfoxide ((CD3)2SO) were purchased from Eurisotop. PVDF-b-PEG-b-PVDF block copolymer was synthesized as reported previously. [START_REF] Folgado | One-Pot" Aminolysis/Thiol-ene preparation of well-defined amphiphilic PVDF-b-PEG-b-PVDF triblock copolymers: Self-assembly behaviour in mixed solvents[END_REF] 2.2. Methods 2.2.1. Preparation of high molar mass PVDF mother solution.

3.75g of PVDF (Kynar 761) was dissolved in 20g of NMP under magnetic stirring at 80 °C for 24h (final PVDF concentration = 15.79 % w/w).

Preparation of dope solutions containing the triblock copolymer.

The dope solutions were prepared by blending 4 g of the PVDF mother solution with different amounts of PVDF-b-PEG-b-PVDF triblock copolymer solution (1 to 25% w/w) in NMP (see Table 1). To ensure homogeneous mixing, the mixture was stirred for 24h at 80 °C.

Preparation of control dope solutions containing PEG homopolymer.

A 15% (w/w) high molar mass PVDF solution in NMP was blended with two different concentrations of PEG138 for 24h at 80 °C (see Table 1).

Blade casting

Membranes of 20 cm in 20 cm were prepared using a blade with a 250 µm clearance using dope solutions stored at 80 °C on a glass plate. 60 seconds after film casting, the substrate was transferred to a water coagulation bath thermostated at 25°C. The membrane was left in this coagulation bath for 24 h, prior to drying at room temperature for another 24h (see Scheme 1).

Water Contact angle (WCA)

The contact angle measurements, which quantify the wettability/hydrophilicity of the membrane surface, were assessed using a monochrome camera B-CAM-21-BW (CCCIR) and a Led R60 lamp purchased from CONRAD. For each sample, 10.0 μL of ultra-pure water was deposited on a polymercoated silica wafer using a micro needle. The images were recorded using One Touch Graber software and treated using Image J software.

NMR spectroscopy

NMR spectra were acquired in either D2O or a mixture of D2O and (CD3)2SO using a Bruker 300 MHz spectrometer. All chemical shifts are reported in ppm.

Scanning electron microscopy (SEM)

The porous structure of virgin PVDF membrane and modified PVDF membranes were characterized using microscopy techniques. SEM analyses were conducted using a Hitachi S-4500 instrument operating at a spatial resolution of 1.50 nm at 15 kV energy. The samples were dried and coated with an ultrathin layer of electrically conducting platinum deposited by high vacuum evaporation.

Water filtration experiments

For filtration tests, Circles of 2.5 cm in diameter were cut from each membrane and were placed in sealed petri dishes filled with distilled water. This water was replaced weekly with fresh distilled water during the study period of 9 months. The membrane was fitted in a 10 mL filtration cell (Amicon 8010 stirred cell). Filtration cell was then connected to a water reservoir and compressed air line. The measurements were performed at pressures between 0.1 and 2.0 bar. The mass of the water passing through the membrane (permeate) was recorded by the Sarto Connect software at regular time intervals. All filtration experiments were performed at room temperature with dust free ultrapure water (filtered through a 400-μm filter). Before recording the water flux, the membranes were conditioned for 2h at 2 bar. The flux and permeability of the 4 modified membranes were evaluated during the aging period via three cycles of pure water filtration. The volumetric flux and permeability were calculated according to Darcy's law using equations ( 1) and (2):

𝐹𝑙𝑢𝑥 (𝐽 𝑣 ) = 𝑉 𝑝 (𝑡 * 𝑆) ⁄ (L h -1 m -2 ) (1) 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦 (𝐿 𝑝 ) = 𝐽 𝑣 𝛥𝑃 ⁄ (L h -1 m -2 bar -1 ) (2)
Where Vp = Permeate volume, t = Time, S = Surface area and ΔP = pressure difference.

Porosity and Pore Size Determination

The porosity of the membrane was determined from its dry and wet masses. The membrane was immersed in water for 24 h. After that, the mass of the wet membrane was measured after wiping off excess water using filter paper. Then, the wet membrane was dried in an oven under vacuum for 10 h at 25°C and the mass of this dried membrane was measured. The porosity was calculated using the following equation [START_REF] Li | Morphology changes of polyvinylidene fluoride membrane under different phase separation mechanisms[END_REF]:

𝜀(%) = (𝑤 𝑤 -𝑤 𝑑 ) 𝜌 𝑤 (𝑤 𝑤 -𝑤 𝑑 ) 𝜌 𝑤 + 𝑤 𝑑 𝜌 𝑝 × 100% ( 3
)
where ԑ is the membrane porosity, 𝑤 𝑤 is the wet membrane mass (g), 𝑤 𝑑 is the dry membrane mass (g), 𝜌 𝑤 is the pure water density while 𝜌 𝑝 is the density of the polymer. The densities of the polymer blends were estimated using the weight fractions of PEG and PVDF. Since 𝜌 𝑃𝑉𝐷𝐹 = 1.78 g•cm -3 and 𝜌 𝑃𝐸𝐺 = 1.20 g•cm -3 . Thus, the density of the blend containing 2 % w/w of triblock (i.e. 0.81 % w/w of PEG) was estimated to be 1.775 g•cm -3 while the density of the blend containing 5% w/w of triblock (or 1.95 % w/w of PEG) was 1.769 g•cm -3 .

The mean pore radius (rm) was calculated from the pure water flux (PWF) value and porosity data obtained previously using the Guereout-Elford-Ferry equation as follows [START_REF] Feng | Preparation and properties of microporous membrane from for membrane distillation[END_REF]:

𝑟 𝑚 = √ (2.9 -1.75 • 𝜀) • 8 • η • 𝑙 • 𝑄 𝜀 • 𝐴 • 𝛥𝑃 (4)
Where η is the viscosity of water (mPa•s) (1.002 at 20°C), 𝑙 is the membrane thickness (m) (150-200µm), 𝑄 is the pure water flux (m

Note: The integral of PVDF signal at 2.8 ppm were used as reference.

Results and discussion

The well-defined triblock copolymer PVDF50-b-PEG136-b-PVDF50 was synthesized according to our previously published work. [START_REF] Folgado | One-Pot" Aminolysis/Thia-Michael Addition preparation of well-defined amphiphilic PVDF-b-PEG-b-PVDF triblock copolymers: Self-assembly behaviour in mixed solvents[END_REF] This triblock copolymer was then used as the hydrophilic additive in the preparation of PVDF membranes. A series of dope solutions (composed of high molar mass PVDF and the triblock copolymer) containing different mass fraction of this triblock copolymer was prepared. The mass fraction of the high molar mass PVDF in NMP was fixed at 15 % (w/w). To ensure complete dissolution of PVDF in NMP, the solutions were heated at 80 °C under stirring for 24h. Different amounts of the triblock copolymer PVDF50-b-PEG136-b-PVDF50 (1, 2, 5, 10, 15 and 20 % w/w) were then added to the homogeneous PVDF solution. These mixtures were stirred at 25°C until homogeneous. A series of membranes were prepared via the NIPS process using dope solutions containing different amounts of the triblock copolymer (see Table 1). Additionally, control membranes were made from dope solutions containing commercially available PEG. The quantities of PEG added to the casting solutions were carefully calculated to match the amount of PEG present in the casting solutions containing the triblock copolymer as additive.

1.1.1. Effect of PVDF50-b-PEG136-b-PVDF50 on the membrane formation.

The membranes were prepared via the NIPS process using a water coagulation bath. As mentioned before, different concentrations of the triblock copolymer (1, 2, 5, 10, 15, 20% w/w) were employed to prepare the membrane casting solutions (Table 1). Membranes prepared from triblock copolymer solutions of concentration higher than 5% w/w were very soft and swollen after immersion in the coagulation bath. They seemed to retain large amounts of water that caused the membranes to become unstable and fall apart (Figure S4). For this reason only the membranes prepared with 1, 2 and 5% (w/w) of triblock copolymer were used in the rest of the present study.

SEM images of the stable membranes (Figure 1) showed asymmetric structures with a thin top skin porous layer supported by a finger-like sublayer. Formation of finger-like structure is typical in membranes prepared via a liquid-liquid phase separation process such as NIPS, in which polymer-rich and polymer-poor phases are developed. The surface and cross-section images of the membranes showed that increasing the PVDF-b-PEG-b-PVDF content in the dope solution led to more porous surfaces. This observation is in agreement with previous reports [START_REF] Kim | Effect of PEG Addative on Membrane Formation by Phase Inversion[END_REF][START_REF] Wang | Recent advances in preparation and morphology control of polymeric membranes formed by nonsolvent induced phase separation[END_REF][START_REF] Matsen | Effect of Architecture on the Phase Behavior of AB-Type Block Copolymer Melts[END_REF][START_REF] Loh | Effects of Additives and Coagulant Temperature on Fabrication of High Performenace PVDF/Pluronic F127 Blend Hollow Fiber Membranes via Nonsolvent Induced Phase Separation[END_REF] and is not surprising as the PEG segment of the triblock copolymer retains water and facilitates the formation of pores. In NIPS process the phase separation happens very fast. This causes the PVDF blocks of the copolymer to precipitate along with the high molar mass PVDF forming the matrix of the membrane. This co-precipitation of the two types of PVDF chains is also likely accompanied by co-crystallization. This combined phenomena likely result in the immobilization of the triblock copolymer in the membranes matrix, reducing the possibility for this copolymer additive to migrate and leach out. The relevant changes in the number and size of the macro voids with the increasing amount of the additive could not be calculated using the SEM cross-section images. All the analyzed membranes presented large macro voids. This may be ascribed to the presence of the hydrophilic PEG segments in the triblock copolymer which enhances the in-flow/out-flow of solvent and non-solvent molecules in the membrane structure resulting in high porosity.

1.1.2. Effect of PVDF50-b-PEG136-b-PVDF50 on the surface hydrophilicity of the PVDF membranes.

As explained above the addition of PEG-containing triblock copolymer resulted in a general higher surface porosity. As observed by SEM (Figure 1), membranes formed from solutions containing larger mass fractions of triblock copolymer displayed higher surface porosity. This porosity was rather irregular which resulted in large pore size distribution as well as increased roughness of the membrane surface. As expected, the hydrophilicity of the surface of the membranes increased with increasing the additive concentration in the dope solutions.

Table 1. Dope solution formulations for the preparation of the hydrophilic PVDF membranes.

Note: *calculated masses of PEG from copolymer additive or commercial PEG.

Water contact angle (WCA) measurements (Table 1 and Figure 2) were performed to evaluate the membrane surface hydrophilicity/wettability.

WCA of the pure PVDF membrane (Triblock0 in Table 1) at t = 10 s was (105 ± 18°). The PVDF surface of this membrane was thus slightly hydrophobic as expected. The membrane prepared using dope solution containing 1% (w/w) triblock copolymer displayed a much lower water contact angle (74 ± 6°) at t = 10 s. This value decreased to 54±5°, 39±6° and 10±10° at t = 10 s when dope solutions containing 2, 5 and 10% (w/w) triblock copolymer were used respectively. When more concentrated dope solutions were used (> 5% w/w) the water droplets were rapidly absorbed into the membranes (≤10 s). Figure 2 shows the evolution of WCA with time for pure PVDF membrane and those prepared using solutions containing 2 and 5% triblock copolymer. In the case of the pure PVDF membrane the WCA remained stable at around 105 ° for the duration of the monitoring (25 s). In contrast, the WCA decreased drastically reaching 0° within 25 s for the membranes prepared from dope solutions prepared with 2 and 5% (w/w) triblock copolymer.

These results show that addition of even small amounts (1-5% (w/w) in the dope solution) of triblock copolymer enhances the hydrophilicity of the PVDF membranes drastically. This is probably due to the fact that most of the added triblock copolymer stays in the membrane matrix thanks to the presence of the two short PVDF blocks acting as anchoring points for the PEG segments to the high molar mass PVDF chains forming the bulk of the membrane. In addition, the presence of this short PVDF blocks likely promotes a more homogeneous distribution of the PEG segments in the membrane matrix compared to the commercially available PEG used as additive.

1.1.3. Membrane ageing and its effect on WCA, flux, and permeability.

To test the hypothesis that the presence of the short PVDF blocks of the triblock copolymer is responsible for the strong interaction of the triblock copolymer with the high molar mass PVDF leading to immobilization of the PEG segments in the membrane matrix, ageing experiments were carried out over 9 months. For these experiments two 10 cm x 10 cm membranes were prepared using dope solutions with 2 and 5% w/w triblock copolymer (Triblock2 and Triblock5 in Table 1). In addition, two membranes (100 cm 2 ) were prepared from dope solutions containing equivalent amounts of commercial PEG homopolymer. These membranes (PEG2eq and PEG5eq in Table 2) were used as control experiments.

WCA was monitored over 9 months to assess the loss of hydrophilicity with time. The WCA measurements (Table 2) indicate a small loss of hydrophilicity for membranes containing triblock copolymer;

the WCA increased by 13° and 14° in 9 months for membranes prepared from the 2 and 5 % (w/w) dope solutions (Triblock2 and Triblock5 in table 2). These membranes remained hydrophilic during the study period with WCA < 70°. In contrast, over the same period of time, the WCA of the membranes prepared with PEG homopolymer increased much more: 29° and 41° respectively (PEG2eq and PEG5eq membranes). At the end of the study period the WCA reached values (90°) close to those observed for pure PVDF membranes (ca. 105°). It is important to note that the PEG2eq and PEG5eq membranes were less hydrophilic (showed higher WCA) than their triblock copolymer counterparts at t=0. This indicates that a significant quantity of the PEG homopolymer is lost during the preparation process (likely in the coagulation bath). In all membrane samples the additives (both the commercial PEG homopolymer and the synthesized triblock copolymer) did leach out of the membrane matrix within the first 60 days. However, the WCA results clearly show that the triblock copolymer loss is marginal (8-9°) compared to that of the PEG homopolymer (17-26°) confirming that the presence of the short PVDF blocks does help in retaining the hydrophilic PEG chains in the membrane matrix. 47 73 [START_REF] Ghaemi | Surface Modification of Polysulfone Membranes Using Poly ( Acrylic Acid ) -Decorated Alumina Nanoparticles[END_REF] 88 [START_REF] Wang | Recent advances in preparation and morphology control of polymeric membranes formed by nonsolvent induced phase separation[END_REF] To quantify the amount of the additive (triblock copolymer and PEG homopolymer) loss, 1 H NMR experiments were carried out on the membranes before and after the ageing process. Integrals of the PVDF ) was taken as reference, and the mass fraction of PEG loss was calculated using equation 5. The data summarized in Table 3 shows that the loss of triblock copolymer is much lower than that of the PEG homopolymer. For the membrane prepared from the 2 wt % triblock copolymer dope solution (Triblock2), about 21 % of the PEG was lost during the study period (9 months) while about 59 % of the PEG homopolymer was lost over the same time for the membrane prepared using PEG homopolymer (PEG2eq). This loss rose to 27 % and 71 % for the membranes prepared from the 5 wt % triblock copolymer (Triblock5) or PEG homopolymer (PEG5eq) dope solution respectively. This set of data indicates that 2 wt % of triblock copolymer additive is sufficient to confer long-lasting hydrophilicity to the PVDF membranes. Higher triblock copolymer weight fraction leads to higher loss of the additive. These results show that more atom-efficient (and maybe cheaper) PVDF membranes endowed with long lasting hydrophilicity could be prepared using such PVDF50-b-PEG138-b-PVDF50 triblock copolymer additive.

For the Triblock2 membrane, flux reached up to 1400 L/h•m 2 at 2 bar right after preparation (t0). After 9 months the flux value halved to around 600 L/h•m 2 . This decrease is mainly due to the loss of hydrophilicity caused by additive leaching (see Table 2 and3 The permeability plots (Figure S7) show that the membranes modified with the PEG homopolymer were almost insensitive to pressure with a low permeability (110-150 L h -1 m -2 bar) over the pressure range tested (0-2 bar). The permeability profile of the membranes prepared using the 2 and 5% (w/w) triblock copolymer dope solution showed moderate fluctuations. This is most probably due to the pore size and pore density of the membrane rather than their degree of hydrophilicity. However, the structural integrity of the membranes prepared using PEG homopolymer as additive was higher than that of the membranes containing the triblock copolymer. This is likely due to the fact that the former effectively do not retain much PEG in their structure compared to the latter. In consequence their pores do not swell as much when in contact with water.

Since the SEM images of the membranes prepared using different triblock copolymer dope solutions (Figure 1) did not show major changes in structure or porosity, water uptake was used as an indirect method to estimate pore size and porosity (Table 4). Membranes prepared from 5 wt. % triblock copolymer dope solution were more porous (76%) than the membranes prepared from the 2 wt. % triblock copolymer dope solution (61%). However, membrane Triblock2 had larger average pore size (62-73 nm) compared to membrane Triblock5 which featured an average pore size of 43 -50 nm. These data are in agreement with the flux values obtained for each membrane. The Triblock5 membrane had lower flux than the Triblock2 membrane over the entire ageing period. The PEG2eq and PEG5eq membranes had the smallest mean pore radii (ca. 17 nm for PEG2eq and 23 nm for PEG5eq) hence their low flux and permeability values.

Conclusions

In summary, a well-defined PVDF-PEG-PVDF ABA triblock copolymer synthesized using RAFT and thia-Michael addition click chemistry was used as a hydrophilic additive for PVDF water filtration membranes. The triblock copolymer was blended with high molar mass PVDF in different proportions, and membranes were prepared via the NIPS process. The resulting porous membranes were fully characterized using SEM, 1 H NMR, WCA and filtration tests. These tests showed that the hydrophilicity of the membranes increased with increasing additive content. Control experiments performed on PVDF membranes prepared using PEG homopolymer as additive showed that most of the PEG leached out during the first 60 days following membrane preparation. It was also demonstrated that only small amounts of the triblock copolymer (2-5% w/w) were required to confer sufficient and long-lasting hydrophilicity to the PVDF membranes. Larger amounts of triblock copolymer led to membrane swelling and structural instability. This study suggests that migration of the copolymer additive towards the airpolymer interface or water-polymer interface and subsequent leaching still occurs. However, these phenomena are significantly decreased thanks to the presence of the short PVDF segments covalently attached to the PEG. These PVDF segments likely anchors the PEG block in the PVDF matrix by coprecipitation and co-crystallization with the high molar mass PVDF forming the bulk of the membrane.

The strategy proposed here, based on the use of amphiphilic PVDF-containing block copolymers as hydrophilic additives, is able to significantly extend the operational lifetime of PVDF membranes for water purification applications.

Scheme 1 .

 1 Scheme 1. Membrane preparation via NIPS process from a blend of high Mw PVDF and PVDF50-b-PEG136-b-PVDF50 triblock copolymer in NMP.

Figure 1 .

 1 Figure 1. SEM images of the surface and cross-section of PVDF membranes prepared from dope solutions containing (a, a') 0%, (b, b') 2% and (c, c') 5% w/w of PVDF-b-PEG-b-PVDF triblock copolymer.

Figure 2 .

 2 Figure 2. Evolution of the WCA versus time: Pure PVDF membrane (black squares), and membranes prepared using 2 and 5% (w/w) triblock copolymer solutions (blue triangles and red dots respectively).

( 2 . 6 - 3 .

 263 0 ppm) and PEG (3.5 ppm) signals were used to determine the percentage of additive loss (see

  ) and water-retaining ability of the membrane, and to a lower extent to fouling (bacteria growth) as shown in Figure S5. The Triblock5 membrane followed a similar trend although the decrease in flux during the ageing period was lower (it only decreased by 200 L/h•m 2 ). However both membranes present an improvement in terms of flux compared to membranes prepared from PEG homopolymer as additive. The highest flux values obtained for membranes prepared using the PEG homopolymer were 100 and 200 L/h•m 2 (for PEG2eq and PEG5eq membranes during the first filtration experiments) while the membranes containing the triblock copolymer reach flux values of 650 and 1400 L/h•m 2 (for Triblock2 and Triblock5 respectively).

Figure 3 .

 3 Figure 3. Flux vs. pressure of PVDF membranes prepared using dope solutions containing 2% w/w (left) and 5% w/w (right) PVDF-b-PEG-b-PVDF block copolymer. The flux measurements were performed right after membrane preparation (black squares), after 9 months (blue triangles) and compared to flux values obtained for membranes prepared using dope solutions containing PEG homopolymer concentrations equivalent to the total PEG concentrations of triblock doped casting solutions (green inverted triangle) just after preparation (t=0).

  3

  •s -1 ), 𝐴 is the area of membrane (m 2 ) (d = 2.5cm) and 𝛥𝑃 is the

	operating pressure (mPa) (2 bar = 2•10 8 mPa).
	2.2.9. Additive Loss Assessment
	𝐴𝑑𝑑𝑖𝑡𝑖𝑣𝑒 𝑙𝑜𝑠𝑠(%) = 100 -	∫ 3.47 3.55 ∫ 3.47 3.55	𝐶𝐻 2 (𝑃𝐸𝐺)𝑎𝑔𝑒𝑑 𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝐶𝐻 2 (𝑃𝐸𝐺)𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝑡 = 0	𝑥 100

Table 2 . WCA evolution of the membranes during the ageing experiment.

 2

	Membrane ID	Mean WCA at 10 s [° of change]. 0 month 2 month 9 month
	Triblock2	54	63 [9]	67 [13]
	Triblock5	39	47 [8]	53 [14]
	PEGeq2	61	78 [17]	90 [29]
	PEGeq5

Table 3

 3

	and S6). The CH2 signal of the PVDF (∫ 2.66 3.01	CH 2 (PVDF)

Table 3 . PEG loss after the ageing process.

 3 Note: the signals of the high molar mass PVDF and the short PVDF blocks of the triblock copolymer overlap. *PEG from the triblock copolymer additive.

	Membrane ID	MPEG*/MPVDF	Additive loss after 2 months (%)	Additive loss after 9 months (%)
	Triblock2	5.63 10 -2 *	9.1	21.4
	PEG2eq	5.64 10 -2	41.2	58.8
	Triblock5	12.51 10 -2 *	21.4	27.3
	PEG5eq	12.50 10 -2	41.7	70.8

Table 4 . Membrane porosity (ε) and mean pore size (rm).

 4

	PVDF Membrane	Porosity	Mean pore
		(%)	radii (nm)*
	Triblock2	61	62-73
	PEG2eq	41	17-19
	Triblock5	76	43-50
	PEG5eq	48	23-26

*Pore size estimated using two thicknesses

(150 and 200µm)

mat, the LabEx CheMISyst (ANR-10-LABX-05-01), IEM and ICGM for funding the PhD of EF.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.