Langmuir 1999, 15, 1763-1769
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Relationship between Morphology of Microphase-Separated Structure and Phase Restructuring at the Surface of Poly[2-hydroxyethyl methacrylate-block-4-(7′-octenyl)styrene] Diblock Copolymers Corresponding to Environmental Change Kazuhisa Senshu,† Motoyasu Kobayashi,‡ Noriko Ikawa,‡ Shuzo Yamashita,† Akira Hirao,‡ and Seiichi Nakahama*,‡ Research & Development Center, Terumo Corporation, 1500 Inokuchi, Nakai-machi, Ashigarakami-gun, Kanagawa 259-01, Japan, and Department of Polymer Chemistry, Faculty of Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152, Japan Received July 6, 1998. In Final Form: October 28, 1998 Hydrophilic-hydrophobic diblock copolymers composed of 2-hydroxyethyl methacrylate (HEMA) and 4-(7′-octenyl)styrene with 26, 47, and 82 wt % of poly[4-(7′-octenyl)styrene], HO-30, -50, and -80, respectively, were synthesized by anionic living polymerization. The surface structures of their solvent-cast films under dry and wet conditions were analyzed by contact angle measurements, transmission electron microscopic (TEM) observation, and X-ray photoelectron spectroscopic (XPS) measurements. The analytical results of HO-30 and HO-50 substantiate surface restructuring in response to environmental change. When the films were soaked in water, very quick hydration of the surface was observed. However, dehydration of the surface occurred slowly even by annealing in air. TEM observation was successfully performed by staining technique with OsO4 and introduction of a carbon-carbon double bond in the hydrophobic segment. Cross sectional TEM images of near film surfaces suggested that the phase restructuring at the outermost surface corresponding to environmental change occurred at the microdomain scale of the block copolymer. The block copolymer film of HO-80 retained a hydrophobic surface under various conditions and never experienced surface restructuring.
Introduction Multicomponent polymeric materials have been utilized in the fields of adhesives, surfactants, composites, and membranes for their characteristic surface properties. In addition, block copolymers, which exhibit excellent nonthrombogenic activity, have been evaluated for possible use as biomedical materials.1-6 The triblock copolymer, poly(2-hydroxyethyl methacrylate) (dHEMA)-block-polystyrene-block-polyHEMA (HEMA/st), is one of the most well-known blood compatible materials. It was synthesized by Okano et al. and was found to exhibit unique interaction with blood and cells. Okano et al. also found that HEMA/ st with lamellar microdomain structure exhibited excellent blood compatibility in in vitro,3,7,8 ex vivo,9 and in vivo10 * To whom correspondence should be addressed. † Terumo Corporation. ‡ Tokyo Institute of Technology. (1) Imai, Y.; Watanabe, A.; Kojima, K.; Masuhara, E. Jpn. J. Artif. Organs 1972, 1, 140. (2) Lyman, D. J.; Metcalf, L. C.; Albo Jr, D.; Richard, K. F.; Lamb, J. Trans. ASAIO 1975, 20, 474. (3) Okano, T.; Nishiyama, S.; Shinohara, I.; Akaike, T.; Sakurai, Y.; Kataoka, K.; Tsuruta, T. J. Biomed. Mater. Res. 1981, 15, 393. (4) Yui, N.; Tanaka, J.; Sanui, K.; Ogata, N. Makromol. Chem. 1984, 185, 2259. (5) Grainger, D. W.; Nojiri, C.; Okano, T.; Kim, S. W. J. Biomed. Mater. Res. 1989, 23, 979. (6) Furuzono, T.; Yashima, E.; Kishida, A.; Maruyama, I.; Matsumoto, T.; Akashi, M. J. Biomater. Sci., Polym. Ed. 1993, 5, 89 (7) Okano, T. Aoyagi, T.; Kataoka, K.; Abe, K.; Sakurai, Y.; Shimada, M.; Shinohara, I. J. Biomed. Mater. Res. 1986, 20, 919. (8) Okano, T.; Suzuki, K.; Yui, N.; Sakurai, Y.; Nakahama, S. J. Biomed. Mater. Res. 1993, 27, 1519. (9) Nojiri, C.; Okano, T.; Grainger, D.; Park, K. D.; Nakahama, S.; Suzuki, K.; Kim, S. W. Trans. ASAIO 1987, 33, 596. (10) Nojiri, C.; Senshu, K.; Okano, T. Artif. Organs 1995, 19, 32.
experiments by suppressing adhesion and activation of platelets and subsequent thrombus formation. However, when HEMA/st is applied to medical devices as a coating material, it easily cracks on manipulation, since it is brittle due to the high Tg of its polystyrene segment. Thus, HEMA/st has not been widely applied to medical devices so far. To improve the mechanical properties of HEMA/st, we have synthesized a new amphiphilic triblock copolymer composed of poly(4-octylstyrene) as a hydrophobic segment (HEMA/oct). Since the introduction of a long alkyl side chain lowers the Tg to -42 °C, HEMA/oct exhibits elastomeric properties compared with HEMA/st at room temperature. After the kink resistance test of polyurethane tube with a coating layer of HEMA/oct, there were no cracks at the surface, although a number of cracks were observed on the HEMA/st surface.11 From the result of in vitro evaluation, HEMA/ oct also exhibited nearly the same excellent blood compatibility as HEMA/st.11 These results suggest that HEMA/oct has a significant potential for wide application as a coating material on blood contacting surfaces of various medical devices. Block copolymers as biomaterials have also been paid attention from a viewpoint of surface characterization.12 Understanding of their surface structures and the interactions between surfaces and biological systems are greatly important in the design of new biomaterials and medical devices. However, it is very difficult to characterize (11) Nojiri, C.; Nakahama, S.; Senshu, K.; Okano, T.; Kawagoishi, N.; Kido, T.; Sakai, K.; Koyanagi, H.; Akutsu, T. ASAIO J. 1993, 39, M322. (12) Ratner, B. D., Ed. In Surface Characterization of Biomaterials; Elsevier: NY, 1988.
10.1021/la9808162 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/06/1999
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the surface interacting with biological systems in situ. Alternatively, we have investigated the surface structures of amphiphilic block copolymers in water and the hydration process of their surface, which may estimate the surface behavior in biological conditions. We have been studying the surface characterization of various hydrophilic-hydrophobic block copolymers by cross sectional transmission electron microscopic (TEM) observation using a staining technique, X-ray photoelectron spectroscopy (XPS), static secondary ion mass spectrometry (SSIMS), and contact angle measurements.13-15 TEM is useful for visualizing of the microphaseseparated structures of block copolymers both in bulk and at surfaces,16-18 and staining with osmium tetraoxide (OsO4) makes observation of the microdomain structure at the surface possible under both dry and wet conditions. The as-cast film surface of HEMA/st was found to be covered completely with polystyrene layer due to minimization of surface free energy.13,14 Furthermore, when the as-cast film of HEMA/st was soaked in water, the hydrophobic polystyrene layer at the top surface was replaced to hydrophilic polyHEMA microdomain.14 Similarly, surface restructuring of HEMA/oct in response to environmental change was demonstrated by the results of XPS measurement.19 However, we could not obtain clear TEM images of the hydrated HEMA/oct surface due to poor staining with OsO4. To improve staining and quick fixation of surface structures, we synthesized a new hydrophilic-hydrophobic block copolymer composed of HEMA and 4-(7′-octenyl)styrene containing a carboncarbon double bond in the alkyl side chain. We strongly believe that this block copolymer has properties advantageous for characterization of surface structures and their restructuring in response to environmental change. Moreover, we ensure that the results obtained in this study are useful for designing new polymeric materials with highly valuable properties not only in the biomedical field but also for adhesives, surfactants, composites, and membranes fields. In the current study, we describe the results of surface characterization of solvent-cast films of poly[HEMA-block4-(7′-octenyl)styrene]s with different bulk compositions under dry and wet conditions by measurement of contact angle, TEM, and angular-dependent XPS. And their surface restructuring corresponding to environmental change is examined. The influence of bulk composition on surface properties is also discussed. Experimental Section Polymers. The diblock copolymer, poly[HEMA-block4-(7′-octenyl)styrene], was synthesized by anionic living polymerization using a protection technique for the hydroxyl group of HEMA as shown in Scheme 1.20,21 Prior to the polymerization, the hydroxyl group of HEMA was protected as the trimethylsilyl ether linkage (HEMA(13) Castner, D. G.; Ratner, B. D.; Grainger, D. W.; Kim, S. W.; Okano, T.; Suzuki, K.; Briggs, D.; Nakahama, S. J. Biomater. Sci., Polym. Ed. 1992, 3, 463. (14) Senshu, K.; Yamashita, S.; Ito, M.; Hirao, A.; Nakahama, S. Langmuir 1995, 11, 2293. (15) Senshu, K.; Yamashita, S.; Mori, H.; Ito, M.; Hirao, A.; Nakahama, S. Langmuir 1999, 15, 0000. (16) Kato, K. Polymer 1967, 8, 33. (17) Hasegawa, H.; Hashimoto, T. Macromolecules 1985, 18, 589. (18) Hasegawa, H.; Hashimoto, T. Polymer 1992, 33, 475. (19) Nakahama, S.; Hirao, A.; Caster, D. G.; Ratner, B. D. 1994. Unpublished observations. (20) Hirao, A.; Kato, H.; Yamaguchi, K.; Nakahama, S. Macromolecules 1986, 19, 1294. (21) Mori, H.; Hirao, A.; Nakahama, S. Macromol. Chem. Phys. 1994, 195, 3213.
Senshu et al. Scheme 1. Anionic Synthesis of Poly[HEMA-block-4-(7′-octenyl)styrene]
Table 1. Characterization of Poly[HEMA-block-4-(7′-octenyl)styrene]s Mn (obsd)
sample
polyHEMA blocka
poly[4-(7′octenyl)styrene] blocka
HO-30 HO-50 HO-80
8700 6500 2300
3200 6000 11800
Mw/Mna
wt % of poly[4-(7′octenyl)styrene] unitb
1.11 1.15 1.05
26 47 82
a M and M /M were obtained by GPC, using standardized n w n polystyrene for calibration. b The compositions were determined by 1H-NMR.
TMS). Using sec-butyllithium as an initiator, living poly[4-(7′-octenyl)styrene] was prepared in THF at -78 °C. Since the polymerization progressed without attack of the propagating end on the vinyl group of the side chain, the resulting polymer possessed a linear structure, a predictable molecular weight, and a very narrow molecular weight distribution.22 The propagating end was capped with 1,1diphenylethylene, and LiCl was added to avoid side chain reactions in the block copolymerization. Addition of HEMA-TMS to the resulting solution afforded the block copolymer. Living polymerization was quenched by adding methanol. The protective group attached to the polymer produced was quantitatively removed by acid treatment, and poly[HEMA-block-4-(7′-octenyl)styrene] with a predictable molecular weight and a narrow molecular weight distribution was obtained. In this study, three block copolymers with different amounts of poly[4-(7′-octenyl)styrene], denoted HO-30, -50, and -80, containing 26, 47, and 82 wt % of poly[4-(7′-octenyl)styrene], respectively, were prepared, and the surface structures of their solventcast films were analyzed. The results of characterization of block copolymers are summarized in Table 1. Contact Angle Measurement. The static contact angle was measured by the sessile drop method with a CA-A contact angle meter (Kyowa Kaimen Kagaku Co. (22) Ikawa, N.; Kobayashi, M.; Hirao, A.; Nakahama, S. 1996. Unpublished data.
Poly[HEMA-block-4(7′-octenyl)styrene]
Ltd., Japan). The specimen was prepared on a clean glass disk (18 mm diameter) by spin coating (4000 rpm, 20 s) from a 3 wt % solution in benzene/methanol (1/5, 2/1, and 9/1 (v/v) for OH-30, -50, and -80, respectively). The solvent was allowed to evaporate overnight in a clean bench. The prepared films were typically 200-800 Å thick and optically transparent. Before contact angle measurement, the samples were annealed at 100 °C for 30 min in vacuo and were placed under the following environments successively, i.e., soaking in water, drying in air, and annealing again at 100 °C in vacuo. To elucidate the repeating rearrangement of the surface structure, these treatments were repeated consecutively and the contact angles were measured. For the hydrated sample, the water drops on the specimen were quickly removed using a filter paper and a blower after soaking the film in water, and then a water droplet (1 µL) was placed on the specimen for measurement. All measurements were carried out within 30 s after placing a water droplet and repeated at least five times on other places on the same specimen. TEM Observation. A polyurethane (PU) sheet was employed as a substrate for easy sample making. The block copolymers were cast onto clean PU sheets from the filtered 3 w/v% solutions in N,N-dimethylformamide (DMF) at room temperature and 40-50% relative humidity. The cast film was kept at room-temperature overnight, vacuum-dried at 60 °C for 10-12 h, and finally annealed at 100 °C for 30 min in vacuo. The prepared film on PU sheet is ca. 3 µm thick. After the film samples were placed under various conditions, i.e., soaking in water, air-drying, and annealing again at 100 °C in vacuo, they were fixed and stained with OsO4. The dry film samples were stained with OsO4 crystal vapor under dry conditions for 2-3 days. In the case of wet specimen, the annealed films were soaked in water for 10 s, 1 or 30 min, and were immediately dipped in 4% aq OsO4 solution for 2 days to fix the structure. The stained films were rinsed with distilled water followed by drying. After degassing, the specimens were embedded in epoxy resin (Quetol 812, Nisshin EM Ltd., Japan). Ultrathin sections (500-800 Å thickness) were obtained from the trimmed blocks by an ultratome (2088V, LKB Ltd., Sweden) and observed under TEM (JEM 1200-EX, JEOL Ltd., Japan) at an accelerating voltage of 80 kV without poststaining. XPS Measurement. The samples for XPS were prepared by the same method for contact angle measurement and were placed under various environmental conditions similarly for contact angle measurement. The angulardependent XPS data were collected by using an X-ray photoelectron spectrometer (5500MT, Perkin-Elmer Co. Ltd., MN) with a monochromatic Al KR (1486.7 eV) X-ray source. The takeoff angle (TOA) was defined as the angle between the horizontal axis of the surface and the axis of the analyzer lens system. The TOAs of 10°, 20°, 45°, and 80° lead to sampling depths of approximately 15, 35, 70, and 100 Å, respectively. A low-energy (5-20 eV) electron flood gun was used to minimize sample charging on the nonconducting polymers, and survey scans (0-1200 eV) were acquired at an analyzer pass energy of 187.85 eV and an X-ray spot size of 0.4 mm for all four TOAs to determine the surface composition. Results and Discussion Contact Angle Measurements. We have reported that a series of hydrophilic-hydrophobic block copolymers, e.g.; poly(HEMA-block-styrene-block-HEMA),14 poly(HEMA-block-isoprene),15 poly(2, 3-dihydroxypropyl meth-
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acrylate) (dDIMA)-block-styrene),23 poly(DIMA-blockisoprene),23 and poly(DIMA-block-4-octylstyrene),23 experience surface restructuring in response to environmental change on the basis of the results of contact angle measurement. In this section, the solvent-cast film surfaces of poly[HEMA-block-4-(7′-octenyl)styrene]s with different contents of poly[4-(7′-octenyl)styrene] were also characterized by the sessile drop method. When a water droplet was placed on as-cast films of HO-30 and -50, the contact angles rapidly lowered within a few seconds it was correspondingly difficult to determine accurate contact angles for the as-cast films. This suggests the occurrence of rapid restructuring at the water-polymer interface, which may have been due be to the low Tg of poly[4-(7′octenyl)styrene] component (-36 °C, determined by DSC). After annealing the film at 100 °C for 30 min in vacuo, the surfaces became somewhat stable, and the shape of a water droplet on the annealed film did not change within ∼30 s. Consequently, reproducible contact angles were obtained. This phenomenon was also recognized for poly(DIMA-block-4-octylstyrene) and poly(DIMA-block-isoprene),23 which had lower Tg’s. Deformation of water droplets on poly(DIMA-block-4-octylstyrene) and poly(DIMA-block-isoprene) as-cast films occurred faster than on poly[HEMA-block-4-(7′-octenyl)styrene] as-cast film, since polyDIMA segment is more hygroscopic than polyHEMA segment.23 Figure 1 shows the cos θ of static contact angle θ of a water droplet on the HO-30, -50, and -80 films as a function of treating period under dry and wet conditions. The surfaces of HO-30 and HO-50 exhibited very similar behaviors (Figure 1A and B). At the HO-30 and -50 film surfaces, very quick restructuring from a dry surface to a hydrated one occurred at the water-polymer interface within 1 min. When the hydrated films were air-dried at room temperature, the hydrophilicities of the film surfaces of HO-30 and -50 exhibited little change, suggesting slow response from a hydrated surface to a dehydrated one during air drying. After annealing the hydrated films of HO-30 and -50 at 100 °C in vacuo, however, the hydrated surfaces reverted again to hydrophobic one. Such surface restructuring occurred repeatedly. On the other hand, the film surface of HO-80 did not exhibit drastic rearrangement. When the as-cast film of HO-80 was soaked in water for 30 min, the contact angles were nearly unchanged and remained hydrophobic. The hydrophilic polyHEMA spheres completely surrounded with hydrophobic poly[4-(7′-octenyl)styrene] matrix could not become in contact with water. Hence, the surface of HO-80 might be able to retain its hydrophobicity even on soaking in water. From contact angle measurements, it was elucidated that the surfaces of poly[HEMA-block-4-(7′-octenyl)styrene] films exhibited rearrangement corresponding to environmental change except for the case of extremely low content of hydrophilic segment. Although similar surface restructuring was observed for HO-30 and HO-50 on contact angle measurements, interesting behavior was recognized by XPS measurement, as described later; the hydrated HO-30 reverted more quickly to a hydrophobic surface than HO50 by annealing at 100 °C in vacuo. TEM Observation. The poly[HEMA-block-4-(7′-octenyl)styrene]s were designed to include a carbon-carbon double bond in the hydrophobic segment to improve surface characterization by TEM observation. In our previous study, we found that the polyisoprene microdomain of poly(HEMA-block-isoprene) was stained and fixed with (23) Mori, H.; Hirao, A.; Nakahama, S.; Senshu, K. Macromolecules 1994, 27, 4093.
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Figure 1. Static contact angle (cos θ) of a water droplet on (A) HO-30 film cast from benzene/methanol (1/5, v/v) solution, (B) HO-50 film cast from benzene/methanol (2/1, v/v) solution, and (C) HO-80 film cast from benzene/methanol (9/1, v/v) solution as a function of treating period under various conditions.
OsO4 in a very short time, and that the time-resolved structures at the outermost surface were visually captured through the hydration process.15 On the other hand, the polyHEMA microdomain of poly(HEMA-block-styreneblock-HEMA) was selectively stained by long exposure of OsO4.14 We therefore examined which microdomain of poly[HEMA-block-4-(7′-octenyl)styrene] was effectively stained with OsO4. Figure 2 shows TEM cross sectional images of the annealed films of HO-30, -50, and -80 cast from DMF solution. The as-cast samples were annealed at 100 °C for 30 min in vacuo and stained with OsO4 vapor. The upper part of the picture is embedded epoxy resin, and the interface of the resin and the film sample corresponds to the top surface facing the air. With increasing of poly[4-(7′-octenyl)styrene] composition, the morphology of microphase-separated structure in bulk changes from stained spherical (Figure 2A), to lamellar (Figure 2B) and unstained spherical (Figure 2C) microdomain according to Molau’s law.24 This indicates that the poly[4-(7′-octenyl)styrene] domain is selectively stained with OsO4, as expected, due to the quick reaction of carbon-carbon double bonds with OsO4. Regardless of the morphology in bulk, the top surfaces of the annealed films of HO-30 and -50 were predominantly covered with thin poly[4-(7′-octenyl)styrene] layers (black layers stained with OsO4). For HO-80 film, the matrix microdomain of (24) Molau, G. E. In Block Polymers; Aggarwal, S. L., Ed.; Plenum Press: New York, 1979; p 79.
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Figure 2. Transmission electron micrographs (cross sectional view) of the ultrathin sections of HO-30 (A), HO-50 (B), and HO-80 (C) films cast from DMF solution. The as-cast films were annealed at 100 °C for 30 min in vacuo and then stained with OsO4 vapor. Black area stained with OsO4 is poly[4-(7′-octenyl)styrene] microdomain, and white area corresponds to polyHEMA microdomain.
Figure 3. Transmission electron micrograph (cross sectional view) of the ultrathin section of HO-30 film after hydration for 1 min. The sample was soaked in water for 1 min and then stained with aqueous OsO4 solution.
poly[4-(7′-octenyl)styrene] continued from bulk to the top surface. By TEM observation, we reconfirmed that the top surfaces of the annealed films were completely covered with a hydrophobic microdomain to minimize surface free energy regardless of bulk composition. This result agrees with those obtained from contact angle measurement. To elucidate surface restructuring, HO-30 film cast from DMF was annealed and then soaked in water for 1 min and stained with aqueous OsO4 solution to fix the hydrated structures intact under wet conditions. Figure 3 shows a TEM cross sectional image of HO-30 film after soaking in water for 1 min. The outermost poly[4-(7′-octenyl)styrene] thin layer of the annealed film disappeared completely, although the morphology in bulk was unchanged. Alternatively, the hydrophilic polyHEMA microdomain covered the top surface. This indicates surface restructuring corresponding to environmental change due to minimization of total surface free energy. Such surface restructuring
Poly[HEMA-block-4(7′-octenyl)styrene]
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Figure 4. Transmission electron micrographs (cross sectional view) of the ultrathin sections of HO-50 film after soaking in water for (A) 10 s, and for (B) 30 min.
in a short time was also observed in HO-50 film. As shown in Figure 4, when HO-50 film was soaked in water for only 10 s, the film surface completely replaced from a poly[4-(7′-octenyl)styrene] layer to a polyHEMA layer (Figure 4A). Furthermore, when the film was soaked in water for 30 min, the top surface showed a hydrated surface structure similar to that soaked in water for 10 s (Figure 4B). However, when the observed surface structures were checked in detail, we found a difference between Figure 4A and B. At the top surface of hydrated film after soaking in water for 10 s, the edges of lamellae (indicated by arrows in Figure 4A) were observed everywhere. On the other hand, after soaking in water for 30 min the film showed a flat surface and few edges appeared at the top surface (Figure 4B). The annealed film before soaking in water also showed a flat surface. We suppose that these lamellar edges at the top surface observed in hydrated film after soaking in water for 10 s correspond to the turned up parts of microdomains through the surface rearrangement. A plausible process of microdomain rearrangement is illustrated in Figure 5. When the film surface contacts with water, the water cluster penetrates into bulk through a surface defect (indicated by asterisk in Figure 5). A polyHEMA microdomain beneath the top poly[4-(7′octenyl)styrene] layer is plasticized and swollen by hydration, which causes turn up and sliding of the microdomain to afford a terrace-like structure at the top surface, resulting in a hydrophilic surface with minimized polymer-water interfacial energy. Finally, the terracelike structures slide and combine with each other to make a flat surface as shown in Figure 4B. In our previous study, the hydrated surfaces of poly(HEMA-block-styrene-block-HEMA)14 and poly(DIMAblock-styrene)23 showed rugged surfaces consisting of micelle-like structures with a polystyrene core and polyHEMA or polyDIMA shell. The hydrated film of poly(HEMA-block-isoprene),15 on the other hand, showed the flat surface covered with polyHEMA microdomain, similar to the hydrated surface of poly[HEMA-block-4-(7′-octenyl)styrene] film (Figure 4B). We guess that this morphological feature of the top surface might be attributable to the difference in molecular mobility of the hydrophobic domain. Low molecular mobility of the polystyrene domain due to its higher Tg may prevent the microdomain from
Figure 5. A schematic representation of the surface restructuring in hydration process.
Figure 6. Transmission electron micrographs (cross sectional view) of the ultrathin sections of HO-30 film (A) air-dried for 2 h and (B) annealed again at 100 °C for 30 min in vacuo after soaking in water for 30 min. The sample preparation was carried out as follows; The as-cast film was annealed at 100 °C for 30 min in vacuo, and soaked in water for 30 min, and then the air-drying treatment was carried out for (A) 2 h at room temperature and (B) annealed again at 100 °C for 30 min in vacuo.
sliding at the top surface, resulting in the formation of a micelle-like structure.14,23 While the higher molecular mobilities of the domains of polyisoprene, poly(4-octylstyrene), and poly[4-(7′-octenyl)styrene], due to their lower
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Figure 7. Transmission electron micrographs (cross sectional view) of the ultrathin sections of HO-50 film (A) air-dried for 2 h and (B) annealed again at 100 °C for 30 min in vacuo after soaking in water for 30 min. The sample preparation was carried out by the same method with HO-30 film in Figure 6.
Tg, may make sliding of the microdomain possible at the top surface, leading to formation of a flat surface. Although the surface rearrangement from dry to wet conditions occurs very quickly, the inverse rearrangement from a hydrophilic surface to a hydrophobic one proceeds very slowly. As shown in Figure 6, when the hydrated HO-30 film was air-dried for 2 h at room temperature, the film surface remained hydrophilic (Figure 6A). By annealing at 100 °C for 30 min in vacuo, the hydrophilic surface reverted to a hydrophobic one (Figure 6B). These TEM images were also observed for HO-50 film with a lamellar microphase-separated structure as shown in Figure 7. On the other hand, the surface of HO-80 film with spherical microdomain of polyHEMA in poly[4-(7′octenyl)styrene] matrix did not exhibit structural change on soaking in water, indicating the absence of surface restructuring (TEM images are not shown). These results of TEM observation, quick hydration, and slow dehydration for HO-30, and -50, and the absence of surface restructuring for HO-80, agree well with those of contact angle measurements described above. The difference in rates of hydration and dehydration may be attributable to the affinity of water for polyHEMA. Since polyHEMA has high affinity for water, it can absorb water very quickly. On the other hand, hydrated polyHEMA cannot desorb water easily due to the hydrogen bond between water and the hydroxyl group of the HEMA unit. Reversion of the hydrated films of poly(HEMA-blockstyrene-block-HEMA) and poly(DIMA-block-styrene) to hydrophobic surfaces was traced by contact angle and XPS measurements.14,23 However, we have not succeeded in observing reverted surface structures under TEM. In contrast, clear TEM images of the reverted hydrophobic layer by annealing were observed at the top surfaces of poly(HEMA-block-isoprene)15 and poly[HEMA-block-4-(7′octenyl)styrene] in this study (Figures 6 and 7). XPS Measurement. Phase-restructuring corresponding to environmental change at the top surface of the poly[HEMA-block-4-(7′-octenyl)styrene] films was confirmed by the results of contact angle measurements and TEM observation. However, these results are not quantitative. Hence, surface restructuring was investigated by means of angular-dependent X-ray photoelectron spectroscopy (XPS). XPS is useful for quantitative surface analysis, in
Senshu et al.
particular, angular-dependent XPS has provides nondestructive depth profiling of the composition.25 The higher takeoff angles (TOAs) penetrate deeper into the surface, e.g., 80° TOA is present in the outer ∼100 Å of the sample surface, while 10° TOA decreases the sample depth ∼15 Å. Since XPS measurement is carried out under highvacuum conditions, it is impossible to analyze hydrated surfaces directly. Recently, using the freeze-drying technique, Lewis and Ratner reported the method of characterization at hydrated surface.26 They described that rearrangement of the hydrated surface structure of hydrophilic-hydrophobic block copolymer film back to a dehydrated one under vacuum conditions at room temperature proceeded rather slowly. In addition, we have succeeded in direct measurement for hydrated film surfaces of poly(DIMA-block-styrene) and poly(HEMAblock-isoprene) under high-vacuum conditions at room temperature without the freeze-drying technique.15,23 In this section, the surface composition of poly[HEMA-block4-(7′-octenyl)styrene] films under various conditions was examined by XPS without the freeze-drying technique. Because of the slow dehydration from wet to dry conditions as shown by the results of contact angle measurement and TEM observation, this method is possible for block copolymer films. The as-cast film was placed successively as follows, i.e., annealing at 100 °C for 30 min in vacuo, soaking in water for 30 min, air-drying for 30 min, and annealing again at 100 °C for 30 min in vacuo, and the surface composition of each sample was acquired by XPS at 80, 45, 20, and 10° TOAs. All XPS survey spectra of the HO-30, -50, and -80 films contained only two elements, oxygen and carbon, indicating freedom from surface contamination. Monomer unit percent was determined from the composition of carbon and oxygen. Figure 8 shows the surface composition of all block copolymer films as a function of TOA. The enrichment of poly[4-(7′-octenyl)styrene] component was not remarkably high for as-cast film surfaces. The unequilibrium structures near the free surfaces of as-cast HO30, -50, and -80 films contained 73, 26, and 3 unit % of polyHEMA component at a depth of ∼15 Å, respectively (Figure 8A). The angular-dependent XPS results for ascast films quite corresponded with those of contact angle measurement. As described in the section on contact angle measurements, when a water droplet was placed on the as-cast film surfaces, the contact angles rapidly lowered within a few seconds. This phenomenon is attributable to the phase mixing of hydrophobic and hydrophilic segments at the as-cast film surfaces. When the films were annealed at 100 °C for 30 min in vacuo, significant surface enrichment of poly[4-(7′-octenyl)styrene] component was recognized (Figure 8B). Annealing treatment leads to clear phase separation and concentration of the hydrophobic segment at the surface due to minimization of surface free energy. When the annealed films were soaked in water for 30 min, the surface composition of poly[4-(7′-octenyl)styrene] component of HO-30 and -50 drastically decreased (Figure 8C), in particular, HO-30 surface at a depth of ∼15 Å was completely occupied by polyHEMA component. While, the surface composition of HO-80 was almost unchanged by soaking in water. In the case of air-drying treatment for 30 min after soaking in water for 30 min, the monomer unit % of 4-(7′-octenyl)styrene increased slightly in HO(25) Briggs, D., Seah, M. P., Eds. In Auger and X-ray Photoelectron Spectroscopy, Practical Surface Analysis; Wiely: Chichester, 1990; Vol. 1. (26) Lewis, K. B.; Ratner, B. D. J. Colloid Interface Sci. 1993, 159, 77.
Poly[HEMA-block-4(7′-octenyl)styrene]
Langmuir, Vol. 15, No. 5, 1999 1769
Figure 9. Surface composition of HO-30, HO-50, and HO-80 films determined by XPS at 10° TOA under various conditions. (-9-; HO-80; --2--; HO-50; sbs; HO-30.)
Figure 8. Surface composition of HO-30, HO-50, and HO-80 films as a function of TOA (sin θ). The sample was placed under various conditions as follows. The as-cast films were spin-coated from benzene/methanol solution (A). Then the samples were annealed at 100 °C for 30 min in vacuo (B) and soaked in water for 30 min (C). The air-drying treatment was carried out at room temperature for 30 min (D). The sample was annealed again at 100 °C for 30 min in vacuo (E) (9, HO-80; 2, HO-50; b, HO-30).
30 and -50, and was unchanged in HO-80 (Figure 8D). Furthermore, when the hydrated films were annealed again at 100 °C for 30 min in vacuo, the film surface exhibited enrichment of poly[4-(7′-octenyl)styrene] component, although the magnitude of the surface concentration was slightly lower than that after annealing of ascast film (Figure 8B and E). The results of angulardependent XPS measurement, the surface restructuring of HO-30 and -50 films and the absence of the surface restructuring of HO-80, also agreed with the results of contact angle measurements and TEM observation. Here, we noticed that the surface composition of reverted 4-(7′-octenyl)styrene unit in HO-30 film was higher than that in HO-50 (Figure 8E). Hydrated films of HO-30 and
-50 were therefore annealed at 100 °C in vacuo for 5, 10, 15, 30, and 60 min, and their surface compositions were determined by angular-dependent XPS measurement. Figure 9 shows the surface composition determined by XPS at 10° TOA as a function of various treating period. This result suggests that the hydrated film surface of HO-30 reverted to a hydrophobic surface more quickly than that of HO-50 by annealing at 100 °C in vacuo. This tendency was also confirmed by contact angle measurements. As shown in Figure 1, the contact angle of HO-30 film annealed again for 10 min after soaking in water was higher than that of HO-50. From these results, it is concluded that the film surface with spherical microdomain structure reconstructed faster than that of lamellar microdomain structure when the hydrated film was annealed. This may have been due to difference in the process of surface restructuring, the number of surface defects, and molecular mobility of hydrophobic domain, though we have no direct evidence for this. It is difficult, at this stage, to clearly explain the differences in surface behaviors of block copolymers with different bulk components. In this study, surface characterization of solvent-cast films of poly[HEMA-block-4-(7′-octenyl)styrene]s with 26, 47, and 82 wt % of poly[4-(7′-octenyl)styrene] component was carried out by contact angle measurement, TEM observation, and XPS measurement. In the cases of the solvent-cast films HO-30 and -50, all the analytical results showed surface restructuring in response to environmental change, while the film surface of solvent cast film of HO-80 remained hydrophobic under various conditions, and never exhibited surface restructuring. The process of surface restructuring from dry to wet conditions was estimated by TEM observation. Furthermore, we also elucidated that the speed of surface restructuring in the microdomain scale from wet to dry conditions differed depending on bulk composition. LA9808162
