Novel Fluorinated Polymers Containing Short ... - ACS Publications

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Novel Fluorinated Polymers Containing Short Perfluorobutyl Side Chains and Their Super Wetting Performance on Diverse Substrates Jingxian Jiang,† Guangfa Zhang,† Qiongyan Wang,†,‡ Qinghua Zhang,*,† Xiaoli Zhan,† and Fengqiu Chen† †

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

‡

S Supporting Information *

ABSTRACT: Because the emission of perfluorooctanoic acid (PFOA) was completely prohibited in 2015, the widely used poly- and perfluoroalkyl substances with long perfluoroalkyl groups must be substituted by environmentally friendly alternatives. In this study, one kind of potential alternative (i.e., fluorinated polymers with short perfluorobutyl side chains) has been synthesized from the prepared monomers {i.e., (perfluorobutyl)ethyl acrylate (C4A), (perfluorobutyl)ethyl methacrylate (C4MA), 2[[[[2-(perfluorobutyl)]sulfonyl]methyl]amino]ethyl acrylate (C4SA), and methacrylate (C4SMA)}, and the microstructure, super wetting performance, and applications of the synthesized fluorinated polymers were systematically investigated. The thermal and crystallization behaviors of the fluoropolymer films were characterized by differential scanning calorimetry and wide-angle X-ray diffraction analysis, respectively. Dynamic water-repellent models were constructed. The stable low surface energy and dynamic water- and oil-repellent properties of these synthesized fluorinated polymers with short perfluorobutyl side chains were attributed to the synergetic effect of amorphous fluorinated side chains in perfluoroalkyl acrylate and crystalline hydrocarbon pendant groups in stearyl acrylate. Outstanding water- and oil-repellent properties of fabrics and any other substrates could be achieved by a facile dip-coating treatment using a fluorinated copolymer dispersion. As a result, we believe that our prepared fluorinated copolymers are potential candidates to replace the fluoroalkylated polymers with long perfluorinated chains in nonstick and self-cleaning applications in our daily life. KEYWORDS: fluoropolymers, short perfluorobutyl chains, crystalline structures, dynamic water repellency, surface molecular reconstruction

1. INTRODUCTION Fluorinated polymers have attracted significant attention in the areas of science and industry over recent decades because of their ultralow surface energy, unique water and oil repellency, self-cleaning property, and excellent thermal and chemical stability as well as the potential for practical applications in functional coatings, fabric finishing, biochemical systems, aerospace engineering, and the microelectronic field.1−7 Perfluoroalkyl-substituted acrylate polymers are considered to be one of the most effective fluoropolymer resins that maintain both the original performance of acrylate and the special surface physicochemical properties.8−13 Especially, poly(perfluoroalkyl acrylate)s with side chains of more than eight fluorinated carbon atoms (C8) exhibit quite low surface energy of 8−10 mN/m, which makes them promising as ideal materials for the production of functional coatings with low surface energy.14 The uniformly organized structure and the crystallization of the perfluoroalkyl side chains actually account for their special surface wettability.10,15,16 Therefore, these fluoropolymers have been extensively researched and applied in industry for the production of various surface modifiers, such as water and oil repellents for textiles, paper, and liquid-crystal-display screens.17−21 The tight packing of perfluoroalkyl groups and high area density of trifluoromethyl terminal groups are © XXXX American Chemical Society

important factors that determine the hydrophobicity and nonstickiness.22,23 Unfortunately, the production of these fluoropolymers may result in the release of perfluoroalkyl acid and perfluoroalkyl sulfonate [i.e., perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS)] by oxidative degradation, and this release has a potential environmental risk for their bioaccumulative potential in wildlife, long-distance migration, and biomagnification potential in food webs.24−27 Considering the biohazardous influence of PFOA and PFOS, the European Union issued bans on PFOS in October 2006 and restrictions on PFOA as well as its salts according to the 2006/122/ECOF decree. Recently, the PFOA Stewardship proposed by the U.S. EPA aimed to reduce the release of the currently abundant PFOA into the environment by more than 100% by 2015.28−30 As a result of these regulatory actions, the long-chain perfluoroalkyl acids and their precursors should be replaced with other fluorinated and nonfluorinated chemicals. The bioaccumulative potentials of perfluorinated compounds are highly dependent on the fluorinated chain length, and these Received: January 27, 2016 Accepted: April 6, 2016

A

DOI: 10.1021/acsami.6b01102 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces potentials remarkably decrease for six fluorinated carbons or less.26,31 Although perfluorinated substances with shorter carbon chains are present in much lower concentrations in biota, these shorter-chained fluorinated compounds are also persistent and difficult to remove.32,33 In comparison to perfluorohexyl substances in acute toxicity tests, compounds with four-carbon chain lengths were significantly less toxic and could be expected to have a higher security.33 Previous studies revealed that short fluorinated side chains are highly flexible and trend toward forming amorphous structures, which can suffer surface reconstruction in contact with water or other liquids.34−39 Poor dynamic water-repellent properties and nonpersistent low surface energy limit their wide usage in commercial applications.11,28,29 Therefore, the key challenge is to elaborate dynamic water-repellent surfaces by using short fluorinated chains and, in particular, perfluorobutyl chains.40−42 It has been demonstrated that the α-substituent and spacer groups between the polymer and fluorinated chain have a significant effect on the wetting behavior and molecular motion.23,43 Furthermore, our previous study confirmed that the crystalline long hydrocarbon side chains in the acrylate comonomer can prohibit surface fluorinated molecular motion and promote dynamic water repellency for fluorinated copolymers.25 Few studies that demonstrate the wettability behavior caused by the microstructure of fluoropolymers with short perfluoroalkyl side chains, especially those containing only four fluorinated carbon atoms, and achieve outstanding water- and oil-repellent properties along with environmentally friendly characteristics have been reported.29,41 Our aim was to develop a fluoropolymer design principle for high dynamic water repellency with short perfluoroalkyl side chains. The key work of this paper is to disclose the effects of the α-substituent of the fluorinated acrylate, the spacer group, and the crystalline hydrocarbon pendant groups of the (co)polymer on the wettability performance and surface molecular rearrangement. The microstructures, surface wettability, and molecular mobility of the prepared fluoropolymers were investigated in detail. Inspired by various methods related to superhydrophobic and superoleophobic modification,44−46 we confirmed the possibility of imparting superamphiphobic properties to fabric, paper, and sponge surfaces by coating with the fluorinated copolymer dispersion containing a short perfluorobutyl side chain, which promises these fluorinated copolymers with short perfluorobutyl side chains extensive application potential in the fabric-finishing industry as well as other possible fields.

Scheme 1. Chemical Structures of C4A, C4MA, C4SA, and C4SMA

nonylphenol polyoxyethylene ether, isopropyl alcohol, an aqueous methylamine solution (25−30 wt %), sodium hydroxide, anhydrous ethyl ether, triethylamine, hydrochloric acid (36 wt %), potassium carbonate, sodium iodide, sodium bicarbonate, hydroquinone, acetone, glacial acetic acid, ethanol, methanol, methyl methacrylate, and butyl acetate (BuOAc; purchased from Shanghai Chemical Reagents Co., Shanghai, China) were of analytically pure (AP) grade and were used as received. Azobis(isobutyronitrile) (AIBN) and 2,2′-azobis(2methylpropionamidine) dihydrochloride (AIBA; Beijing Chemical Reagents Co.) were AP grade and were used after recrystallization with ethanol. 2.2. Synthesis of Fluorinated Homopolymers and Copolymers. The PC4A, PC4MA, PC4SA, and PC4SMA homopolymers were obtained from free-radical polymerization. Using AIBN as the initiator, 5 g of fluorinated monomer (C4A, C4MA, C4SA, and C4SMA, respectively) was mixed with 30 mL of BuOAc in a 50 mL three-neck, round-bottomed flask that was equipped with a mechanical stirrer and reflux condenser. The polymerization was initiated when the temperature reached 80 °C in the thermostatic water bath and continued for 24 h. The conversion, which was tested during the experimental process, was greater than 95%. The precipitated PC4A and PC4MA that were obtained from the polymerization process were dissolved in 1,1,2-trichlorotrifluoroethane, and PC4SA and PC4SMA were dissolved in TFA. The purified polymers were obtained after reprecipitation in acetone, followed by drying in a vacuum oven at 100 °C for 24 h. PC4A was obtained in a rubbery state, and the other homopolymers consisted of a white powder at room temperature. The fluorinated copolymer solution was synthesized via miniemulsion polymerization as described in the literature.25 The mixed emulsifiers [0.33 g of CTAB, 0.66 g of Brij 30; CTAB:Brij 30 = 1:2 (w/w)] were dissolved in 46 mL of deionized water and stirred for 10 min at 50 °C. Then a mixture consisting of 13.4 g of monomers [FA:SA = 1:1 (mol/mol); FA refers to C4A, C4MA, C4SA, and C4SMA] and 0.1 g of SH was dissolved in 6.7 g of isopropyl alcohol, followed by dropwise addition to the surfactant solution under absolute stirring. After 30 min, the emulsion was homogenized by ultrasonication for 2 min to yield a monomer preemulsion. Unless otherwise stated, the monomer accounted for 20 wt % of the entire solution. Then, this preemulsion was transferred to a 100 mL fourneck, round-bottomed flask that was equipped with a mechanical stirrer, a thermometer, and a flux condenser. As the start of the polymerization, high-purity nitrogen was bubbled into this system for 30 min. In addition, the water bath was heated to 70 °C prior to the addition of a mixture of 0.1 g of AIBA and 10 mL of deionized water, and the polymerization then began. After 3 h, the reaction temperature gradually increased to 80 °C and continued for 2 h. The conversion, which was tested during the experimental process, was greater than 95%. The copolymer dispersion was acquired from the cooling process, and the addition of acetone led to precipitation. This precipitant was purified by dissolution in tetrahydrofuran (THF) and reprecipitation in methanol. The pure copolymer was precipitated from a THF solution with excess methanol and dried at 50 °C under vacuum.

2. EXPERIMENTAL SECTION 2.1. Materials. The commercially available perfluorobutanesulfonyl fluoride (≥95%) was provided by Hubei Hengxin Chemical and purified by distillation (bp 65−66 and 114−115 °C). The fluorinated (meth)acrylate monomers [i.e., (perfluorobutane)ethyl (meth)acrylates (C4A and C4MA) and stearyl acrylate (SA; ≥95%)] were provided by Juhua Group Corp. The [N-methylperfluorobutane-1sulfonamide]ethyl (meth)acrylates (C4SA and C4SMA) were synthesized according to our previously described protocol.47 The synthesis route for C4SA and C4SMA is shown in Scheme S1 in the Supporting Information. The chemical structures of C4A, C4MA, C4SA, and C4SMA are shown in Scheme 1. 2-Bromoethanol, acryloyl chloride, and methacryloyl chloride (≥95%, Shanghai Jiachen Chemical Co.) and 1,1,2-trichlorotrifluoroethane, trifluoroacetic acid (TFA), mineral oil, n-hexadecane, n-tetradecane, n-dodecane, n-decane, n-octane, and n-heptane (≥99.9%, Aladdin Industrial Corp.) were used as received. 1-Dodecanethiol (SH), cetyltrimethylammonium bromide (CTAB), B


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Figure 1. (a and b) FTIR spectra of PSA and fluorinated monomers and copolymers. 1H NMR (CDCl3, 400 MHz) spectra of fluorinated monomers (c) C4SA and (d) C4SMA. 2.3. Preparation of Polymer Films. The polymer solutions were prepared by dissolving PC4A and PC4MA in 1,1,2-trichlorotrifluoroethane (1 wt %). In addition, PC4SA and PC4SMA were dissolved in TFA (1 wt %), and the PC4A/SA, PC4MA/SA, PC4SA/SA, and PC4SMA/SA copolymers were dissolved in THF (1 wt %). The homopolymer and copolymer films were prepared by a spin-coating method (2000 rpm, 30 s) on clean glass slides, and then these films underwent a slow dehydration process for 24 h at room temperature. The glass slides with polymer films coated on them were placed in a vacuum oven for 24 h at 100 °C for the purpose of removing solvents completely, followed by an annealing process at 120 °C for 30 min to finally afford the polymer films. 2.4. Characterization and Analysis. The IR spectra of the polymers were recorded on a Nicolet 5700 Fourier transform infrared (FTIR) instrument. The intermediate products and target polymer solutions were cast onto KBr disks for analysis. 1 H NMR analysis was measured using a Bruker 500 MHz NMR spectrometer (Advance DMX500) with a 5 wt % solution in CDCl3 or acetone-d6 at room temperature. The molecular weights and polydispersity indexes (PDIs) of the copolymers were measured using gel permeation chromatography (a Waters 1525 binary HPLC pump, a Waters 717 autosampler, a Waters 2414 refractive index detector, and a Waters 2487 dual λ absorbance detector for UV 311 signals). Detailed data are shown in Table S1 in the Supporting Information. The molecular weights of the copolymers range from 0.85 × 104 to 1.41 × 104, and the PDIs range from 1.79 to 2.42. A CAM 200 optical contact-angle goniometer (KSV Co., Ltd.) was used to determine the contact angles of the homopolymer and copolymer coatings. The static contact angles of water and nhexadecane (each droplet volume was 2 μL) were determined through the sessile-drop method. The measurements were performed on four drops of each liquid, and eight readings of the contact angle (on the

front and back of each drop) were recorded. The average value was determined to be the contact angle. The dynamic contact angles were determined using a tilting-plate method. On an inclinable plane, the sample on the stage was tilted from zero to higher angles until a 50 μL water droplet began to slide down onto the sample. Subsequently, the advancing contact angle (θa), receding contact angle (θr), and sliding angle (θs) were calculated. The differential scanning calorimetry (DSC) data were obtained from a DSC Q200 system (TA Instruments). Prior to the DSC measurements, the samples were preheated to 200 °C to eliminate the effects of thermal history. The heating rate was 10 °C/min for a 4−10 mg sample in an aluminum pan. Then, the homopolymer samples were cooled to −90 °C and measured from −90 to +200 °C at a heating rate of 10 °C/min. In addition, the copolymer samples were cooled to −80 °C and measured from −80 to +200 °C at a heating rate of 5 °C/min. The wide-angle X-ray diffraction (WAXD) measurements of the powdered homopolymers and copolymers were performed on a D/ Max-2550pc X-ray diffractometer (Rigaku Denki Co., Ltd.) with a Cu Kα X-ray source (40 kV, 250 mA). The wavelength (λ) of the incident X-ray was 0.1541 nm. The data collection time was 3 s/step at 0.25° intervals. The diffractograms were collected over a 2θ range from 0.5° to 40°. The water- and oil-repellent properties of fabric surfaces were tested as follows. First, a piece of polyester-mixed cotton cloth, which has a size of 30 cm × 30 cm, was immersed into the copolymer solutions (1 wt %) with a liquor detention of 60−80%. Then the cloth was dried at 160 °C for 5 min. According to the AATCC22-2005 (water-repellent) and AATCC118-2007 (oil-repellent) methods, the water- and oilrepellent properties were evaluated, respectively. The AATCC22-2005 and AATCC118-2007 experimental methods are provided in the Supporting Information (Table S2 and Figures S1 and S2). C


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Table 1. Static Contact Angles θ and Surface Tension Data for Fluorinated Polymers with Perfluorobutyl Groups (T = Room Temperature)a polymer PSA PC4A PC4MA PC4SA PC4SMA PC4A/SA PC4MA/SA PC4SA/SA PC4SMA/SA

θwater (deg) 113 109 112 114 113 109 114 114 118

± ± ± ± ± ± ± ± ±

2 1 2 2 3 1 1 2 1

θhexadecane (deg) 41 76 73 74 72 72 71 75 72

± ± ± ± ± ± ± ± ±

γS (mN/m)

1 1 1 2 1 2 1 1 2

21.06 12.26 12.36 11.83 12.35 13.14 12.57 11.35 11.91

± ± ± ± ± ± ± ± ±

0.62 0.43 0.54 0.74 0.62 0.65 0.37 0.54 0.62

γSd (mN/m) 21.01 10.53 11.40 11.11 11.69 11.69 11.99 10.53 11.69

± ± ± ± ± ± ± ± ±

0.27 0.29 0.29 0.58 0.30 0.59 0.30 0.29 0.59

γSp (mN/m) 0.05 1.73 0.96 0.72 0.76 1.45 0.58 0.82 0.22

± ± ± ± ± ± ± ± ±

0.35 0.14 0.25 0.16 0.33 0.06 0.08 0.21 0.02

a γS refers to the surface free energy. γSd refers to the dispersion component of the surface free energy. γSp refers to the polar component of the surface free energy. Two-liquid model: γS = γSd + γSp and γL(1 + cos θ) = 2(γSdγLd)1/2 + 2(γSpγLp)1/2.

Figure 2. DSC thermograms of fluorinated homopolymers and copolymers: (a)PC4A, PC4MA, PC4SA, and PC4SMA; (b) PC4A/SA, PC4SA/SA, PC4MA/SA, and PC4SMA/SA. The heating rate was 10 K/min.

structures of the fluorinated monomers were successfully confirmed. The PSA homopolymers and copolymers were also analyzed by a FTIR instrument, and the results are shown in Figure 1b. The clear vision of the characteristic absorptions of CO (1732 cm−1), aliphatic C−H stretching (2850−2960 cm−1), and C−O stretching (1163 cm−1) as well as the peaks corresponding to SA, which appeared at 1451, 1386, and 725 cm−1, confirmed the structure of PSA. The characteristic absorption peaks of C−H (2850−2960 cm−1) and CO (1740 cm−1) stretching and the overlapping peaks (1100−1250 cm−1) of F−C−F and C−O−C stretching were clearly visible in the spectra for the copolymer. What is more, C−F stretching (569 and 745 cm−1) appeared in the fingerprint region. Furthermore, the peaks corresponding to the in-plane and outof-plane blending bands of −CH2− were observed at 1465 and 721 cm−1, and the characteristic C−O−C absorptions appeared at 1164 and 1241 cm−1. In addition, the 1H NMR (CDCl3, 400 MHz) spectra of the copolymers (Figure S3) are provided in the Supporting Information. The FTIR spectra, combined with the 1H NMR (CDCl3, 400 MHz) spectra, could prove the successful copolymerization of fluorinated and nonfluorinated monomers. 3.2. Wettability and Surface Free Energies of Fluoropolymer Films. Fluorinated polymers possess a low intermolecular cohesion force with potential for use in oil- and water-repellent applications because of the small atomic radius

The surface compositions of the cotton fabric samples were analyzed using X-ray photoelectron spectroscopy (XPS; PerkinElmer Phi1600 ESCA system) with Mg Kα (1245.0 eV) as the radiation source. Survey scans were recorded in a range of 0−1100 eV at a takeoff angle of 90°. Scanning electron microscopy (SEM) was employed to characterize the surface morphology of the pristine and treated cotton fabrics. SEM was performed on a S-570 scanning electron microscope (Hitachi) with an accelerating voltage of 5 kV. The cotton fabric samples were coated with gold in a vacuum prior to scanning.

3. RESULTS AND DISCUSSION 3.1. Structure Characterization of Fluorinated Monomers and Polymers. The structures of the synthesized fluorinated monomers as well as the subsequent homopolymers and copolymers were characterized by FTIR spectroscopy and 1 H NMR analysis. Parts a and b in Figure 1 show the FTIR spectra of these monomers and polymers. For C4SA and C4SMA, the typical adsorption peak at 1630 cm−1 was designated as the characteristic of double bonds in acrylic and methacrylic monomers, which suggested successful esterification. Parts c and d in Figure 1 show the 1H NMR spectra of C4SA and C4SMA, which confirmed the structures of these fluorinated monomers. As shown in Figure 1d, the signals at 5.70−6.60, 4.30−4.50, and 3.30−4.20 ppm were assigned to −CHCH2, −CH2CH2O−, and −NCH2−, respectively. Furthermore, the CCH3 group signals appeared at 1.90− 2.10 ppm. In combination with FTIR spectroscopy, the D


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ACS Applied Materials & Interfaces and high electronegativity of the fluorine atom as well as the small polarizability of the C−F bonds. In our experiment, the wettability of the polymer films was measured by contact-angle analysis and the surface free energies calculated according to the Kelen−Tudos method.48 The water and hexadecane contact-angle data for the homopolymer and copolymer (molar ratio, 1:1) films were measured, and the results are listed in Table 1. In contrast to other common nonfluorinated polymers, PSA exhibited a relatively low surface energy of 21.06 ± 0.62 mN/m due to the long pendant group of PSA, which is considered to be a smectic B phase.25 Almost no polar carbonyl groups were exposed on the outermost surface of the annealed PSA film, which resulted from the oriented organization of the long side chains that were packed on each side of the backbone. Therefore, the surface tension of PSA was almost completely dominated by the dispersion component (γSd), as described in Table 1. Although the annealed PSA films demonstrated hydrophobicity with a water contact angle of 113 ± 2°, the oilrepellent property of PSA was very poor. It is noticeable that the PC4MA, PC4SA, and PC4SMA fluorinated homopolymers had similar contact angles for a given liquid. The water contact angle of PC4A was a bit smaller, but the hexadecane contact angle was slightly larger than that of the three other kinds of homopolymers. As shown in Figure 2a, PC4A had the lowest glass transition temperature (Tg) of −21.8 °C, and Tg values for the other three homopolymers were 25.1 °C (PC4MA), 31.6 °C (PC4SA), and 43.6 °C (PC4SMA). That is, at room temperature, the mobility of the molecular PC4A is active compared with that of the other three homopolymers. Because of the lack of crystallinity of the short perfluorobutyl groups and when PC4A is exposed to water, a smaller contact angle will be exhibited because of surface reorganization,39 while the high oil contact angle of the PC4A homopolymer should be attributed to relatively more fluorine content than the other three homopolymers. It is of great significance that the surface free energy values for these fluorinated homopolymers are much lower than that of PTFE (18 mN/m), which has closely packed −CF2 groups on its surface. The surface free energy of closely packed −CF3 groups (6 mN/m) is lower than that of the −CF2− surface.49,50 The high packing density of the perfluorocarbon groups with the −CF3 terminal group is supposed to get extremely low surface free energy. Although a polar N-methylsulfonamide group was located in the side chains of PC4SA and PC4SMA, the polar component (γSp) decreased signally compared to those of PC4A and PC4SA. The main reason for these behaviors should be that the Nmethylsulfonamide groups between the polymer backbone and perfluoroalkyl side chain promote enrichment of the perfluorinated segments on the outermost surface. Although 50 mol % PSA was incorporated, the four kinds of fluorinated copolymers in Table 1 showed similar contact-angle values for a given liquid and surface tension with the corresponding homopolymers. Meanwhile, all of these copolymers had lower surface energies than PSA. The ratio the polar component (γSp) accounted for in the surface tension of PC4A/SA and PC4SA/SA was slightly larger than that of the fluorinated homopolymers. However, for copolymers PC4MA/ SA and PC4SMA/SA containing methyl substituents in the main chain, the ratio of the polar component was smaller than that of the corresponding homopolymers, as proof of less accumulation of polar groups in the outermost layer of the copolymer films. As a consequence, although PSA in

copolymers with long alkyl side chains had a higher surface free energy, the relative copolymers simultaneously maintained a low surface free energy and good oil resistance. Especially, the polar component (γSp) in the surface energy values of the copolymers was much smaller than that for the corresponding fluorinated homopolymers. More interestingly, the surface energies of PC4SA/SA and PC4SMA/SA were even lower than those of PC4SA and PC4SMA. The reason should be the presence of an oriented hydrocarbon side chain in SA with a smectic B phase, which restrains the molecular mobility on the surface. For PC4A/SA, PC4SA/SA, PC4MA/SA, and PC4SMA/ SA copolymers, the PC4SMA/SA copolymer showed a relatively high water contact angle. The α-methyl group can decrease the flexibility of the main chain, and the Nmethylsulfonamide spacer group also plays a positive role in the conformational arrangements of the fluorinated side chains.23,39 The mobility of the molecular chains of the PC4SMA/SA copolymer is inactive compared with that of the other three copolymers. This causes an increase in the contact angle against water by minimum surface reorganization. On the other hand, although the PC4SMA/SA copolymer has the most well-organized molecular structure and is inactive in surface reorganization, it has relatively less fluorine content compared to the other three copolymers. Therefore, the oil contact angle for the PC4SMA/SA copolymer film is relatively low compared with the others. 3.3. Thermal Analysis. DSC measurement is a widely used method to discover the thermal properties of polymer materials. Figure 2 exhibits the DSC thermograms of fluorinated homopolymers and copolymers. As shown in Figure 2a, only one glass transition temperature was obtained for each kind of homopolymer. The glass transition temperature for PC4MA was 25.1 °C, which was larger than that of PC4A (−21.8 °C) because of the decrease in the molecular flexibility caused by the introduction of a methyl substituent. Obviously, the appearance of N-methylsulfonamide groups in the fluorinated groups significantly improved the glass transition temperature of the fluorinated polymers. The N-methylsulfonamide groups could not only elongate side chains as well as increase the steric hindrance but also enlarge the potential barrier of internal rotation and intermolecular forces so as to affect the transformation of molecular conformation. As a consequence, the glass transition temperatures of PC4SA and PC4SMA increased to 31.6 and 43.6 °C distinctly. No significant ordered arrangement with a smectic phase was observed in these four fluorinated homopolymers, which was confirmed by the lack of a phase transformation peak appearing in the DSC curve. Figure 2b provides the DSC thermograms of the fluorinated copolymers with perfluorobutyl groups. The start (Tonset) and end (Toffset) temperatures of the transitions as well as the melting range (Tr), melting point (Tm), and enthalpy variation (ΔHf) of the copolymers are listed in Table 2. The DSC test result confirmed that the perfluorobutyl chains in the copolymers cannot even crystallize because of their melting enthalpy along with the degree of crystallinity being zero. Also, no glass transition temperature or phase transition peak was observed for the corresponding fluorinated homopolymers in the DSC curves of the copolymers. As can be seen from the data in Table 2, compared with PSA, Tonset, Tr, and ΔHf of the copolymers decreased remarkably, while Toffset and Tm only decreased slightly. The reason for this behavior may be that the fluorinated acrylate comonomers dilute the concentration of SA E


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PC4SMA/SA, the crystalline interplanar spacings were 53.8 and 54.2 Å, which is slightly bigger than the double length of the PSA side chain (46 Å). Because of the introduction of the Nmethylsulfonamide groups, the side chain was elongated and the interlamellar spacing increased. In the wide-range region, the diffraction peak at 5.3 Å was relatively wide, while the peak at 4.1 Å was sufficiently sharp, which demonstrated the crystalline structure of the SA side chain and the disordered arrangement of the perfluorobutyl group. On the basis of the WAXD result, the octadecyl side chains in the copolymers formed a crystalline arrangement, and the regular interdigitated structure disappeared. The model of sidechain packing for the fluorinated copolymers is shown in Figure 3. For the PC4A/SA and PC4MA/SA copolymers, as described in Figure 3a, the interlamellar spacing is similar to that of the PSA homopolymer, and disordered fluorinated side chains arranged among the crystalline octadecyl side chains. However, for the PC4SA/SA and PC4SMA/SA copolymers containing Nmethylsulfonamide groups (Figure 3b), the interlamellar spacing was a little larger than that of the PSA homopolymers, and the disordered fluorinated side chains also arranged among the crystalline octadecyl side chains. 3.5. Dynamic Surface Property. The advancing contact angle (θa), receding contact angle (θr), and sliding contact angle (θs) are often applied to characterize the dynamic surface properties of solid surfaces. Besides, the difference between θa and θr (i.e., the contact angle hysteresis Δθ) is a relevant factor. The dynamic contact-angle values of water at room temperature for fluorinated polymers with prefluorobutyl alkyl side chains are shown in Table 4. As we all know, the wettability of a solid surface is controlled by its chemical composition and geometrical structure. According to the atomic force microscopy (AFM) images (Figure S5) of copolymer films provided in the Supporting Information, the root-mean-square roughness ranges from 0.5 to 3 nm. The pure copolymer films are relatively smooth, and the surface roughness has almost no influence on contact-angle hysteresis. The contact-angle hysteresis is thus ascribed to the surface reorientation of the perfluoroalkyl side chain that accumulated on the outer surface. As shown in Table 4, the four fluorinated homopolymers with short perfluorobutyl groups exhibited very high advancing contact angles (above 120°). However, the receding contact angle (θr) for PC4A was smaller than those of the other three homopolymer samples. The sliding contact angle (θs) for PC4A reached 35°, which is markedly higher than the other three angles. The reason for this behavior may be attributed to the low glass transition temperature of PC4A (below 0 °C). It is interesting to note that the four fluorinated copolymers displayed good water-repellent properties, which were the same or possibly exceeded that of the corresponding fluorinated homopolymers. Especially for PC4A/SA, a high θa of 122 ± 2° was observed, while Δθ decreased from 85 ± 4° to 34 ± 4° and θs decreased from 35 ± 1° to 23 ± 2°. These results suggested that the PC4A/SA copolymer had better dynamic water repellency because of the crystalline side chain of SA being on the outermost surface, which prevented the outstretched perfluorobutyl side chain from reconstruction and returning back to the bulk. As can be seen from Table 4, the PC4MA/SA, PC4SA/SA, and PC4SMA/SA fluorinated copolymers also displayed good dynamic water repellency. In particular, PC4SMA/SA exhibited outstanding dynamic water repellency with high θa and θr. Compared to PC4SMA, Δθ and θs of

Table 2. Thermal Data of the Fluorinated Copolymers with Perfluorobutyl Groups by DSC first transition copolymer

Tonset (°C)

Toffset (°C)

Tr (°C)

Tmelting (°C)

ΔHf (J/g)

PSA PC4A/SA PC4MA/SA PC4SA/SA PC4SMA/SA

41.8 21.9 7.0 26.5 21.9

58.9 45.7 51.0 50.7 47.8

17.1 23.8 44.0 24.2 25.9

51.6 39.7 44.9 45.2 43.1

108.7 45.1 34.0 42.7 27.2

and disturb the formation of an integrated crystal structure. On the other hand, fluorinated acrylates can inhibit the growth of already crystallized grains in three-dimensional space, so as to decrease the crystal grain size. Then, the corresponding melting ranges of the copolymers become wider, and ΔHf significantly decreased. Furthermore, the difference between Tm and Tonset for the copolymers was larger than that for PSA (9.8 °C), which indicated that the cooling rate for copolymer crystallization was also higher than that for PSA. 3.4. WAXD. The X-ray scattering for the fluorinated copolymers at 25 °C is shown in Figure S4 in the Supporting Information. The result of WAXD for PSA was reported in previous literature.25 Table 3 exhibits the Bragg spacings and Table 3. Bragg Spacings and Approximate Intensities from X-ray Diffraction Experimentsa PSA

PC4A/SA

40.7 (s) 30.2 (s)

48.8 (s) 17.1 (w)

4.1 (s)

5.3b 4.1 (s)

PC4MA/SA

PC4SA/SA

Small-Angle Region 40.1 (s) 53.8 (s) 16.9 (s) 19.6 (w) 12.5 (w) Wide-Angle Region 5.3b 5.3b 4.1 (s) 4.1 (s)

PC4SMA/SA 54.2 (s)

5.3b 4.1 (s)

a

Bragg spacings are given in angstroms. The intensities are classified as sharp (s) or weak (w). bBroad peak.

approximate intensities from X-ray diffraction experiments. According to our previous work,25 there was a bilayer structure with pendant groups either titled or interdigitated in the PSA homopolymer. As described in Figure S4 in the Supporting Information, high intensity diffraction peaks appeared in the small-angle (2θ < 10°) region, which indicated that the pendant hydrocarbon chains in the fluorinated copolymers formed a well-organized lamellar structure. The peak at 2θ = 15−20° was broad, and the peak at 2θ = 20−25° was really sharp, reflecting the low-level coordination of the perfluoroalkyl group. However, the side chains in SA formed crystalline structures. As shown in Table 3, the crystalline interplanar spacing of 30.2 Å in the interdigitated structure of PSA was absent in the copolymers, which indicated the disappearance of the interdigitated structure of the long nonfluorinated side chains. The crystalline interplanar spacings for PC4A/SA and PC4MA/ SA were 48.8 and 40.1 Å, respectively. Then, in contrast to the double spacings (46 Å) of the side chain in PSA, the side chains of PC4A/SA and PC4MA/SA might display an end-to-end structure, arranged vertical or tilted in the direction of the main chain. However, the treatment process of the samples affected the crystalline structure and morphology of the polymers as well as the crystalline interplanar spacing. For PC4SA/SA and F


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Figure 3. Schematics of side-chain packing for fluorinated copolymers: (a) PC4A/SA and PC4MA/SA; (b) PC4SA/SA and PC4SMA/SA.

The copolymers of crystalline SA and fluorinated acrylate with perfluorobutyl side chains may have both a stable low surface free energy and an excellent dynamic water-repellent performance. Although the PC4A, PC4MA, PC4SA, and PC4SMA homopolymers are amorphous, the crystalline nonfluorinated side chains in the PC4A/SA, PC4MA/SA, PC4SA/SA, and PC4SMA/SA copolymers could inhibit the fluorinated side chains from migrating to the polymer bulk when contacting polar liquid, such as water. The dynamic water-repellent model demonstrating the arranging and packing states of perfluorobutyl and octadecyl side chains in copolymer molecules is shown in Figure 4. As shown in Figure 4a, upon contact with water, the flexibility of the amorphous fluorinated side chains of the PC4A/SA and PC4MA/SA copolymers was restrained to some extent and could not easily move from the copolymer surface to bulk, endowing the PC4A/SA and PC4MA/SA copolymers with water repellency. However, for PC4SA/SA and PC4SMA/SA, with the combined synergy of a

Table 4. Dynamic Contact Angles for Fluorinated Polymers with Perfluorobutyl Groups (Room Temperature) polymer PSA PC4A PC4MA PC4SA PC4SMA PC4A/SA PC4MA/SA PC4SA/SA PC4SMA/SA

θa (deg) 117 125 120 121 122 122 126 125 123

± ± ± ± ± ± ± ± ±

2 1 1 1 3 2 2 1 2

θr (deg) 89 40 85 81 83 88 82 83 103

± ± ± ± ± ± ± ± ±

1 3 2 2 1 2 2 1 1

Δθ (deg) 28 85 35 40 39 34 44 42 20

± ± ± ± ± ± ± ± ±

3 4 3 3 4 4 4 2 3

θs (deg) 22 35 23 24 22 23 27 28 16

± ± ± ± ± ± ± ± ±

1 1 2 1 1 2 2 1 2

PC4SMA/SA decreased to 20 ± 3° and 16 ± 2°. The crystalline nonfluorinated side chains on the outermost surface and high glass transition temperature of PC 4 SMA/SA played a synergistic role in the superb dynamic water repellency of its films.

Figure 4. Models for the water repellency of (a) PC4A/SA and PC4MA/SA and (b) PC4SA/SA and PC4SMA/SA. G


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ACS Applied Materials & Interfaces rigid main chain, crystalline nonfluorinated side chains, and repulsion between the N-methylsulfonamide groups and the perfluorobutyl side chain, the copolymer maintained a larger receding contact angle (θr). For the PC4SMA/SA copolymer, the contact-angle hysteresis (Δθ) of 20 ± 3° and the sliding contact angle (θs) of 16 ± 2° were relatively low. This behavior could endow the substrate with a significantly stable waterrepellent property. 3.6. Water/Oil-Repellent Properties of Treated Cotton Fabrics. As we all know, a fluorinated copolymer dispersion, which has extremely low surface energy, has been widely used in the waterproofing after-treatment process of fabric. Although perfluorooctyl acrylates have remarkable surface properties, their negative effects on the environment limit their further applications under the ban that was mentioned above, and environmentally friendly alternatives are urgently needed. The prepared environmentally friendly fluorinated copolymer emulsions were diluted to a 1 wt % aqueous bath. Cotton fabrics were impregnated in the bath for 3 min and padded to wet pickup at approximately 70 wt % on the weight of the dry fabrics. The treated cotton fabrics were then dried at 160 °C for 5 min and cooled to room temperature for water- and oilrepellent characterization. The main performances of treated cotton fabrics include water repellency (AATCC22-2005) and oil repellency (AATCC118-2007). Table 5 shows the results

Figure 5. Photographs of the liquid repellency of the untreated and treated specimens with PC4SMA/SA emulsion: (A) untreated cotton fabric; (A1) treated cotton fabric; (B) untreated filter paper; (B1) treated filter paper; (C) untreated PVA sponge; (C1) treated PVA sponge.

with excellent water- and oil-repellent properties hold great application potential in the finishing industry on various kinds of material surfaces. The surface composition of the cotton fabric treated by the fluorinated copolymer latex was examined using XPS characterization. The XPS spectra of the cotton fabric samples are recorded in Figure 6. As can be seen from Figure 6, the mole

Table 5. Water- and Oil-Repellent Properties of Fluorinated Copolymer Latexes copolymer latexes

water repellency

oil repellency

PC4A/SA PC4MA/SA PC4SA/SA PC4SMA/SA

50 70 80 100

3 4 4 3

for the water- and oil-repellent properties of cotton fabrics treated by fluorinated copolymer emulsion. All of the treated cotton fabrics exhibited some hydrophobic and oleophobic properties. After treatment by PC4SA/SA dispersion, the fabric samples exhibited a water-repellent degree of 80 and an oilrepelleny degree of 4, which meet the requirements of most commercial applications. The water-repellent degree of the fabric treated by PC4SMA/SA even reached 100. Photographs of the different liquid droplets on various untreated specimens, and specimens treated with PC4SMA/SA dispersion are shown in Figure 5. The surfaces of the untreated cotton fabric, filter paper, and PVA sponge exhibited hydrophilic and oleophilic properties. When the model liquids contacted with these specimen surfaces, the droplets spread quickly and even permeated the specimens. The specimens treated with PC4SMA/SA showed significant lyophobic properties to the common liquids in our daily life, like milk tea, coffee, salad oil, and ink. As shown in Figure 5, all of the model liquids on the surface of the treated specimens maintained the state of liquid beads, which is the same result observed for water droplets on the superhydrophobic surface of a lotus leaf. However, the contact angle of water on the smooth surface of the copolymer film coated on glass was less than 120°. This indicated that the specimens treated by the fluorinated copolymer possessed a superlyophobic property, which should be attributed to the roughness structure of the material surface of the fabric samples covered with low-surface-energy groups. This result indicates that the prepared fluorinated copolymers

Figure 6. Surface XPS spectra of the cotton fabric treated by the fluorinated copolymer latex. The inset table shows surface elemental concentrations of the samples (F* is the theoretically calculated value).

percentage of fluoride at the surface was much higher than the theoretical value for the copolymers. Compared with the untreated fabric, the carbon and oxygen contents on the surface decreased significantly. Moreover, the appearance of sulfur and nitrogen signals in the XPS spectra of the cotton fabric treated with the PC4SMA/SA latex illustrated the existence of a Nmethylsulfonamide linking group in the copolymer composition. The accumulation of fluoride on the surface could provide a low surface energy for the treated fabric surface. The morphology of the treated cotton fabric sample is shown in Figure 7. As we all know, the cotton fabrics have a rough H


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perfluorobutyl side chain, which was of great industrial interest for practical application with low bioaccumulative potential.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01102. Synthesis route of [N-methylperfluorobutane-1sulfonamide]ethyl (meth)acrylate (C4SA and C4SMA), molecular weights and PDIs of the copolymers, spray experiments and the test standard of water repellency (AATCC22-2005), test standard of oil repellency (AATCC118-2007), 1H NMR (CDCl3, 400 MHz) spectra of copolymers, powder X-ray diffraction diagrams at room temperature for PSA and copolymers, tappingmode AFM images of the fluorinated copolymer films (PDF)

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Figure 7. SEM images of (A and C) pristine cotton fabric and (B and D) treated cotton fabric.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. texture that is woven from plant fibers. Furthermore, many nanostructure papillae exist on the surface of the fibers. This multiscale roughness texture played a significant role in the formation of superlyophobic surfaces on the fabrics. Figure 7 shows the SEM images of the pristine and treated cotton fabric samples, and these images reveal the differences in their morphology. Figure 7A shows an enlargement of the image in Figure 7C. The fibers of the pristine cotton fabric were slightly rough, with the presence of some grooves. However, from Figure 7B,D, the morphology reflection of cotton fabric dip-coated with the fluorinated polymer latex, we could find a much rougher surface with more grooves. The treated process with the fluorinated polymer latex gave a much rougher surface to that of the pristine fabric cotton, and the increased roughness of the treated fabric was supposed to contribute to the achievement of excellent water- and oil-repellent properties.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21476195 and 21576236) and Zhejiang Provincial National Science Foundation of China (Grant Y14b060038). The authors also gratefully thank Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University for providing XPS measurement.

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REFERENCES

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4. CONCLUSIONS Here, we report the possibility of obtaining highly dynamic water-repellent properties with fluorinated polymers containing a short perfluorobutyl tail. The effects of the α-substituent of the fluorinated acrylate, the spacer group, and the crystalline hydrocarbon pendant groups of SA on the wettability performance and surface molecular rearrangement of the fluorinated copolymer were evaluated by dynamic contactangle, DSC, WAXD, and XPS measurements. The prepared fluorinated copolymers exhibited wettability performances similar to those of the corresponding fluorinated homopolymers with a short perfluorobutyl side chain. The surface molecular motion was restrained to some degree by the Nmethylsulfonamide spacer group and the crystalline hydrocarbon component when in contact with water. A high receding contact angle and low contact-angle hysteresis against water for PC4SMA/SA with a relatively high Tg were observed. PC4SMA/SA dispersion may endow the cotton fabric with a water-repellent grade of 100 points and an oil-repellent grade of 3 according to the AATCC22-2005 and AATCC118-2007 methods. We confirmed the possibility of obtaining superlyophobic properties on the fabric, paper, and sponge surfaces coated by the fluorinated copolymers containing a short I


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