Article pubs.acs.org/Macromolecules
Reversible Photomodulation of the Swelling of Poly(oligo(ethylene glycol) methacrylate) Thermoresponsive Polymer Brushes Ali Dirani, Xavier Laloyaux, Antony E. Fernandes, Bertrand Mathy, Olivier Schicke, Olivier Riant, Bernard Nysten, and Alain M. Jonas* Bio & Soft Matter, Institute of Condensed Matter, Nanosciences, Université catholique de Louvain, Croix du Sud 1/L7.04.02, B1348 Louvain-la-Neuve, Belgium ABSTRACT: Photothermoresponsive polymer brushes were synthesized by coupling azobenzene derivatives to oligo(ethylene glycol) methacrylate (OEGMA) based copolymer brushes grown from silicon substrates. Varying the length and chemical ending of the lateral chains (using different OEGMA comonomers) afforded a collection of thermoresponsive copolymer brushes with predetermined fraction of hydroxyl side groups. These pendent hydroxyl functions were subsequently used to anchor the photochromic modules onto the brushes via classical activation/coupling chemistry, albeit in modest to moderate yields depending on the brush composition, thickness and grafting density. The extent of swelling of the functionalized brushes in water could be reversibly modulated by photoisomerization of the azobenzene motifs, even though the amplitude of the observed variation remained small. Decreasing the grafting density of the brush afforded improved photoresponse possibly through decreased steric crowding between the chains, facilitating azobenzene coupling and isomerization in the confined system.
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INTRODUCTION
Among the library of available photochromes, azobenzenes have been the most extensively studied as a result of their generally clean, efficient and reversible UV/visible isomerization (Scheme 1). The geometry and polarity changes following isomerization from a lower energy trans isomer to a higher energy cis isomer may alter considerably the internal energy balance and morphology of the brush, which can be restored by visible light. Azobenzenes have already been grafted onto a variety of thermoresponsive polymers such as poly(N,Ndimethylacrylamide) (PDMAA),27−34 poly(N-isopropylacrylamide) (PNIPAM),26,33−42 poly(N-hydroxyethylacrylamide),37 poly(2-(dimethylamino)ethyl-methacrylate),43 poly(oligo-(ethylene glycol)methyl ether-methacrylate) P(OEGMA),44 and poly(2-(2-methoxyethoxy)ethyl-methacrylate) P(MEO2MA)34 at concentrations varying between 1 and 12 mol %. The temperature of collapse of gels, or the LCSTs of dilute solutions, was reported to shift upon UV irradiation by −15 to +20 °C. Nevertheless, it remains extremely difficult to predict the behavior of such systems, minute variations of structure often resulting in strongly different photoresponses as nicely illustrated by azobenzene-substituted PDMAA’s.27−34 Examples of photoswitchable polymer brushes are fairly limited.27,28,45−48 For instance, polymer brushes decorated with spiropyransanother popular family of photochromic moleculeswere capable of reversibly modulating the pore size of glass filters45 and to switch surface wettability45−47 by
Responsive polymer brushes are attractive systems for devices such as sensors and actuators,1−5 microfluidic systems,6 smart membranes,7 or biomedical devices.8−10 These brushes are dense layers of end-tethered macromolecules, whose degree of swelling in a given solvent can be abruptly and reversibly modified by an environmental trigger such as temperature, pH, ionic strength, or light.11−13 For instance, thermoresponsive polymer brushes can be stretched or collapsed in a proper solvent by adjusting the temperature below or above a lower critical solution temperature (LCST), respectively.14 Light-responsive polymer brushes are particularly appealing systems when remote actuation is desirable or for the miniaturization of the responsive area. Since the hallmark paper by Lovrien in 1967 on the reversible control by light of the viscosity of polymer solutions,15 photoswitchable polymers have attracted considerable attention, as testified by the series of reviews which regularly appeared over the last decades.16−25 Most previous studies on photothermoresponsive polymeric materials have concentrated on thermoresponsive solutions and gels containing photochromic groups. Unfortunately, in most cases, the slow response of gels has prevented their development for technological applications.26 In this context, switching to polymer brushes would appear as a logical step, since the generally much smaller thickness of brushes should favor higher response rates. Furthermore, the configuration of such photothermoresponsive surfaces would permit independent, simultaneous, and minute modulation of surface properties, which is of interest for biological applications. © XXXX American Chemical Society
Received: October 8, 2012 Revised: November 21, 2012
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Scheme 1. Schematic Representation of the Preparation of OEGMA-Based Photothermoresponsive Copolymer Brushes from Silicon Surfacesa
a
The brush grafting density is controlled by dilution of the ATRP initiator silane with benzyldimethylchlorosilane.
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photoinduced isomerization. Dual responsive surfaces obtained by copolymerization of thermoresponsive PDMAA and photochromic azobenzene−methacrylate derivatives were shown to shift their collapse transition by a few degrees upon irradiation.27,28 Recently, thermoresponsive PNIPAM polymer brushes decorated with fluorescence acceptors and photoswitchable donors in their inner and outer layers, respectively, were capable to emit multicolor fluorescence through response to temperature and light stimuli.48 Herein, photothermoresponsive polymer brushes were synthesized by incorporating azobenzene derivatives onto thermoresponsive oligo(ethylene glycol) methacrylate (OEGMA)-based copolymers brushes49−52 grown from silicon substrates (Scheme 1). Diluting hydroxy-terminated OEGMAs (HOEGMA, and/or hydroxy ethyl methacrylate HEMA) with methoxy-ended OEGMAs (MEO2MA and/or OEGMA) in the copolymerization medium afforded a collection of thermoresponsive brushes with a priori determined concentrations of postreactive pendent −OH functions located more or less away from the chain backbone depending on the oligo(ethylene glycol) side chain length. These brushes were used for the grafting of the azobenzene units through classical activation/coupling chemistry, thus merging thermo- and photoresponse potential within a single framework. In the present study, we particularly focus on the impact of the brush composition and grafting density in order to attain optimal azobenzene grafting, isomerization and associated photomodulation of the collapse transition of the thermoresponsive systems.
EXPERIMENTAL SECTION
Materials. All reagents were from commercial sources and used without further purification. 2-(2-methoxyethoxy)ethyl methacrylate (MEO2 MA), oligo(ethylene glycol)methyl ether methacrylate (OEGMA) with an average molar mass of 300 g/mol, 2-hydroxyethyl methacrylate (HEMA), and hydroxyoligo(ethylene glycol) methyl ether methacrylate (HOEGMA) with an average molar mass of 360 g/ mol were purchased from Aldrich. The synthesis of the ATRP initiator silane, 3-(chlorodimethylsilyl)propyl-2-bromo-2-ethylpropanoate, was described elsewhere.53 Milli-Q water (resistivity: 18.2 MΩcm) was obtained from a Millipore system. Single-side polished silicon wafers (⟨100⟩ orientation) and quartz (Suprasil) were from ACM and Hellma, respectively. Quartz crystal sensors covered by a layer of SiO2 (QSX 303) were purchased from Q-Sense (Sweden) and rinsed with ethanol before use. The different steps of the surface preparation are described below and are identical for silicon wafers, quartz substrates and the QCM-D sensors, unless otherwise stated. Brush Growth. Polymer brushes were grown according to a protocol published before.49 Si wafers, quartz substrates and glassware were cleaned by immersion in a piranha solution (Caution! piranha solution is a strong oxidant which reacts violently with organic compounds), whereas quartz sensors were cleaned by UV/ozone. The substrates were immediately silanized with a mixture of the ATRP initiator silane and benzyldimethylchlorosilane by gas phase silanation. Briefly, the substrates were placed on a Teflon holder and the whole placed in a Schlenk tube. Three cycles of argon/vacuum (5/15 min respectively) at 80 °C were performed on the system before injection of the silane mixture. After 2 h reaction, the thickness of the silane monolayer was 0.8 ± 0.2 nm as determined by X-ray reflectometry. Different series of polymer brushes were synthesized by controlled radical polymerization, using a mixture of MEO2MA, OEGMA, HOEGMA, and/or HEMA in the feed solution, keeping the total molar content in methacrylate units constant. For each series of brushes, a series of initiator-covered silicon substrates were immersed B
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50 °C, using a previously published method.14 The photomodulation of the collapse transition was studied under UV or visible irradiation for ca. 50 min at a given temperature. The optical fiber was placed at a distance preventing possible thermal heating of the QCM-D module due to the irradiation. In the sequel, the frequency shifts of the nth overtone, Δf n, are given after multiplication by n. UV Spectroscopy. UV absorbance spectra of the polymer brushes on quartz substrates in air or in solution were recorded with an UV− vis spectrometer (Varian Cary 50) at a wavelength scan rate of 4 nm/s. The absorbance spectra of the polymer brushes containing pendent azobenzene groups were obtained after subtraction of the absorbance spectra of pure polymer brushes. The absorbance of the polymer brushes in the dry state was measured by placing the substrate on the appropriate holder. For measurement in solution, the substrate was immersed vertically in a UV cuvette filled with water. The yield of azobenzene coupling was extracted as follows. The azobenzene concentration in a dry polymer brush is cazo= APB/(2h·ε) where h is the polymer brush thickness, and APB is the absorbance of the azobenzene-grafted dry polymer brush, integrated from 265 to 400 nm (which is the spectral region of the main absorbance peak of the trans-azobenzene). The factor 2 is due to the presence of a brush on both sides of the quartz substrate. The extinction coefficient ε of the azobenzene molecule in ethanol, integrated over the 265−400 nm spectral range, was determined to be 1.02 × 104 m2·mol−1·nm. On the other hand, the molar concentration of hydroxyl functions in the polymer brushes is cOH ≈ ρ·yn/Mw where yn is the molar ratio of monomers bearing hydroxyl functions in the brush, Mw is the average molar mass of the repeat units of the chains, and ρ is the mass density of the dry polymer brushes evaluated to be 1.3 g/cm3 in a prior work.14 The average yield of the azobenzene coupling in a brush of total thickness h is then defined as Y(h) = cazo(h)/cOH. Photoisomerization. Photoisomerization experiments were conducted using a Lightningcure UV Spot light source LC5 from Hamamatsu equipped with a 150 W xenon bulb (L8253) of wavelength ranging from 240 to 2000 nm and assorted optical fibers. A colored glass filter (Lot Oriel) with a pass band from 290 to 400 nm was used for the trans−cis isomerization, while a filter with a pass band from 400 to 500 nm was used for the cis−trans isomerization. The light intensity is ca. 300 mW/cm2 at the end of the optical fiber.
in the feed solution in an oxygen-free atmosphere (Schlenk tubes) and removed at increasing times to check the polymerization kinetics. When the ellipsometry-determined thickness reached about 80 nm, the polymerization was stopped and the resulting ca. 80 nm-thick brushes were used for further experiments. In parallel, polymer brushes were also grown from quartz substrates and from acoustic QCM-D sensors for characterization. The brush growth rates on silicon substrates, quartz surfaces and QCM-D sensors were similar. The polydispersity of the chains could not be measured but is expected to be small since the brush growth is well-controlled. Azobenzene Synthesis and Grafting. The synthesis of the amino-azobenzene derivative utilized in this study was described elsewhere.33 The 1H NMR spectrum was similar to the one reported in the literature. The brushes were first activated prior to coupling with the azo compound. The surfaces were immersed in a solution of N,N′disuccinimidyl carbonate (DSC) (0.20 mmol per sample) and 4dimethylaminopyridine (DMAP) (0.20 mmol per sample) in DMF (2 mL per sample) for 1 h. The samples were then recovered, washed with DMF and directly immersed into a solution containing the aminoazobenzene (0.15 mmol per sample) in DMF (2 mL per sample). After 24 h of reaction, the samples were removed and thoroughly washed into a Soxhlet apparatus (CH2Cl2) to give the ultimate photothermoresponsive surfaces. Ellipsometry. The film thickness was measured with a spectroscopic ellipsometer (Uvisel from Horiba-Jobin-Yvon, France) at an incidence angle of 70° and in a wavelength range from 400 to 850 nm. The ellipsometric data were fitted by the DeltaPsi 2 software of the apparatus. The ellipsometric model consists of three layers: silicon (bulk), native silicon oxide (1.5 nm thickness) and polymer brush. The complex index of refraction of Si and native SiO2 were taken from tabulated data provided by the manufacturer. The complex index of refraction of the brushes, n − jk, was modeled by a transparent Cauchy layer with n(λ) = A + Bλ−2 + Cλ−4 and k(λ)= 0, with A, B, and C being three fitted parameters and λ the wavelength. The measurement was carried out three times at different spots on the substrate. For a few selected samples, the evaluated ellipsometric thickness of the polymer brushes was compared to the thickness measured by X-ray reflectometry. Both measurements generally agreed within better than 10%. The brush thickness on the QCM-D sensor was likewise evaluated by a model using four layers: gold, titanium, silicon oxide and polymer layer (Cauchy layer). The values of the index of refraction of Ti, Au, and SiO2 were again tabulated values. No ellipsometry measurement was attempted to characterize the polymer brushes on quartz by ellipsometry. However, the thickness of the polymer brushes on quartz was determined by X-ray reflectometry for some samples, and was not different from the values determined by ellipsometry on silicon wafers processed in parallel. X-ray Reflectometry (XRR). XRR measurements were carried out with a modified Siemens D5000 2-circle goniometer (0.002° positioning accuracy). X-rays of 0.15418 nm wavelength (Cu Kα1) were obtained from a Rigaku rotating anode operated at 40 kV and 300 mA, fitted with a collimating mirror (Osmic, Japan) delivering a close-to-parallel beam with ∼0.0085° vertical angular divergence. The XRR data were analyzed according to our previously reported method.54 Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). QCM-D measurements were performed in water with a QSense E4 microbalance. The AT-cut quartz crystal sensor of 14 mm diameter was oscillating at its fundamental frequency ( f 0 = 5 MHz) or one of its overtones. All overtones were acquired, although the third, fifth, and seventh overtones were generally selected for analysis unless otherwise mentioned. The thickness of the SiO2 layer covering the sensor was evaluated by ellipsometry on five different sensors to be 23 ± 2.5 nm. The quartz sensor was maintained in a flow module with one side exposed to the solution. The module temperature was maintained with a precision of 0.02 °C. A glass window mounted on the module allows in situ UV/light irradiation. The determination of the temperature of the collapse transition was realized in dynamic mode with heating and cooling ramps of 0.2 °C/min, between 15 and
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RESULTS AND DISCUSSION Growth of Thermoresponsive Polymer Brushes Incorporating Hydroxyl Functions. Thermoresponsive polymer brushes were synthesized by surface-initiated atom transfer radical polymerization (SI-ATRP) according to a wellestablished procedure.49 Briefly, an ATRP-initiator monolayer was first self-assembled by gas phase silanation. The active surfaces were subsequently used to initiate the brush polymerization by immersion into a concoction of OEGMA monomers, bipyridine and copper catalysts.49 The OEGMAs were selected so as to create a thermoresponsive brush framework14,49−52 which can be straightforwardly postderivatized to graft the azo photochromes. Therefore, our choice was directed toward hydroxy-ended OEGMAs (HEMA, HOEGMA) where the pendent −OH groups can be reacted with a broad range of primary amines using robust activation/coupling chemistry (Scheme 1).53,55−57 Additionally, in the aim to adjust the collapse transition of the brush (TCT)49,58 as well as to vary the degree of postfunctionalization, the hydroxy-terminated OEGMAs were diluted with “inert” methoxy-ended OEGMA monomers (MEO2MA, OEGMA) while keeping constant the methacrylate concentration in the feed solution. Overall, OEGMAs of various side chain lengths and endings were screened for the preparation of thermoresponsive copolymer brushes amenable to flexible postmodification reactions. This well-controlled living polymerization provides polymer brushes with a thickness of ca. 80 nm after a few hours by C
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slightly adjusting the polymerization time according to the composition of the polymerization solution. In parallel, the hydroxyl function content can be controlled from 0 to 100% according to the molar ratio of hydroxy-substituted monomers. The composition of the polymer brushes is assumed to be similar to that of the polymerization solution, which is reasonable due to the structural similarity of the different monomers. Therefore, a brush denoted P(MEO2MAx1-coOEGMAx2-co-HOEGMAy1-co-HEMAy2) indicates that the polymerization was performed in the presence of x1 mol % of MEO2MA, x2 mol % of OEGMA, y1 mol % of HOEGMA and y2 mol % of HEMA, with x1 + x2 + y1 + y2 = 100. Tuning the Grafting Density of the Polymer Brushes. In the aim to adjust the grafting density of the polymer brushes, the ATRP-initiator silane was diluted with an ATRP-inactive silane depicting similar boiling point and molecular volume, namely benzyldimethylchlorosilane. The composition of the silane solution controls the final density of ATRP initiator anchored on the surface. Because the silanation was performed at high temperature from the gas phase using only monochlorosilanes, reproducible monolayers were obtained; in addition, the silanes should not be microphase-separated in the monolayer, in contrast with liquid phase silanation.59 The surface grafting density (i.e., the number of silanes grafted per unit area) was evaluated by X-ray reflectometry using a methodology presented before;54,60,61 it amounted to ca. 1 silane/nm2 for the pure ATRP initiator silane monolayer. No attempt was made to determine the number of ATRP silane molecules grafted when using mixed silanes. In contrast, the grafting density of the polymer brushes, which is defined as the number of polymer chains per unit area, was characterized by measuring the dry thickness of the polymer brushes grown from the silanized substrates. The chain grafting density is σ = ρh/Mw, where ρ is the specific mass of the polymer, Mw the average molar mass of the chains, and h the brush thickness. Assuming that the molar mass of the chains does not depend on the grafting density, which requires the kinetics of the polymerization not to depend on σ either, the variation of h provides thus a direct view of the relative variations of σ. An absolute determination of σ can be obtained by measuring the molar mass of the polymer chains, either by addition of sacrificial initiator in the polymerization solution, or by cleavage of the silane to collect the polymer chains.62,63 However, these measurements are not accurate, either due to different kinetics between solution and surface-initiated polymerization,64 or due to the very low amount of matter grafted on silicon wafers. Therefore, in this work, the dry thickness of the polymer brushes was solely employed as an indicator of the relative grafting density for the different polymer brushes. Figure 1 presents the dry thickness of P(MEO2MA) and P(MEO2MA70-co-HOEGMA30) brushes depending on the composition of the silane mixture. The dry thickness increases progressively from 0 to 65 and 82 nm, respectively, when the content in ATRP initiator silane of the silanation solution increases from 0 to 100 mol %. The increasing dry thickness of the polymer brushes results from a larger number of polymer chains initiated from the surface when the silane monolayer is richer in ATRP initiators. Indeed, a higher content of ATRP initiator silane decreases the average distance between the growing chains, which results in polymer brushes of larger grafting density, and therefore higher thickness.62 The brush thickness (hence grafting density) is only slightly sensitive to the concentration of the ATRP
Figure 1. Dry thickness of P(MEO2MA70-co-HOEGMA30) (open circles) and P(MEO2MA) (closed circles) brushes depending on the content in ATRP initiator silane (mol %) of the silanation solution. The polymerization time was fixed at 4 h.
initiator silane above 50 mol % of initiator in the silanation solution. This is because the chain grafting becomes dominated by the steric hindrance between chains when the density of the initiating sites is large enough.62,63,65,66 Overall, Figure 1 indicates that variations of σ by a factor of 5−10 are easily achievable for our brushes. Collapse Transition of the Thermoresponsive Polymer Brushes. The collapse transition of the brushes was characterized by QCM-D, a technique often used to characterize adsorption/desorption phenomena. However it is also very attractive to monitor the property variations of adsorbed layers, and specially to probe the collapse transition of responsive polymer brushes.14,67−69 The control unit of the QCM-D measures the shift of resonance frequency, Δf, due to the (de)swelling of the brush, as well as the dissipation shift, ΔD, which corresponds to the variation of damping of the oscillation. The determination of the collapse transition temperature by QCM-D is explained in previous work.14,67,70−73 P(MEO2MA) and P(OEGMA) brushes display a bulk collapse transition around 22 and 72 °C, respectively.14 By random copolymerization, the temperature of the collapse transition of (P(MEO2MA-co-OEGMA) brushes varies linearly with the monomer ratio and can be adjusted between 22 and 72 °C.14,49,74 The addition of HEMA sequences tends to decrease the temperature of the collapse transition while HOEGMA sequences increase it.10 P(MEO2MA-co-OEGMA-co-HOEGMA-co-HEMA) brushes thus provide platforms with a finely adjustable thermocollapse transition and an independently varying content in postfunctionalizable moieties. Table 1 shows different compositions of polymer brushes all having an average collapse transition around 38 °C, but combining different amounts of hydroxyl groups. The investigated range of grafting densities was found not to affect the temperature of the thermocollapse transition (results not shown). Azobenzene Coupling in the Thermoresponsive Polymer Brushes. Preliminary studies of the coupling of the amine-substituted azobenzene to the hydroxyl groups of the brush (Scheme 1) were performed on dense N,N′-disuccinimidyl carbonate (DSC)-activated53,55−57 P(HOEGMA) homopolymer brushes. The coupling reaction was relatively inefficient compared to a similar procedure performed on chains in solution, with a coupling yield amounting to ca. 10% only. This low yield most probably results from the lack of D
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the grafting yield is first and foremost controlled by the brush grafting density, and not by the intrinsic reactivity of the amine and hydroxyl functional groups. In order to investigate more carefully the penetration of the polymer brushes by DSC and azobenzene molecules, the yield of the azobenzene coupling was also evaluated at constant grafting density for different thicknesses of P(MEO2MA70-coHOEGMA30) brushes (grown from a substrate silanized with a solution containing 60 mol % of ATRP initiator silane for different polymerization times). Figure 3 shows that, for thin
Table 1. Characteristics of Various Polymer Brushes Exhibiting a Collapse Transition around ca. 38 °C, with a Hydroxyl Group Content Ranging from 10 to 50 mol %a nomenclature P(MEOMA75-coOEGMA15-coHOEGMA10) P(MEOMA73-co-OEGMA7co-HOEGMA20) P(MEOMA70-coHOEGMA30) P(MEOMA30-coOEGMA40-co-HEMA30) P(MEOMA50-coHOEGMA20-co-HEMA30)
−OH content (mol %)
TCT (°C)
TCT (°C) after azobenzene coupling
10
37
24
20
38
21
30
39
16
30
40
27
50
38
