Article pubs.acs.org/Langmuir
Nanostructure and Salt Effect of Zwitterionic Carboxybetaine Brush at the Air/Water Interface Hideki Matsuoka,*,† Yuta Yamakawa,† Arjun Ghosh,† and Yoshiyuki Saruwatari‡ †
Department of Polymer Chemistry, Kyoto University, Katsura, Nishikyo, Kyoto 615-8610, Japan Osaka Organic Chemical Industries Ltd., 7-20 Azuchi-Machi, 1-Chome, Chuo-ku, Osaka 541-0052, Japan
‡
ABSTRACT: Zwitterionic amphiphilic diblock copolymer, poly(ethylhexyl acrylate)-b-poly(carboxybetaine) (PEHA-bPGLBT), was synthesized by the reversible addition− fragmentation chain transfer (RAFT) method with precise control of block length and polydispersity. The polymers thus obtained were spread onto the water surface to form a polymer monolayer. The fundamental property and nanostructure of the block copolymer monolayer were systematically studied by the surface pressure−molecular area (π−A) isotherm, Brewster angle microscopy (BAM), and X-ray reflectivity (XR) techniques. The π values of the monolayer increased by compression in relatively larger A regions. After showing a large plateau region by compression, the π value sharply increased at very small A regions, suggesting the formation of poly(GLBT) brush formation just beneath the water surface. The domain structure of μm size was observed by BAM in the plateau region. XR profiles for the monolayer at higher surface pressure regions clearly showed the PGLBT brush formation in addition to PGLBT carpet layer formation under the hydrophobic PEHA layer on the water surface, as was observed for both anionic and cationic brush layer in the water surface monolayer studied previously. The critical brush density, where the PGLBT brush is formed, was estimated to be about 0.30 chains/nm2 for the (EHA)45-b-(GLBT)60 monolayer, which is relatively large compared to other ionic brushes. This observation is consistent with the fact that the origin of brush formation is mainly steric hindrance between brush chains. The brush thickness increased by compression and also by salt addition, unlike the normal ionic brush (anionic and cationic), whose thickness tended to decrease, i.e., shrink, by salt addition. This might be a character unique to the zwitterionic brush, and its origin is thought to be transition to an ionic nature from the almost nonionic inner salt caused by salt addition since both the cation and anion of the GLBT chain obtained counterions by the addition of salt. This stretching nature of the PGLBT brush depends on the ion species of the salt added, and it followed the Hofmeister series, i.e., more stretching in the order of Li+ > Na+ > K+. However, it was rather insensitive to the anion species (Cl−, Br−, SCN−), which suggests that the carboxylic anion has a more dominant effect than the quaternized cation in GLBT although the former is a weak acid and the latter is believed to be a strong base.
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structures, occurs by changing brush density10,11 and by adding salt.12 By using the water surface monolayer, the brush density can be easily and precisely controlled by compressing/ expanding the monolayer by the Langmuir−Blodgett trough. The critical brush density, at which transition occurs from carpet-only structure to carpet + brush structure, and vice versa, has been found for poly(methacrylic acid) (PMA),13 poly(styrenesulfonate) (PSS),14 poly(acrylic acid) (PAA),15 and cationic QBm brushes.16 Chain length dependence and salt concentration dependence of critical brush density differ with the type of brush. Hence, a different brush formation mechanism has been proposed.15 Electrostatic repulsion was concluded to be the main factor in the strength of PSS brush since very low and length-insensitive critical brush density was
INTRODUCTION
The polymer brush is attracting attention for potential application to novel surface modification, stimuli-responsive surface, biomaterials, etc.1−4 The polyelectrolyte brush has been extensively studied.5,6 Its high ability to reduce surface friction has been reported,7 and attempts have been made to use it as a nanoreactor8 to synthesize gold nanoparticles with high efficiency. We have been investigating the nanostructure, its transition, and stimuli responsibility of polyelectrolyte brush, both anionic and cationic, by utilizing an ionic amphiphilic diblock copolymer monolayer on the water surface.9−16 The monolayer consists of a hydrophobic layer and hydrophilic carpet layer or one with an additional third layer of polyelectrolyte brush under the carpet layer.9 The carpet layer is a layer of densely packed hydrophilic blocks, which is formed to avoid direct contact between hydrophobic blocks and water. The transition between these structures, i.e., double-layer and triple-layer © XXXX American Chemical Society
Received: February 17, 2015 Revised: April 13, 2015
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Langmuir Scheme 1. Synthesis of PEHA-b-PGLBT via the RAFT Technique
observed. A weak PAA and PMA brush had a rather large (0.3− 0.5 chain/nm2) and length-dependent critical brush density,13,15 which led us to conclude that steric or interface stabilization (in the sense to avoid direct contact between hydrophobic layer and water) is important for brush formation. The QBm brush was not stretched, but rather mushroom-like, which might be due to the very low effective charge on QBm brush chains. The nanostructure change of polyelectrolyte brush by salt addition also differed with the strength of the acid.12,15 The critical salt concentration for a strongly ionic PSS brush has been found.12 Up to the critical salt concentration, the brush structure is unaffected by salt addition to the water subphase. However, at a higher salt concentration, the brush chains start to shrink rapidly. This phenomenon is explained by the very high ion concentration in the polyelectrolyte brush and electrostatic shielding effect of salt ions. In the PSS brush layer, ion concentration in the brush layer was simply calculated to be about 2.2 M by PSS brush length and brush density. At such a high ion concentration, addition of a small amount of salt does not influence the brush nanostructure because small salt ions cannot go into the brush layer. The critical salt concentration for the PSS brush was found to be 0.2 M; at 0.2 M NaCl, salt ions go into the brush layer, and the brush chains shrunk due to the shielding effect of salt ions on an electrostatic repulsion between PSS brush chains. From this observation, the effective ion concentration in the PSS brush layer is accounted to be about 0.2 M, which gives 92% of counterion condensation of PSS brush chain. A different salt effect was observed for a weakly ionic PAA brush.15 Brush chains first stretched, and after passing a maximum, it shrunk with further increasing added salt concentration. This curious behavior at first sight was explained by superposition of two factors, which act in opposite directions: an increase of effective charge number on the PAA chain to stretch the brush chain and an electrostatic shielding effect to shrink the brush chain. A similar observation has also been reported for the PMA brush.13 The present study is an extension of our polyelectrolyte brush study. We investigated the nanostructure and formation mechanism of the zwitterionic brush. Zwitterionic substances such as betaine have both an anion and a cation in the molecule, so they exist as an inner salt in a normal condition. Polyzwitterions, also called an amphoretic polyelectrolyte or a polyampholyte, attracted attention as a model of proteins.17 We first studied copolymers of anionic and cationic monomers18,19
and shifted to betaine-type zwitterions with pyridine or imidazole as a source of cationic charge.20,21 Then, we studied mainly poly(sulfobetaine)s of quaternary ammonium cation type extensively.22−26 Now, they are applied to biomaterials by utilizing their high biocompatibility, probably due to large hydration,27−29 and also theoretical studies on its unique characters have been reported.30,31 Recent progress has already been summarized.32 Micellization behavior of a betainecontaining surfactant was reported by Dalbiez et al.,33 and block copolymers containing polybetaine and their selfassembling behavior were also studied.34,35 In the present study, we synthesized diblock copolymers of carboxybetaine, a kind of betaine, and 2-ethylhexyl acrylate, whose polymer is very flexible and also very hydrophobic, with a different block length and length ratio by reversible addition− fragmentation chain transfer (RAFT) polymerization and prepared a polymer monolayer on the water surface by spread of their solution. A hydrophobic layer is expected to be formed in the monolayer and a poly(betaine) layer to be formed just beneath the water surface. The betaine layer is expected to be a “carpet-only” structure or “carpet + brush” structure depending on the situation such as brush density and ion concentration. Transition between these two structures is also supposed to occur if it behaves as a ionic (anionic or cationic) brush. However, betaine is a zwitterion, so a quite different behavior and nanostructure are expected. By application of π−A isotherm measurement and X-ray reflectivity (XR) technique, we systematically investigated the nanostructure and its transition and compared the zwitterionic brush with the ionic brush.
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EXPERIMENTAL SECTION
Materials. Carboxybetaine and 2-ethylhexyl acrylate monomers were kindly donated by Osaka Organic Chemical Industries. The chain transfer agent (CTA) 2-cyanopentanoic acid dithiobenzoate was synthesized as reported. Solvents and salts used were purchased from Wako (Osaka, Japan) and used as received.38−40 The initiators 2,2′azobis(isobutyronitrile) (AIBN) and 4,4′-azobis(cyanopentanoic acid) (ACVA) were also obtained from Wako (98%). Deuterated water was obtained from Cambridge Isotope Laboratory (CIL) (99.9%). Both deuterated chloroform and methanol-d4 were obtained from EURISCO-TOP (99.8%). The water used for sample preparation and dialysis was ultrapure water obtained with a Milli-Q system (Millipore, Bedford, MA), whose conductance was 18.2 MΩ·cm. Polymer Synthesis. The PGLBT homopolymer was the synthesized by the RAFT technique. The solutions of 2 g of GLBT B
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Figure 1. 1H NMR spectra for (a) GLBT homopolymer and (b) PEHA-b-PGLBT synthesized. dissolved in 6 mL of pure water and 0.5/100−2/100 equiv of ACVA and 1/100−2/100 equiv of CTA dissolved in 1.5 mL of DMF were mixed in a Schlenk tube. Then, the solution was degassed by three freeze−pump− thaw cycles (10 min cooling in dry ice/ethanol bath/ 10 mim degassing by pump/10 min Ar flow at room temperature). A polymerization reaction was performed at 70 °C in an oil bath. The reaction was stopped by ice bath cooling. The reaction product was purified by dialysis against pure water for 1 week and then lyophilized. (Scheme 1). The PGLBT homopolymer thus obtained was characterized by GPC and NMR. PGLBT homopolymer was used as a macro-CTA for block copolymerization by RAFT. PGLBT, AIBN, and EHA (1:2:200− 250) were dissolved in 2 mL of methanol in a Schlenk tube and then degassed by four cycles of freeze−pump−thaw treatment. The polymerization reaction was carried out at 70 °C in an oil bath for 24 h. The reaction was stopped by ice-bath cooling, and the reaction mixture was poured into diethyl ether to precipitate. A block copolymer solid was obtained by suctional filtration and dried under vacuum. The yield was 20−40%. 1 H Nuclear Magnetic Resonance (NMR). 1H NMR spectra for GLBT homopolymer and block copolymers were measured with a JEOL 400WS (JEOL, Japan) using as a solvent CD3OD and CDCl3/ CD3OD (1/1, v/v), respectively. GPC. The molecular weight and distribution of PGLBT were estimated by the GPC method. The GPC system, a product of JASCO Co. (Tokyo, Japan), consisted of a Shodex SB-804 HQ column, RI2031 Plus RI detector, UV-2075 Plus UV detector, PU-2080 Plus pump, DG-2080-53 degasser, and CO-2065 Plus column oven. Eluent was pH 3 buffer solution (0.5 M acetic acid aq/0.3 M sodium sulfate). Poly(2-vinylpyridine) samples were used as a standard. Preparation of Monolayer and π−A Isotherm. A specially designed Langmuir−Blodgett (LB) trough (USI System, Fukuoka, Japan) was used for the surface pressure−area per molecule measurement (π−A isotherm), which was equipped with an XR instrument. Typically 20 μL of polymer solution (1 mg/mL) in methanol/chloroform (1/9 v/v) was spread on the water surface by
microsyringe to prepare monolayer. After waiting 15 min for solvent evaporation, we obtained the π−A isotherm by moving the Teflon barrier with speed of 0.030 mm/s. The surface pressure was monitored by the Wilhelmy method with 1 cm × 1 cm filter paper as a sensor plate. The water subphase temperature was kept at 25 °C. The details have been described elsewhere.41 Brewster Angle Microscope (BAM). The in-plane structures of the monolayer on the water surface at low and middle surface pressures were observed with a Brewster angle microscope (BAM), Multiskope System (Optrel, Sinzing, Germany),42 with a specially designed Langmuir−Blodgett (LB) trough (USI System, Fukuoka, Japan). X-ray Reflectivity (XR). The XR instrument41 used for the water monolayer study and data analysis43 were fully described previously. Fundamental experimental conditions are as follows. Measurements were performed for q = 0−0.42 −1, where q is a scattering vector: q = 4π sin θ/λ, where θ is the reflection angle and λ the wavelength of Xray (1.5406 Å). The accumulation time was 10 s at lower angles with 0.01° angular step interval and was 20 s for larger angles with 0.02° step. Hence, almost 11/2 h was required for one measurement. The trough was in constant pressure mode during the XR measurements. The slit width was 50 μm. The box model based on Parratt theory was used for XR profile fitting. For the details, refer to our previous paper.43
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RESULTS AND DISCUSSION Block Copolymer Synthesis. Two PGLBT homopolymers were synthesized. They had a degree of polymerization of 60 and 91 and polydispersity index (Mw/Mn) of 1.21 and 1.15, respectively, estimated by GPC. Figure 1a,b shows 1H NMR spectra for PGLBT homopolymer (macro-CTA) and PEHA-bPGLBT block copolymer. Both spectra indicated successful synthesis of polymers. From the peak area ratio, the block length, m:n, was determined to be 45:60 and 45:91 for the two block copolymers obtained. C
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Figure 2. π−A isotherms of EHA120 and EHA45-b-GLBT60 monolayer on the water surface with BAM images.
Figure 3. (a) XR profiles of EHA45-b-GLBT60 monolayer on the water surface. Each profile shifted downward by 1 decade for clarity. (b) Density profile normal to the water surface of EHA45-b-GLBT60 monolayer on the water.
very small A. This means that the first increase and the following plateau region of π−A isotherm contributes to the surface pressure from EHA chain on the water surface. From BAM observation information, this process might be domain formation of EHA chain (homopolymer or block) on the water surface. Since the EHA homopolymer did not have a sharp increase of π−A isotherm, this sharp increase reflects the contribution of GLBT block to the surface pressure, and its origin should be brush formation under the PEHA hydrophobic layer on the water surface. The sharp increase of π−A isotherm at a small A started at about A = 1 nm2 for block copolymer of GLBT with a degree of polymerization of 60. This value is so small compared to those for other block copolymers. For EHA143-b-QBm52,16 it was about 4 nm2, and for (hydrogenated isoprene))141-b-SS51, it was about 51 nm2.14 Since polyQBm is cationic and PSS (poly(styrenesulfonate) is anionic, the observation here is due
Monolayer Formation. Figure 2 shows the π−A isotherm of EHA45-b-GLBT60 with compression together with the BAM image at several A values. The π−A isotherm of EHA homopolymer (EHA120) is also shown for comparison. The π−A isotherm of the block copolymer first increased with compression, then showed a wide plateau region, and finally increased very sharply in the small A region. BAM revealed domain formation in the plateau region, but no domain formation in the larger A region where π increased. It is interesting to note that the π−A isotherm of GLBT homopolymer also showed similar behavior but without the final very sharp increase at small A. The A value where π started to increase is different for the block copolymer (17 nm2) and homopolymer (47 nm2). This is due to the difference in the degree of polymerization of EHA chain. If this difference is corrected by multiplying by a factor of 45/120, these two π−A isotherms almost superimposed except for sharp increase at D
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Figure 4. (a) XR profiles of EHA45-b-GLBT91 monolayer on the water surface. Each profile shifted downward by 1 decade for clarity. (b) Density profile normal to the water surface of EHA45-b-GLBT91 monolayer on the water.
Figure 5. Thickness of each layer of the monolayer on water as a function of the brush density for (a) (Ip-h2)220-b-(SS)55, (b) (Ip-h2)74-b-(AA)54, and (c) (EHA)45-b-(GLBT)60 monolayer. The critical brush densities were estimated to be (a) 0.12, (b) 0.44, and (c) 0.30 nm−2. (Ip-h2) is hydrogenated isoprene.
carboxybetaine “brush” layer was formed under the carpet layer, and the thickness of brush layer increased with increasing brush density. These observations, i.e., triple-layered structure formation and brush stretching by compression, are the same as other anionic and cationic brushes studied previously.9−16 Since the transition from a two-layered structure to a three-layered structure, in other words, a formation of brush layer, was observed around 0.30 nm−2, this value can be regarded as the critical brush density. For a EHA45-b-GLBT91 monolayer, XR measurements were performed at five brush densities, and XR profiles obtained, and the density profiles evaluated from XR profiles are shown in Figure 4a,b. The same tendency as EHA45-b-GLBT60 monolayer was perceived, and the critical brush density was appraised to be about 0.20 nm−2, which is smaller than that for the GLBT60 brush. The trend observed here, i.e., the longer the hydrophilic chain, the smaller the critical brush density, is the same as for poly(methylmethacrylic acid) (PMA)13 and poly(acrylic acid) (PAA)12 brush, in which an electrostatic interaction is not an important factor for brush formation but is different from that for PSS brush, in which an electrostatic repulsion is the major factor for brush formation. The characteristics of the betaine brush and other ionic brushes are summarized in Figure 5, where the thickness of each layer is shown as a function of brush density. Here, the
to the almost nonionic nature of GLBT chain. The main driving force of brush formation for strong polyelectrolyte is electrostatic repulsion, but for the zwitterionic brush, it is not an electrostatic but might be steric. Nanostructure and Its Brush Density Dependence by XR. To investigate the nanostructure of the monolayer, we measured XR as a function of a brush density, i.e., molecular area or surface pressure. The measurements were carried out under four conditions: 0.18 nm−2 (3 nm2, 17.5 mN/m), 0.30 nm−2 (1.5 nm2, 18 mN/m), 0.36 nm−2 (1.2 nm2, 19 mN/m), and 0.75 nm−2 (0.7 nm2, 22 mN/m). The first condition was before the increase of surface pressure (i.e., in the plateau region), the second was the starting point, the third was after the starting point, and the fourth was after the increase of surface pressure. Figure 3 shows (a) XR profiles and (b) density profiles normal to the water surface evaluated from XR analysis for EHA45-b-GLBT60 monolayer on the water surface. The vertical axis in the density profiles is δ, which is defined by n = 1 − δ − iβ, where n is the complex refractive index. At a low brush density (0.18 nm−2), the monolayer consisted of two layers: a hydrophobic poly(EHA) layer on the water surface and a hydrophilic, zwitterionic, dense carpet layer of poly(betaine) just beneath the water surface. However, with the increase of brush density by compression, the third E
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behavior to PSS and QBm brushes, and this means that brush chains expanded with increasing salt concentration. The origin of this behavior is not yet perfectly clarified, but it is obvious that electrostatic repulsion is a unique factor. The stretching of brush chains should be due to an enhancement of electrostatic repulsion between GLBT brush chains. Hence, the GLBT chain, which is almost neutral at no salt condition, should become (totally) ionic by salt addition. Since betaine exists as an inner salt under the no-salt condition, the almost neutral nature is reasonable. By adding a salt such as NaCl, the situation changes from inner salt to “two salt ions”, i.e., (COO− + Na+) and (N+ + Cl−). Both salts might be totally nonionic as an ion pair. However, if the effective degree of dissociation is different, the overall nature might be ionic. In fact, the totally anionic nature of betaine systems was reported.36,44−47 It is interesting to note the special nature of betaine systems reported as an “antipolyelectrolyte effect”. This is an effect of adding salt to polybetaine solutions, which results in an increase of solution viscosity.23,25,26 Also, the solubility increased by addition of salt.26,36 These are opposite trends to polyelectrolyte systems.48−50 Effect of Salt Ion Species. To determine the “balance” of ions in the betaine brush, we examined the effect of ion species of salts by π−A isotherm using various salts as additives. The effect of cation species examined using LiCl, NaCl, and KCl is shown in Figure 7. The isotherm shifted toward larger A with Li+ and Na+ salts, but its effect was stronger with Li+ salts. On the other hand, almost no change was detected with KCl as a salt. In this experiment, since the anion is common to all salts (Cl−), only the effect of the cation is observed. Cation species affect the brush stretching in the order of Li+ > Na+ > K+. This order follows the Hofmeister series.51−53 Liaw reported the same cation order for solubility and solution viscosity of sulfobetaine systems. They concluded that a larger ion is more efficient to enhance solubility and pointed out the importance of hydration water molecules.26 On the other hand, Figure 8 shows the effect of anion (Cl−, Br−, SCN−). Anion species have no effect on the betaine brush irrespective of anion species: for all of chloride, bromide, and thiocyanic salts, no significant difference was observed for the cation species of salt, i.e., Li+, Na+, and K+. The cation of the salt becomes a counterion of the carboxylic acid unit of betaine, while the anion becomes a counterion of the quaternary ammonium cation of betaine. Our observation here, i.e., cation of salt is active and the anion of salt is inactive in the betaine nanostructure change, means that the carboxylic acid anion might be more effective than the quaternary ammonium cation. Carboxylic acid is generally considered to be a “weak” acid, and the quaternary ammonium unit is a “strong” cation. However, this “common sense” may not apply here, which may mean that this manner on the strength of ions is not always correct, but at least, carboxylic acid is more strong ion than quaternary ammonium cation in carboxybetaine. In fact, as described above, the anionic nature of betaine with carboxylic acid has been reported recently.16,46,47 Effect of Salt on the Nanostructure. XR profiles and corresponding density profiles for the EHA45 -b-GLBT60 monolayer as a function of added LiCl and NaCl are shown in Figures 9 and 10, respectively. As expected from the π−A isotherm in Figure 6, stretching of the betaine brush with increasing added salt concentration was observed both for LiCl and NaCl as a salt. Thus, salt addition caused stretching of the brush, which justifies our discussion above on the ion balance in
data for ionic brush which has a length (degree of polymerization) similar to that of a betaine brush are compared. For the GLBT case, i.e., Figure 5c, only carpet layer is formed at brush density 0.18 nm−2 (0.18 GLBT chains are located in the unit area (i.e., 1 nm2)). On the other hand, both carpet and brush layers were found at 0.30 nm−2. Hence, the critical brush density for (GLBT)60 is estimated to be 0.30 nm−2, which is close to that for (AA)54 but larger than that for (SS)55. The very low critical brush density of the PSS brush is due to the electrostatic nature of PSS brush formation. The main factor of brush formation for the PSS brush was thought to be the steric factor, which results in a rather large critical brush density value. The betaine brush had a critical brush density close to that of PAA, which is consistent with the zwitterionic nature (overall almost neutral) of the betaine chain. By simple calculation, we can estimate the portion of GLBT block which is contained in the brush layer. For example, for the GLBT brush with degree of polymerization of 60, 17 units are in brush (other 43 units are in the carpet layer) at 0.30/nm2 and 42 units in brush (other 18 units in carpet) at 0.75/nm2. Salt Effect. The effect of salt on polyelectrolyte brush has been studied for strongly anionic,12 cationic,16 and weakly anionic15 brushes. The strongly anionic PSS brush and strongly cationic QBm brush had a critical salt concentration. The brush nanostructure was not affected by addition of salt into the water subphase up to the critical salt concentration. However, addition of more salt caused the brush to shrink rapidly. This phenomenon was interpreted by a very high ion concentration in the brush and an electrostatic shielding effect of added salt. Since polyelectrolyte chains are densely packed in the brush, ion concentration in the brush is so high. Hence, salt ions cannot go into the brush layer when salt concentration is low. However, when the salt concentration becomes high enough, salt ions enter the brush layer and shield an electrostatic repulsion between ions on the brush chains, which results in shrinking of brush chains. A different behavior was observed for the weakly anionic PAA brush.15 PAA brush chains first stretched, showed a maximum thickness, and then decreased. This curious behavior was duly interpreted by the superposition of two factors: an increase of effective charge of carboxylic acid and an enhancement of electrostatic shielding effect by salt ions with increasing added salt concentration. A similar salt effect was reported also for PMA brush,9 which is also weakly anionic. However, the zwitterionic betaine brush showed a different behavior by salt addition as shown in Figure 6. The π−A isotherm of GLBT brush shifted toward larger A regions with increasing salt concentration. This is perfectly opposite
Figure 6. π−A isotherms of EHA45-b-GLBT60 monolayer on NaCl(aq). F
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Figure 7. π−A isotherms of EHA45-b-GLBT60 monolayer on (a) LiCl(aq), (b) NaCl(aq), and (c) KCl(aq). (d) Comparison of isotherms at 0.10 M salt solutions for each ion.
Figure 8. π−A isotherms of EHA45-b-GLBT60 monolayer on (a) LiCl and LiBr(aq), (b) NaCl and NaBr(aq), and (c) KCl and KBr(aq).
higher solvent power for betaine chains. Sivanantham et al. also reported stretching of the GLBT brush in micelle systems in solution by addition of NaCl up to 1.0 M.54
betaine with salt. However, on the other hand, it should be noticed that addition of salt at a high concentration has a shielding effect on the electrostatic interaction between polyions. Both the strong and weak acid brush shrunk by the addition of a large amount of salt. Further stretching of the betaine brush occurred even by addition of 0.10 M NaCl or LiCl. Even if the betaine chain becomes more ionic by salt addition, this salt concentration is considered to be high enough to cause a shielding effect on an electrostatic repulsion, which should result in brush chain shrinking. Another factor might be the increase of solvent power of the salt solution.16,26,46,47 A higher salt concentration might have a
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CONCLUSIONS The zwitterionic amphiphilic diblock copolymer, PEHA-bPGLBT, was synthesized by the RAFT technique with precise control of the degree of polymerization and its distribution. Monolayer formation on the water surface was confirmed. The monolayer consisted of two layers, i.e., hydrophobic EHA layer on the water surface and hydrophilic GLBT carpet layer just beneath the water surface at low brush density conditions. G
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Figure 9. (a) XR and (b) density profiles of EHA45-b-GLBT60 monolayer on LiCl(aq). In (a), Each profile shifted downward by 1 decade for clarity.
Figure 10. (a) XR and (b) density profiles of EHA45-b-GLBT60 monolayer on NaCl(aq). Each profile shifted downward by 1 decade for clarity.
However, the GLBT brush was formed under the carpet layer under higher brush density conditions, and brush thickness increased with further compression of the monolayer, which increased the brush density. The critical brush density, where the brush layer started to form, was 0.30 nm−2 for the GLBT60 brush and 0.20 nm−2 for the GLBT91 brush. Such a high value compared to that for a strongly ionic brush such as PSS brush indicates the nonionic nature of betaine brush. In addition, these values are close to those for the PAA brush, in which a steric factor is thought to play an important role in brush formation. Unlike anionic and cationic brushes, the GLBT brush expanded by salt addition. Brush expansion at low salt concentrations was attributed to the ionic nature of brush chains due to transition from an “inner salt” condition to “ion pair” situation by salt addition. However, further expansion at a higher added salt concentration may be due to an increase of solvent power to salt solution against GLBT polymer. The brush expansion behavior has a cation species dependence of added salt, while it is not influenced by an anion species. This means that in the betaine molecule the anion has a stronger ionic nature than the cation. This might be the origin of the ionic nature of GLBT chain by salt addition. Although the anion here is a carboxylic acid, which is a weak acid, and the cation here is a quaternized ammonium cation, which is a strong base, some studies showed that a quaternized ammonium cation behaves like a weak base.16,47
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported by a grant-in-aid for Scientific Research on Innovative Areas “Molecular Soft-Interface Science” (20106006) from the Ministry of Education, Culture, Sports, Science and Technology of Japan, to which our sincere gratitude is due. This work was also supported by the Global COE Program, GCOE, for the International Center for Integrated Research and Advanced Education in Material Science. A.G. expresses his sincere thanks to the Japan Society for the Promotion of Science (JSPS) for the Postdoctoral Fellowships for Foreign Researchers. We express sincere thanks to Professor Ken-ici Iimura (Utsunomiya University, Japan) for his kind support in the BAM observation.
(1) Advincula, R. C., Brittain, W. J., Caster, K. C., Rühe, J., Eds.; Polymer Brushes; Wiley: New York, 2004. (2) Hamely, I. Block Copolymers in Solution: Fundamentals and Applications; Wiley: New York, 2005. (3) Halperin, A.; Tirrell, M.; Lodge, T. P. Thetherd Chains in Polymer Microsructures. Adv. Polym. Sci. 1992, 100, 31−71. (4) Milner, S. T. Polymer Brushes. Science 1991, 251, 905−914. (5) Rühe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gröhn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Polyelectrolyte Brushes. Adv. Polym. Sci. 2004, 165, 79−150.
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DOI: 10.1021/acs.langmuir.5b00637 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.5b00637 Langmuir XXXX, XXX, XXX−XXX
