Thermoresponsive PDMAEMA Brushes: Effect of Gold Nanoparticle

Article pubs.acs.org/JPCB

Thermoresponsive PDMAEMA Brushes: Effect of Gold Nanoparticle Deposition Zuleyha Yenice,† Sebastian Schön,† Hakan Bildirir,‡ Jan Genzer,§ and Regine von Klitzing*,† †

Stranski-Laboratorium für Physikalische und Theoretische Chemie and ‡Department für Chemie, Technische Universität Berlin, D-10623 Berlin, Germany § Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States ABSTRACT: The paper addresses the effect of gold nanoparticle (Au-NP) deposition on the thermoresponsive volume phase transition of the weak polyelectrolyte poly(2(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. PDMAEMA brushes were synthesized via surface-initiated atom transfer radical polymerization (SI-ATRP). The PDMAEMA/Au-NP composite brushes were fabricated by immersing the brush modified wafer in the Au-NP suspension. Atomic force microscopy (AFM), ellipsometry, and scanning electron microscopy (SEM) have been employed to characterize the neat PDMAEMA brushes and PDMAEMA/Au-NP composite brushes. All neat PDMAEMA brushes swelled below the volume phase transition temperature and collapsed with increasing temperature over a broad temperature range independent of the initial brush thickness. Water uptake of the brushes is also independent of initial brush thickness. The adsorption of the charged Au-NPs significantly affects the degree of swelling and the thermoresponsive properties of the brushes. PDMAEMA/Au-NP composite brushes do not exhibit any noticeable phase transition at the experimental temperature range irrespective of the initial brush thickness. The reason for this behavior is attributed to a combination of the following: the decreased conformational entropy of the Au-NP adsorbed polymer chains, the increased hydrophilicity of the system due to the charged Au-NPs, and the ≈13 nm diameter Au-NPs causing steric hindrance. We have also shown that the AFM full-indentation method can be successfully applied to determine the polymer brush thicknesses.

1. INTRODUCTION Polymer composite thin films comprising polymer brushes and noble metal nanoparticles (NPs) have become increasingly important due to their unique properties such as nanosensor applications based on localized surface plasmon resonance,1−6 surface-enhanced Raman spectroscopy substrates,7 and catalytic activity.8 The NP type, size, shape, organization, and interparticle distance play an important role for applications based on the localized surface plasmon resonance. Localized surface plasmons are charge density oscillations confined to coinage metal NPs. A plasmon that oscillates locally around the NP occurs when light interacts with particles smaller than the incident wavelength.9−12 The effect in conformational changes in polymeric chains to actuate the deposited metal particles and modulate the interparticle spacing has been summarized in a review by Tokarev and Minko.13 Polymer brushes respond to changes in temperature, solvent polarity, pH, and other stimuli, depending on their chemistry. Embedding NPs in a stimuli-responsive polymer matrix offers the opportunity of surface functionalization and therefore altering the properties of the system. Adsorption of NPs onto/ into polymer brush matrices is an ongoing research topic. For example, Bhat et al. investigated lateral property polymer brush gradients and reported the effect of molecular weight and © 2015 American Chemical Society

grafting density of the brush on the deposition density of the NPs.14,15 They reported that NPs larger than the dimension of the grafted polymer reside primarily on the top of the brushes; the penetration of the particles increases with increasing molecular weight and decreasing grafting density. After an intermediate grafting density the further increase in the grafting density leads to a decrease in particle loading inside the brushes.15 Roiter et al. observed segregation of Au-NPs on the collapsed state of polymer brushes, whereas the swollen brush system engulfed the particles, despite the large entropic energy penalty associated with this process.1 Filippidi et al. reported that the penetration depth of the NP depends on the size of the NPs.16 Others have undertaken theoretical studies regarding polymer brush−NP interactions, in which the solvent quality and the effect of particle size and shape were discussed.17−19 Most studies pertain to studying the stimuli-responsive properties of these polymer−NP hybrid systems and using optical methods to interrogate the properties of such composites. In the literature it is demonstrated that some hybrid brush systems can sustain their stimuli-responsive properties. Gupta et al. investigated solvent-induced swelling/ Received: May 19, 2015 Revised: June 30, 2015 Published: July 1, 2015 10348

DOI: 10.1021/acs.jpcb.5b04757 J. Phys. Chem. B 2015, 119, 10348−10358

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The Journal of Physical Chemistry B deswelling of Au-NP deposited polystyrene chains3 and the thermoresponsive behavior of Au-NP deposited poly(Nisopropylacrylamide) brushes.2 They observed the typical responsive properties of the brushes also after the adsorption of the ≈5−6 nm diameter Au-NPs onto/into the polymer brushes. The sensing ability of polymer brush/NP composites was demonstrated by UV−vis spectroscopy, through a shift in the plasmon resonance band due to the position and interparticle distance change of the immobilized Au-NPs after swelling or deswelling.2,3 The thermoresponsive properties of polymer brushes were also sustained for grafted PDMAEMA brush−Au hybrid systems, where the Au-NPs (2−3 nm of diameter) were synthesized directly on the polymer brushes by in situ reduction of HAuCI4.20,21 The brush−Au hybrid systems showed LCST based volume phase transition and thermo-adjustable catalytic activity.20,21 However, to our knowledge there is no study that reports a possible shift or hindrance of the thermoresponsive properties of polymer brushes as a result of the adsorption NPs (colloids) or macromolecules. The main goal of this study is to shed light on the mutual effects of NPs and polymer brushes by investigating the thermoresponsive properties of neat and nanocomposite PDMAEMA brushes. Possible effects of the adsorbed Au-NPs on the thermoresponsiveness of PDMAEMA brushes are discussed. The temperature-dependent volume phase transition was measured by AFM using the full-indentation method as an alternative method to ellipsometry.22 To our knowledge, this is the first work in which the thickness of polymer brushes was measured by the full-indentation method. The full-indentation method has the advantage that the brush is not compressed by scanning, and the brush thickness is not affected by the measurement itself.22

synthesis of the brushes, the initiator was chemically attached to the silicon surface as a monolayer, which was subsequently used to polymerize the DMAEMA. 2.2.2. Preparation of the Initiator. The initiator amidebased BMPUS, 2-bromo-2-methyl-N-(11-(trichlorosilyl)undecyl)propanamide, was synthesized according to the procedure described in ref 23 using 10-undecenyl azide. 10Undecenyl azide was synthesized by dissolving NaN3 (1.63 g, 25.08 mmol) in DMSO (50 mL) at 25 °C, adding undecyl bromide (5 mL, 22.80 mmol), and stirring at room temperature until the starting material was fully consumed (≈6 h). The reaction was quenched with H2O (100 mL) and stirred until it cooled to room temperature. The mixture was extracted with Et2O (3 × 30 mL). The combined organic layers were washed with water and brine and dried with MgSO4. After removing the solvent in vacuo, 10-undecenyl azide was obtained as a colorless oil. 2.2.3. Building of Initiator Self-Assembled Monolayer (SAM). Prior to the deposition of the initiator, the wafers were etched with RCA to ensure a high coverage of hydroxyl groups on the surface. A monolayer of the ATRP initiator amide-BMPUS was deposited on to the silicon wafer surface using the self-assembly method for producing monolayers.26 The etched wafers were placed into a 0.001 wt % initiator solution of anhydrous toluene and kept in the fridge. After the deposition time (15 h) required for the initiator molecules to form a monolayer, the wafers were removed from the solution, rinsed with toluene, cleaned by ultrasonification in toluene for 1 min, and dried under a nitrogen gas stream. 2.2.4. General Procedure for SI-ATRP of DMAEMA. A polymerization solution of 80 mL of DMAEMA (0.475 mol), 80 mL of DMSO, 4.65 g of 2,2′-bipyridine, and 1.411 g of Cu(I)CL was prepared under a nitrogen atmosphere and degassed for 4 h. The initiator-deposited silicon wafers were then placed in the polymerization solution. The polymerization was carried out at room temperature for 6, 10, and 48 h, yielding 36.2, 61.3, and 157.8 nm thick brushes, respectively. The wafers were removed from the polymerization solution and rinsed with ethanol, cleaned by ultrasound in ethanol for 3 min, and dried under a nitrogen stream (similar to the procedure described in refs 23−25). 2.2.5. Synthesis of Au-NP Suspension and Preparation of Au-NP−Brush Composites. Au-NP were synthesized by citrate reduction of HAuCl4 according to the procedure described in refs 27 and 28. An aqueous solution of gold(III) chloride hydrate (HAuCl4·xH2O) (0.5 mM, 100 mL) was heated up while stirring at 500 rpm. At the boiling point, 5 mL of 1 wt % trisodium citrate dihydrate (C6H5Na3O7·2H2O) was added. Over a period of a few seconds the boiling solution turned first gray then to ruby red, indicating the formation of the Au-NPs. Three minutes after the color change the temperature was lowered, and the stirring speed was reduced to 150 rpm. The solution was heated in total for 20 min after the citrate addition, and then the heater was turned off, letting it stir at 150 rpm overnight. A lid made of aluminum foil kept the system closed to prevent evaporation of the water during the whole procedure. Particle size (diameter of 13 ± 1.8 nm) was determined by transmission electron microscopy (TEM). After preimmersing in Milli-Q water for 30 min, the brush modified Si wafer was immersed for 4 h into the Au-NP suspension. The Au-NP suspension used for immersion had a pH value of ≈5.2 and a zeta-potential value ≈−36.50 mV. Afterward, the brush-

2. EXPERIMENTAL SECTION Surface grafted PDMAEMA brushes with three initial thicknesses were synthesized by the SI-ATRP (grafting from) technique. SI-ATRP proceeds in a controlled manner and allows for the molecular weight of the grafted polymer chain to be tailored.23−25 Surface grafted polymer brushes with ambient dry thicknesses of 36.2, 61.3, and 157.8 nm were obtained after polymerization of 6, 10, and 48 h, respectively. The brushmodified silicon wafers were cut into two pieces: the first piece was left pure, while the second was deposited with Au-NPs by immersing the brushed wafer into the Au-NP suspension. In the present work the thermoresponsive behavior was investigated for both the neat and Au-NP deposited PDMAEMA brushes by the full-indentation method.22 2.1. Materials. 2-(Dimethylamino)ethyl methacrylate, 2,2′bipyridine, copper(I) chloride, sodium azide, trisodium citrate dihydrate, gold(III) chloride hydrate, toluene, and ethanol were purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO) was purchased from Fisher Scientific. The silicon wafers (diameter: 150 mm; thickness: 650−700 μm; dopant: boron; crystalline orientation: [100]) were purchased from LG Siltron Inc., Korea. The initiator Amide-BMPUS, 2-bromo-2-methylN-(11-(trichlorosilyl)undecyl)propanamide, was synthesized as explained below. 2.2. Preparation. 2.2.1. Surface-Grafted PDMAEMA Brushes. PDMAEMA brushes were grown on silicon wafers by the “grafting from” technique using SI-ATRP (surfaceinitiated atom transfer radical polymerization).23−25 For the 10349


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The Journal of Physical Chemistry B

For the measurements in water, Olympus OMCL-TR series cantilevers with a triangular shape, Cr/Au coatings, a resonance frequency of 10 kHz, and a spring constant of 0.02 N/m were used. Data analysis were performed with the program Igor Pro 6.1.2.1. The surface roughness was calculated by this program which uses the root-mean-square formula: σ = (1/N ∑yi2)1/2. σ is the root-mean-square roughness of the scan area, N is the number of pixels of this area, and yi is the z value of a specific pixel. Full Indentation: The temperature-dependent thickness measurements were conducted using the full-indentation method, which has been thoroughly introduced by Uzum et al.22 An AFM force measurement setup was used for indentation in the z-direction. The experiments were carried out in a BioHeater closed fluid cell from Asylum Research for the MFP-3D AFM. The stiffness of the cantilever and the length of the tip are dependent upon the sample that is to be indented. We used an AC160 with a spring constant of 29 N/ m. Prior to the measurements, the cantilever had to be calibrated to get the inverse optical lever sensitivity (InvOLS). The InvOLS is needed in order to calculate the correct indentation length. The calibration was done in Milli-Q water on a clean silicon wafer. Structural changes on the polymer film occur over an area of ≈50 nm2 due to indentation.22 The measurements were performed between 25 and 65 °C with incremental steps of 5 °C, using a temperature ramp of 6 °C min−1. The temperature was kept constant for at least 10 min prior to each measurement in order to ensure thermal equilibrium. Each measurement consists of 16 individual force curves taken at different lateral positions on a 100 μm × 100 μm area on the sample. The film thickness was calculated by the full-indentation method as shown in Figure 1. The red curve is a typical force curve (force versus indentation depth) which was measured in the present work for sample 3 in Milli-Q water at 25 °C. The figure shows how the indentation region was calculated. The swollen thickness of the brush was calculated as h ≈ 380 nm. Details are described in ref 22. 3.3. Electron Microscopy Measurements. SEM measurements were carried out on a Hitachi SU8030 and a Zeiss DSM 982 GEMINI, and TEM measurements were carried out on a FEI Tecnai G2 20 S-TWIN, both at the Zentraleinrichtung Elektronenmikroskopie (ZELMI), TU-Berlin.

modified surfaces were taken out of the suspension and were rinsed with Milli-Q water.

3. APPARATUS AND MEASUREMENT PROCEDURES 3.1. Ellipsometry Measurements. All ellipsometric measurements were performed with a polarizer−compensator−sample analyzer (PCSA) ellipsometer, Multiscope from Optrel GbR (Wettstetten, Germany) in Null ellipsometry mode. The measurements were carried out at an incident angle of 70° (wavelength 632.8 nm) at ambient dry (relative humidity (rh) ≈ 30%) conditions. The tempered measurements were performed between 15 and 65 °C with incremental steps of 5 °C. Once the desired temperature had been achieved, it was held constant for 10 min prior to each measurement to ensure thermal equilibrium. For data handling, the software “Ellipsometry: simulation and data evaluation” was used. The data were analyzed by using a layer model, comprising air or water (depending on measurement condition)−brush−initiator−SiO2−Si, as shown in Table 1. The refractive index (n) and the thickness (d) of the polymer brush layers were fitted by the program, while other values were held constant. Table 1. Layer Model Used for Analyzing the Ellipsometric Dataa medium

n

k

air water brush initiator SiO2 Si

1.000 1.333 n(brush) 1.460 1.460 3.885

0.000 0.000 0.000 0.000 0.000 −0.018

d (nm)

d(brush) 2.0 1.3

The refractive index and the thickness of the brushes were fitted. The thickness of the initiator layer was measured prior to synthesis and added to the model. a

3.2. Atomic Force Microscopy (AFM) Measurements. Scanning: The instruments used for the AFM scanning measurements were a Cypher scanning probe microscope (for ambient dry measurements) and a MFP-3D AFM (for the liquid measurements), both from Asylum Research. For scanning in ac mode in air, Olympus AC160TS cantilevers, with dimensions of 160 × 50 × 4.6 μm, made of silicon with a reflective coating of aluminum, a spring constant of 26 N/m, and resonance frequency (in air) of 300 ± 100 kHz were used.

Figure 1. Calculation of the brush thickness by the full-indentation method. The red graph (curve) is the measured force curve. The dashed lines show the contact points of the AFM tip, with the brush (left) and with the silicon substrate (right). 10350


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Figure 2. TEM image of Au-NPs (a) together with the particle size distribution graph (b). Low polydispersity is reflected by particle crystallization.

4. RESULTS 4.1. Au-NP Characterization. Negatively charged Au-NPs were synthesized by citrate reduction of HAuCl4.27,28 Particle sizes, determined by TEM, had a diameter of 13 ± 1.8 nm (see Figure 2). The low polydispersity is also reflected by particle crystallization. The zeta-potential of the Au-NPs was measured as ≈−36.50 mV. 4.2. Characterization of Neat PDMAEMA Brushes. Surface grafted PDMAEMA brushes with three initial thicknesses were synthesized by the SI-ATRP technique. 4.2.1. Neat PDMAEMA Brushes in Ambient Condition. Under ambient condition (rh ≈ 30%) the dry thicknesses of the neat PDMAEMA brushes were 36.2, 61.3, and 157.8 nm for 6, 10, and 48 h polymerization times, respectively. The thicknesses were determined by ellipsometry and are shown in Table 2 (both thicknesses and refractive indices vere fitted).

All brushes collapse over a range of temperatures (Figure 3). The measured thicknesses (shown with symbols) are fitted with a sigmoidal fit (solid thin line). The dashed line shows the first derivatives of these fits (∂d/∂T), where d is the thickness and T the temperature. The first derivatives (∂d/∂T) are used for the determination of the LCST values of 42.1 ± 1.7, 44.1 ± 1.0, and 41.8 ± 1.0 °C for the 36.2, 61.3, and 157.8 nm brushes, respectively. These values are consistent with the literature for the LCST of PDMAEMA brushes29 and are independent of the brush thicknesses. The refractive indices are fitted by the ellipsometry program and are shown with the triangles; also, these values are fitted by a sigmoidal curve (solid lines). Because of water loss upon shrinking of the brush, the refractive index increases as the brush collapses (Figure 3). As shown in Figure 3, all neat PDMAEMA brushes shrink upon heating but not exactly back to the value of their ambient dry thicknesses. The refractive indices after collapsing above the LCST are lower than the ambient dry values, which is consistent with the thickness results. This means that the shrunken brush above the LCST is still quite hydrated. The water content of a swollen brush can be calculated as water content % = [(dswollen brush − ddry brush)/dswollen brush] × 100. In order to apply this equation, one requires estimates of the dry thickness: the water content of the ambient dry brushes (rh = 30%) is ≈2.5% is extracted from ref 30 and was used to calculate the dry thicknesses. The water content results of the brushes (samples 1−3) in swollen below and shrunken states above the LCST are shown in Table 3. These results clearly reveal that there is quite a bit of water left inside the brushes even at collapsed state above the LCST. The water uptake of the brushes in Milli-Q water is independent of the brush thicknesses. The volume phase transition upon heating was also measured by AFM with the full-indentation method and was compared to the ellipsometric results using sample 3 (Figure 3c). The brush collapses gradually over a broad range of temperatures from ≈30 to 60 °C during both measurements. Both curves fit very well for temperature values close to ambient temperatures (25− 30 °C), but for elevated temperatures the curves differ as a result of the different heating rates of these distinct methods. The first-derivative calculations for both methods give LCST values of 40.3 °C (full indentation) and 41.8 °C (ellipsometry), which are in good agreement.

Table 2. Ambient Dry Thicknesses of Neat PDMAEMA Brushes (Samples 1−3) Used in the Present Study, Measured by Ellipsometry Together with the Refractive Indices (n)a sample

thickness (nm)

refractive index, n

1 2 3

36.2 ± 2.5 61.3 ± 1.5 157.8 ± 4.4

1.52 ± 0.02 1.52 ± 0.02 1.49 ± 0.00

a

The refractive indices as well as the thickness of the brushes were fitted.

The grafting density was determined as follows: (1) The initiator coverage was determined by the thickness of the initiator monolayer which was ≈2 nm measured by ellipsometry. The theoretical chain length of the initiator is ≈3.2 nm; from this we can estimate the surface coverage of the initiator as ≈60%. (2) One initiator chain takes about 2 nm2 space, which suggests a maximum grafting density of σ ≈ 0.3 chains/nm2 at the silicon/brush grafting plane. 4.2.2. Temperature-Dependent Behavior of Neat PDMAEMA Brushes. The temperature-dependent behavior of neat PDMAEMA brushes was investigated first using ellipsometry and afterward using the full-indentation method. This allowed for the investigation of the volume phase transition of the polymer brushes in Milli-Q water as shown in Figure 3. 10351


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Figure 3. Temperature-dependent volume phase transition of neat PDMAEMA brushes (samples 1−3) in Milli-Q water measured by ellipsometry (black circles); sample 1 (a), sample 2 (b), and sample 3 (c) together with the refractive indices “n” (red triangles). The solid lines show the sigmoidal fits, and the dashed lines show the first derivatives (∂d/∂T) of the ellipsometric data. The comparison of ellipsometry (black circles) and the AFM full-indentation method (blue rectangles) measured for sample 3 (c). The solid lines of the graphs show the sigmoidal fit.

Table 3. Water Content of the Neat PDMAEMA Brushes (Samples 1−3) in Milli-Q Water in Swollen State below the LCST and in Shrunken State above the LCST, Calculated Using the Ellipsometric Measurements sample

water content below LCST (%)

water content above LCST (%)

1 2 3

57 56 59

39 40 41

To assess the effect of heating, both methods were used at room temperature at ≈24.1−25 °C and the thickness results compared for all brushes (Figure 4). The results of the two independent methods at 25 °C are consistent within the experimental errors. This demonstrates that the full-indentation method was successfully applied to measure the thickness of polymer brushes. The volume phase transition of the brush (sample 3) upon heating was also investigated by the AFM scanning method. As shown in Figure 5, the root-mean-square (rms) roughness decreases as the brush collapses upon heating. 4.3. Characterization of PDMAEMA/Au-NP Composite Brushes. For the characterization of the composite brushes, AFM was used for both scanning and thickness determination. The reason for using AFM instead of ellipsometry is the complex refractive index (ñ = n + ik) which is related to the optical properties of the NP deposited composite system. The attached Au-NPs induce an absorption term k into the hybrid

Figure 4. Comparison of ellipsometry (circles) and the AFM fullindentation method (rectangles) measured for all samples at ca. 25 °C in Milli-Q water.

system as explained in ref 30. Having three unknown parameters (thickness (h), the real part n, and the imaginary part k of the complex refractive index) makes the application of ellipsometry challenging. 4.3.1. PDMAEMA/Au-NP Composite Brushes in Ambient Conditions. AFM height images of the 157.8 nm brush (sample 3) before and after Au-NP deposition are shown in Figure 6 for comparison. The roughness (rms) of the ambient dry brush was 0.80 ± 0.10 nm and increased to 4.50 ± 0.11 nm after AuNP deposition (Figure 6). The deposited Au-NPs show a 10352


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SEM was used to examine the Au-NP deposition on a larger scale. The SEM images of the three brushes (samples 1−3) deposited with Au-NPs are shown in Figure 7. The Au-NPs deposited onto the brushes, become more aggregated as the brush thickness increases. The particles are distributed as a monolayer on the thin brush (sample 1) with a particle number density of 2534 particles/μm2 (Figure 7a). The particle number density increases with increasing brush thickness and is calculated as 2851 and 3252 particles/μm2 for samples 2 and 3, respectively (Figure 7). These results are in very good agreement with the work of Christau et al., who investigated the effect of polymer brush thickness on the distribution of the deposited nanoparticles.30 They concluded that the particle number density increases with brush thickness, which can be attributed to the increased roughness of the thicker brush. The particle number density was calculated using ImageJ.31 4.3.2. Temperature-Dependent Behavior of PDMAEMA/ Au-NP Composite Brushes. After Au-NP deposition, the volume phase transition of the polymer brush nanocomposite material was measured by the AFM full-indentation method. The specific parameters of the measurement (heating rate and degree, force map size) were identical to those of the neat brushes. Figure 8 shows the temperature dependent measurements of the PDMAEMA/Au-NP composite brushes (sample 1−3 after Au-NP deposition). There is no collapse of the PDMAEMA/Au-NP composite brushes upon heating up to 65 °C. The brushes are hydrated (in swollen state) at all measured temperatures (25−65 °C). Higher temperatures could not be measured on our system due to water bubble formation on the measured substrate at temperatures around 70 °C. The temperature-dependent measurements for the PDMAEMA/ Au-NP composite brushes (Figure 8), when compared to those of the neat PDMAEMA brushes (Figure 3), suggest that the volume phase transition of the brush nanocomposite was affected by the Au-NPs. The swollen thickness increases after the attachment of the Au-NPs up to 20%.

Figure 5. AFM height images (3-dimensional) of the temperaturedependent behavior of neat PDMAEMA brush (sample 3) in Milli-Q water at 30 °C (a) and at 50 °C (b). Roughness (rms): 2.80 ± 0.30 nm at 30 °C and 1.11 ± 0.06 nm at 50 °C, decreases with shrinking of the brush.

particle layer, which mimics the underlying brush topography (Figure 6b).

Figure 6. AFM height images of the 157.8 nm thick brush (sample 3), before Au-NP deposition (a) and after Au-NP deposition (b) together with the cross-section images. 10353


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Figure 8. Temperature-dependent measurements of PDMAEMA/AuNP composite brushes using the AFM full-indentation method (sample 1 (circles), sample 2 (triangles), and sample 3 (rectangles) after Au-NP deposition). Single measurement points below every curve with the same symbol show the swollen thickness of the neat PDMAEMA brushes for comparison reasons.

geometry of the substrate, the molecular weight, the grafting density, and the polydispersity of the polymer.32−36 In this study the quite broad phase transition region of the PDMAEMA brushes could depend on the different collapse behavior of the different brush regions toward the water subphase. The different brush regions have (1) different monomer densities, the brush gets more diluted toward the water subphase; (2) increased hydration toward the water subphase due to this dilution; and (3) inhomogeneous charge densities toward the subphase. In the literature polyelectrolyte brushes in the swollen state are well described by the Gaussian monomer density profile; close to the grafting plane the brush is densely packed, and the outermost region is the most dilute part.37−41 The Gaussian monomer density profile is also in good agreement with the osmotic brush regime which indicates an inhomogeneous charge density along the brush.40,41 Literature work about PDMAEMA brushes and their thermoresponsiveness is mostly reporting about the effect of pH and mostly on brushes grafted on NPs. Zhang et al. reported that the PDMAEMA brushes grafted on NPs showed no temperature-responsive character at pH 3 where the brush is fully charged, but at pH 10 the LCST was observed at about 31 °C showing a sharp change in transmittance. Zhang and coworkers reported slight thermoresponsive behavior at pH 8 for their PDMAEMA system.42 Dong et al. investigated PDMAEMA grafted silica−NP systems and observed a shift in transition temperature region depending on the pH. They reported that for the same molecular weight the increase in pH led to a shift in the LCST to lover temperatures. When the solution is basic enough, the hydrophobic interactions will dominate and the chains will collapse faster. At pH values of 6, 7, and 8 the LCST was reported to be between 20 and 68 °C.43 Dong et al. also reported that at constant pH an increase in molecular weight led to a decrease in the LCST; at pH 6 the decrease was from 53 to 48 °C. The most investigated temperature responsive polymer brush in the literature is PNIPAM, which is a neutral polymer. Yim et al. reported that the largest conformational changes upon heating were observed for intermediate grafting densities and high molecular weights of the PNIPAM brushes.36 Here the effect of molecular weight and grafting density was well

Figure 7. SEM images of the PDMAEMA/Au-NP composite brushes with different thickness: sample 1 (a), sample 2 (b), and sample 3 (c) after Au-NP deposition.

5. DISCUSSION 5.1. Volume Phase Transition of Neat PDMAEMA Brushes. PDMAEMA has a hydrogen-bonding-based volume phase transition. The transition occurs when it becomes thermodynamically more favorable for the system to phase separate by breaking the hydrogen bonding between the water molecules and the polymer chains. The polymer film dehydrates and collapses above the LCST.29 The change in brush thickness as the temperature increases is shown in Figure 3. The brushes collapse over a broad range of temperatures, which is also indicated by the broad peaks of the first derivatives (∂d/∂T). For polymer brushes in aqueous solution the phase transition is thought to be dependent on the 10354


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for the same time (10 min) for thermal equilibrium, but the heating rates were different. By AFM the heating was done by using a BioHeater closed fluid cell and a temperature ramp of 6 °C min−1 which is relatively fast, whereas by ellipsometry an open cuvette and a water thermostat were used, and the time for reaching elevated temperatures was much longer. These methods suggest that the kinetic properties of the volume phase transition of PDMAEMA brushes play a role and need to be investigated in more detail in the future. The AFM full-indentation method is a relatively easy and fast method that offers the possibility to measure the film (brush) thickness before and after scanning without changing any experimental setup (using the AFM scanning and indentation methods without taking the sample out of the cell). In addition, it avoids the difficulty of modeling ellipsometric data as multilayer structures incorporating NPs. The full-indentation method has previously been used to determine the thickness for micron-scale polymeric films (polyelectrolyte multilayers).22 To our knowledge, there is no work published, in which the AFM full indentation method has been applied to measure the thickness of polymer brushes. The volume phase transition of sample 3 is also investigated by the AFM scanning method as shown in Figure 5. The 3-D images provide good understanding of how the surface roughness changes upon collapse as a result of heating. The reason for the increased roughness upon swelling can be attributed to the dilute ends of the swollen brush toward the water subphase. 5.2. PDMAEMA/Au-NP Composite Brushes. The deposition of Au-NPs on PDMAEMA brushes was achieved primarily by (1) the electrostatic attractive interaction between the negatively charged citrate groups and the tertiary amine functionalities of the polymer chain and (2) hydrogen bonding between the polar groups of the polymer chain and the surface functionalities (−COOH groups) of the Au-NPs. The positively charged tertiary amine functionalities occur due to protonation of the polymer chain at pH ≤ 7. The pH value of the nanoparticle suspension used for the deposition was pH ≈5.2. At this pH the polymer chains are partially charged. The dissociation of citric acid capping molecules on Au-NPs also depend on the pH value. As the Au-NP suspension becomes more basic, the degree of dissociation of citrate molecules covering the Au-NP increases, leading to a larger negative charge density on the particle. The zeta-potential value of the Au-NP suspension used for the deposition was ≈−36.50 mV. As explained above, the PDMAEMA brush is partially charged while it is immersed in the Au-NP suspension which makes the electrostatic deposition possible. The deposition can be also due to H bonding between the carboxyl groups of the surface of citric acid-capped molecules on Au-NPs and the tertiary amine groups and polar groups of the PDMAEMA chains. After NP deposition the thickness of the brushes in water at 25 °C increased by ≈20% compared to the hydrated neat brush. The results of temperature-dependent measurements of the PDMAEMA/Au-NP composite brushes are shown in Figure 8. As shown in the figure, no volume phase transition occurs in the experimental temperature range (25−65 °C). Comparing these results with the measurements of the neat PDMAEMA brushes (Figure 3), it is concluded that the volume phase

described, but the collapse occurred over a quite narrow temperature range (less than 10 °C), as opposed to a broad range like in our system. Balamurugan et al., who also investigated PNIPAM brushes, reported the effect of polymer brush density on the collapsing properties. They suggested that densely packed brushes are less hydrated than those with a lower grafting density, which causes them to collapse at lower temperatures than the dilute brushes. After using temperature-dependent water contact angle measurements, they concluded that the polymer segments in the outermost region of the brush (more dilute parts of the brush) remain highly hydrated, while densely packed, less solvated segments (close to the grafting surface) undergo dehydration and collapse over a range of temperatures.34 In the present PDMAEMA system the phase transition region is broader than the aforementioned PNIPAM systems. In addition to the factors outlined for PNIPAM brushes, the charge density of the PDMAEMA brush will affect the collapse behavior. PDMAEMA brushes are weak polyelectrolyte brushes: Their protonation starts at pH values pH ≤ 7, and as the pH declines the protonation increases.44,45 The pH of Milli-Q water was measured after the immersion of the brush modified wafer and was pH ≈6.1. At this pH the polymer brush is partially charged. The charge density increases along the polymer brush toward the water subphase.41 This means that the outermost brush region which is quite dilute is also more charged and an inhomogeneous charge density is present. The broad phase transition region of the PDMAEMA brushes could depend on the different collapse behavior of the various brush regions, as a result of different monomer and charge densities. The broad phase transition region is also the reason for not seeing a molecular weight effect on the LCST. The LCST values of 42.1 ± 1.7, 44.1 ± 1.0 and 40.3 ± 1.3 °C for the 36.2, 61.3, and 157.8 nm brushes, respectively, evaluated from the first derivatives (Figure 3) show that the effect of molecular weight on the LCST of the brushes is not very pronounced for the PDMAEMA system. As shown in Figure 3, even after collapsing above the LCST, the polymer brushes are still hydrated. The PDMAEMA brush shrinks upon heating but not exactly back to the value of its ambient dry thickness. The water inside the collapsed brushes can be clearly revealed by (1) the water content results of the collapsed brushes, which show clearly the amount of water left inside the brush (≈40%) (Figure 3), and (2) the refractive indices of the collapsed brushes (n ≈ 1.42−1.44), which also clearly reveal the water inside the system. At ambient dry state the refractive indices of the brushes are n ≈ 1.50−1.46 (Table 2). In the literature this effect was explained by the fact that high grafting density brushes cannot collapse completely due to the high entropic cost.46 In addition, ionizable polymers are strongly hydrated even at a low grafting densities. (Note that the polymer brush being partially charged may also has an effect.) The volume phase transition was also measured by the AFM full-indentation method and compared to ellipsometry (Figure 3c). Both curves deviate at elevated temperatures but not at low temperatures. The results at 25 °C are in very good agreement (Figures 3c and 4). (Note that by the full-indentation method a 100 μm × 100 μm area was investigated for each measurement, and the experimental error is the standard deviation.) The reason for the deviation at elevated temperatures could be due to the tempering properties of these independent methods. In both methods the temperature was kept constant 10355


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polymer or the pH of the environment is varied.53−58 In the literature the increase in hydrophilicity increased the amount of H bonds between water and the polymer, which resulted in an increased LCST.51

transition (or LCST) of the brush nanocomposite is hindered or shifted to much higher temperatures by the Au-NPs. The reason for the hindered volume phase transition could be attributed to the following: (1) The conformational entropy of the polymer chains: Unfavorable polymer chain stretching effects can occur due to the NP adsorption.47 The polymer chains stretch around the NPs, causing a loss in conformational entropy which increases with particle diameter.48,49 An increase in Au-NP size will introduce greater restrictions on the number of conformations the polymer brush chains can undergo. The diameter of the AuNP used in this study is ≈13 nm. This is much larger than the Au-NPs used in other studies with composite brush systems, which show temperature-dependent volume phase transition close to the LCST of neat PDMAEMA brushes.2,20,21 Conformational entropy is an important factor governing the thermodynamic response of synthetic polymers to biomacromolecules.50 At low temperatures these systems are usually in a low energy and low entropy state and have the characteristics of high energy and high entropy at high temperatures.50 At higher temperatures the phase transition occurs due to the competition of these two conditions.50 How the conformational entropy will affect the phase transition in complex systems is rather complex. To understand polymer conformations in solution, the possible mechanisms involving changes in enthalpy and entropy needs to be understood.50 In the literature the increase in the number of H bonds between water and the polymer resulted in an increased LCST.51 Increasing the number of hydrogen bonds leads to an increased ordering of water around the chains and therefore to a loss in conformational entropy of these chains. In this case, the entropic loss comes from the reduction in degrees of freedom due to hydrogen bond formation. The temperature required for the polymer to break these H bonds in order to phase separate increases.51 In other words, the more cross-links which are added to a system, the greater the decrease in the conformational entropy and energy. By adding energy to the system, cross-links will break, and the conformational entropy will increase.50 However, in thermodynamics the phase transition is a balance between entropy effects and energy (or enthalpy) effects. Therefore, the enthalpy contribution of the adsorbed NPs needs to be investigated. (2) The increased steric interactions of the polymer chains due to the adsorbed Au-NPs: The type of NP adsorption, like adsorption of the particle at the brush−solvent interface (secondary adsorption), and adsorption of the particles within the grafted chains (ternary adsorption), explained in ref 52, will affect the system and the possible chain conformations of the brushes. Secondary adsorption on the brush surface or ternary on the dilute outermost parts can occur in the present system. Compressing the chains due to secondary adsorption or crowding of the polymer chains due to ternary adsorption of NPs can reduce the conformational entropy of the brush chains. These adsorptions can also lead to increased steric interactions of the chains (polymer−polymer steric interactions),52 which could hinder the collapse of the chains. The increased polymer−polymer steric interactions due to NP adsorption can also explain the increase in brush height after NP adsorption. (3) Charged Au-NPs contributing to the hydrophilicity of the polymer: Increased hydrophilicity can affect the thermoresponsive properties of the brushes. The LCST shifts to higher values when more hydrophilic monomers are introduced to the

6. CONCLUSIONS The thermoresponsive behavior of surface grafted PDMAEMA brushes and PDMAEMA/Au-NP composite brushes was investigated. The adsorption of the charged Au-NPs and the interaction of these particles with the polymer chains significantly changed the degree of swelling and the thermoresponsive properties of the brushes. The ambient and tempered (LCST) studies show the following. 1. The neat PDMAEMA brushes swell in Milli-Q water below the phase transition regime. The water uptake of the brushes is independent of the initial brush thickness. 2. Neat PDMAEMA brushes shrink upon heating but not exactly to the value of their ambient dry thicknesses. The partially hydrated shrunken state was explained by the relatively high grafting density of the polymer brushes causing a high osmotic pressure within the brush, thereby hindering the total collapse of the system. 3. Neat PDMAEMA brushes show an LCST-type volume phase transition. The broad temperature range of the collapse was explained by (a) the decreased monomer density toward the water subphase. The more dilute parts will be more hydrated and therefore collapse at higher degrees. (b) The charge inhomogeneity within the PDMAEMA brush, which is a weak polyelectrolyte. The inhomogeneous charge distribution along the brush affects the hydrophilicity toward the water subphase and therefore influences the collapse behavior. 4. The initial brush thickness did not affect the volume phase transition. The reason was attributed to the weak polyelectrolyte character of the brush and the broadness of the transition temperature region. 5. The deposited Au-NPs and the particle number density increased with the brush thickness. 6. The swollen thickness of PDMAEMA/Au-NP composite brushes in Milli-Q water was increased by ≈20% compared to the neat PDMAMEMA brushes. This was explained by the increased polymer−polymer steric interactions. 7. PDMAEMA/Au-NP composite brushes did not show any volume phase transition at the experimental temperature range (25−65 °C). This result was quite surprising, and the explanations are to a large extent speculative. The explanations for this observation included (a) the adsorbed NPs reduce the number of conformations the polymer brush can undergo, therefore decreasing the conformational entropy of the polymer chains; (b) the bulky NPs crowding the polymer chains and therefore causing steric hindrance; and (c) the increase in hydrophilicity of the polymer brushes due to the adsorbed charged NPs. The present study also revealed that the AFM fullindentation method successfully measured the thickness of PDMAEMA brushes. The full-indentation method has many advantages, such as quick measurements, relatively easy use, and AFM scanning possibilities without the need to change the experimental setup. These advantages could make the fullindentation method the preferred option for thickness measurements of thin films in the future. 10356


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AUTHOR INFORMATION

Corresponding Author

*Phone +49 (0)30 314 23476; Fax +49 (0)30 314 26602; email [email protected] (R.v.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Casey Galvin, from the Micro/Bio/ Nanofluidics Unit, Okinawa Institute of Science and Technology, for many helpful discussions. We also thank Christoph Fahrenson, from Zentraleinrichtung fur Elektronenmikroskopie (ZELMI), Technische Universität Berlin, for the SEM measurements. Financial support was provided by the German Research Foundation (DFG) via the International Research Training Group (IRTG) 1524 at the Technische Universität Berlin and by BCP, a program by the Berlin State Government for the promotion of equal opportunities for women in research and teaching (Berliner Programm zur Fö r derung der Chancengleichheit für Frauen in Forschung und Lehre), at the Technische Universität Berlin.

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Thermoresponsive PDMAEMA Brushes: Effect of Gold Nanoparticle

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