Rehydration of Thermoresponsive Poly(monomethoxydiethylene

Article pubs.acs.org/Macromolecules

Rehydration of Thermoresponsive Poly(monomethoxydiethylene glycol acrylate) Films Probed in Situ by Real-Time Neutron Reflectivity Qi Zhong,†,‡,§ Ezzeldin Metwalli,† Monika Rawolle,† Gunar Kaune,∥ Achille M. Bivigou-Koumba,⊥ André Laschewsky,⊥,# Christine M. Papadakis,† Robert Cubitt,% and Peter Müller-Buschbaum*,† ‡

Key Laboratory of Advanced Textile Materials & Manufacturing Technology, Ministry of Education, and §Engineering Research Center for Eco-Dyeing & Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, 310018 Hangzhou, China † 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 # Fraunhofer-Institut für Angewandte Polymerforschung, Geiselberg -Str. 69, 14476 Potsdam, Golm, Germany % Institut Laue-Langevin, 6 rue Jules Horowitz, 38000 Grenoble, France ABSTRACT: The rehydration of thermoresponsive poly(monomethoxydiethylene glycol acrylate) (PMDEGA) films exhibiting a lower critical solution temperature (LCST) type demixing phase transition in aqueous environments, induced by a decrease in temperature, is investigated in situ with real-time neutron reflectivity. Two different starting conditions (collapsed versus partially swollen chain conformation) are compared. In one experiment, the temperature is reduced from above the demixing temperature to well below the demixing temperature. In a second experiment, the starting temperature is below the demixing temperature, but within the transition regime, and reduced to the same final temperature. In both cases, the observed rehydration process can be divided into three stages: first condensation of water from the surrounding atmosphere, then absorption of water by the PMDEGA film and evaporation of excess water, and finally, rearrangement of the PMDEGA chains. The final rehydrated film is thicker and contains more absorbed water as compared with the initially swollen film at the same temperature well below the demixing temperature.

1. INTRODUCTION Stimuli-responsive polymers mark one class of polymers, which are able to change their chemical and physical properties notably by application of an external stimulus, such as temperature, pH value, light, or magnetic field.1−10 Most often, these stimuli-induced changes aim at the variation of the hydrophilicity of the polymer chain.4−6,11,12 This is typically accompanied by a change of chain conformation and results in a change of volume1 and the alteration of surface properties of a corresponding film, which is in contact with water.13,14 Because of this exceptional behavior, which can be induced by a minor change of the control parameter (e.g., temperature), stimuliresponsive polymers receive a growing interest. Numerous applications arise from the stimuli-responsive behavior, e.g., in the field of biology, medicine, and sensor technology.3,15−19 Among these stimuli-responsive polymers, due to the fact that temperature is easy to control and vary, thermoresponsive polymers with a lower critical solution temperature (LCST) in aqueous solution have been thoroughly investigated during the past decades.2,3,5,20−25 Typically these polymers form hydrogen © 2015 American Chemical Society

bonds with water at low temperatures. When the temperature is increased above the demixing phase transition of LCST type, the homogeneous system phase separates into a highly dilutedor even puresolvent phase and a highly concentrated polymer phase.20 Many of the previously formed hydrogen bonds will be broken, and the polymer chain conformations will switch from a swollen state to a collapsed state. Because of this special LCST type behavior, a large variety of applications are emerging, such as drug delivery systems,26,27 valves to control liquid transfer,28 artificial muscles,29 microfluidics,30 optical switching,31 and smart textiles.32−34 Until now, investigations about thermoresponsive polymers with LCST-type behavior have focused on poly(N-isopropylacrylamide) (PNIPAM).2,11,12 PNIPAM features a sharp transition behavior as well as a LCST in an experimentally easily accessible range of about 32 °C, which exhibits an exceptionally weak dependence on molar mass and concenReceived: March 27, 2015 Published: May 22, 2015 3604

DOI: 10.1021/acs.macromol.5b00645 Macromolecules 2015, 48, 3604−3612

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region, and is also reduced to the same final value. This comparison enables decoupling of changes of the behavior induced by changes of the chain conformation from changes of the relative humidity accompanying the change in temperature. With respect to application, of course, both scenarios will be encountered by a thermoresponsive polymer coating, and thus, it is important to identify possible differences. The PMDEGA films were prepared by spin coating onto a substrate out of solution, meaning that physically attached polymer films are addressed. The responsive behavior is probed in situ with timeresolved neutron reflectivity measurements to detect the kinetics. Vapor of deuterated water (D2O) is applied to generate high contrast with the protonated PMDEGA films during these measurements.

tration.35 As a consequence, various applications of PNIPAM arise in the field of biomedicine.36 However, PNIPAM also has several drawbacks, limiting its applications. The LCST of PNIPAM of about 32 °C is relatively low. Hence, in tropical countries, where the average temperature is already above its LCST, the switching of PNIPAM by varying the temperature is difficult to be realized. Additionally, the glass transition temperature (Tg) of PNIPAM of 140 °C is very high.37 Thus, dry PNIPAM is in the glassy state at room temperature. If, for instance, PNIPAM is used for smart textiles, these will not feel soft and comfortable. Because of these drawbacks, alternative thermoresponsive polymers with a higher LCST and a lower Tg have been explored.20,38,39 Recently, a poly(monomethoxydiethylene glycol acrylate) (PMDEGA) was introduced as a new thermoresponsive polymer.40−44 In our previous investigation the used PMDEGA exhibited a LCST of 39.9 °C and a Tg of −50 °C.45 Indeed, it overcomes the described weaknesses of PNIPAM and can be a promising candidate not only in the field of biomedicine, sensors, and switches but also in textile industry. Whereas a large body of work about thermoresponsive polymers is addressing polymer solutions,3−5 most applications require a thin film format of the polymer. The behavior of these thermoresponsive coatings may, however, deviate from the one observed in solution. To realize such coatings, mostly two different approaches were used: The polymers were chemically attached to the substrate yielding a polymer brush,46−48 or they were physically attached to the substrate yielding a polymer film.44,49−56 Typically spin coating was used for thin film preparation, including the realization of thin PMDEGA films.57,58 In previous studies of these thermoresponsive polymer coatings, most investigations were focused on the transition behavior between the swollen state and the collapsed state, when the temperature was increased either gradually or abruptly.11,38−41,56−60 In our previous investigation, we observed that swollen PMDEGA films showed a complex response to a sudden thermal stimulus introduced by a temperature jump from below the demixing temperature (23 °C) to above the demixing temperature (45 °C): Three stages of the induced effect were resolved, namely film shrinkage, rearrangement, and reswelling.57 So far, only little attention has been paid to the opposite change of temperature, which is a temperature decrease from above the demixing temperature to below the demixing temperature. Regarding applications of thermoresponsive polymer coatings (e.g., in smart textiles), which are foreseen to undergo frequent transitions across the demixing temperature in either direction, such information will be essential. Today, it is still unclear how a collapsed thermoresponsive polymer film will respond to a temperature decrease crossing the demixing temperature, i.e., a temperature drop from above the demixing temperature to below. Will the film only show a simple (re)swelling process or exhibit a more complex rehydration process? In order to answer this question, in the present work we focus on thin PMDEGA films and their responsive behavior to a decrease in temperature. We compare two different scenarios, which differ in the starting conditions. In a first set of experiments, the start temperature is above the demixing temperature; i.e., the initial PMDEGA film is composed of polymer chains in their collapsed state, before the temperature is rapidly decreased to a value below the demixing temperature. In a second set of experiments, the start temperature is already below the demixing temperature, but still within the transition

2. EXPERIMENTAL SECTION 2.1. Materials. The homopolymer of poly(monomethoxydiethylene glycol acrylate), denoted as PMDEGA, was synthesized according to ref 61. The number-average molar mass and the polydispersity of PMDEGA were 24 000 g/mol and 1.7, respectively.45 The demixing temperature of the PMDEGA film was found to depend on the film thickness and for the thickness used in the present investigation to be 39.9 °C.57 1,4-Dioxane was purchased from Acros. Ammonia solutions (NH3, 30−33%), dichloromethane, and hydrogen peroxide (H2O2, 30%) were obtained from Carl Roth GmbH. Deuterated water (D2O) (purity 99.95%) was received from Deutero GmbH. Silicon (Si 100, p-type) substrates were purchased from Silchem. 2.2. Substrate Cleaning and Film Preparation. Silicon (Si) was precut into pieces (7 × 7 cm2) and cleaned afterward. For cleaning it was first placed in dichloromethane at 46.0 °C for 30 min to clean from organic traces. After that it was rinsed with Millipore water shortly to remove the residual dichloromethane. Then, Si was placed in a basic solution (350 mL of water, 30 mL of H2O2, and 30 mL of NH3 at 76.0 °C for 2 h). The cleaned Si was stored in the deionized water (Millipore water). Before spin-coating, the Si was rinsed again with deionized water and dried by compressed nitrogen. Because of this cleaning protocol, a hydrophilic Si oxide layer with a thickness of 20 Å was formed on the surface.62 PMDEGA was dissolved in 1,4-dioxane with a concentration of 8 mg/mL. Thin PMDEGA films were prepared by spin-coating (2000 rpm, 30 s) onto the precleaned Si substrates. Film preparation was carried out at room temperature at a relative humidity of 40%. The asprepared PMDEGA film had a thickness of 359 Å, as measured by neutron reflectivity. 2.3. Neutron Reflectivity. The neutron reflectivity (NR) measurements were performed in time-of-flight (TOF) mode at the D17 reflectometer in ILL, Grenoble, France.63 With a double chopper system, the necessary pulsing of the neutron beam was realized. A broad wavelength range (from 2 to 24 Å) was used simultaneously, and the neutrons were registered as a function of their respective timeof-flight. A sample-to-detector distance of 3.4 m was selected. A twodimensional (2D) detector was used to collect the scattering patterns. Because of the applied slab-shaped neutron beam (oriented in vertical direction, parallel to the vertical sample surface), data were integrated in vertical direction. Thus, the scattering intensity was measured as a function of neutron wavelength λ and exit angle αf (measured from the sample surface along the surface normal).63 The used TOF mode allowed for fast NR measurements because at a fixed incident angle a sufficient large portion of the NR curve was probed simultaneously without the need for any motor movement. During rehydration the kinetic changes of the thermoresponsive PMDEGA films were followed with reflectivity measurements taken every 20 s. During the measurements, in order to minimize the influence of the strong off-specular scattering to the specular peak, both the sample angle and detector angle were set to 1.5° and 4°, respectively. All reflectivity curves were fitted with the Motofit package based on the software Igor pro.64 During the fitting, the values of the 3605


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Macromolecules scattering length densities (SLD) were kept fixed, matching with literature values: SLD values of Si, SiO2, PMDEGA, and D2O were 2.07 × 10−6, 3.47 × 10−6, 1.06 × 10−6, and 6.36 × 10−6 Å−2, respectively.65 2.4. Rehydration Protocol. The as-prepared PMDEGA films were mounted in a customized aluminum chamber thermostated at 23.0 °C. The bottom edge of the Si wafer was in direct contact with the aluminum chamber. After evacuating the chamber, liquid D2O was injected into the water reservoir of the chamber. An unsaturated D2O vapor atmosphere (79%) was installed by the amount of injected water and the position of the reservoir.57,58 Because of the D2O vapor atmosphere surrounding the PMDEGA film, the film swelled. When the swelling process reached an equilibrium state (14 400 s after the injection, final film thickness 413 Å),57 the chamber temperature was set to the desired starting value for the rehydration experiment. We selected two different start temperatures, namely 35.0 °C (below the demixing temperature) and 45.0 °C (above the demixing temperature). For both start temperatures, the PMDEGA film was allowed to equilibrate, which, of course, resulted in different starting conditions: At 35.0 °C the polymer chains in the film were still in a partially swollen state, whereas at 45.0 °C they were in a collapsed state. The decrease in temperature was realized with a cooling rate of 1 °C/min of the chamber temperature from its start value (35.0 or 45.0 °C) to 23.0 °C. The temporal starting point of the rehydration experiment (time t = 0) was defined by the start of this temperature decrease. The whole rehydration process of the PMDEGA films was recorded in situ by real-time NR with a time resolution of 20 s.

3. RESULTS 3.1. PMDEGA Film in Collapsed State at 45.0 °C. In the first rehydration experiment the film of collapsed PMDEGA is exposed to a sudden temperature decrease from above the demixing temperature to below the demixing temperature in a humid atmosphere. To have a well-defined starting point of the rehydration, the PMDEGA film is characterized by NR at 45.0 °C (above demixing temperature). Figure 1a presents the NR data with the best fit based on the scattering length density (SLD) profile of the collapsed PMDEGA film given in Figure 1b. The total PMDEGA film thickness is 378 Å. The applied fitting model for the PMDEGA film is a threelayer model, which was necessary to fit the NR data. The top and bottom layers exhibit slightly higher SLD values than the middle one, namely 1.9 × 10−6 and 1.8 × 10−6 Å−2, respectively. The SLD of the middle layer is slightly lower, namely 1.7 × 10−6 Å−2. Higher SLD values indicate that more D2O is incorporated in these layers, which means that close to the Si substrate as well as close to the film surface more D2O is present inside the PMDEGA films. For the top layer, it can be attributed to the direct contact with the D2O vapor atmosphere. A possible reason why the bottom layer also contains a slightly higher amount of D2O than the middle layer is the hydrophilicity of the Si substrate after basic cleaning.62 Thus, the substrate is somewhat attractive to water and more D2O can reside inside the layer being in contact with the substrate. The D2O volume fraction (V%D2O)z of each layer is calculated from eq 1: (V %D2 O)z =

nz − nPMDEGA n D2O − nPMDEGA

Figure 1. (a) NR data of the initially collapsed PMDEGA film at 45 °C (black dots) shown together with a model fit (red line). (b) Resulting scattering length density (SLD) profile along the surface normal (Z direction) of the collapsed film. The position Z = 0 Å indicates the top surface of the silicon oxide layer, formed by the basic cleaning. 3

V %D2 O =

∑ (V %D2O)z z=1

× (V %)z

(2)

(V%)z is the volume percent of each layer. The resulting average value of V%D2O of the entire collapsed PMDEGA film at 45.0 °C is 12.9%. In our previous study,57 the swollen PMDEGA film in the same vapor atmosphere at 23.0 °C had a film thickness of 413 Å and a value of V%D2O of 17.7%. Comparing these values with the ones from the fit at 45.0 °C, it is confirmed that in our present study, the PMDEGA film is in a collapsed and less hydrated state than at 23.0 °C, as expected. 3.2. Rehydration of the Collapsed PMDEGA Film. After characterizing the initially collapsed PMDEGA film, rehydration is induced by a temperature decrease from 45.0 to 23.0 °C. Because the Si wafer coated with the PMDEGA film is in direct contact with the aluminum chamber and the heat conductivity of Si is higher than that of air, the temperature of the collapsed PMDEGA film decreases faster than in the surrounding air. Hence, for a certain time after the onset of temperature decrease, the temperature of the collapsed PMDEGA film is lower than the temperature of the D2O vapor surrounding it. As a consequence, D2O condensation on the PMDEGA film surface gives rise to contact between water and the collapsed polymer chains, so that the rehydration of the collapsed PMDEGA film takes place. The PMDGA chains can adapt a swollen chain conformation and form hydrogen bonds with water molecules. The rehydration is followed during 5700 s until equilibrium is reached. Figure 2a shows 11 selected NR curves for the real-

(1)

where nz is the SLD of each layer obtained from the fit model, and nPMDEGA and nD2O are the SLD of PMDEGA and D2O, respectively. Afterward, V%D2O of the film is obtained by eq 2: 3606


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indicating that the film becomes thicker in the beginning of rehydration. Afterward, the intensity oscillations almost stay in the same position (the next three curves in the middle of Figure 2a), meaning the film thickness does not show prominent changes in the second stage. Finally, the intensity oscillations shift toward lower qz values again (in the top four curves in Figure 2a), showing that the film swells again in the late stage. In order to obtain more information about the rehydration process, selected SLD profiles obtained from the fit to the NR curves are analyzed more closely. The curves in Figure 2b are plotted from the beginning (on the bottom) to the end (on the top) of the rehydration process. Except for the top (i.e., the final measurement) SLD curves, a prominent SLD peak is observed at the high Z values of the SLD profiles. As position Z = 0 Å marks the silicon oxide surface, this SLD peak can be attributed to a layer of condensate D2O on the PMDEGA film surface. The bottom curve represents the initial PMDEGA film in the collapsed state 80 s after the temperature decrease, and at this time only a very thin D2O layer is condensed. With increasing rehydration time (from bottom to top in Figure 2b), the SLD peaks related to the condensed D2O layer become weaker and broader, indicating that D2O is gradually absorbed by the PMDEGA film. At the end of rehydration (the uppermost curve in Figure 2b), the peak has vanished, and only a D2O enriched PMDEGA layer is observed near the upper interface of the rehydrated film. Hence, it can be concluded that the condensed D2O has completely disappeared, being absorbed by the film. In addition, evaporation of D2O into the surrounding vapor atmosphere can contribute to the disappearance of the condensed D2O layer after the temperature was equilibrated. In Figure 2c, a 2D intensity representation (mapping) of all NR data, measured in situ during the rehydration, is shown as a function of time with a logarithmic qz axis, highlighting the low qz values in the NR data. The scattering intensity of the film is low at the beginning of rehydration, which can be attributed to the lack of D2O in the collapsed PMDEGA film. After the temperature decreases from 45.0 to 23.0 °C, the scattering intensity is remarkably enhanced (marked by the red arrow in the graph), which is caused by the condensation of D2O on the PMDEGA film surface. With increasing rehydration time, the strong scattering intensity in the range of 0.01−0.03 Å−1 gradually vanishes. The scattering pattern turns out to be similar to the one recorded for the swollen PMDEGA film. However, the intensity is significantly enhanced as compared to the start due to the amount of D2O embedded now inside the PMDEGA film. To summarize the whole rehydration process for the temperature decrease from 45.0 to 23.0 °C, i.e., across the demixing temperature, the film thickness and D2O volume fraction (V%D2O) resulting from the model fits are plotted as a function of time in Figures 3a and 3b, respectively. As marked by the dashed lines, the whole rehydration process can be divided into three stages. The film thickness and V%D2O presented in Figure 3 both include the condensed D2O layer on top. The film thickness dramatically increases in the first stage (from 378 to 440 Å), as the condensation of D2O sets in when the temperature drops. Simultaneously, V%D2O shows the same tendency, rising strongly from 12.9 to 32.9%. Afterward, in the second stage the film thickness almost stays constant with time, whereas V%D2O increases further. The possible explanation for such a behavior is the complex rehydration process induced by the temperature

Figure 2. Rehydration of the collapsed PMDEGA film in D2O vapor atmosphere when temperature decreases from 45.0 to 23.0 °C: (a) Selected NR curves (dots) are shown together with model fits (gray lines). The color of the curves is gradually varied from red (high temperature, initial collapsed film before rehydration) to navy blue (low temperature, final rehydrated film measured after 5600 s). The curves are shifted vertically for clarity of the presentation. The related times during the rehydration process are indicated. (b) The resulting SLD profiles corresponding to the selected NR curves (position 0 in the Z-axis means the substrate surface) using the same color code as in (a). (c) 2D intensity presentation (mapping) of the NR data as a function of time with a logarithmic qz axis. The red arrow marks the starting point of the rehydration. Different scattering intensities are displayed on a gray scale (bright and dark mean high and low intensity, respectively).

time investigation together with their model fits. The NR data are fitted with the same model for the PMDEGA film as for the collapsed film at 45.0 °C. In order to describe the condensation process, a thin D2O layer at the top of the PMDEGA film is added to the fitting model. It is well observable in the resulting SLD profiles (Figure 2b). A possible reason for the initial presence of this thin D2O layer on top of the PMDEGA film surface can be the hydrophobicity of the polymer chains in the collapsed conformation when rehydration is started. Inspecting the time behavior of the intensity oscillations of the NR curves, the whole rehydration process can be divided into three stages. In the first stage (the first three curves at the bottom of Figure 2a) the intensity oscillations shift toward lower qz values, 3607


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Table 1. Film Thickness and V%D2O of the Swollen (23.0 °C), Collapsed (45.0 °C), and Rehydrated (23.0 °C) PMDEGA Film after the Temperature Decrease from 45.0 to 23.0 °C swollen PMDEGA film at 23.0 °C collapsed PMDEGA film at 45.0 °C rehydrated PMDEGA film at 23.0 °C

film thickness (Å)

V%D2O

413 378 464

17.7 12.9 31.8

chain conformation of the PMDEGA, since the initial films were prepared via spin coating, which typically results in nonequilibrium conditions. Anyhow, the different level of hydration is very interesting with respect to application and might be attributed to the different level of saturation being present during the hydration process. 3.3. PMDEGA Film in the Partially Swollen State at 35.0 °C. To gain further insight, in a second rehydration experiment, a film consisting of partially swollen PMDEGA chains is exposed to a temperature decrease from slightly below the demixing temperature to well below the demixing temperature. To determine the starting point of the rehydration, the PMDEGA film is first characterized with NR at 35.0 °C. Figure 4a presents the NR data of the partially swollen PMDEGA film (black dots) together with model fit (red line) at 35.0 °C. The same model as the one used for the initially collapsed PMDEGA film at 45.0 °C is applied to fit these NR data. The extracted value of the film thickness is 391 Å. Thus,

Figure 3. Rehydration of the collapsed PMDEGA film due to a temperature decrease from 45 to 23 °C shown as a function of time: (a) film thickness including the condensed D2O layer on top; (b) D2O volume fraction (V%D2O) in the film including the condensed D2O layer. The dashed lines divide the whole process into three different stages as explained in the text.

decrease, which can be divided into three subprocesses: condensation of water, absorption of water and evaporation of excess water, followed by a rearrangement of polymer chains. When the temperature is reduced from 45.0 to 23.0 °C, condensation occurs on the film surface because the temperature of the PMDEGA film is lower than that of the surrounding water vapor. Part of the condensed water enters the polymer and occupies free volume of the film.66 Afterward, as the temperature continues to decrease, the PMDEGA film gradually becomes more hydrophilic (the temperature is below its demixing temperature); it absorbs more D2O and thereby increases in film thickness. In the second stage, the temperature is equilibrated and the polymer rearranges, accommodating slightly more water without further film swelling. Moreover, excess water evaporates. Thus, the film thickness and V%D2O stay constant in the second stage and even slightly increase. In the third stage, V%D2O remains constant, which can be attributed to the end of the absorption process. The film thickness increases due to changes of the polymer chain conformation, which turns into a more expanded and thus more space requiring conformation. Table 1 lists the film thickness and V%D2O values of the films containing swollen (23.0 °C), collapsed (45.0 °C), and rehydrated (23.0 °C) PMDEGA chains. As mentioned before, the film composed of collapsed chains is thinner and contains less D2O than that of swollen chains. Interestingly, the rehydrated film is even thicker and contains significantly more D2O than the initial swollen film before passing the demixing temperature forth and back. Thus, by the applied temperature protocol the initial conditions are not regained, but the PMDEGA films exhibits a different level of hydration. The origin for this difference may be attributed to changes in the

Figure 4. (a) NR data of the partially swollen PMDEGA film at 35.0 °C (black dots) shown together with the model fit (red line). (b) Resulting scattering length density (SLD) profile along the surface normal (Z direction) of the collapsed film. The position Z = 0 indicates the top surface of the silicon oxide layer, formed by the basic cleaning. 3608


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Macromolecules comparing to the initially collapsed PMDEGA film at 45.0 °C (378 Å), the partially swollen PMDEGA film is thicker. The resulting SLD profile is plotted in Figure 4b. Although this partially swollen film at 35.0 °C still has the three-layer structure, the SLD values of each layer are, of course, different from the ones of the collapsed film. The obtained SLD values of these three layers from top (close to the air) to bottom (close to the substrate) are 2.1 × 10−6, 1.85 × 10−6, and 2.1 × 10−6 Å−2, respectively. Therefore, all three layers show higher SLD values than those probed for the initially collapsed film at 45.0 °C, indicating that the film contains more D2O, as expected. Thus, the film is in a partially swollen state. Nevertheless, we find an enrichment of D2O close to the substrate interface and close to the film surface, very similar to the enrichment observed for the collapsed PMDEGA film. Using eq 1, the average value of V%D2O for the film is calculated to be 15.6%, which is higher than the one of the collapsed film (12.9%, see Table 1), as also expected. However, it is important to note that although the film is still in the partially swollen state, the values of both film thickness and V% D2O are lower than those of the swollen film at 23.0 °C (Table 1). The reason for these lower values originates from the broad temperature range of the transition of PMDEGA films. In comparison to the most investigated system PNIPAM, which shows a rather sharp transition in such thin films,2,49 PMDEGA tends to dehydrate much more gradually after passing the demixing temperature when the temperature is further increased.38 This is in agreement with the much more pronounced concentration dependence of the demixing temperature for PMDEGA.35,44 Hence, even though the temperature is set to 35.0 °C, which is still lower than the demixing temperature (39.9 °C), a part of D2O has already been repelled from the film, inducing both the film thickness and V%D2O to decrease. Therefore, this initial film, which we denote as “partially swollen” in the present article, is as well partially collapsed and partially hydrophobic. This initial partial hydrophobicity is also the reason that in the following rehydration experiment the condensed D2O can still form a thin layer on the PMDEGA film surface. 3.4. Rehydration of the Partially Swollen PMDEGA Film. After having studied the partially swollen PMDEGA film at 35.0 °C, the temperature is reduced from 35.0 to 23.0 °C. In situ NR is applied to follow in real time the kinetics of the PMDEGA film under the conditions of this decrease in temperature. Figure 5a shows 11 selected NR curves together with model fits. Similar to the rehydration of the PMDEGA film in the collapsed state, the rehydration of the partially swollen PMDEGA film also proceeds in three stages; however, details are different between both temperature change experiments. Again more details about rehydration of the partially swollen PMDEGA film can be obtained from the SLD profiles (Figure 5b). The three stages mentioned above are clearly visible along with the thin condensation layer of D2O on the PMDEGA film surface at the starting point of the temperature decrease. With increasing rehydration time, more D2O gets absorbed by the PMDEGA film, which is seen from the broadening of the SLD peak at the film surface and its shift to larger Z values. Then, the film thickness relaxes back by absorption and evaporation processes. Finally, the D2O layer is completely absorbed by the PMDEGA film and vanishes from the SLD profiles. Although condensation of D2O is observed in both rehydration processes, the thickness values of the D2O layers are different due to the

Figure 5. Rehydration of the partially swollen PMDEGA film in D2O vapor atmosphere when temperature is reduced from 35.0 to 23.0 °C: (a) Selected NR curves (dots) are shown together with model fits (gray lines). The color of the curves is gradually varied from red (high temperature, initial collapsed film before rehydration) to navy blue (low temperature, final rehydrated film measured after 4100 s). The curves are shifted vertically for clarity of the presentation. The related times during the rehydration process are indicated. (b) Corresponding SLD profiles of the selected NR curves (position Z of the Z-axis means the substrate) using the same color code as in (a). (c) 2D intensity presentation (mapping) of the NR data as a function of time with a logarithmic qz axis. The red arrow marks the starting point of the rehydration. Different scattering intensities are displayed on gray scale (bright and dark mean high and low intensity, respectively).

different start temperatures. For the temperature decrease from 35.0 to 23.0 °C, the maximum thickness of the D2O layer observed is 10 Å. In contrast, the maximum D2O layer can reach 16 Å (60% thicker), when the temperature drops from 45.0 to 23.0 °C. Thus, the hydration state of the thermoresponsive polymer is important for its rehydration. Figure 5c depicts the two-dimensional intensity representation (mapping) of the NR data as a function of rehydrated time with a logarithmic qz axis. The observable patterns are similar as compared the other rehydration process (see Figure 2c). Because of the condensation of D2O on the PMDEGA film surface, the scattered intensity increases strongly in the first stage. With ongoing rehydration time, the intensity due to the condensed D2O layer gradually weakens, and the initial 3609


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on the PMDEGA film surface in the case the temperature decrease starts at lower temperature. Third, the values of the final film thickness and V%D2O are lower for the partly swollen film, which can be also related to the thinner D2O layer condensed on the film surface. Table 2 lists the values of the final film thickness and V%D2O of the films containing swollen (23.0 °C), partially swollen

scattering pattern of the swollen film is observed again, as observed in the other temperature change experiment. The film thickness and V%D2O are plotted as a function of rehydration time in Figures 6a and 6b, respectively. As marked

Table 2. Film Thickness and V%D2O of the Swollen (23.0 °C), Partially Swollen (35.0 °C), and Rehydrated (23.0 °C) PMDEGA Film after the Temperature Decrease from 35.0 to 23.0 °C swollen PMDEGA film at 23.0 °C partially swollen PMDEGA film at 35.0 °C rehydrated PMDEGA film at 23.0 °C

film thickness (Å)

V%D2O

413 391 417

17.7 15.6 23.5

(35.0 °C), and rehydrated (23.0 °C) PMDEGA chains. When the temperature decreases from 35.0 to 23.0 °C, it is clear that both film thickness and V%D2O increase. Although the PMDEGA film shows only a slightly increased film thickness as in the initial swollen PMDEGA film at the same temperature, the amount of D2O in the film is notably enhanced from 17.7 to 23.5% in the final rehydrated state. This interesting behavior may be related again to the nonequilibrium chain conformation installed with spin coating, the complex rehydration process, and changes in the chain conformation, which causes the PMDEGA film to absorb more D2O as compared to a film that had undergone a different thermal history. Figure 7 sketches the rehydration processes of PMDEGA films of partially swollen and of collapsed chains. Although the rehydration processes in both cases exhibits three similar stages, first condensation, then absorption with potential evaporation, and finally rearrangement, prominent differences are observed. Because of the different initial temperature (below and above the demixing), the extent of D2O condensation is different. A longer time is required to complete the condensation process, and a thicker D2O layer is condensed in the case of a higher initial temperature. A higher amount of D2O causes the final rehydrated PMDEGA film to be significantly thicker and to contain a higher amount of D2O.

Figure 6. Rehydration of the partially swollen PMDEGA film during the temperature decrease from 35.0 to 23.0 °C shown as a function of time: (a) film thickness including condensed D2O layer on top; (b) D2O volume fraction (V%D2O) in the film including the condensed D2O layer. The dashed lines divide different stages as explained in the text.

by the dashed lines, the whole process can again be divided into three stages, namely (1) condensation of D2O, (2) absorption of D2O and evaporation, and (3) rearrangement of the rehydrated PMDEGA chains. Although these three stages are similar to the other rehydration experiment starting with collapsed PMDEGA chains (temperature decrease from 45.0 to 23.0 °C), there are still remarkable differences between both rehydration processes, which can be attributed to the different initial chain conformations and hydration states of the polymers introduced by the applied different thermal protocols. The strongest difference is found in the second stage, which is characterized by a constant film thickness in the case of the collapsed PMDEGA film but shows a strong decrease in film thickness in the case of the partially swollen PMDEGA film (compare Figures 3a and 6a). An additional difference is that in the second stage V%D2O slightly increases for the collapsed film but remains constant for the partially swollen film. With respect to the first stage, the condensation time is shorter if the temperature decrease starts at lower temperature. The condensation of D2O occurs during 800 s, when the temperature drops from 45.0 to 23.0 °C. In contrast, it only takes 500 s, when the temperature is reduced from 35.0 to 23.0 °C. Moreover, in the temperature decrease from 35.0 to 23.0 °C, the maximum amount of V%D2O, which can be reached, is lower than the one in the change from 45.0 to 23.0 °C. This difference may be related to the thinner D2O layer condensed

4. CONCLUSION The rehydration processes of PMDEGA films that were either initially in the collapsed or in a partially swollen state are investigated with in situ NR for two different starting temperatures. The rehydration turns out to be a complex process, which can be divided into three stages: The first stage is dominated by the condensation of D2O on the PMDEGA film surface, which sets in after the decrease in temperature due to the temperature difference of the film and the surrounding vapor atmosphere. Because of the condensation, the film thickness and V%D2O significantly increase. Afterward, the second stage begins, which is characterized by simultaneous absorption of D2O by the PMDEGA film and its evaporation. Because of the decrease in temperature, the PMDEGA film gradually becomes more hydrophilic and, consequently, absorbs D2O. The excess D2O, which cannot be absorbed, starts to evaporate into the surrounding atmosphere. Combining these two effects, the film thickness and V%D2O values almost stay constant in this stage or even decrease. Finally, a third stage is observed which is characterized by the rearrangement of the 3610


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Figure 7. Schematic summary of the rehydration process of the PMDEGA film containing partially swollen (upper panel, temperature change from 35.0 to 23.0 °C) and collapsed polymer chains (lower panel, temperature change from 45.0 to 23.0 °C).

polymer chains in the rehydrated PMDEGA film, causing the film to thicken again at constant V%D2O. In both rehydration experiments the films achieved after the rehydration are different from the initial swollen films. This underlines the importance of the details of the thermal history for the level of hydration of the PMDEGA films. Concerning applications, e.g. for textile coatings or sensors, which should undergo multiple usages, this is an important information. In future experiments it might be of interest to see to what extent the reported findings can be generalized to other thermoresponsive systems.

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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 This work was supported by the DFG priority program “Intelligente Hydrogele” (Mu1487/8, Pa771/4, La611/7). Q.Z. is thankful for support by Zhejiang Provincial Natural Science Foundation of China (Grant LQ14E030009).

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Rehydration of Thermoresponsive Poly(monomethoxydiethylene

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