Characterization of the Morphologies and Nanostructures of Blends of

Feb 12, 1998 - David B. Grandy, Douglas J. Hourston, Duncan M. Price, Michael Reading, Glaura Goulart Silva, Mo Song, and Paul A. Sykes. Macromolecule...
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Langmuir 1998, 14, 1219-1226

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Characterization of the Morphologies and Nanostructures of Blends of Poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) with Isotactic and Atactic Polypropylenes by Tapping-Mode Atomic Force Microscopy G. Bar* and Y. Thomann Freiburger Materialforschungszentrum, Stefan-Meier-Strasse 21, D-79104 Freiburg, Germany

M.-H. Whangbo* Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received October 23, 1997. In Final Form: December 23, 1997 Blends of poly(styrene)-block-poly(ethene-co-but-1-ene)-block-poly(styrene) (SEBS) with isotactic polypropylene (i-PP) and with atactic polypropylene (a-PP) were prepared under different thermal treatments. On the basis of the phase imaging of tapping-mode atomic force microscopy (TMAFM), we investigated the nanostructures and the morphologies of these blends as well as their dependence on blending history. The observed macrophase separation in i-PP/SEBS blends is caused by the incompatibility of the two polymer components. The microphase separation of the SEBS component depends on the thermal treatment. The morphology of a-PP/SEBS blends exhibits domains that span a wide range of sizes from 15 µm down to 15 nm. Our work shows that TMAFM phase imaging is an important and competitive tool for studying the microphase separation of polymers.

Introduction Morphologies of block copolymers and their blends in bulk and at various interfaces have received considerable attention in recent years.1 Block copolymers are widely used for interface adhesives and compatibilizers in polymer blends to improve the particle size and uniformity of the dispersion. The mechanical properties of polymer blends are governed by their composition, morphology, and interface structure, and the morphology at a given composition is determined by the blending history and the interface properties.2 Thus, in understanding phase separation, miscibility, adhesion, and interface phenomena of polymers, it is important to study nanometer-scale structures and morphologies of block copolymers and their blends. Bulk nanometer-scale structures and morphologies of polymer bulks have been investigated mainly by transmission electron microscopy (TEM), and those of polymer surfaces by scanning electron microscopy (SEM).3 Small-angle X-ray scattering (SAXS) has also been used to study microphase structures of block copolymers.4-6 Atomic force microscopy (AFM) 7 is a rather new technique and has been widely used for high-resolution imaging of polymer surfaces. Images of polymer single (1) Lohse, D. J.; Hadjichristidis, N. Curr. Opin. Colloid Interface Sci. 1997, 2, 171. (2) Paul, D. R.; Newman, S. Polymer Blends; Academic Press: New York, 1978. (3) Sawyer, L. C.; Grubb, D. T Polymer Microscopy; Chapman and Hall: New York, 1987. (4) Meier, H.; Strobl, G. R. Macromolecules 1987, 20, 649. (5) Nojima, S.; Roe, R. J. Macromolecules 1987, 20, 1866. (6) Heck, B.; Arends, P.; Ganter, M.; Kressler, J.; Stu¨hn, B. Macromolecules 1997, 30, 4559. (7) Binnig, G.; Quate, C. F.; Gerber C. Phys. Rev. Lett. 1986, 56, 930.

crystals have revealed large-scale morphologies.8-11 For polymers such as polyethylene and poly(tetrafluoroethylene), it was possible to visualize their chain order in molecular resolution.12-15 These studies were performed using contact-mode AFM in which the probe tip mounted on a cantilever scans a sample surface while maintaining contact with the surface. Contact-mode AFM imaging has serious limitations because the tip can exert considerable forces to a sample surface thereby damaging surfaces of soft materials such as polymers. To alleviate this problem, tapping-mode AFM (TMAFM) was developed. In TMAFM the cantilever oscillates vertically near its resonance frequency so that the tip makes contact with a sample surface only briefly in each cycle of oscillation. A short intermittent tip-sample contact reduces lateral forces during scanning, thus preventing sample damage, which enables one to image weakly bound surface layers and structural details.16,17 The recent development of TMAFM allows one to record shifts in phase angles of (8) Stocker, W.; Bar, G.; Kunz, M.; Mo¨ller, M.; Magonov, S. M.; Cantow, H.-J. Polym. Bull. 1991, 26, 215. (9) Reneker, D. H.; Chun, I. Polym. Prepr. 1996, 37, 546. (10) Nisman, R.; Smith, P.; Vancso, G. J. Langmuir 1994, 10, 1667. (11) Kajiyama, T.; Ohki, I.; Takahara, A. Macromolecules 1995, 27, 4773. (12) Magonov, S. N.; Qvarnstro¨m, K.; Eilings, V.; Cantow, H.-J. Polym. Bull. 1991, 25, 689. (13) Jandt, K. D.; Buhk, M.; Miles, M. J.; Petermann, J. Polymer 1994, 35, 2458. (14) Magonov, S. N.; Kempf, S.; Kommig, M.; Cantow, H.-J. Polym. Bull. 1991, 26, 715. (15) Vancso, G. J.; Fo¨rster, S.; Leist, H. Macromolecules 1996, 29, 2158. (16) Sheiko, S. S.; Mo¨ller, M.; Cantow, H.-J.; Magonov, S. N. Polym. Bull. 1993, 31, 693. (17) Wawkuschewski, A.; Cra¨mer, K.; Cantow, H.-J.; Magonov, S. N. Ultramicroscopy 1995, 58, 185.

S0743-7463(97)01154-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/12/1998

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vibration,18 and the resulting phase images provide enhanced image contrasts for heterogeneous surfaces.19-24 Poly(styrene)-block-poly(ethene-co-but-1-ene)-blockpoly(styrene) (SEBS) is a thermoplastic triblock copolymer composed of a hard segment, polystyrene (PS), and a soft rubbery segment, poly(ethene-co-but-1-ene) (PEB), with typical weight of PS in the range 20-35 wt %.25 SEBS has been blended with isotactic polypropylene (i-PP) to improve the mechanical properties (e.g., impact strength)26 under the assumption that the PEB block has a good compatibility with i-PP. However, the mechanical properties of the binary blends i-PP/SEBS are unsatisfactory and exhibit macrophase separation, indicating incompatibility. Indeed, our recent study shows that blends of i-PP and PEB are miscible only with a certain butene content (miscibility window).27 Far less is known about the miscibility of atactic polypropylene (a-PP) with block copolymers, and nothing is known about the miscibility between a-PP and SEBS. In the present work, we examine the nanostructures and morphologies of several i-PP/SEBS and a-PP/SEBS blends based on TMAFM phase imaging. Using the capability of TMAFM to visualize different components of polymer blends in nanometer resolution, we probe the effect of a polymer film-glass interface and that of thermal treatment on the morphologies of the polymer blends. The present study is not concerned with the methodological aspects of phase imaging. Our results are compared with recent TEM6,28 and SAXS6 studies of i-PP/SEBS. The present study differs substantially from contact- and tapping-mode AFM studies of block copolymers.24,29,30 In our study, polymer films were prepared on a glass substrate, and the flat surfaces of the films on the side of the film-glass interface were employed for TMAFM imaging, whereas other studies used free surfaces of polymer films.

blends of a-PP/SEBS were obtained by depositing a solution of both components in xylene at 130 °C on a glass substrate and then evaporating the solvent slowly. The estimated thickness of the resulting films was several microns as judged from light microscopy. The flat surfaces of films on the side of the filmglass interface were examined in our TMAFM measurements. In addition, SEBS samples were examined in two ways: (1) SEBS samples were cut by an ultramicrotome, and the surfaces thus obtained were investigated by AFM. (2) Solutions of SEBS in xylene at 130 °C were deposited on a glass substrate and then quenched to room temperature. The resulting film-glass interface was examined by TMAFM. TMAFM experiments were performed using a Nanoscope III scanning probe microscope. Height and phase images were recorded simultaneously while the instrument was operated in the tapping mode under ambient conditions. We used commercial Si cantilevers with force constants of 13-70 N/m. Images were acquired using the fundamental resonance frequency of these cantilevers, ω0, which was typically around 300 kHz. The phase angle shifts were measured using the resonance frequency of the free cantilever, the driving amplitudes A0 ≈ 60 nm, and the setpoint amplitudes Asp ≈ 40-45 nm, unless mentioned otherwise. Images were recorded with typical scan speeds of 1/2-1 line/s using a scan head with a maximum range of 170 × 170 µm or 16 × 16 µm.

Experimental Section

Low-Resolution Images of i-PP/SEBS

Important chemical and physical properties of i-PP, a-PP, and SEBS are listed in Table 1. Commercially available SEBS was used in our study. We prepared i-PP/SEBS blends of composition ratios 80:20, 50:50, and 70:30 wt % and a-PP/SEBS blends of composition ratio 50:50 wt %. Three types of i-PP/SEBS blends were obtained by (1) depositing a solution of both components in xylene at 130 °C on a glass substrate and then evaporating the solvent slowly, (2) depositing a solution of both components in xylene at 170 °C on a glass substrate, evaporating the solvent, and then annealing at 170 °C for 4 h, and (3) depositing a solution of both components in xylene at 190 °C on a glass substrate, evaporating the solvent, and then quenching from the melt. The

i-PP crystallizes at 130 °C and melts at temperatures above 166 °C (see Table 1). Thus, it is of interest to compare the resulting morphologies of the i-PP/SEBS blends obtained at 130 °C and at temperatures above 166 °C. In this paragraph, we describe the overall morphologies that result from the different thermal treatment. As presented in Figure 1, macrophase separation of i-PP and SEBS is observed in all cases. It should be noted that the images are presented at different scales because the observed morphologies are quite different. Figure 1a shows a typical phase image of an i-PP/SEBS (80:20 wt %) sample prepared at 130 °C. The phase image reveals that SEBS and i-PP undergo macrophase separation: SEBS forms roughly spherical domains of dark contrast whereas the major component i-PP appears in bright contrast. The spherical domains are well separated from each other and exhibit a typical size of 100-400 nm. The assignment of the domains to SEBS is possible simply by changing the sample composition and comparing the sizes of the major and minor components (Figure 1b-e). Figure 1b shows a typical phase image of an i-PP/SEBS (50:50 wt %) sample prepared at 130 °C. The phase image reveals domain structures of dark contrast embedded in an area of bright contrast. The increased SEBS content results obviously in a macrophase separation exhibiting domains of dark contrast which are much larger in size compared with those of Figure 1a (note the different scales). Consequently, the domains of dark contrast are assigned to the SEBS component. The size distribution of the SEBS domains is also not as uniform as in Figure 1a. Large nonspherical domains up to 5 µm in size coexist with small

(18) Chernoff, D. A. Proceedings Microscopy and Microanalysis; San Francisco Press: San Francisco, CA, 1995. (19) Magonov, S. N.; Elings, V.; Papkov, V. S. Polymer 1997, 38, 297. (20) Lecle`re, Ph.; Lazzaroni, R.; Bre´das, J. L.; Yu, J. M.; Dubois, Ph.; Je`roˆme, R. Langmuir 1996, 12, 4317. (21) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H.-J.; Whangbo, M.-H. Langmuir 1997, 13, 3807. (22) McMaster, T. J.; Hobbs, J. K.; Barham, P. J.; Miles, M. J. Probe Microscopy 1997, 1, 43. (23) Magonov, S. N.; Elings, V.; Whangbo, M.-H. Surf. Sci. Lett. 1997, 375, L385. (24) Magonov, S. N.; Cleveland, J.; Elings, V.; Denley, D.; Whangbo, M.-H. Surf. Sci. 1997, 389, 201. (25) Holden, G.; Legge, N. R.; Quirk, R. P.; Schroeder, H. E. Thermoplastic Elastomers; Hanser Publishers: Munich, Germany, 1996. (26) Gupta, A. K.; Purwar, S. N. J. Appl. Polym. Sci. 1986, 31, 535. (27) Thomann, Y.; Suhm, J.; Kressler, J.; Bar, G.; Mu¨lhaupt, R. To be published. (28) Setz, S.; Stricker, F.; Kressler, J.; Duschek, T.; Mu¨lhaupt, R. J. Appl. Polym. Sci. 1996, 59, 1117. (29) Stocker, W.; Beckmann, J.; Stadler, R.; Rabe, J. P. Macromolecules 1996, 29, 7502. (30) Motamatsu, M.; Mizutani, W.; Tokumoto, H. Polymer 1997, 38, 1779.

Table 1. Chemical and Physical Properties of SEBS, i-PP, and a-PP SEBSa composition mol wt Mw Mw/Mn Tg (°C) Tm (°C) a

29 wt % styrene 87 000 2 × 12 600 for PS 61 800 for PEB 1.04 83 for PS -49 for PEB

i-PP

a-PP

87 100

50 000

2.95 3

2.0 -2.4

166

SEBS is Kraton G 1652 (Shell Chemical Co.).

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Figure 1. AFM phase images of i-PP/SEBS blends. The blend composition ratios of i-PP:SEBS are 80:20 wt % in (a), 50:50 wt % in (b) and (c), 30:70 wt % in (d), and 50:50 wt % in (e). The samples in (a) and (b) were prepared at 130 °C, the samples in (c) and (d) were annealed at 170 °C, and the sample in (e) was annealed at 190 °C and quenched to room temperature. The scan size was 16 µm in (a), 30 µm in (b)-(d), and 5 µm in (e). The contrast covers phase angle variations in the 100° range in all images.

nearly spherical domains of 100-200 nm in size. The large SEBS domains of dark contrast enclose wellseparated i-PP domains of bright contrast. Figure 1c presents a phase image of a i-PP/SEBS (50:50 wt %) sample

which was prepared at 170 °C. The morphologies observed in Figure 1b,c are similar. Large nonspherical and small spherical SEBS domains of dark contrast coexist. The i-PP component appears in a bright contrast. The large

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Figure 2. AFM phase images of i-PP/SEBS blend (50:50 wt %) prepared at 130 °C. The scan size was 2.3 µm in (a) and 0.8 µm in (b). The contrast covers phase angle variations in the 180° range in (a) and (b).

Figure 3. AFM phase images of i-PP/SEBS blend (50:50 wt %) annealed at 170 °C. The scan size was 6 µm in (a) and 1.8 µm in (b). The contrast covers phase angle variations in the 180° range in (a) and (b).

SEBS domains contain small i-PP domains as in Figure 1b. Figure 1d shows a phase image of an i-PP/SEBS (30: 70 wt %) sample prepared at 170 °C. The major component SEBS appears in dark contrast containing large nonspherical and small spherical i-PP domains of bright contrast. As in Figure 1b,c, the large domains enclose small spherical domains of opposite contrast. Finally, Figure 1e shows the phase image of an i-PP/SEBS (50:50 wt %) sample that was prepared at 190 °C and subsequently quenched. The phase image reveals SEBS domains of dark contrast embedded in an i-PP of bright contrast. The dark domains exhibit a circular shape and a typical size of a few hundred nanometers in diameter. High-Resolution Images of i-PP/SEBS In probing the microphase separation within the SEBS area, any nanostructure within the i-PP area, and the morphology at the interface between SEBS and i-PP, it is necessary to rely on high-resolution phase images. Phase images of a 50:50 wt % sample prepared at 130 °C are shown in parts a and b of Figure 2, which clearly distinguish the morphology and nanostructure in different regions. The i-PP areas of bright contrast exhibit a

crosshatched arrangement of lamellae with a typical diameter of 20 nm. Within the SEBS areas of dark contrast, wormlike microphase domains in dark and bright contrast are present. The separation between the bright domains is typically in the range 40-50 nm. Figure 3 shows phase images of a 50:50 wt % sample annealed at 170 °C for 4 h. The annealing changes the morphology within the SEBS region of dark contrast from the wormlike microphase domains (Figure 2) to meshlike domains in dark and bright contrast. The diameter of the meshlike domains is typically about 20 nm, and the distance between the bright domains is in the range 3040 nm. Within the i-PP areas of bright contrast the crosshatched lamellae morphology is observed. Figure 4 shows phase images of a 50:50 wt % sample that was heated to 190 °C and subsequently quenched to room temperature. The microphase separation within the SEBS area of dark contrast is affected considerably. Small circular domains in bright contrast with a diameter of 7-15 nm are regularly dispersed in a matrix of dark contrast. These bright domains form a pseudohexagonal close-packing pattern within the dark matrix, and the nearest-neighbor distance is about 30 nm.

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Figure 4. AFM phase images of i-PP/SEBS blend (50:50 wt %) annealed at 190 °C and quenched to room temperature. The scan size was 2.8 µm in (a) and 0.47 µm in (b). The contrast covers phase angle variations in the 180° range in (a) and (b).

Phase Images of a-PP/SEBS Figure 5 shows phase images of a 50:50 wt % sample of the a-PP/SEBS blend. A low-resolution image is presented in Figure 5a. As in the case of the i-PP/SEBS image (Figure 1b), large domains of dark contrast are embedded in an area of bright contrast, and the dark domains contain smaller domains of bright contrast. The major differences in the overall morphologies of a-PP/ SEBS and i-PP/SEBS are that in the case of a-PP/SEBS the dark domains are larger in size and span a wider range of sizes (from 15 µm to a few hundred nanometers). Parts b and c of Figure 5 show high-resolution phase images, which demonstrate that the characteristic morphology is preserved at smaller scales, as in a fractal system: larger domains of dark contrast coexist with smaller domains embedded in an area of bright contrast. Again various sizes of domains are found, the smallest of which have a diameter as small as 15-30 nm. Discussion Contrasts of TMAFM height and phase images depend in a complex way on the driving amplitude A0 and the set-point ratio rsp ) Asp/A0.17-21,32-39 For large A0 and moderate to small rsp, the tip-sample interaction becomes repulsive so that the phase image contrast is related to the variation of the local surface stiffness on the sample surface. In this case, a larger stiffness leads to a more positive phase shift and thus to a brighter contrast in phase image. However, when A0 is small and the sample is very soft, the tip-sample interaction can be dominated by attractive forces (e.g., adhesion and capillary forces) leading to a negative phase shift. In particular, soft (31) Thomann, Y.; Cantow, H.-J.; Bar, G.; Whangbo, M.-H. Appl. Phys. A, in press. (32) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Mo¨ller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54, 8908. (33) Sarid, D.; Ruskell, T. G.; Workman, R. K.; Chen, D. J. Vac. Sci. Technol. B 1996, 14, 864. (34) Anczykowski, B.; Kru¨ger, D.; Fuchs, H. Phys. Rev. B, 1996, 53, 15485. (35) Burnham, N. A.; Behrend, O. P.; Oulevey, F.; Gremaud, G.; Gallo, P.-J.; Gourdon, D.; Dupas, E.; Kulik, A. J.; Pollock, H. M.; Briggs, G. A. D. Nanotechnology 1997, 8, 67. (36) Brandsch, R.; Bar, G.; Whangbo, M.-H. Langmuir 1997, 13, 6349. (37) Van Noort, S. J.; Van der Werf, K. O.; Degrooth, B. G.; Van Hulst, N. F.; Greve, J. Ultramicroscopy 1997, 69, 117. (38) Tamayo, J.; Garcia, R. Langmuir 1996, 12, 4430. (39) Tamayo, J.; Garcia, R. Appl. Phys. Lett. 1997, 71, 2394.

samples would have a long tip-sample contact time when A0 is small, so that the cantilever’s vibrational characteristics may be strongly affected by adhesion and capillary forces or even by the viscosity of a sample.37-39 As a consequence, the phase image contrast recorded at small A0 can be opposite to that obtained with moderate to large A0.21 This situation is demonstrated in Figure 6, which should be compared with Figure 4b. The phase image of Figure 4b was recorded with A0 ≈ 60 nm and Asp ≈ 40 nm, but that of Figure 6 with A0 ≈ 30 nm and Asp ≈ 20 nm. Both figures show the phase images of the same sample recorded with the same tip, but the image contrasts of the domains are opposite. The above discussion of image contrasts allows one to assign the individual polymer components. Since the PEB block is by far the most compliant component, the regions of the most negative phase shift are assigned to the PEB component (i.e., darkest regions within the domains in Figures 2-4). The bright regions are thus assigned to the i-PP component. This assignment is independently confirmed by comparing the relative sizes of the dark and bright areas with the composition of the blends (Figure 1a-e), which makes it clear that the dark and bright areas are related to the SEBS and i-PP components, respectively. This is further supported by the high-resolution images of Figures 2-4, which reveal the lamella structure of i-PP in the bright areas. The morphologies and mechanical properties of 30:70 and 70:30 wt % i-PP/SEBS blends were studied by TEM.6,28 This study showed large irregularly formed domains of more than 1 µm in size together with spherical domains of a few hundred nanometers in size, all of which are very similar to those found in Figure 1a-e. Since the TEM images were obtained for ultrathin cryosections of a sample, the overall morphology observed in our study represents that of the bulk closely. Therefore, the use of the film-glass interface appears relevant for learning about the bulk morphology of polyolefinic samples by AFM, as also shown previously.21,31 This finding is important because many properties of polymers depend on their bulk structures. i-PP used in our investigation melts at temperatures above 166 °C. Annealing at 170 °C for 4 h leads to similar macrophase separation of the SEBS and i-PP components, as does the sample preparation below the crystallization temperature (Figure 1b,c). However, the annealing

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Figure 5. AFM phase images of a-PP/SEBS blend (50:50 wt %) prepared at 130 °C. The scan size was 30 µm in (a), 4.3 µm in (b), and 2.2 µm in (c). The contrast covers phase angle variations in the 100° range in (a) and in the 45 ° range in (b) and (c).

strongly affects the microphase separation of SEBS, as seen in high-resolution images (Figures 2-4). The preparation by heating the components to 190 °C and subsequent quenching to room temperature affects the morphology considerably (Figure 1b,c versus Figure 1e) and leads to the formation of circular SEBS domains embedded in the i-PP matrix. Parts b-d of Figure 1 show further that the interface region between the SEBS and i-PP regions consists mainly of one component, because the small domains enclosed in the large domains are found away from the interface. This indicates that the mechanism leading to phase separation during the evaporation of the solvent is indeed caused by immiscibility of the two polymer components. Solventinduced phase separation can be excluded because no mixing is observed after annealing the melt. The high-resolution images of Figures 2-4 reveal the nanostructure and morphology of the i-PP and SEBS phase in more detail. The effect of various interfaces on the transcrystallinity of i-PP was studied in detail.40,41 Nucleation (for crystallization) and transcrystallinity were observed on Au, Teflon, and Cr surfaces but not on the (40) Setz, S.; Schnell, R.; Thomann, R.; Kressler, J.; Mu¨lhaupt, R. Macromol. Rapid Commun. 1995, 16, 81. (41) Thomann, R. Private communication.

Figure 6. AFM phase image of i-PP/SEBS blend (50:50 wt %) annealed at 190 °C and quenched to room temperature. The scan size was 2.4 µm. The contrast covers phase angle variations in the 180° range. The image was obtained using A0 ≈ 30 nm and Asp ≈ 20 nm, but that of Figure 4b was obtained with A0 ≈ 60 nm and Asp ≈ 40 nm.

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Figure 7. AFM phase image of SEBS. The scan size was 0.8 µm in (a) and in (b). The contrast covers phase angle variations in the 45° range in (a) and in the 80° range in (b). The image in (a) was taken on a cryogenically cut surface, while (b) was taken on the film-glass interface.

glass surface used in this study. Figures 2-4 clearly show that the i-PP phase exhibits the typical crosshatched lamella structure. At the i-PP/SEBS interface the i-PP lamella grow mostly in the directions perpendicular to the interface, which is typical for transcrystallinity and indicates that the SEBS interface acts as a nucleating agent. This, also reported by a TEM study,28 shows that transcrystallinity at a lamella scale can be visualized by AFM. It is of interest to examine the observed morphology of the SEBS phase in more detail. Figures 2-4 show that the SEBS regions are microphase-separated; i.e., they exhibit PS and PEB regions. The morphology of the SEBS region depends on the molecular weight composition of PS and PEB as well as on annealing temperature. Wormlike structures have been reported for the SEBSair interface of a SEBS sample containing 30 wt % PS.30 SEBS films of various PS contents have been prepared by spin-coating, and the film-air interface was studied recently by using contact-mode AFM.30 This study showed that the morphology of PS at the air interface changes from wormlike to meshlike structures as the PS content is increased. SEBS films containing 30 wt % PS exhibit a lamellar structure very similar to that shown in Figure 2. Generally, the bulk morphology of triblock copolymers is governed by the volume ratio of the components. The morphology of the minor component exhibits phaseseparated domains in the form of spheres, cylinders, or lamellae dispersed in the major component. In our samples, the minor and major components of each SEBS region are given by PS and PEB, respectively. Figures 2-4 show that the morphology of PS changes from a lamellar structure to a meshlike structure to a spherical structure, as the temperature of the film preparation is increased. As the temperature is increased, the morphology change is accompanied by a decrease in the spacing between the lattice points made up of PS domains from 50 nm (Figure 2) to 30 nm (Figure 4). Such a dependence of the domain separation on temperature was also observed in a recent SAXS study.6 A possible reason for this observation might be that the flexibility of the polymer chains is different at different temperatures, and the block copolymer chains are more extended at lower temperatures. The spherical morphology observed in Figure 4b

is particularly interesting. The pseudohexagonal packing, the diameter of the PS spheres (about 15 nm), and the lattice spacing (30 nm) are in perfect agreement with the corresponding values obtained for the bulk by SAXS.6 Let us now examine how the observed morphology of the SEBS phase is influenced by the glass interface. Phase images taken for cryogenically cut SEBS surfaces are shown in Figure 7a. The image shows a spherical or cylindrical morphology of PS. The preferred orientation of the PS cylinders is induced most likely by the cutting action with an ultramicrotome. The diameter of the cylinders is about 15 nm, and the distance between the cylinders is in the range 22-27 nm. These values agree well with those determined by SAXS and TEM measurements. Figure 7b shows a phase image taken at the filmglass interface of a SEBS film prepared on the glass substrate at 130 °C. The PS component forms lamellae, suggesting that the glass interface and/or the preparation temperature leads to a lamellar arrangement of PS. Further, the distance between adjacent PS lamellae is large, which might be due probably to the more extended block copolymer chains at the interface. Especially the PS phase is enriched at the interface, as can be observed in Figure 7b, which is consistent with the observations in Figures 2c, 3b, and 4b showing an increase in the PS component as the temperature is decreased. The interface between the SEBS and i-PP regions is of particular interest. As already mentioned, the PEB block should be more compatible with i-PP than with the SS block. According to the high-resolution images of Figures 2c, 3b, and 4b, it is the PS component that forms the interface between SEBS and i-PP. Obviously, the PS arranges in the form of lamella oriented parallel to the interface to i-PP. The driving force for this effect is explained in terms of mixing of the PEB block into the i-PP phase as well as the repulsion between PS and i-PP.6,28 To our knowledge, the miscibility of a-PP with SEBS has not been studied yet. The most striking observation for the a-PP/SEBS blends is the fractal character of the morphology: the domains span a wide range of sizes from 15 µm down to 15 nm. The driving force for this behavior is most likely the good compatibility of the PEB block with the amorphous PP, which might be due to the smaller molecular weight of the a-PP used here. It was demon-

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strated 42 that the compatibility of i-PP and ethene-but1-ene copolymers depends largely on the butene content, and butene-rich copolymers are compatible with the amorphous regions of i-PP. Our recent investigation shows that i-PP and PEB are indeed miscible for a butene content of 87.5 wt %.27 These studies suggest that the PEB block of SEBS and a-PP should have a good compatibility, leading to the formation of small domains, as observed in Figure 5c. Conclusions Our TMAFM study leads to the following conclusions: (1) TMAFM phase imaging is an important tool for studying the microphase separation of polymers and reproduces important findings observed by TEM and SAXS (42) Yamaguchi, M.; Miyata, H.; Masuda, T. J. Appl. Poym. Sci. 1996, 62, 87.

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studies. (2) For olefinic polymers, the flat surfaces of their films prepared on glass substrates (on the side of the filmglass interface) provide relevant information about the bulk morphology. (3) The phase separation in the i-PP/ SEBS blends during the evaporation of the solvent is caused by immiscibility of the two polymer components. (4) The morphology of a-PP/SEBS blends has a fractal character; i.e., its domains span a wide range of sizes from 15 µm down to 15 nm. This is probably caused by the good compatibility of the PEB block with the amorphous PP. Acknowledgment. The work at North Carolina State University was supported by the Office of Energy Research, Basic Energy Sciences, Materials Science Division, U.S. Department of Energy, under Grant DE-FG05-86ER45259. LA9711544