Synthesis and Performance of Novel Fluorinated Acrylate Polymers

Synthesis and Performance of Novel Fluorinated Acrylate Polymers: Preparation .... Journal of Applied Polymer Science 2016 133 (10.1002/app.v133.5), n...
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Synthesis and Performance of Novel Fluorinated Acrylate Polymers: Preparation and Reactivity of Short Perfluoroalkyl Group Containing Monomers Qinghua Zhang,† Qiongyan Wang,†,‡ Xiaoli Zhan,*,† and Fengqiu Chen† †

Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, China Research and Development Center, Zhejiang Sucon Silicone Co., Ltd., Shaoxing, Zhejiang 312088, China

‡

ABSTRACT: Developing novel fluorinated monomers and polymer materials with a short perfluoroalkyl group is attracting the attention of researchers. The synthesis of fluorinated monomers, [N-methyl-perfluorobutane-1-sulfonamide]ethyl acrylate (C4SA) and methacrylate (C4SMA), [N-methyl-perfluorohexane-1-sulfonamide]ethyl acrylate (C6SA), and methacrylate (C6SMA) were presented from a three-step procedure, which combined the reaction of perfluorobutanesulfonyl fluoride or perfluorohexanesulfonyl fluoride with an excess of methylamine, alkylation reaction with 2-bromoethanol, and esterification reaction with acryloyl chloride or methacryloyl chloride. The outcome compounds structures of each step were confirmed by 1H NMR, 19F NMR, FT-IR, and elemental analysis. Free radical copolymerizations of fluorinated acrylates and butyl methacrylate (BMA) in butyl acetate (BuOAc) were performed to determine the comonomers reactivity ratios and Q-e values. The surface wetting properties of fluorinated homopolymers were investigated by surface tension and dynamic contact angles. As a result, the target polymers showed extreme low surface energies and dynamic surface properties.

1. INTRODUCTION Fluorinated polymers have been widely explored because of their unique surface properties.1−4 (Meth)acrylates with perfluoroalkyl side chain has emerged as extensively used as low surface energy polymer coatings due to the substitution of hydrocarbon moieties into fluorinated ones may introduce a number of distinct oil and water repellence, low refractive index, and high chemical resistance.5−7 These properties are related with the high electronegativity of fluorine atom, its strong fluorine−carbon bond, and large van der Waals radius. Both the “shielding” effect of the fluorine atoms and high fluorine−carbon bond energy cause the resistance of perfluoroalkyl chains toward chemical, thermal, and biological degradation.8 (Meth)acrylates with perfluorooctyl side group were perfect components to produce fluorinated functional polymers with super low surface energy. Unfortunately, poly(fluoroalkyl acrylate)s with long fluoroalkyl side chains, such as perfluorooctyl or perfluorooctanesulfonyl groups, have been confirmed to produce perfluoroalkyl acid or perfluoroalkylsulfonate, such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) by hydrolyzation and oxidative degradation.9,10 The chemical substances with perfluorinated carbon chain lengths equal to or greater than seven carbons, especially PFOA and PFOS, can persistent and bioaccumulate in the environment and are harmful to the health of people and other living things.11,12 Therefore, some of the long-chain perfluoroalkyl chemicals have accordingly been limited by restrictions and prohibitions in the world. It is urgent to develop alternative materials to replace the currently used fluorinated substances. According to the technical data bulletin of 3M, the toxicity and bioaccumulation of perfluoroalky compounds toboggan with the decrease of perfluoroalkyl groups.13,14 The perfluorinated carboxylic or sulfonic acids © 2014 American Chemical Society

containing chains with six or less perfluoroalkyl chain length were considered to no significant bioaccumulation in the human body.13,14 In order to find biodegradable and environmentally friendly alternative fluorinated polymers, a lot of new fluorinated monomers and polymers containing the shorter side perfluoroalkyl chain had been prepared and characterized.13−17 Previous research suggested that the fluorinated acrylate polymers with short perfluoroalkyl groups have poor dynamic water repellency due to the high molecular mobility on the surface. It had been confirmed that the water repellency stability of fluorinated derivatives are caused by the flexibility of the backbone, the architectures and length of perfluoroalkyl chains, the type of spacer linker group located between the main chain and the fluorinated side chain.16−19 Accordingly, based on the design of molecule construction, synthesizing novel environmental friendly fluorinated monomers with different main chain and spacer group is very urgent. As we know, fluorinated homopolymer materials, such as polytetrafluoroethylene (PTFE), have perfect surface properties. However, the applications of these materials are restricted by the inferior film, mechanical strength, and stickability with substrates. The copolymerization of fluoroalkyl acrylates with nonfluorinated vinyl monomers is a preferable way to obtain compositive materials with combination properties.20−23 The aim of the present work is developing an efficient synthetic approach to nontoxic or nonbiopersisten fluorinated acrylate C4SA, C4SMA, C6SA, and C6SMA containing a sulfonamide group as a space linker between the fluorinated chain and acrylic group. As we know, the chain architecture and Received: Revised: Accepted: Published: 8026

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triethylamine (10.1 g, 100 mmol) were placed in a glass reaction vessel containing anhydrous diethyl ether (100 mL) and equipped with a reflux condenser. The dry methylamine (6.2 g, 200 mmol, anhydrated by sodium hydroxide) was introduced slowly into the reaction mixture over a period of 4 h at 0−5 °C, and a white precipitate gradually generated in the system. Then the reaction continued for 14 h at ambient temperature and refluxed for 3 h. The reaction mixture was cooled to room temperature and acidified by aqueous hydrochloric acid solution (9 wt %, 100 mL). The product layer was washed by water until it had been neutral and then dried by anhydrous sodium sulfate. The diethyl ether was removed by distillation under reduced pressure, yielding a pale yellow solid product. The solid product was purified by repeated precipitations from acetic acid solution into water and finally dried in vacuum for several hours at room temperature (95% yield). FT-IR spectrum of the product I exhibited that a −CH3 and −NH vibration peak appeared at 2967 and 3334 cm−1, and −SO2F vibration peak at 1474 cm−1 disappeared. 1H NMR (CDCl3, 400 MHz, δ ppm) for N-methyl-perfluorobutane-1sulfonamide: 3.03 (3H, s, −CH3), 5.30 (1H, s, −NH); 1H NMR for N-methyl-perfluorohexane-1-sulfonamide: 3.01 (3H, s, −CH3), 5.28 (1H, s, −NH). 2.2.2. Synthesis of N-(2-Hydroxyethyl)-N-methylperfluorobutane(hexane)-1-sulfonamide (II). The respective dry potassium carbonate (1.4 g, 10 mmol), N-methylperfluorobutane(hexane)-1-sulfonamide (1.25 or 1.65 g, 5.0 mmol), 2-bromoethanol (0.68 g, 5.5 mmol), and a liter of sodium iodide were dissolved in acetone (25 mL). The reaction continued for 24 h under reflux until complete conversion and then cooled to room temperature and purified. The precipitates were washed by acetone and then distillation in vacuum, yielding a pale yellow solid product. The crude product was purified by repeated precipitations from ethanol into water and finally dried in vacuum for several hours at room temperature (97% yield). FT-IR spectrum of product II exhibited that a −OH vibration peak appeared at 3589 cm−1, and a −NH vibration peak at 3334 cm−1 disappeared. 1H NMR (d6-acetone, 400 MHz, δ ppm) for N-(2-hydroxyethyl)-N-methyl-perfluorobutane-1-sulfonamide: 3.21 (3H, s, −CH3), 3.38 (1H, m, −NCH 2), 3.72 (3H, m, −CH 2CH 2OH), 4.10 (1H, s, −CH2OH); 1H NMR for N-(2-hydroxyethyl)-N-methylperfluorohexane-1-sulfonamide: δ 3.23 (3H, s, −CH3), δ 3.42 (1H, m, −NCH2), δ 3.79 (3H, m, −CH2CH2OH), 4.22 (1H, s, −CH2OH). 2.2.3. Synthesis of [N-Methyl-perfluorohexane(butane)-1sulfonamide]ethyl (meth)acrylate. N-(2-Hydroxyethyl)-Nmethyl-perfluorobutane(hexane)-1-sulfonamide (2.15 or 2.75 g, 5.0 mmol), triethylamine (0.65 g, 6.4 mmol), hydroquinone (20 mg), and diethyl ether (50 mL) were mixed in a 100 mL three-necked flask. The reaction temperature was kept in 0 °C by adding acryloyl chloride (0.6 g, 6.65 mmol) or methacryloyl chloride (0.7 g, 6.65 mmol). The reaction mixture was then heated to 35 °C and stirred for 12 h. The product was washed with water (50 mL), hydrochloric acid (1 N, 50 mL), and saturated aqueous sodium bicarbonate (40 mL). Then the organic phase was dehydrated by anhydrous sodium sulfate. After removal of the solvent by distillation under reduced pressure, the residue was purified by repeated precipitations from methanol into water and finally dried in vacuum for several hours at room temperature (85% yield).

the composition of copolymers are significantly affected by the reactivity ratios of the monomers in radical copolymerization.24,25 We showed the copolymerization of the fluorinated monomers with BMA, and the reactivity ratios and the Q-e values will be reported for the first time. In addition, the structures and the surface wetting properties of the fluorinated polymers were investigated. The research work reported here will help in application of these fluorinated monomers in designing and synthesizing novel functional fluoropolymer materials.

2. EXPERIMENTAL SECTION 2.1. Materials. The commercially available perfluorobutanesulfonyl fluoride and perfluorohexanesulfonyl fluoride (⩾98%) are products of Hubei Hengxin Chemical and purified by distillation (bp 65−66 °C and 114−115 °C). 2Bromoethanol, acryloyl chloride (⩾99%, Shanghai Jiachen Chemical Co.), and aqueous methylamine solution (25−30 wt %, Shanghai Wulian Chemical Co.) were used as received. Anhydrous diethyl ether, triethylamine, acetone, hydrochloric acid, sodium hydroxide, acetic acid, potassium carbonate, sodium iodide, ethanol, methanol, n-butyl methacrylate, nbutyl acetate, azobis(isobutyronitrile), and trifluoroacetic acid (Purchased from Shanghai Chemical Reagents Co., Shanghai, China) were AP grade and used as received. 2.2. Synthesis and Analysis of Fluorinated Monomers. The synthesis route of fluorinated monomers C4SA, C4SMA, C6SA, and C6SMA is shown in Scheme 1. They were Scheme 1. Synthesis Route of Fluorinated Monomers C4SA, C4SMA, C6SA, and C6SMAa

(a) Et3N, n = 4 or 6; (b) room temperature, 14 h; (c) NaI, 57 °C; (d) K2CO3, 24 h; (e) Et2O; (f) 35 °C, 12 h.

a

synthesized through three steps. First, the original material perfluorobutanesulfonyl fluoride or perfluorohexanesulfonyl fluoride reacted with methylamine to gain N-methylperfluorobutane(hexane)-1-sulfonamide. Then, N-(2-hydroxyethyl)-N-methyl-perfluorobutane(hexane)-1-sulfonamide was obtained from the N-methyl-perfluorobutane(hexane)-1-sulfonamide by direct alkylation with 2-bromoethanol. At last, C4SA, C4SMA, C6SA, and C6SMA were synthesized by esterification reaction of N-(2-hydroxyethyl)-N-methyl-perfluorobutane(hexane)-1-sulfonamide with acryloyl chloride and methacryloyl chloride, respectively. 2.2.1. Synthesis of N-Methyl-perfluorobutane(hexane)-1sulfonamide (I). Perfluorobutanesulfonyl fluoride or perfluorohexanesulfonyl fluoride (15.1 or 20.1 g, 50 mmol) and 8027

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6.51 (3H, m, −CHCH2); 19F NMR (d6-DMSO, 376 MHz, δppm) for C4SMA: δ −80.74 (CF3), δ −111.89 (α-CF2), δ −121.43 (β-CF2), δ −125.99 (γ-CF2); Anal. Calcd for C4SMA: C, 31.06, H, 2.82, N, 3.29. Found: C, 31.23, H, 2.95, N, 3.10. 1 H NMR for C6SA: δ 3.17 (3H, s, −CH3), δ 3.47−3.93 (2H, m, CH2CH2O), δ 4.35 (2H, m, CH2CH2O), δ 5.83−6.50 (3H, m, −CHCH2); 19F NMR (d6-DMSO, 376 MHz, δppm) for C6SA: δ −80.74 (CF3), δ −111.73 (α-CF2), δ −120.43 (βCF2), δ −121.75 (γ-CF2), δ −122.69 (δ-CF2), δ −126.10 (εCF2); Anal. Calcd for C6SA: C, 28.18, H, 1.96, N, 2.74. Found: C, 28.23, H, 1.99, N, 2.82. 1H NMR for C6SMA: δ 1.97 (3H, m, C−CH3), δ 3.17 (3H, m, −N−CH3), δ 3.42−3.99 (2H, m, CH2CH2O), δ 4.40 (2H, m, CH2CH2O), δ 5.63−6.18 (3H, m, −CHCH2); 19F NMR (d6-DMSO, 376 MHz, δppm) for C6SMA: δ −80.71 (CF3), δ −111.74 (α-CF2), δ −120.42 (βCF2), δ −121.75 (γ-CF2), δ −122.69 (δ-CF2), δ −126.10 (εCF2); Anal. Calcd for C6SMA: C, 29.71, H, 2.29, N, 2.67. Found: C, 29.88, H, 2.32, N, 2.79. 2.3. Determination of Reactivity Ratios in Copolymerization. Free radical polymerizations of fluorinated acrylate (FA) and n-butyl methacrylate (BMA) were performed in a 25 wt % BuOAc solution with 2 wt % (based on monomers) azobis(isobutyronitrile) (AIBN) as initiator. Typically, a mixture of 2.5 g (6.08 mmol) of C4SA and 2.5 g (17.61 mmol) of BMA and 0.10 g (0.61 mmol) of AIBN in 15 g of n-butyl acetate(BuOAc) was subjected to three freeze− pump−thaw cycles to remove oxygen. Polymerization was carried out at 80 °C for 5−10 min to keep the conversion below 10 wt %. Copolymerization was quenched by precipitation in methanol. The precipitates were dried for 12 h in vacuum at 65 °C and then used for determine the reactivity ratios. 2.4. Homopolymerization. The homopolymers PC4SA, PC4SMA, PC6SA, and PC6SMA were prepared in butyl acetate

FT-IR spectrum of product C4SA and C4SMA in Figure 1 exhibited that a CO and CCH2 vibration peak appeared at

Figure 1. The FT-IR spectra of C4SA, C4SMA, C6SA, and C6SMA.

1735 and 1637 cm−1, and −OH vibration peak at 3589 cm−1 disappeared. The 1H NMR of fluorinated monomers was showed in Figure 2. 1H NMR (CDCl3, 400 MHz, δppm) for C4SA: δ 3.22 (3H, s, −CH3), δ 3.30−4.13 (2H, m, CH2CH2O), δ 4.45 (2H, m, CH2CH2O), δ 5.77−6.51 (3H, m, −CH CH2); 19F NMR (d6-DMSO, 376 MHz, δppm) for C4SA: δ −80.75 (CF3), δ −111.89 (α-CF2), δ −121.45 (β-CF2), δ −125.99 (γ-CF2); Anal. Calcd for C4SA: C, 29.20, H, 2.43, N, 3.41. Found: C, 29.31, H, 2.28, N, 3.60. 1H NMR for C4SMA: δ 2.05 (3H, m, C−CH3), δ 3.26 (3H, m, −N−CH3), δ 3.31− 4.10 (2H, m, CH2CH2O), δ 4.45 (2H, m, CH2CH2O), δ 5.74−

Figure 2. The 1H NMR (CDCl3, 400 MHz) spectra of fluorinated monomers. (A) C4SA, (B) C4SMA, (C) C6SA, (D) C6SMA. 8028

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as described in our previeous work.19 The fluorinated polymers were purified by dissolved in trifluoroacetic acid and then reprecipitated in methanol for three times. The purified polymers were dried in a vacuum oven at 45 °C for 2 days. The polymers consisted of white powder. The fluorinated homopolymer films on clean glass slides were prepared for characterization by the same method reported in the literature.19 The polymer films were heat treated in vacuum for 24 h at 80 °C followed by annealed at 130 °C for 30 min. Then the film samples were applied to AFM and contact angles test. 2.5. Characterization. 1H NMR spectra of the samples were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer (Advance DMX400) with CDCl3 or d6-acetone as solvent at room temperature. 19F NMR analysis was conducted with a Bruker 600 MHz nuclear magnetic resonance spectrometer in d6-DMSO. Fourier transform infrared (FT-IR) spectra of the intermediate and final product were recorded on a Nicolet 5700 instrument. The polymer films were cast onto KBr disks from trifluoroacetic acid solutions. Elemental analyses were performed on Flash EA1112 (Thermo Electron SPA). The static and dynamic contact angles were tested by a CAM200 (KSV Co., Ltd.). The static contact angles of water and n-hexadecane with four drops of each liquid on the polymer films were measured. Eight readings of the contact angle were taken to calculate the average value of the contact angle. The wetting liquids were water (γd = 21.6 mN/m; γp = 51.0 mN/m) and n-hexadecane (γd = 21.6 mN/m; γp = 0), and the surface free energy of the polymer was calculated from the following Owens and Wendt’s equation.26 The dynamic contact angles, including advancing contact angle (θa), receding contact angle (θr), and sliding angle (θs), were measured as the same method described in the literature.27 Thermal transitions of the fluorinated polymers were determined with the differential scanning calorimetry (DSC) using Q200 system (TA Instruments) with heating rates of 10 °C/min from −80 to 200 °C. In order to eliminate the effects of thermal history, the sample was preheated to 200 °C and then cooled to −80 °C for the measurements. The tapping-mode atomic force microscopy (AFM) was carried out with a SPA-400 AFM (Seiko Instruments Inc., Japan) at ambient conditions using a scanning nanoprobe microscope.

proceed well at a broad range of solvents and catalysts conditions. As illustrated in Scheme 1, the complex ammonium salt was initially formed and precipitated in the reaction system. The desired amide could be produced by dissolving the ammonium salt in dioxane with passing anhydrous HCl through the solution.8,28 In this work, N-methyl-perfluorobutane(hexane)-1-sulfonamide was synthesized by reaction of perfluorobutane(hexane) sulfonyl fluoride with an excess of methyl amine using triethylamine as base simultaneously. After the methyl amine was introduced in the reaction solution, the white precipitate was produced and considered to be the complex ammonium salt with a proposed formula of CnF2n+1SO2N(CH3)− N(CH3)H3+ N(CH3)H3F.29 The desired intermediate N-methylperfluorohexane(butane)-1-sulfonamide was subsequently isolated after neutralizing with hydrochloric and thermal decomposition of this complex salt. Then, 9 wt % hydrochloric was used to treat the reaction mixture and obtained the target product in 95% yield. The DSC characterizations of perfluorobutanesulfonamide or perfluorohexanesulfonamide revealed their melting temperatures were 32 and 69 °C respectively. 3.1.2. Synthesis of the Fluoroalcohols. N-(2-Hydroxyethyl)-N-alkyl-perfluoroalkane-1- sulfonamide was a valuable intermediate for preparing oil and water repellent, surface active agents, active ingredients in pesticides, and many other useful materials. The N−H group in N-substituted perfluoroalkanesulfonamides could be further modified with an alkyl chain. It is reported that the desired fluoroalcohol (Scheme 1) was obtained in 78% yield with sodium NaOCH3 as a base.30 We synthesized N-(2-hydroxyethyl)-N-methylperfluorobutane(hexane)-1-sulfonamide by alkylation of the N-methyl-perfluorobutane(hexane)-1-sulfonamide with 2-bromoethanol using KCO3 as base and a little sodium iodide in the acetone solvent. The high yield could be arrived in this reaction if only the purity of 2-bromoethanol was enough. The DSC characterizations of perfluorobutane or perfluorohexane fluoroalcohols revealed their melting temperatures were 65 and 72 °C, respectively. 3.1.3. Synthesis of the Fluorinated (Meth)acrylate. The synthesis of fluorinated acrylates containing N-methylsulfonamide in the spacer group has rarely been reported. Based on the prepared fluoroalcohols, [N-methyl-perfluorobutane(hexane)1-sulfonamide]ethyl acrylate and methacrylate (C4SA, C4SMA, C6SA, and C6SMA) were synthesized by esterification reaction of fluoroalcohols with acryloyl chloride and methacryloyl chloride, respectively. At ambient temperature, the outward appearance form of C4SA, C4SMA, C6SA, and C6SMA were white powders. The DSC characterizations revealed their melting temperatures were 49 °C, 51 °C, 48 and 73 °C, respectively. As we know, perfluorobutyl or perfluorohexyl (meth)acrylates with only two methenes in the spacer existed as liquid at ambient temperature. It was noticeable that the introducing of N-methylsulfonamide in the spacer group increases the melting temperatures for fluorinated monomers. With the combined influence of N-methylsulfonamide and length of perfluoroalkyl side chain, C6SMA exhibited a higher melting point than the other three monomers. The synthesized fluorinated monomers were characterized by Fourier transform infrared (FT-IR), 1H NMR spectra analysis. Figure 1 showed the FT-IR spectra of fluorinated monomers. The stretching vibration of CO at 1740 cm−1, the characteristic stretching peaks of C−H at 2850−2960 cm−1,

3. RESULTS AND DISCUSSION 3.1. Synthesis of Fluorinated Monomers. Most of the fluoroacrylate monomers and polymers with methylene as space linker between backbone and fluorocarbon side chain are reported in the literature. However, in these fluoropolymers, only a long fluorinated carbon chain (⩾8) was able to cause the formation of liquid-crystalline phase and stable low surface energy. In the present study, the synthesis of several different fluorinated monomers with short perfluoroalkyl side chains and N-methylsulfonamide as space linker were performed by reaction of perfluorobutane(hexane) sulfonyl fluoride with an excess of methyl amine, followed by an alkylation reaction with 2-bromoethanol leading to fluoroalcohol and then by esterification of the hydroxyl end-group with acryloyl chloride. 3.1.1. Synthesis of N-Methyl Perfluoroalkanesulfonamides. N-Functionalized perfluoroalkanesulfonamides could be synthesized by the substitution reaction of sulfonyl fluoride with primary amine under alkaline conditions. The reaction can 8029

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and the stretching peaks of C−F at 1200 and 1240 cm−1 corresponding to the perfluoroalkyl chain were clearly visible. Meanwhile, there were typical adsorption peaks at 1630 cm−1, which was attributed to the characteristic of double bonds in acrylic and methacrylic monomers. This suggested that the esterification reactions had been succeeded. Moreover, the confirmation for the fluorinated monomers, C4SA, C4SMA, C6SA, and C6SMA were performed by 1H NMR analyses. The 1 H NMR spectra in Figure 2 showed the −OCH(H)CH2 group signals at 5.70−6.60 ppm, the signal at 4.30−4.50 ppm being assigned to −CH2CH2O−, and the signal at 3.30−4.20 ppm being assigned to −CH2CH2O−, and the signal at 1.90− 2.10 ppm being assigned to −OCH(CH3)CH2. This suggested that the essential of the fluorinated monomers had been confirmed. 3.2. Q-e values and Reactivity Ratios of Monomers in Copolymerization. A drawback of fluorinated homopolymers is the poor solubility in common hydrocarbon solvents, which restrict their wide application as coating materials. These limitations could be overcome by copolymerization of fluorinated (meth)acrylates with nonfluorine-containing monomers. The performances of copolymers are mainly related to the chemical composition and sequence structure of the macromolecules. Estimation of the monomer reactivity ratios is crucial in controlling the overall copolymerization process and assessing potential performance in the desired application. This involved the reactivity ratios of the two monomers. The Kelen−Tudos procedure was regarded as one widely accepted method to determine the reactivity ratios of monomers.31,32 This technique was used to determine the reactivity ratios of the synthesized fluorinated (meth)acrylates and BMA in radical solution copolymerization with BuOAc as solvent. There was no precipitation that appeared in the copolymerization process, and the conversions were restricted below 10% to avoid composition drift. 1H NMR was utilized to determine the copolymer composition for at different monomer feed ratios. Figure 3 showed the 1H NMR spectrum and chemical structure formula of copolymer P(C4SA/BMA). The spectra clearly revealed the chemical shift assignments of resonance due to BMA and C4SA copolymer structures. The peak at 3.95 ppm should arise from the typical band of the CH2 group in

COOCH2 of BMA, and the peaks at 3.20 ppm and 3.35−4.40 ppm represent the −NCH3 and −NCH2CH2 group of C4SA in the side chain, respectively. The average molar compositions of the copolymer samples were determined from the corresponding 1H NMR chemical shifts of −COOCH2, −NCH3, and −NCH2CH2 protons in BMA and C4SA. The characteristic spectra of P(C4SMA/ BMA), P(C6SA/BMA), and P(C6SMA/BMA) were similar to P(C4SA/BMA) analyzed by 1H NMR, and the results are shown in Table 1. The Kelen−Tudos method used to determine the reactivity ratios was expressed by eq 133 η = (r1 + r2/α)ζ − r2/α

(1)

where r1 and r2 represent the reactivity ratios of the two monomers, η = G/(F+α), ζ = F/(F+α), and G = X(Y−1)/Y. The α in eq 1 is an arbitrary constant determined by the equation α = (Fmin·Fmax)1/2, where F = X2/Y; Fmax and Fmin represent the maximum and minimum values; X is the monomer molar ratio (f1/f 2) in the feeds; Y is the monomer molar ratio (F1/F2) in copolymers. A linear η/ζ relationship was expected with the elaboration of the experimental data. Then, the reactivity ratios r1 and r2 could be obtained from the slope and the intercept of the fitted linear equation. The experimental parameters are listed in Table 1, and the plots of ζ versus η were provided in Figure 4 for these four copolymerization systems. As shown in Table 2, the reactivity ratios of FA and BMA determined by the Kelen− Tudos method are rC4SA = 0.26/rBMA = 2.34, rC4SMA = 0.42/rBMA = 1.04, rC6SA = 0.86/rBMA = 1.12, and rC6SMA = 0.45/rBMA = 1.04 respectively. The calculation results of monomer reactivity ratios indicate that the copolymer sequence of these four systems tend to nonideal and unstable copolymerization. The system is within the range 0 < rFA·rBMA < 1. The BMA in copolymeric composition was more than it is in the initial monomer composition. BMA was found to be more reactive than the fluorinated monomers (rBMA>rFA). Accordingly, the ultimately obtained products still belonged to the random copolymer category. However, the chain segment of BMA was longer, and the one of FA was shorter. The values of the monomer reactivity ratios reveal the relative macroradicals reactivity toward different monomers. In essence, the values of r1 = k11/k12 and r2 = k22/k21, where k11, k12, k22, and k21 are the kinetic constants of the propagation reactions in copolymerization, k12 and k21 are cross propagation constants, and k11 and k22 are self-propagation rate constants, respectively.34 The greater value of r means the radical tend to self-propagation. In our case, the polymerization reaction activity of BMA was higher than the corresponding fluorinated monomers, and the developing chain radicals containing fluorine were more prone to react with BMA. We note that C4SMA, C6SA, and C6SMA have comparable reactivity. C4SA is slightly less reactive than the others. Based on the determination of reactivity ratios, the Q-e values of each pair of monomers could be calculated by the Alfrey-Price equation.35 The reactivity of monomer and radical are determined by resonance, electro-withdrawing, and interspace effects. The Q-e value of BMA has been reported as 0.82 and 0.28.36 The Q-e values of C4SA, C4SMA, C6SA, and C6SMA monomers by the Alfrey-Price method are shown in Table 3. It is noticeable that the e values of fluorinated monomers are greater than BMA. A possible explanation for these behaviors may be the stronger electron-withdrawing

Figure 3. 1H NMR spectrum of P(C4SA/BMA). 8030

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Table 1. Kelen−Tudos Parameters for C4SA/BMA, C4SMA/BMA, C6SA/BMA, and C6SMA/BMA Solvent Copolymerization System mole fraction in feed copolymerization system C4SA/BMA α = 0.9017

C4SMA/BMA α = 0.2577

C6SA/BMA α = 0.2046

C6SMA/BMA α = 0.2510

FA

BMA

conv%

X

Y

F

G

η

ζ

0.0024 0.0036 0.0049 0.0061 0.0078 0.0097 0.0012 0.0024 0.0047 0.0059 0.0075 0.0090 0.0010 0.0020 0.0029 0.0059 0.0063 0.0078 0.0013 0.0019 0.0042 0.0053 0.0061 0.0074

0.0282 0.0246 0.0211 0.0176 0.0127 0.0070 0.0317 0.0282 0.0211 0.0176 0.0127 0.0085 0.0317 0.0282 0.0246 0.0197 0.0127 0.0070 0.0303 0.0282 0.0197 0.0155 0.0127 0.0077

7.01 5.52 5.46 5.59 5.70 6.59 8.44 5.39 2.83 3.18 2.51 3.96 9.44 2.73 2.53 8.13 9.06 7.66 2.31 5.35 3.70 3.67 5.46 4.56

0.0864 0.1481 0.2303 0.3455 0.6142 1.3820 0.0371 0.0835 0.2227 0.3341 0.5940 1.0580 0.0309 0.0695 0.1191 0.2977 0.4940 1.1115 0.0440 0.0676 0.2125 0.3442 0.4808 0.9590

0.0356 0.0611 0.0918 0.1340 0.2452 0.4926 0.0359 0.0750 0.1840 0.2886 0.4264 0.7874 0.0270 0.0625 0.1136 0.2476 0.4414 1.0440 0.0409 0.0651 0.1818 0.2821 0.3777 0.7273

0.2097 0.3590 0.5778 0.8908 1.5385 3.8772 0.0383 0.0930 0.2695 0.3868 0.8275 1.4216 0.0354 0.0773 0.1249 0.3579 0.5529 1.1833 0.0473 0.0702 0.2484 0.4200 0.6120 1.2645

−2.3406 −2.2758 −2.2784 −0.2329 −1.8907 −1.4235 −0.9963 −1.0298 −0.9876 −0.8236 −0.7991 −0.2857 −1.1135 −1.0425 −0.9293 −0.9046 −0.6252 −0.0468 −1.0318 −0.9708 −0.9564 −0.8759 −0.7922 −0.3596

−2.1060 −1.8052 −1.5400 −1.2457 −0.7748 −0.2979 −3.3655 −2.9368 −1.8732 −1.2779 −0.7363 −0.1701 −4.6405 −3.6983 −2.8207 −1.6082 −0.8253 0.0338 −3.4585 −3.0225 −1.9151 −1.3055 −0.9179 −0.2373

0.1887 0.2848 0.3905 0.4870 0.6305 0.8113 0.1295 0.2651 0.5112 0.6001 0.7625 0.8465 0.1474 0.2742 0.3790 0.6363 0.7299 0.8526 0.1587 0.2185 0.4974 0.6259 0.7092 0.8344

inductive effect of perfluoroalkyl groups and the polar Nmethylsulfonamide groups on the side chains. Simultaneously, the Q values of C4SA and C6SA are less than BMA. Their resonance effects are weaker. The C4SA and C6SA monomer are more stable, but the corresponding radicals are more active. Similarly, the Q values of C4SMA and C6SMA are larger than BMA. The C4SMA and C6SMA monomer are less stable, but the corresponding radicals are less active. These basal Q-e values are available to estimate how difficult and what styles of copolymerization with other vinyl monomers. 3.3. Synthesis and Surface Wetting Properties of Fluorinated Homopolymers. The fluorinated homopolymers were prepared by radical polymerization. At ambient temperature, the purified homopolymers were obtained as white powder. The intrinsic viscosities [η] of the four fluorinated homopolymers were measured using an Ubbelohde viscometer with trifluoroacetic acid as solvent at 30 °C. It is shown that the [η] were between 0.05 and 0.11, and the molecular weights of the fluorinated homopolymers were estimated to between 32000 and 46000 using the relation [η] = KMα, where K = (0.24−0.25) × 10−4 and α = 0.75−0.78. The values of K and R were assumed based on the viscosityaveraged molecular weight of polyalkyl acrylate.37 Figure 5 showed the FT-IR spectra of fluorinated homopolymers. The stretching vibration of CO at 1740 cm−1, the characteristic stretching peaks of C−H at 2850−2960 cm−1, and the stretching peaks of C−F at 1200 and 1240 cm−1 implied the perfluoroalkyl chain were clearly visible. Meanwhile, there was no typical adsorption peak at 1630 cm−1 caused by the double bond in acrylic and methacrylic. The results of FTIR analysis indicated that the fluorinated acrylate monomers had been polymerized.

Contact angles of liquid on the solid surface are commonly used to determine surface wettability properties. The static contact angle, θ, is conventionally measured through the liquid by measuring the angle at the liquid−solid−air boundary. It quantifies the wettability and hydrophobicity of one solid substrate by the Young equation. According to the same methods described in our previous work,19 the static contact angles of water (θs) and hexadecane (θHD) on the surface of fluorinated polymers were tested by a CAM 200 contact angle apparatus. Based on determination of water and hexadecane contact angles on the polymer surface, the surface free energy γ composed of dispersion component γ d and the polar component γp could be calculated by the Owens and Wendt equation.38 The testing data of contact angle and the calculation results of surface free energies were displayed in Table 4. It was noticeable that all four polymers PC4SA, PC4SMA, PC6SA, and PC6SMA showed very low surface free energy as 11.25−12.35 mN/m. As we know, the surface wetting property of solid surface is related to the roughness scale, chemical structure, and atomic composition on the solid surface. In order to study the effects of roughness on the hydrophobicity of the polymer surface, AFM was used for quantitative analysis of the roughness scale of the film samples. AFM observations of the testing film surfaces revealed roughness of standard deviation of the height values (RMS) as 0.5−2 nm, which was considered to have no effect on the contact angles measurements. The critical surface free energies of the synthesized polymers with short perfluoroalkyl chains were found to be significantly lower than the 18 mN/m of PTFE, which is a typical fluoropolymer with repeating chains of −(CF2−CF2)− in it. It is well known that the critical surface free energy of tightly packed −CF3 groups possess the lowest one to be 6 mN/m. 8031

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Figure 4. The Kelen−Tudos plot for C4SA/BMA, C4SMA/BMA, and C6SA/BMA, and C6SMA/BMA copolymerization system.

Table 2. Reactivity Ratios for C4SA/BMA, C4SMA/BMA, C6SA/BMA, and C6SMA/BMA Copolymerization Systems copolymers

r1(FA)

r2(BMA)

r1 × r2

P(C4SA/BMA) P(C4SMA/BMA) P(C6SA/BMA) P(C6SMA/BMA)

0.26 0.42 0.86 0.45

2.34 1.04 1.12 1.04

0.61 0.44 0.96 0.47

Table 3. Q-e Values of the Fluorinated Acrylate and BMA monomers

Q

e

BMA C4SA C4SMA C6SA C6SMA

0.82 0.43 1.02 0.78 1.00

0.28 0.98 1.19 0.48 1.15

Figure 5. The FT-IR spectra of fluorinated homopolymers PC4SA, PC4SMA, PC6SA, and PC6SMA.

The distinct low surface free energy in our case could be attributable to the enrichment of −CF3 groups on the surface. PC4SA and PC4SMA still showed amazing low surface free energies to be 11.83 and 12.35 mN/m though the length of the perfluoroalkyl chain is 4. Comparison of the γ values in Table 4 indicated that the polar component γp for PC4SA and PC4SMA was more than the ones for PC6SA and PC6SMA. The reason for this behavior should lie in less functional polar groups such as the ester group and the N-methylsulfonamide group for PC6SA and PC6SMA existing at the polymer−air interface. Perfluorohexyl chains in PC6SA and PC6SMA are tent to

expose at the outmost surface because of the regular organization of the longer perfluoroalkyl chain. The static contact angle and Young’s equation are applicable for an ideal solid with a smooth surface. As a real surface, accurate determination of contact angles will be challenging due to the contact angle hysteresis caused by irregularity and chemical heterogeneity. The real wettability property of the solid surface was usually characterized through dynamic contact angle, which is the extremes of the possible contact angle range and they occur when the three phase boundary is in motion. 8032

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Table 4. Static/Dynamic Contact Angles and Surface Tension Data for Fluorinated Homopolymers polymer

θS (deg)

θa (deg)

θr (deg)

θs (deg)

H

θHD (deg)

γS (mN/m)

γSd (mN/m)

γSp (mN/m)

PC4SA PC4SMA PC6SA PC6SMA

114 113 121 120

121 122 124 123

81 83 101 105

24 22 15 15

0.33 0.33 0.19 0.15

74 72 76 74

11.83 12.35 10.65 11.25

11.11 11.69 10.53 11.11

0.72 0.76 0.12 0.14

to A transition and smectic A to isotropic transition. The results exhibited the formation of a feature of smectic phases for the fluorinated comb polymers PC6SA and PC 6SMA with perfluorohexyl in side chains. This was the reason that PC6SA and PC6SMA had better dynamic water repellent property than PC4SA and PC4SMA. All of the four fluorinated homopolymers showed high glass transition or crystallization temperatures. The mobility of the backbone and the perfluoroalkyl side chain of the synthesized fluorinated polymers were suppressed at room temperature, and the fluorinated side chains could be organized in an arrangement with −CF3 groups closely packed on the outmost surface caused by phase segregation. Therefore, the synthesized fluoropolymers with short perfluoroalkyl chain showed excellent low surface energy property. These fluorinated monomers should be expected to be widely applied as functional coatings or an efficient modifier of the polymeric surface.

The homogeneity information on the surface could be provided by test dynamic contact angles. Table 4 summarizes the advancing contact angle (θa), mean receding contact angle (θr), mean sliding angle (θs), and mean contact angle hysteresis (H) expressed as (θa − θr) /θa, respectively. As shown in Table 4, the advancing contact angle (θa) for these four samples was similar and high to above 120°, independent of the flexibility of the backbone, and the length of the perfluoroalkyl side chain. This implied that the same amount of perfluoroalkyl chain with −CF3 groups enriched on the surface. On the other hand, the receding contact angle (θr) and sliding angle (θs) showed a smaller value for PC4SA and PC4SMA than PC6SA and PC6SMA. The reason for these phenomena should be the higher molecular mobility of perfluorobutyl than perfluorohexyl in polymers, and the surface reconstruction was easy to occur under wet conditions. As described before, the roughness scale of the surface was considered to be hardly any influence on the contact angles measure. Hence, in our cases, the contribution of reorientation and mobility of the side chain to H is expected. The effect of the side perfluoroalkyl chain on thermal behaviors of fluoropolymer was studied by the DSC method from −55 to 150 °C at a heating rate of 10 °C/min under nitrogen. Figure 6 presented the DSC results of the polymers in

4. CONCLUSION In summary, novel fluorinated monomers C4SA, C4SMA, C6SA, and C6SMA with short perfluoroalkyl side chains were successfully prepared by reaction of perfluorobutanesulfonyl fluoride or perfluorohexanesulfonyl fluoride with an excess of methylamine, followed by alkylation reaction with 2-bromoethanol, and then esterification reaction with acryloyl chloride or methacryloyl chloride. The reactivity of four fluorinated (meth)acrylate monomers toward BMA were determined by using the Kelen−Tudos method. The results indicated the reactivity ratios for the C4SA/BMA, C4SMA/BMA, C6SA/ BMA, and C6SMA/BMA copolymerization systems to be rC4SA = 0.26/rBMA = 2.34, rC4SMA = 0.42/rBMA = 1.04, rC6SA = 0.86/rBMA = 1.12, and rC6SMA = 0.45/rBMA = 1.04, respectively. All of these copolymerization systems were nonideal and unstable copolymerization, and the reactivity of BMA was more than the corresponding fluorinated monomers. Consequently, the Q and e values were obtained by the Alfrey and Price method. Their Q and e values are QC4SA = 0.43, eC4SA = 0.98; QCASMA = 1.02, eCASMA = 1.19, QC6SA = 0.78, eC6SA = 0.48; QC6SMA = 1.00, eC6SMA = 1.15, respectively. Structure and surface properties of the four fluorinated homopolymers with N-methylsulfonamide as a spacer group were investigated. They had lower surface free energy. PC6SA and PC6SMA with longer perfluoroalkyl chains exhibited a better dynamic water repellent property than PC4SA and PC4SMA. They are of great environmental and industrial interests. This suggests that the synthesized monomers in our case could be used to prepare new fluorinated copolymers used as potential substitute coating materials with durable and stable water and oil repellency.

Figure 6. DSC heating traces of fluorinated homopolymers PC4SA, PC4SMA, PC6SA, and PC6SMA.

the present work. The obvious glass transition temperatures of PC4SA and PC4SMA were observed at 31.6 and 43.4 °C respectively. However, the polymers PC6SA and PC6SMA showed two phase transition temperature. To the polymer PC6SMA, as presented in our previous work,19 the glass transition at 107.8 °C and the endothermic transition at 125.2 °C, which was interpreted as smectic A to isotropic transition. The PC6SA was observed to undergo two distinct enthalpy variations at 85.5 and 103.8 °C corresponding to be smectic B

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 8033

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially supported by the National Natural Science Foundation of China (21276224, 21076184, and 21176212).

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REFERENCES

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