Article pubs.acs.org/Langmuir
Sulfone-Containing Methacrylate Homopolymers: Wetting and Thermal Properties Shota Fujii and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Although the sulfonyl functional group has a large dipole moment and compounds containing them (sulfones) have correspondingly high dielectric constants, this chemical structure has been neglected for use as a functional group to render surfaces hydrophilic. We have prepared three methacrylate polymers containing side chains capped with sulfolane, methylsulfone, and ethylsulfone functionality. The sulfolane-containing polymer exhibits an unusually high glass transition temperature (Tg = 188 °C) for a methacrylate polymer and slightly different thermal degradation behavior than the other two sulfone-containing polymers, likely due to the bulky structure of the sulfolane group in the polymer side chain. At macroscopic polymer film/water interfaces, the sulfone-containing side chains exposed to the interface impart hydrophilic properties as assessed by contact angle analysis. The hydrophilicities of sulfolane and methylsulfone surfaces are similar, and greater than the ethylsulfone surface. Although the chemical compositions of the sulfolane and ethylsulfone polymers are almost identical, the five-membered ring structure of sulfolane allows the sulfonyl moiety to be exposed at the interface in a manner similar to that of the methylsulfone polymer. The sulfonyl group at the ethylsulfone polymer/water interface is slightly masked by the ethyl group. Interestingly, the sulfolane surface displays a higher affinity to methylene iodide and n-hexadecane probe fluids compared to the other sulfone surfaces, suggesting that the fivemembered ring structure of the sulfolane moiety can orient in a manner that shelters the sulfonyl group at hydrophobic interfaces.
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INTRODUCTION Significant effort has been devoted to the chemical design and preparation of surfaces that resist protein adsorption and cell adhesion as well as function in materials and devices that exhibit biocompatibility.1−3 Applications of this surface chemistry are in, for examples, prosthetic devices, contact lenses, drug delivery vehicles, and microfluidic devices used for analysis of biological fluids. The specific molecular level nature of the surface chemistry required to promote biocompatibility has been addressed,3 hydrophilic, electrical charge-neutral, hydrogen bond-accepting, and not hydrogen bond-donating chemical functional groups have been identified as useful, and these are appropriate criteria for targeting new systems. Polyethylene glycol (PEG)-grafted surfaces demonstrate low fouling ability for nonspecific protein adsorption and cell adhesion, and their hydrophilicity, electrical neutrality, and steric hindrance have been implicated as the roots of their success.4,5 PEG-based polymers have also been widely used in coating nanoparticles for drug delivery systems to promote biocompatibility.6 PEG, however, is readily subject to oxidation in biological media; thus, these applications are limited. Alternative biocompatible functional species include betainebased zwitterions, such as carboxybetaine, sulfobetaine, and phosphobetaine, as well as amino acid-based zwitterions. These have gained attention, as they significantly improve the surface biocompatibility, arguably because of their electrical neutrality and high hydration capability.7−14 © 2015 American Chemical Society
The sulfonyl functional group (R2SO2) possesses a large dipole moment, and sulfone compounds can have high dielectric constants. Due to their miscibility with water and polar and aromatic solvents, as well as their stability toward strong acid, base, and high temperature, low molecular weight sulfones, especially sulfolane, have found use as solvents in chemical reactions.15−17 The structure of sulfolane as well as its zwitterionic and dritterionic resonance structures are shown in Scheme 1. Sulfones are used not only as solvents for chemical Scheme 1. Sulfolane Structure
Received: November 19, 2015 Revised: December 29, 2015 Published: December 30, 2015 765
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reaction was quenched with saturated NaHCO3 solution (10 mL), and the mixture was washed with a saturated solution of sodium chloride. The organic layer was dried over MgSO4 and concentrated at reduced pressure. The crude product was purified by column chromatography (acetone/hexane 1:2), which afforded a colorless oil (yield: 2.80 g, 85%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 6.18 (s, 1H), 5.68 (s, 1H), 5.59 (m, 1H), 3.47−3.15 (m, 4H), 2.53 (m, 2H), 1.96 (s, 3H). 13 C NMR (125 MHz, CDCl3): δ (ppm) = 166.1, 135.3, 127.3, 69.9, 56.7, 49.5, 29.0, 18.1. ESI-MS (M + Na): calcd for 204.05, found 204.05. Synthesis of Poly(3-sulfolanyl methacrylate) (PSMA). Compound 2 (0.400 g, 1.92 mmol), 2-cyano-2-propyl benzodithioate (CPBD) (1.74 mg, 7.84 μmol), and 2,2′-azobis(2-methylpropionitrile) (AIBN) (0.258 mg, 1.57 μmol) were dissolved in DMSO (1.2 mL). After being degassed using 3 freeze−pump−thaw cycles, the reaction mixture was stirred for 24 h at 60 °C. The reaction was quenched in liquid nitrogen. The polymer solution was precipitated in methanol, filtered, rinsed with acetone and then dried at reduced pressure (yield: 0.301 g, 75%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 5.34 (br, 2H), 3.65− 2.96 (br, 4H), 2.62−2.14 (br, 2H), 2.10−1.39 (br, 2H), 1.20−0.634 (br, 3H). Synthesis of 2-(Methylsulfonyl)ethyl Methacrylate (3). 2(Methylsulfonyl)ethanol (1.12 g, 9.06 mmol) dissolved in dry chloroform (15 mL) was added to triethylamine (2.06 g, 19.9 mmol) at room temperature. The solution was cooled to 0 °C, and methacryloyl chloride (1.04 g, 9.97 mmol) was added slowly. The reaction mixture was stirred for 30 min at 0 °C, allowed to warm to room temperature, and stirred for 30 min. The reaction was quenched with saturated NaHCO3 solution (10 mL), and the mixture was washed with a saturated solution of sodium chloride. The organic layer was dried over MgSO4 and concentrated at reduced pressure. The crude product was purified by column chromatography (acetone/ hexane 1:3), which afforded a colorless oil (yield: 1.24 g, 71%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 6.14 (s, 1H), 5.66 (s, 1H), 4.62 (t, J = 5.75 Hz, 2H), 3.39 (t, J = 5.75 Hz, 2H), 3.00 (s, 3H), 1.96 (s, 3H). 13C NMR (125 MHz, CDCl3): δ (ppm) = 166.6, 135.4, 127.0, 58.2, 54.0, 42.3, 18.3. ESI-MS (M + Na): calcd for 215.05, found 215.05. Synthesis of Poly(2-(methylsulfonyl)ethyl methacrylate) (PMSEMA). Compound 3 (0.400 g, 2.08 mmol), 2-cyano-2-propyl benzodithioate (CPBD) (1.84 mg, 8.33 μmol), and 2,2′-azobis(2methylpropionitrile) (AIBN) (0.274 mg, 1.67 μmol) were dissolved in DMSO (0.6 mL). After being degassed using three freeze−pump− thaw cycles, the reaction mixture was stirred for 24 h at 60 °C. The reaction was quenched in liquid nitrogen. The polymer solution was precipitated in methanol, filtered, rinsed with acetone, and then dried at reduced pressure (yield: 0.280 g, 70%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 4.252 (br, 2H), 3.51 (br, 2H), 3.17 (br, 3H), 2.04−1.43 (br, 2H), 1.29 (br, 2H), 1.09−0.657 (br, 3H). Synthesis of 2-(Ethylsulfonyl)ethyl Methacrylate (4). 2(Ethylsulfonyl)ethanol (1.00 g, 7.24 mmol) dissolved in dry chloroform (15 mL) was added to triethylamine (1.61 g, 15.9 mmol) at room temperature. The solution was cooled to 0 °C, and methacryloyl chloride (0.829 g, 7.97 mmol) was added slowly. The reaction mixture was stirred for 30 min at 0 °C, allowed to warm to room temperature, and stirred for 30 min. The reaction was quenched with saturated NaHCO3 solution (10 mL), and the mixture was washed with saturated solution of sodium chloride. The organic layer was dried over MgSO4 and concentrated at reduced pressure. The crude product was purified by column chromatography (acetone/ hexane 1:3), which afforded a colorless oil (yield: 0.588 g, 39%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 6.13 (s, 1H), 5.66 (s, 1H), 4.61 (t, J = 6.00 Hz, 2H), 3.34 (t, J = 6.00 Hz, 2H), 3.08 (q, J = 7.25 Hz, 2H), 1.96 (s, 3H), 1.42 (s, 3H). 13C NMR (125 MHz, CDCl3): δ (ppm) = 166.6, 135.4, 126.9, 58.0, 51.1, 48.5, 18.1, 6.49. EI-MS (M + H): calcd for 206.06, found 206.02. Synthesis of Poly(2-(methylsulfonyl)ethyl methacrylate) (PESEMA). Compound 4 (0.294 g, 1.43 mmol), 2-cyano-2-propyl benzodithioate (CPBD) (1.26 mg, 5.71 μmol), and 2,2′-azobis(2methylpropionitrile) (AIBN) (0.187 mg, 1.14 μmol) were dissolved in
reactions and separations but also for a wide range of engineering and biomedical applications. In lithium-ion and lithium−oxygen batteries, sulfone-based electrolytes are more useful than other common electrolytes such as ethylene carbonate due to their high ion conductivity, stability toward electrochemical oxidation, nonflammability, and low volatility.18−22 Divinyl sulfones have been used as water-soluble crosslinking agents in the preparation of biomaterials, including extracellular matrixes and hyaluronic acid-based hydrogels.23,24 Sulfones demonstrate the properties of inflammation inhibitors and tumor cell growth inhibitors; thus, they are advantageous in medical adhesive applications.25,26 Polymers containing sulfones, such as poly(ether sulfone), are mechanically stable materials throughout broad pH and temperature ranges and are often used as membrane filtration materials. These polymer membranes exhibit water permeation and low protein fouling due to the sulfone moieties, indicating that the sulfonyl groups impart hydrophilic properties to material surfaces. Surprisingly, however, there has been no focused research on the sulfone as a surface functional group, even though it fits all of the criteria useful to provide biocompatibility to material surfaces: charge neutral, hydrophilic, hydrogen bond accepting, not hydrogen bond donating. We report the synthesis of three methacrylate polymers containing sulfone side chains: sulfolane, methylsulfone, and ethylsulfone. The thermal properties of these polymers were investigated using differential scanning calorimetry (DSC) and themogravimetric analysis (TGA). The surface wetting properties and the effects of the sulfone structure were determined using contact angle analysis of spin-coated silicon wafersupported thin polymer films.
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EXPERIMENTAL SECTION
Materials and Synthesis. 2-(Methylsulfonyl)ethanol (98%), 2,2′azobis(2-methylpropionitrile) (98%), and 2-cyano-2-propyl benzodithioate (>97%) were purchased from Sigma-Aldrich. Butadiene sulfone (sulfolene, 98%), methacryloyl chloride (95%), and all solvents were purchased from Acros Organics. 2-(Ethylsulfonyl)ethanol (95%) was purchased from Oakwood Chemical. Triethylamine was purchased from Fisher Scientific. All reactions were carried out under nitrogen atmosphere, and all solvents were dehydrated by standard methods. The progress of reactions was monitored using thin layer chromatography (TLC) and detected using UV (254 nm) and staining with an ethanol solution of phosphomolybdic acid (5%). Products were purified by column chromatography with silica gel 60 (240−400 mesh). Nuclear magnetic resonance spectra were recorded with a 500 MHz Bruker spectrometer using chloroform-d and DMSOd6 solvents. Chemical shifts (δ) are expressed in parts per million downfield from tetramethylsilane using the solvent resonance as the internal standard. Mass spectral data were obtained at the University of Massachusetts Mass Spectrometry Center. Synthesis of 3-Hydroxysulfolane (1). Butadiene sulfone (20.0 g, 0.169 mol) was dissolved in 2 N NaOH (42 mL) and stirred for 24 h at room temperature. The reaction was quenched with 12 N HCl solution (7.0 mL) and then concentrated at reduced pressure. The crude product was purified by column chromatography (acetone/ hexane 1:1), which afforded a colorless solid (yield: 20.1 g, 87%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 4.70 (m, 1H), 3.41−3.09 (m, 4H), 2.99 (br, 1H), 2.39 (m, 2H). EI-MS (M + H): calcd for 137.02, found 137.11. Synthesis of 3-Sulfolanyl Methacrylate (2). Compound 1 (2.20 g, 16.2 mmol) dissolved in dry chloroform (10 mL) was added to triethylamine (3.63 g, 35.6 mmol) at room temperature. The solution was cooled to 0 °C, and methacryloyl chloride (1.85 g, 17.8 mmol) was added slowly. The reaction mixture was stirred for 30 min at 0 °C, allowed to warm to room temperature, and stirred for 30 min. The 766
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Langmuir Scheme 2. Syntheses of PSMA, PMSEMA, and PESEMA via RAFT Polymerization
DMSO (0.47 mL). After being degassed using three freeze−pump− thaw cycles, the reaction mixture was stirred for 24 h at 60 °C. The reaction was quenched in liquid nitrogen. The polymer solution was precipitated into methanol, filtered, rinsed with acetone, and then dried at reduced pressure (yield: 0.230 g, 71%). 1H NMR (500 MHz, CDCl3): δ (ppm) = 4.25 (br, 2H), 3.51 (br, 2H), 3.17 (br, 2H), 2.04− 1.43 (br, 2H), 1.29 (br, 2H), 1.09−0.657 (br, 3H). Characterization. Size exclusion chromatography (SEC) measurements were carried out using sulfone-containing polymer samples dissolved in DMF containing 0.01 M LiCl at 50 °C with three columns (one 50 mm × 7.5 mm PL gel mixed guard column, one 300 mm × 7.5 mm PL gel 5 μm mixed C column, and one 300 mm × 7.5 mm PL gel 5 μm mixed D column). The polymer solutions were filtered using a 0.2 μm PTFE membrane before injection. The output from the column was then passed through a Knauer refractive index (RI) detector (K-2301). The molecular weight values of the polymers was calculated using polystyrene standards. Fourier transform infrared (FT-IR) measurements were performed on a PerkinElmer Spectrum 100 FTIR spectrometer equipped with an attenuated total reflectance (ATR) cell using four scans at 1 cm−1 resolution. The thermal properties of sulfone-containing polymers were studied using a TA Instruments Q200 differential scanning calorimeter (DSC) and Q50 themogravimetric analysis instrument (TGA). The heating rate for all measurements was 10 °C/min. Film Preparation and Surface Characterization. Polymer thin films were prepared by spin coating at 2000 rpm for 60 s from 1.0 wt % polymer solutions in 2,2,2-trifluoroethanol (TFE) in a nitrogenpurged glovebox. The films were dried for over >24 h under vacuum. AFM analyses were performed with a Digital Instruments Dimension 3000 scanning force microscope operating in tapping mode at room temperature. X-ray photoelectron spectra (XPS) were recorded with a Physical Electronics Quantum 2000. Contact angle measurements were performed with a Ramé-Hart telescopic goniometer and a Gilmont syringe with a 24-gauge flat-tipped needle. Ultrapure water (Milli-Q), methylene iodide, n-hexadecane, and toluene were used as probe fluids. During careful addition and withdrawal of the probe fluid
from a single drop on surfaces, dynamic advancing and receding contact angles were measured. The contact angle values are the average of no less than 10 measurements on different areas of sample surfaces. All values for all sample surfaces were within ±1° of each average.
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RESULTS AND DISCUSSION Synthesis and Characterization of Sulfone-Containing Polymers. Sulfone-containing polymers were prepared as shown in Scheme 2. Methacrylate monomers with sulfolane (SMA), methylsulfone (MSEMA), and ethylsulfone (ESEMA) were prepared by reactions of methacryloyl chloride with alcohol derivatives of the sulfones. All three monomers were polymerized by reversible addition−fragmentation chain transfer (RAFT) polymerization. Chemical structures for each step were confirmed by 1H and 13C NMR as well as mass spectroscopy (see Supporting Information). The sulfone monomers, especially MSEMA and ESEMA, are very reactive after being completely purified and dried, even with an inhibitor present. Due to this instability, only crude purifications of the monomers were carried out and impurities are apparent in the NMR spectra (see Supporting Information). After polymerization of the monomers and isolation of the polymers, 1H NMR spectra confirm that the peaks due to impurities in spectra of the monomers are completely absent in spectra of the sulfone-containing polymers. The number-average (Mn) and weight-average (Mw) molecular weight as well as the molecular weight distribution of the sulfone-containing polymers were measured by SEC with DMF eluent and PS standards (Table 1 and Figure S1, Supporting Information). The weight-average molecular weights of PSMA, PMSEMA, and PESEMA samples were 54100, 44500, and 68800 g/mol, respectively, and the 767
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recorded. The weight-average molecular weight of the polymer in this report27 was determined to be more than three times that which we report, and this difference likely affects the Tg values. There may be differences in tacticity as well because the polymer described here was prepared using RAFT polymerization and that reported27 used AIBN-initiated radical polymerization. Since the sulfone groups in the polymer side chains have a large dipole moment, dipolar interactions should occur among the polymer side chains, resulting in the high Tg values of PSMA and PMSEMA. We attribute the depression of Tg of PESMA to the longer terminal alkyl chain in the side chains, as a longer chain gives a larger volume fraction and makes the interaction among the side chains weaker, as is seen in poly(alkyl methacrylate) series.28,29 The bulkiness of polymer side chains is an additional factor that raises Tg for methacrylate polymers. It is reported that increasing the ratio of comonomers containing the bulky adamantyl and silsesquioxane groups in the side chain of poly(methyl methacrylate) (PMMA) copolymers raises the Tg, as the bulky molecules restrict the side-chain mobility.30−32 The sulfolane group in the side chain of PSMA is bulky compared to the other sulfone side chains in PMSEMA and PESEMA. This bulkiness and the large dipole moment of the sulfonyl moiety in sulfolane contribute to result in the highest Tg in the three polymers. The tacticity of the polymers, which we have not examined, may be a contributing factor as well. Figure 2a shows TGA results for the sulfone-containing polymers. There are two stages in the thermal degradation processes of all three polymers. The first and second degradation stages occur at ∼270 and ∼380 °C, respectively. The mass loss in the first degradation steps is close to the mass values of the side chains in the sulfone-containing polymers, indicating that the side-chain moieties are cleaved in the first stage of the degradation processes. To confirm this mechanistic detail, we recorded FT-IR spectra for the sulfone-containing polymers before and after heating at 300 °C for 30 min in a nitrogen atmosphere (Figure 2b). Before heating, the IR absorbance from the sulfone moieties was observed at ∼1310 (SO 2 , asymmetric stretching) and ∼1120 cm −1 (SO 2 , symmetric stretching) in each sulfone-containing polymer; these bands disappeared after heating. The carbonyl band at ∼1725 cm−1 was still observed after heating but shifted to
Table 1. Molecular Weight and Molecular Weight Distribution of Sulfone-Containing Polymers Determined by SEC sample
Mn (g/mol)
Mw (g/mol)
Mw/Mn
PSMA PMSEMA PESEMA
47500 37700 52900
54100 44500 68800
1.14 1.18 1.30
molecular weight distributions of all three sulfone-containing polymers was relatively narrow, less than 1.3. Sulfonecontaining polymers can be dissolved in some polar solvents, including DMF, DMSO, and trifluoroethanol (TFE) but not in other common polar and nonpolar solvents, including tetrahydrofuran (THF), acetone, methanol, and toluene. Figure 1 shows the DSC curves of the sulfone-containing polymers. The glass transition temperatures (Tg) of PSMA,
Figure 1. DSC thermograms of PEMA, PMSEMA, and PESEMA during second heating measured at a rate of 10 °C min−1 under nitrogen flow.
PMSEMA, and PESEMA are 188, 90, and 52 °C, respectively. PMSEMA has been described recently,27 and according to this report, the Tg is 109 °C, 19 °C higher than the value we
Figure 2. (a) TGA thermograms of PSMA, PMSEMA, and PESEMA measured at a rate of 10 °C min−1 under nitrogen flow. (b) FT-IR spectra of PSMA, PMSEMA, and PESEMA before (black line) and after (red line) heating at 300 °C for 30 min at nitrogen atmosphere. 768
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Langmuir ∼1750 cm−1. A peak at ∼1805 cm−1 is observed in the spectra of all three thermally treated sulfone-containing polymers. We attribute this to anhydride functionality that is present as a component of the carbonyl-containing structures.33,34 These observations led us to believe that the first degradation step involves the cleavage of the side chain in the sulfone-containing polymers. Given the presence of the carbonyl group after heating, we suggest that alcohol radicals (·OR) corresponding to the side chain in each sulfone-containing polymer are produced during the first degradation step. The degradation behavior of all three sulfone-containing polymers is similar; however, a subtle difference warrants comment. The mass loss in the first degradation step in PMSEMA and PESEMA was slightly greater than the mass of the side chain corresponding to the alcohol radicals, indicating that main chain degradation is competitive during this stage. The mass loss in the first degradation step of PSMA, however, was nearly identical with the side-chain mass, and a plateau between the first and second degradation stages is observed in the TGA curve. This indicates that main-chain degradation of PSMA is not significant during the first degradation step. Radicals produced in polymer degradation processes affect and promote degradation.35 Within the series of three sulfonecontaining polymers, the differing degradation behavior between PMSEMA, PESEMA, and PSMA can be rationalized based on the bulky structure of the sulfolane alcohol radical compared to the other sulfone radicals. Surface Properties of Sulfone-Containing Polymer Films. Thin sulfone-containing polymer films were prepared on silicon wafer substrates by spin coating from 1.0 wt % polymer solutions in TFE. The relative humidity during the spin coating influenced the surface roughness of the polymer films. When films were prepared at over 40% relative humidity, the surfaces were obviously not smooth as indicated by reflected light. To remove the humidity effect, we prepared all polymer films in a nitrogen atmosphere (water concn < 0.1 ppm). The thickness of the films was determined by elipsometry to be ∼50 nm. The surface composition of the polymer films was determined using XPS analysis at two different takeoff angles (15° and 75°) (Figure S2, Supporting Information). Experiments using these two conditions assess the composition of the outermost ∼1 nm and outermost ∼4 nm of the samples. The data for all three polymers indicate the presence of only the expected C, O, and S atoms in appropriate ratios. The absence of any pronounced takeoff angle dependence suggests that there is no preferred orientation of the polymer side chains at the polymer−vacuum interface. (Table S1, Supporting Information). The results of the advancing (θA) and receding (θR) water contact angle for the polymer films are summarized in Table 2. Significant hysteresis (θA − θR) was observed in all three
polymer films. Surface roughness and reorganization are generally implicated as the cause of hysteresis of this magnitude.36 As can be seen in the AFM height images shown in Figure 3, the root-mean-square surface roughness values (RMS) for all the polymer films were less than 0.5 nm in 5 × 5 μm2 scanning areas. Therefore, we attribute the observed hysteresis to surface reorganization during contact angle analysis. The sulfone functionality interacts with the probe fluid upon wetting and pins the receding contact line, decreasing the θR value. These receding contact angle values signify the water affinity of the sulfone moieties in these polymers. According to the receding contact angle values, the water affinity order of the sulfones in the polymer side chain is ethylsulfone (PESMA) < sulfolane (PSMA) < methylsulfone (PMSEMA). The methylsulfone and sulfolane surfaces exhibit similar and higher water affinity than the ethylsulfone surface. We also calculated the log P values of each sulfone monomer using the Molinspiration Property Calculation Service,37 as summarized in Table 2. This value permits comparisons in polarity from predictions of the octanol/water partition coefficient (P). The water affinity order indicated by the log P values agrees with the receding contact angle order. We note that the hydrophilicity of the sulfone surfaces, particularly PSMA and PMSEMA films, as assessed by contact angle analysis, is comparable to that of polyethylene glycol (PEG) monolayers prepared on silicon substrates.38,39 The difference in water affinity of the sulfone-containing polymers is reflected by their molecular structures. Figure 4 shows each sulfone monomer structure obtained with ab initio calculation methods. Due to the five-membered ring structure of sulfolane, the sulfone moiety in the monomer is exposed to the free surface in a manner similar to the functional group in the methylsulfone monomer. The sulfone moiety in the ethylsulfone monomer is slightly masked by the terminal ethyl group, resulting in low water affinity compared to the other monomers. Contact angle measurements were also made using methylene iodide, n-hexadecane, and toluene (Table 3). Differences in contact angle values among the sulfonecontaining polymer films were small, but very reproducible, and from our experience worthwhile commenting on. The advancing and receding contact angles of methylene iodide and n-hexadecane for the sulfolane surface are lower than corresponding data for the other sulfone surfaces. This suggests that the five-membered ring structures can orient with methylene groups exposed to the environment and the sulfone group pointing inward. The affinity order of toluene for the sulfone surfaces is the reverse of the other hydrophobic probes. Due to the high affinity of sulfone moieties for aromatic solvents,15 the contact angle values on the methylsulfone (PMSEMA) and ethylsulfone (PESEMA) surfaces for toluene are lower than the contact angle values for n-hexadecane on these surfaces, even though the surface tension of toluene (γ = 28.4 N/m) is greater than that of n-hexadecane (γ = 27.5 N/ m). The toluene contact angle values are higher than the nhexadecane values for the sulfolane (PSMA) surface. On the basis of these measurements, it is reasonable to conclude that the sulfonyl groups on the methylsulfone and ethylsulfone surfaces are more exposed than on the sulfolane surface in oleophilic environments. These results imply that the fivemembered ring structure of sulfolane is advantageous in providing not only hydrodrophilicity but also oleophilicity.
Table 2. Advancing (θA) and Receding (θR) Water Contact Angles in Degrees (deg) on Sulfone-Containing Polymer Films sample
θA
θR
θA − θR
miLog Pa
PSMA PMSEMA PESEMA
52 48 49
25 23 31
27 25 18
0.89 0.77 1.14
a
miLog P was Calculated by using the Molinspiration Property Calculation Service.1 769
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Figure 3. Typical AFM images on PSMA, PMSEMA, and PESEMA films.
that it can be prepared easily from the inexpensive butadiene sulfone. The sulfone-containing polymer films exhibit hydrophilic properties that depend on the sulfone structure in the polymer side chains. Owing to the five-membered ring structure of sulfolane, the sulfone moiety is exposed to the outside in a similar manner as the methylsulfone system, whereas the sulfone moiety of the ethylsulfone surface is slightly masked by the terminal ethyl group. For hydrophobic probes such as methylene iodide and n-hexadecane, the sulfolane surface shows high affinity to the probe fluids compared to the other sulfone surfaces, whereas the affinity order for toluene is the reverse of the other hydrophobic probes. This indicates that the sulfone moiety in sulfolane is masked by hydrophobic moieties in the five-membered ring structures at the hydrophobic interfaces. These results demonstrate that the sulfone functional group is a worthwhile target to functionalize biomaterials and that the properties of this moiety as a surface modifier can be related to the chemical structure of the sulfone.
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04265. 1 H and 13C NMR spectra of compound 1−4, PSMA, PMSEMA, and PESEMA. GPC chromatograms of PSMA, PMSEMA, and PESEMA. XPS survey spectra for sulfone-containing polymers and a summary of the XPS analysis (PDF)
Table 3. Advancing and Receding Contact Angle (θA/θR) in Degrees (deg) of Methylene Iodide (CH2I2), n-Hexadecane (HD), and Toluene on Sulfone-Containing Polymer Films sample
CH2I2 (γ = 50.8 N/m)
HD (γ = 27.5 N/m)
toluene (γ = 28.4 N/m)
PSMA PMSEMA PESEMA
24/8 29/11 29/11
11/8 14/11 14/11
14/11 12/11 12/11
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ASSOCIATED CONTENT
* Supporting Information
Figure 4. Energy-minimized structure of (1) SMA, (2) MSEMA, and (3) ESEMA calculated with ab initio method (HF/6-31 G*). Upper, middle, and lower rows are chemical, ball and stick, and spacefill structures, respectively.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
CONCLUSION We have prepared and studied three methacrylate polymers containing sulfolane, methylsulfone, and ethylsulfone in the polymer side chains. We find that the polymer with sulfolane in the side chains shows a high Tg compared to the other sulfonecontaining polymers due to the large dipole moment of the sulfone moiety and its bulky structure. During the thermal degradation process of the sulfone-containing polymers, alcohol radicals corresponding to the polymer side chains were produced during the first degradation process. The bulkiness of the sulfolane led to a lower reactivity of the radical derivative compared to the other sulfone systems. We want to emphasize the high Tg of the sulfolane-containing polymer (188 °C) and
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Gelest and the NSF-supported Center for Hierarchical Manufacturing (CMMI-1025020) and Materials Research Science and Engineering Center (DMR-0820506) at the University of Massachusetts for support.
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REFERENCES
(1) Chen, S.; Li, L.; Zhao, C.; Zheng, J. Surface hydration: Principles and applications toward low-fouling/nonfouling biomaterials. Polymer 2010, 51, 5283−5293.
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Langmuir
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DOI: 10.1021/acs.langmuir.5b04265 Langmuir 2016, 32, 765−771
