Dramatically Tuning Friction Using Responsive Polyelectrolyte

Article pubs.acs.org/Macromolecules

Dramatically Tuning Friction Using Responsive Polyelectrolyte Brushes Qiangbing Wei,†,‡ Meirong Cai,† Feng Zhou,*,† and Weimin Liu*,† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: We present a paradigm that dramatically tunes friction from superior lubrication (μ ∼ 10−3) to ultrahigh friction (μ > 1) using responsive polyelectrolyte brushes. The tunable friction is based on counterion-driven interactions in polyelectrolyte brushes that can be simply achieved by exchanging the counterions. We systematically investigated the effects of opposite counterions of different types on the friction properties of polyanionic, polycationic, and polyzwitterionic brushes. For cationic brushes with quaternary ammonium groups, the friction coefficient was progressively tuned from ∼10−3 to ∼100 according to the counterions series Cl− < ClO4− < PF6− < TFSI−. The friction of anionic brushes can be tuned by oppositely charged surfactants (tetraalkylammonium) with different length of hydrophobic tails, multivalent metal ions, and protons. The friction increase of cationic brushes is due to the dehydration and the collapse of polyelectrolyte chains induced by ion-pairing interactions. For anionic brushes, the friction increased with the length of hydrophobic tails of surfactants, which resulted from hydrophobicity induced electrostatic interaction among surfactants and polymer chains. The anionic brushes with the carboxylate and the sulfonate side groups revealed different friction responses, which is owing to the carboxylate groups getting stronger specific interaction with the quaternary ammonium and thus with the multivalent metal ions as well. The mechanism of tuning friction was finally concluded; that is, highly hydrated and swelling polymer brushes show superior lubrication, partially collapsed polymer chains have moderate lubrication, and completely dehydrated and collapsed conformation loses lubricating capability.

■

INTRODUCTION The extremely low friction properties supported by water lubrication are usually found in some biological interfaces, for example, the vitreous body of eyes, the organs of human body, and synovial joints such as hip, knees, and finger joints.1 It has shown that the high efficiency of the lubrication in living organism is due to the complex structure of cartilage and the biolubricants such as phospholipids, hyaluronan (HA), lubricins, and bottle-brush-like glycoproteins in synovial fluids.2,3 HA and glycoproteins are both biopolyelectrolytes that contain large amounts of sulfonic and carboxylic groups on the side chains. It is thus not daring to suggest that charged biomacromolecules and brush-like structures are of critical importance for the lubrication of synovial joints.4−7 Understanding the lubricating mechanism of synovial joints will help to develop novel high efficient biomimetic lubricants for artificial joints, to which synthetic polyelectrolytes are very promising alternatives.5,8−12 Among synthetic polymers, polyelectrolyte brushes provide extremely efficient boundary lubrication (μ < 0.001) in water, owing to the high hydration of charged segments on the polymer backbones and the exceptional resistance to mutual interpenetration displayed by the compressed yet swollen brushes.13,14 © 2013 American Chemical Society

Besides the excellent water lubrication, polyelectrolyte brushes grafted on solid surface provide a wide platform for building smart surfaces because of their response to a variety of environmental triggers such as pH, salt concentration, and counterions.15−19 Their responsive behaviors resulting from swelling−collapse transition of end-tethered polymer chains are to a large extent governed by the electrostatic interaction and osmotic pressure in the brushes. The polyelectrolyte brushes show a fully stretched conformation in pure water, as a result of both electrostatic repulsion between neighboring chains and the repulsion between charges in the same chain. In contrast, when it is immersed in electrolyte solution, strong electrostatic screening and exclusion of water from brushes lead to more favorable to collapsed conformation. Potential applications of swelling−collapse transition in polyelectrolyte brushes include smart gates of microchannels,20 controlled release systems,21,22 highly sensitive sensors, and nanoactuations for converting chemical energy to mechanical force.23,24 Zhou et al.23 demonstrated fast and reversible actuation of cantilever driven Received: July 22, 2013 Revised: October 28, 2013 Published: November 27, 2013 9368

dx.doi.org/10.1021/ma401537j | Macromolecules 2013, 46, 9368−9379

Macromolecules

Article

by conformation changes of polyelectrolyte brush modified on single side of cantilever. The magnitude of bending was controlled by placing polyelectrolyte brush into different pH and ion strength solutions. In addition to the collapse resulting from strong electrostatic screening, ion-pairing interaction between involved counterions and charged monomer of the brushes is the other driven force to induce conformation change of polymer chains. Moreover, the fact that the characteristics of ion-paired state are extremely sensitive to counterions reveals the critical role of counterions in determining the overall properties of polyelectrolyte brushes, as recently reported in the literature.25−28 Azzaroni et al. reported a smart surface with switchable surface wettability using the cationic polyelectrolyte brush bearing quaternary ammonium groups undergo ion-pairing interaction.26 Can conformation change of polyelectrolyte brushes be translated to friction control? Several researches related to the issues of tunable friction have been conducted.29−36 The solvents responsive switching of friction by binary polymer brushes was reported by LeMieux29 and Vyas.31 Dedinaite et al. reported temperature and surfactant induced control of friction force of absorbed polyelectrolyte layers.34,35 Dunér et al. also have demonstrated that the friction of polyacrylic brushes can be tuned by pH and calcium ions.36 The high friciton was attributed to energy dissipation mechanism resulting from stronger interaction between calcium ions and carboxylate groups. Despite several works being conducted on tuning friction of polyelectrolyte, the types of counterions and the magnitude of friction change are still very limited. Moreover, control of friction is limited to the micro/nanoscale rather than the macroscale, and organic solvents are also required to switch surface topography or chemical composition of polymer brushes. In this paper, we systematically present feasible surfaces for dramatically tuning macroscale friction from superior lubrication (μ ∼ 10−3) to ultrahigh friction (μ > 1) via exchanging counterions into polyelectrolyte brushes when the surface was sliding against a silicone elastomer ball. We also emphasize the correlation between conformation changes and hydrated state of polymer chains and the friction properties. As described in Scheme 1, the counterions in polycationic and polyanionic brushes can be simply exchanged with oppositely charged molecules. Atom force microscope and quartz crystal microbalance with dissipation were used to monitor the swelling properties and collapse kinetics of polymer chains coordinated with different counterions. The friction coefficient of cationic poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] brush (PMETAC) was progressively tuned from ∼10−3 to ∼100 via exchanging Cl− with ClO4−, PF6−, and bis(trifluoromethanesulfonimide) (TFSI−). Cationic surfactants with different length of hydrophobic tails (tetrabutylammonium bromide (TBAB), dodecyltrimethylammonium bromide (DTAB), and hexadecyltrimethylammonium bromide (CTAB)) and multivalent metal ions were selected to exchange with poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) and poly(methylacrylic acid sodium) (PMAA) brushes. With increasing length of hydrophobic tails (e.g., CTAB), the hydrophobicity induced polymer collapse became progressively evident, and the corresponding friction concomitantly increased resulting from the dehydration of polymer chains. For anionic polymer brushes, PSPMA brush and PMAA brush had differential response to cations or even protons, offering more possibilities to tune friction properties. Finally, a polyzwitterionic brush (poly[2-(methacryloyloxy)-

Scheme 1. Schematic Illustration of Conformation Changes When Exchanging Counterions in Polyelectrolyte Brushes (top) and Chemical Structures of Corresponding Counterions (bottom)

ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (PSBMA)), containing quaternary ammonium groups and sulfonate groups simultaneously, was rectified to be able to provide more stable friction at different counterions. In brief, we herein demonstrate a systematic study of how to dramatically tune surface friction by taking advantage of charged polymer in response to a richness of electrolytes.

■

EXPERIMENTAL SECTION

Materials and Chemicals. 3-Sulfopropyl methacrylate potassium salt (SPMA, 95%, TCI), 2-(methacryloyloxy)ethyltrimethylammonium chloride (METAC, 80% in water, TCI), [2-(methacryloyloxy)ethyl]dimethyl(3-sulfopropyl)ammonium hydroxide (SBMA, 97%, Aldrich), and methacrylic acid sodium (MAA, 99.5%, Aldrich) were used as received. 2,2′-Bipyridyl (bipy, AR), sodium perchlorate (ClO4−, AR), ammonium hexafluorophosphate (PF6−, AR), bis(trifluoromethanesulfonimide) lithium salt (TFSI−, AR), ammonium chloride (NH4+, AR), tetramethylammonium bromide (TMAB, AR), tetraethylammonium bromide (TEAB, AR), tetrabutylammonium bromide (TBAB, AR), dodecyltrimethylammonium bromide (DTAB, AR) and hexadecyltrimethylammonium bromide (CTAB, AR) were purchased from Aldrich and used without any purification. Copper(I) bromide was purified via reflux in acetic acid. Other general reagents and solvents were used as received. The ultrapure water was prepared with a NANOpure Infinity system from Barnstead/Thermolyne Corporation. Silane initiator 3-(trichlorosilyl)propyl 2-bromo-2methylpropanoate and thiol ester initiator ω-mercaptoundecyl bromoisobutyrate were synthesized according previous reports.37,38 Gold film was prepared via thermal evaporation of 100 nm gold on silicon wafers with 5 nm Cr as the adhesive layer. 9369


Macromolecules

Article

Figure 1. (A) Changes in friction coefficients and wettability (inserted images) corresponding to PMETAC brush on Si substrates when the counterions are Cl−, ClO4−, PF6−, and TFSI−. (B) Friction coefficient vs time plot of PMETAC brush in water and then in situ injecting 5 mM solutions of ClO4−, PF6−, and TFSI− in sequence. The friction tests were carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N (0.23 MPa) at 25 °C. Preparation of Polyelectrolyte Brushes. Polyelectrolyte brushes were prepared by versatile surface initiated atom transfer radical polymerization from initiator modified substrates. Homogeneous initiator modified Si substrates were prepared by vapor deposition of silane initiator in a desiccator under vacuum. Patterned initiator modified gold substrates were prepared by microcontact printing (μCP) with a PDMS stamp using thiol ester initiator. The wafers modified with initiator were thoroughly rinsed with ethanol to remove physically absorption of initiator molecules and then dried with N2 flow. The general polymerization procedure was first dissolving the monomer in water or water/methanol mixed solvent at room temperature and degassing with dry N2 for 30 min under stirring condition. Next, bipy and CuBr were added to this solution successively. The mixture was then further stirred and degassed with a stream of dry N2 until a clear solution formed. In the final, the mixture was syringed into a Schlenk flask, where initiator modified substrates were sealed with N2 protection. After polymerization for a certain time, the samples were taken out, washed with water and ethanol, and dried under N2 flow for further characterizations. Polymerization recipes for the four monomers are as follows: METAC (80% in water) 8 mL, bipy 160 mg, CuBr 80 mg, 8 mL of water/ methanol (v/v = 2:1); SPMA 6 g, bipy 80 mg, CuBr 35 mg, 12 mL of water/methanol (v/v = 2:1); MAA-Na 8 g, bipy 120 mg, CuBr 50 mg, 12 mL of water; SBMA 4 g, bipy 120 mg, CuBr 40 mg, and 8 mL of water/methanol (v/v = 2:1). Exchange of counterions was performed by immersing brushes modified wafers in a solution of 50 mM target counterions for 30 min under shaking. Once finished, the wafers were taken out and rinsed with copious pure water. Characterization of Polyelectrolyte Brushes. Static contact angle (CA) was acquired on a DSA-100 optical contact angle meter (Krüss Company, Ltd., Germany) at ambient temperature (25 °C). A droplet of 5 μL of deionized water was used as probe liquid. Five different positions were measured for each sample to get the average CA. All the contact angles were measured with about 5 s of residence time of water droplet on the surface. Thickness of the polymer layer was measured using a spectroscopic ellipsometer (Gaertner model L116E) equipped with a He−Ne laser source (λ = 632.8 nm) at a fixed angle of incidence of 50°. The refractive index of polymer film was 1.46, and the Cauchy model was used to calculate film thickness. Atomic force microscopy (AFM) measurements were performed on an Agilent Technologies 5500 AFM using a MacMode Pico SPM magnetically driven dynamic force microscope. Images were taken using commercially available type II MAC levers with a nominal force constant of 2.8 N/m at a driving frequency of 75 and 24 kHz in ambient condition and in liquid environment. The swelling ratio was calculated as hwater/hdry, where hwater is the height of polymer in water

and hdry is the height in ambient condition (humidity: 15%). The mechanical properties of brushes were probed by adjusting the applied loads (the load was indicated as set point value), which was well controlled to avoid permanent damage to the brushes by changing the set points in voltage. Quartz crystal microbalance with dissipation measurements (QCM-D) was performed using a Q-Sense microbalance (Sweden) at 25 °C. Commercially available (QSX-301, QSense) gold-coated quartz chips were used. Friction Test. A macroscopic friction test was performed on conventional pin-on-disk reciprocating tribometer by recording the friction coefficient (μ) at different sliding conditions using a Universal Micro-Tribometer (UMT-2, CETR). Elastomeric poly(dimethylsiloxane) (PDMS) hemisphere with a diameter of 6 mm was employed as a pin against polymer brushes in water. The PDMS pins were prepared from a commercial silicone elastomer kit (SYLGARD 184 silicone elastomer, base and curing agents, Dow Corning, Midland, MI). In order to prepare PDMS pins with a hemisphere end, a polystyrene 96-well cell culture plate with roundshaped well (Dow Corning) was used as a mold. The base and curing agents of SYLGARD 184 elastomer kit were mixed at 10:1 ratio (by weight). The mixtures were transferred into the mold after removing bubbles under gentle vacuum and then incubated in an oven (70 °C) for 24 h. The distance of one sliding cycle was 10 mm, and the friction coefficient (and the frictional force) vs time plot was obtained. The initial 20 sliding cycles was used to calculate the friction coefficient and at least three friction tests were repeated for each sample to get average value. The friction coefficients showed in the paper were measured under defined load rather than obtained by the ratio of the friction force over load, so is an effective friction coefficient. Each friction test was carried out on a virgin surface area of the brush under water without wear at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N (Hertzian contact pressure ≈ 0.23 MPa) at 25 °C. Reversibly switchable friction by changing pH value was completed in situ on the same area of surface.

■

RESULTS AND DISCUSSION Tuning Friction Using Polycationic Brushes. The asprepared PMETAC brush (about 20 nm thick on Si substrates) having Cl− anions as the counterions can respond to salt concentration due to electrostatic screening. Nevertheless, electrostatic screened polymer with highly mobile counterions is not stable, which will stretch again when immersed in pure water. Another way to make polymer collapse is building strong ion-pair interaction between ammonium cations and more hydrophobic anions (PF6− and TFSI−), where polyelectrolyte brush experiences a dramatic “hydrophobicity induced collapse” rather than an “electrostatic driven collapse”.39 The ion-pairing 9370


Macromolecules

Article

Figure 2. Swelling properties of PMETAC brushes. (A) AFM cross-section analysis of PMETAC brushes in “dry” state (ambient condition, humidity: 15%) and water with different counterions. (B) Corresponding swelling ratios. The height was measured by AFM using patterned brushes on Au substrates and swelling ratio was calculated as hwater/hdry.

the water combined in the polyelectrolyte brushs.26,43 It is the counterions driven collapse and dehydration making the PMETAC brush lose the lubricating ability under water. In short, macroscale friction of polycationic brushes can be progressively tuned from superior lubrication to ultrahigh friction by simple counterions exchange in the order of Cl− < ClO4− < PF6− < TFSI−. In order to further probe counterion-driven collapse and dehydration of polymer chains, patterned PMETAC brushes on Au substrates were also employed to investigate their swelling and elastic properties using AFM. Figure 2A shows the height changes of PMETAC brushes coordinated with Cl− and TFSI− in “dry” state and water, respectively. It is observed that the asprepared PMETAC brushes with Cl− anions was 12 nm thick in ambient condition, while in pure water, it was ∼41 nm, more than 3 times larger, due to the instant swelling with the swelling ratio of ∼3.4. The large swelling ratio reveals that the brush experienced a high level of hydration. However, the height of TFSI− coordinated PMETAC brush in water (∼18 nm) got no apparent difference with that in “dry” state (∼15 nm). The swelling ratio is only 1.2. The swelling ratios decrease from 3.4 to 2.2, 1.4, and 1.2 (Figure 2B) when Cl− anions in PMETAC were exchanged with ClO4−, PF6− and TFSI−, respectively, which was attributed to the increasing hydrophobic characteristics of counterions resulting from counterion-driven interactions (Figure S6). Next, the load dependence AFM analysis was used to detect the rigidity of PMETAC brush under water. During AFM scanning under water, the applied load was increased by decreasing the amplitude set point. The height of the patterned PMETAC brush with Cl− anions decreased from 41 nm, when the applied load was increased by halving the amplitude set point from 6.8 V (this value is the minimum applied load to obtain a clear image, indicating slight contact between AFM tip and brush and no apparent indentation of AFM tip) to 3.4 V. But no clear height change was observed for TFSI− coordinated PMETAC brush when amplitude set point was decreased from 7.4 to 3.7 V (the set point of 7.4 V is the minimum applied load to obtain a clear image). The significant change in height feature of PMETAC-Cl was assigned to the indentation of AFM tip into the fully extended polymer brushes, indicating its extremely soft and highly hydrated nature. However, the surface behavior was completely switched from “soft” to “hard” when exchanging the counterions to TFSI−.41,43 Combined with the friction tests, it is concluded

interaction is sufficiently strong to tune surface properties that they cannot be reversed in water or even at low ion concentration.40 It was found in our case that the conformation change of the brush significantly affected the surface friction. As shown in Figure 1A, the PMETAC brush with Cl− counterions exhibited extremely low friction coefficient under pure water (∼0.006) when the surface was sliding against a silicone elastomer (Sylgard 184) ball. While when the Cl− counterions were simply exchanged with ClO4−, PF6−, and TFSI−, the friction coefficient gradually increased to 0.016, 0.092, and 0.828, respectively. Furthermore, the counterions exchange was also found leading the CA of the surface to increase from ∼10° to 43°, 56°, and 75°, respectively, which follow the same trend with friction coefficients: Cl− < ClO4− < PF6− < TFSI−. Surface XPS analysis of PMETAC brushes (Figure S3 and Table S1) proved that the counterions exchange has occurred, but Cl− is not thoroughly exchanged with PF6− (approximately 60% of Cl− was exchanged). Importantly, in situ friction changes have been observed when the three electrolyte solutions successively flew past (Figure 1B), indicating the friction of the polycationic brushes can be tuned continuously. It is believed that the friction property of PMETAC brush is closely related to the hydration state of the brush, which is to a large extent affected by the interaction between quaternary ammonium groups (QA+) and the surrounding counterions. Actually, the QA+−Cl− ion pair in the as-prepared PMETAC brushes exists as dissociation state in water, which causes the electrostatic repulsion between the cationic charges on neighbor polymer chains and forces polymer chains to swell into the extended conformation.41 As a result, the highly hydrated and extended PMETAC brush with soft and fluid-like characteristic served as an effective boundary lubricating film with superior water lubrication. Hydrophilic Cl− (large hydration radius) can be exchanged by hydrophobic anions (such as ClO4−, PF6−, and TFSI−) according to the well-known Hofmeister series.42 The ClO4− coordinated PMETAC brush showed relatively low friction coefficient as well, suggesting that this brush is partially collapsed and still has hydrated capacity although a little bit weak compared with Cl− coordinated brush. In contrast, PF6− and TFSI− with larger volume are highly polarizable, but less hydrated, which strongly bind to the QA+ pendant groups through ion-pairing interaction and weaken the dissociation and hydration; hence, the polymer chains collapse and exclude 9371


Macromolecules

Article

amounts of combined water.43 The fact of no observable change in frequency in addition the dramatic decrease of dissipation after the PF6− solution was washed away with water confirms that the brush became rigid and complete collapse due to the exclusion of all combined water. Sequentially injecting 50 mM TFSI− solution and then washing with water, the decrease in frequency (Δf 3) was attributed to the stronger interaction of the QA+−TFSI− pair than the QA+−PF− pair and larger molecular weight of TFSI− (FW: 280.0). From the dehydration and collapse process monitored by QCM-D, we can see that PMETAC brush coordinated with ClO4− anions showed partially collapsed conformation while PF6− and TFSI− coordinated brush showed completely collapsed conformation, which is highly consistent with the AFM study. These partially collapsed brushes to some extent possess moderately lubricating capability while the fully collapsed and rigid brushes show ultrahigh friction as depicted in Scheme 2. The QCM results interpreted the frictional data very well.

that the high hydration and very soft behavior of fully extended brush lead to ultralow friction, while oppositely, the dehydrated and rigid polymer film results in high friction. Furthermore, it also indicates that the swelling properties and softness on microscopic level is a powerful and directly evidence to support tunable friction properties of PMETAC brush coordinated with different anions through ion-pairing interactions. The in situ dehydrated and collapsed kinetics of PMETAC brush introduced by ion-pairing effects were further monitored by changes in frequency and dissipation of a quartz chip modified with PMETAC brush using QCM-D. The changes of frequency (Δf) is related to the mass of the adsorbed film on quartz chip for rigid and thin film. Dissipation (defined as ΔD = Edissipated/2πEstored) represents the capacity of brushes releasing mechanical energy and provides information on the rigidity of the film.44,45 Please note that the minor changes of frequency and dissipation induced by the bulk viscosity and density of electrolyte solutions46 were negligible compared with large shifts induced by counterions exchange and exclusion of water from polyelectrolyte brushes. Figure 3 shows the changes in

Scheme 2. Illustration of Correlation between Lubrication and Conformation of Polymer Brushes

Tuning Friction Using Polyanionic Brushes. Considering the importance and ubiquity of anionic glycoproteins in biological systems,2,6 study on the friction properties dependence of polyanionic brushes on polymer conformation change will help to understand the lubricating mechanisms. Two typical brushes, PSPMA and PMAA, were selected. The former is a strong polyelectrolyte containing sulfonate groups on the side chains while the latter is a weak polyelectrolyte bearing carboxylate groups. Although there were some reports on the microscopic interaction of polyelectrolyte−surfactant complexes,27,47−49 only very few systematic investigations on macroscopic properties and the differences between strong and weak polyelectrolytes have been carried out. Herein, ∼45 nm thick PSPMA brush and ∼35 nm thick PMAA brush were prepared by SI-ATRP to perform the study using the ammonium surfactants with different alkyl chains as counteranions source. Figure 4A shows the variations of friction coefficients and wettability of PSPMA brushes with different ammonium cations. The as-prepared PSPMA brush, with K+ as counterions, exhibited excellent water affinity (CA ∼ 10°) and superior lubrication ability (μ ∼ 0.005). Nevertheless, the friction coefficient gradually increased to 0.013, 0.07, and 0.24 when exchanging counterions to TBAB, DTAB, and CTAB, and in the meantime, the CA increased from 10° to 42°, 70°, and 85°. For PMAA brush, the wettability change was similar to that of PSPMA when Na+ was exchanged to TBAB, DTAB, and CTAB, whereas the friction properties were quite different (Figure 4B). PMAA brush with Na+ showed extremely low friction coefficient (μ ∼ 0.006) and superhydrophilicity, too.

Figure 3. Changes in frequency and corresponding dissipation of QCM chip modified with PMETAC brush successively exchanging with 50 mM solutions of ClO4−, PF6−, and TFSI−.

frequency and dissipation of the ∼15 nm PMETAC brush (in ambient conditions) coated quartz chip when successively exchanging Cl− counterions with ClO4−, PF6−, and TFSI−. First, replacing the water by the 50 mM ClO4− solution inside the QCM chamber, a rapid and slight increase in frequency was observed, implying ion exchange together with dehydration process occurred. This was also rectified by the dissipation reduction. When the pure water was changed back, the frequency decreased and the dissipation returned to almost its original value, indicating that the QA+−ClO4− still existed in a loosely paired dissociation state. The eventual frequency increase (Δf1) indicates that the ClO4− remained inside the PMETAC brush. Although the molecular weight of ClO4− anions (FW: 99.5) is over Cl− counterions (FW: 35.5), the frequency increase still indicated the partial exclusion of bounded water.40 Almost no decrease in dissipation further confirms that the film was still very soft although loss of water occurred. When 50 mM solution of PF6− (FW: 145.0) was injected into the QCM chamber, a significant increase of frequency was observed, and a significant loss of water occurred (frequency increase Δf 2). It suggests that the interaction of PF6−−QA+ ion pair is much stronger than that of ClO4−−QA+ ion pair in forcing polymer chains collapse and exclude large 9372


Macromolecules

Article

Figure 4. Changes in friction coefficients and wettability (inserted images) corresponding to polyanionic brushes on Si substrates. The counterions are cationic surfactants (TBAB, DTAB, and CTAB) for PSPMA brushes (A) and PMAA brushes (B). (C) PMAA brushes with NH4+, TMAB, and TEAB as counterions. (D) PMAA and PSPMA brushes coordinated with divalent Cu2+ and trivalent Fe3+ ions. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N (0.23 MPa) at 25 °C.

Figure 5. Swelling properties of PSPMA brushes. (A) Cross-section analysis of PSPMA brushes in “dry” state (ambient condition, humidity: 15%) and water with different counterions. (B) Corresponding swelling ratios. The height was measured by AFM using patterned brushes on Au substrates, and the swelling ratio was calculated as hwater/hdry.

∼3.5 in pure water. The height of the polymer patterns increased from ∼35 to ∼120 nm when immersed in water. This clearly illustrates that the polymer chains experienced a highly extended conformation resulting from the strongly electrostatic repulsion between charged chains because of dissociation of the K+−sulfonate ion pair. The highly hydrated polymer brushes led to excellent lubrication (μ ∼ 10−3).8 This is also suitable for the explanation of ultralow friction properties of Na + coordinated PMAA brush.9,39 Immersing PSPMA brush into 50 mM solutions of ammonium cations with hydrophobic hydrocarbon chain resulted in the partial replacement of K+ by

Then the friction coefficient sharply increased to ∼1.2 if the counterion was TBAB, which is approximately 2 orders of magnitude larger than that of PSPMA brush with TBAB. Afterward, when the counterions were changed to DTAB and CTAB, the friction coefficient did not change that much, only ∼1.5 and ∼1.6, respectively. The massively different friction response must be related to their response to surfactants, which was then further characterized by AFM, QCM, etc., as follows. The swelling and collapse property of anionic brushes was probed by AFM as shown in Figure 5. It was found that K+ coordinated PSPMA brush showed a large swelling ratio of 9373


Macromolecules

Article

Figure 6. Swelling properties of PMAA brushes. (A) Cross-section analysis of PMAA brushes in “dry state” (ambient condition, humidity: 15%) and water with different counterions. (B) Corresponding swelling ratios. The height was measured by AFM using patterned brushes on Au substrates, and the swelling ratio was calculated as hwater/hdry.

Figure 7. Changes in frequency and corresponding dissipation of QCM chips modified with PSPMA brush (A) and PMAA brush (B) in the presence of 50 mM TBAB solution.

PSPMA brush. As shown in Figure 6B, the swelling ratio in water was about 1.2, which is instead very different from that of PSPMA, indicating that the PMAA chains shrunk and experienced a collapsed conformation. From the slight change of the dry thickness, we can deduce that the majority of TBAB molecules bind to the PMAA chains in nonaggregated forms via electrostatic interaction rather than hydrophobic effect because the aggregation of surfactants must be accompanied by a significant increase of brush mass.47,50 In the case of DTAB and CTAB, the hydrophobic interaction was more pronounced, but the friction coefficients only increased from 1.2 to 1.5 and 1.6, respectively, due to the complete collapse of TBAB exchanged PMAA. QCM-D measurements were carried out in situ to monitor the conformation of PSPMA and PMAA brushes when both exchanged with TBAB counterions as additional evidence. Approximately 40 nm of PSPMA brush and 30 nm of PMAA brush on gold-coated quartz chips were employed in the experiments. For PSPMA brush, after exchanged with 50 mM TBAB solution, the decrease in frequency was attributed to the replacement of K+ by heavier TBAB molecules (Figure 7A); meanwhile, the water loss was not prominent. Therefore, after rinsing with water, the frequency increased yet was still lower than the water baseline, indicating slight exclusion of water. Nearly no change in dissipation suggests that the mechanical property of the polymer film did not change much. These changes in frequency and dissipation, in agreement with the swelling properties measured by liquid AFM, strongly confirm

ammonium cations and formation of polyelectrolyte−surfactant complexes. The CA concomitantly increased. It was found that the “dry” thickness of PSPMA increased to 40 and 60 nm for TBAB and DTAB, respectively, owing to the uptake of voluminous surfactant molecules. Instead, the swelling ratios in water reduced to 2.3 and 1.7, respectively. The moderate swelling ratios suggest that the conformation of polymer chains undergoes partially collapsed state. This partially collapsed state is to a certain extent hydrated and has moderately lubricating capability, which was confirmed by the friction coefficient of ∼10−2 (Figure 4A). When the counterions were changed to more hydrophobic and larger volume CTAB, the “dry” thickness of PSPMA dramatically increased to ∼250% (Figure S7). The increase was attributed to the sharp decrease of the osmotic pressure coming from the strong interaction between the hydrophobic CTAB and the polyelectrolyte chains, which rendered the chains to collapse and dehydrate, leading to a fully collapsed conformation. This is confirmed by the AFM test wherein it is found that the swelling ratio of PSPMA was only 1.2 when soaked in water (Figure 5B). The fully collapsed conformation of polymer brushes no longer have effective lubricating capability under water, which was found to have a friction coefficient of >10−1 (Figure 4A). For TBAB exchanged PMAA brush, the friction coefficient is about 2 orders of magnitude larger than that of PSPMA brush with TBAB as counterions. After uptake of TBAB, the dry thickness of PMAA brush only increased from 28 to 32 nm (Figure 6A), which has no clear difference from the case of 9374


Macromolecules

Article

have larger ion pairing equilibrium constant; thus, even TBAB could lead to full collapsed conformation of PMAA brush due to stronger specific interaction between carboxylate groups and ammonium of TBAB and the sharp decreases in osmotic pressure. Water molecules cannot penetrate into inner brush, which leads to the minimum degree of dissociation. Thus, PMAA brush give ultrahigh friction (μ ∼ 1.2), when exchanged with TBAB counterions. Moreover, the contact angle of TBAB exchanged PMAA is ∼62°, larger than that of PSPMA (∼42°), also demonstrating the less dissociation degree due to the strong interaction between TBAB and PMAA chains. Possibly, the stronger specific interaction between TBAB and carboxylate groups is related to the Hofmeister-like effect according to the literature.42,55 In order to tune the friction of PMAA brush in a gradient trend, more hydrophilic ammoniums with short hydrocarbon chains were used to exchange. Figure 4C is the change of friction coefficients and wettability of PMAA with NH4+, TMAB, and TEAB as counterions. NH4+ coordinated PMAA exhibited good water affinity (CA ∼ 14°) and superior water lubrication (μ ∼ 0.008), which is a little larger than that of Na+ coordinated PMAA. For TMAB and TEAB, the friction coefficients gradually increased to 0.013 and 0.14 while the contact angles to ∼20° and ∼48°, respectively. The swelling properties of NH4+, TMAB, and TEAB exchanged PMAA brushes were probed by AFM. It shows a negative correlation between the friction properties and the swelling ratios (Figure 6B). From the cross section analysis of patterned PMAA in Figure S8, the increase of “dry” thickness was slight and neglectable, suggesting the aggregation of ammonium groups did not occur after exchanged with NH4+, TMAB, and TEAB. The swelling ratios in Figure 6B slowly decreased as the order of Na+ > NH4+ > TMAB > TEAB > TBAB. To sum up, the friction coefficient of PMAA brush was tuned from 10−3 to 10−2, 10−1, and finally 100 through the interactions between surfactants and the brushes. A gradual transition from ultralow friction to ultrahigh friction was realized. Tuning Friction of Polyanionic Brushes by Multivalent Metal Ions. Metal ions play a critical role in cell biochemistry by interacting with proteins.56,57 Hence, studies on the interactions between metal ions and polyanionic brushes and its effects on friction properties will get useful insights into the biolubrication mechanisms in living systems and the related bioprocesses. As discussed above, the swelling behavior and friction properties of PSPMA and PMAA brushes with monovalent alkali metal ions were fundamentally altered when ammoniums with different alkyl chain lengths were used. The question arises then, how would the original feature and an increase in the valence of the metal ions influence the swelling behavior and the following friction properties? To make it clear, exchange of the monovalent alkali metal ions (Na+ or K+) with transition metal ions, e.g., divalent Cu2+ and trivalent Fe3+, was conducted. Figure 4D shows changes of friction coefficient and wettability of PSPMA and PMAA brushes coordinated with divalent Cu2+ and trivalent Fe3+, respectively. Only a slight increase in friction coefficient, from 0.005 to 0.008 and 0.011, respectively, was found when K+ in PSPMA replaced with Cu2+ and Fe3+, whereas the exponential variation happened to PMAA brush coordinated with the same metal ions. The friction coefficient sharply increased from 0.006 to 1.05 and 1.32, respectively, when Na+ replaced with divalent Cu2+ and trivalent Fe3+. Since friction properties of polymer brush is

that TBAB exchanged PSPMA brush only showed a partially collapsed conformation and therefore exhibited moderate lubricating ability (μ ∼ 10−2). Figure 7B clearly shows that PMAA brush responds to TBAB differently. During the exchanging process of TBAB, apart from the initial disturbance of frequency decrease, the frequency increased monotonously, meaning a continuous loss of water despite the combination of TBAB with anions. After rinsing with water, the surface cannot recover to its original state: apparent frequency increase and dramatic dissipation reduction (becoming more rigid) were observed. It indicates that the PMAA brush has a more collapsed conformation than the PSPMA in the solution of TBAB. The strong evidence along with different swelling ratios probed by AFM together verify that the PSPMA and PMAA brushes have different lubricity responses to TBAB. The friction coefficient of PSPMA brush can be gradually tuned from ∼10−3 to ∼10−1 by counterions exchange with ammonium surfactants having different hydrophobicity. The three conformational states of polyelectrolyte brushes in Scheme 2 can be still used to depict the formation of PSPMA−surfactant complexes with TBAB, DTAB, and CTAB. The adsorption of oppositely charged surfactants to polyelectrolyte brush surfaces has been reported in theoretical and experimental investigations.51−53 It was found that the adsorption behavior is closely dependent on grafted density, charged sites on polymer chains and concentration of surfactants,50 and the interaction of polymer−surfactant complexes is mainly governed by electrostatic interaction and hydrophobic interaction. Toward our densely packed strong polyelectrolyte brushes PSPMA, the alkaline metal ions gave the maximum charge density because of the full dissociation. According to the swelling properties and collapse kinetics of polyelectrolyte chains interacted with surfactants investigated by AFM and QCM-D, we infer that at the first step the surfactant molecules could bind to the most accessible part of the brush through electrostatic interaction, namely the periphery where the segment density is not as high as the interior. In this case, a small increase in “dry” thickness was generally observed for the polymer−surfactants complexes. Second, the surfactant would aggregate and form micelles via hydrophobic interaction between hydrocarbon tails, leading to an uptake of large amounts of surfactant molecules. This binding process is cooperative, which would lead to dramatic increase of “dry” thickness of polymer film.50,53 For less hydrophobic TBAB and DTAB, the binding is mainly governed by electrostatic interaction. The chains still partially hydrate due to the characteristics of sulfonate groups whose charges are more diffuse and less to be screened. While for CTAB, because of the longer alkyl chain and the higher hydrophobicity, the hydrophobic interaction and aggregation are strongly enhanced.47,54 The dramatic reduction in both frequency and dissipation during CTAB exchanging process with PSPMA (Figure S9) also supports the facts that the increased mass of film due to the uptake of higher molecular weight CTAB molecules was far more than the excluded water and that the polymer−CTAB complex film became rigid. The strongly hydrophobic binding of CTAB in addition the electrostatic interaction resulted in the strong dehydration and the formation of strongly hydrophobic pockets which become largely inaccessible to water molecules. The interaction of PMAA brush with countercations can also be explained by synergistic process of electrostatic interaction and hydrophobic effect.47,50,54 However, weak polyelectrolytes 9375


Macromolecules

Article

Figure 8. Changes in frequency and corresponding dissipation of QCM chips modified with PSPMA brush (A) and PMAA brush (B) in the presence of 50 mM Cu2+ solution.

property was also found in the case of Fe3+. The exact structure of polycarboxynate coordinated trivalent metal ion is unknown yet, but it seems rational to assume a bridging of PMAA chains via trivalent Fe3+ similar to that found for the interaction with Cu2+.61−63 The swelling and collapse states can also be further confirmed by wettability evaluation in Figure 4D. It is seen that both PSPMA and PMAA brushes showed very good affinity with water with CA of ∼100. After coordinated with Cu2+ and Fe3+, the CA of both increased, revealing their less hydrophilicity. PMAA brush got larger CA than PSPMA brush when coordinated with the same ion. For instance, the contact angle of Cu2+ coordinated PMAA brush was ∼48°, whereas for PSPMA it was ∼34°. Reversible Switching Friction by pH. In comparison to strong polyelectrolyte brush, a typical characteristic of weak polyelectrolyte brush is that the local charge density is not fixed. It is a function of local concentration of protons inside the brush.39 So the conformation changes would come out when exposing these brushes in aqueous solution with different pH, due to change of charges density. The pH responsiveness of weak polyelectrolyte brushes is not only of fundamental interest but also relevant to various practical applications such as nanoactuator64 and smart nanochannel with pH controllable ion transportation.65−67 Unlike the counterion-driven interactions in polyelectrolyte brush, pH-induced swelling and collapse of polymer chains via protonation and deprotonation is completely reversible, which can fast switch its practical properties. Taking advantage of the sensitively switchable capability, we herein attempted to switch the friction of PMAA brush via pH value. Figure 9 shows the variation in friction coefficients of PMAA brush over aqueous solutions with different pH values compared with PSPMA brush. It is obvious that PSPMA brush showed a constant friction coefficient (μ ∼ 0.005) when the pH values increased from 2.0 to 12.0. This is due to the fixed charge density inside the strong PSPMA brush, which keeps strong electrostatic repulsion between neighboring charged chains in both acidic and basic aqueous solution. However, the friction of PMAA brush was magically switched by pH value. PMAA brush showed ultralow friction coefficient (μ ∼ 0.006) under both neutral (pH ≈ 7) and basic (pH ≈ 12) solutions. Because the pH value of neutral and basic solution is larger than the apparent pKa of PMAA, deprotonation occurred, and the swelling behavior is similar to that of strong PSPMA brush.68 When the pH value (∼5.5) was around the apparent pKa, the PMAA brushes were partially protonated and showed

closely related to its chain conformation and degree of hydration, swelling−collapse transition of polyelectrolyte brush caused by metal ions driven interaction must be accompanying. From the changes in frequency and dissipation of QCM-D measurements in Figure 8, it can be seen that Cu2+ coordinated PSPMA brushes had only a small decline in frequency, indicating Cu2+ uptake did not lead to apparent water exclusion. The small increment of dissipation verifies that the Cu2+ coordinated PSPMA brush still remained high hydration and extended conformation. The coordination of Fe3+ with PSPMA was very similar to the case of Cu2+. The frequency increased and the dissipation decreased sharply for Cu2+ interacting with PMAA brush (Figure 8B), indicating the brush fully collapsed and excluded large amounts of water molecules from inside and simultaneously the film switched to a more rigid structure, due to the strong interaction between Cu2+ and carboxylate groups, which interprets the high friction of Cu2+ coordinated PMAA brush. The XPS analysis (Figure S4) proved that Na+ was almost completely replaced by Cu2+, which also illustrated the stronger interaction. Because PSPMA brush is a strong polyelectrolyte, Cu2+ and Fe3+ loosely bind to sulfonate groups via electrostatic interaction to form weak ion pairs without precipitation and chelation.58 Moreover, the multivalent ions possibly form crosslinked polymer−metal ion network, which will reduce the osmotic pressure of inner polymer brushes.39,59,60 The weak cross-linking and reduction of osmotic pressure lead to weak collapse of polymer brushes and then slight increase of friction coefficient. The degree of collapse will depend on the subtleties of the interaction of multivalent ions with the polymer backbone, especially the strength of the formed ion pairs. In comparison with PSPMA brush, PMAA showed very strong ion-pairing interaction between Cu2+ (or Fe3+) and carboxylate groups, mainly dominating the collapse of weak polyelectrolyte brush. Rühe et al. have systematically investigated the interaction of PMAA brushes with metal ions and its swelling properties using in situ spectral analysis61−63 and described the formation of stable polycarboxynate−copper complexes and its possible structures. Cu2+ ions can form chelating bidentate complexes where the central ion is coordinated to both oxygen atoms of the carboxylate ligand and bridges bidentate configurations with the two oxygen atoms of the ligands being coordinated to different copper ions.63 The strong ionpairing interaction almost completely neutralized the charges on polymer chains and thus forced them to collapse and dehydrate, which is a reasonable explanation for ultrahigh friction of Cu2+ coordinated PMAA brushes. A similar friction 9376


Macromolecules

Article

Figure 10. Friction coefficients of zwitterionic PSBMA brushes under water after exchanging with different anionic (ClO4−, PF6−, and TFSI−) and cationic (TBAB, DTAB, and CTAB) solutions. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N (0.23 MPa) at 25 °C.

Figure 9. Friction coefficients of PMAA and PSPMA brushes under different pH values. The inset is in situ reversible switch between superior lubrication and ultrahigh friction of PMAA brush. The friction test was carried out by sliding a silicone elastomer ball at a sliding velocity of 2 × 10−3 m/s under loading of 0.5 N (0.23 MPa) at 25 °C.

groups on polymer chains preserved characteristics of polyelectrolyte brushes, which is the most rational explanation for friction-stable feature of the polyzwitterionic brush. In the same way, the friction properties of PSBMA brush exchanged with different surfactant cations were studied as well. No change in friction coefficient was observed. The PSBMA film after exchanged with surfactants still had good affinity to water, and the “dry” thickness did not increase. The QCM-D analysis in Figure 11B confirms that nearly no uptake of CTAB molecules after rinsing with water is due to the weak driven force for counterions exchange of polyzwitterionic brush and larger volume of surfactant molecules, which was further verified by the wettability and mass change of polymer film. Similarly, even though few surfactant molecules bound with sulfonate through the electrostatic interaction, the quaternary ammonium groups can still be hydrated to preserve the properties of polyelectrolyte brushes. Tunable Friction for Controlling Motion of Objects. As proof-of-concept, Figure 12 demonstrates a PDMS block sliding on inclined Si wafers modified with PMETAC brush. The angle of inclination is ∼10°. The PMETAC brush with Cl− as counterion exhibited very good water affinity, and the PDMS block can rapidly slide down on the inclined Si wafer due to the ultralow friction between the contacted PMETAC brush and the PDMS block in the presence of water (Figures 12A and 12A1). The force analysis indicates frictional force (Ff = mg cos θμ ≈ 0.015 mN, where θ is the inclined angle and μ is friction coefficient) is smaller than the component force of gravity in parallel direction of Si wafer (F∥ = mg sin θ ≈ 0.43 mN). For TFSI− exchanged PMETAC, the surface became hydrophobic. This surface has ultrahigh friction according to former discussion. The PDMS block cannot slide down because the frictional force was larger than the component force of gravity in parallel direction of Si wafer (Figures 12B to 12B1) even at larger inclined angles. This qualitative experiment demonstrates that the tunable friction may have potential applications in controlling motion of objects.

moderate friction coefficient. At low pH value (pH ≈ 2), the negative carboxylate groups were rapidly protonated. The PMAA brush fully collapsed and dehydrated due to the strong hydrogen bonding between carboxylic acid groups (intra- and interpolymer chains).58,69 The fully collapsed and rigid polymer film led to ultrahigh friction. Importantly, the friction is reversible, which was fast switched between superior lubrication and ultrahigh friction by pH value for several cycles (Figure 9). Toward Friction-Stable Surface of Polyzwitterionic Brushes. Zwitterionic polymers have been demonstrated as best candidate materials for biomimetic lubrication because of its high level of hydration and good biocompatibility.70−72 Unlike polyelectrolyte brushes, polyzwitterionic brushes have both cationic and anionic groups fixed on the same chain to maintain neutral state. In order to compare with polyelectrolyte brushes and seek a polymer brush surface with stable friction, zwitterionic PSBMA brush containing quaternary ammonium groups and sulfonate groups on the molecule was employed.73−75 We hypothesized that the counterions used in polyelectrolyte brushes have little influence on the friction properties of polyzwitterionic brushes. PSBMA brush can be highly hydrated due to abundant charged groups on the side chains and have potential biological application.76,77 In our experiment, about 15 nm thick PSBMA brush on Si was used for friction tests. The friction coefficient of PSBMA brush was similar to the swelling polyelectrolyte brushes, as low as 0.006. After exchanging with anionic solutions (ClO4−, PF6−, and TFSI−), the changes of its friction coefficient and wettability are very different from the anions exchanged PMETAC brush (Figure 10). The friction coefficient only slightly increased for ClO4−, PF6−, and TFSI− exchanged PSBMA brush, and the amplitude was far less than that of PMETAC brush. There is almost no change in CA of these three anions exchanged PSBMA brushes. Figure 11A is the QCM-D study of PSBMA brush during exchanging with TFSI− anion. The slight increase in frequency (∼10 Hz) and decrease in dissipation due to the loss of water molecules after water rinsing suggest a weak uptake of TFSI− anion actually occurred. However, the very little amount of anion uptake had almost no effects on the hydration and so did the friction properties. In addition, even some anions would partially bind with cationic quaternary ammonium groups and the anionic

■

CONCLUSIONS In this work, we have demonstrated a tunable lubrication platform that the macroscale friction can be dramatically tuned from superior lubrication (μ ∼10−3) to ultrahigh friction (μ > 1) progressively. This tunable friction is based on counterions 9377


Macromolecules

Article

Figure 11. Changes in frequency and corresponding dissipation of QCM chips modified with zwitterionic PSBMA brush after exchanging with TFSI− (A) and CTAB (B) solutions.

would promote applications of these smart surfaces in microfluidic devices, biosensors, and so on.

■

ASSOCIATED CONTENT

* Supporting Information S

XPS spectra, AFM images, additional cross-section analysis of patterned polymer brushes, and additional QCM-D data. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (F.Z.). *E-mail [email protected] (W.L.).

Figure 12. Optical images of sliding a PDMS block from inclined planes with ultralow friction (A, A1) and ultrahigh friction (B, B1). The PDMS cylinder with 6 mm in diameter is about 0.25 g in weight. The inclined angle is ∼10°.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21125316, 21204095, and 51171202), 973 project (2013CB632300), and Key Research Program of the Chinese Academy of Sciences (grant KJZD-EW-M01).

driven interaction with polyelectrolyte brushes. AFM and QCM-D measurements were employed to probe the swelling properties and swelling−collapse process of polyelectrolyte brushes coordinated with different counterions since swelling and hydration of polymers are the most important factors that affect aqueous lubrication of polymer brushes. For PMETAC brush, the friction and wettability were progressively tuned by simple counterions exchange according to the order of Cl− < ClO4− < PF6− < TFSI−. For strong PSPMA brush exchanged with ammonium surfactants, the friction increased with the length of hydrophobic tail of surfactants in the order of TBAB < DTAB < CTAB. Besides the electrostatic interaction between sulfonate groups and QA groups, the hydrophobic aggregation of surfactants was another strong driven force for the collapse and dehydration of polymer chains. While the weak PMAA brush−TBAB complex without hydrophobic interaction exhibited ultrahigh friction, which was verified by the specific interaction between the carboxylate groups and the QA groups that was stronger than the PSPMA−TBAB complex. The specific interaction also happened to the multivalent metal ions. Cu2+ (or Fe3+) coordinated PMAA brushes showed dramatic increase in friction coefficients while only slight change was observed for PSPMA due to their stronger coordination capability with carboxylate groups. Both anionic and cationic counterions have little influence on the friction properties of polyzwitterionic brush. The tunable lubrication platform based on counterions driven interaction with polyelectrolyte brushes is sensitive and stable, which would be helpful to further understand the lubricating mechanism of polymer brushes and

■

REFERENCES

(1) McCutchen, C. W. Wear 1962, 5, 1. (2) Lee, S.; Spencer, N. D. Science 2008, 319, 575. (3) Klein, J. Science 2009, 323, 47. (4) Coles, J. M.; Chang, D. P.; Zauscher, S. Curr. Opin. Colloid Interface Sci. 2010, 15, 406. (5) Dėdinaitė, A. Soft Matter 2012, 8, 273. (6) Rubinstein, M.; Papoian, G. A. Soft Matter 2012, 8, 9265. (7) Klein, J. Friction 2013, 1, 1. (8) Kobayashi, M.; Takahara, A. Chem. Rec. 2010, 10, 208. (9) Heeb, R.; Bielecki, R. M.; Lee, S.; Spencer, N. D. Macromolecules 2009, 42, 9124. (10) Chen, M.; Briscoe, W. H.; Armes, S. P.; Klein, J. Science 2009, 323, 1698. (11) Kyomoto, M.; Moro, T.; Saiga, K.; Hashimoto, M.; Ito, H.; Kawaguchi, H.; Takatori, Y.; Ishihara, K. Biomaterials 2012, 33, 4451. (12) Kobayashi, M.; Terada, M.; Takahara, A. Faraday Discuss. 2012, 156, 403. (13) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J.-F.; Jerome, R.; Klein, J. Nature 2003, 425, 163. (14) Raviv, U.; Giasson, S.; Kampf, N.; Gohy, J. F.; Jerome, R.; Klein, J. Langmuir 2008, 24, 8678. (15) Zhou, F.; Huck, W. T. S. Phys. Chem. Chem. Phys. 2006, 8, 3815. (16) Bünsow, J.; Kelby, T. S.; Huck, W. T. S. Acc. Chem. Res. 2009, 43, 466. 9378


Macromolecules

Article

(52) Xu, L.; Li, X.; Zhai, M.; Huang, L.; Peng, J.; Li, J.; Wei, G. J. Phys. Chem. B 2007, 111, 3391. (53) Naderi, A.; Makuška, R.; Claesson, P. M. J. Colloid Interface Sci. 2008, 323, 191. (54) Ballauff, M.; Borisov, O. Curr. Opin. Colloid Interface Sci. 2006, 11, 316. (55) Lo Nostro, P.; Ninham, B. W. Chem. Rev. 2012, 112, 2286. (56) Soong, R. K.; Bachand, G. D.; Neves, H. P.; Olkhovets, A. G.; Craighead, H. G.; Montemagno, C. D. Science 2000, 290, 1555. (57) Knoblauch, M.; Noll, G. A.; Muller, T.; Prufer, D.; SchneiderHuther, I.; Scharner, D.; van Bel, A. J. E.; Peters, W. S. Nat. Mater. 2003, 2, 600. (58) Liu, Z.; Liu, J.; Hu, H.; Yu, B.; Chen, M.; Zhou, F. Phys. Chem. Chem. Phys. 2008, 10, 7180. (59) Mei, Y.; Lauterbach, K.; Hoffmann, M.; Borisov, O. V.; Ballauff, M.; Jusufi, A. Phys. Rev. Lett. 2006, 97, 158301. (60) Yan, L. T.; Xu, Y. Y.; Ballauff, M.; Muller, A. H. E.; Boker, A. J. Phys. Chem. B 2009, 113, 5104. (61) Konradi, R.; Rühe, J. Macromolecules 2004, 37, 6954. (62) Zhang, H. N.; Rühe, J. Macromolecules 2005, 38, 4855. (63) Konradi, R.; Rühe, J. Macromolecules 2005, 38, 4345. (64) Zou, Y.; Lam, A.; Brooks, D. E.; Phani, A. S.; Kizhakkedathu, J. N. Angew. Chem., Int. Ed. 2011, 50, 5116. (65) Yameen, B.; Ali, M.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. Nano Lett. 2009, 9, 2788. (66) Hou, X.; Liu, Y. J.; Dong, H.; Yang, F.; Li, L.; Jiang, L. Adv. Mater. 2010, 22, 2440. (67) Hou, X.; Yang, F.; Li, L.; Song, Y.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2010, 132, 11736. (68) Santonicola, M. G.; de Groot, G. W.; Memesa, M.; Meszynska, A.; Vancso, G. J. Langmuir 2010, 26, 17513. (69) Schuwer, N.; Klok, H. A. Langmuir 2011, 27, 4789. (70) Chen, M.; Briscoe, W. H.; Armes, S. P.; Cohen, H.; Klein, J. Eur. Polym. J. 2011, 47, 511. (71) Moro, T.; Takatori, Y.; Ishihara, K.; Konno, T.; Takigawa, Y.; Matsushita, T.; Chung, U. I.; Nakamura, K.; Kawaguchi, H. Nat. Mater. 2004, 3, 829. (72) Klein, J. Polym. Adv. Technol. 2012, 23, 729. (73) Shao, Q.; He, Y.; Jiang, S. J. Phys. Chem. B 2011, 115, 8358. (74) He, Y.; Shao, Q.; Chen, S.; Jiang, S. J. Phys. Chem. C 2011, 115, 15525. (75) Mi, L.; Giarmarco, M. M.; Shao, Q.; Jiang, S. Biomaterials 2012, 33, 2001. (76) Hower, J. C.; Bernards, M. T.; Chen, S.; Tsao, H.-K.; Sheng, Y.J.; Jiang, S. J. Phys. Chem. B 2008, 113, 197. (77) Jiang, S.; Cao, Z. Adv. Mater. 2010, 22, 920.

(17) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101. (18) Chen, T.; Ferris, R.; Zhang, J. M.; Ducker, R.; Zauscher, S. Prog. Polym. Sci. 2010, 35, 94. (19) Barbey, R. l.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109, 5437. (20) Hou, X.; Guo, W.; Jiang, L. Chem. Soc. Rev. 2011, 40, 2385. (21) Tokarev, I.; Minko, S. Adv. Mater. 2009, 21, 241. (22) Azzaroni, O.; Trappmann, B.; van Rijn, P.; Zhou, F.; Kong, B.; Huck, W. T. S. Angew. Chem., Int. Ed. 2006, 45, 7440. (23) Zhou, F.; Shu, W.; Welland, M. E.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 5326. (24) Zhou, F.; Biesheuvel, P. M.; Choi, E. Y.; Shu, W.; Poetes, R.; Steiner, U.; Huck, W. T. S. Nano Lett. 2008, 8, 725. (25) Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. T. S. Angew. Chem., Int. Ed. 2005, 44, 4578. (26) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Adv. Mater. 2007, 19, 151. (27) Döbbelin, M.; Arias, G.; Loinaz, I.; Llarena, I.; Mecerreyes, D.; Moya, S. Macromol. Rapid Commun. 2008, 29, 871. (28) Lee, C. H.; Lim, H. S.; Kim, J.; Cho, J. H. ACS Nano 2011, 5, 7397. (29) LeMieux, M. C.; Lin, Y. H.; Cuong, P. D.; Ahn, H. S.; Zubarev, E. R.; Tsukruk, V. V. Adv. Funct. Mater. 2005, 15, 1529. (30) Chang, D. P.; Dolbow, J. E.; Zauscher, S. Langmuir 2006, 23, 250. (31) Kumar Vyas, M.; Schneider, K.; Nandan, B.; Stamm, M. Soft Matter 2008, 4, 1024. (32) Nordgren, N.; Rutland, M. W. Nano Lett. 2009, 9, 2984. (33) Loveless, D. M.; Abu-Lail, N. I.; Kaholek, M.; Zauscher, S.; Craig, S. L. Angew. Chem., Int. Ed. 2006, 45, 7812. (34) Dėdinaitė, A.; Pettersson, T.; Mohanty, B.; Claesson, P. M. Soft Matter 2010, 6, 1520. (35) Dėdinaitė, A.; Thormann, E.; Olanya, G.; Claesson, P. M.; Nyström, B.; Kjøniksen, A.-L.; Zhu, K. Soft Matter 2010, 6, 2489. (36) Dunér, G.; Thormann, E.; Ramström, O.; Dėdinaitė, A. J. Dispersion Sci. Technol. 2010, 31, 1285. (37) Husseman, M.; Malmström, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P.; Hawker, C. J. Macromolecules 1999, 32, 1424. (38) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265. (39) 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. Adv. Polym. Sci. 2004, 165, 79. (40) Azzaroni, O.; Moya, S.; Farhan, T.; Brown, A. A.; Huck, W. T. S. Macromolecules 2005, 38, 10192. (41) Farhan, T.; Azzaroni, O.; Huck, W. T. S. Soft Matter 2005, 1, 66. (42) Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 19. (43) Zhou, F.; Hu, H.; Yu, B.; Osborne, V. L.; Huck, W. T. S.; Liu, W. Anal. Chem. 2006, 79, 176. (44) Zhang, G. Macromolecules 2004, 37, 6553. (45) Zhang, G.; Wu, C. Macromol. Rapid Commun. 2009, 30, 328. (46) Feldoto, Z.; Pettersson, T.; Dėdinaitė, A. Langmuir 2008, 24, 3348. (47) Pyshkina, O.; Sergeyev, V.; Zezin, A.; Kabanov, V.; Gage, D.; Stuart, M. C. Langmuir 2003, 19, 2000. (48) de Vos, W. M.; Kleijn, J. M.; de Keizer, A.; Cohen Stuart, M. A. Angew. Chem., Int. Ed. 2009, 48, 5369. (49) de Vos, W. M.; Meijer, G.; de Keizer, A.; Cohen Stuart, M. A.; Kleijn, J. M. Soft Matter 2010, 6, 2499. (50) Konradi, R.; Rühe, J. Macromolecules 2005, 38, 6140. (51) Tran, Y.; Auroy, P.; Lee, L. T.; Stamm, M. Phys. Rev. E 1999, 60, 6984. 9379


Dramatically Tuning Friction Using Responsive Polyelectrolyte

Recommend Documents