Structure–Rheology Relationship in Weakly Amphiphilic Block

Article pubs.acs.org/Langmuir

Structure−Rheology Relationship in Weakly Amphiphilic Block Copolymer Langmuir Monolayers Giovanni Li Destri,*,† Fausto Miano,‡ and Giovanni Marletta† †

Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN), Department of Chemistry, University of Catania and CSGI, V.le A Doria 6, 95125 Catania, Italy ‡ Novartis Farma S.p.A. Strada Provinciale Schito Torre Annunziata (NA), Napoli, Italy S Supporting Information *

ABSTRACT: The linear viscoelastic behavior in the low-frequency regime at the water/air interface of three different polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA) copolymer monolayers, with block length ratio varying from 66−33 to 50−50 and 25−75 in molecular units, was studied and related to the interfacial behavior, characterized by means of Langmuir isotherms, and their structure, characterized by means of the atomic force microscopy technique. The two monolayers with the highest PMMA amount showed a single phase transition at about 12 mN/m, the viscoelastic behavior changing from a predominantly elastic to a viscoelastic one. This change in the viscoelastic properties was ascribed to the beginning of entanglement among the PMMA coronas of the predominantly circular quasi-2D micelles formed by the two copolymer systems. Conversely, the polymer with the lowest PMMA amount, despite having the same PMMA block length of the PS−PMMA 50−50 block copolymer, was found to behave as a viscoelastic system at any surface pressure value. This characteristic behavior cannot therefore be simply related to the molecular weight difference, but it has been put in connection to the irregular micelle structure observed in this case, consisting of a mixture of spherical and wormlike micelles, and to the different conformation adopted by the PMMA block. By blending this copolymer with an immiscible elastic homopolymer, namely poly(2-vinylpyridine), it was possible to tune the micelle nanostructure, obtaining regular circular quasi-2D micelles, with viscoelastic properties as expected for the PMMA-rich copolymer monolayers. To the best of our knowledge, this study shows for the first time the explicit dependence upon the relative block length and, in turn, upon the nanostructure of the quasi-2D micelles, of the viscoelastic properties of Langmuir monolayers and suggests that molecular weight and intermolecular interactions are not the only parameters governing the polymer conformation and, in turn, the polymer rheology and dynamics in quasi-2D confined systems.

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INTRODUCTION Polymer thin and ultrathin films, i.e., with thickness lower than 100 nm, represent a fascinating system to study the structure and dynamics of macromolecules in a confined environment as a function of thermodynamical parameters. Thus, polymer films on solid substrates have been thoroughly studied in the past decade, and many phenomena, such as nanoconfinement effects on the glass transition temperature1 as well as on characteristic phase separation2−4 and crystallization processes, have been recently found.5−11 A very interesting class of nanoconfined polymer ultrathin films is represented by Langmuir polymer films,12 i.e., polymer monolayers at the air/water interface, representing the extreme case of confinement and generally considered as quasi-2D systems. Langmuir monolayers of amphiphilic block copolymers have been shown to be able to assemble in morphologies and phaseseparated nanostructures which cannot be obtained in bulk or with other thin film preparation methodologies.13−17 Moreover, © 2014 American Chemical Society

these morphologies can be easily varied by simply tuning some simple experimental parameters such as composition,18−26 blocks length,27,28 pH of the subphase,29 spreading solvent,30 and concentration of the spreading solution.31,32 These properties make block copolymers ideal candidates for nanopatternig via the bottom-up technique and as surfactants in biomedicine. However, in most of the quoted articles, the characterization of the monolayers was limited to their morphology and interfacial behavior, i.e., their Langmuir isotherm, so that little information regarding the intermolecular interactions, and their role played during the film formation and compression, was reported. By the way, intermolecular interactions are believed to play a key role in driving the assembly at the water/film interface especially in the case of weakly amphiphilic block copolymers, in which the hydrophilic Received: November 13, 2013 Revised: March 7, 2014 Published: March 12, 2014 3345

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extensively studied and is well-known to form quasi-2D surface micelles whose shape and dimensions can be easily tuned by varying the relative blocks length.27 The viscoelastic properties of such a system have been studied too with the help of surface quasi-elastic light scattering (SQELS)42 which has shown the effect played by the molecular weight on the rheology of the monolayer as well as, because of the presence of the quasi-2D micelles, a significant deviation of the viscoelastic behavior from the one observed for pure PMMA monolayers at low surface pressure values (10 kHz), which is expected to be much higher than the characteristic relaxation times of block copolymer films.33,43 Therefore, the low frequency viscoelastic characterization is expected to be more sensitive to the variation of the degree of the entanglement and to the film nanostructure. To this aim, the low-frequency viscoelastic properties of several monolayers through the oscillating drop technique (ODT) have been studied on both air/water44 and oil/water interfaces.45 This experimental work was aimed to provide detailed characterization of the viscoelastic properties of different PS− PMMA block copolymers, combining ODT measurements with Langmuir isotherms and atomic force microscopy (AFM) images. The dependence of rheology on both surface concentration and block copolymer composition has been studied in detail. To minimize the effect of the molecular weight on the film rheology,42 three block copolymers with different blocks length ratio but similar molecular weight were chosen. Moreover, a clear relationship between interfacial rheology and film structure has been found, thus allowing to modulate the viscoelastic characteristic of the Langmuir monolayers by simply controlling the micellar structure by adding an immiscible amphiphilic homopolymer, poly(2vinylpyridine) (P2VP).

block is not water-soluble and is thoroughly confined at the water/air interface, so that strong variations of the intermolecular interactions are expected upon compression.27 In the very last years, in order to shed light on such interactions, attention has been focused on the viscoelastic characterization of block copolymer Langmuir films19,33 to better understand both the interactions between macromolecules and the subphase and the interactions among the macromolecules. It is to note that almost all the papers published in this field deal with strongly amphiphilic block copolymers, i.e., with a watersoluble hydrophilic block. More pronounced viscoelastic transitions are expected for weakly amphiphilic block copolymers since, as already mentioned, the interactions among water insoluble hydrophilic blocks are not shielded by the water subphase. Moreover, homopolymer and block copolymer Langmuir monolayers are widely used as model membranes to investigate their interactions with biological molecules such as proteins34 and phospholipids35−37 in view of the development of new polymer-based drug carriers and tissues. It is known that the interactions between the polymer and the biomolecules alter significantly the viscoelasticity of the membrane;36 therefore, understanding the correlation between the viscoelastic properties and the intermolecular interactions in a polymer-based Langmuir monolayer will help in clarifying the molecular processes involved when polymer membranes are in contact with biomolecules. Moreover, to engineer new polymer-based membranes, it is necessary to be able to tune their viscoelastic properties since the materials, depending on their application, will be subjected to very different mechanical stresses. Our results will show how it is possible to tune the viscoelastic properties of block copolymer Langmuir monolayers by simply blending it with an immiscible homopolymer. Finally, very recently, the viscoelastic characterization of polymer Langmuir monolayers has allowed to get new insights into the polymer motions which, from a fundamental point of view, are extremely intriguing to better understand the dynamics of macromolecules in a quasi-2D confined system. These seminal studies, which have found new fascinating phenomena such as 2D polymer reptation,38 have shown how the dynamics of the polymers is strongly related to the interactions with the water subphase,39 which may act, depending on the polymer nature, as a “good” or “bad solvent”, and by the polymer molecular weight,40 both affecting the quasi-2D polymer conformation and, in turn, the motion of the confined macromolecules. These new findings are also of extreme importance to shed light on fundamental properties of polymer ultrathin films such as their glass transition temperature, whose behavior as a function of the degree of confinement is still under debate.41 However, so far, only the dynamics of homopolymer monolayers, i.e., of homogeneous layers, was studied, but it is expected that, in the case of nanostructured monolayers, the rheological behavior, and therefore the polymer motion, can change in view of the different conformations the polymer may adopt by changing the nanostructure. It is therefore of extreme interest to study the rheology of block copolymer monolayers with different nanostructure but similar molecular weight in order to shed light on how the nanostructure affects the conformation of the active block. A weakly amphiphilic block copolymer, which is expected to be suitable for the above-described goal, is polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA) which has been

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EXPERIMENTAL SECTION

Three PS−PMMA block copolymers were purchased from Polymer Source (Dorval, Canada); poly(2-vinylpyridine) was purchased from Polyscience Inc. (Warrington, PA). The relevant properties of the copolymers employed in the experiment are reported in Table 1.

Table 1. Mass Average Molecular Mass (Mw), Polydispersity Index (PDI), and PS/PMMA Ratio of the Polymers Employed in This Work Mw (103 Da) ID P722-SMMA P719-SMMA P720-SMMA poly(2vinylpyridine) 3346

monomer units (PS/PMMA ratio) 1410/707 (2:1) 698/709 (1:1) 456/1404 (1:3) 2285

PS

PMMA

PDI

146.7 72.6 47.4

70.7 70.9 140.4

1.11 1.09 1.11 1.7

240

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Interested readers can find more extensive explanation of the ODT theory and explicative drop shapes in ref 44.

All the polymers were purified by dissolving them in chloroform (HPLC grade from Sigma), then by precipitating them by adding methanol (HPLC grade from Sigma), and then by filtrating them. Chloroform was used as solvent in view of its low boiling point; polymers solutions for Langmuir and ODT experiments had concentrations between 0.1 and 1 mg/mL. An LB KSV 5000 minitrough was employed for Langmuir−Blodgett (LB) films preparation and isotherm recording. The surface pressure sensor is a platinum Willhelmy plate which was washed with acetone and Millipore water before each experiment. Ultrapure Millipore water (18.2 MΩ) was used as subphase. Polymers were dissolved in chloroform and then spread on the surface with 10 and 100 μL Hamilton syringes. Monolayers were compressed and expanded at 2 mm/min controlled barriers speed. The area per monomer was calculated by dividing the recorded area per molecule by the number of monomers of each block copolymers. The LB films were deposited on cleaved mica which was immersed below the water surface before spreading the polymer solution and then, upon reached the target surface pressure, gently lifted at a speed of 1 mm/min. After the film deposition the samples were left drying in air. Height and phase images of LB films were recorded with a Multimode/Nanoscope IIIa (Digital Instruments) in tapping mode. Commercial silicon tapping cantilevers were employed (Digital), with pyramidal tips having a curvature radius of 10 nm and a nominal internal angle of 35°. During analysis cantilevers were oscillating at their resonance frequency close to 300 kHz. In order to enhance the phase contrast and to ensure the reproducibility of the AFM images, all the images were recorded at a low set point value (0.9 V). ODT apparatus and theory have been described elsewhere.44 In oscillating drop experiments, a Millipore water sessile drop is kept in a constant temperature and controlled humidity cell and recorded by a CCD camera. After drop stabilization 0.02−0.3 μL of polymer solution was put in contact with it in order to spread on it and to form a stable monolayer whose surface pressure was measured by measuring the drop shape and applying the Young−Laplace equation, which describes the surface tension of a drop subjected to gravity:

γ=

ρgD12 , H(s)

s=

D1 D2

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RESULTS In Figure 1, the surface pressure vs area per monomer curves of each of the three block copolymers are reported. The shapes of

Figure 1. Langmuir isotherms of the three PS-b-PMMA monolayers. The area per monomer is reported to minimize the differences due to the different molecular sizes.

the isotherms for the two copolymers with higher PMMA amount are quite similar, and it is possible to find some general trend: between 0 and 12 mN/m there is the first stroke, with lower slope, which is separated by a kink, i.e., by a phase transition, from the second steeper stroke which lasts until the film collapse. Conversely, the P722-SMMA monolayer does not show any phase transition as the surface pressure increases steadily up to the film collapse. The collapse pressure for the three films has different values and increases by increasing the PMMA content. However, it is worth to mention that the collapse pressure does not scale linearly with the PMMA content, thus suggesting a different conformation of the PMMA blocks spread on the water surface with changing the relative blocks length. The PMMA amount also affects the area per monomer at which the surface pressure starts increasing: once again the area per monomer value increases with the PMMA amount. This trend can be justified by assuming that the onset of the surface pressure changes mostly depends on the length of the PMMA block spread at the air/water interface, while the PS block, according to the literature,27 is in a coiled conformation and exposed to air. In this picture, the surface pressure is attributed to conformational variations or entanglement degree variations of the PMMA blocks. All the isotherms show also a hysteresis cycle, suggesting that, indeed, the film compression is an irreversible process. However, by stopping the compression before the phase transition, which appears as a kink at a surface pressure of 12 mN/m, the hysteresis disappears for the two polymers with highest PMMA amount while it is still present for P722-SMMA (Figure 2), in agreement with the results of Seo et al.27 We therefore expect a strong change in viscoelastic properties of P719-SMMA and P720-SMMA block copolymers below and above the kink. Figure 3 shows the typical morphology of PS−PMMA block copolymers monolayers transferred onto a solid substrate. These macromolecules are known to form quasi-2D surface micelles, which are characteristic features of amphiphilic block copolymers Langmuir films,28,29,46−49 formed by an hydro-

(1)

where D1, the equatorial diameter, is the maximum width of the drop and D2 is the diameter drawn at a distance D1 from the bottom of the drop. The factor H(s) accounts for local drop shape over a broad range of D1/D2. The surface tension is then converted into surface pressure Π as the difference between the surface tension of pure water (72.8 mN/m at 20 °C) and the surface tension of the monolayer. The drop was submitted to periodic oscillations with frequency between 0.005 and 2 Hz with a typical area change (dA/A) between 0.01 and 0.03. The CCD camera recorded 50 pictures/s of the oscillating drop, which were then processed to obtain the geometric parameters and the surface tension of the drop at each frame. In particular, the sinusoidal area change of the drop is described by

A = A 0 + ΔA sin(ωt )

(2)

where ω is the applied frequency. The variation of the area leads to a variation of the interface concentration of the copolymer and, in turn, to a change of the surface pressure which follows the same sinusoidal trend with a delay angle φ

Π = Π 0 + ΔΠ sin(ωt + φ)

(3)

From this variation it is possible to determine the complex interfacial dilatational modulus E*

E* = −

dΠ d ln(A)

(4)

From this modulus the elastic component (E′) and the viscous one (E″) can be obtained from the equation

E* = E′ + iωE″ = E cos φ + iE sin φ

(5) 3347

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by FFT (Figure 3c). This is a typical phenomenon for high PS amount PS−PMMA block copolymers: it is in fact supposed that the enthalpy gain from aggregation of PS blocks is high enough to overcome entropy loss from the PMMA alignment on water interface and allows the formation of bigger aggregates with more macromolecules.27 By shortening the PS block length such gain decreases, and it is not able to compensate the entropy loss from PMMA block alignment: therefore, bigger aggregates are not formed.50 The pressure-dependent different packing can be seen by analyzing the AFM phase images reported in Figure 4 which

Figure 2. Langmuir isotherms of the three PS-b-PMMA monolayers arrested before the phase transition at 12 mN/m. The P722-SMMA shows, unlike the two copolymers with higher PMMA amount, hysteresis.

Figure 3. AFM height images of LB monolayers of P720-SMMA (A), P719-SMMA (B), and P722-SMMA (C) deposited on mica at 15 mN/m. Below each image the corresponding fast Fourier transform is reported, as it is evident P720-SMMA and P719-SMMA monolayers show a certain reciprocal order while P722-SMMA monolayer does not because of the very irregular shape of the micelles.

Figure 4. AFM phase images of P719-SMMA LB monolayer deposited on mica at 10 mN/m (A) and at 15 mN/m (B). At 10 mN/m the bright PMMA corona of each single micelles is still visible around the PS dark core while at 15 mN/m PMMA forms a homogeneous matrix, thus suggesting high entanglement among the coronas. A schematic sketch of this transition is reported in (C, D) where the PS core of the micelles is drawn in blue while the PMMA coronas are red. Below 12 mN/m the micelles are isolated, and voids among them are observed (C); above 12 mN/m the highly entangled PMMA coronas form a homogeneous layer below the PS cores (D).

phobic PS core, exposed to the air interface, and an hydrophilic PMMA corona given by the PMMA blocks spread on the water surface.50 The number of macromolecules forming each single micelle is known to be strongly dependent upon the block length ratio varying from hundreds of molecules for PS-rich polymers to few molecules for PMMA-rich ones.27 Micellar structure is in agreement with the above-described characteristic isotherm shape for PS−PMMA block copolymers monolayers, which, in this framework, is due to conformational changes or entanglement degree changes of the PMMA blocks, while the PS ones are coiled and exposed to air. Under surface compression, the polymer organizes in micelles at the subphase/air interface, and their number increases by increasing the surface pressure up to 7 mN/m; above this value Seo et al. have found the density of micelles to be constant.27 The shape and dimensions of micelles are relatively constant and molecular weight dependent for the two copolymers with higher PMMA amount, leading to a quite regular pattern, as shown by the fast Fourier transform (FFT) of the P720-SMMA block copolymer (quasi-hexagonal packing) (Figure 3a), while P719-SMMA shows a less regular pattern (Figure 3b), which still keeps some preferential repetition direction, probably due to the lower regularity in micelles shape. For P722-SMMA, smaller circular aggregates and bigger wormlike ones are instead simultaneously present which causes an irregular and isotropic pattern as clearly shown

display the different stiffness of the monolayer domains. Indeed, phase images clearly show a darker stiff PS core and a bright soft PMMA corona. Single coronas are visible for each micelle (Figure 4a) below the kink, while above it, we observed hydrophobic cores on a compact more hydrophilic matrix (Figure 4b). In Figure 4c,d a schematic sketch of the micelles below and above the kink is reported. Moreover, the height of PS cores is independent of surface pressure, for the three polymers, and comparable with the height of the corresponding PS random coil (Table 2). The heights have been calculated by applying the equation r = lN1/2

(6)

where r is the end-to-end vector, N is the number of PS monomer, and l is the length of a single PS monomer. From this argument, it is clear that no changes in conformation of PS block take place upon compression, suggesting that PS does not contribute to the isotherm slope and viscoelastic behavior of the films. Only the conformation of the PMMA blocks and the 3348

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by Saulnier et al.,51 it is possible to obtain information about the relaxation time (τ), the conservative interfacial elasticity (Ee), and the dissipative one (Ene) with the three following equations:

Table 2. Average Height of the PS Cores of the Quasi-2D Micelles as a Function of the Relative Blocks Length (Top) and Calculated End-to-End Distance of the PS Block Following Eq 6 (Bottom) P720-SMMA

P719-SMMA

P722-SMMA

5.0 ± 0.6 nm 4.5 nm

6.3 ± 0.4 nm 5.5 nm

8.6 ± 0.6 nm 7.9 nm

lim E′(ω) = Ee

(7)

lim E′(ω) = Ee + Ene

(8)

ω→ 0

ω→∞

interactions among them have therefore to be taken into account to describe the rheology of the monolayers. The viscoelastic behavior of pure PS−PMMA block copolymers monolayers has been studied by means of the oscillating drop technique (ODT). The measurements have been performed in such a way to approach the experimental conditions of the Langmuir experiments. The surface pressure in each experiment was set by dosing the amphiphilic polymer at the drop surface so that the viscoelastic properties of the various systems, before (Figure 5a) and after (Figure 5b) the phase transition, could have been determined.

E″(ω) = Eneτ (9) ω→ 0 ω Here τ is the time the system needs to reach equilibrium after the external deformation, Ee is the energy that the system releases after the deformation to restore the initial conditions, and Ene is the dissipated energy. For the two polymers with higher relative amount of PMMA below the first kink (Table 3) we found relaxation times of few lim

Table 3. Conservative Interfacial Elasticity (Ee), Dissipative Interfacial Energy (Ene), and Relaxation Time (τ) of the Three Block Copolymer Monolayers below and above 12 mN/m P720-SMMA P720-SMMA P719-SMMA P719-SMMA P722-SMMA P722-SMMA

at at at at at at

10 15 10 15 10 15

mN/m mN/m mN/m mN/m mN/m mN/m

Ee (mN)

Ene (mN)

τ (s)

34 86 17 60 65 207

19 430 9 216 15 64

40 40 40 40 200 200

tens of seconds and conservative and dissipative interfacial elasticity of tens of mN/m with a constant Ee/Ene ratio of about 2 for both copolymers. After the phase transition, a typical viscoelastic behavior is evident: φ, E′, and E″ increase and the loss modulus values show a clear dependence from frequency (they increase with increasing the frequency) while the E′/E″ ratio decreases to values lower than 5, thus suggesting that the viscous component increases with compression more than the elastic one. As expected from the hysteresis in the isotherm of P722SMMA block copolymer monolayer, a viscoelastic behavior in the whole surface pressure range (Figure 5a) is observed. Both storage and loss moduli are higher than the moduli observed for the case of the other two monolayers, also showing a low E′/E″ ratio (∼5) which remains constant, unlike the PS− PMMA 50−50 and PS−PMMA 25−75 monolayers in which a decrease E′/E″ ratio with increasing the surface pressure is observed. Since the P722-SMMA monolayer is viscoelastic at any surface pressure values, despite having the length of the PMMA surface active block very close to one of the P719-SMMA block copolymer, it is clear that the viscoelastic properties of amphiphilic block copolymers markedly depend on the relative blocks length, and in turn on the nanostructure of the monolayer, and not only on the molecular weight of the surface active block, as previously reported.40,42 To the best of our knowledge, this is the first report on such dependence for these systems. We propose that the block length dependence of the viscoelastic behavior found for the systems of this study is

Figure 5. Elastic and viscous moduli of P720-SMMA, P719-SMMA, and P722-SMMA monolayers at 10 mN/m (A) and 15 mN/m (B). By increasing the surface pressure, a sharp increase of the viscous moduli of P720-SMMA and P720-SMMA monolayers is observed.

The experimental data showed a predominantly elastic behavior for P719-SMMA and P720-SMMA monolayers below the first kink, while viscoelastic behavior is observed above it. Before the phase transition, we in fact observed low delay angles φ, high ratios E′/E″and no marked frequency dependence of the viscous component. For both P719-SMMA and P720-SMMA below 12 mN/m the ratio E′/E″ is roughly 10. By applying the model proposed 3349

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than in the case of pure block copolymer monolayers, no intermicellar interactions are possible, and the interfacial behavior is mainly governed by the P2VP compression. Only above the plateau at 15 mN/m micelles are close enough to interact (Figure 7c), and as a consequence, the isotherm shows a behavior similar to the one showed by the pure block copolymer monolayers. As expected, the dramatic changes in film morphology also reflect very different viscoelastic behavior (Figure 8). Indeed, a

mainly due to the wormlike micellar structure of the P722SMMA monolayer. In order to verify this hypothesis, we changed the structure of P722-SMMA quasi-2D micelles by diluting the copolymer with an immiscible homopolymer. In order to be employed as a simple diluting agent, the homopolymer must fulfill two fundamental requirements: (1) it must be immiscible with both the types of blocks, in order not to interfere with the phase separation process which generates the micelles, and (2) it must behave in a purely elastic way, in order not to affect the viscoelastic properties of the block copolymer monolayer. P2VP has been chosen, as it fulfills both the requirements (see Supporting Information). An equimolar blend of P722-SMMA with P2VP was prepared; the Langmuir isotherm (Figure 6) allows to identify

Figure 8. Elastic and viscous moduli of P722-SMMA/P2VP equimolar blend monolayer at different surface pressures. Below 20 mN/m very low values (few mN/m) of the loss modulus are observed, while at 20 mN/m, values comparable with the E′ ones are recorded.

purely elastic behavior is observed in the first upward stroke (5 mN/m) while on the plateau (15 mN/m) a certain increase of φ is already observed while relaxation time and dissipative interfacial energy are still zero. Above 15 mN/m the usual viscoelastic behavior appears in agreement with the isotherm shape and with the AFM images which show a strong entanglement among PMMA coronas, as confirmed by the lowering of the E′/E″ ratio (∼2) as well as by the appearance of a characteristic relaxation time and of high values of dissipated energy (Table 4).

Figure 6. Langmuir isotherm of the P722-SMMA/P2VP equimolar blend. In the inset the Langmuir isotherm of the pure P2VP monolayer is reported.

a first stroke very similar to P2VP isotherm and a second one very similar to the block copolymer one, thus confirming the immiscibility between the two macromolecules. This immiscibility is also figured out from the AFM images. When the block copolymer are mixed to P2VP, the observed surface micelles appear smaller and with a much more regular shape than the pure block copolymer case, the effect of P2VP being, indeed, to prevent, at low surface pressures, the aggregation of many macromolecules and the formation of big wormlike micelles. AFM height images (Figure 7) show the

Table 4. Conservative Interfacial Elasticity (Ee), Dissipative Interfacial Energy (Ene), and Relaxation Time (τ) of the P722-SMMA/P2VP Equimolar Blend Monolayer below the 15 mN/m Plateau, at the Plateau, and above the Plateau P722-SMMA:P2VP 1:1 10 mN/m P722-SMMA:P2VP 1:1 15 mN/m P722-SMMA:P2VP 1:1 20 mN/m

Ee (mN)

Ene (mN)

τ (s)

12 17 76

0 0 919

0 0 20

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DISCUSSION The comparison among Langmuir isotherms, nanomorphology, and viscoelastic properties allows to unravel the interfacial behavior of LB monolayers of PS−PMMA block copolymers and, in particular, to shed light on the involved intermolecular interactions. As it is well-known,27 the only block spreading at the air−water interface, and thus responsible of any interfacial transition, is PMMA. Therefore, in the case of P720-SMMA and P719-SMMA, the only phase transition observed at 12 mN/m is due to the onset of interactions among the coronas of the surface micelles which cause the entanglement among the approaching PMMA blocks. Indeed, below 12 mN/m, micelles

Figure 7. AFM height images of P722-SMMA/P2VP equimolar blend monolayer deposited on mica at 10 (A), 15 (B), and 20 mN/m (C). The PMMA coronas on top of the P2VP matrix are clearly visible.

fine structure of the diblock copolymer micelles: in particular, the bright PS cores surrounded by the PMMA crowns are clearly visible, especially in Figure 7c, while below them the P2VP forms a homogeneous dark layer. This morphology further confirms the expected immiscibility between the two polymers. As the distance among these micelles is much higher 3350

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in such isotropic system leading, for instance, to the breakup of some micelle which of course will increase the viscous behavior of the system. As a consequence, the film shows similar viscoelastic behavior both below and above 12 mN/m (Figure 5) without any marked change of the E′/E″ ratio. It is also worth to note that the frequency dependence of the viscous modulus is strikingly different for this system with respect to the two others, especially above 12 mN/m (Figure 5b). This trend, i.e., the decrease of the viscous modulus with increasing the frequency, leads to longer relaxation times (Table 3) and describes the different relaxation mechanism related to a more disordered system without any regular pattern. It is worth to stress that the PMMA block length of P722-SMMA is very close to the one of the P719-SMMA and, therefore, that the observed different rheology is not a molecular weight effect. This suggests that the nanostructure of the monolayer directly affects the spreading behavior of the surface active block and, in turn, its conformation. A confirmation to this hypothesis comes from the analysis of the E′/E″ ratio of the P722-SMMA monolayer which, unlike the other two monolayers, does not show any decrease with increasing the surface pressure, thus suggesting that no entanglement among PMMA coronas takes place. This behavior can be justified by taking into account that the longer PS block makes the P722-SMMA more hydrophobic and, therefore, that the water−air interface acts as a “bad solvent”.39 As a consequence, the PMMA block adopts a coiled conformation which prevents the entanglement among coronas even at high surface pressure values. Finally, the lack of entanglement is suggested by the Langmuir isotherm which does not show any phase transition at 12 mN/m. However, by modulating the structure of P722-SMMA quasi2D micelles, we succeeded to modulate its viscoelastic properties too and to obtain rheological properties similar to the ones showed by the P719-SMMA and P720-SMMA monolayers. This modulation was possible by simply blending the block copolymer with a an immiscible homopolymer in order to reduce the number of macromolecules involved in the formation of the micelles, thus avoiding the formation of the big wormlike ones. Moreover, the P2VP spreads below the P722-SMMA micelles and reduces the interface free energy with the subphase, thus allowing the PMMA to adopt a more expanded conformation. The so-obtained circular micelles do not show any viscoelastic behavior at low surface pressure values (Figure 8) since, as it is clear by AFM images (Figure 7a,b), they are too far to entangle. Moreover, because of the lack of entanglement, the shape of the Langmuir isotherm is identical to the one of pure P2VP. A dramatic transition is however observed when compressing the monolayer above 16 mN/m: the entanglement among the micelles increases (Figure 7c) and affects both the shape of the isotherm and the viscoelastic properties. It is worth to observe that, above 16 mN/m, the complex modulus of the blend monolayer shows the typical frequency dependence of the other two pure entangled macromolecules monolayers with higher PMMA amount (Figure 8). The characteristic frequency dependence of the complex modulus of pure P722-SMMA monolayer, as well as the very high relaxation time, has therefore to be related to the irregular structure of the quasi-2D micelles and therefore suggests that different polymer dynamics are involved when changing the film nanostructure. In conclusion, the structure and the rheology of the monolayers are very tightly connected and lead, in the case

are far from each other (Figure 4a), and only by compressing the system above the phase transition, the entanglement among several PMMA coronas takes place (Figure 4b). The ODT results perfectly agree with the above-described hypothesis. Below 12 mN/m these two copolymers form predominantly elastic monolayers with very small E″ (