Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Swelling and Exchange Behavior of Poly(sulfobetaine)-Based Block Copolymer Thin Films Lucas P. Kreuzer,† Tobias Widmann,† Nuri Hohn,† Kun Wang,† Lorenz Bießmann,† Leander Peis,† Jean-Francois Moulin,‡ Viet Hildebrand,§ Andre ́ Laschewsky,§,∥ Christine M. Papadakis,† and Peter Müller-Buschbaum*,†,⊥ Macromolecules Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 05/01/19. For personal use only.
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Lehrstuhl für Funktionelle Materialien/Fachgebiet Physik weicher Materie, Physik Department, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡ Helmholtz-Zentrum Geesthacht at Heinz Maier-Leibnitz Zentrum, Lichtenbergstr. 1, 85747 Garching, Germany § Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Str. 24−25, 14476 Potsdam-Golm, Germany ∥ Fraunhofer Institut für Angewandte Polymerforschung, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany ⊥ Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1, 85748 Garching, Germany S Supporting Information *
ABSTRACT: The humidity-induced swelling and exchange behavior of a block copolymer thin film, which consists of a zwitterionic poly(sulfobetaine) [poly(N,N-dimethyl-N-(3(methacrylamido)propyl)ammoniopropanesulfonate) (PSPP)] block and a nonionic poly(N-isopropylacrylamide) (PNIPAM) block, are investigated by time-of-flight neutron reflectometry (TOF-NR). We monitor in situ the swelling in the H2O atmosphere, followed by an exchange with D2O. In the reverse experiment, swelling in the D2O atmosphere and the subsequent exchange with H2O are studied. Both, static and kinetic TOF-NR measurements indicate significant differences in the interactions between the PSPP80-b-PNIPAM130 thin film and H2O or D2O, which we attribute to the different H- and D-bonds between water and the polymer. Changes in the chain conformation and hydrogen bonding are probed with Fourier transform infrared spectroscopy during the kinetics of the swelling and exchange processes, which reveals the key roles of the ionic SO3− group in the PSPP block and of the polar amide groups of both blocks during water uptake and exchange.
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INTRODUCTION Poly(sulfobetaine)s belong to the class of zwitterionic polymers, which contain an anionic and a cationic group within one constitutional repeat unit.1,2 Because of their structural similarity to zwitterionic phospholipids, their high chemical stability, and the independence of their zwitterionic character toward pH, they are used as biocompatible lubrication and antifouling materials. Furthermore, many poly(sulfobetaine)s in aqueous solution exhibit an upper critical solution temperature (UCST) in aqueous media and therefore undergo a coil-to-globule collapse transition upon cooling.3,4 This characteristic behavior is highly unusual, as other zwitterionic polymer classes such as poly(carboxybetaines) and poly(phosphatidylcholines) do generally not show thermo-responsiveness.5−7 In comparison, thermo-responsive nonionic polymers feature a lower critical solution temperature (LCST) in aqueous solution, which results in a coil-to-globule collapse transition upon heating.8−11 Poly(N-isopropylacrylamide) (PNIPAM) is among the most thoroughly studied thermo-responsive polymers.8−10 In aqueous environments, PNIPAM undergoes a coil-to-globule © XXXX American Chemical Society
phase transition as the temperature is increased above its cloud point of ∼32 °C (LCST-type), which is attributed to changes in the hydrophilicity and consequently in the number of hydrogen bonds between the water molecules and the polymer chain.9 The cloud point of PNIPAM is rather independent of the molar mass and is only slightly higher in D2O than in H2O.12 The phase behavior of both, poly(sulfobetaine)s and PNIPAM homopolymers in dilute and concentrated solutions has been investigated to a great extent, and also the swelling and deswelling behavior with different solvents are well-known for the case of bulk hydrogels, micellar systems, and brushes.9,13−16 Studies focusing on block copolymers (BCPs) consisting of poly(sulfobetaine) and PNIPAM blocks in aqueous solution are less frequent.17−20 With respect to thin films, reported work about the poly(sulfobetaine) homopolymer is very rare,3 and to the best of our knowledge, literature Received: March 6, 2019 Revised: April 12, 2019
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DOI: 10.1021/acs.macromol.9b00443 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules lacks studies that investigate poly(sulfobetaine)-b-PNIPAM BCPs in thin-film morphology. Thermo-responsive BCP thin films are capable of undergoing drastic changes in properties upon variation of temperature. This behavior is the focus of numerous research activities because it can be adopted in a manifold of application fields such as (nano) sensors and switches, soft-robotics, and artificial pumps or muscles.21−27 For implementing thermoresponsive polymer thin films in these fields, controlled sorption and diffusion of gaseous and liquid penetrants into the polymer are essential. By exposing stimuli-responsive polymer films to a certain atmosphere, small molecules, such as water, are able to penetrate and are adsorbed by the polymer film, which may result in significant changes in the structure and mechanical properties. A general understanding of the underlying mechanisms of the sorption and release of water is crucial, especially for a new type of BCP such as poly(sulfobetaine)-b-PNIPAM. Typically, studies have been focused on the swelling and drying of various BCP thin films in a humidified atmosphere.28−33 Furthermore, organic solvent vapor annealing has been reported, which primarily aims at achieving highly ordered nanostructures of nonresponsive polymer thin films by solvent uptake.34−39 The exchange kinetics of solvent molecules in BCP thin films has only rarely been probed.40 In general, the free energy of mixing, which is dominated by the osmotic pressure, is responsible for the diffusion of penetrants into the material. Regarding bulk polymers, these diffusion processes were studied experimentally and theoretically.27,41,42 In the case of thin films, the situation is more complex. The geometrical restriction of the polymer, the increased interfacial areas, and the additional interactions at the substrate−polymer and polymer−air interfaces may severely affect the diffusion processes as compared to the bulk polymers or their dilute solutions.43−45 Therefore, the water uptake and swelling behavior of thin BCP films cannot simply be extrapolated from the polymer’s bulk or solution behavior. In our previous studies, we observed that when diffusing into the PNIPAM thin film, water first fills the accessible free volume.46−48 This water uptake results in a continuous decrease of the glass transition temperature (TG) of the polymer. After the free volume is filled, further water uptake leads to swelling of the polymer chains and an increase in the film thickness.46−49 By increasing the complexity of the stimuliresponsive polymer, for example, by using amphiphilic BCPs, the water uptake behavior becomes more complex.33,50 Introducing a glassy hydrophobic block into the polymer increases the mechanical stability of the BCP film morphology but simultaneously hampers the extent of swelling. This happens because overall, the interactions between the hydrophobic block and water molecules are less attractive than the interactions among the hydrophobic blocks.32,46,51−53 Moreover, the swelling kinetics are also affected because the water molecules cannot move as freely as in homopolymer films.50,52,53 In the present work, we investigate the swelling and exchange behavior of a BCP thin film that consists of a zwitterionic poly(sulfobetaine) block, namely poly(N,Ndimeth yl - N - ( 3 - ( me t h a c r y l a m i d o ) p r o p y l ) a m m o n i o propanesulfonate) (PSPP), and a PNIPAM block, PSPP80-bPNIPAM130 (Figure 1). In aqueous solution, PSPP displays UCST behavior with its clearing point depending strongly on the molar mass.4 In contrast to PNIPAM that typically exhibits
Figure 1. Chemical structure of the BCP under study, PSPPm-bPNIPAMn (m = 80, n = 130).
a phase transition temperature (TT) of about 32 °C in dilute solution in either solvent, the TT of PSPP is significantly higher in D2O than in H2O.4 For dilute solutions of the homopolymer PSPP85, for instance, the TT is found at 14.7 °C in D2O but at 8.7 °C in H2O.3 These TTs of the PSPP block in the PSPP80-b-PNIPAM130 BCP are rather similar to the ones of the corresponding homopolymers.17,19,20 Thus, at the temperature chosen for our thin film experiments (26.5 °C), both blocks of the BCP are expected to be in the soluble state for D2O and H2O. Consequently, both blocks are expected to take up either solvent readily, which makes this BCP system different from the ones containing hydrophobic blocks. Our experiment is designed as follows: Swelling of the PSPP80-b-PNIPAM130 film is induced by immersing it into a humid atmosphere of H2O. Once the polymer film has undergone significant swelling in H2O, the atmosphere is changed from H2O to D2O. In this step, the D2O exchange in the BCP film is probed. In addition, the reverse process is studied, namely the swelling by D2O, followed by H2O exchange. To develop a fundamental understanding of the behavior of the PSPP80-b-PNIPAM130 thin film, the temperature is kept constant. Hence, the focus lies on the kinetics of the swelling and on the exchange processes in the H2O/D2O atmosphere. As discussed above, in solution, PSPP behaves differently in H2O and D2O; thus, differences in the two swelling processes and changes during the exchange processes are expected.4 Time-of-flight neutron reflectometry (TOFNR) is used to follow the dynamic processes in situ. With this technique, the evolution of the film thickness, the scattering length density (SLD), and the surface roughness can be followed. One key advantage is the simultaneous and independent determination of the film thickness and the SLD of the BCP thin film. Since the latter is related to the composition of the film, the evolving film thickness and the vertical film composition can be determined with dependence on time. This way, the overall water content Φ within the polymer film and the swelling ratio d/dini are determined as a function of time during the kinetic processes. To probe in situ changes in chain conformation and hydrogen bonding between the water molecules and the polymer chains, the in situ TOFNR data are supported by data from Fourier transform infrared (FTIR) spectroscopy. Both, the H2O swelling followed by D2O exchange and the reverse experiment are followed in situ. Because of the differences in the vibrational absorption energy of the O−H and O−D bonds, one can clearly distinguish between H2O and D2O by FTIR measurements. This yields information about the sorption of water molecules within the polymer film during the swelling as well as their desorption during the exchange process. Furthermore, FTIR spectroscopy B
DOI: 10.1021/acs.macromol.9b00443 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
from 500 μm to 1 mm by adjusting the distance between the sample and the exit of the optical fiber. Characteristic intensity oscillations were analyzed in a wavelength range from 400 to 1000 nm. To enable in situ measurements, a custom-made vapor chamber was used. The sample was mounted horizontally on a sample stage, which was surrounded by a solvent reservoir. The solvents were injected into and removed from the closed chamber with a syringe. The relative humidity inside the sample chamber was measured with a SHT3x humidity sensor (Sensirion AG, Staefa, Switzerland). Temperature was controlled by connecting the vapor chamber to a JULABO F12 MC thermal bath (JULABO Labortechnik GmbH, Seelbach, Germany) and was set to 26.5 °C. The incident light beam passed through a glass window on top of the chamber onto the sample. The reflected light exhibits one to several oscillations. The film thickness and refractive index were determined by model fitting, matching their positions and amplitudes. A good fit was accomplished by using a single layer model. TOF-NR. The neutron reflectivity measurements were performed using the REFSANS instrument at Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. A wavelength band from 3 to 21 Å was used with a resolution of Δλ/λ = 10%. Static measurements were performed at two fixed incident angles of αi = 0.6° (20 min) and αi = 2.4° (100 min) to cover a larger qz range. Kinetic measurements were performed at a fixed incident angle of αi = 0.76°. The time binning of the kinetic measurements to 30 s ensured good statistics while maintaining a high time resolution. The average neutron SLD for the as-prepared PSPP80-b-PNIPAM130 thin films was obtained from static TOF-NR measurements and amounts to (0.785 ± 0.011) × 10−6 Å−2. The SLD values for H2O and D2O are −0.559 × 10−6 Å−2 and 6.36 × 10−6 Å−2, respectively.55 A custom-made sample chamber made from high purity aluminum was used and was connected to a JULABO F12 MC thermal bath (JULABO Labortechnik GmbH, Seelbach, Germany) for thermal control. The temperature was set to 26.5 °C. The samples were mounted in horizontal geometry on three aluminum pins. H2O and D2O were injected and removed via a syringe into a reservoir, which was located underneath the sample. The sample environment conditions, namely the temperature and relative humidity, were monitored with a SHT3x humidity sensor (Sensirion AG, Staefa, Switzerland). The gathered static and kinetic TOF-NR data were evaluated using the Motofit package.56 FTIR Spectroscopy. Infrared spectra were recorded with a Bruker Equinox FTIR spectrometer equipped with a deuterated triglycine sulfate detector. Data were collected with a spectral resolution of 2 cm−1, and the signal was averaged over 256 scans. The wavenumber range was set from 600 to 4000 cm−1, which covers all characteristic peaks, such as the O−D and O−H peaks, the amide I and amide II peaks, and the SO3− peak. To avoid small molecules, such as CO2 and H2O, in the optical path of the infrared radiation, the FTIR instrument was purged with dry, CO2-filtered air during all measurements. To enable in situ measurements, a custom-made vapor chamber was used, which consists of two parts. The upper part functions as an enclosed sample chamber, where the BCP thin film was placed vertically on a copper ring between two ZnS windows (thickness of 1 mm). The sample holder was fixed on a second copper block in which a solvent reservoir was located. Furthermore, the copper block was connected to a JULABO F12 MC thermal bath (JULABO Labortechnik GmbH, Seelbach, Germany) to ensure a constant temperature of 26.5 °C. Both parts, the sample holder and the solvent reservoir, were linked with a slender tube. Relative humidity inside the sample chamber was measured with a SHT3x humidity sensor (Sensirion AG, Staefa, Switzerland). The infrared spectrum was acquired within 7 min, and measurements were taken every 30 min for overall 7 days at 26.5 °C. Water content was determined by integrating over the corresponding O−H and O−D peaks with fixed integration limits, while peak positions were determined by a Gaussian fit of the corresponding peak (SO3−, amide I and II).
allows characterizing separately the hydrophobic hydration of the carbon atoms in the backbone and of the isopropyl groups in the side chain of the PNIPAM block, and the hydrogen bonding of water molecules with moieties such this BCP system as the hydrophilic amide groups of both blocks and the SO3− group of the PSPP block. Both, the kinetics of the swelling and especially the exchange process give insights into the mechanism of water uptake. Thus, information on whether the BCP thin film behaves differently in deuterated and protonated atmospheres is obtained. The results are an important step toward a fundamental understanding of the behavior of PSPP-bPNIPAM thin films in different atmospheres.
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EXPERIMENTAL SECTION
Materials. N,N-Dimethyl-N-(3-methacrylamidopropyl)-ammoniopropanesulfonate was kindly provided by Raschig (Ludwigshafen, Germany) and used as received. N-Isopropylacrylamide (NIPAM, TCI, 98%) was crystallized from n-hexane, and 4,4′-azobis(4cyanopentanoic acid) (V501, Wako) was crystallized from methanol. Diethyl ether (VWR, 100%), methanol (Avantor, 99.8%), trifluoroethanol (Roth, 99.8%), sodium chloride (ChemSolute, 99%), deuterated water (D2O, Armar, 99.9 atom % D for synthesis and characterization during synthesis, Deutero GmbH, 99.98% for studying swelling and exchange behavior), and dichloromethane-d2 (CD2Cl2, Armar, 99.5 atom % D) were used as received. Water was purified by a Millipore Milli-Q Plus water purification system (resistivity of 18 MΩ cm−1). The syntheses of the poly(N,Ndimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate) macro RAFT agent PSPP804 and of the BCP PSPP80-b-PNIPAM130 (Mn 34 × 103 Da) in two consecutive polymerization steps started from N,N-dimethyl-N-(3-methacrylamidopropyl)-ammoniopropane sulfonate (PSPP), NIPAM, and chain transfer agent 4-cyano-4(((phenethylthio)carbonothioyl)thio)-pentanoate54 and used established protocols as described in detail before.4,17 Water was purified by a Millipore Milli-Q Plus water purification system (resistivity of 18 MΩ cm−1). Deuterated water (D2O, 99.98%, Deutero GmbH, Kastellaun/Germany) was used as received. Sample preparation for TOF-NR and spectral reflectance (SR): Silicon with an oxide layer on the surface was used as a substrate material for the PSPP130-bPNIPAM80 thin films. Precut silicon substrates (7 × 7 cm2 for TOFNR; 2 × 2 cm2 for SR) were placed in an acid bath consisting of 54 mL of H2O, 84 mL of H2O2, and 198 mL of H2SO4 for 15 min at 80 °C to clean the surface of any organic substances. The substrates were rinsed with Millipore water for at least 10 min to remove possible traces of the acid bath. Before spin-coating, the substrates were treated with oxygen plasma at 200 W for 10 min to install a hydrophilic surface. PSPP80-b-PNIPAM130 thin films were prepared via spin-coating (2500 rpm, 900 s) from a trifluoroethanol solution (c = 25 g/L) at room temperature. To generate a surface of high homogeneity, solvent vapor annealing (900 s) was applied in the trifluoroethanol atmosphere directly after the spin-coating process in a desiccator at room temperature. For TOF-NR and SR, respectively, two samples were prepared following the same preparation protocol, one for each experiment: H2O swelling followed by D2O exchange and D2O swelling followed by H2O exchange. Sample preparation for FTIR Spectroscopy. Precut silicon substrates (1 × 1 cm2) were treated with oxygen plasma for 10 min to create a hydrophilic surface. After a waiting time of 25 min, the PSPP80-b-PNIPAM130 films were prepared by drop-casting from a trifluoroethanol solution (c = 35 g/L) at room temperature. The polymer solution was allowed to dry overnight in a desiccator. Two samples were prepared following the same preparation protocol, one for each experiment: H2O swelling followed by D2O exchange and D2O swelling followed by H2O exchange. Methods. SR. Film thickness d was measured with a Filmetrics F20 Thin Film Measurement System (Filmetrics Inc., San Diego, United States). The spot size of the incident light beam was varied C
DOI: 10.1021/acs.macromol.9b00443 Macromolecules XXXX, XXX, XXX−XXX
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film (III in Scheme 1). D2O exchange is initiated by replacing the H2O in the reservoir with D2O. In analogy to the swelling phase, the exchange kinetics is followed in situ with TOF-NR for another 6 h, until the film has equilibrated (state IV in Scheme 1). A third and final static measurement of the D2Oexchanged film is carried out at the end of the measurement cycle (V in Scheme 1). In a second cycle, the reverse experiment, namely the D2O swelling followed by the H2O exchange, is performed under the same conditions as described above. The reflectivity curves of the static measurements from both experiments (states I, III, and V in Scheme 1) along with their model fits are shown in Figure S2 in the Supporting Information. For the as-prepared film, the best fits are obtained using a two-layer model, whereas for the swollen and exchanged states, a four-layer model has to be used because of water-rich layers that are found near the substrate during the swelling process. These enrichment layers are attributed to hydrophilic interactions between water and the hydrophilic substrate and the SiO2 layer. They are well pronounced after the D2O swelling and D2O exchange because of large differences in the SLDs of D2O and the polymer. Nevertheless, these enrichment layers are also seen after H2O swelling and after the H2O exchange process. During the H2O swelling, the film thickness increases to a higher value (59 ± 1 nm) as compared to the D2O swelling (51 ± 2 nm). However, the film thickness increases further during the exchange with H2O (59 ± 1 nm), whereas the film thickness remains almost constant during the D2O exchange (61 ± 1 nm). Furthermore, the overall SLD values of the polymer films in the exchanged state (V) indicate that after the H2O exchange, D2O molecules still remain in the film and vice versa. The values of the film thicknesses and SLDs are listed in Table S1 in the Supporting Information. These findings suggest different interactions between the polymer and H2O or D2O, which is most likely due to different H- and D-bonds to the polymer chain. Whereas the static TOF-NR measurements give information about the polymer thin film in the equilibrium states, the kinetic measurements (stages II and IV in Scheme 1) offer information about the evolution of the film thickness and the SLD during the swelling and exchange processes. Swelling Behavior. The swelling processes of the PSPP80b-PNIPAM130 film in H2O and D2O atmospheres are followed in situ with SR to obtain first hints about possible differences in the uptake of H2O and D2O. The resulting film thickness as a function of time is shown in Figure 2 for the swelling with H2O (Figure 2a) and D2O (Figure 2b), respectively. Both curves are fitted using a model which considers that, after injection of the solvent, the relative humidity is not immediately constant but needs a certain time to reach its maximum value (Figure S1 in the Supporting Information). This effect has to be considered in addition to the intrinsic swelling kinetics of the polymer film, which is driven by the diffusion of water into the polymer film.42,58,59 The equations used for fitting (eqs S1 and S2) as well as more extensive explanations are found in the Supporting Information. From this model, the effective Flory−Huggins interaction parameter χeff between the polymer and water as well as τ, which is a time constant of the diffusion-limited swelling process, are obtained. (Note that the interactions between the polymer and the substrate are not considered in this model.) To get an insight into the importance of intrinsic processes compared to the build-up of the relative humidity, the
RESULTS AND DISCUSSION To resolve the mechanisms behind the diffusion of water into the PSPP80-b-PNIPAM130 thin BCP film, the swelling and exchange kinetics were analyzed in situ with TOF-NR, SR, and FTIR spectroscopy. TOF-NR measurements were performed with a high time resolution of 30 s. Thus, the kinetic processes inside the polymer film can be followed almost in realtime.50,57 The large SLD differences between D2O and the polymer offer good contrast during the D2O swelling, whereas the SLD differences between D2O and H2O result in good contrast during the exchange kinetics (either way). Although the contrast between H2O and the polymer is smaller, the kinetics may still be followed with sufficient detail. As a result, the TOF-NR measurements yield a vertical SLD profile, from which the vertical film composition and the water content Φ inside the polymer film are obtained. Hence, TOF-NR provides not only the evolution of film thickness and water content Φ as a function of time but also the water content as a function of depth in the polymer film. Even for very thin films, which are in the typical range needed for applications such as nanosensors and nanoswitches (
