Poly(vinylidene fluoride) Containing Phosphonic ... - ACS Publications

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Poly(vinylidene fluoride) Containing Phosphonic Acid as Anti-Corrosion Coating for Steel Sanjib Banerjee, Mohammad Wehbi, Abdellatif Manseri, Ahmad Mehdi, Ali Alaaeddine, Ali Hachem, and Bruno Ameduri ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15408 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

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ACS Applied Materials & Interfaces

Poly(vinylidene fluoride) Containing Phosphonic Acid as AntiCorrosion Coating for Steel Sanjib Banerjee,*,a Mohammad Wehbi,a,b,c Abdellatif Manseri,a Ahmad Mehdi,b Ali Alaaeddine,c Ali Hachemc and Bruno Ameduri*,a a

Ingénierie et Architectures Macromoléculaires Team, Institut Charles Gerhardt, UMR 5253

CNRS, UM, ENSCM, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France b

Chimie Molecularie et Organisation du Solide Team, Institut Charles Gerhardt, Universite´ de

Montpellier, Place Eugène Bataillon, 34095 Montpellier Cedex 05, France c

Department of Chemistry and Biochemistry, Faculty of Sciences 1, Lebanese University, Rafic

Hariri University Campus – Hadas, Beirut, Lebanon.

ABSTRACT: Vinylidene fluoride (VDF)-based copolymers bearing pendant phosphonic acid function for potential application as anti-corrosion coatings was synthesized via free radical copolymerization of VDF with a new phosphorous containing 2-trifluoromethacrylate monomer, (dimethoxyphosphoryl)methyl

2-(trifluoromethyl)acrylate

(MAF-DMP).

MAF-DMP

was

prepared from 2-trifluoromethacrylic acid in 60% overall yield. Radical copolymerizations of VDF with MAF-DMP initiated by tert-amyl peroxy-2-ethylhexanoate at varying ([VDF]0/[MAFDMP]0) feed ratios led to several poly(VDF-co-MAF-DMP) copolymers having different molar percentages of VDF (79-96%) and number average molecular weights (Mns) up to ca. 10,000 g mol-1 in fair yields (47-54%). Determination of the composition and microstructure of all synthesized copolymers were done by 1H and 19F NMR spectroscopies. The monomer reactivity ratios of this new VDF/MAF-DMP pair was also determined (rVDF = 0.76 ± 0.34 and rMAF-DMP = 1 ACS Paragon Plus Environment


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0 at 74 °C). The resulting poly(VDF-co-MAF-DMP) copolymers exhibited high melting temperature (162-171 °C, with respect to the VDF content) and the degree of crystallinity reached up to 51%. Finally, the pendant dimethyl phosphonate ester groups of the synthesized poly(VDF-co-MAF-DMP) copolymer were quantitatively hydrolyzed giving rise to novel phosphonic acid-functionalized PVDF (PVDF-PA). In comparison to hydrophobic poly(VDFco-MAF-DMP) copolymers (the water contact angle, WCA, was 98°), the hydrophilic character of the PVDF-PA was found to be surprisingly rather pronounced, exhibiting low WCA (15°). Finally, steel plates coated with PVDF-PA displayed satisfactory anti-corrosion properties under simulated sea water environment.

Keywords: poly(vinylidene fluoride); phoshonic acid; radical polymerization; reactivity ratio; coating; adhesion; anti-corrosion

INTRODUCTION

Poly(vinylidene fluoride) (PVDF) is the second most produced fluoropolymer, trailing after the deserving winner polytetrafluoroethylene (PTFE).1-3 VDF is regarded as an attractive monomer and one of the most commonly used fluoroalkenes.4-6 The reactivity of VDF is similar to those of tetrafluoroethylene and chlorotrifluoroethylene. Moreover, VDF is less dangerous than the other two: it is non-explosive and non-toxic (the lethal concentration, LC50 > 200,000 ppm). In addition, VDF-based (co)polymers are extensively used in piezoelectric devices,7 Li-ion batteries (as binders and separators),8, 9 membranes for water purification,4, 10, 11 in petrochemical industry and photovoltaic devices.9 2 ACS Paragon Plus Environment

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However, PVDF suffers from (i) increased processing cost (due to its highly crystalline nature), (ii) low solubility (except in selected organic solvents such as dimethyl sulfoxide, N,Ndimethylformamide and N,N-dimethylacetamide), (iii) difficulty in crosslinking (using toxic telechelic diamines or bisphenolates12) and (iv) laborious tuning properties for targeted applications (due to the lack of functionality).4,

9, 13, 14

Therefore, it has been challenging to

covalently bind PVDF with other substrates (generally consisting of polar components) like adhesion promoting agents, printer inks, and paints.15 These above drawbacks can be overcome by (i) incorporating vinyl monomers bearing pendant functional groups4 (for instance acetoxy,16 thioacetoxy, hydroxyl, esters, ethers, halogens, carboxylic acid17 or substituted aryl groups) as the comonomers during the radical copolymerization or (ii) cross-linking via cure site comonomers containing trialkoxysilane,18 cyanato or isocyanato groups and by using bisamines or bisphenates.12 By this approach, some of the properties of the resulting copolymers4, 19, 20 can be improved such as adhesion, thermal stability,21 conductivity22 or hydrophobicity,21 to name a few. Among the various comonomers for VDF, 2-(trifluoromethyl)acrylic acid (MAF) and MAF-esters (alkyl 2-trifluoromethacrylates)) are particularly attractive.20 MAF or MAF-esters containing copolymers exhibit excellent aging resistance and adhesion property.20 Thus, our team has reported the copolymerization of VDF with MAF derivatives via both conventional radical polymerization23-25 and reversible deactivation radical polymerization (RDRP)26,

27

techniques. The monomer reactivity ratios of MAF (rMAF = 0) and VDF (rVDF = 0.33 ± 0.09) at 55 °C were also reported,23 which confirmed inability of MAF to homopropagate under radical polymerization conditions,28 but resulted in copolymers with an unique alternating monomer sequences.20,

23, 26

It was shown that the obtained copolymers could have applications in high 3 ACS Paragon Plus Environment


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performance nanocomposites,29 polymer electrolyte fuel cell membranes,22 Li-ion batteries (as binders).8 Thus, MAF or MAF-esters have emerged as classic comonomers to prepare functional VDF-based copolymer with tunable properties. In recent years, phosphorus-containing polymers30-32 have received great interest33-35 due to their interesting complexing properties36 and have found applications as corrosion inhibiting agents, dispersants, deposit resistance coating,37 flame retardants,38 adhesion promoters for paints,39 superlubricity coatings,40 (super)hydrophobic photostable coatings for stone,41 polymer electrolyte membrane fuel cell,42-45 and in biomedical fields.46 Good adherence to the substrate in presence of high humidity or water (wet adhesion) is the main feature of any anti-corrosion coating.31 Diffusion of water through the coating-metal interface could result in decrease of adhesion and subsequently metal corrosion defects (such as blistering and delamination). Timperley et al.47 prepared polymers bearing bis(fluoroalkyl)acrylic and methacrylic phosphate functionalities for potential application as flame retardant. However, there is no report of phosphorous containing VDF-based copolymers exhibiting anti-corrosion. Hence, the present study aims at the development of novel phosphonic acid functionalized PVDF as anti-corrosion coating for steel in marine environment.

EXPERIMENTAL SECTION

Materials. 2-trifluoromethyl acrylic acid (MAF) and 1,1-difluoroethylene (vinylidene fluoride, VDF) were generously supplied by Tosoh F-Tech (Shunan, Japan) 1 and Arkema (Pierre Benite, France) companies, respectively. Tert-amyl peroxy-2-ethylhexanoate (TAPE, 95%) was purchased from AkzoNobel company (Chalons sur Marne, France). Dimethyl phosphite (DMP, 4 ACS Paragon Plus Environment

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purity 98%), paraformaldehyde (purity 95%), tri(propylene glycol) diacrylate (TGPDA), 1,4butanediol diacrylate (BDDA, purity 90%), 2-hydroxy-2-methylpropiophenone (Darocur 1173, purity 97%) and benzophenone (purity ≥99%) were purchased from Aldrich (Aldrich Chimie, France). Dimethyl carbonate (DMC, reagent Plus grade, purity >99%), methyl ethyl ketone (MEK), dichloromethane, pyridine, thionyl chloride, hydrochloric acid and laboratory reagent grade methanol were purchased from Sigma-Aldrich. The solvents used for NMR spectroscopy such as deuterated dimethyl sulfoxide (DMSO-d6, purity >99.8%) and deuterated chloroform (CDCl3, purity >99%) and were purchased from Euroiso-top (Grenoble, France). Characterization. Unless otherwise stated, the instrumental parametrs for the instruments used for the characterization of the copolymer are provided in our earlier publications.25, 48 Nuclear Magnetic Resonance (NMR) Spectroscopy.

1

H and

19

F NMR spectroscopies were

employed to determine the microstructures of the copolymers were determined by, recorded on a Bruker AC 400 Spectrometer. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR analyses of the copolymers were performed in ATR mode using a PerkinElmer Spectrum 1000. Size Exclusion Chromatography (SEC) Measurements. Number average molecular weights (Mns) and dispersities (Ðs) of the poly(VDF-co-MAF-DMP) copolymers were assessed with triple-detection GPC from Agilent Technologies using DMF (containing 0.1 wt % of LiCl) was used as the eluent at a flow rate of 0.8 mL min-1 and toluene as the flow rate marker, while poly(methyl methacrylate) standards were used for the calibrtaion. Thermogravimetric Analysis (TGA).

The thermogravimetric analysis (under air) of the

synthesized copolymer samples were performed using TA Instruments TGA 51 apparatus. The

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samples were heated from room temperature to 580 °C at 10 °C min−1. Scanning electron microscopy (SEM) and Energy X-Ray dispersive spectroscopy (EDXS). SEM images and EDXS analyses were performed with an environmental scanning electronic microscope Quanta 200 coupled with an Oxford INCA analyzer. Differential Scanning Calorimetry (DSC). DSC analyses, under N2 atmosphere, of the poly(VDF-co-MAF-DMP) copolymers were carried out using a Netzsch DSC 200 F3 instrument. The degrees of crystallinity of the copolymers were determined using equation 1:

() =

∆ × 100 (1) ∆

where ∆Hc (104.5 J g-1) corresponds to the enthalpy of melting of a 100% crystalline PVDF49 and ∆Hm is the heat of fusion (determined by DSC in J g-1), respectively. Water Contact Angle (WCA). WCA measurements were carried out at ambient temperature on Contact Angle System OCA-Data Physics using the water sessile drop method on the polymer coated glass slides. The coated slides were prepared by spin-coating (rpm = 3000, time = 30 s) from acetone solution (20 wt%) of the polymers. Synthesis of (Dimethoxyphosphoryl)methyl 2-(Trifluoromethyl)acrylate. (i) Synthesis of Dimethyl (hydroxymethyl)phosphonate (DHP). DHP was synthesized using a method reported earlier by Jeanmaire et al.50 Typically, dimethyl phosphite (20.8 mL, 227 mmol), paraformaldehyde (6.820 g, 227 mmol) and potassium carbonate (1.560 g, 11.3 mmol) were taken in a round bottom flask containing methanol (50 mL). After stirring at room temperature for 1 h, the reaction mixture was filtrated and the solvent was removed from the filtrate under vacuum using a rotary evaporator. It was then dried under vacuum at 50 °C for 16 h to obtain the product, as a colorless liquid.


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(ii) Synthesis of Dimethyl 2-(trifluoromethyl)acryloyl Chloride (MAF-COCl). MAF-COCl was prepared by reacting 2-trifluoromethyl acrylic acid (MAF) with thionyl chloride following a procedure reported earlier.21 Typically, MAF (25.0 g, 178.5 mmol) and SOCl2 (15.6 mL, 214.2 mmol) were added to a 50 mL round bottomed flask. A vertical condenser was attached to the round bottomed flask and an oil bubbler was attached on the top of the condenser to monitor the evolution of the gases during the progress of the reaction (HCl and SO2). The reaction was stopped when evolution of gases ceased. The product (MAF-COCl, a colorless liquid) was purified by fractional distillation. (iii) Synthesis of Dimethyl (dimethoxyphosphoryl)methyl 2-(trifluoromethyl)acrylate (MAFDMP). DHP (20.0 g, 143 mmol) and pyridine (12.6 mL, 157 mmol) were added to dichloromethane (40 mL) in a two-necked round bottom flask which was equipped with a dropping funnel. The mixture was stirred magnetically and purged with dry nitrogen for 20 min. After cooling at -10 °C in an ice-salt bath, MAF-COCl (24.9 g, 157 mmol) was transferred to the dropping funnel and slowly added (for 30 mins) to the reaction mixture while maintaining the flask at -10 °C. The reaction mixture was kept stirring at -10 °C for 2 h, followed by at room temperature for 16 h. After addition of 8 mL methanol, the reaction mixture was taken in a separatory finnel and washed thrice with 40 mL dilute HCl, once with saturated NaHCO3 solution and then finally with water (until neutral to pH). The collected organic layer was dried over MgSO4. It was then filtered and the solvent was removed under vacuum to obtain MAFDMP (a brownish viscous liquid, yield = 60%). Structural identification of MAF-DMP was performed by 1H and 19F NMR spectroscopies. 1

H NMR (400 MHz, CDCl3, δ ppm, Figure S4): 3.78 (m, 6H, –OCH3); 4.53 (d, 2JHP = 9 Hz, 2H,

−O−CH2−PO(OCH3)2; 6.48 and 6.75 [2 s, 2H, H2C=C(CF3)(CO2CH2PO(OCH3)2].



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19

F NMR (376 MHz, CDCl3, δ ppm, Figure S5): signal centered at -67 (-CF3).

31

P NMR (162 MHz, CDCl3, δ ppm, Figure S6): peak centered at 20.50 [-P(O)(OCH3)2]. Free Radical Copolymerization of MAF-DMP with VDF. The radical copolymerizations of

MAF-DMP with VDF were performed in a 50 mL Hastelloy autoclave Parr system (HC 276), accessorized witha manometer, a mechanical Hastelloy anchor, inlet and outlet valves, a rupture disk (3000 PSI) and a Parr electronic controller (for regulating the stirring and heating rate). Before the start of the reaction, the autoclave was checked for any leaks by pressurizing it with 30 bars of nitrogen. It was then kept under 40 × 10−6 bar vacuum for a period of 30 min to remove the residual trace of oxygen. A typical copolymerization (P2, Table 1) was performed as follows (Scheme 1): A solution of TAPE initiator (0.40 g, 1.8 mmol) and MAF-DMP monomer (5.40 g, 23.4 mmol) in DMC (25 mL) was degassed by N2 purging for 30 min. This homogeneous solution was then introduced (under vacuum) into the autoclave using a connected funnel. VDF gas (6.00 g, 93.7 mmol) was transferred into the autoclave under weight control after cooling the autoclave in a liquid nitrogen bath. After this, the reactor was warmed up to room temperature, stirred mechanically and slowly heated up to 74 °C. The pressure and temperature evolutions were recorded during the progress of the polymerization. The maximum pressure, Pmax reached 20 bar, while the final pressure recorded was 11 bar. The reaction was stopped after 16 h, and then the autoclave was kept in an ice bath for cooling. The unreacted gaseous monomer (VDF) was purged off. Then, the autoclave was opened; the volatiles (solvent and any remaining unreacted liquid monomer) were removed under vacuum. The entire product was dissolved in acetone and subsequently precipitated from chilled pentane. The pure product was collected by filteration, and then dried under vacuum (20 × 10-3 bar at 50 °C) for 16 h. The


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polymerization yield was determined by gravimetry (yield = 50%). The poly(VDF-co-MAFDMP) copolymer (off white powder) was characterized by 1H, 31P and 19F NMR spectroscopy. 1

H NMR (400 MHz, DMSO-d6, δ ppm of P2, Table 1, Figure 1): 2.20 to 2.45 {m,

−CF2CH2−CH2CF2− reverse tail to tail (T-T) VDF−VDF dyad addition}; 2.62 (m, −CH2CF2−CH2C(CF3){CO2CH2PO(OCH3)2}−]; 2.70 to 3.20 (m, −CH2CF2−CH2CF2−, normal H-T VDF−VDF dyad addition, overlapped with -CH2C(CF3)(CO2CH2PO(OCH3)2) of MAFDMP); 3.72 (m, 6H, –OCH3); 4.55 (d, 2H, −O−CH2−PO(OCH3)2; 6.05 to 6.45 (tt, 2JHF = 55 Hz , 3

JHH = 4.6 Hz, −CH2CF2−H end-group corresponding to the proton transfer to solvent or

polymeric macroradical or the back biting.51 19

F NMR (376 MHz, DMSO-d6, δ ppm of P2, Table 1, Figure 2): from -66 to -71 (-CF3 of

MAF-DMP), from -91.5 to -93.5 {−CH2CF2−CH2CF2−normal head to tail (H-T) VDF−VDF dyad addition}; from -92.5 to -94.0 (–CF2 of VDF adjacent to defect); -95.0 (–CF2 of VDF in the VDF–MAF-DMP alternating dyad); -113.2 (−CH2CF2−CF2CH2−CH2, reverse H-H VDF−VDF dyad addition); -114.8 (dtt, 2JHF = 55 Hz, 3JHF = 16 Hz and 4JFF = 6 Hz, CF2CH2CF2-H, chain-end arising from transfer); -116.5 (−CH2CF2−CF2CH2−CH2, reverse H-H VDF−VDF dyad addition). The VDF molar fractions in the copolymers were determined using equation (2) :27 % =

$%& $''* !" + $''+ !" )/2 $%' × $%& $''* $,' ! + ! )/2 + ! /3 " " + $%' $''+ $&&

(

(

100 (2)

31

P NMR (162 MHz, DMSO-d6, δ ppm of P2, Table 1, Figure S8): signal centered at 20.5 ppm

(-CO2CH2PO(OCH3)2 of MAF-DMP).

Determination of the Monomer Reactivity Ratios. For the determination of reactivity ratios of VDF/MAF-DMP monomer pair, seven radical copolymerizations of MAF-DMP with VDF at 9 ACS Paragon Plus Environment


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seven [MAF-DMP]0/[VDF]0 molar feed ratios (15/85, 30/70, 40/60, 50/50, 70/30, 80/20, 90/10) were performed in borosilicate Carius tubes (total volume 8 mL, having the following simensions: length 130 mm, internal diameter 10 mm, thickness 2.5 mm, , see Figure S1 for the image of Carius tube). Typically, the different reactants including the TAPE initiator (1.5 mol % with respect to the total monomer concentrations), solvent, DMC and MAF-DMP were added in the tubes. The tubes were then degassed by three thaw-freeze cycles, the contents of the tubes were frozen in a liquid N2 bath and VDF was transferred via a manifold (see Supporting Information, Figure S1) from an intermediate cylinder. The drop of pressure was calibrated beforehand with the amount of VDF (in g). Subsequently, the tubes were vacuum sealed while keeping the content frozen in a liquid N2 bath. The tubes were then secured in thick metallic tubes and placed in a heating/shaking apparatus (Figure S1) maintained at the reaction temperature (74 oC). After the reaction (ca. 20 min to ensure a monomer conversion

Poly(vinylidene fluoride) Containing Phosphonic ... - ACS Publications

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