Tuning Morphologies of Langmuir Polymer Films Through Controlled

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Tuning Morphologies of Langmuir Polymer Films Through Controlled Relaxations of Non-Equilibrium States Sivasurender Chandran,† Stefanie Dold,† Amaury Buvignier,‡ Kai-Steffen Krannig,§ Helmut Schlaad,§,∥ Günter Reiter,†,#,¶ and Renate Reiter*,†,¶ †

Institute of Physics, Albert Ludwigs Universität Freiburg, Freiburg 79085, Germany Ecole Nationale Superieure de Chimie de Rennes, 35708 Rennes Cedex 7, France § Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Potsdam D-14424, Germany ∥ Institute of Chemistry, Universität Potsdam, Potsdam D-14476, Germany # Freiburg Centre for Interactive Materials and Bio-inspired Technologies, Albert Ludwigs Universität Freiburg, Freiburg 79085, Germany ¶ Freiburg Materials Research Center, Albert Ludwigs Universität Freiburg, Freiburg 79085, Germany ‡

S Supporting Information *

ABSTRACT: Langmuir polymers films (LPFs) frequently form nonequilibrium states which are manifested in a decay of the surface pressure with time when the system is allowed to relax. Monitoring and manipulating the temporal evolution of these relaxations experimentally helps to shed light on the associated molecular reorganization processes. We present a systematic study based on different compression protocols and show how these reorganization processes impact the morphology of LPFs of poly(γbenzyl-L-glutamate)(PBLG), visualized by means of atomic force microscopy. Upon continuous compression, a fibrillar morphology was formed with a surface decorated by squeezed-out islands. By contrast, stepwise compression promoted the formation of a fibrillar network with a bimodal distribution of fibril diameters, caused by merging of fibrils. Finally, isobaric compression induced in-plane compaction of the monolayer. We correlate these morphological observations with the kinetics of the corresponding relaxations, described best by a sum of two exponential functions with different time scales representing two molecular processes. We discuss the observed kinetics and the resulting morphologies in the context of nucleation and growth, characteristic for first-order phase transitions. Our results demonstrate that the preparation conditions of LPFs have tremendous impact on ordering of the molecules and hence various macroscopic properties of such films.

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INTRODUCTION In general, Langmuir polymer films (LPFs) have much higher mechanical, thermal, and chemical stability than their counterparts of low molecular weight compounds with amphiphilic nature and are therefore of interest for various applications. The behavior of LPFs has been intensively studied in the context of controlled self-assembly of homo- and block copolymers,1−3 rheology of monolayers,3−5 glass transition behavior of polymeric systems in 2D- or quasi 2D-confinement,6,7 and structure formation in biological systems.8,9 Monroy and coworkers made pioneering contributions to investigate the dynamic behavior of Langmuir films of flexible polymers prepared by subsequent material deposition to increase the surface density.10−12 They showed that the air−water interface represents good solvent conditions for their chosen polymers [poly(vinyl-acetate) and poly(tert-butyl acrylate)]. Drop-wise, i.e., slow, addition of polymer solution should therefore enable most of the segments of the polymer chains to attach to the water surface, allowing the system to reach a state close to equilibrium. The dynamics of those equilibrated films in the © XXXX American Chemical Society

semidilute regime were probed by small perturbations (e.g., through transient compression steps or oscillatory movements of the barriers) with the aim to test the concept of reptation of polymer chains in quasi 2D systems. These studies also addressed the rheological features of LPFs. In a more recent publication,13 it was shown, that the rigidity of the backbone of the polymer chain plays an essential role in this respect. In addition, the lack of significant attractive interactions with the water surface makes it more difficult to achieve equilibrated films. Such stiff polymer chains are thus prone to adopt collapsed conformations because cohesive intra- and intermolecular forces dominate their behavior. For a multitude of polymeric Langmuir films nonequilibrated conformations were observed. In our study, we therefore concentrate on films out of equilibrium due to their chemical architecture. Additionally the surface concentration was changed through the movement of Received: December 12, 2014 Revised: May 21, 2015

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DOI: 10.1021/acs.langmuir.5b01212 Langmuir XXXX, XXX, XXX−XXX

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discussed together with morphologies of the corresponding transferred films. The next section will describe the results obtained when the compression was performed in well-defined steps. A phenomenological mathematical description of the relaxations is also targeted. Compression under isobaric conditions and the corresponding results will be discussed in the last part of the experimental section. Our data show clearly that for films transferred at identical areas per molecule, these different modes of compression yield different morphologies ranging from fibrillar to compact textures. The observed morphologies were correlated with the surface pressure relaxations and will be discussed in the context of nucleation and growth of ordered domains. As a main conclusion from our results, we propose a novel method for tuning the monolayer compaction through controlled relaxation steps, expressed via a parameter which we call the “monolayer free area”, which in turn governs the self-assembly behavior of LPFs.

two compressing barriers forcing the molecules even further out of equilibrium. As LPFs are prepared on a liquid surface, sufficient mobility is provided for the molecules to reorient and thus adjust to the exerted driving forces, for example, resulting from the polarity of the micro environment and the flow profile created during compression. For densely packed films, reorientation processes are expected to take longer than the time available in the course of LPF compression, even when compression is performed at the slowest accessible speed.4 Thus, nonequilibrated polymer structures, which cause residual stresses within the film, will be an unavoidable consequence when Langmuir films are compressed under conventional conditions, i.e., at a constant rate. It is therefore not astonishing that the surface pressure of such an ”overcompressed” film starts decaying as soon as the movement of the barriers is stopped. This decay of the surface pressure in a 2D LPF is analogous to relaxation experiments in rapidly stressed bulk materials where the observed relaxation behavior was associated with various structural reorganization mechanisms. While the pressure decay was observed for various Langmuir films and notably for PBLG,14 its effect on the concomitant morphologies or structures within the LPFs was not the subject of intensive investigations, with the noteworthy exception of some recent reports.1,2,15−17 In this context, we present a systematic investigation of the relaxation behavior of Langmuir polymer films and discuss possibilities to exploit these findings for tuning morphologies of thin polymer films. We chose the well-studied synthetic homopeptide poly(γ-benzyl-L-glutamate)(PBLG) which adopts α helical conformation under appropriate solvent conditions. It is therefore regarded as a molecule with a rod-like core, formed by the peptide backbone, and decorated with voluminous flexible side chains that can weakly interact with the water surface. Monolayers of PBLG have already been studied by several groups in the past,14,18−22 and it is widely accepted that the α helices survive the procedure of spreading and lay down flat on the water surface.14,23,24 In comparison to flexible polymer chains, due to their intrinsic stiffness, rod-like molecules, and assemblies composed therefrom, should exhibit a reduced number of relaxation modes. Along the helical axis an electric dipole moment of 3.4 D per monomer unit is observed.25 The total dipole moment can thus be tuned with chain length. For long PBLG molecules, the formation of fibrillar networks was observed,26,27 and one naturally arising question is to find out if this can be attributed to the strength of the dipole moment. In this study, we used a small molecule of only 22 repeat units (number-average), thus exhibiting only moderate dipolar interactions among the helices. We will explore if the interconnectivity of the fibrillar network gets lost when the total dipole moment is reduced. One goal was to obtain a monolayer composed of relatively simple building units which thus can equilibrate rapidly by excluding complex cooperative relaxations processes. For our studies, we designed experimental protocols which allowed the system to relax under welldefined conditions. Snapshots of morphologies, taken by means of atomic force microscopy (AFM) after various relaxation steps, were carefully analyzed. The results will be discussed and compared with results from commonly performed compression experiments where two barriers were moved continuously at a constant speed. The paper is organized in the following way. After a brief description of the used techniques, the isotherm resulting from continuous compression will be presented and

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MATERIALS AND METHODS

Materials. PBLG with an average number of 22 units (1H NMR end group analysis), corresponding to a number-average molar mass of 4920 g/mol, an apparent dispersity of 1.15 (SEC; eluent: N-methyl-2pyrrolidone, calibration: poly(methyl methacrylate)) was synthesized by ring-opening polymerization of γ-benzyl-L-glutamate N-carboxyanhydride using 1-hexylamine as the initiator in N,N-dimetheylformamide solution at room temperature, as described by Krannig et.al.28 The PBLG was precipitated into methanol, dried in vacuum at 60−70 °C, and then dissolved in chloroform, which is known to be a helicogenic solvent.14,22 PBLG in helical conformation is mostly treated as a stiff rod with a mean diameter of 15 Å. However, light scattering studies suggest that, especially for long molecules, a certain semiflexibility of the rods should be considered which can be describe by a “stiffness related length” which was termed “persitence length”.29−31 The pitch of an intact helix is made of 3.6 monomer units. The here chosen PBLG with 22 repeating units can form 6 helical turns. According to Papadopoulos et al.,32 a stable helix needs a minimum of 5 turns (18 monomer units), thus we expect that the here studied molecules form helices. Langmuir Technique. A solution of 60 μmol/L PBLG in chloroform was spread onto the water surface of a Langmuir trough built by Riegler and Kirstein. Low boiling point and low surface tension of chloroform facilitated the spreading of molecules on the surface of high purity water. The whole setup was kept inside a closed Plexiglas box for reducing contamination of the water surface and noise due to acoustic vibrations. To allow for complete solvent evaporation, the isotherm measurements were started after a waiting time of 30 min. The surface pressure (Π) was measured using the Wilhelmy technique. A small piece of completely wetted filter paper was used as a Wilhelmy plate. The temperature of the subphase was maintained at 20 °C by circulating thermostated water through the base plate of the trough. The PBLG molecules on the water surface were compressed by movable barriers and the surface pressure was recorded as a function of area and/or time. The films were compressed at different compression rates and it emerged that a compression rate of 5 cm2/min is free of artifacts (refer to SI Figure-S1). Hence, all the films discussed in the manuscript were compressed at a rate of 5 cm2/ min, which corresponds to 10 Å2/min, when 100 μL of 0.3 mg/mL solution were spread on the trough having a surface area of 192 cm2 which are experimental parameters typically used in our experiments. Substrate Preparation. Silicon wafers from Silchem Handelsgesellschaft GmbH (Freiberg, Germany) were cut into small pieces (2 × 1.5 cm2). These pieces were immersed in Piranha solution (3:1 mixture of H2SO4 and H2O2, respectively) for 30 min and subsequently rinsed 3 times with high purity water. The wafers were then dried by a flow of dry nitrogen. The procedure for forming selfassembled monolayers of octadecyltrichlorosilane (OTS) was adapted from Rozlosnik et al.33 Freshly cleaned wafers were immersed for 45 min in a freshly prepared solution of 500 μmol/L OTS in n-heptane B

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Langmuir allowing for a covalent attachment of OTS molecules to the hydrophilic silicon substrate. Exposure to moisture was avoided to suppress the formation of OTS aggregates in solution. Unbound and loosely bound OTS molecules were washed off by immersing the OTS-coated wafers in pure n-heptane for 15 min. After withdrawal, the wafers were again dried by a flow of nitrogen gas and stored in a dry desiccator until use. Transfer. The films were transferred from the water surface to the OTS coated silicon substrates, through the Langmuir−Schaefer method, also known as horizontal lifting.34,35 The substrate was positioned parallel to the water surface by clamping to a motor driven holder above the water surface. The substrate was approached to the water surface at the lowest speed to avoid large perturbations of the water surface, which could create modifications of the morphology of LPF formed at the water surface. After a contact time of 60 s the substrate was retracted. This time interval is sufficient to account for a damping of the surface waves created through the approaching wafer and should therefore reduce possible artifacts due to the transfer protocol (refer SI Figure-S2). During the transfer procedure, the wafer substrate never immersed into water. The surface of PBLG helices is basically hydrophobic with side chains of minor polarity sticking out to all sides. The change in polarity at the air−water interface might induce some bending of the side chains but the expected hydrophobic interaction with the OTS molecules on the wafer provide sufficient adhesion, as we observe transfer ratios of about 1. After retraction of the wafer, the bottom side, which was initially in contact with water, becomes water repelling as it dries off quickly. As long as the film stays in this quasi 2-dimensional geometry and no material piles up above or below this plane, inversion of structure due to the transfer should not occur. The obtained films showed high stability for periods of several months. Atomic Force Microscopy and Image Analysis. Atomic force microscopy (AFM, JPK, Germany) was used in tapping/intermittent contact mode for acquiring the surface morphology of the samples. Cantilevers (with a resonance frequency of 150−160 kHz) holding tips with a radius of curvature ∼10 nm were used. All images were measured at a scan frequency of 1−3 Hz and at a high set point ratio (representing a low tapping force) in order to avoid both tip-induced structural changes and artifacts due to tip dragging. For a free amplitude of 1 V, the set point was usually varied between 0.7 and 0.9. Each sample was imaged several times and at several positions to ensure that the images were representative. The positions chosen were always far from the edges of the substrate to avoid regions containing artifacts introduced during transfer via the water meniscus. The obtained images were analyzed by JPK image processing software. All images were presented after flattening but without any further processing. We have also calculated the surface coverage of the films from their respective AFM images using Gwyddion software36 by converting the images to binary maps. The threshold was chosen in such a way that the monolayer appeared bright and the background substrate was dark. An analogous procedure with two thresholds was followed for evaluating the coverage of films having a two-layer morphology.

Figure 1. (a) A characteristic isotherm, surface pressure (Π) versus area per monomer unit, of PBLG-22 molecules is shown. (b) The morphology of the film transferred at 24 Å2 is shown together with a representative height trace (c). Domains with diameters of 25 ± 3 nm can be seen. The size of the image is 0.4 × 0.4 μm2. A0, A1, and A2 are explained in the text.

which is considered as a gaseous phase for many systems as reported in the literature.1,4,6,15 Figure 1(b) shows the morphology of the film transferred at an area of 24 Å2 resulting from material deposition only, i.e., without conducting any movement of the barriers. The formation of slightly elongated molecular aggregates with a typical diameter of 25 ± 1 nm and a height of 2.5 ± 0.4 nm is revealed (refer to Figure 1(c)). The formation of these aggregates is the result of a complex interplay between the propensity of the molecules to selfaggregate due to dipolar- and π−π-stacking interactions on one hand and a thinning/spreading process of the film solution that eventually leads to a dewetting process during solvent evaporation on the other hand. A careful inspection of the obtained structures (Figure 1(b)) shows some indications of the beginning of local alignment of these molecular aggregates. We consider dipolar interactions between the helices (with a significant dipole moment of 3.4 D/monomer) as the major driving force for the observed self-assembly. The comparatively small aspect ratio of 2 (assuming 1,5 nm for the diameter of the helix and a length of the molecule of ca. 3 nm) most likely induces the observed anisotropy and directionality of the aggregates. The typical width of the aggregates corresponds to a bundle of about 15 helices. Initially, the surface pressure remained zero even when reducing the area per monomer by compressing the film (to values still higher than A0 in Figure 1). This is due to incomplete coverage of the surface. On reducing the area even further (i.e., upon increasing the surface concentration), to values below A0, the pressure started to increase with surface concentration in a nonlinear fashion. This monotonous increase in surface pressure is generally interpreted as an increase in compaction and is expected to saturate when a full monolayer coverage is obtained. On compressing further, the isotherm deflects from the monotonous increase and exhibit a plateau like behavior.4,6,14,22 Figure 2(a) shows the morphology of the monolayer transferred at 18.5 Å2, the point where the deflection occurs. The presence of fibrillar structures (diameter of ca. 25 nm and a height of 2−3 nm) can be seen. The inset of Figure 2(a) clearly shows the internal structure of the fibrils and a characteristic period of their diameter of ∼25 nm, which is similar to the size of the aggregates observed in the gaseous phase. Hence, the marginal local alignment of the aggregates observed in the gaseous phase was enhanced upon compression, resulting in fibrillar structures. It is important to

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RESULTS AND DISCUSSION PBLG−Isotherm under Continuous Compression. The initial and crucial step in preparing a Langmuir polymer monolayer is the spreading of a dilute polymer solution onto the water surface. After solvent evaporation, the water surface will be populated with insoluble polymer molecules, which can be compressed by movable barriers. The surface tension of water (γ0) decreases when the film is formed and thus exerting a surface pressure, Π = γ0 − γ, where γ is the surface tension of the water covered with the polymer. Figure 1 shows the surface pressure isotherm, which records the evolution of surface pressure as a function of the surface area. In order to interpret the isotherm correctly, it is important to explore first the state of the molecules on the water surface right after spreading, C

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Figure 2. Morphologies of films transferred at 18.5 Å2 (a,d), 12.7 Å2 (b,e), and 11 Å2 (c,f) are shown. Images with different sizes are shown to highlight both (i) ordering of the monolayer (1.5 × 1.5 μm2) and (ii) the coverage by the second layer (10 × 10 μm2). A representative height trace for indicating the dimensions of the fibrils is also shown. Inset of a: Morphology of (a) at a smaller scale (0.2 × 0.2 μm2) to highlight the substructure of the fibrils.

note that fibril formation generates large empty areas between fibrils i.e., a large “monolayer free area”. We have analyzed Figure 2(a) to quantify the fraction of “free area” in the monolayer and found that 30 ± 2% of the film surface was empty, i.e., not occupied by fibrils. This observation is in contrast to earlier reports on similar rod-like polymers like poly-L-alanine and poly-L-leucine22 and coil-like polymers,4,6 where a complete coverage was claimed when reaching the plateau pressure. Incidentally, these reports have quantified the coverage either by performing numerical calculations based on the A1 values in the isotherm or have used measurement techniques with comparatively low spatial resolution like Brewster angle microscopy. Given the dimensions of the fibrils observed in our system, Brewster angle microscopy could not have sensed their presence as the film would look homogeneous given a lateral resolution of the order of the wavelength of light. Upon compression beyond the area A1, a plateau-like region became apparent. Such plateau regions were observed previously for PBLG14 and also for low molecular weight organic molecules like surfactants,37,38 fatty acids,39 and lipids.40 Plateau regions are usually interpreted as zones of phase coexistence. For PBLG molecules (of rather high molecular weights), this region was reported to correspond to a “monolayer to bilayer” transition.14,22 However, as shown in Figure 2(b) and (c), the morphologies of the films transferred at 12.7 and 11 Å2 (close to the high density boundary of the plateau) suggest a different interpretation. There, the bright regions correspond to a second layer (with a total height of 6− 7 nm) grown out of the monolayer (darker background). This second layer covered the monolayer only partially and in a random fashion. In contrast to the underlying monolayer, this second layer did not depict well aligned structural elements. In addition, we could observe that the fibrillar character of the monolayer was lost in the morphology of the film transferred at 11 Å2 (Figure 2(c)). Thus, we conclude that compressing

through the plateau does not transform a monolayer into a complete and homogeneous bilayer. To obtain a better understanding, we have studied the morphologies of films transferred within the whole range of the plateau at a larger scale (10 × 10 μm2). Figure 2(d−f) show the morphologies of the films transferred at 18.5, 12.7, and 11 Å2, respectively. The bright dots in the background of the dark monolayer seem to be formed from material being squeezed out from the monolayer. The height of the bright islands was 6−7 nm. From Figure 2(d−f), it becomes clear that the number of these islands increased upon compression within the plateau region. To quantify this increase in island number, we have performed a statistical analysis of the images (Figure 2(d−f)) and extracted the percentage of surface covered by these islands. Even at 11 Å2, the percentage of the area covered by such squeezed out surface islands was found to be 15% only. So, for our case of PBLG-22, it is obvious that we did not observe the formation of a homogeneous second layer even at the high density end of the plateau. However, the number of islands increased along the plateau (ref Figure 2). Upon further compression to areas smaller than A2, the layer formed on the water surface is believed to collapse by forming irregular threedimensional assemblies, buckles, and cracks. From our observations (not shown here), we can confirm that an irreversible collapse takes place producing random structures which were not considered any further in this study. In this section, we have witnessed two striking observations viz., (i) ∼30% ”free area” at A1, where a complete coverage is expected in literature and (ii) only 15% coverage by the second layer (in form of islands) even at 11 Å2. In this context, it is natural to ask the following questions: Is it possible to reduce the ”free area” in the monolayer? Can we increase the coverage of the second layer? Will the orientation of the fibrils of the first layer be preserved in the second layer? Let us discuss these questions in the forthcoming sections. D

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Figure 3. (a) Isotherms of surface pressure (Π) versus area per monomer for stepwise compression measurements are shown. At each step, the barriers were stopped and the pressure was allowed to relax for 1000 s. The traces of three independent measurements with a different number of relaxation steps are depicted. The inset shows the same curves with shifted (Π) values for clarity. (b) To highlight the time scales involved in the process, the evolution of Π is presented as a function of time for a stepwise compression (black line) and for a continuous compression (red line) experiment.

Figure 4. Morphology of films transferred at 12.7 Å2 from a continuous compression (a,e) is shown and compared to the morphology of films obtained by stepwise compression with one (b,f), three (c,g), and six (d,h) steps. Images with different sizes are presented to highlight the bimodal distribution of the thickness (15 × 15 μm2) and to demonstrate the conservation of orientation of the fibrils, as opposed to a continuous compression (2 × 2 μm2).

Step-Wise CompressionMerging of Fibrils. Compressing a polymeric film at a constant rate up to high packing densities will trap molecules in metastable states because the molecules cannot reorient fast enough to attain their minimum in free energy. This metastability of Langmuir monolayers is reflected in a decay of the surface pressure with time, once the film is kept at a constant area (by stopping the compression process). Instead of compressing the film monotonously, it can also be compressed in a sequence of well-defined steps, interrupted for a finite period of time to allow for relaxations after each step. In this section, we discuss such step-wise compression experiments where a finite waiting time of 1000 s was allowed after each step. Interesting influences of this experimental protocol on the morphologies should be expected in the plateau region. To ensure that the monolayer did not store stress during initial compaction, an additional relaxation step was performed for 500 s at an area close to A1 (at 20.9 Å2) where equilibrium could be attained rapidly. Figure 3(a) shows a typical surface pressure isotherm (Π vs area per monomer). In Figure 3(b), the corresponding evolution of Π with time is shown. In these experiments, relaxations were not recorded long enough to reach the final equilibrium value, because the

main focus was put on short time scale processes. For systematically tuning the pathway, three different protocols were chosen as shown in Figure 3(a), i.e., isotherms with one, three, and six intermediate steps before compressing to the final transfer area of 12.7 Å2. To determine the differences in terms of morphology caused by these three different pathways, all films were transferred at 12.7 Å2 and compared to results for a conventionally (monotonously) compressed monolayer. Fresh monolayers were prepared for each measurement to avoid interference with effects created by previous manipulations. Figure 4 compares the resulting AFM images on two different scales, 15 × 15 μm2 and 2 × 2 μm2. As all films were transferred at the same area per monomer, these films only differed in the chosen relaxation pathway. For monotonous compression, the second layer appeared amorphous with randomly distributed aggregates covering barely 10% of the film (Figure 4(a)). However, the underlying monolayer still showed fibrillar character (Figure 4(e)). On comparing morphologies of a nonrelaxed film (Figure 4(a)) with a film transferred after one step (Figure 4(b)), it is remarkable to observe that already after only one intermediate relaxation step a layer of comparatively large fibrils E

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concentrated on investigations of the relaxation processes and the changes in morphologies caused by these relaxations. Surface Pressure Relaxations. Figure 6(a) shows the normalized surface pressure Π/Πs,max (Πs,max is the maximal pressure at the beginning of the relaxation process, s denotes step compression) as a function of time. These traces were recorded after the last relaxation in stepwise compression experiments and therefore some influence of prior relaxation steps is expected. The figure shows that at smaller areas per monomer the surface pressure relaxed at a faster rate. As can be seen in Figure 3(b), relaxations were not allowed to attain a steady Π-value, they were stopped by starting a subsequent compression. This instantaneously creates a jump in Π until the plateau value is recovered and the course of the isotherm is retraced until the next relaxation step is initiated. The time interval allowed for relaxations between each compression step was kept constant at 1000 s which is not long enough to obtain a full decay of the relaxation processes. Therefore, some degree of metastability was passed on to subsequent relaxation steps. To disentangle this complexity, we have carried out relaxation studies for films obtained by continuous compression at various mean molecular areas. Each relaxation experiment was carried out on a freshly prepared monolayer. This allowed us to understand the dependence of relaxations on lateral packing density. Figure 6(b) shows the decay of the normalized pressure (Π/Πc,max; where Πc,max is the maximal pressure at the beginning of the relaxation process, c denotes continuous compression) with time. With increasing compression the film becomes more compact as the “free surface area per monomer” gets smaller and thus the rearrangement of the molecules is constrained. This is clearly reflected in the prolonged duration to approach steady Π values for increased compactness. To investigate the behavior of the relaxation curves, we have plotted the surface pressure decay corrected for the background (Π−Π∞) on a logarithmic scale as a function of time as indicated in Figure 6(c). Two straight lines with different slopes can be detected (dashed lines in Figure 6(c)), hinting to the existence of two relaxation processes at different time scales. For a simple relaxation scenario, like for bulk liquids, the spectra should be modeled by an exponential function with a single relaxation time as described by (eq 1). However, the log−linear plot strongly suggests the use of a model taking a two step process into account. Figure 6(d) shows modeling performed with both, a single and a double exponential function (eqs 1 and 2).

grew out of the matrix of the underlying monolayer with much slimmer fibrils. Interestingly, approximately the same percentage of the surface was covered by either aggregates or larger fibrils after both compression protocols, i.e. with and without an intermediate relaxation step. When the number of relaxation steps was increased to 3 (Figure 4(c)) or to 6 (Figure 4(d)), we could observe that the area covered by the thicker layer was increasing accordingly. The area coverage after 6 relaxation steps was as large as 52 ± 2%. On closer inspection of these images (Figure 4(f−h)), it became clear that the regions with larger fibrils resulted from coalesced fibrils and were not the result of squeezed out material as it was observed for films transferred after monotonous compression. A dotted ellipse is drawn in Figure 4(f) to guide the reader where larger fibrils have formed by merging of slimmer ones. It should also be noted that the thickness of the fibrils observed in Figure 4 was ca. 4 ± 1 nm within the thinner layer and ca. 8 ± 1 nm within the thicker layer. Step compression promoted the growth of a thicker layer which also spread out over larger areas and eventually might evolve into a quasi 2D-like morphology. While the merging of the fibrils became evident from the images (Figure 4) we also attempted to quantify the changes in size of the fibrils. Figure 5 shows the power spectral density (PSD), i.e., the plot represents the azimuthally averaged FFT intensities as a

Figure 5. Power spectral density, PSD versus k, for stepwise compressed films is compared with a continuously compressed film. The interfibrillar distances corresponding to the changes of slope (as indicated by the dashed lines) are also displayed.

function of the wave vector k (= 1/distance). While the PSD of a continuously compressed film showed a change in slope corresponding to ∼40 ± 3 nm, the films obtained by stepwise compression showed an additional slope change corresponding to ∼140 ± 5 nm. The value of 40 nm represent the interfibrillar distance between fibrils with diameters of ∼25 nm, while the additional change in steepness at the larger length scale implies thickening or coalescence of the initially formed fibrils, thereby changing the interfibrillar separation. Thus, the step compression mode enhanced merging of the fibrils. Increasing the number of steps led to an increase of the percentage of the film area covered by larger fibers which did not significantly change their enlarged width. However, it is not yet clear which driving force was responsible for this merging process, causing a transition form a quasi-two-dimensional monolayer to a more three-dimensional structure. To gain further insight, we

Π(t ) = Π 0e−(t − t0)/ τ + Π∞

(1)

Π(t ) = Π1e−(t − t0)/ τ1 + Π 2e−(t − t0)/ τ2 + Π∞

(2)

where Π0 and τ are the strength and characteristic time for the relaxation in eq 1, while Π1(2) and τ1(2) are the relaxation strengths and relaxation times for the respective relaxations in eq 2. During the fitting routine, the time t0 is fixed at 100 s, to account for the time required for damping the surface fluctuations caused by the sudden stop of the barriers.1,15 It is clear that the double exponential function describes the data better than a single exponential function (cf. Figure 6(d)). The analysis of the data clearly show that the observed relaxations are not due to a single reorganization mechanism, but consists of a combination of two different processes and therefore fitting with eq 2 is justified. The deduced parameters are displayed in Table 1. F

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Figure 6. Normalized pressure decay (at various areas within the plateau of the isotherm) is shown for both (a) stepwise and (b) continuous compression. Two ways to analyze the data are shown: (c) Log−linear plot of the surface pressure decay corrected for the background (Π−Π∞) as a function of time. (d) Description of the surface pressure relaxation (Π vs t at 18.5 Å2) with single (blue line) and double (red line) exponential functions are shown. Πs,max and Πc,max are defined in the text.

Table 1. Fit Results of the Relaxation Spectra from Figure 6(b)a

a

area (Å2)

Π (mN/m)

Π∞ (mN/m)

τ1 (s)

τ2 (s)

Π1 (mN/m)

Π2 (mN/m)

Dr

16.3 15.7 10.5

8.75 8.9 11.1

5.94 4.87 4.07

829 ± 110 665 ± 70 632 ± 80

10332 ± 800 12935 ± 600 12970 ± 500

1.49 ± 0.1 2.59 ± 0.2 2.85 ± 0.25

1.22 ± 0.12 1.61 ± 0.15 3.88 ± 0.4

0.31 ± 0.02 0.46 ± 0.04 0.62 ± 0.05

Dr = (Π1+Π2) /(Π1+Π2+Π∞) is the dynamic range of the relaxation, where Π∞ is the minimum value to which the surface pressure is decayed.

Figure 7. Morphologies of films transferred at 18.5 Å2 after continuous compression and a relaxation time of trel = 0 s(a) and trel = 10 000 s (b) are shown. The white regions (encircled by dotted ellipses) depict the onset of thickening of fibrils, which could potentially act as nuclei for further growth. Inset: Morphology of (b) at a comparatively large scale (5 × 5 μm2) showing the distribution of such nuclei. (c) Morphology of a film transferred at 12.7 Å2, after a single step compression.

aligned parallel to each other. Our AFM measurements clearly show that the lateral self-aggregation of the fibrils leads to fairly parallel alignment with respect to the immediate neighbors. However, on the larger scale the formation of micrometer sized domains with a preferred fibrillar orientation is observed (see Figure-S3 and S3a in the SI). Within a single domain, parallel alignment should be accomplished relatively fast because the orientations of the fibrils do not deviate much from the one of the director. At the boundaries of such domains, however, fibrils with very different orientations meet and substantial reorientation or even rotation of a number of fibrils might be

With increasing compactness, no obvious trend for the relaxation times τ1 and τ2 can be observed. However, the dynamic range (Dr) seems to increase as the area per monomer is reduced. What kind of structural reorganizations can be attributed to theses observations? The large values of the relaxation times (Table 1) suggest slow cooperative phenomena rather than processes involving individual molecules. As one potential process, we may consider the coalescence of fibrils leading to growth processes in the lateral and vertical direction with respect to the plane of the monolayer. In order to merge efficiently, fibrils should be G

DOI: 10.1021/acs.langmuir.5b01212 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

Figure 8. (a) Surface pressure Π as a function of area per monomer is shown. The activation of isobaric compression (ISB) is indicated by a vertical dashed line. (b) Evolution of surface pressure with time is also shown. The different waiting times prior to film transfer are indicated with colored arrows and proper numbers. The corresponding morphologies are shown in Figure 9. (c) Area reduction versus waiting time during isobaric compression is displayed together with the double (continuous line) and single (dashed line) exponential fits for the data (open symbols).

shown that manipulating the relaxations has tremendous impact on the final morphology of the film. It is thus natural to ask if relaxation kinetics can be manipulated in such a way, that a decrease of the ”free monolayer area” occurs. In this context, we have counteracted the pressure decay and maintained a constant pressure i.e., the monolayer was compressed under isobaric conditions (Figure 8). In this experiment, isobaric conditions were switched on when an area of 18.5 Å2 was reached through continuous compression. Figure 8(a) shows the isotherm in a pressure−area presentation and in Figure 8(b) the same isotherm is depicted in a pressure-time presentation. The vertical dashed line (at area 18.5 Å2) marks the activation of the isobaric compression mode (ISB). The targeted value for the constant surface pressure was set to 8 mN/m and during the time period when ISB was activated (20 000 s), the area was compressed from 18.5 to 13 Å2 (see Figure 8(c)). It was observed, that the rate of compaction was not linear in time, but exponentially decreasing. Similar to the pressure relaxation, the area compaction was modeled with a double exponential function (continuous red line in Figure 8(c)) and the evaluated relaxation times were τ1 = 800 ± 80 and τ2 = 8200 ± 300 (see Figure 8(c)). These values are again too large to reflect reorganization processes on the molecular scale. However, they are a bit smaller than the ones observed for continuous compression which are given in Table 1. In our opinion, they also describe reorientation and coalescence of fibrils within domains and across domain boundaries for a system which was forced to densify after admitting only limited Π relaxations caused by pressure adjustment in isobaric compression. To observe the changes in morphology, we have transferred films after performing isobaric compression after different waiting times. As stated before, each transferred film was obtained from a freshly deposited Langmuir layer. Figure 9 shows the morphologies of films transferred after various

necessary which should occur on a longer time scale. Thus, we assign the two time regimes to be dominated by coalescence of fibrils within domains for short times and across boundaries of adjacent domains for the longer time scale. The mentioned observation, that the dynamic range Dr increases with compression could also be explained within this model. At elevated packing densities, a comparatively large number of domains were pushed together by means of continuous compression without relaxation steps in between. Therefore, a variety of relaxations within domains of different sizes and across domain boundaries should occur simultaneously. In order to strengthen our proposed model, further supporting measurements using a different experimental protocol were performed. Therefore, we have transferred a film at a large area of 18.5 Å2 after 10 000 s of relaxation (before the second slower relaxation process started to dominate, refer to τ2 in Table 1). The morphology is shown in Figure 7(b). The presence of small domains of increased thickness (increased brightness) can be seen. To guide the reader, dotted circular lines are drawn around some of these small domains. The inset in Figure 7(b) is a larger image (5 × 5 μm2) of the same area, where the distribution of such domains is clearly visible. These regions could act as nuclei for further growth, especially when molecules are supplied abundantly. In addition, these regions showed signs of orientation and order and thus were different from the squeezed out rounded islands observed in continuous compression experiments. To understand the growth of fibrils with about twice the initial thickness during a step compression experiment, let us compare the morphologies of the nonrelaxed monolayer (Figure 7(a)) with the ones of films transferred after a long time relaxation (Figure 7(b)) and a sample transferred after a single step in a step compression experiment (Figure 7(c)). It can be seen that upon relaxation merging of fibrils started from nucleating centers and compression assured that sufficient material was provided for consequent growth. Consequently, we propose a nucleation and growth process for fibril coalescence, which takes place during relaxation and consecutive compression. In step compression experiments, this process will be repeated with every step and hence allows for enhancing the lateral coverage of the merged fibers. Thus, the length scales of regions of ordered and assembled molecules can be tuned by manipulating relaxations within Langmuir polymer film. Isobaric CompressionTuning the “Free Area” in a Monolayer. As described earlier, for the continuous compression experiment, the surface coverage of the monolayer is of the order of 70%, even at 18.5 Å2, where a complete coverage would be expected. In the previous section, we have

Figure 9. Morphologies of films transferred at 8 mN/m after isobaric compression of 0 (a), 500 (b), 3000 (c) and 10 000 s (d) are shown. The corresponding areas per monomer are 18.5, 16.8, 14.2, and 13.1 Å2, respectively. The increase in compactness of the film with longer waiting time is obvious. The size of all images is 1 × 1 μm2. H

DOI: 10.1021/acs.langmuir.5b01212 Langmuir XXXX, XXX, XXX−XXX

Langmuir isobaric compression times in comparison with a nonrelaxed film. The morphology of the film transferred after 500 s (cf. Figure 9(b)) shows small changes like perturbations in the arrangement of the fibrils and associated variations in height, as indicated by dotted circles. On increasing the isobaric compression time to 3000 s, the presence of a compact region was witnessed (Figure 9(c)). The size of this compact region increased with isobaric compression time as shown by the morphology of the film transferred after 10 000 s (Figure 9(d)). To further evaluate the extent of compaction, we have analyzed the morphology of the film transferred at 10 000 s and found a coverage of 92 ± 2%. In the isobaric compression mode, the changes in the initial fibrillar structure started by small perturbations (500 s), followed by the formation of nuclei which grew upon further compaction, i.e., with isobaric compression time increasing from 3000 to 10 000 s. The fraction of the remaining “free area” within the Langmuir polymer film was inversely proportional to the waiting time of the ISB-mode. This opens up new possibilities for tuning the “free area” of a polymer monolayer. The dependence of macroscopic properties like the rheological behavior, the glass transition temperature and mechanical properties on the fraction of “free area” is already established for bulk materials.41 Hence, these experiments provide analogous opportunities for tuning the macroscopic properties of the monolayer.

ACKNOWLEDGMENTS

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REFERENCES

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SUMMARY AND OUTLOOK In conclusion, we have demonstrated that through manipulations of the relaxational pathways, morphologies of LPFs can be tuned from fibrillar to compact or from a homogeneous monolayer to a patchy composite bilayer-like structure. The observed compaction during isobaric compression was correlated with the waiting time. Thus, isobaric compression allowed tuning the fraction of “free area” within polymer monolayers. This opens up new possibilities to influence the thermo-mechanical properties of polymer thin films with thicknesses of the order of molecular dimensions. The tunability of the layer morphologies can be of vital importance for adjusting macroscopic behavior of a polymer film, like its glass transition temperature, rheological behavior, and optical properties. Further work on flexible coil-like polymers like poly(vinyl acetate) (PVAc) is in progress in order to verify if the proposed approach can be generalized to other polymer architectures. ASSOCIATED CONTENT

S Supporting Information *

Morphology of the films transferred after different compression speeds, different contact time during transfer, effect of relaxation time on the morphologies, and a higher resolution image of a fibril is shown. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01212.

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The authors thank the “Competence Network of Functional Nanostructures” financed through the Landesstiftung BadenWürttemberg for financial support. The authors are also thankful for the financial support provided by the Sino-German Research Centre under the project “Concepts for controlling nucleation in systems of biodegradable polymers”. The authors are grateful for the fruitful discussions with members of the International Research Training group (IRTG)Soft Matter Science, in particular to Prof. Jörg Baschnagel.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 761 203 5863. Fax: +49 761 203 5855. E-mail: [email protected]. Notes

The authors declare no competing financial interest. I

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