Structure and Thermal Response of Thin Thermoresponsive

Article pubs.acs.org/Macromolecules

Structure and Thermal Response of Thin Thermoresponsive Polystyrene-block-poly(methoxydiethylene glycol acrylate)-blockpolystyrene Films Qi Zhong,† Ezzeldin Metwalli,† Monika Rawolle,† Gunar Kaune,‡ Achille M. Bivigou-Koumba,§ André Laschewsky,§ Christine M. Papadakis,† Robert Cubitt,∥ and Peter Müller-Buschbaum*,† †

Physik-Department, Lehrstuhl für Funktionelle Materialien/Fachgebiet Physik Weicher Materie, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡ Martin-Luther-Universität Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany § Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany ∥ Institut Laue-Langevin, 6 rue Jules Horowitz, 38000 Grenoble, France ABSTRACT: Thin thermoresponsive films of the triblock copolymer polystyreneblock-poly(methoxydiethylene glycol acrylate)-block-polystyrene (P(S-b-MDEGA-bS)) are investigated on silicon substrates. By spin-coating, homogeneous and smooth films are prepared for a range of film thicknesses from 6 to 82 nm. Films are stable with respect to dewetting as investigated with optical microscopy and atomic force microscopy. P(S-b-MDEGA-b-S) films with a thickness of 39 nm exhibit a phase transition of the lower critical solution temperature (LCST) type at 36.5 °C. The swelling and the thermoresponsive behavior of the films with respect to a sudden thermal stimulus are probed with in-situ neutron reflectivity. In undersaturated water vapor swelling proceeds without thickness increase. The thermoresponse proceeds in three steps: First, the film rejects water as the temperature is above LCST. Next, it stays constant for 600 s, before the collapsed film takes up water again. With ATR-FTIR measurements, changes of bound water in the film caused by different thermal stimuli are studied. Hydrogen bonds only form between CO and water in the swollen film. Above the LCST most hydrogen bonds with water are broken, but some amount of bound water remains inside the film in agreement with the neutron reflectivity data. Grazing-incidence small-angle X-ray scattering (GISAXS) shows that the inner lateral structure is not significantly influenced by the different thermal stimuli.

1. INTRODUCTION During the past years stimuli-responsive polymers have attracted an immense attention.1 In particular, due to their huge volume change in response to an external stimulus such as temperature,2,3 light,4,5 or pH value,6,7 a broad range of potential applications has been proposed for these polymers.8−13 Among these stimuli-responsive polymers, thermoresponsive hydrogels are a class of special interest.14−21 Because of their three-dimensional network structure, these hydrogels can swell but do not dissolve in water. Upon an external thermal stimulus, the hydrogel can switch between a collapsed and an extended chain conformation.2 In particular, the collapse transition of polymers with a lower critical solution temperature (LCST) behavior is interesting for a large variety of applications, such as drug delivery systems,22−24 valves to control liquid transfer,25,26 or optical devices.27,28 However, so far, most of the investigations have focused on polymer solutions and on bulk polymer samples.29−33 Compared to these bulk hydrogel samples, thin hydrogel films can swell and collapse only in one direction (along the surface normal), which makes them promising for applications like nanosensors and nanoswitches.9,10,12,34 Among the frequently investigated © 2013 American Chemical Society

polymers with a LCST behavior, poly(N-isopropylacrylamide) (PNIPAM) is still the most extensively investigated one.3,29,35 PNIPAM has a LCST of 32 °C, which is slightly below the body temperature, which limits biomedical applications.9,14,21,25,36,37 Moreover, the moderate to low value of the LCST also imposes limits to the usability of PNIPAM in devices or in tropical countries, where the average temperature is above the transition temperature. In these cases, PNIPAM will only stay in the collapsed conformation, and switching cannot be realized. Consequently, novel thermoresponsive hydrogels with a somewhat higher LCST are desirable. In this context, the promising novel thermoresponsive homopolymer poly(monomethoxydiethylene glycol acrylate) (PMDEGA) has been explored, as it exhibits a LCST around 40 °C.39−44 In solution, PMDEGA has a broader transition than PNIPAM and virtually no hysteresis upon cooling. Within the class of poly((oligoethylene glycol) [meth]acrylate)s,45 PMDEGA has the shortest possible side chain to achieve water solubility. Thus, PMDEGA is sterically much Received: March 26, 2013 Published: May 3, 2013 4069

dx.doi.org/10.1021/ma400627u | Macromolecules 2013, 46, 4069−4080

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The number-average molar mass of the block copolymer was determined as 24 000 g/mol by analyzing the 1H NMR spectra, consisting of a large inner block of PMDEGA with a number-average degree of polymerization of 125, which is framed by two short hydrophobic blocks of polystyrene, with an number-average degree of polymerization of 11 each. Deuterated water (D2O) (purity 99.95%) was used as received from Deutero GmbH. 1,4-Dioxane (purity 99.5%) was received from Acros. Dichloromethane (purity 99.9%), ammonia solutions (NH3, 30−33%), and hydrogen peroxide (H2O2, 30%) were purchased from Carl Roth GmbH. Silicon (Si 100, p-type) was supplied by Si-Mat. 2.2. Substrate Cleaning and Film Preparation. Silicon (Si) with a native oxide layer on the surface (p-doped and purchased from Si-Mat with a diameter of 100 mm and a thickness of 525 ± 25 μm) was used as the solid support for thin hydrogel films. The precut Si was placed in dichloromethane at 46 °C for 30 min and afterward rinsed several times with Millipore water. Next, Si was stored for 2 h at 76 °C in a basic solution which contains 350 mL of water, 30 mL of H2O2, and 30 mL of NH3 to remove organic traces. Then, Si was stored shortly in Millipore water. Before spin-coating, Si was thoroughly rinsed with Millipore water to remove all possible traces of the basic bath.54 Compressed nitrogen was used to dry the Si substrate. Because of this cleaning protocol, a hydrophilic oxide layer with a thickness of 5 nm was installed at the Si surface.40,42 Thin P(S-b-MDEGA-b-S) films were prepared by spin-coating (2000 rpm, 30 s) from 1,4-dioxane solutions at room temperature (relative humidity 40%) onto the precleaned Si substrates in a series of concentrations from 1 to 15 mg/mL. For ATR-FTIR measurements, Kapton foils were used as flexible substrates. The precut Kapton foils were placed in acetone overnight for cleaning. Afterward, they were rinsed with Millipore water to remove possible traces of acetone. Before solution casting, the Kapton foils were dried with compressed nitrogen to remove residual water. Thick P(S-b-MDEGA-b-S) films were prepared from solution casting (concentration 60 mg/mL). The solution used was transparent and did not show any sign of aggregates. The obtained film thickness was 5 μm. Several identical films were prepared under identical conditions to verify reproducibility. 2.3. White-Light Interferometry. Sample thickness and index of refraction (n) for optical wavelengths were measured with the Filmetrics F20 ThinFilm Measurement System (Filmetrics Inc., San Diego, CA). By adjusting the distance between the sample and the fiber-optic cable, the spot size of the incident white-light beam was varied from 500 μm to 1 cm. A wavelength range from 400 to 1100 nm was used for the measurement. An as-prepared P(S-b-MDEGA-bS) film was mounted in a small customized aluminum chamber (size length × width × height = 60 × 60 × 17 mm3), and water or aqueous salt solutions were injected into this chamber to install a water vapor atmosphere. The incident white-light beam was focused through the glass windows on top of the chamber to illuminate the sample. As the light makes multiple reflections between different parallel surfaces, the multiple beams interfere with each other, resulting in a net transmission and reflection amplitude depending on the wavelength of the incident beam. The data show a series of fringes as a function of wavelength, where the positions and amplitudes are related to the thickness and optical properties (n) by a model fitting using a single layer model. The four-layer structure found with neutron reflectivity is not resolved due to the small differences in the refractive indices of PS and PMDEGA. The refractive index of the triblock copolymer film is 1.59, while for the homopolymer PMDEGA film it is 1.60. On top of the white-light interferometry cell a small hole was located, which was tightly sealed by rubber. The water or aqueous sodium chloride solution was injected by a syringe through this hole into the cell for the swelling experiments. The desired humidity was typically reached within 2 min. Equilibrium of film swelling was determined from reaching a constant film thickness. The film thickness reached a constant value depending on the humidity latest after 300 min. According to this, during the white-light interferometry investigation of the collapse behavior, first the film was swelled in the vapor atmosphere for 300 min to reach its equilibrium state, and

closer to the group of thermoresponsive poly(Nalkylacrylamide)s as compared to the rather comb- or brushlike polymers derived from the homologous PEG macromonomers.43 In order to achieve a network structure with physical crosslinks and avoid the necessity of chemical cross-linking, amphiphilic block copolymers can be used.46−51 Among the amphiphilic block copolymers, the symmetrical ones of BAB type, with hydrophobic outer blocks (type B) and a hydrophilic inner block (type A), are particularly suited.47 In aqueous solution, both hydrophobic outer blocks B can be either in the core of one micelle or in the core of two different micelles, which thereby are bridged. Consequently, a physically crosslinked network is established by these bridged micelles.52 For PMDEGA, such BAB-type triblock copolymers have been realized by the addition of short polystyrene blocks, yielding a polystyrene-block-poly(methoxydiethylene glycol acrylate)block-polystyrene (P(S-b-MDEGA-b-S)) copolymer.41,43 Until now, only few investigations have focused on the responsive behavior of thermoresponsive hydrogel films to a sudden change of temperature.42 With respect to applications such as thermal sensors, temperature may undergo a sudden change instead of a slow change. For this reason, the investigation of how a hydrogel film reacts to a sudden thermal stimulus is of high interest. In the present work, we focus on the thermoresponsive behavior of thin films of P(S-b-MDEGA-b-S). Because of its inner structure, the triblock copolymer films are expected to stay homogeneous and not exhibit dewetting as the PMDEGA homopolymer films. The surface structure and the stability against dewetting from a Si substrate are measured with optical microscopy and atomic force microscopy. The internal structure investigation of the thin films is based on scattering techniques, such as X-ray reflectivity and grazing-incidence small-angle X-ray scattering (GISAXS).53 The LCST behavior of the thin P(S-b-MDEGA-b-S) films is probed by white-light interferometry in a water vapor atmosphere. To investigate the responsive behavior of the thin films to sudden temperature stimuli (temperature jumps from 25 to 35 or 45 °C), in-situ neutron reflectivity measurements are performed. Deuterated water (D2O) vapor is used to generate contrast with the (protonated) polymer films. In static neutron reflectivity measurements, the initially prepared dry films are characterized in detail. Moreover, bound water in the film is investigated by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) for different thermal stimuli. This article has the following structure: The introduction is followed by an experimental section describing the sample preparation and the experimental techniques. The next sections show results and discussion on the collapse transition and thermoresponsive behavior of thin P(S-b-MDEGA-b-S) films with thickness of 39 nm in a D2O vapor atmosphere to temperature jumps from 25 to 45 or 35 °C. After that, the characterization of bound water is addressed. Furthermore, the morphology of thin P(S-b-MDEGA-b-S) films is discussed with a focus on the change of structure due to the different temperature stimuli.

2. EXPERIMENTAL SECTION 2.1. Materials. The synthesis of the symmetrical triblock copolymer polystyrene-block-poly(methoxydiethylene glycol acrylate)-block-polystyrenedenoted as P(S-b-MDEGA-b-S)by RAFT polymerization was similar to a procedure described elsewhere.41,43 4070


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2.7. Thermal Stimulus Protocol. An as-prepared P(S-b-MDEGAb-S) film was mounted in the customized aluminum chamber (size 120 × 100 × 87 mm3). Before the neutron reflectivity measurements were carried out, 8 mL of D2O was injected by a syringe through a rubbertight hole, located at the cell ceiling, into the water reservoir of the preevacuated chamber to install an unsaturated D2O vapor atmosphere surrounding the P(S-b-MDEGA-b-S) film (relative humidity of 79% measured by a humidity sensor). The desired humidity was typically reached within 8 min. Initially, the temperature was thermostated to 23 °C. Because of D2O vapor atmosphere inside the chamber, the film started to absorb D2O, swelled, and reached the equilibrium state. To start the kinetic experiment, a sudden change of temperature was applied in the chamber. The temperature was increased from 23 to 45 °C. Thus, the final temperature is well above the LCST of P(S-bMDEGA-b-S) (36.5 °C). This increase in temperature was taken as the start of the switching kinetics (time = 0). Because of the thermal conductivity of the sample chamber, the LCST was passed after 110 s.62,63 During the whole time, the sample was monitored by in-situ neutron reflectivity. In addition, to understand the response of the swollen P(S-b-MDEGA-b-S) films to different thermal stimuli (above or below LCST), a second experiment was performed in which the temperature was increased from 23 to 35 °C instead to 45 °C. As a consequence, the final temperature remained below the LCST of the P(S-b-MDEGA-b-S) film. The uniformity of the temperature in the chamber was within the given error bar of the temperature (±0.2 °C). 2.8. ATR-FTIR Spectroscopy. With ATR-FTIR spectroscopy (JASCO FTIR-4100 spectrometer), water bound to P(S-b-MDEGA-bS) was detected. The scanned wavelength covered a range from 650 to 4000 cm−1. To ensure identical conditions to the neutron scattering experiment, the water vapor treatment was performed in the customized aluminum chamber following the thermal stimulus protocol used in the neutron scattering experiments.

then the temperature was increased in small steps. The uniformity of the temperature in the chamber was within the given error bar of the temperature (±0.2 °C). 2.4. X-ray Reflectivity. X-ray reflectivity curves were measured with a Siemens D5000 diffractometer operating a reflectivity extension in a θ/2θ geometry. The measurements were performed in air with a scintillation counter. The sample was fixed on the sample stage by a vacuum chuck. A tantalum knife edge was mounted above the sample surface to block the direct beam, to control the footprint of the X-ray beam on the sample, and to avoid over-illumination. The measurements were performed with a wavelength of λ = 0.154 nm (secondary graphite monochromator, Cu Kα). The incident angle θ was varied from 0° to 6°. X-ray reflectivity data were fitted with the program Parratt32.55 A model with several layers was necessary to fit the data. 2.5. Grazing-Incidence Small-Angle X-ray Scattering (GISAXS). GISAXS measurements were carried out at the beamline BW4 of the DORIS III storage ring at HASYLAB (DESY, Hamburg).56 The selected wavelength was λ = 0.138 nm. The beam divergence in and out of the plane of reflection was set by two entrance cross-slits. To operate a microbeam, the X-ray beam was moderately focused to the size of (height × width) 40 × 80 μm2 by using an assembly of refractive beryllium lenses.57 Each sample was placed horizontally on a goniometer. A beam stop was used to block the direct beam in front of the detector (MARCCD; 2048 × 2048 pixels). A second, point-like movable beam stop was used to block the specular peak on the detector. The incident angle was set to αi = 0.35°, which is well above the critical angles of PMDEGA (0.15°) and of Si (0.20°). Therefore, the X-ray beam penetrates into the film and probes the full film. At the chosen sample−detector distance of 2.22 m, the Yoneda peak and the specular peak are well separated on the detector.58 From the twodimensional (2D) GISAXS data, structural information is obtained from vertical and horizontal cuts of the 2D intensity distribution. 2.6. Neutron Reflectivity. At the D17 reflectometer at ILL (Grenoble, France) neutron reflectivity measurements were performed in time-of-flight (TOF) mode.59 A broad wavelength range (from 0.2 to 2.4 nm) was used simultaneously for TOF mode, and neutrons were registered at the detector as a function of their respective time-offlight. The necessary pulsing of the beam was realized by a double chopper system. The longest sample−detector distance available at the instrument (3.40 m) was used in the measurements. The scattered intensity was recorded on a 2D detector as a function of wavelength λ and the exit angle αf.59 The selected TOF mode in combination with an experimental instrument resolution, optimized to the thickness of the probed P(S-bMDEGA-b-S) films, allowed the detection of kinetic changes of hydrogel films during the swelling process and the collapse transition of the swollen film with a high time resolution. Neutron reflectivity scans were performed at one fixed incident angle every 20 s. The initially prepared films were measured with a longer counting time of 7200 s to have good statistics and thus gain well-defined starting conditions for the switching kinetics. The probed qz range covers a range from 0.01 to 0.1 Å−1, which was selected to cover the critical edges of protonated (Si, PS, and PMDEGA) and deuterated (D2O) substances. All reflectivity curves were fitted by using the Motofit software based on the Parratt algorithm.60 In the automated batch fit approach of this software, all the data sets were analyzed in series. The scattering length density (SLD) values were fixed from the fit to the initial static sample and matched with the values from literature, which are listed in Table 1.61

3. RESULTS 3.1. As-Prepared Films. By base-cleaning and spin-coating, thin and homogeneous P(S-b-MDEGA-b-S) films are successfully prepared on silicon substrates. Figure 1 shows optical micrographs of the identical film stored in ambient conditions for different storage times. The as-prepared film is homogeneous on large length scale (Figure 1a). After storage in ambient conditions for 1 day, the film changes its surface morphology (Figure 1b). Plenty of tiny holes and rings are

Table 1. SLD Values of the Sample Substances material

SLD value (10−6 Å−2)

Si SiO2 PS PMDEGA D2 O

2.07 3.47 1.42 1.06 6.36

Figure 1. Optical micrographs of a thin P(S-b-MDEGA-b-S) film with a thickness of 39 nm: (a) as-prepared, stored in ambient conditions for (b) 1 day, (c) 4 days, and (d) 30 days. The micrographs are measured with a magnification of 2.5 and show an area of 2560 × 1920 μm2. 4071


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Figure 2. AFM images of thin P(S-b-MDEGA-b-S) film (thickness of 39 nm) stored in ambient conditions for 30 days. The scan size varies from (a) 1 × 1 μm2 to (b) 2 × 2 μm2 to (c) 4 × 4 μm2. The color code is adjusted independently to emphasize the surface morphology.

observable on the surface. Further prolonging the storage time to 4 days or even 30 days does not cause any continuous transformation of the film. No pronounced changes of the surface morphology are traced on large scale (Figure 1c,d), indicating film stability against dewetting. However, although still intact, the film appears less homogeneous. To obtain a view of the film on a local scale, the film stored in ambient conditions for 30 days is investigated with AFM (Figure 2). Although the film is slightly heterogeneous on a large scale, a complete destruction of the film into isolated polymer drops, which would be typical for dewetting, is not found. As the rms roughness obtained from the AFM data is less than 4 nm, the surface is still very flat. Instead of a transformation into isolated drops, many tiny holes (diameter on the order of 80 nm) are visible on the surface. This change in surface morphology is attributed to the mobility of the PMDEGA chains in the film. Because the glass transition temperature Tg of PMDEGA is very low (−50 °C), the PMDEGA chains exhibit high mobility in the film instead of having a frozen-in chain conformation, as for instance PNIPAM. As a consequence, the chains can rearrange themselves to a more favorable conformation after the preparation. However, this rearrangement does not result in the dewetting of the copolymer film as it was observed for the homopolymer PMDEGA. Moreover, PMDEGA is watersoluble. When it is stored in ambient conditions, it will absorb water vapor from air. The absorbed water even accelerates the mobility of the PMDEGA chains.40 However, in the present investigation, as the PMDEGA block is hydrophobically modified by two PS end-blocks, physical cross-links are expected to be formed in the film. Thus, the rearrangement of the PMDEGA chains is hindered by these cross-links, and thin P(S-b-MDEGA-b-S) films are more stable as compared to thin PMDEGA homopolymer films.40 With optical microscopy and AFM, only the surface structure is probed. To gain information about the internal structure such as the film thickness and the density profile, a series of thin P(Sb-MDEGA-b-S) films with different thicknesses are investigated with X-ray reflectivity (XRR). Figure 3a shows the XRR curves of thin P(S-b-MDEGA-b-S) films (black dots) together with the fits (red lines). From these curves, it is obvious that even for the thinnest film (bottom curve in Figure 3a), the intensity oscillations (Kiessig fringes) are well pronounced. It indicates that even for the lowest concentration (1 mg/mL) used in the spin-coating, a homogeneous film is present on the substrate. Further increasing the concentration (from bottom to top), more and more fringes are observed due to the increasing film thickness. Generally, the XRR data are well described by the applied fitting model (two-layer model): the bottom layer is a thin (few nanometer) PMDEGA layer, whereas the top layer is a mixture of PMDEGA and PS constituting the main part of the film. The

Figure 3. (a) X-ray reflectivity curves (black dots) are shown together with model fits (red lines) for thin P(S-b-MDEGA-b-S) films (6, 16, 30, 47, 54, and 82 nm from bottom to top). The curves are shifted vertically for clarity of the presentation. (b) Film thickness as a function of P(S-b-MDEGA-b-S) solution concentration used for spincoating. The solid line is a linear fit.

morphology of the top layer is not resolved with XRR. From the fits, the total film thickness is obtained. It varies between 6 and 82 nm. Figure 3b shows the resulting film thicknesses as a function of the concentration of the solution used for spincoating, together with a linear fit to the data points represented by a solid line. In the range of concentrations investigated, the overlap concentration is not reached, and a linear dependency on the concentration is observed. Thus, the behavior of P(S-bMDEGA-b-S) solutions in the spin-coating process is similar to many common homopolymer solutions.40 Because of this linear dependency, the film thickness required can be easily obtained by selecting the appropriate concentration of the solution used for spin-coating. In contrast to XRR, with neutron reflectivity (NR) measurements, a weak contrast between the blocks of the investigated copolymer P(S-b-MDEGA-b-S) is achieved.42 Figure 4 shows a neutron reflectivity curve (black dots) together with the model fit (red line) for an as-prepared P(S-bMDEGA-b-S) film. The density profile of the film is achieved from this fit. In contrast to the XRR experiments, a four-layer profile (corresponding sketch shown as inset in Figure 4) is necessary for the fitting due to the increased contrast between PMDEGA and PS: The bottom layer, with a direct contact to the substrate, has a SLD of 1.06 × 10−6 Å−2 and a thickness of 4.1 nm. This SLD value suggests that this layer consists of pure 4072


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Figure 4. Neutron reflectivity curve (black dots) together with model fit (red line) of the as-prepared P(S-b-MDEGA-b-S) film. The fitting model is described in the text. The inset shows a sketch of the film morphology with PMDEGA being displayed in red and PS in black.

PMDEGA (Table 1). Because of base cleaning, the substrate is expected to be hydrophilic. As PMDEGA is also hydrophilic, it is reasonable that the bottom layer is composed of PMDEGA (in agreement with the XRR experiments). On top of this PMDEGA layer, a layer of 5.1 nm thickness and an SLD of 1.42 × 10−6 Å−2 are found, i.e., a thin PS layer which is a consequence of the preferential wetting of the substrate by PMDEGA and the block copolymer architecture. Above, there is a thick layer with an SLD value of 1.06 × 10−6 Å−2 and a thickness of 25.1 nm. This SLD value indicates that the central layer is composed mainly of PMDEGA with little amount of PS. The short PS blocks are presumably located in this domain but do not have a strong influence on the SLD since their volume fraction is small. A micellar structure is assumed similar to what is present in solution. At the film surface, another PS layer of thickness 5.1 nm is found. The total thickness of the thin P(S-b-MDEGA-b-S) film is determined to be 39.4 nm. The high thicknesses of the PS layers might indicate a perforated lamellar type; however, we have no direct evidence from AFM or the diffuse neutron scattering measured along with the NR. 3.2. Collapse Transition of Thin P(S-b-MDEGA-b-S) Films. The swelling behavior of thin P(S-b-MDEGA-b-S) films is investigated at room temperature, i.e., below the LCST. Films with a fixed initial thickness of 39 nm are measured in water vapor at different vapor pressures. Figure 5a presents the swelling behavior of thin P(S-bMDEGA-b-S) films exposed to sodium chloride solutions with different concentrations. From top to bottom, the sodium chloride concentration increases up to 0.36 g/mL (saturated sodium chloride solution). With increasing concentration, the number of water molecules in the vapor atmosphere decreases. As P(S-b-MDEGA-b-S) films cannot absorb sufficient water molecules, the swelling capability is hindered. In Figure 5b, the swelling capability (swollen film thickness normalized to the as-prepared film thickness) is plotted as a function of the vapor pressure. Similar to thin PMDEGA films, the whole pressure region can be divided into pressure sensitive and nonsensitive regions.40 Beside this similarity, one pronounced difference should be noted. Because of the lack of physical cross-links, the swollen PMDEGA films are unstable and start to dewet when the exposure to saturated water vapor lasts for a sufficiently long time (90 min). In contrast, thin P(S-b-MDEGA-b-S) films show no prominent dewetting of the swollen films due to physical cross-links in the film. Thus, we conclude that the physical cross-links inside the films do not influence the general swelling

Figure 5. (a) Swelling behavior of thin P(S-b-MDEGA-b-S) films when exposed to aqueous sodium chloride solutions with different concentrations at room temperature. From top to bottom, the concentrations of sodium chloride are 0 (black), 0.022 g/mL (magenta), 0.045 g/mL (cyan), 0.09 g/mL (blue), 0.18 g/mL (green), and 0.36 g/mL (red). (b) Swelling capability (swollen film thickness normalized to the as-prepared film thickness) plotted as a function of the vapor pressure. The solid line is a guide to the eye.

behavior of thin P(S-b-MDEGA-b-S) films but prevent the dewetting of the swollen films. It has been reported that by hydrophobic modification of the PNIPAM chains with PS blocks the LCST is shifted to slightly lower temperatures.34 For PMDEGA as a novel thermoresponsive polymer, it seems that hydrophobic modification has a much more pronounced effect on the LCST transition, at least in aqueous solution.41,43 To clarify this aspect, white-light interferometry is used to measure the transition behavior of a thin P(S-b-MDEGA-b-S) film with a film thickness of 39 nm. Figure 6a shows the temperature-dependent change of the film thickness when exposed to a saturated water vapor atmosphere. When the temperature approaches the LCST, due to the breaking of hydrogen bonds, the film repels water and shrinks. In contrast to PNIPAM-based thermoresponsive hydrogel films, which exhibit a sharp transition region, thin P(S-bMDEGA-b-S) films show a very broad transition region, which is similar to thin PMDEGA films reported previously by our group.40,42 This variation can be attributed to the different chemical structure of the polymer chains. In PMDEGA, only the ether group can act as H-acceptor and form hydrogen bonds with water. However, in the case of PNIPAM, not only the amide groups (H-donor) and the ether groups (Hacceptor) can form hydrogen bonds but also water cages can be formed around the isopropyl group on side chains of PNIPAM.64 Thus, it is understandable that more water is bound in case of swollen PNIPAM chains. Moreover, the volume phase diagrams of PEO, which is in some sense similar to PMDEGA, and PNIPAM are significantly different.65 Considering these two differences, a much broader transition region in the PMDEGA-based polymers can be explained.42 4073


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Figure 7. Exposure of the as-prepared P(S-b-MDEGA-b-S) film to D2O vapor atmosphere at 23 °C: (a) Selected neutron reflectivity curves (black dots) are shown together with model fit (red lines). The as-prepared film is presented on the bottom, while the final, swollen film is on the top. With increasing time the curves are shifted vertically for clarity of the presentation. (b) Two-dimensional intensity presentation (mapping) of the neutron reflectivity data as a function of swelling time with a logarithmic qz axis. The red arrow marks the initial position of the critical angle. Different scattering intensities are displayed with different color (bright means high and dark low intensity).

Figure 6. (a) Temperature-dependent film thickness measured for the P(S-b-MDEGA-b-S) films with a thickness of 39 nm when exposed to saturated water vapor. (b) The corresponding first derivative of the data in (a) indicates that the PTT is 36.5 °C.

To determine the phase transition temperature (PTT), the first derivative of thickness over temperature is plotted as a function of temperature in Figure 6b. From the curve, the PTT of the P(S-b-MDEGA-b-S) film with thickness of 39 nm is determined to 36.5 ± 0.2 °C. As the PTT of thin PMDEGA films with a thickness of 41 nm is 40.6 °C, we conclude that due to the hydrophobic PS blocks at both ends of the PMDEGA block, the PTT shifts to lower temperature. This behavior is similar to the PNIPAM-based thermoresponsive polymers.34 3.3. Kinetic Measurement under a Sudden Thermal Stimulus. The response of thin P(S-b-MDEGA-b-S) films with a thickness of 39 nm to a sudden thermal stimulus is measured with in-situ NR. In contrast to XRR, with NR measurements, a strong contrast between protonated P(S-b-MDEGA-b-S) and deuterated water is achieved.42 When the PMDEGA block absorbs deuterated water, the contrast between the swollen PMDEGA block and the nonswollen PS block is enhanced. Thus, not only the internal film structure and the density profile of the films are resolved but also kinetic changes of the amount of water inside the films can be traced during the swelling and the LCST-type transition. However, NR experiments are limited in time resolution due to the available neutron flux. As a consequence, the kinetic thermal response measurements were performed at undersaturated water vapor conditions (relative humidity 79%) to slow down all kinetic processes and thereby allow for a detection of the individual stages of swelling and deswelling. For the kinetic measurements, the as-prepared P(S-bMDEGA-b-S) film is mounted in a customized sample chamber. Afterward, the cell is evacuated and 8 mL of D2O are injected into the reservoir to install the nonsaturated D2O vapor atmosphere. Simultaneously, the absorption of D2O by the film is monitored with in-situ neutron reflectivity. Figure 7a shows 11 selected neutron reflectivity curves (black dots) together with model fits (red lines) from the beginning

(bottom) to the end (top) of the D2O exposure process. The intensity oscillations do not show a pronounced shift with respect to the scattering vector component qz, indicating that the film thickness exhibits no prominent change during the exposure. Thus, the film does not significantly change its thickness in the nonsaturated D2O vapor atmosphere. However, in the two-dimensional intensity representation (mapping) of the neutron reflectivity data which enhances the region of low qz values (Figure 7b) changes are traced: the critical angle (marked with red arrow) shifts toward higher qz values, and the overall reflected intensity increases with time. These observations show that, though the film does not strongly react with a change in film thickness, it still absorbs D2O vapor, causing the shift of the critical angle and the enhancement of the overall scattering intensity. By fitting of the individual neutron reflectivity curves with the model described above, more details such as the film thicknesses and the SLD profiles of the films are obtained. Furthermore, the D2O volume fraction (V% D2O) can be calculated from the equation n − n1 V % D2 O = × 100% n2 − n1 (1) where n is the SLD obtained from the fits, while n1 and n2 are the SLDs of the as-prepared film and D2O. Figure 8a presents the resulting relative film thickness d/dinitial and Figure 8b the D2O volume fraction (V% D2O) as a function of time after the film was exposed to water vapor. As seen in Figure 8, the response of the thin P(S-b-MDEGAb-S) film to water vapor exposure can be divided into two stages (separated by dashed line). In the first stage, from 0 s (D2O is injected into the reservoir) to 2000 s, the film thickness 4074


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kinetics.68−71 In this model, Li and Tanaka suppose that the swelling or the shrinking process is not a pure diffusion process, but that it rather follows first-order kinetics. According to their assumption, the shear modulus (Mshear) is related to the net osmotic modulus (Mos) and the osmotic bulk modulus (Kbulk). Their relation can be described by69 R=

Mshear Mshear = 4 Mos Kbulk + 3 Mshear

(2)

The swelling or shrinking processes are described by VD2O(∞) − VD2O VD2O(∞)

∞

=

t⎞ ⎟ ⎝ τn ⎠

⎛

∑ Bn exp⎜− n=1

69

(3)

In eq 3, VD2O and VD2O(∞) are the solvent absorbed at time t and infinite time (the equilibrium state of swelling), respectively. B is related to the shear modulus (Mshear) and the osmotic modulus (Mos), while τn is the relaxation time of the nth mode. If all high-order terms (n ≥ 2) can be neglected, eq 3 can be simplified to ⎛ VD O(∞) − VD O ⎞ 2 ⎟⎟ = ln B1 − t /τ ln⎜⎜ 2 VD2O(∞) ⎝ ⎠

Figure 8. Response of thin P(S-b-MDEGA-b-S) film to water vapor exposure at 23 °C: (a) relative film thickness and (b) D2O volume fraction (V% D2O) shown as a function of swelling time. The dashed line separates two stages and solid line is a fit with a model as explained in text.

(4)

In our investigation, the value of VD2O(∞) is obtained directly from the final equilibrium state of V% D2O (Figure 8b). The values of B1 and τ are determined by fitting (B1 = 0.41 and τ = 0.56 × 104 s). According to the literature,69 B1 and τ are 0.71 and 1.3 × 104 s for very thick, disk-like gels (thickness of 1.33 mm). Because the film investigated here is much thinner (∼40 nm), the smaller values of B1 and τ appear reasonable. When it reaches the equilibrium state, V% D2O of the whole swollen film has increased to 12%. In comparison, a pure PMDEGA film had uptaken 18% water (increase in V% D2O), but the film thickness had increased by 15%.42 Considering that the film thickness did not increase for P(S-b-MDEGA-b-S) films, the water content absorbed by the film is comparably high. The possibility to take up water without a significant increase in film thickness makes thin P(S-b-MDEGA-b-S) films promising for applications such as water storage in device structures because no strain will be imposed. After the equilibrium state of D2O absorption is reached, a kinetic investigation during a temperature jump from 23 to 45 °C is performed. The response of the swollen film is again monitored with in situ neutron reflectivity. After the sudden jump of the temperature, a prominent shift of the intensity oscillations toward lower qz values is visible, especially when comparing the first and second curve from the bottom of Figure 9a. Afterward, the intensity oscillations slowly shift back to slightly higher qz values. The curve for the final, collapsed state (the top one in Figure 9a) does not show a pronounced difference to the one in the initial, swollen state (the bottom one). Therefore, we conclude that for temperatures above the PTT the film immediately shrinks due to the collapse of the swollen PMDEGA chains. Afterward, the film slowly recovers to its initial, swollen thickness. Simultaneously, the critical angle of total external reflection shifts (marked with red arrow in Figure 9b) toward lower qz values and then slowly shifts back to higher qz values with time. This shift indicates that D2O is first repelled and then reabsorbed by the film, which results in a higher film thickness. In order to obtain more details, all reflectivity data are fitted by the model explained

decreases from 39.4 to 37.8 nm, showing that the film actually shrinks by 4% instead of increasing its thickness. Simultaneously, the amount of D2O (V% D2O) increases significantly from 0 to 6%. This behavior is different from the thin PMDEGA films investigated previously, which exhibited swelling accompanied by an increase in film thickness.42 It should be noted that at higher humidity a swelling consisting of a film thickness increase and an uptake of water occurs. The absence of an increase in film thickness although water is incorporated into the film is attributed to two main factors. When the film is exposed to water vapor atmosphere, the D2O molecules immediately occupy the free volume inside the P(Sb-MDEGA-b-S) film, similar to the behavior of PNIPAM-based copolymer films.66 Therefore, the water content increases without a significant increase in film thickness. Because the vapor pressure installed is unsaturated, only a limited number of water molecules surround the film, which limits the swelling capability of the P(S-b-MDEGA-b-S) film as compared to saturation condition. Moreover, the physical cross-links introduced into the film by the PS end-blocks do not plasticize and thereby prevent strong swelling of the film. Similar constraints imposed by a glassy PS part were already reported for PNIPAM-based block copolymer films.34,67 These P(S-bNIPAM) films were able to uptake water without any change in film thickness. The small change observed in the present investigation might be attributed to the lower PS content of the P(S-b-MDEGA-b-S) copolymer. In the second stage, from 2000 to 24 000 s, the PMDEGA chains absorb water as PMDEGA is hydrophilic. The film thickness and water content increase with time. The final swollen thickness is 38.2 nm, which is however still 3% thinner than the one of the as-prepared, dry film (Figure 8a). From our previous study,34,42 the second stage can be described by a diffusion process with a model explaining gel swelling 4075


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higher than the PTT of the thin P(S-b-MDEGA-b-S) film (36.5 °C). In the first stage (0−160 s), the water content drops from 11% to 6%, which is accompanied by an additional shrinkage of the film thickness by 2%. Therefore, this stage is denoted as shrinkage stage. Rejection of water and decrease in thickness are expected due to the collapse of the PMDEGA chains. When the film thickness reaches its minimum value, still 6% of D2O remain inside the film (Figure 10b). This amount of residual water is exactly the same as the amount of water occupying the free volume in the P(S-b-MDEGA-b-S) film during the swelling process. Thus, we conclude that this amount of D2O is bound that strongly to the film that it is not repelled from the polymer. In the second stage (160−2300 s), both the thickness and the water content stay virtually constant. The collapsed chains, which are in an energetically unfavorable state, can slowly rearrange to a more favorable conformation. Thus, the film reorganizes itself in D2O vapor atmosphere without any change in thickness or water content. For this reason, this stage is denoted as reorganization stage. Afterward, the film enters the third stage. Both the film thickness and the water content slowly increase again, indicating that the film reabsorbs water and becomes thicker. Similar relaxation processes were observed in thin PNIPAM and PMDEGA films previously.34,42 From this interesting relaxation behavior, it can be understood that, when temperature suddenly jumps above the LCST, the swollen film immediately collapses and repels water. This collapse takes place so rapidly that the polymer chains do not have time to rearrange themselves to approach an optimal chain conformation. However, as the film is still mounted in D2O vapor atmosphere, the collapsed chains slowly rearrange themselves to a more relaxed conformation with time. During this rearrangement process, the film reabsorbs D2O and becomes thicker. One difference between the relaxation of thin homopolymer PMDEGA and block copolymer P(S-b-MDEGA-b-S) films has to be noted. The relaxation process of collapsed PMDEGA films can be described by the model from Li and Tanaka.71 This is not the case for thin P(S-b-MDEGA-b-S) films. In these films not only the swelling is hindered by the PS blocks located at both ends of the PMDEGA block but also the relaxation process is affected. From the above discussion, it is clear that the response of a swollen P(S-b-MDEGA-b-S) film to a sudden thermal stimulus above the PTT proceeds in three stages. However, the response of a swollen P(S-b-MDEGA-b-S) film to a sudden change of temperature below the PTT is still unknown. In order to address this question as well, in-situ neutron reflectivity measurements are repeated with an as-prepared P(S-bMDEGA-b-S) film. The protocol for the first step (water exposure step) is identical, whereas in the second step, the temperature jumps only from 23 to 35 °C instead of 45 °C; i.e., the final temperature of the experiment is still below the PTT of 36.5 °C of the copolymer film. Therefore, one may not expect a collapse of the swollen film. Yet surprisingly, the shrinkage of the swollen film is still observed (Figure 11). When the temperature increases, the film immediately responds by shrinkage. In the first 300 s, the film shrinks by 4%. This shrinkage is accompanied by decrease of the water content from 12% to 6%, which is similar as observed in the temperature jumps experiment above the PTT. Possibly, the unexpected drop of water content can be attributed to the characteristic broad transition region of the P(S-b-MDEGA-bS) film. From Figure 3a, it is clear that even at low temperature

Figure 9. Kinetic changes of the P(S-b-MDEGA-b-S) film exposed to a temperature jump from 23 to 45 °C in D2O vapor atmosphere: (a) Selected neutron reflectivity curves (black dots) shown together with model fits (red lines). The initial film is presented on the bottom, and the film in the final, collapsed state at the top. With increasing time the curves are shifted vertically for clarity of the presentation. (b) Twodimensional intensity representation (mapping) of the neutron reflectivity data as a function of time on a logarithmic qz axis. The red arrow marks the initial position of the critical angle. Different scattering intensities are displayed with different color (bright means high and dark low intensity).

before. Figures 10a and 10b show the evolution of the relative film thickness d/dinitial and the D2O volume fraction (V% D2O), respectively, as a function of time. According to Figure 10, the response of the swollen film can be divided into three stages (separated by dashed lines) when the temperature jumps from 23 to 45 °C, i.e., to a temperature

Figure 10. Response of the swollen P(S-b-MDEGA-b-S) film to a jump of temperature from 23 to 45 °C shown as a function of time: (a) relative film thickness and (b) D2O volume fraction (V% D2O). The dashed lines separate different stages as explained in the text. 4076


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Figure 11. Response of a P(S-b-MDEGA-b-S) film to a jump of temperature from 23 to 35 °C as a function of time: (a) relative film thickness and (b) D2O volume fraction (V% D2O). The dashed lines separate different stages as explained in the text.

Figure 12. ATR-FTIR spectra of P(S-b-MDEGA-b-S) films: (a) whole spectra ranging from 650 to 4000 cm−1 and (b) zoom-in from 650 to 2000 cm−1. From bottom to top, (1) as-prepared film, (2) the swollen film at 23 °C, (3) the film which was first exposed to water vapor and then subjected to a temperature jump from 23 to 35 °C, and (4) the film which was first exposed to water vapor and then subjected to a temperature jump from 23 to 45 °C. The curves are shifted vertically for clarity.

(32 °C) the swollen polymer chains has already started to collapse and to repel water. For this reason, the film collapse still may start even when the temperature only reaches 35 °C. The shrunken film will also slowly rearrange in a D2O vapor atmosphere. But one pronounced difference is the time needed for this rearrangement (700 s), which is far shorter than in the case of temperature jumps above the PTT (2000 s). This phenomenon may be attributed to the degree of collapse of the PMDEGA chains. When the final temperature is above the PTT, the chains are in the completely collapsed state, so that it takes much longer time to rearrange them. When, however, the final temperature is kept closely below the PTT, the PMDEGA chains do not fully collapse, and thus, the time required for the rearrangement is much shorter. As the film is still placed in vapor atmosphere, it can reabsorb water and recover from the collapsed to the swollen state. Both the film thickness and water content increase back to their initial values. 3.4. Bound Water in P(S-b-MDEGA-b-S) Films. With neutron reflectivity measurements, the changes of the film thickness and water content are monitored. According to our previous investigation on thin PMDEGA films, though there are CO and C−O moieties in the PMDEGA chains, water can only form hydrogen bonds with CO.42 As P(S-bMDEGA-b-S) is obtained by the hydrophobic modification of both ends of PMDEGA block with PS blocks, it is not clear whether these hydrophobic PS blocks will influence the formation of hydrogen bonds. To address this question, ATR-FTIR measurements are performed on thick P(S-bMDEGA-b-S) films. Figure 12a presents the obtained spectra in the wavenumber range from 650 to 4000 cm−1. Curve 1 presents the whole spectrum of the as-prepared film at room temperature. It is similar to the one of the PMDEGA homopolymer films probed in an earlier investigation.42 The two characteristic peaks related to CO and C−O bonds are observed at 1728 and 1104 cm−1 in the spectrum, respectively.

Besides these peaks, the absorption bands in the range of 2800−3000 cm−1 are assigned to C−H stretching. However, one prominent difference between P(S-b-MDEGA-b-S) and PMDEGA is an additional peak which is observable at 700 cm−1. This peak is attributed to the benzene ring from polystyrene.72 After exposure to water vapor atmosphere for 2 h, a band in the range of 3300−3600 cm−1 emerges for the swollen film (curve 2), which is related to the O−H bond in water. Thus, it is obvious that water is absorbed by the film. Moreover, a characteristic peak at 1640 cm−1 arises. This peak is assigned to hydrogen bonds between CO and water. The characteristic peak related to the C−O bond seems to stay unchanged after swelling in water atmosphere. Therefore, similar to PMDEGA, hydrogen bonds seem to be only formed between CO and water in P(S-b-MDEGA-b-S). To compare the change of bound water for different thermal stimuli, additional ATR-FTIR measurements are preformed: A thick film is exposed to water vapor atmosphere at 23 °C for 2 h; afterward, the temperature jumps to 35 °C (below the PTT of the P(S-b-MDEGA-b-S) film), followed by waiting for 2 h to ensure equilibrium (curve 3). The fourth film (curve 4) is exposed to the same swelling process, except for that the temperature jumps to 45 °C (above the PTT). To obtain a better view, a zoom-in of the spectra from 650 to 2000 cm−1 is plotted in Figure 12b. Besides the as-prepared film (curve 1), the characteristic peak at 1640 cm−1 is observed in the other three curves, with the peak area varying with temperature. The amount of bound water is deduced by integration of the peak area around 1640 cm−1. For the as-prepared P(S-b-MDEGA-b-S) film (curve 1), the peak area is 6.6. After swelling in water vapor atmosphere 4077


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(curve 2), the peak area dramatically increases to 57.4, indicating that a significant amount of water is bound to C O. After the temperature jumps from 23 to 35 °C (curve 3), the characteristic peak at 1640 cm−1 remains unchanged, and the peak area reduces slightly to 53.3. However, when the temperature jumps from 23 to 45 °C (curve 4), the peak area significantly drops to 30.0. Thus, the amount of bound water is reduced, but a certain fraction remains in the film. These results confirm the results from neutron reflectivity measurements. 3.5. Inner Lateral Structure of P(S-b-MDEGA-b-S) Films. With optical microscopy and AFM, the surface structure of thin films is probed. In contrast to X-ray reflectivity, which can only detect the structure along the surface normal, grazingincidence small-angle X-ray scattering (GISAXS) is a powerful tool, well suited to investigate the internal structure parallel to the surface, called lateral structure, and the long-ranged correlation in P(S-b-MDEGA-b-S) films. Several reports demonstrate that if the polymer chains do not feature mobility, a roughness correlation between the solid substrate and the polymer surface is installed when using spincoating to prepare the films.73−76 Such correlated roughness causes intensity oscillations along the vertical direction (qz axis) in GISAXS images. In the case of PMDEGA homopolymer films, our previous measurements have not shown such roughness correlation, irrespective of whether the films were as-prepared or treated with different thermal protocols.40 This behavior was attributed to the low glass transition temperature of PMDEGA (Tg below −50 °C). The resulting high mobility of the PMDEGA chains in the films erases the energetically unfavorable roughness correlation. In order to investigate the behavior in case of the triblock copolymer films, analogous GISAXS measurements are performed, measuring the asprepared P(S-b-MDEGA-b-S) film and P(S-b-MDEGA-b-S) films after treatment by different thermal protocols. Figure 13 shows the corresponding data. Similar to thin PMDEGA films, no correlated roughness is observed in the GISAXS data. Thus, the energetically unfavorable state of correlated roughness is not conserved, although the PS end-blocks provide physical cross-linking, which significantly changes the swelling behavior and the response to a sudden change in temperature as compared to PMDEGA homopolymer films. Obviously, the film surface still has sufficient mobility to allow for a relaxation of the lateral structure, which erases the roughness correlation. Moreover, the inner film morphology is not significantly altered by water vapor exposure or by the applied thermalresponse protocols as the GISAXS data stay unchanged.

Figure 13. GISAXS data of thin P(S-b-MDEGA-b-S) films with a thickness of 39 nm: 2D GISAXS images of (a) the as-prepared film, (b) a film after exposure to D2O vapor, (c) a film first exposed to D2O vapor and then subjected to a temperature jump from 23 to 35 °C (below the PTT), (d) a film first exposed to D2O vapor and then subjected to a temperature jump from 23 to 45 °C (above the PTT), and (e) the vertical cuts from these two-dimensional images taken at qy = 0. Cuts from images a−d are shown from bottom to top. The Yoneda and the specular peak are indicated with Y and S, respectively.

4. CONCLUSION Thin and homogeneous P(S-b-MDEGA-b-S) films are obtained by spin-coating from 1,4-dioxane solutions on silicon substrates. As-prepared films exhibit a smooth surface. Because the glass transition temperature of PMDEGA is very low (Tg below −50 °C), the PMDEGA chains are expected to have a high mobility. Thus, energetically unfavorable states, such as those induced by correlated roughness, are not present. Because of the triblock copolymer with hydrophobic end groups an inner microphase separation structure is present which prevents dewetting. If the hydrophobic blocks would have been replaced by hydrophilic ones, this film stability might not have been reached. The P(S-b-MDEGA-b-S) films show LCST behavior with PTTs lower than in PMDEGA films. Thus, the presence of

hydrophobic end-groups decreases the PTT, as known for other thermoresponsive polymers. The swelling behavior of the films is anomalous and shows complex behavior. In the first stage of the swelling process, water occupies the entire free volume in the film. As water accelerates the mobility of the PMDEGA chains, the rearrangement of the chains causes the film to shrink by 4% instead of swelling further. Afterward, the hydrophilic PMDEGA chains absorb water, inducing the film to reswell. Simultaneously the water content increases in the film. When the swelling reaches an equilibrium state, the film is still 3% thinner than its initial value. The film response to a temperature jump from 23 to 45 °C (above the PTT) consists of three stages. The film immediately collapses and repels water. As the film is still mounted in the water vapor 4078


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atmosphere, a slow rearrangement of the polymer chains gives rise to a relaxation of the film. During the relaxation process, the film structure slowly changes, and the film reabsorbs water. But overall, due to the collapse of the swollen chains, the final film thickness and water content are lower than the initial values. When the temperature jumps below the PTT (from 23 to 35 °C), the response of the swollen film unexpectedly consists of similar three stages; however, the recovery proceeds faster. Because of the relaxation behavior, which is similar to the one of thin PMDEGA films, the block copolymer films containing a long central PMDEGA block can appear suitable for drug delivery systems which require a delayed release. Moreover, thin P(S-b-MDEGA-b-S) films have the ability to absorb 12% water while keeping the film thickness unchanged. Therefore, these films are promising candidates for water storage in microelectronics as constant thickness will prevent internal stress.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +49 89 289 12451; Fax +49 89 289 12 473 (P.M-B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank C. Wiegand (TU München) for help with the ATRFTIR measurements. We thank J. Perlich, S. Prams, and M. A. Ruderer for the help during the BW4 experiments. This work is supported by the DFG priority program “Intelligente Hydrogele” (Mu1487/8, Pa771/4, La611/7).

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