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Self-Organization of Polystyrene‑b‑polyacrylic Acid (PS‑b‑PAA) Monolayer at the Air/Water Interface: A Process Driven by the Release of the Solvent Spreading Zineb Guennouni,†,‡,○ Fabrice Cousin,*,‡ Marie-Claude Fauré,†,§ Patrick Perrin,∥,⊥ Denis Limagne,† Oleg Konovalov,# and Michel Goldmann*,†,§,∇ †

Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, 4 place Jussieu F-75005 Paris, France ‡ Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif sur Yvette Cedex, France § Faculté des Sciences Fondamentales et Biomédicales, Université Paris Descartes, 45 rue des Saints Pères , 75006 Paris, France ∥ Ecole Supérieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI), ParisTech, PSL Research University, Sciences et Ingénierie de la Matière Molle (SIMM), CNRS UMR 7615, 10, Rue Vauquelin, F-75231 Cedex 05 Paris, France ⊥ Sorbonne-Universités, UPMC Univ Paris 06, SIMM, 10, Rue Vauquelin, F-75231 Cedex 05 Paris, France # European Synchrotron Radiation Facility, 6 rue Jules Horowitz 38000 Grenoble, France ∇ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette, France S Supporting Information *

ABSTRACT: We present an in situ structural study of the surface behavior of PSb-PAA monolayers at the air/water interface at pH 2, for which the PAA blocks are neutral and using N,N-dimethyformamide (DMF) as spreading solvent. The surface pressure versus molecular area isotherm shows a perfectly reversible pseudoplateau over several cycles of compression/decompression. The width of such plateau enlarges when increasing temperature, conversely to what is classically observed in the case of an in-plane first order transition. We combined specular neutron reflectivity (SNR) experiments with contrast variation to solve the profile of each block perpendicular to the surface with grazing-incidence small-angle scattering (GISAXS) measurements to determine the in-plane structure of the layer. SNR experiments showed that both PS and PAA blocks remain adsorbed on the surface for all surface pressure probed. A correlation peak at Qxy* = 0.021 Å−1 is evidenced by GISAXS at very low surface pressure which intensity first increases on the plateau. When compressing further, its intensity decays while Qxy* is shifted toward low Qxy. The peak fully disappears at the end of the plateau. These results are interpreted by the formation of surface aggregates induced by DMF molecules at the surface. These DMF molecules remain adsorbed within the PS core of the aggregates. Upon compression, they are progressively expelled from the monolayer, which gives rise to the pseudoplateau on the isotherm. The intensity of the GISAXS correlation peak is set by the amount of DMF within the monolayer as it vanishes when all DMF molecules are expelled. This result emphizes the role of the solvent in Langmuir monolayer formed by amphiphilic copolymers which hydrophobic and hydrophilic parts are composed by long polymer chains.

1. INTRODUCTION Interfacial monolayers of diblock copolymers have a broad domain of technological applications ranging from pharmaceutical1 to ferrofluids. Indeed, they enable the tuning of interfacial properties such as wettability, lubricity,2,3 adhesion, protein repelling, or the fine control of colloidal stability. They are also quite useful to prepare templates for nanomolding.4 This comes both from the specific properties of each block and from the large variety of ordered morphologies they can form.5 For instance, amphiphilic copolymers have an enhanced surface activity at the air/water interface compared to small surfactant molecules. Diblock copolymers were widely studied at the air− liquid interface by the Langmuir monolayer method, a technique allowing the control of the surface density of the © XXXX American Chemical Society

monolayer, compared to what can be obtained by spin-coating. Among the different systems studied,6−8 polystyrene-bpolyacrylic acid (PS-b-PAA) has been the focus of a lot of attention because it is made of two flexible blocks. The PS chain is neutral and highly hydrophobic while PAA block is a weak polyelectrolyte whose pKa is located at intermediate pH (4.7), enabling an easy tuning of the charge density from completely neutral at low pH up to fully charged at high pH. These copolymers have been used in several applications such Received: August 10, 2015 Revised: January 28, 2016

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DOI: 10.1021/acs.langmuir.5b02652 Langmuir XXXX, XXX, XXX−XXX

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Langmuir as “template” for polyelectrolyte layer-by-layer (LBL) deposition.9 A refined understanding of the behavior of the diblock copolymers molecules at the air−water interface is however tricky because of the numerous physicochemical parameters involved in these systems such as the respective masses of the blocks, the pH, or the ionic strength of the subphase. The spreading solvent used for the deposition may also play a role. So far, various studies aimed at deciphering the respective roles of each, yet some questions are still open. Currie et al.10 have investigated the influence of pH on the PS-b-PAA surface pressure isotherm. At low pH, that is, when PAA blocks are neutral, the isotherm evidence an almost constant surface pressure plateau, which is usually the signature of a first-order transition. These authors assigned it to a transition from a mushroom regime to a brush regime (see Figure SI.1 in the Supporting Information), referring to a similar plateau observed on monolayers of poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO). Indeed, Bijsterbosch et al.11 identified three different regions on the compression isotherm: (i) a first region at low surface density interpreted as the adsorption of the PEO chains in a “mushroom” structure, (ii) a plateau assigned to a first order transition between this mushroom conformation to a brush conformation, driven by steric repulsions between PEO chains, and (iii) a third region at high surface density where all hydrophilic PEO chains are desorbed from interface and stretched in brushes conformation. For PS-b-PAA at high pH, when the PAA blocks are ionized, no plateau was observed on the isotherm. Currie et al.10 interpreted this behavior by a brush conformation of the PAA blocks, whatever the surface pressure, induced by electrostatic repulsions between charged PAA blocks. However, to the best of our knowledge, no structural study has been performed on PS-b-PAA monolayer at low pH by scattering techniques. It is thus legitimate to wonder if the interpretation of the PS-b-PAA isotherms based on the behavior of the PS-bPEO system is valid. Focusing on the high pH regime, Theodoly et al.12 demonstrated by grazing incidence small angle X-ray scattering (GISAXS) that PS-b-PAA film was not homogeneous at the molecular scale. They interpreted their results by the presence of hemimicelles of copolymers at the air−liquid interface which aggregation number increases under compression. To the best of our knowledge, this is the only determination of selfassembled aggregates performed in situ directly at the air/liquid interface on this system. However, they used a spreading solvent for which micelles of copolymers already existed in the deposition solution. Then, this result may be not representative of the equilibrium state of the film. Many studies aimed in achieving an in-plane morphological characterization of the PS-b-PAA13,14 or PS-b-PEO15−17 monolayers transferred onto solid substrates by microscopy (AFM and TEM). In all cases, surface aggregates were observed with various morphologies, from spheres15 to dots, spaghetti up to large domains.16 Such a variety of morphologies raises the question of whether such aggregates pre-existed on the liquid substrate or if they result from the transfer procedure or from the interaction with the solid substrate that would induce dewetting. For PS-b-PEO, Cox et al.15 assumed the absence of such rearrangements and attributed the spontaneous surface aggregation along with removal of spreading solvent to the concurrent interactions occurring simultaneously during evaporation. Cheyne and Moffitt18−20 proposed a non-

equilibrium mechanism for the formation of the surface aggregates, based on a two-step process: an initial dewetting of PS chains from the water during spreading solvent evaporation followed by a morphological evolution related to the interfacial adsorption of PEO chains. The authors considered that the aggregates become kinetically frozen upon solvent evaporation. This solvent removal assumption dismisses the simple vision of the mushroom−brush transition in the regime where the hydrophilic block of the copolymers is neutral. It highlights the key role of the spreading solvent during self-assembling process at the interface. Choosing a spreading solvent for the PS-b-PAA copolymer is tricky since, due to its high polarity, PAA is not dissolved in the usual solvents like chloroform or toluene which are used for neutral blocks.14 A common choice is the use of hydrophilic−hydrophobic solvent mixtures such as dioxane + toluene,10 dioxane + HCl,21 or THF + toluene.22 This strategy is however strongly system-dependent because the ratio of solvents in the mixture must be adjusted to the ratio of block lengths. Recently, Wang et al.14 obtained PS-b-PAA monolayer by using N,N-dimethyformamide (DMF) as spreading solvent. However, although DMF is a good solvent for both blocks, it is miscible into water which may modify the formation of the monolayer compared to the cases using volatile solvents. Working at pH 7, they observed by Brewster angle microscopy (BAM) large surface domains at the air−water interface and several surface aggregate morphologies by atomic force microscopy (AFM) measurements after transfer on solid substrate. They inferred that the surface aggregation is induced by the diffusion of DMF into water. In summary, it is still difficult to fully puzzle out the whole mechanisms that drive the PS-b-PAA monolayer at the air/ water interface given the disparity of pH and spreading solvent used in published studies. In particular, the following pending questions remain: First, is it correct to interpret the plateau by a mushroom−brush transition, as well as its appearance/ disappearance with respect to the pH? Indeed, as there is no clear order parameter describing this configuration change, it should not be considered as a first order transition in the Landau model? Second, do the surface aggregates exist prior to their transfer onto solid surfaces? If they do, how do they rearrange during the successive compression/decompression cycles at the water surface? Indeed, the removal of the spreading solvent from the monolayer should lead to the freezing of the system as PS blocks are expected to be in a glassy state at room temperature (Tg ∼ 100 °C). Does the spreading solvent influence the formation process? In this work, we aim for performing a full in situ structural characterization of PS-b-PAA monolayers, using DMF as spreading solvent, for different surface densities and surface pressures, combining specular neutron reflectivity (SNR) experiments using contrast variation to solve the profile of each block perpendicular to the surface with grazing-incidence small-angle scattering (GISAXS) measurements to determine the in-plane structure of the layer. Our strategy is to decouple spreading solvent effects from electrostatic ones. For this reason, we present here results achieved in neutral conditions (pH 2).

2. EXPERIMENTAL SECTION 2.1. Materials. Polystyrene-poly(acrylic acid) diblock copolymer PSD-b-PAA was provided by Polymer source Inc., Canada. It consists of 31 deuterated styrene repetition units (3500 g/mol) and 145 acrylic B


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hydrogenated PAA and deuterated water and between PSD and deuterated water allow to neglect the PSD contribution in the modeling of the SNR curves. The SLD of the hydrogenated DMF used for the deposit is 0.70 x10−6 Å−2. Before any modeling, the incoherent scattering was subtracted from the SNR curves. It was measured in an off-specular geometry, as depicted in reference,23 and has a value of respectively 9 x10−6 for the 67%/33% H2O/D2O mixture and 3.5 x10−6 for the deuterated water. Data were analyzed by using the Parratt formalism. The interface was described by a serie of layers, each of them being characterized by a thickness, a scattering length density and a roughness. The experimental resolution of the spectrometer was taken into account in the calculation. Grazing Incidence Small Angle X-ray Scattering. GISAXS measurements were performed on the ID10B beamline at the ESRF synchrotron source (Grenoble, France). The trough was mounted on an active antivibration system (Accurion) and was enclosed within a gastight box with Kapton windows and flushed with water-saturated helium gas. The energy of the incoming X-ray beam, 8 keV (0.155 nm), was selected using a double diamond crystal monochromator. The incident beam was slightly deflected downward by a double mirror setup to an incidence angle with the water surface of 2.4 mrad, which is below the critical angle of total reflection on the air/water interface (2.8 mrad at 8 keV). The incident beam dimensions, defined by optical slits, were 200 μm vertically and 300 μm horizontally. Collimation was achieved with two horizontal gap slits (200 and 300 μm separated by 475 mm) for the scattered intensity. Such a setup leads to an in-plane angular resolution of about 1 mrad. This glancing incidence method is a standard technique to reduce the bulk scattering and to reveal the surface signal.24 The scattered signal was recorded using a one-dimensional (1D), gas-filled (Ar/CO2) position-sensitive detector (PSD) with vertically counting wires. In this configuration, the spectra are obtained by scanning over the in-plane 2θ angle. At each 2-theta point, the vertical scattered intensity (along Qz) is recorded. The respective electronic scattering densities of the components are respectively ρe(DMF) = 8.30 × 10−6 Å−2, ρe(PS) = 9.58 × 10−6 Å−2, ρe(PAA) = 9.89 × 10−6 Å−2, and ρe(H2O) = 9.43 × 10−6 Å−2, according to the following equation:

acid repetition units (10 800 g/mol) with polydispersity index, Pi = 1.12. The copolymer was dissolved in N,N-dimethyformamide (DMF) (Sigma-Aldrich, purity better than 99.9%) to form the spreading solutions. We checked the perfect solubilization of the copolymer chains in DMF solvent by small-angle neurton scattering (SANS) experiments, using two contrast conditions to probe selectively either the PS part or the PAA part of the copolymer chains (see Supporting Information Figure SI.2). The masses of the blocks derived from the SANS experiments are in perfect accordance with the masses provided by the manufacturer. 2.2. Methods. Isotherm. Concentration of spreading solutions is typically 2 × 10−4 mol/L. A few microliters of the solution were deposited drop by drop with a microsyringe (Hamilton) onto the air/ liquid interface of a Langmuir trough equipped with a movable barrier for film compression. The surface pressure was measured according to the Wilhelmy plate method. The plate was made of filter paper and the measurement device was a microbalance from Riegler&Kirstein GmbH (Germany). The subphase consisted of Milli-Q Millipore ultrapure water (18.2 MΩ·cm) into which HNO3 (Sigma-Aldrich, purity better than 98%) was added to set the pH to 2. The temperature was regulated at ±0.5 °C using a circulating water bath. Isotherms have been recorded in the range of temperatures 10 °C ≤ T ≤ 40 °C. The balance measuring the surface tension has been calibrated for each temperature. When the temperature was changed in situ on the same monolayer, the temperature change induced a continuous variation of the water level and therefore a shift of the zero level of the surface pressure in this case. Prior to the deposit of each monolayer, the Teflon trough was cleaned with Hellmanex and rinsed several times with ultrapure water. Before any experiment, a compression of water surface at pH 2 was performed to check that the surface was not polluted. Each isotherm was performed three or four times to check the reproducibility of the results. A similar setup and procedure were used for SNR and GISAXS/GIXD measurements. All isotherms were performed at a constant speed compression of 35 Å2/mol/min. The typical time of isotherm cycle (compression− expansion) is about 1 h 15 min. The speed compression has no influence on the isotherm. We checked it by performing isotherms at two very different compression speeds, 3 Å2/molecule/min and 400 Å2/molecule/min (10 times lower and greater than the usual speed), and we did not detect any change in the isotherms (data not shown). Specular Neutron Reflectivity. SNR experiments were carried out on the horizontal time-of-flight reflectometer EROS at LLB.23 The horizontally collimated beam was deflected on the sample by two neutron supermirrors to collect data at a fixed angle of incidence. For a given experiment, two different angles were used (respectively, 1° and 3.08°, or 1.5° and 3.08°, depending on the contrast) to access a Qz range of 0.008−0.18 Å−1 with a neutron white beam covering wavelengths from 3 to 25 Å.We used a constant δλ/λ resolution of 0.11 and a collimation giving a δθ/θ angular resolution of 0.03. Then, the δQ/Q resolution is of ∼0.12. The experiments were performed in situ on a Langmuir trough mounted on an active antivibration system (Accurion), and placed in a sealed PMMA box to prevent dust contamination with two silicon windows to let pass the neutron beam. Once an experiment has begun, it was checked from the position of the critical wavevector Qc that there was no exchange of deuterated water with atmospheric light water over a scan of ∼24 h. A SNR measurement of the water surface was performed prior the spreading of the PS-b-PAA layer. Since the PS part of the copolymer was deuterated, it was possible to use contrast variation to get a specific insight on either the profile of the PS part or on the profile of the PAA part of the copolymer monolayer. Two conditions of subphase contrasts were used one with a mixture of 67%/33% H2O/D2O whose Scattering Length Density (SLD) NB of 1.7 x10−6 Å−2 matches exactly the SLD of the PAA chains. The second one was pure deuterated water NB of 6.37 x10−6 Å−2. It does not exactly matched the SLD of deuterated polystyrene (6.5 x10−6 Å−2) which is too high to be matched with pure water. However, in such a contrast, the respective differences of SLD between

ρe =

re ∑ ZkNk V k

Zk is electron number of each atom k, Nk is atom number in the molecule, re is the electron radius, and V is the volume of molecule. Grazing incidence X-ray diffraction (GIXD) was also performed on the same monolayer using the “classical” Soller’s slits setup but the spectra (not showed) did not present any signal indicating an order at the molecular atomic range distance. The surface pressure was kept constant during a scan.

3. RESULTS 3.1. Isotherm. Figure 1 presents the surface pressure versus area per molecule (π−A) isotherms for PSD-b-PAA copolymer and for three compression−expansion cycles at T = 18 °C. The general shape of the first compression isotherm is in agreement with previous experiments.10,13,21 Three regions can be identified: (i) at low density surface, the surface pressure increases continuously and relatively slowly under film compression compared to the high density surface region (iii). The compressibility in the first zone (at low density region, between 5000 and 3500 Å2 per molecule) is 4 times larger than that in the third zone (measured between 750 and 450 Å2 per molecule). Both regimes are separated by a region (ii) with a slight increasing surface pressure commonly called pseudoplateau (π ≈ 2.5 mN/m). The compressibility 1 ∂π χ = − A ∂A of regions (i), (ii), and (iii) range at about 285, 700, and 65 m/N, respectively. From the three

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temperature is reached.25,26 Also, one should observe at constant pressure a shift to lower surface density with increasing temperature while one obtains here the opposite effect. This indicates that the monolayer has not a classical behavior. However, we underline that the transition associated with these isotherms remains partially reversible with the temperature. Indeed, we have followed the behavior of the monolayer along a temperature cycle consisting of a first measurement at 10 °C, an increase up to 40 °C and a return to 10 °C. The obtained isotherms are shown in Figure 3. One

Figure 1. Surface pressure isotherm cycles of PS-b-PAA copolymers at pH 2 and T = 18 °C. The different arrows correspond to the different points of GISAXS and SNR measurements. The colors code is respected for GISAXS and SNR figures.

compression−expansion cycles, we find no hysteresis regardless of the density surface, which illustrates the reversibility of the monolayer organization at the interface. Figure 2 shows the isotherms obtained for temperatures of 10, 20, and 30 °C. The limits of the pseudoplateau are

Figure 3. Measurements of isotherms performed on the same monolayer of PS-b-PAA at pH 2 on a temperature cycle. The temperature of the subphase is first at T = 10 °C, then at T = 40 °C and cooled back to 10 °C.

recovers the pseudoplateau at about the same surface pressure, although its large area limit is decreased from about 3500 to 2000 Å2. However, the low area limit corresponding to the dense phase surface density remains unchanged meaning that the number of PS-b-PAA on the surface remains constant. These results again show that the pseudoplateau is not arising from a mushroom-brush transition as usually described in the literature. Indeed, as the mushroom area should remain constant from one cycle to another, the large area limit of the plateau should return to the same value. This particular behavior will be discussed later. 3.2. Specular Neutron Reflectivity. The three typical regions of the isotherm were probed by SNR measurements: (i) at a 2D pressure of π = 1.4 mN/m, before the plateau of the isotherm for the two contrasts; (ii) at π = 2.5 mN/m, in the middle of the plateau for the two contrasts and (iii) at the end of the plateau. For this last condition, we measured at π = 4 mN/m (shoulder of the plateau) for the 67%/33% H2O/D2O subphase and at π = 10 mN/m (beyond the plateau) for the 100% D2O subphase. The different points of measurements are indicated by arrows on Figure 1. The SNR curve of the aqueous subphase was always measured first. The layer was then deposited, first compressed up to 10 mN/m to check the isotherm, and expanded up to the larger molecular area. It was then compressed again for neutron scattering measurements. Each measurement was performed at constant surface pressure, enabling the movement of the trough barrier. The reflectivity curves are shown in Figure 4. They are displayed in the Fresnel representation (R(Qz)Qz4 versus f(Qz)), which enables one to focus on the scattering coming only from the layer present at

Figure 2. Compression isotherms (π−A) of a monolayer of PS-b-PAA formed at pH 2 and compressed at various temperatures.

identified by an increase of the compressibility at large area and a decrease at smallest area, although the surface pressure does not remain constant along the compression. One clearly observes on these isotherms that the average surface pressure of the pseudoplateau decreases unexpectedly when the temperature rises from 10 to 20 °C and that its width decreases sharply at 30 °C. At 40 °C, the three regions of the isotherm still exist, but in the second one (ii), the slope was significantly larger than for other temperatures. Moreover, one observes that the isotherms tend to be shifted toward low surface pressure when the temperature increases (as mentioned in the Materials and Methods subsections, the shift to negative surface pressure at large area for T = 40 °C results from a variation of the liquid surface level). These results are opposite to what is expected in the case of a simple in-plane first order transition. Indeed, if this were the case, one would observe a rise of the plateau surface pressure associated with a decrease of its width when the temperature increases, up to its disappearance when a critical D


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Figure 4. SNR of the single subphase at pH 2 and T = 18 °C, covered by a deuterated PS copolymer monolayer at various surface pressures. Continuous lines correspond to the best fit (see main text): (a) 100% D2O; (b) 67%/33% H2O/D2O.

Figure 5. (a) Typical 2D GISAXS pattern for a PSD-b-PAA monolayer at pH 2, at T = 18 °C and at 2.5 mN/m. (b) X-ray scattered intensity integrated along Qz versus Qxy for a PSD-b-PAA monolayer at different surface pressures. Each curve is shifted for clarity by a factor 3 with respect to the previous curve. An example of reproducibility is presented at 1.4 mN/m, with “Back” indicating the expansion cycle.

the interface by compensating the Qz−4 scattering decay from the pure air/water interface. Let us now describe the results obtained using the 100% D2O aqueous subphase where only the PSD part of the copolymer chains was almost matched (Figure 4a). All scattering curves superimpose within errors bars with the one of the free aqueous subphase. Such SNR curve of water can then be modeled by a pure Fresnel one without any adjustable parameter, proving that the subphase was perfectly clean and that the subtraction of incoherent scattering was properly done. The SLD for modeling the subphase was 6 × 10−6 Å−2 slightly lower than of pure D2O (6.37 × 10−6 Å−2). It indicates that the solvent contains 6% of H2O, introduced when setting the pH of the subphase to 2. As it was not possible to detect any significant change of the SNR curve when the copolymer film is added, it appears that the thickness of the layer of PAA should remain below 5 Å, the smallest possible thickness that can be detected with SNR. An unambiguous determination of a smaller thickness would indeed require a measurement up to larger Q, which is experimentally impossible due to incoherent scattering. Thus, the SNR measurements reveal the clear-cut behavior of the PAA parts of the copolymer chains when they are neutral: They are adsorbed at the air/water interface,

regardless the surface pressure, even up to 10 mN/m, after the pseudoplateau. The behavior of the PSD parts of the copolymer is almost similar to the one of the PAA part (Figure 4b). Here again the scattering curve of the subphase without copolymer film can be perfectly modeled by the Fresnel curve corresponding to a 67%/33% H2O/D2O mixture. Adding the copolymer, the SNR curve is quasi similar at 1.4 and 2.5 mN/m to the one of the pure subphase. Any attempt of modeling of the SNR curve with a different model from the Fresnel one would be senseless. As for their PAA counterparts, the PSD parts of the copolymer chains remain adsorbed at the air/water interface in a thin layer whose maximal thickness range to a few Å. At 4 mN/m, the situation is different. The intensity of the SNR curve is increased compared to the one of subphase. Such an increase is expected when enough deuterated PS chains are located at the interface as the SLD is much higher in this case than the one of the 67%/33% H2O/D2O mixture. Therefore, it unambiguously demonstrates the presence of a PSD layer. Its thickness and SLD range respectively in 15−20 Å and (2.7−3.3) × 10−6 Å−2. Indeed, several profiles with thicknesses lying in this range enable to model the experimental curve within errors bars. Figure 4b presents the best adjustment, that is, the one that E


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Langmuir leads to the lowest χ2, achieved for a layer of 18 Å thickness with a SLD of 3 × 10−6 Å−2. At this stage, considering GISAXS results (see below), one may wonder if there are still some DMF molecules within the copolymer layer and how this would affect the SLD of the adsorbed layer. Given that the SLD of DMF (0.70 x10−6 Å−2) is slightly below the one of H2O/D2O mixture (1.7 × 10−6 Å−2) whereas the SLD of PSD is much higher (6.5 × 10−6 Å−2), the potential changes of the SNR curve induced by DMF molecules is probably weak but of course dependent on its relative concentration. At low density, the monolayer appears too thin to be detectable and consequently, the DMF concentration cannot be evaluated. At high surface pressure, the adjustment results are satisfying when considering a pure PS monolayer indicating that if DMF is present, its concentration would be very weak compared to the ones of PSD (see Supporting Information Figure SI.3). The main result of these SNR measurements is to demonstrate that the chains are neither in a mushroom regime nor in a brushes regime. They appear to present a “pancake” structure (see Figure SI.1 in the Supporting Information), the copolymers remaining adsorbed at the interface for all measured surface pressures when the PAA chains are neutral. Thus, they do not undergo mushrooms to brushes transition under compression. 3.3. Grazing Incidence Small Angle X-ray Scattering. The in-plane structure of the adsorbed layer of copolymers on the pH 2 subphase was probed for several surface pressures from 1.4 mN/m up to 6 mN/m by GISAXS. A readily marked correlation peak appears on GISAXS spectra at low surface pressure, beyond the plateau of the isotherm (Figure 5b). The observed signal arises only from the scattering of the surface. Indeed, it appears on the Qxy − Qz pattern as a vertical rod at constant Qxy with its maximum at Qz = 0 (see a typical 2D pattern in Figure 5a). Conversely, a “bulk” scattering of disorganized objects beneath the surface would lead to a circle in the Qxy − Qz pattern (powder ring). In order to obtain the characteristic thickness of the scattering layer we have plotted the rod scan, I(Qz) at Qxy* which corresponds to the Qxy position of the maximum intensity of the peak (see Supporting Information Figure SI.4). Since the resolution is very poor along the Qz direction for such Qxy, the smearing of such scattering curve is very important. We have thus decided to adjust it by a Gaussian function which half width indicates the range of thickness of the probed layer. One obtains a value of 40 Å, slightly lower than the expected penetration length of the X-rays evanescent wave, but also in the range of the thickness deduced from the neutron reflectivity measurement. Figure 5b shows the X-ray integrated intensity I versus Qxy for all the surface pressures. I(Qxy) has been obtained by integrating on Qz from Qz = 0 Å−1 to Qz = 0.2 Å−1. At 1.4 mN/ m, the peak is located at Qxy* = 0.021 Å−1, which corresponds to a characteristic distance d* of ∼30 nm (d* = 2π/Qxy*). Such distance is very large compared to the typical size of a copolymer (Rg ∼ 3 nm). This indicates that the copolymer chains self-assemble in nanostructures involving several chains, even at low surface pressure. At 2 mN/m, before the plateau on the isotherm, the GISAXS spectrum is almost identical with a peak located at the same Qxy* indicating a similar surface structure that at 1.4 mN/m. However, when compressing further and entering in the plateau, the system undergoes a strong change of its surface structure. At the onset of the plateau at 2.2 mN/m, the correlation peak is strongly shifted toward low Qxy, that is, the characteristic distance of the system

increases! Such concomitant increase of the characteristic distance with the decrease of the film area may indicate unusual in-plane reorganization in the monolayer. This process is however perfectly reversible. The in-plane scattering curve at 1.4 mN/m obtained after decompression down from 3 mN/m was indeed almost perfectly similar to the one obtained at the same surface pressure under the first compression, in accordance with the perfect reversibility of the isotherms at this pH. When compressing further along the plateau of the isotherm, the peak continues to be shifted toward low Qxy to reach very low values (0.014 Å−1 at 2.7 mN/m, i.e, a distance of 43 nm in direct space). Simultaneously, one observes a progressive decrease of the intensity of the peaks that almost vanishes at the end of the plateau. At higher surface pressures, after the plateau has been passed, the correlation peak has completely disappeared. The disappearance of the peak at the border of the pseudoplateau can be interpreted by resulting from an in plane first-order transition but also from a progressive loss of the electronic contrast between the scattering species at the surface. Given that the combination of SNR and GISAXS measurements clearly dismiss the possibility of a simple mushroom-brush transition and that the results from Wang et al.,14 suggest a continuous reorganization of surface aggregates in the presence of DMF, we dismiss the first hypothesis. Indeed, in a simple first order transition, one does not expect a shift of the peak when the transition proceeds. Moreover, this shift is in accordance with the fact that the surface pressure does not remain constant on the pseudoplateau but is continuously increasing. We underline that the measured d* unexpectedly increases when the pressure is increased. Also, such a first order transition would not be in agreement with the evolution of the isotherms when varying temperature. The plateau appears then more as a region of large compressibility as an in-plane phase transition. We associate thus the intensity decrease to the loss of electronic contrast in the plane of the surface. It is reasonable to consider that the contrast is generated by the electronic density difference between the aggregates and their surrounding vicinity. The shift of the peak to low Qxy values indicates a reorganization of the self-assembly (as mentioned above, similar result at high pH has been interpreted by Theodoly et al.12 as the growth of predeposited surface micelles). We propose a new interpretation that explains both effects. We consider that some DMF solvent remains within the PS part of the copolymer in the monolayer. We also consider that these DMF molecules are expelled from the surface when a defined surface pressure value is reached. This will lead to a variation of the in-plane solvent concentration. Such variation should lead to a change of the copolymer self-assembly and then of the correlation peak scattering vector. Moreover, such concentration variations also modify the contrast between the aggregates and their surroundings. Note that the DMF should not be within the PAA part since PAA and DMF both are soluble in water. Thus, the surface is composed by four different chemical components (PS, PAA, DMF, and H2O) which are split in two surface phases, one made of PAA and water, the other made of PS and DMF. From the electronic contrast aspect, the electronic scattering density ρPAA‑water of the PAA/water phase should vary slightly upon compression since the PAA chains remain hydrated, with a mean value ranging from 9.43 × 10−6 Å−2 (pure H2O) to 9.83 × 10−6 Å−2 (pure PAA). On the reverse, F


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Langmuir the electronic scattering density ρPS‑DMF of the PS/DMF phase will strongly change upon a variation of DMF concentration within PS upon compression due to DMF expulsion since DMF has a very different scattering length density (8.30 × 10−6 Å−2) than PS (9.58 × 10−6 Å−2), and also from PAA and water. Thus, a swelling/deswelling of PS by DMF will strongly change the scattering intensity which is proportional to (ρPAA‑water − ρPS‑DMF)2. Remarkably, if all DMF is expelled from PS, the electronic density of the pure PS is almost equal to ρPAA‑water. In other words, there will be no more electronic contrast between the two surface phases and the scattering intensity will vanish, irrespective to its structural organization. Such variation of ρPS‑DMF rends a refined modeling of the scattering very speculative since the exact contrast between the two phases is unknown. Thus, when changing the surface pressure, the evolution of the scattering intensity will be simultaneously modified by contrast change and by the structural modification. Therefore, we have simply modeled the scattering curve by a Gaussian function to extract the Qxy* value and its intensity (in arbitrary units), instead of a complex analysis in the DWBA approximation (see an example in Figure SI.5A in the Supporting Information). We do not extract other parameters as it would be overinterpretation. Figure SI.5B−D reports, respectively, the I(Qxy*), Qxy*, and d* values with respect to the surface pressure.

decreasing on the plateau cannot be interpreted by a simple compression of self-assembled micelles with a constant aggregation number. Indeed, the typical distance would decrease upon compression for such a process. In ref 12, the authors were then constrained to introduce a variation of the micelle’s agregation number under compression. Although this is possible, one should consider that pure PS should be in a glassy state at room temperature. As mentionned above, it is more reasonable to consider that the variation of the peak intensity on the plateau results from a variation of the layer composition. In a recent paper, Wang et al.14 underlined the important role of the DMF used as spreading solvent and in particular how the PS-b-PAA molecules self-assemble when the DMF dissolves in water. We interpret our results by the following assumption that, at low surface pressure (high surface tension), DMF does not completely dissolve in the water subphase and that a part of it remains at the surface in the monolayer. This is supported by bulk experiments on PS-bPAA aggregates in water that showed that DMF molecules remain in the PS core of these aggregates30 (the authors give also the solubility of various solvents in PS). This is not surprising since DMF is also a good solvent for PS which prompted us to choose it because it prevents the use of solvents mixture. Such assumption allows understanding that the film is not chemically homogeneous after deposition at large area and at low surface pressure. When the surface pressure is increased, the film composition and structure remain unchanged until a critical compression threshold where the DMF is expelled and merge into the water subphase (similar to the expulsion of water when a sponge is pressed). Indeed, DMF is less hydrophobic than the PS or PAA blocks. Such expulsion of DMF induces two effects: First, it decreases the number of molecules adsorbed onto the surface leading to a flattening of the surface pressure versus area curve. This appears as a plateau on the isotherm which should be at least considered as the signature of the “squeeze out pressure” of the DMF surface molecules. Second, it varies the film composition by lowering the DMF surface concentration. This explains the variation of the self-assembled organization of PS-b-PAA, leading to a larger d* value (2π/Qxy*). We note that the surface pressure for desorbing the DMF should vary with the DMF concentration in the film or with the temperature, which explains the “sloping plateau” observed on the isotherm and its unexpected variation with temperature. We underline that the PS blocks cannot be considered as in the glassy state because of the presence of DMF in the film. This also explains the perfect reversibility of the system when the film is expanded. It results from the readsorption of DMF from the subphase into the film which leads to the reorganization of the copolymers. Such a model allows a new interpretation of the isotherms: (i) At low surface pressure the film composition is PS-b-PAA and DMF leading to a constant self-organization. The compression does not change signif icantly the correlation length. Qxy* remains constant but the peak intensity increases as the surface density of domains is increasing and the film becomes more dense. (ii) When the surface pressure reaches a defined value, DMF is expelled from the surface which lowers the slope of the isotherm. Indeed, the progressive expulsion enables a large reduction of the molecular area with a small increase of surface pressure: it gives rise to the appearance of the pseudoplateau. This pseudoplateau is thus not an in-plane first-order transition plateau as the surface aggregates evolve continuously on it along compression but is related to a transition from the surface to the subphase. The

4. DISCUSSION The refined detailed structural determination of the PS-b-PAA monolayer at the air/water interface coupling SNR and GISAXS has unambiguously demonstrated two important features that require to revisit the usual description of the system. First, the SNR experiments demonstrate unambiguously the absence of brushes configuration of the PAA block in neutral conditions although PS-b-PAA monolayer was described as undergoing a transition from mushroom to brush configuration when the film is compressed. They appear as adsorbed at the surface in a “pancake” structure for all surface pressures before collapse of the copolymer. This is thus clearly different from the PS-b-PEO monolayer for which it is proposed that PEO blocks significantly dive into the water subphase.11,27−29 It points a difference of hydrophilicity between neutral PAA and PEO, and a subsequent difference of behavior in water, that prohibits interpretating the compression isotherm of PS-b-PAA by referring to the one of PS-b-PEO. Second, GISAXS experiments demonstrate that the film is not homogeneous at the molecular scale at low surface density, even when the surface pressure is increasing along compression indicating a monophasic layer. Although similar observation has been obtained by Theodoly et al.,12 we underline that they studied the system at pH 11 where the isotherm does not evidence any plateau. The correlation peak was interpreted by the presence of aggregates at the air/water interface. Our results remove the ambiguity on the other mechanism of surface aggregation that may interfere, that is, the formation of structures resulting from their former presence within the spreading solution prior to the deposit. This suggests to be cautious when considering literature describing the possible observation of self-organized structures in the monolayer. At low pressure and up to the end of the plateau (e.g., at 1.4 mN/m), the correlation peak has already a large intensity. It is thus an intrinsic feature of the system. Upon compression, the growth of the characteristic distance and the peak intensity G


Article

Langmuir

Figure 6. Schematic representations of the surface evolution of the PS-b-PAA at pH2 under surface pressure variations. (a) At low pressure (large molecular area). (b) At an intermediate pressure (on the plateau). (c) At high pressure (small molecular area). PS is depicted in red domains, PAA in green domains, DMF in yellow, and water in blue. Black arrows indicate the barriers displacement and yellow arrows indicate the DMF motion. The color of PS domains changes with 2D pressure, as it depends on the amount of DMF that remains trapped inside the monolayer.

decrease of solvent concentration in the film induces surface reorganizations with an increase of rich-PS domains dimensions, that tried to minimize conctact with water, and rich PAA domains. Such explusion has not a high energetic cost as DMF is miscible with water, and also because reorganizations are favored by the progress increase of hydrophobicity of the DMF/water solvent in the vicinity of the layer. (iii) When all DMF solvent molecules are expelled, no reorganizations are any longer possible and the surface pressure starts to strongly increase upon compression as the copolymer remains adsorbed at the surface. Moreover, from the point of view of electronic contrast, the film appears as homogeneous as only PS-b-PAA remains present on the surface, consequently no GISAXS signal is detected. The reversibility of the system upon compression/decompression is striking given that the surface morphology is driven by DMF solvent. We underline that this solvent is miscible in water and its boiling point temperature is very high (153 °C). Then, it cannot evaporate at room temperature at which the experiments are performed (10−40 °C). The DMF molecules can only diffuse from the interface to the subphase. We also note that the amount of DMF in the monolayer is much lower than that contained in the subphase (mainly defined by the volume of deposited solution). For example, by depositing 25 μL of DMF solution at the air−liquid interface of a Langmuir trough for which the surface area is 150 cm2, the feature volume of DMF remaining in the monolayer at the air−liquid interface is 1.5 × 10−2 μL, assuming a maximal thickness of 1 nm of DMF, as determined by SNR measurements. Therefore, it can be concluded that the concentration of DMF in the subphase is almost constant when the DMF in the monolayer is expelled into the water under the effect of 2D compression. DMF molecules turn back to the rich-PS domains part of the surface aggregates upon decompression because it is energetically favorable. This comes from the solubilities of DMF in PS and in water respectively but also from energy cost of the air/water interface. As DMF has a higher solubility in PS than in water, a part of the water-dissolved DMF comes back (as for a Gibbs film) swelling the layer as soon as the PS domains become accessible again. Since the amount of copolymer stays constant at the interface, the deswelling/swelling of the layer is perfectly reversible upon compression/decompression. This explains the reversibility of the measurements on several cycles. Moreover, the amount of DMF in PS is strongly dependent on the physical-chemistry conditions (temperature in this case but probably pH, PS block mass, etc.) as mentionned.26 This explains the unexpected variation of the pseudoplateau when

the temperature is modifed. Lowering the temperature would lead to a lower concentration of DMF in the PS phase with respect to the one in the water subphase. This description of the role of the DMF underlines the problem of the solvent for such block copolymer Langmuir monolayer. Indeed, Langmuir monolayer are usually obtained by spreading a solution of amphiphilic molecules which dimension range in the nanometer scale. In the case of amphiphilic copolymers, each block is rather long and behaves as a polymer. Then, their interaction with the solvent is more complex (as developed by Flory).31 Obtaining a good solvent for both hydophobic and hydrophilic polymers and not soluble in the water subphase is not obvious. One can choose a pure solvent but without satisfying all these requests, as DMF which is miscible in water. The other way is the use of solvents mixture but one cannot ensure that the relative concentration remains constant along the evaporation or dissolution in the subphase.10,21,22 The effect we observed indeed emphizes the important role of the solvent which deserves to be probed in all the Langmuir films formed by amphiphilic diblock copolymers. At this stage, it is difficult to assess a clear morphology of the surface aggregates from GISAXS measurements. It is indeed possible to have either surface hemimicelles with increasing number of aggregation upon compression (the PS glassy state being suppressed by the presence of DMF) or a phase on the surface, equivalent to 2D slide of a 3D bicontinuous phase. However, considering the continuous change of the peak under compression associated with its strong intensity and the fact that we did not succeed in measuring the second order peak, the film is probably organized as in such a slide of a continuous phase. Indeed, Theodoly et al.12 proposed a model of surface micelles since they observed the second order peak associated with an hexagonal structure. We propose the possible surface evolution of the nanoaggregates upon compression/decompression that corresponds to the continuous phase in Figure 6.

5. CONCLUSIONS By neutron reflectivity, we observe that PS-b-PAA copolymers in DMF solution and deposited onto water subphase at pH 2 are not in brushes or mushroom configuration but remain adsorbed as pancakes at the interface whatever the 2D pressure. This result reads out the usual interpretation of the isotherms, extrapolated from the PS-b-PEO, which present a similar plateau associated with a mushroom-brush transition in this case. GISAXS experiments demonstrate that the PS-b-PAA film is not homogeneous at the molecular scale for low surface pressures, before the pseudoplateau, and that the overall typical H


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Langmuir

(2) Klein, J.; Kamiyama, Y.; Yoshizawa, H.; Israelachvili, J. N.; Fredrickson, G. H.; Pincus, P.; Fetters, L. J. Lubrication Forces between Surfaces Bearing Polymer Brushes. Macromolecules 1993, 26, 5552−5560. (3) Pincus, P. Colloid Stabilization with Grafted Polyelectrolytes. Macromolecules 1991, 24, 2912−2919. (4) Goodwin, J. W. Colloidal Dispersions. R. Soc. Chem., London 1982, 197−217. (5) Kita-Tokarczyk, K.; Junginger, M.; Belegrinou, S.; Taubert, A. Amphiphilic Polymers at Interfaces. Adv. Polym. Sci. 2010, 242, 151− 201. (6) Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H. Salt Effect on the Nanostructure of Strong Polyelectrolyte Brushes in Amphiphilic Diblock Copolymer Monolayers on the Water Surface. Langmuir 2007, 23, 7065−7071. (7) Mouri, E.; Kaewsaiha, P.; Matsumoto, K.; Matsuoka, H.; Torikai, N. Effect of Salt Concentration on the Nanostructure of Weak Polyacid Brush in the Amphiphilic Polymer Monolayer at the Air/ Water Interface. Langmuir 2004, 20, 10604−10611. (8) Xie, D. H.; Rezende, C. A.; Liu, G. M.; Pispas, S.; Zhang, G. Z.; Lee, L. T. Effect of Hydrogen-Bonding Complexation on the Interfacial Behavior of Poly(isoprene)−b-Poly(ethylene oxide) and Poly(isoprene)−b-Poly(acrylic acid) Langmuir Monolayers. J. Phys. Chem. B 2009, 113, 739−744. (9) Safouane, M.; Miller, R.; Moehwald, H. J. Surface viscoelastic properties of floating polyelectrolyte multilayers films: A capillary wave study. J. Colloid Interface Sci. 2005, 292, 86−92. (10) Currie, E. P. K.; Sieval, A. B.; Fleer, G. J.; Cohen Stuart, M. A. Polyacrylic Acid Brushes: Surface Pressure and Salt-Induced Swelling. Langmuir 2000, 16, 8324−8333. (11) Bijsterbosch, H. D.; de Haan, V. O.; de Graaf, A. W.; Mellema, M.; Leermakers, F. A. M.; Cohen Stuart, M. A.; van Well, A. A. Tethered Adsorbing Chains: Neutron Reflectivity and Surface Pressure of Spread Diblock Copolymer Monolayers. Langmuir 1995, 11, 4467− 4473. (12) Theodoly, O.; Checco, A.; Muller, P. Charged diblock copolymers at interfaces: Micelle dissociation upon compression. EPL 2010, 90, 28004. (13) Joncheray, T. J.; Bernard, S. A.; Matmour, R.; Lepoittevin, B.; El-Khouri, R. J.; Taton, D.; Gnanou, Y.; Duran, R. S. Polystyrene-bPoly(tert-butyl acrylate) and Polystyrene-b-Poly(acrylic acid) Dendrimer-Like Copolymers: Two-Dimensional Self-Assembly at the Air− Water Interface. Langmuir 2007, 23, 2531−2538. (14) Wang, X.; Ma, X.; Zang, D. Aggregation behavior of polystyrene-b-poly(acrylic acid) at the air−water interface. Soft Matter 2013, 9, 443−453. (15) Cox, J. K.; Yu, K.; Constantine, B.; Eisenberg, A.; Lennox, R. B. Polystyrene−Poly(ethylene oxide) Diblock Copolymers Form WellDefined Surface Aggregates at the Air/Water Interface. Langmuir 1999, 15, 7714−7718. (16) Devereaux, C. A.; Baker, S. M. Surface Features in Langmuir− Blodgett Monolayers of Predominantly Hydrophobic Poly(styrene)− Poly(ethylene oxide) Diblock Copolymer. Macromolecules 2002, 35, 1921−1927. (17) Price, E. W.; Harirchian-Saei, S.; Moffitt, M. G. Strands, Networks, and Continents from Polystyrene Dewetting at the Air− Water Interface: Implications for Amphiphilic Block Copolymer SelfAssembly. Langmuir 2011, 27, 1364−1372. (18) Cheyne, R. B.; Moffitt, M. G. Novel Two-Dimensional “Ring and Chain” Morphologies in Langmuir−Blodgett Monolayers of PS-bPEO Block Copolymers: Effect of Spreading Solution Concentration on Self-Assembly at the Air−Water Interface. Langmuir 2005, 21, 5453−5460. (19) Cheyne, R. B.; Moffitt, M. G. Self-Assembly of Polystyreneblock-Poly(Ethylene Oxide) Copolymers at the Air−Water Interface: Is Dewetting the Genesis of Surface Aggregate Formation? Langmuir 2006, 22, 8387−8396.

self-organized structure formed at low pressure is maintained along it. The isotherms and evolution of the position and intensity of the GISAXS correlation peak on the pseudoplateau can be explained by considering that DMF molecules remain solvating the PS block in the film. These molecules start to be desorbed from the surface when a critical surface pressure is reached. This corresponds to the onset of the pseudoplateau along which the DMF molecules are continuously desorbed under compression. The resulting variation of composition of the film induces the reorganization of the copolymers, as observed by GISAXS. It also explains the slight increasing pressure observed on the pseudoplateau and the perfect reversibility of the system under expansion−recompression as the DMF molecules are readsorbed when the surface pressure is lowered. Moreover, it is also coherent with the change of the isotherm when the temperature is modifed. We consider that such solvent effect cannot be systematically neglected and deserves to be probed in various diblock copolymer systems. This first comprehensive structural study on PS-b-PAA copolymers at the air−water interface performed with neutral PAA blocks demonstrates that the DMF solvent concentration controlled by the 2D pressure is a major parameter for the selforganization at the surface. When the PAA blocks are charged, it is likely that electrostatic repulsions between blocks will counterbalance the in-plane reorganizations of aggregates driven by DMF behavior, making the surface behavior of PSb-PAA at the air-interface very different. This charged case will be study in a forthcoming paper.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02652. Scheme of the brush, mushroom, and pancake conformations of polymer, SANS experiment results and analysis demonstrating the absence of PS-b-PAA agregation in the DMF spreading solution, attempt for determining the DMF concentration in the monolayer from the neutron reflectivity curves (and its limitation), and rodscan of the GISAXS correlation peak and graphics representing its intensity, Q*, and d* values with respect to the surface pressure (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address ○

Z.G.: LICORNE, ECE-Paris, 37 quai de Grenelle, 75725 Paris Cedex 15, France.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Rana Farha and Dr. Philippe Fontaine for their helpful assistance. REFERENCES

(1) Kuhl, T. L.; Leckband, D. E.; Lasic, D. D.; Israelachvili, J. N. Modulation of interaction forces between bilayers exposing shortchained ethylene oxide headgroups. Biophys. J. 1994, 66, 1479−1488. I


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