Combined Cononsolvency and Temperature Effects on Adsorbed

Article Cite This: Langmuir 2017, 33, 14269−14277

pubs.acs.org/Langmuir

Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels Sebastian Backes,†,‡ Patrick Krause,† Weronika Tabaka,† Marcus U. Witt,†,‡ and Regine von Klitzing*,†,‡ †

Stranski-Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany ‡ Institut für Physik, Technische Universität Darmstadt, Alarich-Weiss-Strasse 10, 64287 Darmstadt, Germany ABSTRACT: The present study addresses the multiresponsive behavior of poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this socalled cononsolvency effect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specific surface details, as well as the sample preparation. Thereby, the microgels are deposited on two different kinds of surfaces: on gold surface and on polycation (PAH) coating which show different interactions with the microgels in terms of electrostatic interaction and wettability. In addition, the microgels were deposited from different solvent mixtures. This influences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T > VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

1. INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) is a smart polymer that is sensitive to external stimuli, most prominently to temperature, as it exhibits a lower critical solution temperature (LCST) around 32 °C in water. It is soluble below and insoluble above this temperature.1−6 This behavior is explained by a balance of hydrogen bond energy (between water and polymer) and entropy. Above the LCST, the hydrogen bonds between water and PNIPAM break and the polymer becomes insoluble. PNIPAM-based microgels can be synthesized by adding a cross-linker and thereby building a three-dimensional polymer network. Those microgels have a volume phase transition temperature (VPTT) of around 32 °C, above which they expel water and shrink. Microgels combine the advantages of a fast stimuli response and being colloidally stable even above the VPTT. The responsiveness of PNIPAM offers numerous potential applications, like in medicine and drug delivery7−9 or sensors.10,11 A very interesting phenomenon, the so-called cononsolvency effect, can be observed when adding certain organic solvents, such as short-chained alcohols, to an aqueous PNIPAM solution.12−15 Both water and alcohol are good solvents for PNIPAM, but when a small amount of alcohol is added to water, this causes a shrinking of microgels. Upon further addition of alcohol, the microgels reswell. © 2017 American Chemical Society

There are several approaches to explain the cononsolvency phenomenon, but a generally accepted theory of the molecular driving forces behind it is still missing. As the solvent quality is steadily improving when more and more alcohol is added, classical mean field theory cannot describe the miscibility gap for intermediate alcohol content.16 One category of explanations is focused on the solvent structure. It is assumed that solvent−cosolvent complexation is preferred over bonds of solvent or cosolvent to the polymer, and that those complexes are a poor solvent for the polymer.14,17,18 Another approach in this category by Hao et al. is concentrated on composition fluctuations in THF−water solutions, which were assumed to be the main reason for PNIPAM cononsolvency.19 A second category of explanations concerns the microscopic solute− solvent interactions. Tanaka et al. considered competetive adsorption of solvent and cosolvent as the cause for cononsolvency.20−22 In their theoretical approach, cooperative adsorption leads to a higher probability of one solvent molecule to adsorb in the vicinity of another molecule of the same kind along the polymer chain. This nonlinearly amplifies the changes in solvent composition. However, they neglect solvent− Received: August 17, 2017 Revised: November 21, 2017 Published: November 22, 2017 14269

DOI: 10.1021/acs.langmuir.7b02903 Langmuir 2017, 33, 14269−14277

Article

Langmuir cosolvent interactions, which contradicts the first category of explanations, as well as simulations with perturbed chain statistical association fluid theory (PC-SAFT) as studied by Arndt et al.23 Mukherji et al. have found preferential interactions of one solvent with the polymer as a generic phenomenon to explain cononsolvency.16,24 Rodriguez-Ropero et al. however considered the chemistry-specific balance between solvation free energies and configurational entropy as an explanation for cononsolvency.25 Bischofberger et al. examined the water-rich and alcohol-rich phases separately. They found hydrophobic hydration and the kosmotropic effect of alcohols in the first case and classical mixing contributions to the thermodynamics of polymer solutions in the second case to explain the cononsolvency phenomenon.26 Some experimental studies have examined the effect of cononsolvency on the VPTT of PNIPAM microgels. By addition of up to 30 mol % (≈49 vol %) of methanol or 8 mol % (≈22 vol %) of ethanol to aqueous solutions, the VPTT decreases, as has been shown by dynamic light scattering (DLS).26,27 At higher alcohol concentration, no VPTT is observed, as PNIPAM does not show a VPTT in alcohol in the common measuring ranges. This can be explained by the stronger hydrogen bonds of PNIPAM with alcohol compared to water.16 Consequently, the plateau minimum of the hydrodynamic radius at high temperatures increases with increasing alcohol concentration. These characteristics for microgels are generally similar to linear chains.27 Few studies have been carried out so far concerning the cononsolvency effect on microgels which are adsorbed to solid surfaces. Adsorption of microgels to a surface can lead to a compression by more than 1 order of magnitude.28,29 The higher polymer density in adsorbed microgels can affect their stimuli response in several ways. Therefore, studying the behavior of adsorbed particles is of great importance, especially for potential applications like sensors. One approach was based on microgels adsorbed between two gold layers like an etalon in mixtures of water and methanol.30 The VPTT was shifted to lower temperatures with increasing methanol concentration, and comparison to microgels in bulk solution did not show any significant difference. An effect of confinement was however found in a combined DLS and atomic force microscopy (AFM) study of PNIPAM and P(NIPAM-co-AAA) microgels on silicon wafers coated with the cationic polyelectrolyte poly(allylamine hydrochloride) (PAH).31 The minimum volume of PNIPAM microgels in bulk was reached at 10 vol % ethanol. Confined to the surface, however, the minimum is found at 40 vol % ethanol. This shift in the minimum volume toward higher organic solvent concentrations is explained by the higher polymer density in confined gels and a preferential water uptake of PNIPAM-rich phases. This is derived from the larger particle volume in pure water compared to pure organic solvents. This finding is in contradiction to the results described above. Additionally, the sizes of microgels spin-coated from different water−solvent solutions have been compared under ambient conditions. An increase in ethanol concentration during spin-coating leads to higher particles with a reduced width and an overall increased volume. A possible explanation provided for this phenomenon is the entrapment of organic solvent molecules due to their bigger size. The contradiction of those findings from previous studies of adsorbed microgels leads to the present work. The question is if the degree of hydrophilicity of the surface has an effect on the solvent composition within the adsorbed microgel. The

influence of adsorption to both PAH and gold surfaces on the cononsolvency effect in water−ethanol mixtures is studied by AFM. Additionally, the temperature effect in cononsolvent systems is analyzed systematically for the first time on both surfaces, and the influence of the sample preparation is examined. Those findings are compared to the behavior of PNIPAM microgels in bulk solution, which has been studied by DLS. Additional insights are provided by dynamic force measurements with AFM, which have been performed on microgels for the first time with varying temperature in different solvent compositions. By those measurements,the storage and loss modulus can be obtained separately, giving information about elastic and viscous properties. This allows one to draw conclusions about the internal structure of the polymer− solvent system.

2. MATERIALS AND EXPERIMENTAL METHODS 2.1. Materials. N-Isopropylacrylamide (NIPAM), N,N′-methylene bis(acrylamide) (BIS), potassium persulfate (KPS), acrylic acid (AAc), and poly(allylamine hydrochloride) (PAH, Mw ≈ 58,000) were purchased from Sigma-Aldrich. The used ethanol was purchased from Chemsolute. Silicon wafers (orientation: 100) were purchased from Siltronic AG (Munich, Germany). Gold-coated silicon wafers (polycrystalline gold with 100 nm thickness on 100 silicon wafer, precoated with titanium for better adhesion) were purchased from Georg Albert PVP-Beschichtungen (Silz, Germany). For water purification, a three-stage Millipore system (Milli-Q Plus 185) was used. 2.2. Microgel Synthesis. Synthesis of the microgels used in this work was done via surfactant-free precipitation polymerization.32 PNIPAM and P(NIPAM-co-AAc) microgels were synthesized with the negative initiator KPS. The NIPAM monomers, AAc comonomers if applicable (5 mol %), and cross-linker BIS (5 mol %) were dissolved in Milli-Q water, and the solution was degassed for 60 min with nitrogen under constant stirring. The solution was transferred into a reactor and heated to 70 °C. To start the reaction, 1 mL of the initiator KPS dissolved in Milli-Q water was transferred into the reactor. The reaction temperature was maintained for 3 h. Then, the heat was turned off and the reaction volume was allowed to cool to room temperature overnight. At room temperature, the microgels were purified by dialysis against Milli-Q water for 10 days. Afterward, they were freeze-dried at a temperature of −85 °C and a pressure of 0.001 mbar. 2.3. Sample Preparation. The gels were deposited on silicon wafers with two kinds of coatings, namely, PAH and gold. For the PAH coating, silicon wafers were cleaned in a plasma cleaner and then deposited in a 0.01 M PAH solution (with 0.1 M NaCl) for 30 min. Afterward, they were deposited in Milli-Q water for 1 min to remove residual PAH and then dried in a nitrogen stream. The microgels were then spin-coated on the wafers. To do so, they were solved in Milli-Q water unless noted otherwise. The contact angle of water was measured via the sessile drop method. PAH is a hydrophilic surface with a contact angle of 44°, while gold is less hydrophilic and has a contact angle of 87°. 2.4. Scanning Force Microscopy. The topography of microgels adsorbed to a surface was scanned with an atomic force microscope (AFM). All measurements were done with a Nanowizard II (JPK, Berlin, Germany) using HQ:NSC18/CR-AU BS tips (Mikromash, Sofia, Bulgaria) in intermittent contact mode. The tips have a typical radius of 8 nm and a chromium−gold coating on their backside for better reflection. Measurements were performed in a temperaturecontrolled JPK liquid cell in solutions of water and ethanol. At least three microgels were scanned for each water−ethanol composition at each temperature, and the average was taken. The particle analysis was performed with Gwyddion (free software). The imaging of soft microgels in liquid is rather challenging and time-consuming. Therefore, the results presented in the plots of Figures 2 and 3 could not be achieved within 1 day. As a consequence, the AFM cell 14270


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Figure 1. (a) Height profile from force map on microgel particle. The red dot indicates the location of the tip, where dynamic force measurements are performed. (b) Example force against time plot during a dynamic force measurement. with the sample had to be reset another day with another cantilever and the apparatus parameters like set point and gain had to be adjusted to get the optimum results. Therefore, absolute values for the dimensions of microgels were not always comparable, and only the volume relative to the minimum value of the collapsed particles at 50 °C is shown for the temperature dependent measurements to ensure comparability. In contrast, the data shown in Figure 4 could be obtained with the same cantilever and the same (or at least very similar) apparatus parameters (set point, gain). Therefore, absolute values are shown there. The adsorbed microgels analyzed in this study possess a contact angle of 20−40°, which is well below 90°. Therefore, no major artifacts resulting from the tip geometry are expected, which might arise in systems possessing sharp edges. 2.5. Dynamic Force Measurements. Dynamic force measurements were done with an MFP-3D AFM (Asylum Reasearch, Oxford Instruments, Santa Barbara, CA) with a CoolerHeater (Asylum Research) for temperature control in liquid, with a drop of the respective solvent being placed upon the sample. HQ:CSC38/NO AL tips (Cantilever C, MikroMasch, Sofia, Bulgaria) were used for all measurements. Those tips are uncoated and have a half tip cone angle of 20° and a reference spring constant of 0.05 N/m. Before the measurements, the tips had to be calibrated against a hard surface to obtain the inverse optical lever sensitivity (InvOLS). Then, the exact spring constant was determined by fitting the thermal noise spectrum of the cantilever. Those procedures are built in the Asylum Research software. The dynamic modulus G* = G′ + iG″ can be obtained from dynamic force measurements.33−36 First, a standard force map was recorded in the area around a microgel particle. From the height profile, the center of the particle can be identified, which is where the tip is then placed for dynamic measurements, as shown in Figure 1a. All dynamic procedures were controlled with the built-in dwell panel of the Asylum Research software. The procedure is exemplarily shown in Figure 1b. The tip is approached to the surface with a velocity of 2 μm/s until a load force of 2 nN is exerted on the cantilever (area I in Figure 1b). Then, the tip is kept here to equilibrate for 5 s (area II), until the cantilever starts with a sinusoidal oscillation for 10 s (area III). The amplitude is 10 nm, and the frequency is varied between 5, 10, 20, 40, 60, 80, and 100 Hz, which is well below the resonance frequency of the cantilever (≈5 kHz). After 10 s of oscillation, the cantilever is retracted from the microgel (area IV). Force and indentation signals were used for data analysis. The recorded values depend to some extent on the load force exerted by the tip. Higher forces yield slightly higher values for G′, which might be referable to the finite size of the microgels. Measurements with a very low load force however produce an unstable oscillation signal. Therefore, a constant value of 2 nN was chosen for all measurements to ensure reasonable and comparable results. The indentation was around 100 nm, which is significantly larger than the tip radius, so a conical indenter shape was assumed. Because of their rather small size, pure PNIPAM microgels are not well suited for indentation measurements with AFM. Therefore,

P(NIPAM-co-AAc) microgels were synthesized. Because of the negative charge of the comonomers, a bigger radius is achieved. The cononsolvency behavior of P(NIPAM-co-AAc) is similar to pure PNIPAM. Data Analysis. The Hertzian contact mechanics model37,38 allows the calculation of the complex modulus of the sample for a conical indenter by G* = G′(ω) + iG″(ω) =

1 − ν F(ω) 3δ tan(ϕ) δ(ω)

(1)

with the Poisson ratio ν, which was assumed to be 0.5 (value for incompressible materials because of the high water or ethanol content in the gels), the indentation depth δ, and the half tip cone angle ϕ. F(ω) and δ(ω) are the force and indentation signals recorded during the oscillation of the cantilever, which are given by A (ω) i(φF (ω) − φδ(ω)) F(ω) = F e δ(ω) Aδ (ω)

(2)

with the respective amplitudes A and phases φ of the force and indentation signals which have been fitted with a sine function. The complex modulus consists of a real part, the storage modulus (G′), and an imaginary part, the loss modulus (G″). Dynamic force measurements allow one to separately obtain both parts. G′ represents the elastic properties, and G″ represents the viscous properties of the sample. A sample which acts perfectly elastic would give zero phase shift between force and indentation, thus giving only a real part for the complex modulus. On the other hand, a purely viscous sample would give a phase shift of 90°, leaving only an imaginary part for the complex modulus. Real samples like the microgels observed in this study give a phase shift between 0 and 90° according to the respective influence of elasticity and viscosity. The quotient G″/G′ is called the loss tangent and gives information about whether elastic or viscous behavior is dominating. By fitting the approach part of the curves (area I in Figure 1b), the elastic modulus E could be calculated additionally to the complex modulus, using the Hertz model37 implemented in the Asylum Reasearch software: F=

2Eδ 2 tan ϕ π(1 − ν 2)

(3)

Three different microgel particles were analyzed for every system, and the average was taken. The hydrodynamic drag which acts upon the cantilever was found to be insignificant for the present system, so no systematic hydrodynamic drag correction as has been done in similar studies33−35,39 was performed. 2.6. Dynamic Light Scattering. The hydrodynamic radius of microgels in bulk solution was measured by dynamic light scattering (DLS). Measurements were performed on an LS spectrometer (LS Instruments, Fribourg, Switzerland) with a HeNe laser at λ = 632.8 nm with 21 mW. For the correlation function, the LS spectrometer was 14271


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Figure 2. Volume relative to the minimum volume of PNIPAM microgels on (a) PAH and (b) gold surfaces and (c) hydrodynamic radius in bulk in varying water−ethanol solutions at 20 °C. The volume of the adsorbed microgels is normalized with respect to the minimum, since not all curves in the paper could be measured with the same tip, leading to slight deviations.

Figure 3. Volume relative to the minimum volume of PNIPAM microgels on (a) PAH surfaces after spin-coating from aqueous solution, (b) PAH surfaces after spin-coating from solutions in the respective water−ethanol mixture, (c) gold surfaces, and (d) hydrodynamic radius in bulk during heating in 0, 10 and 20% ethanol. 50 °C was the maximum temperature in AFM experiments in order to avoid evaporation. used. The scattering intensity was measured for 30 s at angles between 30 and 120° in steps of 5°. In temperature dependent measurements, all samples were heated from 14 to 60 °C and then cooled again in steps of 2 °C. The data were fitted using a self-written script with the cumulant fit procedure.

the minimum volume in bulk and on gold is observed at 30 vol % ethanol, on PAH, 40 vol % ethanol is needed for the maximum collapse. P(NIPAM-co-AAc) microgels show a similar trend, with a plateau minimum on PAH and gold between 30 and 50 vol % ethanol. The temperature dependent swelling with varying solvent composition is shown in Figure 3. The VPTT in pure water is always located between 30 and 35 °C. On PAH, 10 vol % ethanol shows no effect on the VPTT, whereas 20 vol % ethanol decreases the VPTT by 5 °C. No VPTT is observed for 30 vol % ethanol or more. This is in contrast to the bulk behavior, where 10 vol % ethanol already leads to a decrease of the VPTT by 4 °C and 20 vol % ethanol leads to a decrease by more than 10 °C. To investigate the influence of the type of surface as well as the sample preparation, measurements were also performed on gold wafers and on particles which were spin-coated from the same water−ethanol mixture that was used in the swelling experiments afterward. In both cases, the resulting temperature dependence resembles the bulk behavior,

3. RESULTS 3.1. Microgel Topography. The topography of adsorbed microgels was scanned with AFM. Figure 2 shows the relative volumes of PNIPAM microgels on PAH and gold compared to the hydrodynamic radius in bulk solution. The ethanol concentration is varied at a constant temperature of 20 °C. For the surface data, the same set of particles was observed throughout the whole composition cycle. On PAH, the ethanol concentration was first increased from 0 to 100 vol % and then decreased to 0 vol % again. No significant hysteresis is observed. On gold, the microgels are detaching in pure ethanol, so no full composition cycle as on PAH could be performed here. A slight shift of the composition where the minimum is located can be seen for PAH compared to gold and bulk. While 14272


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Figure 4. Comparison of (a) volume, (b) projected surface area, (c) height, and (d) flatness (rSurface/height) of PNIPAM microgels on gold and PAH. The microgels were spin-coated from aqueous solutions containing 0 (not further indicated), 10, or 20 vol % ethanol and were measured under ambient conditions (dry), in water, or in ethanol. The measurements were carried out at 20 °C.

with a reduction of the VPTT by 5 °C for 10 vol % ethanol and 15 °C for 20 vol % ethanol. Apart from the transition temperature, the swelling ratio varies depending on the surface and sample preparation conditions as well. Figure 3 shows that on PAH microgels only swell by a factor of 2.25, while on gold the swelling factor is 3.75 compared to the collapsed state above the VPTT. Furthermore, microgels spin-coated on a PAH surface from an aqueous solution containing 10 vol % ethanol show a higher swelling, with a factor of nearly 3. A reswelling of PNIPAM microgels in bulk solutions of ethanol and water at temperatures above 50 °C has been reported recently.40 Because the AFM liquid cell used in the present study is not fully closed, evaporation made measurements above 50 °C impossible, so no reswelling at high temperatures could be observed at the surface. To better understand the behavior of microgels on surfaces and the influence different surfaces have on the swelling, the gel dimensions are compared on gold and PAH. They are analyzed in ambient (called “dry” in the following) state after spincoating from pure water, 10 or 20 vol % ethanol, and after swelling in water or ethanol (except on gold, because immersion in ethanol leads to detachment of the microgels here). The results are presented in Figure 4. Generally, the microgels are always bigger on PAH than on gold, and when spin-coated from pure water, they are flatter on gold. Their dimensions in both lateral and vertical directions are larger in water than in ethanol. As reported before, microgels in the dry state are bigger and have a smaller surface area and a larger volume when the ethanol concentration in the spin-coating solution is increased.31 The gels are much flatter (with flatness being defined as the quotient of the microgel radius on the surface and the particle height) under ambient conditions than in solution, which means that swelling occurs mainly in the vertical direction.

A good indicator for interactions between microgels and surfaces is the swelling ratio fA for the surface areas A, which can be calculated as fA =

Aliquid Adry

(4)

as well as the swelling ratio f V concerning the volumes V, given as fV =

Vliquid Vdry

(5)

The swelling ratios given in Table 1 clearly show that microgels on gold swell more in the lateral direction than microgels on PAH. Table 1. Particle Swelling Ratio from Ambient to Liquid Conditions at 20 °C fA,water fA,ethanol f V,water f V,ethanol

on PAH

on gold

2.53 2.47 14.18 11.83

3.59 20.25

3.2. Storage and Loss Moduli of Microgels. Dynamic force measurements were carried out with AFM on P(NIPAMco-AAc) microgels deposited on gold surfaces. AAc monomers add electric charges to the microgels. Therefore, electrostatic forces increase the adhesion to the surface, and those gels stick to gold surfaces even in ethanol, in contrast to pure PNIPAM gels. Figure 5 shows the cononsolvency behavior of P(NIPAMco-AAc) microgels at a constant temperature of 20 °C. A minimum volume is observed between 30 and 60 vol % ethanol. 14273


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Table 2. Average Values of G′, G″, E, and the Ratio E/G′ for P(NIPAM-co-AAc) Microgels for Different Water−Ethanol Mixtures at 20 and 50 °C temperature (°C)

vol % EtOH

G′ (kPa)

G″ (kPa)

E (kPa)

E/G′

20 20 20 20 50 50

0 30 60 100 0 30

98 156 208 177 544 448

6 14 108 62 374 246

183 198 320 293 732 680

1.87 1.27 1.54 1.66 1.35 1.52

4. DISCUSSION A central aspect of this work is the influence of confinement at a surface on the cononsolvency effect for microgels. A slight shift in the concentration at which the particle size minimum occurs has been detected between PAH, with 40 vol %, and gold and bulk, with 30 vol %. This is consistent with the literature, as a shift between PAH and the bulk has been reported before,31 while no shift has been found between gold surfaces and the bulk.30 This already indicates that not only confinement to a surface but also the specific properties of the surface play a role in influencing the cononsolvency effect. The shift of 10 vol % for PAH which is found here is however much less significant than 30 vol % found in ref 31. The reason for this deviation might be the smaller increments in ethanol concentration measured in the present work in comparison to the former work. Temperature dependent measurements and the shift in VPTT provide further insight into the impact of surfaces. The fact that a deviation from bulk behavior can be observed for microgels on PAH surfaces but not on gold is consistent with the results from cononsolvency experiments at constant temperature. Accordingly, adsorption to PAH surfaces has a repressive impact on the cononsolvency effect. A higher ethanol fraction is needed to achieve a particle shrinking and a decrease of the VPTT, respectively. Thus, there have to be stronger interactions between PNIPAM and PAH than between PNIPAM and gold. This is supported by the swelling ratio in the lateral direction fA, which is higher for gold surfaces than for PAH. Microgels are flexible particles, so they flatten upon adsorption, depending on internal factors like the cross-linker concentration,42 and on the respective surface. This can also be seen in the gel dimensions after spin-coating. Microgels on gold are significantly smaller and flatter than the same microgels on PAH. A further hint for the weaker interaction between

Figure 5. Volume relative to the minimum volume of P(NIPAM-coAAc) microgels on gold surfaces at 20 °C.

Figure 6a shows the storage modulus (G′) for different water−ethanol mixtures and different temperatures. At 20 °C, surprisingly, the highest values for G′ are not observed at 30% ethanol, where the volume minimum is located (see Figure 2), but for 60 and 100% ethanol. No frequency dependence of G′ is observed at 20 °C. The G′ values obtained at 50 °C are significantly higher than those at 20 °C. In contrast to the values at 20 °C, they increase with increasing frequency. The loss tangent (G″/G′) is shown in Figure 6b. Although the G″ data scatter more strongly than the G′ data, several trends can be observed. The loss tangent for the microgels in 0 and 30% ethanol at 20 °C is the lowest with values below 0.1, which indicates a mostly elastic behavior. For microgels in a solution with higher ethanol content (60 and 100%), the loss tangent is up to 0.3, which still implies a behavior dominated by elasticity rather than by viscosity. At 50 °C, however, the loss tangent for both 0 and 30% ethanol is significantly higher. This means that, in a highly shrunken state, the role of viscous behavior is more pronounced. From the force curves obtained during the indentation process of the dynamic AFM measurements, the elastic modulus E could be extracted as well. The obtained values, alongside averaged values for G′ and G″, are given in Table 2. For homogeneous and isotropic materials, E is connected to G′ by the relation E = 2G(1 + ν) = 3G for a Poisson ratio ν of 0.5.41 The obtained E/G′ values are between 1.27 and 1.57, which is lower than expected. This would lead to negative E values for the apparent Poisson ratio ν = 2G ′ − 1 between −0.07 and −0.37.

Figure 6. (a) G′ and (b) G″/G′ of P(NIPAM-co-AAc) microgels in dependence of frequency for different water−ethanol mixtures at 20 and 50 °C. The microgels are spin-coated from aqueous solutions on gold surfaces. 14274


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Further insights into the swelling mechanisms have been gained by considering the rheological properties and measuring the storage and loss modulus of microgels at different temperatures and ethanol contents. It is known from temperature dependent measurements in water that the elastic modulus of PNIPAM microgels increases when particles shrink with increasing temperature.45,46 Accordingly, the highest storage modulus G′ is observed at 50 °C, both in 0 and 30 vol % ethanol. The breaking of hydrogen bonding between PNIPAM and solvent molecules at T > VPTT leads to a smaller number of solvent molecules remaining within the collapsed structure. Therefore, more monomer−monomer contacts occur, and the gel becomes stiffer. Additionally, the chain conformation changes, so that hydrophobic parts come closer together to avoid exposure to the solvent, and hydrogen bonds between PNIPAM monomers form. This also explains the higher loss tangent at 50 °C, as PNIPAM chains are rubbing against each other in the absence of solvent molecules. Thereby, they are causing a higher friction and a higher loss modulus G′′. Regarding the cononsolvency effect, G′ of the microgels at 30 vol % ethanol, where their volume is minimal at 20 °C, is not significantly decreased compared to G′ in pure water. This finding stresses how the shrinking mechanism for the cononsolvency effect differs from the shrinking upon temperature increase. The low storage modulus despite the small size in the case of cononsolvency leads to the conclusion that the remaining number of solvent molecules must efficiently lubricate the polymer network. A probable explanation of this is the presence of polymer loops bridged by ethanol molecules. Preferential adsorption of ethanol molecules to PNIPAM monomers compared to the adsorption of water to PNIPAM has been proposed as a reason for the formation of loops in several theoretical studies.16,24,47−49 The microgel subsequently remains softer in the low-temperature cononsolvency case than in the high-temperature case. Preferential adsorption of one solvent seems however not to be the sole requirement for the appearance of the cononsolvency effect. Scherzinger et al. have analyzed the cononsolvency effect for polymers with secondary amide side chains, i.e., hydrogen bond donor and acceptor functions, and tertiary amide side chains, which only have a hydrogen bond acceptor function.50 Although all analyzed polymers have a preference toward methanol compared to water, only those with a secondary amide group show a significant cononsolvency effect. An additional requirement therefore seems to be the ability of the polymer to form intramolecular hydrogen bonds to support the shrinking. The temperature effect however is stronger, and all systems, including those without intramolecular hydrogen bonds, do show a VPTT. At 20 °C, the highest moduli were surprisingly observed for 60 and 100 vol % ethanol, i.e., the systems with the highest ethanol content, whereas it was lower for 30 vol % despite the smaller size. This leads to the assumption that the water which is present in the microgel is mainly responsible for its softness, so it might act like a plasticizer for PNIPAM. The shift in the ethanol concentration where the minimum modulus is observed compared to the minimum volume shows parallels to the shift in transition temperature of PNIPAM microgels in pure water. The transition from low to high modulus takes place at higher temperatures than the volume phase transition.45,46 Both results show that there is no direct

PNIPAM microgels and gold is the fact that they can be easily removed by exposure to ethanol, while they remain fixed at the PAH surface. The attraction of PNIPAM to gold, which is a good conductor, can be referred to image charges. On PAH, however, microgels are adhering to the surface at first contact, preventing them from spreading out and resulting in larger particles which are less flat. Both the opposite charges of PAH (+) and PNIPAM gels (− due to negatively charged initiator) as well as interdigitation between chains of those two polymers might be responsible for this adherence. PNIPAM microgels are consequently not in an equilibrium state after spin-coating on a PAH surface. The chain conformation obviously influences the swelling behavior more strongly than the presence of a surface itself. The theory that simply a higher polymer density is responsible for the shift in microgel volume minimum31 no longer holds, especially since microgels are even more compressed on gold. Bonds of PNIPAM to ethanol are preferred over bonds to water.16 Furthermore, findings of a nuclear magnetic resonance (NMR) study43 suggest that the respective methanol or ethanol concentration confined inside a PNIPAM macrogel is higher than that in bulk solution. This might indicate that ethanol is even more enriched in PNIPAM microgels of higher PNIPAM density like after adsorption at a surface. According to our knowledge, there is no study about the water/ethanol ratio as a function of PNIPAM density. Therefore, we can only speculate about this point. Measurements with azobenzene-containing surfactant have also shown that the microgel is more hydrophobic than the aqueous environment.44 It is rather the nonequilibrium microscopic chain arrangement and the reduced flexibility that might increase the water ratio inside the gel and hinder the cononsolvency effect on PAH. The solution from which the microgels are spin-coated influences the overall structure and thereby also the internal microscopic chain arrangement. Microgels spin-coated from mixtures are larger in the dry state than microgels spin-coated from pure water, so the meshes of the polymer network might be less compressed. It is assumed that this is caused by the larger ethanol molecules. Those larger meshes can take up the respective water−ethanol mixture in the swelling process. If spin-coated from pure water, however, the meshes are more compressed, and a smaller fraction of ethanol molecules can fit in. In the polymer confined to PAH after spin-coating from water, the ethanol concentration might therefore be lower than that in polymers in bulk solution. This might explain the slight shift of the cononsolvency minimum to higher ethanol concentrations. In summary, the microscopic structure of the gel network, influenced by the surface coating and the spincoating conditions, terminates the cononsolvency behavior of adsorbed microgels. A second factor which might influence the shift on PAH compared to bulk and gold is the hydrophilicity of the surface. The water contact angle on PAH is 44°, compared to 87° on gold. PAH is more hydrophilic, which might lead to a higher water concentration next to the PAH surface compared to a gold surface. Such small changes in composition can be amplified inside the microgel because of cooperative adsorption.20 This might explain why higher ethanol concentrations are needed for PAH to reach a minimum volume, or to achieve a certain VPTT decrease. The RMS roughness for all surfaces is in the sub-nanometer range, so it is not expected to play a role here. 14275


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correlation between size (or swelling ratio) and elastic modulus or shear modulus. The frequency dependence of the storage modulus for high temperatures and its absence for low temperatures is consistent with theory, which predicts a more pronounced frequency dependence of G′ for materials with a higher loss tangent.51 The obtained E values are obviously smaller than 3G which would be assumed for homogeneous polymer bulk material. This is indicative of the finite size and the inhomogeneous structure of the microgels, which consist of a highly cross-linked core and a weakly cross-linked shell.6,52 Additionally, apparent Poisson ratios are negative, which is only observed for specially structured materials, and very unlikely here. Hence, this is another hint at the invalidity of the relationship known from ideal conditions, and therefore indicates nonideal conditions. Due to adsorption, the material properties within the gel are far from being isotropic and cannot be described with an average number.

Sebastian Backes: 0000-0001-5400-4937 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Research Council (DFG) for financial support via the project KL1165/12-3.

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5. CONCLUSION The combination of temperature response and cononsolvency effect of PNIPAM microgels has been compared on different surfaces and in bulk solution. It has been shown that the type of surface and the sample preparation process influence the swelling behavior. A slight shift in the ethanol concentration where the minimum microgel volume occurs can be observed for PAH surfaces compared to gold and bulk phase. Additionally, more ethanol is needed for microgels spin-coated from water on PAH to decrease the VPTT. A probable reason for this is a strong interaction of the microgels with the surface which stems from electrostatic attraction and interdigitation of PNIPAM and PAH chains. In this way, the incorporation of ethanol could be sterically hindered, leading to a higher ethanol concentration necessary to achieve a certain effect. Additionally, the higher hydrophilicity of PAH compared to gold might lead to water enrichment near the PAH surface, which would explain the need for a higher ethanol concentration to shrink microgels. Dynamic force measurements on PNIPAM microgels have revealed a nontrivial relationship between size and elasticity. Microgels at T < 32 °C at their cononsolvency volume minimum have a much lower shear modulus than microgels at T > VPTT. This is caused by the different mechanisms underlying the shrinking. Cononsolvency is explained by preferential ethanol adsorption, leading to ethanol bridging of PNIPAM chains, with many solvent molecules still present. At high temperatures, however, many PNIPAM−solvent hydrogen bonds break, and the solvents are being expelled from the microgel. This makes the microgels much stiffer compared to the cononsolvency case. The subject of further studies in this field will be microgels with a homogeneous cross-link density, in contrast to standard microgels, where the core is more densely cross-linked than the shell. Because the present study has shown an influence of the microscopic gel structure on the swelling behavior, homogeneous microgels are likely to behave differently. This could shed more light on the cononsolvency mechanism.

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Combined Cononsolvency and Temperature Effects on Adsorbed

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