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Effect of Charged Group Spacer Length on Hydration State in Zwitterionic Poly(sulfobetaine) Brushes Yuji Higaki,†,‡,§,∥ Yoshihiro Inutsuka,§ Tatsunori Sakamaki,§ Yuki Terayama,§ Ai Takenaka,∥ Keiko Higaki,‡ Norifumi L. Yamada,⊥ Taro Moriwaki,# Yuka Ikemoto,# and Atsushi Takahara*,†,‡,§,∥ †
Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan § Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project, CE80, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ⊥ Neutron Science Laboratory, High Energy Accelerator Research Organization, Ibaraki 319-1106, Japan # Japan Synchrotron Radiation Research Institute/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ‡
S Supporting Information *
ABSTRACT: Effect of alkyl chain spacer length between the charged groups (CSL) in zwitterionic poly(sulfobetaine) (PSB) brushes on the hydration state was investigated. PSB brushes with ethyl (PMAES), propyl (PMAPS), or butyl (PMABS) CSL were prepared by surface-initiated atom transfer radical polymerization on silicon wafers. Hydration states of the PSB brushes in aqueous solutions and/or humid vapor were investigated by contact angle measurement, infrared spectroscopy, AFM observation, and neutron reflectivity. The PSB brushes are swollen in humid air and deionized water due to the hydration of the charged groups leading to the reduction of hydrated PSB brushes/water interfacial free energy. The hydrated PSB brushes exhibit clear interface with low interfacial roughness due to networking of the PSB brush chains through association of the SBs. The hydrated PSB brushes produce diffusive swollen layer in the presence of NaCl because of the charge screening followed by SB dissociation by the bound ions. The ionic strength sensitivity in the hydration got more significant with increasing the CSL in SBs because of the augmentation in partial charge by charged group separation.
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INTRODUCTION Zwitterionic betaines are ubiquitous in organisms and are found in cell membrane, proteins, and osmolytes.1 Polymers with betaine pendant groups have unique characters such as biocompatibility,2−5 lubrication,6−9 and antifouling10−15 in the hydrated state. Because the betaines consist of cationic and anionic charged groups linked with an alkyl chain spacer, they associate through electrostatic interaction of the cationic and anionic groups. The betaine-association force and the ionic strength dependence associate with the identity of the charged groups and alkyl chain spacer length between the charged groups (CSL).16−23 The electrostatic association of betaines causes inverted ionic strength dependence in solubility to polyelectrolytes, a so-called antipolyelectrolyte effect.24−26 Net positive polycations and net negative polyanions are wellhydrated in deionized water, and the chain dimension expands because of the electrostatic repulsive force, whereas they shrink under high ionic strength by a charge screening effect of the bound ions. Poly(sulfobetaine) (PSB)s, a polybetaine family with sulfobetaine (SB) pendant groups that consist of a quarternary ammonium cation and a sulfonate anion, aggregate in deionized water by the SB pairing, whereas they dissolve in © 2017 American Chemical Society
salt solution owing to the dissociation of the SB pairing by the charge screening effect. 2 7 − 3 4 Meanwhile, poly[2(methacryloyloxy)ethyl phosphorylcholine] (PMPC) dissolves in aqueous solutions regardless of the ionic strength and have weak ionic strength dependence in the solubility and chain conformation.35 Jiang et al. demonstrated by molecular dynamics simulation that the betaine-pairing force associates well with the charge-density contrast between the cationic and anionic groups.21,23 The symmetric charge densities in SBs result in the strong association, while the unbalance charge densities in carboxybetaines (CB) lead to the weak association. They also reported the effect of CSL in CB for the electrostatic potential, hydration, and the degree of association with Na+.22 The simulation indicated that the CB molecules with short CSL are less charged and have low hydration free energy due to the charge interplay. The cohesive interaction of CBs with Na+ also depends on the CSL, and the CB molecules with longer CSL showed large interaction with Na+. Received: June 15, 2017 Revised: July 23, 2017 Published: July 24, 2017 8404
DOI: 10.1021/acs.langmuir.7b01935 Langmuir 2017, 33, 8404−8412
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measurements. Water uptake into the PSB brushes and hydrogen bonding network structure of the hydrated water were explored by infrared (IR) spectroscopy under water vapor. The thickness and the volume fraction profiles in the dried and swollen states were figured out complementarily by atomic force microscope (AFM) observation and NR. We show that the CSL in SBs associates with the hydration states of the PSB brushes, and the impact of ionic strength to the hydration state is well consistent with the ion-binding efficiency of SBs modulated by the partial charge of the charged groups.
Recent progress in polymer brush preparation via surfaceinitiated controlled radical polymerization affords well-defined charged polymer brushes with precise molecular weight control.36−39 The charged polymer brushes with high graftdensity are well-hydrated, and the surface-tethered chains are extended by the excluded volume effect and osmotic pressure.39−41 The excellent antifouling and lubrication properties of zwitterionic polymer brushes have been reported.8−13,15 The hydration states of the zwitterionic polymer brushes has been regarded as a crucial factor behind the antifouling and lubrication performances because the strongly hydrated water prevents the adhesion of foreign substances. But the hydration states of the charged polymer brushes have been poorly understood yet. Liu et al. reported an ion-specific hydration state of a wide variety of charged polymer brushes by use of quartz crystal microbalance with dissipation (QCM-D).42−45 The ion-specific weight loss and dissipation factor change in the charged polymer brushes under aqueous solutions were demonstrated. The effect of CSL in the SB groups was also examined, and they demonstrated that the amount of hydrated water and the ionic strength sensitivity depends on the CSL.45 However, the clear insight on the polymer density profile in the hydration layer was not elucidated. Not only the degree of hydration but also the hydrogen bonding network structure of waters in hydrated charged polymer brushes is of wide interest, and still under debate.46−50 Neutron reflectivity (NR) is a powerful tool to shed light on the hydration states of polymer brushes at liquid interface.51 Because the large difference between the scattering length density (SLD) of hydrogen and deuterium provides contrast between hydrogenated and deuterated species, the SLD profile of the hydrogenated polymer brushes in contact with deuterated liquids is obtained with high resolution. The specular NR affords not only the total thickness of the swollen brush but also the polymer volume fraction profiles normal to the interface.52−57 Dunlop et al. tackled the heterogeneous chain dimension analysis of poly(2-(methacryloyloxy)ethyltrimethylammonium chloride) (PMTAC) brushes by NR.52 They demonstrated that the hydrated PMTAC brushes adopt a two-layer model smoothed with stretched parabolic function that consist of a dense layer at the substrate-side interface and a diffuse extended brush layer. Genzer et al. reported the swelling behavior of charged polymer brushes under water vapor by NR, X-ray reflectivity, and ellipsometry over a wide range of relative humidity (RH).53 PSB brushes showed complex water uptake stages along with the RH and the behavior associated with the SB pairing state depending on the grafting-density. The ionic strength dependent chain dimension in PMTAC and PSB brushes and the ionic strength-inert chain dimension in PMPC brushes were demonstrated.54 However, the effect of CSL in PSB brushes on the swollen polymer density profile and the ionic strength dependence have not been explored. In this paper, effect of CSL in PSB brushes on hydration states and the ionic strength dependence were investigated. PSB brushes with ethyl [poly(3-(N-2-methacryloyloxyethylN,N-dimethyl) ammonatoethanesulfonate), PMAES], propyl [poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl) ammonatopropanesulfonate), PMAPS], and butyl [poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl) ammonatobutanesulfonate), PMABS] alkyl chain spacers were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) on silicon wafers. The wetting behavior was elaborated by contact angle
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EXPERIMENTAL SECTION
Materials. Thin plate silicon wafers (single side polished, 40 mm length × 10 mm width × 0.5 mmt) and thick silicon blocks (single side polished, 50 mm length × 20 mm width × 8 mmt) were used as polymer brush substrates. 3-(N-2-Methacryloyloxyethyl-N,N-dimethyl) ammonatoethanesulfonate (MAES), 3-(N-2-methacryloyloxyethylN,N-dimethyl) ammonatopropanesulfonate (MAPS), and 3-(N-2methacryloyloxyethyl-N,N-dimethyl) ammonatobutanesulfonate (MABS) monomers were prepared following the previous reports.58 PMAES, PMAPS, and PMABS brushes were prepared on the silicon wafers through SI-ATRP according to previously reported procedures.59 Chemical structures of the polymer brushes are shown in Figure 1. Milli-Q water (Millipore Inc., Billerica, MA) with a resistance
Figure 1. Chemical structure and abbreviation of PSB brushes. above 18.2 MΩ cm was used throughout the study. Deuterium oxide (Merck, 99.96%, Darmstadt, Germany) and sodium chloride (Kanto Chemical, Molecular biology grade, Tokyo, Japan) were used as received. Measurement. Characteristics of the polymer brushes used in this paper are summarized in Table 1. We adopted two series of PSB brushes, low molecular weight (thin brush) series and high molecular weight (thick brush) series. The NR experiments were carried out by the low molecular weight series because the thickness range (thickness in ambient condition: 20−30 nm) is preferable to obtain clear Kiessig
Table 1. Characteristics of PSB Brush Samples polymer brushes
Mna
Mw/Mna
thicknessb (nm)
graft densityc (chains nm−2)
PMAESd PMAESe PMAPSd PMAPSe PMABSd PMABSe
157 000 605 000 156 000 326 000 99 600 243 000
1.16 1.17 1.33 1.20 1.22 1.32
27 132 26 70 34 107
0.14 0.18 0.14 0.17 0.27 0.35
a
Mn and Mw/Mn were determined by SEC equipped with a refractive index detector. The molecular weights were calibrated with the absolute molecular weight of PMAPS obtained by a SEC-MALS (multiangle light scattering) measurement. bThickness was estimated by ellipsometry under ambient condition (relative humidity: ca. 40% RH) at 298 K. The refractive index of PMAPS, 1.50,59 was applied to the calculation. cGraft density, σ, calculated from the Mn and thickness (L) using equation σ = ρLNA·10−21/Mn, where ρ is the bulk density of the free (unbound) polymer and NA is Avogadro’s constant. The bulk density of PMAPS, 1.34 g mol−1,59 was applied to the calculation. d Samples for NR measurement. eSamples for AFM observation, IR, and contact angle measurements. 8405
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SLDs of Si, SiO2, and D2O were set to 2.07 × 10−4, 3.47 × 10−4, and 6.36 × 10−4 nm−2, respectively.
fringes in the reflectivity, and the slight structure variation is able to be acquired from the drastic change in the fringed reflectivity profile. The AFM observation, IR measurement, and contact angle measurements were carried out by the high molecular weight series (thickness in ambient condition: 100−150 nm) because the thick polymer brushes are preferable to measure the thickness variation from the topographic images, and the thick hydrated layer is necessary to acquire enough IR absorption of water in the swollen layer. Contact Angle Measurement. Contact angles of water droplet were recorded with a droplet placing and droplet shape analysis system equipped with a video camera (Attension Theta; Biolin Scientific AB, Stockholm, Sweden). RH and ambient temperature were conditioned approximately 40%, and 298 K, respectively. Advancing contact angle (θA) and receding contact angle (θR) were measured by expansion/ shrinkage method.60 Contact angle of a captive air bubble was measured by placing an air bubble (10 μL) using a microsyringe with curved tip on the polymer brush substrate facing downward in a chamber with a transparent glass window filled with Milli-Q water. The contact angle of the air bubble was calculated similarly with the water droplet by the reversed bubble image. Temperature is controlled at 298 K by circulating water to the chamber jacket. Infrared Spectroscopy. Microscopic IR spectroscopy was conducted at BL43IR beamline of SPring-8 (Hyogo, Japan) with a Fourier transform IR microscope system (VERTEX 70 and HYPERION 2000, Bruker) equipped with a MCT detector. A fine-focused IR beam from the synchrotron radiation was narrowed to 20 × 20 μm by apertures. A polymer brush sample was installed into a homemade humid vapor flow cell with a BaF2 window (illustration and picture of the vapor flow cell are provided in Figure S1). The humid vapor passing gap was adjusted by a silicone rubber sheet spacer to be 1 mm. The RH of the flow gas was controlled by humidity control apparatus HUM-1 (RIGAKU, Tokyo, Japan). The temperature and humidity were monitored at the gas outlet by a small-sized data logger. IR spectrum was recorded in transmission mode with resolution of 4 cm−1 and 512 times integration within 90 s under successive humidity ascent in 1.0% min−1. The humid vapor temperature was kept in a range of 27.5 ± 0.2 °C. AFM Observation. AFM images were observed using a scanning force microscope installed with environment control scanner (Cypher ES; Asylum Research, Santa Barbara, CA). Topographic images were obtained in contact mode at 298 K with a back-side Au-coated silicon nitride triangular cantilever (OMCL-TR800PSA, Olympus Corporation, Tokyo, Japan; spring constant, 0.57 N m−1; tip radius, less than 20 nm) under ambient (298 K, approximately 40% RH) or aqueous solutions. The ambient and swollen thicknesses were determined from a height gap at a scratch track on the polymer brush. The scratch track was introduced by scratching with a sharp needle. Neutron Reflectivity. NR was conducted using a reflectometer (BL16 SOFIA, Materials and Life Science Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), Tokai, Japan) providing a 25 Hz pulsed neutron beam.51 The wavelength (λ) of the incident neutrons was selected to around 0.20−0.88 nm using a disk chopper. The reflected neutrons were recorded by a twodimensional position-sensitive scintillation detector. The neutron momentum transfer vector is defined by q = (4π/λ)sin θ, where 2θ is the specular reflection angle with respect to the polymer brush interface. A silicon block with polymer brush (20 mm × 50 mm × 8 mmt) was covered with a quartz trough filled with pure D2O or NaCl D2O solutions to produce a polymer brush/liquid interface. A Viton rubber frame was sandwiched between the trough and silicon block, and the set was fixed with aluminum plates to prevent the solution leakage. Neutron beam was incident from downward of the sample cell in a temperature-controlled chamber at 298 K and was reflected at the polymer brush/liquid interface to exit in a downward direction. A 12 mm width and 30 mm transverse footprint area was adjusted by a slit width. NR of the air/polymer brush interface was performed under dry nitrogen gas flow in a temperature control chamber at 298 K, and the neutron beam was incident from upward (air side). The MOTOFIT program was applied to fit the reflectivity profiles to the model SLD profiles composed of thickness, SLD, and Gaussian roughness.61 The
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RESULTS AND DISCUSSION Solubility Test. PMAPS is a well-known thermoresponsive polymer that the aqueous solution exhibits upper critical solution temperature (UCST) at around 310 K. Schulz et al. reported comprehensive experiments on the solubility of PMAPS to water, and they demonstrated that the UCST depends on the molecular weight and polymer concentration.62 The solubility of PMAES, PMAPS, and PMABS to Milli-Q water were examined (Figure S2). The UCSTs of the 1.0 wt % Milli-Q water solution were 288 K in PMAES and 312 K in PMAPS, whereas PMABS shows no UCST below 333 K. The solutions were opaque below UCST because the PSB chains aggregate through electrostatic association of the SBs. The dissolution of PSB chains is triggered by dissociation of the SB pairing. The PSB aggregates provide coarse particles enough to cause light scattering to make the solution opaque. Thermal activation causes dissociation of the SB pairings, resulting in the dissolution in water. The solubility of the PSBs in water suggests the order of the association efficiency of the SB groups, so that the UCST order of PMAES < PMAPS < PMABS would correspond to the aggregation force of the SBs. The solubility is also sensitive to ionic strength. Addition of NaCl to the aqueous solution promotes the solubility of the PSBs. The sodium cations and chloride anions are bound to the sulfonate anion and quarternary ammonium cation in SB groups, respectively. Localization of ions to the SBs in PSB chains induces charge screening effect in the SBs to disengage the pairings leading to dissolution of the PSBs. The bound ion effect exists in the all PSBs, and the all PSBs dissolved in 100 mM NaCl solution. NaCl concentration for the dissolution of PSBs follows the order PMABS > PMAPS > PMAES, while PMAES dissolves in deionized water at the test temperature. The critical [NaCl] would associate with both the association strength and ion-binding efficiency of the SBs. Chain dimension analyses of the isolated single PSB chains in the dilute solutions through light and X-ray scattering measurements are required to figure out the detailed ionic strength sensitivity of the chain conformation. NaCl concentration dependence in the chain stiffness and excluded volume strength of PMAPS chains in NaCl aqueous solutions were already reported. PMAPS chains have theta NaCl concentration at 74 mM, and the radius of gyration expands with increasing [NaCl] due to the excluded volume effect by long-range repulsive interaction.28 The chain characteristics of PMAES and PMABS are worth understanding the hydration state of polymer brushes, and it would be plausible extension of this work. Wettability. Aggregation state of the SBs associates with the wetting behavior of the PSB brushes because the solubility of the PSB chains relates with the affinity of PSB chains to water. Water droplets spread on PMAES and PMAPS brushes immediately after the attachment, whereas water droplets on PMABS brushes slowly spread on the surface. Hydration of the PSB brushes induces the chain rearrangement, and the hydrated PSB brushes reduce the interfacial tension leading to the advance of the contact line. PMABS brushes exhibited large advancing contact angle of water droplet, whereas water droplet immediately spread on the other PSB brushes (Table 2, Figure S3). All the PSB brushes showed extremely low receding water droplet contact angles below 10° and identical large air bubble contact angle in Milli-Q water (Table 2, Figure S4). The large 8406
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water was hardly observed in the case of pristine Si water, while the spiky absorptions assigned to water vapor were observed over 3800 cm−1, so that the O−H stretching absorptions of liquid water observed in the IR spectra of PSB brushes were derived from the hydrated waters. The water vapor absorptions were subtracted from the raw data by using the IR spectrum of pristine Si wafer to give the background-subtracted IR spectra shown in Figure 2. The O−H stretching absorption intensity (i.e., the volume of water uptake) depends on the PSB brush thickness, so that the simple comparison of the absorption intensity is meaningless. All PSB brushes uptake water under high humidity, and the water uptake became significant over 60% RH. This result suggests that the water uptake behavior is identical in the PSB brushes, so that it is independent of the CSL, whereas the UCST of the PSB aqueous solutions exhibit significant dependence on the CSL. This is well consistent with the finding from the contact angle measurement. The hydration state of the PSB brushes is further explored by AFM observation and NR in the following sections. The IR absorption was deconvoluted by two Gaussian peaks at around 3450 cm−1 (Amid) and 3250 cm−1 (Alow), and the peak shift of the Amid was tracked with respect to the RH (Figure 2d, a typical peak deconvolution result was shown in Figure S5). The Amid peak wavenumber almost remained unchanged in all PSB brushes, and CSL dependence was hardly observed in the peak wavenumber and the RH dependence. The shoulder peak at the lower wavenumber side (Alow) also
Table 2. Dynamic Contact Angles of Water Droplets and Static Contact Angle of Air Bubble in Milli-Q Water on the PSB Brushes PMAES PMAPS PMABS
θAa (deg)
θRb (deg)
θbubblec (deg)
28 ± 5.5 46 ± 6.3 70 ± 1.1
7 ± 1.3 8 ± 1.5 13 ± 4.5
174 ± 2.6 173 ± 2.2 173 ± 1.4
a
Advancing contact angles of water droplets. bReceding contact angles of water droplets. cStatic contact angles of air bubbles in Milli-Q water.
advancing contact angle in PMABS brushes is attributed to the slow dynamics of the hydration in PMABS brushes. The hydrated PSB brushes/water interfacial tension are identical regardless of the CSL as shown in the receding contact angles of water droplet and air bubble contact angle in water. The wetting phenomena indicate that the swollen state of PSB brushes in contact with water would be identical and unconnected with CSL. IR Measurement. Humidity-dependent water uptake in the PSB brushes and hydrogen-bonding network structure of the hydrated water were explored by O−H stretching absorption in the IR spectra. The IR absorptions at around 3250 cm−1, 3450 cm−1, and 3600 cm−1 have been assigned to waters with highly structured, partially distorted, nearly isolated hydrogen-bonding network structure condition.63,64 Figure 2 shows IR spectra of the swollen PSB brushes under various humidity. The IR absorption assigned to O−H stretching vibration of liquid
Figure 2. IR spectra of hydrated (a) PMAES, (b) PMAPS, and (c) PMABS brushes under various relative humidity (10, 20, 30, 40, 50, 60, 70, and 80% RH). The vapor temperature was controlled to 27.5 ± 0.2 °C. (d) Evolution of the peak wavenumber of the O−H stretching vibration absorption assigned to waters with distorted hydrogen bonding network structure (Amid, see Figure S5) for PMAES (1, red ●), PMAPS (2, blue ▲) and PMABS (3, black ■). The lines were provided for the ease of tracking. 8407
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swollen thickness in Milli-Q water is shown in Figure 3c. All the PSB brushes were swollen with water even in Milli-Q water without salts, although water is a poor solvent for the PMAPS and PMABS at 298 K. The thickness increased approximately 1.6 times the thickness in ambient regardless of the CSL. All the PSB brushes showed similar [NaCl] dependence in the swollen thickness except for the threshold [NaCl] for the drastic swelling. The swollen thickness increased with increasing [NaCl]. PMABS brushes showed drastic swollen thickness increase at 100 mM [NaCl], whereas PMAES brushes caused relatively weak thickening at the [NaCl]. This result is well consistent with the preceding study of PSB brush hydration by QCM-D.45 The ionic strength responsive water uptake was significant in PSB brushes with long CSL SBs. The low threshold [NaCl] in the drastic swelling of PMABS brushes indicates that the association of PMABS brushes is more sensitive to the ionic strength than PMAES and PMAPS brushes. At the extremely high [NaCl] of 1000 mM, the PSB brushes showed identical normalized swollen thickness regardless of the CSL. It should be noted that the AFM images were taken by contact mode under liquid, and the cantilever tip may penetrate into the soft low-density polymer brush layer to some extent. Therefore, the relatively flat plane of the hydrated PSB brush layer was observed as topographic images, and the thicknesses would be underestimated.39 The height gap data has large error due to the tip penetration. More than ten height gap data were measured to ensure the statistical accuracy. Although the error range was wide, the ionic strength dependence in the averaged PSB brush thickness seems to be reliable because the change in error range along with the NaCl concentration corresponded with the averaged thickness. The AFM results are further discussed along with the NR results. Neutron Reflectivity. NR provides detail insights for the hydration structure of the PSB brushes including the polymer density profiles normal to the substrate. Figure 4 shows NR curves of PSB brushes under dry nitrogen, D2O, and NaCl D2O solutions with fitting curves based on the model SLD profiles. The SLD profiles were obtained by curve fitting of the NR curves on the basis of a layered model structure. A two-layer SLD model consists of a SiO2 layer, and a polymer brush layer was applied to the polymer brush/air interface. The NR profile with clear Kiessig fringes in a wide q range was obtained in all the PSB brushes and successfully fitted with a single box model with Gaussian roughness. Damping of the fringes at high q range indicates the large-scale lateral inhomogeneity in the layered structure over the beam footprint area. The thicknesses in ambient atmosphere were yielded from the NR analysis to be 26.4, 30.5, and 31.5 nm for PMAES, PMAPS, and PMABS brushes, respectively. The thicknesses were well consistent with thickness measured by ellipsometer (see Table 1). The SLDs of PMAES, PMAPS, and PMABS were determined by the NR analysis to be 0.84 × 10−4 nm−2, 0.84 × 10−4 nm−2, and 0.85 × 10−4 nm−2, respectively. The SLDs estimated from the NR analysis disagree with calculated SLDs probably due to the presence of hydrated water remaining even under dry nitrogen flow field. The NR profiles of PSB brushes under salt-free pure D2O exhibited clear Kiessig fringes, and the decay slope was kept q−4 in all the PSB brushes. A three-layer model consisting of a SiO2 layer, a substrate interface layer, and a swollen brush layer was applied to the SLD model. The polymer volume fraction (VF) Φ(z) profiles were calculated by the following simple equation.54,56
kept the peak wavenumber independent of the RH. The hydrogen-bonding configuration of hydrated water in charged polymer brushes has been elaborated by various spectroscopic analyses, and it has not concluded yet.46−49 Red shift of the O− H stretching IR absorption peak and the increase in the integration ratio of Alow/Amid suggest the growth of the hydrogen-bonding network structure. However, the wavenumber of O−H stretching absorption peak and the Alow/ Amid integration ratio were unchanged along with RH in all PSB brushes (Figure 2d), indicating the hydrogen-bonding network structure of the hydrated water is independent of the RH. The CSL seems to have negligible effect in the hydrogen-bonding network structure of hydrated water. Since the net charge of the zwitterionic SBs is almost neutral, the hydrated waters in PSB brushes produce a hydrogen-bonding network structure similar to the bulk state without perturbation through electrostatic interaction with SBs. Although the separation of the charged groups in SBs changes the partial charge, the charged state hardly associates with the hydrogen-bonding network of the hydrated water in the PSB brushes. AFM Observation. The thicknesses of PSB brushes in ambient and swollen states were determined by height gap at a PSB brush region and substrate in the AFM topographic image (Figure 3, panels a and b). The [NaCl] dependence on the swollen thickness in NaCl aqueous solutions normalized by
Figure 3. AFM topographic images of the PMAPS brushes under (a) ambient atmosphere and (b) 100 mM NaCl solution. The lower panels are the line profiles along the dot lines in upper panels. (c) Normalized swollen thickness of the PSB brushes (left red column: PMAES, center blue column: PMAPS, right gray column: PMABS) in 10 mM, 100 mM, and 1000 mM NaCl aqueous solutions. The swollen thickness was normalized with the thickness in Milli-Q water. 8408
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Figure 4. NR curves of (A) PMAES, (B) PMAPS, and (C) PMABS brushes under (a) dry nitrogen gas, (b) D2O, (c) 10 mM, (d) 100 mM, and (e) 1000 mM NaCl D2O solutions. Open symbols are experimental data points. Solid curves are calculated reflectivity curves based on model SLD profiles. NR intensities are offset by two decades for the sake of clarity.
Figure 5. Volume fraction profiles of (a) PMAES, (b) PMAPS, and (c) PMABS brushes under dry nitrogen gas, D2O, and NaCl D2O solutions calculated from the SLD profiles following eq 1.
ρN (z) = ϕ(z)ρN,PSB + [1 − ϕ(z)]ρN,D2O
exhibited weak progress in swelling at 100 mM. This result is well consistent with AFM observation and implies that the ionic strength sensitivity of the SB association corresponds to the CSL in SBs. Because the SBs consist of a negatively charged sulfonate anion and a positively charged quarternary ammonium cation, the PSB brushes are well hydrated by local hydration of the charged groups even in deionized water. The hydrated waters around the charged groups promote the excluded-volume effect of the hydrated chains leading to the thickening of the PSB brushes. Because of the concentration gradient in the PSB brushes and bulk water phase, the hydration occurs spontaneously by osmotic pressure. However, the electrostatic inter- and/or intrachain associations of the SBs produce the network of the PSB brushes to prevent the extension of the swollen diffusive layer in salt-free water. The SB pairings are supposed to dissociate in the presence of ions because of the charge screening by the bound ions. The PSB brushes are released from the network as a result of the SB dissociation to produce thick swollen diffusive layer. Molecular weight distribution in the PSB brushes also induces the significant extension of the polymer fraction. The excluded volume of PSB brushes increases with increasing [NaCl] by the long-range electrostatic repulsive interactions resulting in the thickening.28 The contrast in the ionic strength sensitivity in CSL of PSB is attributed to the ion-binding efficiency of SBs modulated by the charge interplay and partial charges. The weak interaction
(1)
where the ρN(z), ρN,PSB, and ρN,D2O are the SLD at a position(z), SLD of the PSB brush, and SLD of D2O, respectively. Figure 5 shows the polymer VF profiles as a function of the perpendicular distance from the edge of the SiO2 layer. The thickness increased about twice accompanied by reduction in the polymer VF by hydration of the PSB brushes. As seen in the clear Kiessig fringes in the reflectivity, the hydrated PSB brush layer exhibited clear interface with low interfacial roughness under D2O and 10 mM NaCl D2O solution. The CSL of the SBs hardly associate with the degree of swelling in the salt-free D2O, whereas solubility of the PSBs to water obviously depends on the CSL. The fringes smeared at high [NaCl], and the hydrated PSB brushes produced diffusive swollen layer. The polymer VF profiles consist of a single low VF layer with diffusive tail followed by the adsorbed layer. Multilayered structure models were not required to fit the reflectivity in contrast with previously reported PNIPAM brushes that consist of bilayer structure produced by vertical phase separation in the transition state.56 The diffusive decaying profile indicates that the extremely low polymer VF layer is produced at the D2O interface. All the PSB brushes showed similar behavior except for the threshold [NaCl] for the drastic hydration. PMABS brush showed drastic swelling and extension of the diffusive swollen layer at 100 mM, whereas PMAES and PMAPS brushes 8409
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Figure 6. Schematic representation for the hydration states of PSB brushes and the effect of CSL in the SB groups.
All the above discussions on the hydration states of PSB brushes with different CSLs are illustrated in Figure 6.
between the less charged carboxylate group in short CSL CBs and Na+ was demonstrated by quantum chemical calculations and molecular dynamics simulations.22 CBs with longer CSL preferentially associated with Na+ nearly five times than CBs without the alkyl chain spacer. If the similar charge interplay effects exist in SBs, the SBs with long CSL exhibit large partial charges to have strong interactions with sodium cations and chloride anions. The preferential interaction between the PMABS and ions would result in the low threshold [NaCl] for drastic swelling in PMABS brushes. Although the threshold [NaCl] for the drastic hydration of PSB brushes decreased with increasing the CSL, the PMAES brushes showed more diffusive swollen layer than PMAPS brushes at 100 mM [NaCl]. The drastic hydration would associate with the loss of network structure through the nearly complete sulfobetaine dissociation. As seen in the solubility to water, the association efficiency of PMAES is less than PMAPS and PMABS. Therefore, PMAES brushes show thicker diffusive swollen layer than PMAPS and PMABS brushes at low [NaCl] because the PMAES brushes produce more dangling chains, those which are not involved in the network, than PMAPS and PMABS brushes. Meanwhile, PMAES brushes show high threshold [NaCl] for drastic hydration because of the low ionic strength sensitivity of the short CSL SB. The SBs with a long flexible alkyl chain spacer implies the preferential formation of intrabetaine pairing (backbiting) structure.20 The intrabetaine pairing cyclic structure requires bending of the alkyl chain in the sulfobetaine for the engagement of the charged groups. The conformational entropy loss by chain folding and excluded volume expansion around the charged groups by hydrated waters would make the cyclic intrabetaine coupling unfavorable, leading to the extended form of SBs.16,19 Therefore, it is reasonable to assume that the ion-binding efficiency of SBs is dominated by the partial charge modulated by the charge separation with an alkyl chain spacer rather than the preferential intrabetaine pairing conformation. A thin high polymer VF layer at substrate interface (z < 2 nm) was required to fit the broad single peak at high q range in the reflectivity. The interface layer has been recognized in previous work of NR studies for polyelectrolyte brushes, and it was associated with the less swollen fraction that has strong attractive interaction with the SiO2 substrate.56 The adsorbed layer thickness was subtle compared to those found in polyelectrolyte brushes.52 The net neutral zwitterionic PSB brushes would have weak electrostatic interaction with solid substrate to exhibit thin less-swollen solid−substrate interface layer, and the solid−substrate interface layer thickness hardly associates with the CSL in SBs.
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CONCLUSIONS Effect of CSL in zwitterionic SBs on the hydration states of PSB brushes was investigated. PSB brushes were well hydrated, and the hydrated PSB brushes/water interfacial free energy was significantly reduced under water regardless of the CSL. The PSB brushes sucked up water with increasing RH, and the hydrogen bonding network structure of the hydrated water was independent of the RH and CSL. The PSB brushes are well hydrated in deionized water due to the hydration of charged groups, whereas the hydrated PSB brush layer exhibits clear interface with low interfacial roughness because of networking of the PSB brush chains through association of the SBs. The hydrated PSB brush produced diffusive swollen layer in the presence of NaCl because of the charge screening followed by SB dissociation by the bound ions. The ionic strength sensitivity in the hydration got more significant with increasing the CSL because of the augmentation in partial charge by charged group separation. The variation in hydration states of PSB brushes depending on the CSL and ionic strength is essential for the design of antifouling and well-lubricating hydrated soft interfaces applied in a physiological condition and the marine field.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01935. Illustrations of the vapor flow cell for IR measurement; appearance of PSB solutions; side view of water droplets on the PSB brushes during dynamic contact angle measurement; side view of air bubbles on the PSB brushes in Milli-Q water; a typical peak deconvolution of O−H stretching IR absorptions (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Atsushi Takahara: 0000-0002-0584-1525 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 8410
DOI: 10.1021/acs.langmuir.7b01935 Langmuir 2017, 33, 8404−8412
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Langmuir Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. Part of this work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan). This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. FT-IR measurements were performed at BL43IR (2013B1177, 2015B1313, 2016A1329, and 2016B1703) in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). NR measurements were performed on BL16 in Materials and Life Science Facility (MLF), J-PARC, Japan (program nos. 2009S08 and 2014S08). We gratefully acknowledge Y. Harada, K. Yamazoe, and Y. Cui (Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo) for kind support for humidity controlled IR measurements in BL43IR.
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ABBREVIATIONS SB:sulfobetaine; CB:carboxybetaine; PSB:poly(sulfobetaine); CSL:charged group spacer length; NR:neutron reflectivity; RH:relative humidity; SLD:scattering length density
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