Article pubs.acs.org/Langmuir
Poly(N,N‑dimethylaminoethyl methacrylate) Brushes: pH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte Marc Thomas,†,§,‡ Martyna Gajda,‡,§ Crispin Amiri Naini,†,§ Steffen Franzka,§,∥ Mathias Ulbricht,‡,§ and Nils Hartmann*,†,§,∥ †
Physical Chemistry, Department of Chemistry, and ‡Technical Chemistry II, Department of Chemistry, University of Duisburg-Essen, 45117, Essen, Germany § Center for Nanointegration Duisburg-Essen (CENIDE) and ∥Interdisciplinary Center for Analytics on the Nanoscale (ICAN), University of Duisburg-Essen, 47057 Duisburg, Germany S Supporting Information *
ABSTRACT: The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times.
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INTRODUCTION Stimuli-responsive polymer brushes offer unique perspectives in designing smart interfaces and devices.1−10 Examples include switchable membranes, microactuators, photochromic displays, responsive cell culture substrates, and protein-repellent surfaces.11−16 A detailed knowledge of the underlying physicochemical constraints which rule the switching processes of these films is essential in order to advance the development of such applications.17−19 In addition to the thermodynamics, the switching kinetics is of particular interest, especially for those applications demanding fast response times.7−10,20−23 Different techniques have been used in order to study the temporal switching behavior of stimuli-responsive polymer brushes, including atomic force microscopy, ellipsometry, and quartz crystal microbalance measurements.24−33 Commonly structural changes have been monitored on a second and minute time scale. In a previous contribution we reported on a novel laser temperature-jump technique, whichfor the first timeprovided insight into the intrinsic switching kinetics of ultrahin thermoresponsive polymer brushes.18 The results demonstrated fast swelling and deswelling of poly(N-isopropyl© 2015 American Chemical Society
acrylamide) (PNIPAAm) brushes on a micro/millisecond time scale. In this contribution we employ this technique in order to investigate the intrinsic switching kinetics of poly(N,Ndimethylaminoethyl methacrylate) (PDMAEMA) brushes. PDMAEMA is a weak polyelectrolyte, which exhibits a volume phase transition in water at a critical temperature Tc in the range of 35−40 °C, where deswelling of the polymer takes place.34−38 Note that this critical temperature commonly is also referred to as the lower critical solution temperature (LCST) of the polymer.17 Contrary to PNIPAAm, the thermoresponsive switching behavior of PDMAEMA is strongly affected by the pH, i.e., via protonation of the amine groups at acidic conditions.34 Potentiometric titration profiles and 1H NMR measurements allow one to measure the protonation as a function of the pH.39,40 Protonation increases the charge density and osmotic pressure in the polymer and hence its Received: September 13, 2015 Revised: November 11, 2015 Published: November 16, 2015 13426
DOI: 10.1021/acs.langmuir.5b03448 Langmuir 2015, 31, 13426−13432
Article
Langmuir degree of hydration and swelling ratio.37 As a result, a shift of the phase transition to higher temperatures is observed.34 A recent theoretical study supports a strong hydration of the protonated amine groups throughout the temperature-induced coil-to-globule transition of PDMAEMA in solution, whereas the unprotonated amine groups only marginally participate in H-bonding to water molecules.41 In addition, as depicted in Figure 1, along with protonation also conformational changes
for about 5 min and dried in a stream of high-purity argon (5.0, Messer). Then the samples were cleaned for 45 min in a low-pressure oxygen plasma (Femto with a 40 kHz generator, Diener Electronic) at a generator power of 100 W and an oxygen gas flow and pressure of about 9 sccm and 0.5−0.6 mbar. Prior to atom transfer radical polymerization (ATRP), the cleaned substrates were silanized in an argon stream with aminopropyltriethoxysilane (APS, 96%, ABCR) using a gas flow cell with a volume of 150 mL.18 For saturation with the silane the argon stream is led over a container with 40 μL of APS at a flow rate of 3.1 L min−1. The silanization time is 45 min. This yields silane monolayers with a thickness of 0.6−0.7 nm. Finally, the silanized substrates were stored for 1 day in air in order to allow for cross-linking of the organic monolayer. Surface-initiated ATRP was carried out following a routine as described in the literature.50 For immobilization of the ATRP initiator, the samples were put in a solution of 100 μL of α-bromoisobutyryl bromide (98%, Aldrich), 1390 μL of triethylamine (99.5%, Fluka), and 61.0 mg of 4-(dimethylamino)pyridine (>98%, Fluka) in dry acetonitrile (p.a., 99.99%, Acros) for 2 h. After reaction, the samples were washed twice in acetonitrile and twice in methanol. ATRP grafting then was carried out in a 100 mL three-neck flask with septum under an argon atmosphere. A solution of N-dimethylamino methacrylate (DMAEMA, 97%, Aldrich) and bipyridine (Bipy, >99%, Sigma-Aldrich) in a methanol/water mixture 7:3 (v/v) was prepared first. After 30 min degassing with argon (5.0, Messer), copper(II) chloride (99.999%, Aldrich) and copper(I) chloride (>99.9%, Acros) were added successively with strong stirring under a continuous argon stream. The molar ratio of DMAEMA/Bipy/copper(I) chloride/copper(II) chloride was 100:1.2:0.7:0.1. When a dark brown solution was formed, a volume of 10 mL was injected with help of a gastight syringe through the septum into the flask containing the prefunctionalized substrates. To stop the polymerization, 10 mL of a solution of 250 mg of copper(II) chloride (>99.999%, Acros) in methanol/water 7:3 (v/v) was added quickly. The washing procedure was as follows: After quenching the reaction, the samples were washed twice in water and twice in pure methanol. Drying was done with a dry argon stream (5.0, Messer). Typical polymer layers exhibited a dry film thickness between 44 and 90 nm. Considering the thickness of swollen polymer layers, as obtained from AFM measurements, a grafting density of 5.2 × 1013 cm−2 is calculated (cf. Supporting Information). Dynamic and static micromanipulation experiments employing a novel laser temperature-jump technique were carried out in order to characterize the pH-dependent thermoresponsive switching behavior of the polymer brushes.18,19 The pH was adjusted using acidic and alkaline aqueous solutions as prepared by mixing appropriate amounts of deionized water (18 MΩ·cm, Milli-Q) and standard HCl (1 M, Bernd Kraft) and NaOH (0.1 M, Waldeck) solutions, respectively. The pH values were measured employing an electronic pH meter (pH 213, Hanna Instruments). Experiments at pH < 4 and pH > 10 were not feasible using the Si/Ti-coated glass plates due to detachment and crack formation of the metal film. A microfocused CW-laser beam at λ = 532 nm was used for backsurface heating of the substrate/water interface. The 1/e laser spot exhibited a size d1/e2 = 2.8 μm, and the laser power was adjusted in the range P = 2−10 mW. During laser irradiation a stationary temperature profile is rapidly established inducing local deswelling of the polymer layer. Melting standards and calculations on the basis of the underlying heat conduction equation were used in order to determine the temperature profile at a given laser power. In addition, the phase transition temperature of PNiPAAm brushes was used for temperature calibration. Because of interference effects, respective thickness changes of the polymer layer can conveniently be monitored using a reflective optical microscope with water immersion optics (Nikon 40× Fluor 0.80W ∞/0) and a CCD camera. The reflectance is proportional to changes of the polymer thickness. Analysis of the deswollen polymer structures in static micromanipulation experiments upon continuous laser irradiation yields the temperature-dependent scaled swelling ratio.18 Analysis of local intensity changes in dynamic
Figure 1. Conformational changes (top) and relative degree of protonation α (bottom) of PDMAEMA as a function of the pH from ref 40 (the fitted line is to guide the eyes only).36,39,40
of the side groups are proposed to take place.36,42 Whereas a cyclic arrangement is assumed to form at pH > 7, a linear conformation is expected in acidic solutions, i.e., at pH < 7.36 Besides from PDMAEMA solutions, gels and thin films,34−38 PDMAEMA brushes and related tertiary amine methacrylate systems have been studied in some detail.28−33,43−48 Previous work mainly focused on the dependence of the switching behavior of PDMAEMA brushes on pH and ionic strength. This includes studies on the pH-dependent switching kinetics.28−33 In contrast, only comparatively few contributions addressed the thermoresponsive switching behavior and its pH dependence.46−48 Moreover, the temperature-dependent switching kinetics of PDMAEMA brushes still is largely unexplored. The investigation of the intrinsic switching kinetics of thermoresponsive polymer brushes is particularly challenging as it makes high demands on the overall temporal resolution of the experimental technique.18,19 Both rapid heating and fast detection are essential in order to instantly establish a constant stimulus and follow the switching process.18 Laser temperaturejump techniques in this respect have been proven to offer unique opportunities.18,19
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EXPERIMENTAL SECTION
As substrates Si/Ti-coated glass plates and Si substrates were employed. Si/Ti-coated glass plates were used for laser temperaturejump experiments.18 Deposition of 150 nm Ti and 150 nm Si onto glass plates (BK7, Schott) was carried out via physical vapor deposition. Once the substrates were transferred to air, surface oxidation set in forming a thin SiO2 layer. Si substrates were used for experiments employing atomic force microscopy (AFM). Commercial Si(100) wafers (p-type, 1−20 Ω·cm) exposing a native oxide layer were cut into pieces and used as substrates.49 For cleaning, the substrates were immersed in a supersonic bath of ethanol (p.a., VWR) 13427
DOI: 10.1021/acs.langmuir.5b03448 Langmuir 2015, 31, 13426−13432
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Langmuir micromanipulation experiments upon intermittent laser irradiation, i.e., upon rapidly switching the heating on or off, allows one to monitor the temporal switching process on a micro/millisecond time scale. Fitting of the temporal intensity curves considering first-order kinetics yields the temperature-dependent rate constants k for swelling and deswelling (cf. Supporting Information).18 Complementary thickness measurements of the polymer layers were carried out using an AFM (Dimension Icon, Bruker) operated in peak force tapping mode. The peak force was set to low values in order to avoid compression of the polymer layer. For these measurements, the polymer layers were locally removed via photothermal laser patterning with a microfocused CW-laser operated at λ = 532 nm and d1/e2 = 2.6 μm.49,51 Hence, bare substrate areas were created, which served as a reference level in the AFM measurements. Dry polymer layers were measured in air. For in situ measurements in aqueous media a special cantilever holder was used. Starting in water at a pH of 7, the pH in subsequent measurements was adjusted using appropriate amounts of diluted HCl and NaOH, respectively. No background electrolyte was added. All AFM measurements were carried out at room temperature. In situ measurements at higher temperatures close to Tc and above are not feasible.
as observed in static laser micromanipulation experiments upon continuous irradiationare formed. Noteworthy, deswelling remains confined to a much smaller area at pH = 4, indicating a significantly higher phase transition temperature in acidic solutions. We note that a very faint dark spot remains visible in the center of the irradiated areas after the laser has been switched off and swelling of the polymer brush has taken place. This is attributed to the overshooting effect of PDMAEMA as reported in the literature.36 The pH-dependent shift of the phase transition temperature has been investigated in detail by static laser micromanipulation experiments. The analysis of the data from such experiments yields the scaled swelling ratio S as a function of the temperature. As displayed in Figure 3a, the phase transition becomes sharper and is shifted to higher temperatures from Tc = 60 °C to Tc = 75 °C in the pH range from 10 down to 4. Note, Tc here is defined at S = 0.5.
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RESULTS AND DISCUSSION First experiments are carried out with PDMAEMA brushes exhibiting a dry thickness of 55−65 nm. Figure 2 displays two
Figure 2. Image series of optical micrographs from dynamic laser micromanipulation experiments of PDMAEMA brushes: (a) at pH = 4 and (b) at pH = 10. The dry polymer layer thickness is 55 nm. For laser manipulation a laser power of 5.3 mW has been used.
Figure 3. Temperature- and pH-dependent swelling behavior of PDMAEMA brushes: (a) scaled swelling ratio from static laser micromanipulation experiments and (b) swelling rates (left) and deswelling rates (right) from dynamic laser micromanipulation experiments. The dry polymer thickness is 55 nm. For laser manipulation a laser power of 5.3 mW has been used. The 2σ error of the measurement in (a) is equal to the symbol size or smaller. The error bars in (b) refer to 2σ.
optical micrograph series recorded in dynamic laser micromanipulation experiments at acidic and alkaline conditions, i.e., at pH = 4 and pH = 10, respectively. Generally, two regions can be distinguished. In the inner areas photothermally induced deswelling of the polymer film takes place. Because of reflectance changes at the water/polymer/substrate interface, these areas appear in dark gray, whereas the surrounding outer areas with the swollen polymer film are light gray. Note that the somewhat brighter spot in the middle of the micrographs at pH = 10 corresponds to a defect in the polymer layer (cf. Supporting Information). Note also, in contrast to our study on PNIPAAM brushes,18,19 the current experiments have been carried out using a higher focusing optics. In addition, in view of the larger side groups, the absolute grafting density of PDMAEMA brushes is expected to be somewhat lower. For these reasons, the intensities and the contrast are lower when compared with our previous work. As evident from these data, rapid switching takes place on a millisecond time scale. At longer times stationary structures
Particularly strong changes are evident at pH ≤ 7, where protonation of the amine groups becomes significant (cf. Figure 1). For a discussion of these results, it is illustrative to consider the structural changes, which are observed in alkaline and acidic solutions. At pH ≫ 7, PDMAEMA essentially is uncharged.39 At room temperature the polymer chains are strongly hydrated and extend into the solution.46−48 Hydrogen bonds stabilize additional water within the polymer brush.41 With increasing temperature, dehydration and collapse of the polymer chains take place.46,47 The release of water during this phase transition is endothermic but entropically favorable.19 Protonation of the amine groups at pH < 7 is very much expected to affect this hydrophobic collapse, i.e., increasing the amount of bound water and strengthening the bonding of the water molecules.41 Increased hydration of the PDMAEM brushes in acidic 13428
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Langmuir solutions is supported by complementary AFM measurements (cf. below). As a result, the temperature-induced phase transition becomes more endothermic and is shifted to higher temperatures. This is in agreement with a recent dynamic light scattering study focusing on spherical PDMAEMA brushes.48 Along with the shift of the phase transition temperature, also the temperature-dependent rate constants for swelling and deswelling change. Figure 3b displays respective data from dynamic laser micromanipulation experiments. Thisfor first timeprovides insight into the intrinsic temperature-induced switching kinetics of PDMAEMA brushes and its dependence on the pH. Both the swelling and the deswelling rates decrease when decreasing the pH from 10 to 4, again suggesting that protonation and/or conformational changes affect the thermoresponsive switching behavior. Depending on the temperature, changes are in the order of one magnitude and larger. Slower temperature-induced switching at pH = 6 as compared to pH = 10 has also been observed in a previous study on membranes functionalized with polystyrene-blockpoly(N,N-dimethylaminoethyl methacrylate) diblock copolymers.52 Slow deswelling at pH ≤ 7 can be attributed to an increased repulsive electrostatic interactions of the grafted chains (Figure 4).32 In addition, a linear conformation of the
Figure 5. In situ AFM measurements of pH-dependent thickness changes of a PDMAEMA polymer brush with a dry layer thickness of 65 nm: (a) averaged height profiles across a laser-fabricated line exposing the bare substrate surface at pH = 7 and 2; (b) thickness of the swollen polymer layer upon increasing (red symbols/arrow) and decreasing (blue symbols/arrow) of the pH.
It could be argued that the chains are still entangled after synthesis and hence become fully disentangled when exposed to low pH.31,32 The polymer layer used here, however, was used in swelling/deswelling experiments before. Moreover, complete deswelling is observed after immersion in pure water (cf. below). Hence, such an effect can be ruled out. Note that data from micromanipulation experiments support the observation reflected in Figure 5, i.e., show a higher brightness in the surface areas of the swollen polymer layer in acidic solutions, indicative for an increased polymer layer thickness (cf. Supporting Information). Complementary experiments also reveal an increased thickness of dry PDMAEMA brushes after immersion in acidic solutions (cf. Supporting Information). In agreement with previous studies,28−33 all these data demonstrate an increased hydration, i.e. swelling, of the polymer brush after protonation of the amine groups in acidic solutions. Noteworthy, the thickness changes observed in situ are not reversible via successive addition of small amounts of NaOH (Figure 4). A pronounced hysteresis is evident. At pH = 6 a film thickness of 185 nm is reached which, however, does not decrease further up to pH = 10. Only upon rinsing in pure water is the original film thickness of 160 nm restored. The hysteresis could point to the formation of a hydrophobic skin layer during brush collapse or steric effects,29 which hinder equilibration of the polymer layer. Interestingly, the addition of NaOH up to pH = 12 increases the polymer layer thickness up to 200 nm. The cause for this thickness change in strongly alkaline solutions remains unclear. Because of the low concentration of the counterion species involved and their neutral nature with respect to Hofmeister effects, though, salt effects are excluded at this point.19,53 This refers also to the observed thickness changes in acidic solutions (cf. above).
Figure 4. Schematic presentations of the brush structure illustrating repulsive electrostatic interactions and steric effects in acidic solutions: (a) protonated amine side groups in stretched conformation at pH ≪ 7; (b) unprotonated amine side groups in cyclic conformation at pH > 7.
protonated side groups at pH ≤ 736 could well result in steric effects, which shift the phase transition to higher temperatures and decrease the switching rates. In order to obtain information on pH-dependent thickness changes, AFM measurements are carried out. For these measurements, the polymer layers are locally removed via photothermal laser patterning with a microfocused CW laser.49 Hence, bare substrate areas are created, which serve as a reference level in the AFM measurements. Figure 5 displays data from in situ experiments using a PDMAEMA brush with a swollen film thickness of about 160 nm at pH = 7 and room temperature. Successive addition of small amounts of HCl shifts the pH to 2. Note that no background electrolyte is used here. Hence, the experiment at pH = 7 starts in the osmotic brush regime, where counterions are bound inside the brush and largely compensate the charge of the protonated amine groups.29,43−45 Decreasing the pH increases the degree of protonation of the brush. Also, the ionic strength of the solution increases. As a result, the osmotic pressure and the repulsive electrostatic interactions of noncompensated charged groups increase.29,44,45 For this reason, when approaching a pH of 2 the film thickness shows a steep jump up to a value of about 210 nm. 13429
DOI: 10.1021/acs.langmuir.5b03448 Langmuir 2015, 31, 13426−13432
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Swollen polymer layer thicknesses and swelling ratios at pH = 7 and pH = 2 are compared for samples exhibiting different dry layer thicknesses as synthesized at identical conditions but different polymerization times (Figure 6). Note that the
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (N.H.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Professor Werdelmann foundation and the Fonds der Chemischen Industrie (FCI, Forschungsfördermittel und ChemiefondsStipendium).
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Figure 6. Swelling ratios of PDMAEMA polymer brushes at pH = 2 and pH = 7. The polymer layers are synthesized at identical conditions but distinct polymerization times resulting in different dry layer thicknesses. Error bars reflect uncertainties because of the polymer brush roughness.
swelling ratio SR here is calculated via division of the swollen polymer layer thickness by the dry polymer layer thickness. An increased swelling ratio at pH = 2 when compared with the value at pH = 7 is observed for all samples. In addition, the data display an increasing swelling ratio with increasing polymer layer thickness. This could point to a varying brush profile because of termination reactions and buried chain ends during growth of thicker polymer layers.54 This may result in lower densities at the outer interface of such polymer layers.
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CONCLUSIONS
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ASSOCIATED CONTENT
REFERENCES
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The pH-dependent thermoresponsive switching behavior of PDMAEMA brushes including the intrinsic temperatureinduced switching kinetics is investigated. The data suggest that protonation of the amine groups and accompanied conformational changes of the side groups increase hydration and repulsive electrostatic and steric interactions between the polymer chains. As a result, structural, thermodynamic and kinetic parameters are strongly altered in acidic solutions. The polymer layer thickness increases significantly when approaching a pH of 2. Also, a shift of the volume phase transition to higher temperatures is observed in acidic solutions. Most notably, both the swelling and the deswelling rates strongly decrease with decreasing pH. Depending on the temperature, changes are in the order of 1 magnitude and larger. This emphasizes the strong impact of the pH on the response time of sensors and devices on the basis of thermoresponsive polyelectrolyte brushes.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03448. Estimation of the grafting density of PDMAEMA layers, laser manipulation experiments, pH-dependent changes of the polymer layer thickness (PDF) 13430
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DOI: 10.1021/acs.langmuir.5b03448 Langmuir 2015, 31, 13426−13432
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Langmuir (54) Mastan, E.; Xi, L.; Zhu, S. What Limits the Chain Growth from Flat Surfaces in Surface-Initiated ATRP: Propagation, Termination or Both? Macromol. Theory Simul. 2015, 24, 89−99.
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DOI: 10.1021/acs.langmuir.5b03448 Langmuir 2015, 31, 13426−13432
