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Design of self-oscillating polymer brushes and control of the dynamic behaviors Tsukuru Masuda, Aya Mizutani Akimoto, Kenichi Nagase, Teruo Okano, and Ryo Yoshida Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03228 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015

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Chemistry of Materials

Design of self-oscillating polymer brushes and control of the dynamic behaviors Tsukuru Masuda1, Aya Mizutani Akimoto1, Kenichi Nagase2, Teruo Okano2 and Ryo Yoshida1* 1

Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2

Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo 162-8666, Japan

ABSTRACT: A polymer brush surface with autonomous function has been designed by using a self-oscillating polymer that we developed. The self-oscillation is induced by chemomechanical energy conversion from an oscillating chemical reaction (the Belousov-Zhabotinsky (BZ) reaction) to conformational changes of polymer chains. In this study, the surface nano-structure of polymer brushes were regulated and the spatio-temporal behaviors of self-oscillation were investigated. The target polymer brush surfaces were prepared through surface-initiate atom transfer radical polymerization (SI-ATRP) of Nisopropylacrylamide (NIPAAm) and N-(3-aminopropyl) methacrylamide (NAPMAm), and the subsequent conjugation of Ru(bpy)3 to the amino group of NAPMAm. The characterization of the prepared polymer brush and the free polymer was determined by X-ray photo electron spectroscopy, atomic force microscope, attenuated total reflection Fourier transform infrared spectroscope, UV-Vis spectrophotometer, gel permeation chromatography, and 1H NMR. Their dynamic properties were estimated by quartz crystal microbalance with dissipation and fluorescence microscopy. The amounts of Ru(bpy)3 immobilized to polymer brush surfaces could be controlled by adjusting the reaction conditions of SI-ATRP and conjugating Ru(bpy)3. Importantly, an appropriate structure of polymer brush to give stable oscillation has been indicated from image analysis of chemical wave propagation. Further, several physicochemical parameters to control the oscillating behaviors, including the rate constant of the autocatalytic reaction, the diffusion constant of the activator, and the activation energies for the reaction and diffusion, have been obtained from theoretical consideration. These results will be helpful for developing subsequent applications such as autonomous transport systems.

INTRODUCTION

Recently, we created such “self-oscillating” polymer brushes15 by grafting a self-oscillating polymer that we have In the past few decades, well-defined polymer brushes been developping16,17 onto a glass substrate through surfacedesigned by living radical polymerization methods have been initiated atom transfer radical polymerization (SI-ATRP). actively studied. By introducing polymer brushes on materiThe basic chemical structure of the self-oscillating polymer is al surfaces, the surface can achieve unique properties such as 1-3 the copolymer of N-isopropylacrylamide (NIPAAm) and lubrication, superhydrophilicity, and colloidal stability. In Ru(bpy)3 as a catalyst for Belousov-Zhabotinsky (BZ) reacparticular, stimuli-responsive polymer brushes, for which tion. The BZ reaction is a well-known chemical oscillating physiochemical surface properties change in response to 4-9 reaction accompanying a spontaneous redox oscillation of external stimuli (temperature, light, pH, chemicals, etc.), the catalyst which creates temporal rhythms or spatiotempohave attracted much attention. They have the potential to ral patterns (called chemical waves) as one of dissipative realize oil-repellent or lubricative surfaces, mass transport structures.18,19 Coupled with the BZ reaction, the redox control on the surface, and novel bioseparation systems by changes of the copolymerized Ru(bpy)3 moiety induces hyutilizing chromatography or cell culture dishes.10-14 In these dration/dehydration changes of the polymer chains because systems, the surface properties can be controlled by the exhydrophilicity of the polymer varies with the charge number ternal stimuli. But on the other hand, if polymer brush surwhile the LCST changes.20 This change in hydrophilicity of faces can change the properties periodically without any onthe polymer exhibits various aspects of self-oscillation in the off switching of external stimuli, novel applications such as polymer systems such as swelling-deswelling oscillation of autonomous ciliary-like actuators and mass transport systhe gel,16, 21 peristaltic motion of the larger-sized gel with a tems the in micro- or nano-order scale are expected. ACS Paragon Plus Environment


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Scheme 1. Preparation of self-oscillating polymer brushes. (1) Preparation of ATRP-initiator immobilized glass substrates. (2) Preparation of poly(NIPAAm-r-NAPMAm) grafted substrates by SI-ATRP. (3) Conjugation of Ru(bpy)3 to poly(NIPAAm-r-NAPMAm) grafted glass substrates.

propagating chemical wave,22,23 transmittance or viscosity oscillation of the polymer solution,24 dispersion/flocculation oscillation of the microgels,25 formation/breaking of polymer micelles or vesciles,26,27 etc. In a previous report, we successfully demonstrated autonomous propagation of a chemical wave on a selfoscillating polymer brush prepared by SI-ATRP. However, the frequency was quite low and the oscillation gradually attenuated. In order to improve the self-oscillating behaviors and obtain stable oscillation, it is necessary to clarify the correlation between the surface nano-structures and the resulting oscillatory behaviors as a guide to design the optimal polymer brush surface. The architecture of polymer brush can be precisely controlled by changing polymerization condition in ATRP.28, 29 In this study, self-oscillating polymer brushes with various amounts of Ru(bpy)3 were prepared and characterized by Xray photoelectron spectroscopy (XPS), atomic force microscope (AFM), attenuation total reflection Fourier-transform infrared (ATR/FT-IR) spectroscopy, and UV-vis spectroscopy. Further, swelling-deswelling behavior of the selfoscillating polymer brush was analyzed by using a quartz crystal microbalance with dissipation (QCM-D). Chemical wave propagation on the polymer brush was observed by fluorescence microscope and the self-oscillating behaviors were investigated by spatio-temporal image analysis. From these results, guidelines for a suitable design of a polymer brush surface with autonomous function has been indicated. EXPERIMENTAL

Materials. N-Isopropylacrylamide (NIPAAm) was kindly provided by KJ Chemicals (Tokyo, Japan) and purified by recrystallization in toluene/hexane. N-3(Aminopropyl) methacrylamide (NAPMAm) was purchased from Polysciences (Warrington, PA, USA) and used as received. Tris(2-N,N-dimethylamino)ethyl)amine (Me6TREN) was purchased from Aldrich (St. Louis, MO, USA). Bis(2,2’-bipyridine) (1-(4’-methyl-2,2’-bipyridine- 4carbonyloxy)-2,5-pyrrolidinedione) ruthenium(II) bis (hexafluorophosphate) (abbreviated as Ru(bpy)3-NHS), and [11-(2-bromo-2-methyl)propionyloxy] undecyltrichloro silane (abbreviated as BrC11TCS) were synthesized according to previous reports.30,31 Ethyl 2-bromoisobutyrate (EBiB), copper (I) bromide (CuBr), dehydrated toluene, dehydrated dimethyl sulfoxide (DMSO), dimethylformamide (DMF), acetone, hexane and ethanol were purchased from Wako Pure Chemical Industries (Osaka, Japan). Sodium carbonate (Na2CO3) and sodium bromate (NaBrO3) were purchased from Kanto Chemical (Tokyo, Japan). HNO3 aqueous solution (1.0 M) and malonic acid (MA) were purchased from Wako Pure Chemical Industries (Osaka, Japan). Glass coverslips (size. 24 mm x 50 mm; thickness 0.2 mm) were purchased from Matsunami Glass Industry (Osaka, Japan). Preparation of ATRP initiator immobilized glass substrates. ATRP initiator (BrC11TCS) was immobilized on the surface of glass substrates by silane coupling reaction. Glass cover slips were cleaned by oxygen plasma irradiation (400 W, 0.1 mmHg, 180 sec) in a dry plasma cleaner PC-1100 (SAMCO, Kyoto, Japan) and placed into a separable flask, in which the relative humidity was 60%, for 2 h. Toluene solution containing 0.3 v/v%

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Table 1. Elemental analyses by X-ray photoelectron spectroscopy for PB-22.4 Sample a) Atom (%) Si Br C N O C/Si N/C Unmodified Glass 30.2 n.d. b) 3.0 0.3 65.8 0.1 ATRP Initiator modified glass 22.9 0.4 26.5 0.5 49.3 1.2 0.02 PB 0.9 n.d. 75.2 12.0 11.9 0.16 PB-Ru 2.6 n.d. 74.0 11.2 11.9 0.15 Take off-angle of 90˚. a) PB and PB-Ru represent poly(NIPAAm-r-NAPMAm) brush modified glass and the poly(NIPAAm-rNAPMAm-r-Ru(bpy)3NAPMAm) brush modified glass, respectively. b) n.d. means “not detected”.

BrC11TCS was poured into the separable flask, and the solution was stirred for 18 h at 25 ˚C. The ATRP initiator immobilized glass substrates were washed with toluene and acetone and dried in a vacuum oven for 2 h at 110 °C. Preparation of poly(NIPAAm- r -NAPMAm) grafted substrates by SI-ATRP. Poly(NIPAAm-rNAPMAm) grafted surfaces with various chain lengths were prepared by modulating the feed monomer or initiator concentration in SI-ATRP (Table S1 in Supporting Information). The typical preparation procedure was as follows: NIPAAm (8.56 g, 57.6 mmol) and NAPMAm (1.50 g, 8.40 mmol) were dissolved in DMF:water 1:1 mixed solvent (30 mL/30 mL). The oxygen in the solution was removed by argon gas bubbling for 1 h. CuBr (121 mg, 0.84 mmol) and Me6TREN (224 µL, 0.84 mmol) were added under an argon atmosphere, and the solution was stirred for 15 min to obtain a CuBr/Me6TREN catalyst system. The prepared ATRP solution was poured into a separable flask containing the ATRP initiator immobilized glass substrates, and EBiB (12.2 µL, 0.084 mmol), as an unbound sacrificial ATRP initiator, was added to the solution under an argon atmosphere in a globe box. The ATRP reaction was conducted for 18 h at 25 ˚C. After the reaction, poly(NIPAAm-r-NAPMAm) grafted glass substrates were washed with acetone, methanol, 50 mmol L-1 EDTA solution, and water. Poly(NIPAAm-rNAPMAm) grafted substrates were immersed in 0.1 M Na2CO3 aqueous solution for 1 day, and washed with water. After the washing, poly(NIPAAm-r-NAPMAm) grafted surfaces were dried in a vacuum oven for 3 h at 100 °C. After polymerization, the reaction solution was dialyzed against 0.1 M Na2CO3 aqueous solution for 1 day and distilled water for 5 days using dialysis membrane (MWCO: 1000) (Spectrum Laboratories, Rancho Dominguez, CA), and the polymer was recovered by freeze-drying. Numberaverage molecular weights and polydispersity index (PDI) of the copolymers were determined by a gel permation chromatography (GPC) (Tosoh, Tokyo, Japan) using DMF containing 50 mM LiCl as a mobile phase. A calibration curve was obtained using poly(ethylene glycol) standards. NAPMAm content in the copolymer was determined by 1H NMR (JOEL, JNM-LA400WB) using D2O as a solvent. Conjugation of Ru(bpy) 3 to poly(NIPAAm- r NAPMAm) grafted glass substrates. Poly(NIPAAm-

co-NAPMAm) grafted surfaces were contacted with 400 µL DMSO solution containing Ru(bpy)3-NHS (28 mg) and triethylamine (33.3 µL) for 4 h at 25 ˚C. After the reaction, the substrates were washed with DMSO and water, and dried in a vacuum oven for 3 h at 100 °C. Measurements of LCST behaviors of the polymer solution. The optical transmittance of the poly(NIPAAm-r-NAPMAm-r-Ru(bpy)3NAPMAm) solution was measured by using a UV-Vis spectrophotometer (UV-2500PC, Shimazu, Japan). Transmittance changes at 583.5 nm as a function of temperature were measured in the oxidized Ru(III) and reduced Ru(II) states in 1 M HNO3 solutions containing 3 mM Ce(SO4)2 and 3 mM Ce2(SO4)3, respectively. Elemental analyses by X-ray photoelectron spectroscope (XPS). Elemental analyses were performed on the surfaces of the unmodified glass, the ATRP initiator immobilized glass substrate, the poly(NIPAAm-rNAPMAm) grafted substrates, and the poly(NIPAAm-rNAPMAm-r-Ru(bpy)3NAPMAm) grafted substrates by Xray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Fisher Scientific, Waltham, MA) using a monochromatic AlKα X-ray source and a take-off angle of 90°. Thickness measurement of the polymer brush. The thickness of the grafted polymer brushes on the glass substrates was measured using an atomic force microscope (AFM) (NanoScope, Veeco, Santa Barbara, CA, USA) in air at 25 °C, using the tapping mode measurement (silicon cantilever, scan rate : 0.5 Hz, tip velocity : 10.0 mm s-1). Part of polymer brush was cleaved from the glass substrate by scratching with a scalpel blade. Quantitative estimation of poly(NIPAAm- r NAPMAm) on glass substrates. Amount of grafted poly(NIPAAm-r-NAPMAm) was determined by an attenuated total reflection Fourier transform infrared spectroscope (ATR/FT-IR) (Nicolet 6700) (Thermo Fisher Scientific, Waltham, MA). Strong absorption of Si−O derived from glass substrates appeared at 1000 cm−1. Absorption of amide carbonyl derived from poly(NIPAAm-r-NAPMAm) on glass substrates appeared in the region of 1650 cm−1. The amount of poly(NIPAAm-r-NAPMAm) on glass substrates were determined by the peak intensity ratio of I1650/I1000, using a calibration curves (Figure S1 in Supporting Information).



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Table 2. Characterization of self-oscillating polymer brush surfaces. Amount of Reaction ratio of immobilized Sample Ru(bpy)3 d) amino group [%] [nmol cm-2] PB-7.7-0.15 0.7 0.15 2.2 1.4 /14 7.7 ± 1.4 30.3 PB-7.7-17 70 1.7 25 PB-16.2-3.8 1.4 /1.4 16.2 ± 4.6 74.1 70 3.8 22 PB-22.4-5.6 2.8 /1.4 22.4 ± 3.0 98.5 70 5.6 22 a) All samples are referred as PB-x-y where x and y represent the amount of grafted polymer brush and the amount of immobilized Ru(bpy)3, respectively. b) Determined by ATR/FT-IR measurement before introduction of Ru(bpy)3. c) Determined by AFM. d) Determined by UV-vis measurement. a)

[M]0 / [ I ]0 [M / mM]

Amount of grafted polymer b) [µg cm-2]

Thickness of polymer layer c) [nm]

Table 3. Characterization of free poly(NIPAAm-r-NAPMAm). [M]0 / [ I ]0 M w /M n NIPAAm / Sample a) Mn b) b) [M / mM] NAPMAm c) FP-19k 1.4 /14 1.9 x 104 1.20 89.9 / 10.1 FP-41k 1.4 /1.4 4.1 x 104 1.51 87.5 / 12.5 FP-67k 2.8 /1.4 6.7 x 104 1.48 86.6 / 13.4 a) All samples are referred as FP-x where x represents the numberaveraged molecular weight (Mn). b) Determined by GPC using DMF containing 50 mmol L-1 LiCl. c) Determined by 1H NMR.

Quantitative estimated amount of Ru(bpy) 3 immobilized to polymer brush surface. The amount of Ru(bpy)3 immobilized to polymer brush surface was determined by measuring the absorbance of the substrate at 460 nm by using a UV-Vis spectrophotometer (UVPC-2500, Shimazu, Kyoto, Japan) (the illustration of the experimental set up shown as Figure S2 in Supporting Information) and a calibration curve (Figure S3 in Supporting Information) prepared from a series of Ru(bpy)3 solution with known concentration. The amount of Ru(bpy)3 per area was estimated with the following equation (Lambert-Beer’s Law):

A = εCL = εC’ (1) where A is absorbance, ε is adsorption coefficient, C is concentration of Ru(bpy)3 [mmol L-1], and C’ is amount of Ru(bpy)3 per area [µmol cm-2]. Quartz crystal microbalance with dissipation (QCM-D) measurement. QCM-D measurements were conducted by using the Q-sense D300 system (Biolin Scientific, Gteborg, Sweden). In the measurement, a quartz crystal sensor coated with SiO2 thin layer (QSX303, Q-sense) was used. The self-oscillating polymer, poly(NIPAAm-rNAPMAm-r-Ru(bpy)3NAPMAm), was grafted on the surface of sensor by using the same procedure as mentioned above. The sensor substrate was installed into the open chamber, and the frequency shift (Δf) and dissipation shift (ΔD) were monitored in aqueous solution containing HNO3 (1 M) and Ce2(SO4)3 (3 mM) at respective temperatures. To measure the Δf and ΔD values in response to the oxidization of Ru(bpy)3, an aqueous solution containing NaBrO3 was added to the aqueous solution containing HNO3 at

[Ru(bpy)3-NHS] 0 [mM]

25 °C. The data measured at the seventh overtone (35 MHz) was used for the analysis. Observation of the BZ reaction on the selfoscillating polymer brush. The BZ reaction on the self-oscillating polymer grafted glass slides was observed by using fluorescence microscope (DFC 360FX, Leica, Mannheim, Germany) (excitation wavelength: 425nm, fluorescence wavelength: 585 nm) in a solution containing 0.81 M HNO3, 0.15 M NaBrO3, and 0.10 M malonic acid at 25 ˚C. Spatio-temporal image analyses were made by using PC software (Image J, National Institute of Health, Bethesda, MD, USA). RESULTS AND DISCUSSION Characterization of self-oscillating polymer brush surfaces. For one of the prepared self-oscillating polymer brushes, the elemental compositions were determined by X-ray photoelectron spectroscopy (XPS) (Table 1). The composition of carbon increased after immobilizing the ATRP initiator onto the glass substrate. After modifying poly(NIPAAm-r-NAPMAm) to the glass substrate through SI-ATRP, the compositions of carbon and nitrogen increased (Figure S4 in Supporting Information), but the composition of silicon, which derives from glass substrate, decreased. Additionally, higher absorbance at 460 nm attributed to Ru(bpy)3 were detected by UV-vis measurement after conjugation of Ru(bpy)3-NHS to the amino group of the grafted poly(NIPAAm-r-NAPMAm) (Figure S5 in Supporting Information). The result indicated that Ru(bpy)3 was successfully incorporated into the copolymer brush through the reaction between amino group of NAPMAm and NHS of Ru(bpy)3-NHS. Table 2 summarizes the results of surface characterizations for the self-oscillating polymer brushes prepared under several reaction conditions. The samples are referred as PBx-y, where x and y represent the amounts of grafted polymer brush and immobilized Ru(bpy)3, respectively. In the process of grafting poly(NIPAAm-r-NAPMAm) onto glass substrates by SI-ATRP, unbound sacrificial initiator (EBiB) was added to the ATRP reaction solution to promote polymerization.32 Additionally, the free polymer obtained from the


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Figure 1. (a) Temperature dependence of optical transmittance for the aqueous solutions containing poly(NIPAAm-co-NAPMAm-coRu(bpy)3) with the similar composition to the grafted polymer. (b) Oscillating profile of optical transmittance for 0.05 wt% poly(NIPAAm-co-NAPMAm-co-Ru(bpy)3) solution at 25 ºC. BZ reaction substrates: [HNO3] = 0.81 M, [NaBrO3] = 0.15 M, [MA] = 0.1 M.

Figure 2. (a) Temperature dependence of frequency shift (Δf) and dissipation shift (ΔD) for the self-oscillating polymer brush in the reduced state. Outer solution: [HNO3] = 1 M (1 mL). (b) Frequency shift (Δf) and dissipation shift (ΔD) in response to addition of oxidant (NaBrO3) at 25 °C. Outer solution before adding oxidant: [HNO3] = 1 M (1 mL). Additional solution: [NaBrO3] = 2 M (50 µL).

ATRP reaction solution was used for estimating the molecular weight and the composition of the grafted polymer. As for the structure of the polymer, the previous study indicated that the arrangement of the polymer was random by analyzing the monomer reactivity ratios for NIPAAm and NAPMAm.33 The amount of grafted polymer and the thickness of polymer layer increased by reducing unbound initiator concentration ([I]0) or increasing monomer concentration ([M]0) during ATRP. The number-averaged molecular weight (Mn) also increased by reducing [I]0 or increasing [M]0 (Table 3). As the concentration of Ru(bpy)3-NHS in the reaction solution increased from 0.7 mM to 70 mM, the amount of Ru(bpy)3 immobilized to the copolymer brush showed an approximately tenfold increase. The amount of Ru(bpy)3 also increased with the amount of grafted copolymer when the concentration of Ru(bpy)3-NHS was constant (70 mM). This is because the number of amino groups which act as a reaction site for the Ru(bpy)3-NHS increased as the amount of grafted polymer increased.

Characterization of free self-oscillating polymer. At the same time, a free self-oscillating polymer with a similar composition to the grafted polymer brushes was prepared in order to estimate the dynamic behaviors of the grafted polymer. The composition ratio of the free self-oscillating polymer was determined to be NIPAAm : NAPMAm : Ru(bpy)3NAPMAm = 89.9 : 7.3 : 2.8 by 1H NMR and UVvis measurements. The composition ratio of NIPAAm and NAPMAm was approximately the same as that of the feed composition. The reaction ratio of feed Ru(bpy)3-NHS was 93%. Figure 1(a) shows the temperature dependence of optical transmittance for the free polymer solution in both of the reduced and oxidized states. In both states, the transmittance of the polymer solution suddenly decreased as temperature increased. This is due to thermoresponsive phase transition properties of NIPAAm in the copolymer. When the Ru(bpy)3 moiety was kept in the oxidized state, the phase transition temperature shifted higher than that in the reduced state. As a result, when the polymer is dissolved in a catalyst-free BZ reaction solution (i.e., an aqueous solution



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Figure 3. (a) Chemical wave propagation on self-oscillating polymer brush (PB-7.7-1.7) observed by a fluorescence microscope. Scale bar: 500 µm. A dark band (an excited pulse of the oxidized state) moved from left to right. Time interval between each image: 10 s. (b) Spatio-temporal pattern of chemical wave propagation (PB-7.7-1.7).

containing HNO3, NaBrO3, and MA) at an appropriate constant temperature, the transmittance of the polymer solution oscillates coupled with redox changes of Ru(bpy)3 conjugated to the polymer (Figure 1(b)) (overall oscillating profile shown in Figure S6 in Supporting Information). This result suggests that conformation changes occur for the grafted self-oscillating polymer. Swelling-deswelling behavior of the selfoscillating polymer brush. To confirm that the grafted polymer actually exhibits swelling-deswelling behavior, the self-oscillating polymer brush was prepared on the surface of a SiO2 coated QCM-D sensor. In the process of SI-ATRP

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and Ru(bpy)3 conjugation, the concentrations of the reagents were the same as those for PB-7.7-1.7. Figure 2(a) shows temperature dependence of the frequency shift (Δf) and dissipation shift (ΔD) for the self-oscillating polymer brush in the reduced state. As temperature increased from 15 to 35 °C, Δf value increased while ΔD value decreased. The increase in Δf value indicates that the effective mass of the grafted polymer layer decreases due to dehydration. The dehydration with increasing temperature was also indicated by the decrease in ΔD value. The ΔD value represents the degree of energy dissipation derived from the viscoeleticity of polymer layer on the surface of sensor. Decrease in the ΔD value suggests that the self-oscillating polymer brush changed to a collapsed state as temperature increased. These changes due to hydration/dehydration of polymer chains are reversible with cyclic cooling-heating changes (Figure S7 in Supporting Information). Swelling behavior of the grafted polymer in response to the oxidization of Ru(bpy)3 was observed. At constant temperature, the reduced Ru(bpy)3 moiety can be oxidized by adding aqueous solution containing an oxidant (NaBrO3). Immediately after the addition of NaBrO3, the Δf value decreased and the ΔD value increased (Figure 2(b)). These results indicated that self-oscillating polymer brush swelled in the oxidized state more than in the reduced state. The swelling kinetics from oxidization was faster than those observed in the previous gel systems.34 This is because the thickness of the grafted polymer layer was much smaller than the size of bulk gels. The faster diffusion of the oxidant (NaBrO3) into the polymer brush is effective for the faster swelling kinetics. Evaluation of the dynamic behaviors of selfoscillating polymer brush. To evaluate the dynamic behaviors of the self-oscillating polymer brushes, the glass substrates grafted with self-oscillating polymer brush were

Figure 4. Oscillating profiles of self-oscillating polymer brushes. (a) PB-7.7-0.15, (b) PB-7.7-1.7, (c) PB-16.2-3.8, (d) PB-22.4-5.6. Outer solution: [HNO3] = 0.81 M, [NaBrO3] = 0.15 M, [MA] = 0.1 M. Temperature: 25 ºC.


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immersed in a catalyst-free BZ reaction solution and observed by fluorescence microscope. The fluorescence of Ru(bpy)3 (585 nm) was strong intensity in the reduced state only.15 Autonomous propagation of the chemical wave, which was observed as periodic changes of fluorescence intensity at a fixed point, successfully occurred on PB-7.7-1.7, PB-16.2-3.8, and PB-22.4-5.6 (Figures 3 and 4). However, oscillation was not observed on PB-7.7-0.15, because the amount of conjugated Ru(bpy)3 was not enough. The selfoscillation for PB-22.4-5.6 attenuated to the reduced state and stopped within 1000 s. The duration was shorter than those for PB-7.7-1.7 and PB-16.2-3.8. A possible reason for this attenuation is that the amount of immobilized Ru(bpy)3 is too large to induce stable oscillation. A previous study on the BZ reaction suggested that the oscillation does not take place at lower concentrations of the substrates of the BZ reaction (steady state). As the concentration exceeds a certain value, a stable and periodic oscillation occurs (oscillating state). However, if the concentration becomes much higher, the oscillation stops and the system stays at the steady state again.35 This would be applied to adjust the amount of Ru(bpy)3 and the the design of polymer brush systems. Thus, these results indicate that appropriate amount of Ru(bpy)3 should be immobilized in the polymer brush to induce stable oscillation. It is also possible that properties of the polymer chain such as polymer mobility have some effects on the oscillating behaviors. The feedback from mechanical motion of the polymer chain to the chemical reaction is also expected. Further study to reveal the details of the dynamic behavior of the self-oscillating polymer brush is currently being investigated. Considering potential applications to autonomous ciliarylike actuators, it is important to understand the spatiotemporal behaviors of the self-oscillating polymer brush and how these dynamics can be controlled. Figure 3(b) shows the spatio-temporal pattern constructed by sequentially lining up one-line images along the direction of wave propagation in each frame of the recorded movie. From this pattern, the period and the velocity was estimated to be 63.0 s and 48.5 µm s-1, respectively, for PB-7.7-1.7 at 25°C (values for each self-oscillating polymer brush in Table S2 in Supporting Information). The period was shorter than that typically obtained in gel systems.22 This is because the thickness of the polymer brush (30-100 nm) is much smaller than the size of bulk gels (100 – 1000 µm). It is suggested that an effective increase in concentration of the substrates by faster diffusion resulted in the shorter period. Such effects were already reported in our previous report, in which the period for self-oscillating nano-gel particles was shorter than that for the bulk gels.36 Next, temperature dependence of the oscillation frequency Fosc (Fosc = 1/P osc, P osc: oscillation period) and the wave velocity v were investigated. In general, both Fosc and v increase as temperature increases in accordance with the Ar-

Figure 5. Arrhenius plot for (a) the self-oscillation frequency (Fosc) and (b) the propagating wave velocity (v).

rhenius equation.37 As shown in Figure 5, this relationship was maintained in the self-oscillating polymer brush system. From the slope of the plots, the activation energy of autocatalytic reaction (Ek) and that of chemical wave propagation (EV) were estimated to be 94.7 kJ mol-1 and 73.4 kJ mol-1, respectively. The wave velocity is theoretically described as the following equation:

v = (4kD[H+][BrO3-])1/2 (2) where k is the rate constant of the autocatalytic reaction for the reaction intermediate (HBrO2) which acts as an activator, and D is the diffusion constant of the activator. The k at 20 °C was reported to be 42 M-2 s-1.38 By using the k value at 20 °C and Ek, k at any temperature can be calculated from the Arrhenius equation. Further, from the equation above, the D at 25 °C was estimated to be 6.0 x 10-7 cm2 s-1 by using the measured wave velocity v, the calculated k at 25 °C, and the concentration of the substrates. Oscillating behaviors under different concentrations of the substrates were investigated (Figure S8 in Supporting Information). As a result, wave velocity and the oscillating period at other concentration conditions is also predictable. The direction of chemical wave can be controlled by designing the shape of glass plate or patterning the polymer brush region on the plate. Such attempts are currently being investigated. In these ways, important physicochemical parameters can be decided, which will be helpful for several applications utilizing selfoscillating polymer brushes as novel functional surfaces. CONCLUSIONS In this study, self-oscillating polymer brushes with various immobilized amounts of Ru(bpy)3 were prepared for investigating a correlation between the surface nano-structure and the spatio-temporal behaviors of self-oscillation. The selfoscillating polymer brushes with several graft amounts of polymer and Ru(bpy)3 were prepared through SI-ATRP and the subsequent Ru(bpy)3 conjugation. For the prepared polymer brushes and free polymer, characterization was done by using several analyses. The free self-oscillating polymer with a composition similar to the grafted polymer brush exhibited soluble-insoluble oscillation in a catalyst free BZ reaction solution. QCM-D measurements revealed that



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the self-oscillating polymer brush actually undergoes swelling-deswelling changes in response to temperature change or oxidization of the Ru(bpy)3 moiety. By immobilizing an appropriate amount of Ru(bpy)3 to the grafted polymer, stable chemical wave propagation can be obtained. Inadequate or excessive amount of Ru(bpy)3 resulted in no oscillation or a shorter duration of oscillation. Activation energies of the reaction and diffusion for the BZ reaction occuring in the polymer brush were also obtained by analyzing the spatio-temporal behaviors. They can be utilized as guide parameters for designing autonomous functional polymer brush systems.

ASSOCIA_T ED C ONTENT Supporting Information. Experimental, FT-IR spectra, calibration curve for determining the amount of grafted polymer, calibration curve for determining the amount of immobilized Ru(bpy)3, XPS peaks, UV-vis spectra, QCM-D measurement, and the detailed oscillating profiles of the self-oscillating polymer brush. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research to R.Y. from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant No.15H02198) and Research Fellowships to T.M. from the Japan Society for the Promotion of Science for Young Scientists (No. 14J09992). We are grateful to Prof. Takashi Miyata (Kansai University) for letting us use the QCM-D instrumentation, and Ph. D student Ms. Catherine Shasteen for English editing.

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