Microstructure and Microtribology of Polymer ... - ACS Publications

Chapter 22

Microphase Separation and Morphological Transitions at the Surface of Block Copolymers 1,4

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Ph. Leclère, J. L. Brédas , G.Moineau ,M.Minet ,Ph.Dubois ,R. Jérôme , and R. Lazzaroni 1

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Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: December 10, 1999 | doi: 10.1021/bk-2000-0741.ch022

Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Place du Parc 20, B-7000Mons,Belgium Centre d'Etude et de Recherche sur les Macromolécules (CERM), Institut de Chimie, Bât B6, Université de Liège, B-4000 Liège, Belgium Service des Matériaux Polymères et Composites(SMPC),Université de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium 2

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Tapping-Mode Atomic Force Microscopy is used to study the microphase separation in thin films of symmetric triblock copolymers synthesized via a two-step « living » radical polymerization of nbutylacrylate and methylmethacrylate (MMA). This straightforward synthetic pathway allows for strict control of molecular weight, molecular weight distribution, and composition. The presence in the copolymer of immiscible segments covalently bound to each other leads to phase separation on the nanometer scale. Measuring the phase of the oscillating cantilever simultaneously with the topography allows us to determine the microdomain morphology at the surface. Here this approach is applied to all-acrylate (PMMA-b-poly-n-butylacrylate-bPMMA) thermoplastic elastomers. The data point to a strong contrast in the local mechanical properties, corresponding to the microphase morphology. Cylinders of the minority phase are found to orient perpendicular to the surface, due to the surface energy difference between the constituents. Lamellae are also arranged perpendicular to the surface, in contrast to what is usually observed in block copolymers. This particular orientation is thought to result from the symmetric character of these triblock systems, with the outer blocks more polar than the central sequence. The increasing importance of block copolymers arises mainly from their unique properties in solution and in the solid state, which is a consequence of their molecular structure. In particular, sequences of different chemical composition are usually incompatible and therefore have a tendency to phase segregate. The microdomain formation process in the solid state is directly related to the specific molecular 4

Corresponding author.

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© 2000 American Chemical Society

Tsukruk and Wahl; Microstructure and Microtribology of Polymer Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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architecture, which can be designed by using existing monomers or polymers. Block copolymers can produce numerous phase-separated nanostructures that present high fundamental and technological interest [1]. Block copolymers have been prepared classically via living anionic polymerization, as pioneered by Szwarc [2]. This opens the way to afinetailoring of synthetic polymers and copolymers in terms of precise control of molecular weight, molecular weight distribution, molecular architecture, and chain-end functionalization. Due to the thermodynamic immiscibility of their constitutive components, block copolymers combine the intrinsic properties of the parent homopolymers along with the additional benefit of new properties appearing in relation to the phase morphology. For instance, combining thermoplastic and elastomeric sequences into block copolymers, called thermoplastic elastomers (TPEs) [3], results in rubber materials with thermally reversible crosslinking (the crosslinked centers being the thermoplastic domains below their glass transition temperature). Commercially-available TPEs prepared by anionic polymerization are typically of the polystyrene-polydiene-polystyrene type of triblock copolymers (such as polystyrenepolybutadiene-polystyrene, SBS). The work presented here stems from the need to improve the processability and thermoresistance to degradation of TPEs, relative to the SBS copolymers of the first generation. Because of the temperature limitations imposed by both the poor thermal and oxidative resistance of polydienes and the relatively low glass transition temperature T of the polystyrene end blocks, a major loss of strength is usually observed above 60°C. Consequently, there is much interest in: (i) replacing the polystyrene blocks with thermoplastic polymers with higher values of T ; and (ii) building elastomeric sequences which better resist oxidation. These two requirements can be fulfilled by acrylate-based polymers: on one hand, the presence of an alkylchain (at least four carbon long, i.e., /i-butylacrylate (Λ-BUA)) on the ester groups provides elastomeric properties, with a strongly reduced possibility of oxidation or crosslinking; on the other hand, predominantly syndiotactic polymethylmethacrylate) PMMA shows a T which can be as high as 125°C, Le., significantly higher than that of polystyrene (T = 100°C). We have therefore synthesized a series of such copolymers associating poiyalkylacrylate elastomeric sequences and polymethacryiate thermoplastic sequences. g

g

g

g

Here, we report on the synthesis and characterization of PMMA-poly(/ibutylacrylate)-PMMA (MBuM) symmetric triblock copolymers, as a new family of thermoplastic elastomers. These compounds have been prepared by a novel route based on controlled radical polymerization [4]. Compared to "classical" anionic living polymerization [5], this new route, sketched in Scheme la, appears particularly appealing since the triblock copolymers are prepared in a two-step process instead of the usual three steps required in anionic polymerization. In the latter case, as Λbutylacrylate cannot be polymerized via a living process, the MBuM symmetric triblocks have to be synthesized by sequential copolymerization of teri-butylacrylate (tBuA) and MMA followed by the transalcoholysis of the tBu esters with Λ-butanol


358 (Scheme lb). An added advantage of the two-step controlled radical polymerization is the possibility to work in much less stringent experimental conditions (higher reaction temperature; no need for strictly anhydrous medium).

Controlled Radical way

Anionic way

H ι Br— C


Lidiarrion +tBuA THF, -78°C LiCl

Li

H / CttjCHj— C ~ Br

diethylmeso-2,5-dibromoadipate α-BuA NiBr (PPh3>2 toluene, 85°C 2

PtBuA Li THF, -78°C MMA

-Br

Br-

isolation of the difunctional poly(n-BuA)

poly(MMA-b-tBuA-b-MMA)

MMA NiBx (PPh3)2 toluene, 85°C 2

a-butanol p-toluenesuUbnic acid, rcfkix

poly(MMA-b-nBuA-b-MMA)

pol>(MMA-b-nBuA-b-MMA) (b)

(a) Scheme 1

The performances of these new materials as thermoplastic elastomers are intimately related to the extent of phase separation and the resulting microstructure. It is thus of prime importance to characterize in detail the microscopic morphology of thin films of the block copolymers described above. Atomic Force Microscopy (AFM) and derived techniques have recently appeared as powerful tools for the morphological characterization of polymer surfaces. In this work, Tapping-Mode (TM) AFM [6] is used and particular attention is devoted to measurements of the dephasing of the cantilever oscillation relative to the signal exciting the piezo driver. This approach, called "phase detection imaging-PDI", is a very sensitive technique to probe the local mechanical properties at the surface [7-10]; it is thus well adapted to investigations of microphase separation in TPEs.


359 Experimental Section The synthesis of the block copolymers is described in detail elsewhere [4]. The MBuM macromolecules were synthesized from a poly(n-butylacrylate) difunctional macroinitiator. NiBr2(PPh3)2 and diethyl meso-2,5-dibromoadipate were introduced in a glass tube equipped with a magnetic stirrer. Vacuum-nitrogen cycles were repeated to remove residual molecular oxygen. n-Butylacrylate was added under nitrogen via a syringe and the tube was placed in a oil bath maintained at 85°C. After 24 hour polymerization, the residual monomer was extracted by pumping and the polymer was recovered with a 70% yield. The molecular weight of the macromolecular initiator ( M = 63,000; M / M = 1.1) was determined by Size Exclusion Chromatography (SEC) calibrated with PMMA standards. This difunctional macroinitiator and NiBr2(PPh3)2 were then introduced in a round-bottom flask equipped with a magnetic stirrer. Toluene was added and the solution was stirred until complete dissolution of the macroinitiator. MMA was introduced under nitrogen and the reaction mixture was heated up to 85°C for 24 hours. The copolymer was finally recovered by precipitation from methanol. The copolymers presented in this study are described in Table 1. Thin films of the copolymers (typically 500 nm-thick) were prepared by solvent casting from a 2mg/mL toluene solution on a freshlycleaved mica substrate. Such a thickness was chosen in order to ensure that: (i) the film surface is smooth enough (thicker films tend to be rougher and the topographic contrast can perturb the phase image); and (ii) the morphology is not influenced by specific interactions with the substrate, as is the case when the thickness is of the same order of magnitude as the microdomain size [11]. (here, the film thickness is about 15 times larger than the domain size). Moreover, PDI images on 1 mm-thick samples, albeit affected by topographic features, qualitatively show the same morphology as the 500 nm-thick deposits, confirming that the observations on the latter are not influenced by the substrate. Toluene is chosen as solvent since it is a good solvent for both components; no selective precipitation is thus expected to influence the morphology. The samples were first analyzed after evaporation of the solvent at room temperature. In order to investigate the effect of annealing above the T of PMMA on the surface morphology, they were then treated at 140°C under high vacuum (10 Torr) for 48 hours.


n

w

n

g

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Table I. Description of the copolymers considered in this study. Sample A Β

PBu M„ 63,000 63,000

PBu MJM

n

1.11 1.11

PMMA M n

14,500 30.000

Copolymer MJM n

1.18 1.3

Volume %

PMMA 30 47

All AFM images were recorded in air with a Nanoscope III microscope from Digital Instruments Inc. operated at room temperature in Tapping Mode (TMAFM), using microfabricated cantilevers with a spring constant of 30 Nm"l. The system is


360 equipped with the Extender TM Electronics Module to provide simultaneously height and PDI cartography. Images of each sample were taken at several locations with scanning time of about 5 minutes. The A / A Q value was set to 0.95, where Ao is the free oscillation amplitude and A is the setpoint amplitude selected for the measurement. Repeated scans indicated that the observed structures were stable. All images were recorded with the maximum available number of pixels (512) in each direction. The images presented here were culled from several recorded files. For image analysis, the Nanoscope image processing software was used. The data were notfilteredand are shown as captured. SP

s p


Results and Discussion Figure 1 compares the topographic and PDI images, obtained simultaneously on the same area of the surface, for a thin film of MBuM containing 30 % of MMA (see Table 1, sample A). While the topographic image is featureless, which indicates that the surface is flat on a lateral scale of micrometers (the RMS roughness is typically 1-2 nm), the PDI image is highly contrasted: it shows an homogeneous distribution of bright, round-shaped objects in a darker matrix. In TMAFM, the phase contrast originates from local differences in tip-surface interactions, occurring as die tip probes chemicallydifferent domains. The PDI image thus constitutes clear evidence that microphase separation has taken place in this system. It must be stressed that PDI-TMAFM appears as a unique tool for investigating the microscopic morphology of such compounds since, on the one hand, the electron density difference between the different monomer units is too small to lead to significant contrast in X-ray scattering measurements and, on the other hand, no selective staining agent is available, which precludes morphological characterization by means of transmission electron microscopy (the only results reported on the phase separation of these materials are comingfromsolid state NMR studies [12]). Over the last couple of years, the origin of the phase contrast in TMAFM has been actively investigated [8-10, 13-17]. It has been shown that the contrast is related to the local dissipation of the energy brought to the surface by the oscillating tip [16]. It thus strongly depends on the details of the tip-sample interactions. In turn, these interactions depend on the intrinsic properties of the analyzed material, in particular its elastic and viscoelastic moduli, but also on the mechanical properties of the tip and on the existence of tip-sample adhesion. Quantitative measurement of the sample mechanical properties on the local scalefromPDI-TMAFM data is therefore a very delicate task. Nevertheless, a qiialitative interpretation of the PDI images in terms of the spatial distribution of domains on an heterogeneous surface is possible. In particular, in the "soft tapping" regime we used, i.e., when the amplitude of the oscillating cantilever is only slightly reduced (for instance, for Aep/Ao = 0.95) upon interaction with the surface, it has been shown that the magnitude of the phase shift is directly related to the elastic modulus of the sample [9, 16]. On this basis, we assign the bright spots in Figure lb,



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Figure 1. TMAFM height (a) and phase detection (b) images ( l x l μπι ) of the 14,500-63,000-14,500 MBuM copolymer (Sample A). The Root Mean Square roughness of the height image is about 1.6 nm illustrating the flatness of such a thin film. The phase contrast shows the morphology of the copolymer. The inset shows that the PMMA domains are locally arranged in a centered hexagonal structure. 2



362 which correspond to a larger phase shift, to domains of die harder, thermoplastic component (PMMA), while the darker matrix, showing a smaller phase shift, is made of the softer, elastomeric component (poly(n4mtylacrylate)). This assignment has been confirmed by recording approach-retraction curves over the different microdomains [18]. As reported previously [14, 17], we observe that decreasing the A^/Ao value leads to the reversal of the contrast in the phase image and the appearance of contrast between domains in the topographic image, due to changes in the mechanisms governing the tipsample interaction. Data analysis indicates that the mean diameter of the PMMA domains is about 30 (+/- 2) nm and the distance between the centers of adjacent spots is around 46 (+/- 2) nm. The chemical composition of sample A corresponds to a 30:70 PMMA/poly(n-butylacrylate) volume ratio; in this composition range, phase-segregated block copolymers are expected to present a "eylinder-in-a-marrix" morphology. The image of Figure lb should thus not represent spheres of PMMA embedded in an elastomeric matrix, but rather PMMA cylinders standing upright, perpendicular to the surface, so that only their apex is visible. The measured diameter/distance ratio is fully consistent with the theoretical value when assuming hexagonal packing of the cylinders. Such an arrangement likely arisesfromthe difference in surface energy between the two components: the surface energies, as estimated with the method described in reference [19], are 43 mJ/cm and 39 mJ/cm for PMMA and poly(/i-butylacrylate), respectively. PMMA being slightly more polar than poly(n-butylacrylate), it tends to be less present at the interface with air; the PMMA cylinders are thus expected to align perpendicular to the outer surface, so that only their upper section is exposed. 2

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Figure lb does not show any long-range hexagonal compact organization of the cylinders, as would be expectedfromtheoretical consideration [20]; nevertheless, on the local scale, PMMA domains are occasionally found to be arranged as centered hexagons (see inset of Figure lb). The lack of long-range ordering may be due to the fact that in such thin films, the evaporation of the solvent is too fast to allow the system to reach thermodynamic equilibrium (even though phase separation takes place effectively). Note also that annealing the film above the T of PMMA does not lead to significant changes in the morphology. Figure 2a contains a PDI image of the same compound as in Figure 1, annealed for 48 hours at 140°C; it shows no clear evolution towards a more regular arrangement of the PMMA domains. g

The annealing experiments are, however, indicative of the occurrence of morphological changes at high temperature. Figure 2b corresponds to a copolymer film annealed at 140°C and then quenched with liquid nitrogen. In contrast, the sample corresponding to Figure 2a was allowed to cool down much more slowly as it was kept in the oven after annealing. The quenched surface consists of a mixture of isolated bright spots, as found before, and elongated bright objects, surrounded by darker areas. The elongated domains appear to be cylinders of PMMA lying flat on the surface; their width (30 +/- 2 nm) agrees well with the diameter of the spots corresponding to cylinders standing upright.



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Figure 2. TMAFM phase detection images (2x2 μην ) of the 14,500-63,000-14,500 MBuM copolymer (Sample A) annealed at 140°C for 48 hours and then either : (a), slowly cooled or (b) quenched with liquid nitrogen.


364 The presence of such cylinders parallel to the surface is likely related to the fact that the cooling rate was too high to allow for complete rearrangement into the vertical configuration. This in turn indicates that the surface morphology at high temperature consists of a mixture of flat and standing cylinders (note that very long annealing treatments followed by quenching do not produce an "all-flat" morphology). This mixed morphology may result from the interplay between entropie (i.e., surface disordering) and enthalpic (i.e., surface energy) factors. Because the difference in surface energy between the components is quite small (43 vs 39 mJ/cm ), it is reasonable to think that the entropie gain corresponding to the formation of a disordered assembly of cylinders can overcome the enthalpic loss of bringing more PMMA to the surface, even at moderate temperatures. It must be noted that mixed arrangements of cylinders have been observed previously, in very thin thin layers of styrene-butadiene-styrene copolymers [21]. This morphology has been explained in terms of film thickness influence. Parallel orientations can only exist at specific values of thickness. If the average thickness of the film is not compatible with the repeat distance, the film develops macroscopic variations in thickness, by a 2-D coarsening process, such that the thicker regions are an integer of the repeat distance. In our opinion, this process is not at work here because the film thickness is much larger than the domain size [22]. It is therefore unlikely that the surface morphology we observe is a reflection of the adsorption of the macromolecules on the mica substrate. Instead, it is more likely to be governed by processes taking place at the polymer/air interface. Further heating to around 200°C leads to complete mixing of the two components (order-disorder transition) [23].


2

Another remarkable feature of Figure 2b is the fact that the poly(/î-butylacrylate) domains adjacent to the flat PMMA cylinders appear darker than the rest of the matrix. This suggests that the local elastic modulus of the matrix, as probed by the AFM tip, is slightly reduced in the vicinity of the flat cylinders. Assuming a tip radius of about 20 nm (which is typical for TMAFM tips), one can imagine that, as it probes a domain of the poly(n-butylacrylate) matrix in a region where PMMA cylinders stand upright, it also interacts laterally with those rigid cylinders, so that the phase shift is not as small (the probed area does not appear as dark on the PDI image) as it would be on a pure elastomeric surface. Conversely, if die soft domains are elongated, they are more easily probed by the tip (there is less disturbance by adjacent PMMA cylinders) and the corresponding phase signal is more typical of their mechanical properties (i.e., those areas appear darker). Along the same line, most of the flat PMMA cylinders appear slightly less bright than their vertical counterparts: as the tip scans the surface, the former are probably not as well supported by die underlying layer as the latter, which form columns. As a result, the local elastic modulus is slightly reduced over the flat cylinders and the phase shift is smaller. Increasing the PMMA content in the block copolymers to around 50% leads to the morphology illustrated in Figure 3. The alternation of dark and bright stripes corresponds to lamellae of the soft and hard components, assembled perpendicular to the surface. This regular lamellar arrangement is characterized by a typical period length of



365

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Figure 3. TMAFM phase detection image ( 2 x 2 urn ) of the 30,000-63,000-30,000 MBuM copolymer film (Sample B). 50 (+/- 2) nm. We also observe the presence of a number of topological defects at the surface: dislocations, corresponding to connections between lamellae, are visible (one such defect clearly appears near the center of the image) and lamellae occasionally bend completely into U-tums"; some locations apparently show a few isolated PMMA cylinder^ standing perpendicular to the surface. The other possible organization of lamellae, with the lamellae lying on top of each other parallel to the substrate, is always observed [24] for films of 50/50 diblock copolymers; in that case, the films organize in such a way that a lamella of the component with smaller surface energy occupies the outer surface (this situation is sketched in Figure 4a). tr

In our block copolymers, a similar arrangement, with only the less polar segments (i.e., the central poly(w-butylacrylate sequence) exposed to the surface, would imply that all chains form loops so that both their external PMMA sequences are accommodated in a lamella below the surface (Figure 4b). Such an organization corresponds to a significant loss in conformationalfreedomand is therefore expected to be entropically unfavorable. Instead, in a lamellar arrangement perpendicular to the surface (Figure 4c), the central segments can either be extracted or looped; this should thus be more favorable entropically. Since the difference in surface energy between the components is small, this entropy gain probably overcomes the energetic destabiiization due to the presence of the most polar component at the surface. It is important to notice that this lamellar organization is a direct consequence of the specific molecular architecture of these block copolymers.



366

Figure 4. Schematic representation of lamellar arrangements of block coplymers. The light gray and dark sequences correspond to the components with the highest and the lowest surface energy, respectively. The outer surface is the area above each sketch, (a) in a diblock copolymer, lamellae arrange parallel to the surface, exposing only the low-energy component ; (b) in a symmetric copolymer with polar outer segments, parallel arrangement of the lamellae implies that all central segments form loops ; (c) alternative arrangement of lamellae of the symmetric triblock copolymer, with both components exposed to the surface. Conclusion Microphase separation has been clearly observed in "all-acrylic" triblock copolymers, by measuring the local phase shifts of the oscillating probe in Tappingmode atomic force microscopy. Cylinders of PMMA are found to arrange perpendicular to the film surface at room temperature while it is suggested that their arrangement is more disordered above the glass transition temperature of PMMA. Lamellae are also found to stack perpendicular to the surface. The existence of such well-defined phase segregations indicates that these compounds should be promising as a new generation of thermoplastic elastomers.


367 Acknowledgements


We are grateful to J.P. Aimé and S. Kopp-Marsaudon for fruitful discussions. The collaboration between Mons and Liège is conducted in the framework of the Belgian Federal Government Office of Science Policy (SSTC) "Pôle d'Attraction Interuniversitaire en Chimie Supramoléculaire et Catalyse Supramoléculaire" (PAI 4/11). Research in Mons is also partly supported by the European Commission and the Government of the Région Wallonne (Project NOMAPOL-Objectif 1-Hainaut), the Belgian National Fund for Scientific Research FNRS/FRFC, and an IBM Academic Joint Study. GM is indebted to "ELF-ATOCHEM" (France) for financial support. RL is Maître de Recherches du Fonds National de la Recherche Scientifique (FNRS - Belgium). References [1]

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