Morphology of Films of SEBS Triblock Copolymers

Chinese J. Chem. Eng., 13 (4) 498 - 503 (2005)Morphology of Films of SEBS Triblock Copolymers*HAN Xia LIU Honglai DONG Yarning and HU YingDepartment of Chemistry, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, ChinaAbstract Surface morphologies of the films of poly [styrene-6-(ethylene-cp-butene)-b-styrene] (SEBS) have been studied by using tapping-mode atomic force microscopy (TM-A FM). The films of block copolymer were prepared both by spin-coating on mica and by solvent-casting on different solution surfaces. For spin-coating samples, the effect of solution concentration, solvent, and annealing temperature are investigated. It is shown that changing the concentration of the solution makes no difference on the morphology of the film of the block copolymer. The microstructures are quite stable during thermal annealing; only the size of the domains changes toward the equilib-rium configuration. However, solvent annealing can notably change the microstructures. When different selective solvents are used for film spin-coating, different morphologies can be obtained and explained by the different solu-bility parameters of the solvents. A s expected, significant different morphologies in the top and the bottom surfaces of the casting films were observed. The images of the top surfaces reveal cylinder microdomains, while those of the bottom surfaces were spherical morphologies.Keywords atomic force microscopy (A FM), morphology, block copolymer, spin-coating, solvent-casting1 INTRODUCTIONBlock copolymers are an interesting class of materials for their rich and varied regularly or-dered microdomain structures at equilibrium and non-equilibrium state[1-3]. In understanding the mechanism of phase separation, miscibility, adhe-sion, and interfacial phenomena of block copoly-mers, morphologies in bulk and at interfaces have re-ceived considerable attention both theoretically[2] and experimentally[4,5] in recent years. A tomic force mi-croscopy (A FM) is a rather new technique and has been widely used since its invention in 1986. Due to its simplicity of preparing the specimens and its abil-ity to obtain intuitive topographical images, it has become an important tool of imaging the surface of materials to an atomic level resolution. Operation of the AFM in tapping mode enables one to minimize lat-eral forces and to investigate soft surfaces, especially for the soft elastomers without plastic deformation. It has been successfully used for typical thermoplas-tic elastomers such as polystyrene-b-polybutadiene-b-polystyrene (SBS)[6-8] and Poly [styrene-b-(ethylene-co-butene)-b-styrene] (SEBS)[9-11].SEBS is a thermoplastic triblock copolymer com-posed of a hard segment, polystyrene (PS), and a soft rubbery segment, poly(ethylene-co-butene) (PEB). An exclusive concern was focused on this kind of block copolymers in recent years. Van der Berg et al.[6] have revealed the chemical nature of the dif-ferent, phases of SBS triblock copolymer films by im-age analysis. They determined the domain structures quantitatively and compared them with theoretical re-sults. Konrad and coworkers[12] established volume morphologies of a thin SBS triblock copolymer film by using surface morphologies obtained by tapping mode AFM measurements. Jeong et al.[13] studied the kinetics and mechanism of morphological transition from non-equilibrium lamella to cylinder microdomain in a SEBS triblock copolymer by using time-resolved synchrotron small-angle X-ray scattering, transmis-sion electron microscopy, and rheology. Recently, Mo-tomatsu et al.[14] and Wang et al.[10] have successfully studied surface morphologies using A FM.In this work, TM-A FM is used to image the surface morphologies of the SEBS thin films prepared not only by spin-coating on freshly cleaved mica substrate, but also by solvent-casting onto different solution surfaces. In contrast to conventional casting techniques on solid substrate, use of water or aqueous solution substrates will minimize the deformation of the resulted soft film upon removal of the film from the casting vessel. For the unconfined free surface of the block copolymer, there are long-range and short-range interactions oc-curred at the air-polymer and substrate-polymer in-terfaces, respectively, competition of them controls the morphology formed. Interactions between sub-strate and polymer influence the regularity of the mor-phology of the block copolymer. Besides, when block copolymers are cast from solution with a selective sol-vent, non-equilibrium morphology can be formed.2 EXPERIMENTAL SECTION2.1 MaterialsSEBS triblock copolymer (Kraton G-1650) wasReceived 2004-06-30, accepted 2005-05-17.* Supported by the National Natural Science Foundation of China (No. 20476025, No. 20236010, No. 20490200) and Shanghai Municipal Education Commission of China.** To whom correspondence should be addressed. E-mail: hlliu@Morphology of Films of SEBS Triblock Copolymers 499supplied by Shell Development Co. without fur-ther treatment, the number-average molecular weight,M n , the polydispersity index, M w /M n , and the mass fraction of polystyrene (PS), were determined to be 8.78 × 104, 1.27, and 0.29, respectively. They are measured by gel permeation chromatography with polystyrene standards and 1H nuclear magnetic res-onance spectroscopy.2.2 Sample preparation SEBS was dissolved in toluene to prepare 0.005,0.06 and 0.09 g·ml -1 solutions respectively. The film samples subject to AFM measurements were prepared by solution-casting or spin-coating at room temper-ature and then dried in vacuum for several days to eliminate the last trace of solvent.2.3 Atomic force microscopy The AFM topography images were obtained in the constant repulsive force mode by an AFM (AJ- Ai-jian nanotechnology Inc., China) with a triangular micro fabricated cantilever (Mikro Masch Co., Rus-sia) with a length of 100μm and a spring constant of48N·m -1. The measurements were carried out in airunder normal conditions. All images presented in thepaper are height images.3 RESULTS AND DISCUSSION3.1 Effect of ConcentrationTo study the effect of the initial SEBS/toluene so-lution concentration on the film morphology formed inthe spin-coating, six concentrations between 0.003 and0.09g·ml -1 were used. Fig. 1 only gives the AFM im-ages of the spin-coated films from (a) 0.005 g·ml -1 and(b) 0.06 g·ml -1 solutions of SEBS in toluene, whileothers are not shown here.Figure 1 AFM ima ges (500 × 500 nm 2) of films spin-coated from (a) 0.005 and (b) 0.06 g·ml -1SEBS/toluene solutions (the height scale is 2 nm)From Fig. 1 we can see that no essential difference has been observed between the morphologies of spin-coated films from these two concentrations. They have both spherical and cylindrical domains representing rubbery microphase, while the black background is plastic microphase. In fact, the image from dilute solution has more spherical domains than that from higher concentrated solution; while the image from the latter has more cylindrical domains. This proba-bly because there are more solvent molecules disperse the polymer chains in dilute solution. When the solu-tion was spin-coated, solvents evaporated rapidly and more dispersed morphology was formed by phase sep-aration. Obviously, that from the more dense solu-tion favors the longer agglomerates. The results ob-tained here are consistent to the conclusions by Inoue et al.[15] based on transmission electron microscope (TEM) data of poly(styrene-b-isoprene) (SI) diblock copolymer films.3.2 Effect of thermal annealing Figure 2 is the AFM surface images of spin-coated films measured after annealing at different tempera-tures where Fig. 2 (a) represents the sample without annealing, while Figs. 2(b)—2(d) are those annealed at 353.15 K, 413.15 K and 453.15 K, respectively, un-der vacuum for 24 h. It is demonstrated that the film without annealing treatment shows an ambiguous phase-separation morphology, which can be elucidated by solvent effect, i.e., toluene, not a good solvent to PEB block, hinders the thorough phase-separation.Figure 2 AFM images (1.00×1.00 μm 2) of the spin-coated films from 0.06 g·ml -1 toluene solutions.(a) una nnea led, a nd a nnea led a t (b) 353.15 K, (c)413.15 K a nd (d) 453.15K for 24 hours [the height scale is 4nm for (a )-(c) a nd 10nm for (d)]Annealing at low temperature can improve the phase-separation and then gives a vivid PS/PEB interface.The dia meter of discrete microdoma ins, 13 nm, a l-most has no changes before and after annealing under this tempera ture. When the a nnea ling tempera ture is higher than their glass transition temperatures, the domains begin to grow and fuse together. La rger ag-glomerates with dia meters a bout 30 nm a re formed.We can see that the higher annealing temperature fa-vors the formation of larger domains both for rubberyphase a nd plastic phase corresponding to lower freeChinese J. Ch. E. 13 (4) 498 (2005)500Chinese J. Ch. E. (Vol. 13, No. 4)energy and closer to equilibrium. The diameters of microdomains increase from 12 nm to 48 nm when an-nealing temperature increases from room temperature to 453.15 K, as shown in Table 1.Table 1 Average diameter of the discrete microdomains obtained from AFM height imagesAnnealing temperature, K Average diameter, nm 303.15 12353.15 13413.15 30453.15 483.3 Effect of solventThe morphology of the block copolymer film varies from solvent to solvent as reported in literatures [15-17]. To investigate the effect of casting solvent on the domain structure, five solvents with dif-ferent affinity to the PS and PEB blocks were chosen to dissolve SEBS for preparing the spin-coating solu-tions. Solvent affinity and the solubility parameters of solvents are shown in Table 2. Topographical images and their section analysis graphs are shown in Fig. 3.Among those solvents, we can see from Table 2that cyclohexane is the best one for the rubbery PEB.Its solubility parameter is 8.2, while that of PEB is es-timated as 7.9—8.1. The corresponding image of the film is shown in Fig. 3(b) exhibiting mostly spherical domains indicating the excellent dispersing properties of the solvent for PEB. Although solubility parameter of n-heptane, 7.4, is lower than that of PEB, it is more affinitive to PEB. PEB segments are stretched, while PS segments are collapsed coiled. The microphase separation is confined at a certain extent. The cor-responding image shown in Fig. 3(a), demonstrates a morphology with PEB cylinders parallel to the film surface. CCl 4, neutral solvent with solubility param-eter of 8.6, is in the middle of the PS and PEB,which may be good solvent for both segments. PS and PEB segments are in free stretched state. When solution is spin-coated, they are microphase separated due to their inherent thermodynamic incompatibility.A co-continuous morphology is induced by the dras-tic phase-separation as shown in Fig. 3(c). Solubility parameters of toluene, 8.9 and 1,2-dichloroethane, 9.8,Figure 3 AFM images (1.00 × 1.00 μm 2) and their section analysis graphs of the spin-coated films from 0.06 g·ml -1 solutions of SEBS in (a) n-heptane, (b) cyclohexane, (c) CCl 4, (d) toluene and (e) 1,2-dichloroethane (The height scales are 4.5 nm)August, 2005Morphology of Films of SEBS Triblock Copolymers501 Table 2 Solubility parameters, solvent affinity, morphology and average diameter of the discrete microdomainsSpin-coating solvent Solubility parameter(cal/cm3)1/2Solvent affinity Morphology Averagediameter, nmn-heptane,cyclohexane, carbon tetrachloridetoluene1,2-dichloroethaneMEKPSPEB7.48.28.68.99.89.38.7—9.1affinitive to PEBaffinitive to PEBmiddleaffinitive to PSaffinitive to PSdissolve PS onlyPEB cylinders parallel tosurfacePEB spheres in PS matrixPS-PEB bicontinuousmicrodomainsPEB cylinders in PS matrixPEB cylinders in PS matrix9.711.812.313.517.7Solubility parameter of PEB is estimated to be 7.9—8.1.are far beyond of the range of 7.9—8.1. The two sol-vents act as a precipitator to PEB segments and cause phase-separation. The corresponding images shown in Figs. 3(d) and 3(e), exhibit cylindrical domains, the larger the difference between solubility parameter of the solvent and that of PEB, generally, the larger the average diameter of the domains will be. This can be clearly seen both from Table 2 and from their section graphs in Fig. 3. The surface morphologies of SEBS films cast from toluene and cyclohexane shown in this work are the same as those by Wang et al.[10] Those from other solvents in this work have not been found previously. In principle, the film should have unique morphology in equilibrium state. The different mor-phologies observed when use different solvents indi-cate that the films are mostly in non-equilibrium state. Hasegawa and Hashimoto[16] considered the phenom-ena a memory effect, i.e., the domain structure which existed in solution is frozen-in during the process of solvent evaporation.3.4 Effect of solvent annealingWe used films cast from 0.09 g·ml-1 solutions of SEBS in toluene; the surface image of them has been shown in Fig. 4(a). The film thickness is 1mm. Two pieces of these films about 2 × 2 cm2 each were im-mersed in n-heptane and methyl ethyl ketone (MEK) for 30min, respectively, then dried immediately under vacuum at room temperature for a week to remove the residual solvent. Corresponding images of these two samples are shown in Figs. 4(b) and (c). Com-paring Fig. 4(c) with Fig. 4(a), which has similar mor-phology to the latter, we find that the former has finer microstructure. Morphology of PEB cylinders in a PS matrix keeps stable to MEK, which is proba-bly because the solubility parameter of MEK, 9.3, is near to the solubility parameter range of PS, 8.7—9.1. However, MEK cannot dissolve any of PS and PEB blocks due to its higher polarity fraction. It affects the surface morphology not so much but a little more smooth. What is interesting is Fig. 4(b) for the case of n-haptane, the image shows fine PS spheres dis-persed in a PEB matrix. On the one hand, n-haptanecan dissolve SEBS, the top surface layer with cylindri-cal structure may be dissolved and the underlayer be-neath the surface layer appeared shows spherical mor-phology. On the other hand, n-haptane is a better sol-vent to PEB than to PS. The dissolved PEB domains flow to joint together at the new polymer-air inter-face in order to decrease the whole surface free energy. The mesh-like morphology with PS spheres in PEB matrix then comes into being. A lthough the same solvent is used in solvent treatment and spin-coating, the film morphologies are not the same in Fig. 4(b) and Fig. 3(a). From another point of view, this phenom-ena testifies Hasegawa and Hashimoto's[16] hypothe-sis which is about a solvent memory effect discussed above.Solvent rinsing the film can change the original morphology. Similar to thermal annealing, it is some-times named as "solvent annealing" [18]. In section 3.2, we have mentioned that thermal annealing does not destroy the morphology structure but change the do-main size. However, solvent annealing can change the film morphology.3.5 Effect of casting substrateFilms of SEBS were prepared by casting a solu-tion of 0.06 g·ml-1 in toluene onto a solution sur-face. Three different solutions as casting substrate were used, deionized water, sulfuric acid and sodium hydroxide aqueous solutions. Evaporating the solvent from SEBS/toluene solution, the films were formed on the aqueous solution surfaces because the density of SEBS/toluene solution is lower than that of aqueous solutions. Films prepared in this way can effectively minimize the deformation after removal of the film from the substrate.Figure 5 shows the AFM images of the films cast upon sulfuric acid solution, 5(a1) and 5(a2), deionized water, 5(b1) and 5(b2), and sodium hydroxide solu-tion, 5(c1) and 5(c2). The top images denote mem-brane's top surfaces, while the bottom images denote their bottom surfaces. We can see from the figures that different solutions do not change the morpholo-gies notably. The images of the top surfaces exhibitChinese J. Ch. E. 13 (4) 498 (2005)502Chinese J. Ch. E. (Vol. 13, No. 4)cylindrical structures as shown in Figs. 5(a1), 5(b1) and 5(c1). The images of the bottom surfaces demon-strate close packed hexagonal nearly spherical aggre-gates as shown in Figs. 5(a2), 5(b2) and 5(c2). Be-cause the top surfaces contact with air during the evaporation, the solvent escapes faster than that on bottom surfaces where the polymer solution spreads on the aqueous solution. Similar to the explanation of Fig. 3, the bottom surfaces have relatively more sol-vent during the preparation favoring the dispersion of PEB. While the top surfaces have less solvent, PEB thus grows to form longer cylinders.In addition, the average diameters of the mi-crodomains are obtained by images analysis listed in Table 3. Generally, the domain size of the top surfaces is smaller than that of the bottom surfaces. On the other hand, the size decreases from sulfuric acid solu-tion, then pure deionized water, to sodium hydroxideFigure 4 AFM images (2.00 × 2.00 μm2) of the films cast from 0.09g·m l-1 solutions of SEBS in toluene,(a) as-cast, and then immersed in (b) n-heptane, (c) MEK for about 0.5 h(the height scales are 20 nm)Figure 5 AFM images (1.00 × 1.00 μm2) of films cast on solutions substrates from 1.0% SEBS in toluene (the top images are for membrane's top surface, while the bottom ones for their bottom surface. The height scales are 10 nm)Table 3 Average diameter of the discrete microdomains obtained from AFM height imagesSample membrane H2SO4-top H2SO4-bottom H2O-top H2O-bottom NaOH-top NaOH-bottomAverage diameter15 data points were on average in each image to obtain data presented. August, 2005Morphology of Films of SEBS Triblock Copolymers503solution either for the top surfaces or the bottom sur-faces. Khulbe and coworkers[19] studied surface mor-phology of membranes prepared from poly(phenylene oxide) (PPO) by TM-AFM. They found that the nod-ules on the bottom surface were twice as large as those on the top surface. The substrate effect on morphol-ogy of the solvent-casting film was well studied by many research groups, but comparison between the morphologies of the top and bottom surfaces is rare, especially for the films cast on liquid substrates.4 CONCLUSIONSThe tapping mode A FM is especially suitable for samples that are soft or liable to mechanical dam-age. The surface morphologies of the SEBS thin films prepared by spin-coating or solvent-casting onto dif-ferent substrates were examined by TM-A FM. High-resolution images of morphologies and microstructures of surfaces were obtained by changing solution con-centration, altering selective solvent and thermal or solvent annealing. From the measurements, we found that the microstructures are quite stable during ther-mal annealing; only the size of the domains changes toward the equilibrium configuration. However, sol-vent annealing can notably change the microstruc-tures. For films casting on soft substrates, the top sur-faces have a significant different morphology from that on the bottom surfaces. The differences are caused by the different degrees of solvent evaporation. 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