Solvent-Induced Morphology of the Binary Mixture of Diblock

2712

J. Phys. Chem. B 2009, 113, 2712–2724

Solvent-Induced Morphology of the Binary Mixture of Diblock Copolymer in Thin Film: The Block Length and Composition Dependence of Morphology Rui Guo,† Haiying Huang,† Binyang Du,‡ and Tianbai He*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China, and Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang UniVersity, Hangzhou, 310027, People’s Republic of China ReceiVed: September 26, 2008; ReVised Manuscript ReceiVed: NoVember 28, 2008

A series of binary SB blend samples with various overall volume fraction of PS (ΦPS) and different discrete distribution of the block length (denoted as dPS or dPB) were prepared by mixing various asymmetric poly(styrene)-block-poly(butadiene) (SB) block copolymers with a symmetric SB block copolymer. The influences of the external solvent field, composition, and the block length distribution on the morphologies of the blends in the thin films were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The experimental results revealed that after solvent annealing, the interface of the blend thin films depended mainly on the cooperative effects of the annealing solvent and the inherently interfacial curvature of the blends. Upon exposure to the saturated vapor of cyclohexane, which has preferential affinity for the PB block, a “threshold” of ΦPS (approximate 0.635∼0.707) was found. Below such threshold, the influence of the annealing solvent played an important role on the interfacial curvature of the blend thin film. The morphologies of the thin films and the long-range order of the structures were related to the value of dPS, regardless of the change of dPB. 1. Introduction Block copolymers, the representative soft matter, have the property of strong response to weak external signals and can spontaneously self-assemble into various morphologies and structures on the nanometer scale.1-10 Many reports have investigated the influences of the external fields11-22 on the selfassemble behavior of the block copolymers. Among them, the effect of the solvent field, such as solvent evaporation rate,12,19 the solvent-induced ordering,13,15,16,21 and the nature of the solvent17,18,20 on the microstructures of block copolymer has been systematically investigated. The different chain mobility (via controlled solvent evaporation) and the discrepant swollen extent of the blocks (due to the existence of the solvent with different affinity for the blocks) can lead to various metastable microphase structures of the block copolymers. However, most of these studies were focused on the neat block copolymers, and very few reports discussed the blend systems, for example, the binary blends of block copolymer with homopolymer23,24 or the mixtures of two block copolymers.25-28 Because of the competitions of short-range interactions between the blocks and the longrange interactions between the components, the phase separation of the blend systems is much more complicated and differs from the neat block copolymer. For example, Hashimoto et al.29-35 have done a series of works on the morphology of binary mixtures of two polystyrene-block-polyisoprene diblock copolymer (SI) in the bulk state. The authors proved that the chain length ratio of the two components can induce special effects, such as “cosurfactant effect”,30,32,34,35 which will influence the curvature of the interface, leading to the different morphologies. On the other hand, Spontak et al.36 have shown that the bulk * To whom correspondence should be addressed. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Zhejiang University.

morphology of the binary SI mixtures with various composition but similar molecular weight can be tuned by changing the relative amounts of the two SI block copolymers. In our previous works,18,20 we have reported that for the solution-cast film of the single block copolymer polystyreneblock-polybutadiene (SB) or polystyrene-block-poly(methyl methacrylate) (SMMA), the preferential affinity of the casting solvent for a certain block will induce significant change in the interfacial curvature, leading to the formation of the “inverted phase”. Similar influence of the solvent field has also been investigated for a binary blend thin film of SB block copolymers with complementary compositions by solvent annealing.28 The presence of an annealing solvent with preferential affinity for a certain block made the morphology of this blend thin film complex and varied. If the chain length ratio and the composition of the block copolymer blends were changed, would the influence of the solvent field still exist? In other words, how do the chain length and the composition of the binary blends of block copolymers influence the morphology and the interfacial curvature under the solvent field with preferential affinity for a certain block? In the present work, a series of binary SB blend samples with various overall volume fractions of the PS block were prepared by selecting a symmetric SB as the fixed component and mixing with another asymmetric SB block copolymer with various chain length ratios of PS/PB block. The effects of block length and composition of the binary blends on the morphologies of their thin films under solvent field were then investigated and discussed. 2. Experimental Section 2.1. Materials. Five poly(styrene)-block-poly(butadiene) (SB) diblock copolymers were purchased from Polymer-Source Inc. One had the symmetric block length and was named as s. The

10.1021/jp808551j CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009

Binary Mixture of Diblock Copolymer in Thin Film

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2713

TABLE 1: Characteristics of the PS-b-PB Diblock Copolymers Used in this Work code

Mn,PS-Mn,PBa × 10-3(Da)

s as1 as2 as3 as4

66.9-75.0 16.1-78.8 28.0-60.0 63.5-33.0 61.0- 9.0

wPSb

fPSc

Mw/Mn

0.471 0.170 0.318 0.658 0.871

0.432 0.149 0.284 0.621 0.852

1.08 1.05 1.19d 1.09 1.06

a

Mn,k, number-average molecular weight of the PS or PB. b wPS, PS weight fraction. c fPS, volume fraction of the PS block calculated from fPS ) (wPS/FPS)/((wPS/FPS) + (1 - wPS)/FPB) by using the following densities for the PS and PB block: FPS ) 1.05 g cm-3 and FPB ) 0.895 g cm-3. d The polydispersity index of PS block in this sample is 1.07, still near monodispersity.

TABLE 2: Characteristics of the Blends as1/s,as2/>s, as3/s, as4/s Studied in This work morphologyd

was1/wsa

ΦPSb

dPS/dPBc

20/80 33/67 50/50 67/33 80/20

0.374 0.335 0.287 0.239 0.203

1.181/1.001 1.307/1.001 1.465/1.001 1.584/1.000 1.581/1.000

was2/ws

ΦPS

dPS/dPB

20/80 33/67 50/50 67/33 80/20

0.402 0.382 0.357 0.332 0.313

1.100/1.009 1.152/1.012 1.194/1.012 1.197/1.010 1.160/1.007

was3/ws

ΦPS

dPS/dPB

morphology

20/80 33/67 50/50 67/33 80/20

0.469 0.493 0.525 0.557 0.583

1.001/1.086 1.001/1.130 1.001/1.170 1.001/1.175 1.000/1.144

Lamella Lamella Lamella Lamella Lamella

was4/ws

ΦPS

dPS/dPB

morphology

20/80 33/67 50/50 67/33 80/20

0.513 0.564 0.635 0.707 0.764

1.002/1.350 1.002/1.615 1.002/2.016 1.001/2.423 1.001/2.611

PS PS PS PS PS

dispersed dispersed dispersed dispersed dispersed

morphology PS PS PS PS PS

PB PB PB PB PB



a

Weight ratio of the components. b ΦPS, overall volume fraction of PS in the blend systems. c The discrete distribution of the molecular weight of the PS and PB block in the blend systems, respectively. d The interface information of the blend thin films obtained at the extremely slow evaporation rate.

other four had similar molecular weights and asymmetric block lengths with various ratios of the PS/PB block, which were named as, as1, as2, as3, and as4, respectively. Selected characteristics of these block copolymers were summarized in Table 1. The block copolymers were separately dissolved in toluene to obtain solutions with the concentration of 5 mg/mL. A series of as1/s, as2/s, as3/s, and as4/s mixtures with various weight fractions were prepared by simply mixing the solutions. The details of the blend samples were listed in Table 2. All the blend samples were mixed at a molecular level regardless of the compositions of the mixing solutions.29 2.2. Sample Preparation. The thin films were prepared by spin-casting the mixing solution onto the carbon-coated mica at 2000 rpm for 30 s. Such fast solvent evaporation resulted in the insufficient time for the rearrangement of the configuration of the chains, leading to a disordered pattern of the as-cast thin films. Cyclohexane was chosen as the annealing solvent. Table

3 listed the properties of cyclohexane. The as-cast thin films were directly exposed to saturated cyclohexane vapor (≈ 100 mmHg) in an airtight vessel (approximately 31 cm3) at room temperature (≈ 25 °C). After being exposed to the saturated cyclohexane vapor for approximately 12 h, the block chains of the systems had the sufficient time to relax and rearrange their configurations. The morphologies of all series blend samples did not change with further prolonged annealing treatment within the time scale of the experiment (e72 h), which indicated that they all reached the metastable state. After the solvent annealing treatment, the samples were removed from the vessel as quickly as possible and air-dried at ambient atmosphere. No morphological changes were observed during the air-drying process, which indicated that the resulting microdomain structures after cyclohexane annealing were well preserved. The thicknesses of the initial as-cast thin films were approximately 40 nm as measured by D8 X-ray reflectometry. The dewetting phenomenon was not observed for the thin films during the solvent annealing process. Although several films with varied local thicknesses are produced after solvent annealing, that is, forming the usual “terrace” phenomenon in such annealing treatment,37 no obvious change in the morphology had been observed in the different terraces. For the solution-cast experiment, a 30 µL pipet was used to cast equal-sized droplets of the blend toluene solutions with the concentration of 0.5 mg/mL onto the carbon-coated mica. An extremely slow solvent evaporation condition (≈0.125 µL/ h), which was achieved by allowing the solution-cast films to be exposed to a toluene vapor atmosphere (a few drops of toluene were precast around the substrate) in a small dish completely covered with the lid, was adopted. The solutioncast films were completely dried at room temperature after approximately 10 days. After complete toluene evaporation, the thicknesses of the blend thin films were approximately 40∼60 nm. 2.3. Instruments. The tapping mode atomic force microscopy (AFM) measurements were carried out on a commercial SPA-300HV/SPI3800 N station (Seiko Instruments Inc., Japan). The AFM cantilever (spring constant 2 N/m, Olympus Co., Japan) was driven to oscillate at ∼70 kHz, close to the cantilever’s resonant frequency. Transmission electron microscopy (TEM) experiments were performed on a JEOL 1011 TEM with an accelerating voltage of 100 kV in the bright-field mode. For plane-view TEM studies, the thin film and its carbon support were floated off in a pool of distilled water and then picked up with copper grids. To enhance the contrast between the PS and PB phases, the specimens were stained with osmium tetraoxide (OsO4) for few hours to stain the PB block, thus the PB regions appeared dark and the PS regions appeared bright in all the TEM micrographs presented here. For cross-sectional TEM experiments, some portions of the floated film were collected onto a piece of cured epoxy resin and dried. After staining with OsO4, these epoxy pieces were embedded in the epoxy resin and subsequently heated to 35, 45, and 55 °C, respectively, for 12 h. The ultrathin sections with approximately 50 nm thickness were microtomed using a LEICA Ultracut R microtome and a glass knife at room temperature and collected onto the carbon-coated copper grids. 3. Results As it has been known, the relative affinity of solvent for a certain block is governed by polymer-solvent interaction parameter, χP-S. For the SB block copolymer and the annealing solvent, i.e. cyclohexane used here, the value of χPS-cyclohexane

2714 J. Phys. Chem. B, Vol. 113, No. 9, 2009

Guo et al.

Figure 1. Tapping mode AFM height images of thin films of as1/s blend systems after exposure to the saturated vapor of cyclohexane for 12 h at room temperature. The weight ratios of as1 and s are (a) 20/80, (b) 33/67, (c) 50/50, (d) 67/33, and (e) 80/20, respectively. The inset of each figure shows the FFT pattern of the image.

TABLE 3: Characteristics of the Annealing Solvent

cyclohexane

molar volumeV (mL/mol)

polymer-solvent interaction parameter χPS-cyclohexaneb/χPB-cyclohexanec

16.8

108.6

0.5322/0.3417

Obtained from Polymer Handbook Fourth Edition. Cyclohexane is a near Θ solvent for PS at 34.5 °C, χPS-cyclohexane ) - 0.556 + 324.3/T. Calculated from χ ) V(δ - δPB)2/RT + 0.34 at the annealing temperature 25 °C, and δPB ) 17.0 (J/cm3)1/2. a

c

solubility parametera δ (J/cm3)1/2

b



Figure 2. (a-c) Plain-view bright-field TEM micrographs of the same as1/s blend thin films shown in parts (a) 20/80, (c) 50/50 and (d) 67/33 of Figure 1, respectively.

and χPB-cyclohexane are listed in Table 3. According to the Flory-Huggins criterion, cyclohexane is a good solvent for PB block and (near) Θ solvent for PS block at the annealing temperature (25 °C). The AFM and TEM results of different morphologies of the blend thin films as a function of composition after annealing in cyclohexane vapor for 12 h are summarized in Figure 1 to Figure 6. (The controlled experiments are also carried out for the neat block copolymers s, as1 to as4. All of the thin films of the neat block copolymers, except for as4, formed the morphologies of dispersed PS phases in the PB matrix. For the neat as4 block copolymer, morphology with the PB phase dispersed in PS matrix without long-range order was observed. These data are not shown here.) Figure 1 displays the typical AFM height images of five representative as1/s blend thin films after exposure to the saturated solvent vapor of cyclohexane for 12 h. Both the as1/s blend thin films with the weight ratios of 20/80 (ΦPS ) 0.374) and 33/67 (ΦPS ) 0.335) exhibited “flowerlike” patterns. Each “flower” comprised of six uniform isolated bright spots and each bright spot was attributed to three flower28,38 (Figure 1a,b). The three-dimensional (3D) video of the cross-sectional view of the flowerlike structure of as1/s (20/80) thin film confirms that these bright spots are spheres (see the Supporting Information). The diameters of the spherical domains in Figure 1a,b are 33.7 ( 2.9 and 31.7 ( 2.5 nm, respectively. The uniform size of the spheres indicates that the two different kinds of block copolymers, that is, as1 and s, are mixed at a molecular level, forming a single spherical microdomain. As the weight ratio of as1/s

increases to 50/50 (ΦPS ) 0.287), the spherical structures in the blend thin films vary from the flowerlike pattern to simple dispersed domains, which are less orderly. The average diameter of these dispersed spheres is 26.7 ( 2.4 nm (Figures 1c). When the weight ratio of as1/s reaches 67/33 (ΦPS ) 0.239), the coexistence of two types of dispersed spherical microdomains is clearly observed as shown in Figure 1d. The average diameter of the large spheres is 30.4 ( 4.4 nm and the smaller ones is 15.0 ( 2.4 nm. Similar morphology and size of spherical domains are also observed in 80/20 (ΦPS ) 0.203) blend thin film with exception of the decreasing number of the larger spheres (Figure 1e). Although the large and small spheres are uniformly dispersed without extending to the level of macrophase separation, the morphologies are obviously less longrange order as indicated by the fast Fourier transform (FFT) patterns (insets of Figure 1d,e). Parts a, b, and c of Figure 2 displays the TEM images obtained from the same as1/s blend thin films displayed in parts (a) 20/80, (c) 50/50 and (d) 67/33 of Figure 1, respectively. The structures revealed by TEM are consistent with those observed by AFM. The TEM images further confirm that the bright domains are dispersed PS spheres. Figure 3 and Figure 4 show the AFM and TEM images of as2/s blend thin films with different compositions, respectively. Similar flowerlike structures with regularly arranged PS spherical domains are observed for the blend thin films with weight ratios of 20/80 (ΦPS ) 0.402) (Figure 3a and 4a), 33/67 (ΦPS ) 38.2%) (Figure 3b and 4b), and 50/50 (ΦPS ) 0.357) (Figure 3c and 4c), respectively. The diameters of these spherical


Guo et al.

Figure 3. Tapping mode AFM height images of thin films of as2/s blend systems after exposure to the saturated vapor of cyclohexane for 12 h at room temperature. The weight ratios of as2 and s are (a) 20/80, (b) 33/67, (c) 50/50, (d) 67/33, and (e) 80/20, respectively. The inset of each figure shows the FFT pattern of the image.

domains are 35.5 ( 2.1, 33.0 ( 2.9, and 28.0 ( 2.5 nm, respectively. Further increasing the weight ratio to 67/33 (ΦPS ) 0.332) and 80/20 (ΦPS ) 0.313), the irregular-shaped spherical PS domains (Figure 3d and 4d) or wormlike PS cylindrical domains (Figure 3e and 4e) dispersed in the PB matrix are observed. The diameters of these PS spherical domains are 27.5 ( 2.7 and 24.2 ( 1.9 nm, respectively. The AFM and TEM results also indicate that the morphologies shown in Figures 1 to 4 are the “single-layer” structures,

which are different from the overlapping phenomena observed by Luchnikov et al.39 Figure 5 shows the representative TEM images of as3/s blend thin films with weight ratios of 20/80 (ΦPS ) 0.469), 50/50 (ΦPS ) 0.525), and 80/20 (ΦPS ) 0.583), respectively. The PS cylindrical morphologies are observed. The cross-sectional TEM micrograph (inset of Figure 5c) clearly shows one layer of spherical-like PS domain, which proved that the PS cylinders are parallel to the plane of the thin film. For as3/s blends with



Figure 4. (a-e) Plain-view bright-field TEM micrographs of the same as2/s blend thin films shown in parts (a) 20/80, (b) 33/67, (c) 50/50, (d) 67/33 and (e) 80/20 of Figure 3, respectively.

weight ratios of 33/67 and 67/33, similar morphologies are observed (data not shown). For the as4/s blend system, no evident structures are observed by the in-plain view TEM for 20/80 (ΦPS ) 0.513), 33/67 (ΦPS ) 0.564), and 50/50 (ΦPS ) 0.635) blend thin films after exposure to the saturated solvent vapor of cyclohexane for 12 h. However, the cross-sectional TEM micrographs revealed the formation of the lamellar morphologies throughout the films. Figure 6a shows the in-plain and cross-sectional TEM micrographs of 50/50 (ΦPS ) 0.635) blend as an example. With a further increase of the weight ratio to 67/33 (ΦPS ) 0.707), the spherical PB domains dispersed in the PS matrix are observed (Figure 6b). The average diameter of the PB spherical domains is 38.1 ( 5.2 nm. In some parts of this thin film, the flowerlike

patterns comprising six isolated PB spheres with less order are observed. The similar morphology is also observed in the as4/s 80/20 (ΦPS ) 0.764) blend thin film. 4. Discussions Figure 7 summarizes the metastable morphologies of the above four binary blend thin films of SB block copolymers as a function of blend composition, namely, the overall volume fraction of PS block (ΦPS). For the as1/s and as2/s blend thin films, the compositions are in the range of 0.203 e ΦPS e 0.402. Almost all the blend thin films form the PS spherical domains dispersed in the PB matrix. For the as3/s blend systems with the compositions in the range of 0.469 e ΦPS e 0.583, the PS cylinders in the PB matrix are formed. For as4/s blend thin films,


Guo et al.

Figure 5. (a-c) Plain-view bright-field TEM micrographs of thin films of as3/s blend systems with weight ratios (a) 20/80, (b) 50/50, and (c) 80/20 of as3 and s, respectively, after exposure to the saturated vapor of cyclohexane for 12 h at room temperature. The inset in part c shows the cross-sectional TEM micrograph corresponding to its plain-view micrograph.

Figure 6. Plain-view bright-field TEM micrographs of thin films of as4/s blend systems with weight ratios (a) 50/50 and (b) 67/33 of as4 and s, respectively, after exposure to the saturated vapor of cyclohexane for 12 h at room temperature. The inset in the part (a) shows the corresponding cross-sectional TEM micrograph.

two kinds of morphologies are observed. When the composition is in the range of 0.513 e ΦPS e 0.635, the parallel lamellar structure are generated, whereas for 0.707 e ΦPS e 0.764, the blend system will form the spherical PB domains dispersed in the PS matrix. Note that the binary blend systems studied here are totally miscible on the molecular level at all compositions, and only the microphase separation occurs under the solvent annealing treatment. As we have known, the morphological behavior of the system will be affected by the preferential affinity for the PB block

after solvent annealing in the saturated vapor of cyclohexane, since the relatively selective swelling of PB block will lead to an increase of the effective volume fraction of PB domain, and hence influence the interfacial curvature of the system.18-20 Accordingly, the morphology and phase behavior of the blend thin films are mainly dependent on the inherently interfacial curvature of the blend systems and the influence of the annealing solvent. Therefore, before we discuss the influence of solvent on the final morphologies of blend thin films, it is necessary to elucidate



Figure 7. Summary of morphological behavior of the binary blend thin films of PS-b-PB block copolymers with the increasing of the overall volume fraction of PS block (ΦPS). The differences of molecular weight of the PS and PB block chains are represented by the different lengths of the straight lines in the inset of figure.

the inherently interfacial curvature of the blend thin films without the solvent influence. In order to achieve this, we adopted the usual slow drying from a nonselective solvent as an alternative route for equilibrium morphology, instead of thermal treatment. The blend thin films were prepared by casting their toluene solution onto the mica surface and then dried with extremely slow solvent evaporation rate (about 0.125 µL/h for ten days). Toluene is considered to be a good solvent for both blocks, and the influence of the discrepant affinity of toluene for the PS and PB block can be neglected.18,40 The morphologies obtained at this condition reflect the spontaneous domain structures of the blends in the thin films without solvent treatment. It should be noted here that the structures obtained in this way do not necessarily correspond to the thermodynamic equilibrium morphology because free surfaces and film thickness are equally important parameters upon which the final structure is dependent.41-45 Here, we focused mainly on the comparison of the spontaneous curving of the interface of blends with the one obtained after solvent annealing at the same film thickness, with all the other parameters kept constant. The information of the interface of the blend thin films obtained at the extremely slow evaporation rate are summarized in Table 2, and some typical morphology of them are listed below. Figure 8 shows the TEM images of as1/s blend thin films obtained at the extremely slow evaporation rate. The 20/80 (ΦPS ) 0.374) and 33/67 (ΦPS ) 0.335) blend thin films show that the structures of the PS spherical and PS cylindrical microdomains coexisting in the PB matrix, whereas the blend thin films with 50/50 (ΦPS ) 0.287), 67/33 (ΦPS ) 0.239) and 80/ 20 (ΦPS ) 0.203) show only the PS spherical microdomain in the PB matrix. The cross-sectional view of TEM micrograph in the inset of Figure 8d further confirms that the PS block form dispersed spheres. Apparently, it seems that these blend systems allowed the asymmetric block, as1, to take on its spontaneous curvature at all compositions. The same conclusion is also valid

for the as2/s blends with compositions range of 0.313 e ΦPS e 0.402, which form PS cylinders in PB matrix under the extremely slow evaporation rate. It is because the two components of the block copolymer blends, that is, as1 (or as2) and s, are miscible at a molecular level, they will therefore share the common interface, and the “cosurfactant effect” will determine the inherently interface curvature of the blend systems.34,35,46,47 When the long symmetric block copolymer (bcp) is mixed with the asymmetric short block copolymer, the large symmetric bcp s will fill the “density dip” caused by the short asymmetric bcp as packing of many chains with its curvature. Such blending will then promote the asymmetric bcp as to take on its spontaneous curvature, that is, the inherently interfacial curvature of the blend systems under “cosurfactant effect” is similar to that of the asymmetric bcp as.34,48 For as3/s blends, the compositions are in the range of 0.469 e ΦPS e 0.583. The lamellae structures parallel to the free surface are always obtained after extremely slow solvent evaporation. A TEM micrograph of 80/20 blend thin film is given in Figure 9. The formation of the terrace is due to the mismatch between the film thickness and the lamellar period (Figure 9a). In the cross-sectional view of TEM images, the two wetting layers form a lamella in T1 (Figure 9b) and an alternative lamellae in T2 (Figure 9c), respectively. A thin PB layer covers the sample surface due to the lower surface energy of PB.49 For as4/s blends, the compositions reach the range of 0.513 e ΦPS e 0.764 and PB domains dispersed in PS matrix are always observed for all compositions. Figure 10 clearly indicates a trend of morphological transition from parallel PB cylinders (20/80, ΦPS ) 0.513) to PB spheres (50/50, ΦPS ) 0.635) and finally to the coexistence of large and small sphere structures (80/20, ΦPS ) 0.764). Similar phenomena of the coexistence of the “large and small spherical phase” have been observed by Hashimoto et al.29 in the binary blend systems of SI block copolymers in the bulk state. They termed it the “local


Guo et al.

Figure 8. Plain-view bright-field TEM micrographs of thin films of as1/s blend systems with weight ratios (a) 20/80, (b) 33/67, (c) 50/50, (d) 67/33 and (e) 80/20 of as1 and s, respectively, obtained at extremely slow evaporation rate. The inset in the part d shows the cross-sectional TEM micrograph corresponding to its in-plain view micrograph.

scale” phase separation, which may be triggered by the fluctuation-induced segregation effect. The different inherently interfacial curvature of as3/s and as4/s can also be attributed to the cosurfactant effect, which helps the asymmetric bcp as3 and as4 to keep their spontaneous interfacial curvature in the blend systems. It should be noted here that the substrate may have some effect on the final structures,49,50 however, no obvious change in morphologies is observed in the parallel experiment on the different substrates, such as the carbon-coated mica nearly neutral for both PS and PB, and silicon wafer preferentially wet by PB. Therefore, the influence of the substrate for the inherent morphologies of the blends can be ignored.

With the knowledge of the inherent morphologies of the series of blends at different compositions, we can now investigate the mechanism of the morphological transition of the blend systems under the solvent field. For the as1/s blends, the influence of the solvent field with preferential affinity for PB block making the interface curving for PB phase is consistent with the inherently interfacial curving of the systems, as shown in the schematic illustration of Figure 11a. From the morphologies of as1/s blend thin films after solvent annealing (Figure 1 and Figure 2), it is found that the 20/80 (ΦPS ) 0.374), 33/67 (ΦPS ) 0.335), and 50/50 (ΦPS ) 0.287) blend thin films show the ordered flowerlike spherical morphology or the dispersed PS spherical morphology with uniform domain dimension (Figure



Figure 9. (a) Plain-view bright-field TEM micrograph of thin film of as3/s blend system with weight ratio of 80/20 obtained at extremely slow evaporation rate. (b,c) The cross-sectional view TEM micrographs of the parts marked T1 and T2 in panel a, respectively.

1a-c, Figure 2a,b), whereas the 67/33 (ΦPS ) 0.239) and 80/ 20 (ΦPS ) 0.203) blend thin films display the coexistence of large and small PS spheres with different domain dimensions. Such phenomena could be attributed to the packing frustration induced by the difference between the PS chain lengths. The chemical junctions of as1 and s may share the same interface, which will in turn drive the PS block of different components, that is, PSas1 and PSs to segregate into the respective domains,27 as illustrated in Figure 12. The configurations of PS chains are stretching and comfortless rearranged under the solvent field of cyclohexane,28 the differences of the PS chains length thus became noticeable. In order to qualitatively understand such interpretation, an artificially created parameter, dP, is introduced in this work to address the discrepant degree of the chain length and the discrete distribution of the PB or PS block in the blend systems. Here, d(P) ) Mw(P)/Mn(P), while Mn(P) ) ΣniMn(Pi)/Σni, and Mw(P) ) ΣniMn(Pi)2/ΣniMn(Pi). The subscript P represents the block of the PS or PB, and ni is the molar quantity of the block copolymer component i in the blend systems, and Mn(Pi) is the number-averaged molecular weight of block P in the block copolymer. The values of the calculated dP are listed in Table 2. dP is somewhat similar to the polydispersity index (PDI) of a polymer sample. The variation of the dP value comes from two origins, that is, the blending process of two block copolymers with different PS/PB block lengths and the polydispersity of each diblock copolymer sample before mixing. In our present work, the dP value is mainly affected by the first factor, since the PDI value of the neat bcp is very small. Therefore, the uniform PS domains of the as1/s blends (0.287 e ΦPS e 0.374) show that the packing frustration in the systems induced by the discrete distribution of the PS block (dPS 1.181∼1.465) is not strong enough to cause the segregation of PSas1 and PSs into their respective domains in these cases (Figure 11d). For the as1/s blends with the coexistent large and small PS spheres, the dPS value (1.584 and 1.581) is evidently larger than the systems with uniform PS spherical domains dimension, whereas the dPB value is nearly constant (dPB 1.001). In comparison with the inherent morphologies of these blend systems without solvent treatment (Figure 8a-d), it seems that the presence of the solvent field intensify the influence of the

discrete distribution of the nonpreferential affinity PS block. The PSas1 and PSs segregate into their respective domains in order to eliminate the fluctuations of the chain density (Figure 11e). For the as2/s blend systems, the influence of the solvent field with preferential affinity for PB block on the interface is also consistent with the inherently interfacial curving of the systems. Although the morphological transition of these blend thin films (Figure 3 and Figure 4) are similar to those of the as1/s blend systems, that is changing from flowerlike pattern to dispersed PS spheres, the dimension of the PS domain are always uniform (the coexistent large and small spheres structures have not been observed). Since the dPS ranges from 1.100 to 1.160, which is much lower than the value of the as1/s blends, the small density frustration does not drive the PSas2 and PSs to form their respective domains. Moreover, for both the as1/s and the as2/s blend systems, the ordered PS spheres arranged as flowerlike patterns are observed in the composition range of 0.335 e ΦPS e 0.402. It is very likely that the flowerlike pattern is a metastable structure, which is mainly dependent on the blend composition and the preferential affinity of solvent for the PB block. The possible reason for the formation of flowerlike pattern is that the inherent morphology of the blend system has the spontaneous tendency to fuse PS domains together (e.g., Figure 8a,b) and hence the dispersed PS spheres tend to adopt a tight arrangement in order to decrease the space between PS microdomains under the solvent field. For the as3/s blends (0.469 e ΦPS e 0.583), the influence of the solvent field with preferential affinity for PB block on the interface is different from the inherently interfacial curving of the systems (Figure 11b). Their inherent morphologies in thin films show the lamella structure (Figure 9); however upon exposure to cyclohexane vapor, the morphologies of the inplain PS cylinders dispersed in PB matrix generated as observed for all compositions (Figure 5). It seems that the effect of the solvent field acts as a dominating role. Meanwhile, the dPS values are kept constant (≈1.000) and dPB values possess a narrow range from 1.086 to 1.144. Thus, the small density frustration in the systems leads to the uniform PS domain (Figure 11f).


Guo et al.

Figure 10. Plain-view bright-field TEM micrographs of thin films of as4/s blend systems with weight ratios (a) 20/80, (b) 33/67, (c) 50/50, (d) 67/33, and (e) 80/20 of as4 and s, respectively, obtained at extremely slow evaporation rate.

For the as4/s blends (0.513 e ΦPS e 0.764), the influence of the solvent field with preferential affinity for PB block on the interface is opposite to that of the inherently interfacial curving of the systems (Figure 11c). The influence of the solvent on the interfacial curvature is most pronounced in the range of 0.513 e ΦPS e 0.635. After solvent annealing, the systems generate the lamella structures, although the inherent morphologies of them are PB dispersed domains. In these cases, the dPS values are still kept constant (≈1.002), but dPB ranges from 1.350 to 2.016. The single lamella structures indicate that the discrete distribution of the PB block has little effect on the morphologies under the solvent field with preferential affinity for PB block (Figure 11g). However, with ΦPS g 0.707, the systems produce

the dispersed PB domains even when the solvent field has a preferential affinity for PB block (Figure 11h). It proves that the influence of the solvent field on the final morphologies of blend thin films will no longer play the decisive role. It is most likely that the preferential affinity of cyclohexane for PB block is not strong enough to alter the interfacial curvature when the composition ΦPS g 0.707. In this case, the large dPB value (2.423∼2.611) will result in the large density fluctuation of PB blocks in the blends and hence the domain structure with lack of long-range order (Figure 6b). The experimental results indicate that for as/s blends, the varied morphologies in thin films can be achieved by solvent annealing in cyclohexane vapor, varying the composition and



Figure 11. Schematic illustration of the interfacial transformation of the blends. (a-c) The inherently interfacial curving of the (a) as1/s (as2/s), (b) as3/s, and (c) as4/s blend systems, respectively. (d-h) The interface of (d) as1/s (as2/s) with lower dPS, (e) as1/s with higher dPS, (f) as3/s, (g) as4/s with ΦPS e 0.635, and (h) as4/s with ΦPS g 0.707 blend systems, respectively, after solvent annealing.

Figure 12. Schematic illustration of the packing frustration induced by the difference of the chain length of the PS block, driving the PSas1 and PSs to segregate into the respective domains.

chain length of the blend systems. The preferential affinity of solvent for PB block can drive the systems to alter their inherently interfacial curvature, and intensify the response of the morphology for the discrete distribution of the nonpreferential affinity PS block, even though the difference of ∆χ (|χPSsolvent-χ PB-solvent|) is small. 5. Conclusions By selecting a symmetric PS-b-PB block copolymer as the fixed component and then mixing with another asymmetric SB block copolymer with the various chain length ratios of PS/PB blocks, a series of binary SB blends with different volume fraction of PS were prepared. The influences of the composition and the chain length distribution of the mixtures on the morphologies and phase interfaces of the blend thin films under saturated vapor of cyclohexane (preferential affinity for PB block) were investigated. The morphologies of the cyclohexane-annealed blend thin films are mainly dependent on the cooperative effects of the “cosurfactant effect” of the blends, which determines the inherently interfacial curvature of the systems, and the nature of the annealing solvent. When the overall volume fraction of PS, ΦPS, is within the range of 0.203 to 0.635, all of the blend thin films form the dispersed PS phase in the PB matrix or lamellar structures. In such cases, the effect of the solvent field plays an important

role. Meanwhile, the large discrete distribution of PS block, dPS, will drive the PSas and PSs to segregate into their respective domain, regardless of the value of dPB. However, if ΦPS is larger than 0.707, the systems tend to take their inherently interfacial curvature and form the dispersed PB domains even the annealing solvent has a preferential affinity for PB block. In such cases, the large dPB will lead to the large fluctuation of PB block and the less long-range order of the morphologies. The influence of the discrete distribution of PB or PS block on the morphology of blend block copolymer thin films can provide some reference for the effect of polydispersity on the phase behavior of the bcp thin films under the external field, which has not yet been reported to the best of our knowledge. Acknowledgment. We are grateful to Professor Joachim Loos and Dr. Kangbo Lu of Eindhoven University of Technology for assistance with the TEMT measurement and Ms. Guifen Sun for technical help with the microtomy. This work is supported by National Natural Science Foundation of China (20774095) and subsidized by National Basic Research Program of China (2005CB6238). B.D. thanks the National Natural Science Foundation of China (20604022), Scientific Research Foundation for Returned Overseas Chinese Scholars (State Education Ministry), and Zhejiang Provincial Natural Science Foundation of China (Y406029) for financial supports. Supporting Information Available: The 3D video of the structure of the binary blend poly(styrene-block-butadiene) block copolymer thin film of as1/s (20/80) systems. Bright particles are polystyrene phase. Box size: 260 × 260 × 40 nm. The ortho slices go along the Y axis, from back to front and go back. The information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. (3) Ruzette, A.; Leibler, L. Nat. Mater. 2005, 4, 19. (4) Abetz, V.; Simon, P. F. W. AdV. Polym. Sci. 2005, 189, 125. (5) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401.

2724 J. Phys. Chem. B, Vol. 113, No. 9, 2009 (6) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (7) Jeong, U.; Kim, H.; Rodriguez, R. L.; Tsai, I. Y.; Stafford, C. M.; Kim, J. K.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2002, 14, 274. (8) Park, C.; Yoon, J.; Thomas, E. L. Polymer 2003, 44, 6725. (9) Olson, D. A.; Chen, L.; Hillmyer, M. A. Chem. Mater. 2008, 20, 869. (10) (a) Zhang, Q.; Tsui, O. K. C.; Du, B.; Zhang, F.; Tang, T.; He, T Macromolecules 2000, 33, 9561. (b) Huang, H.; Zhang, F.; Hu, Z.; Du, B.; He, T.; Lee, F. K.; Wang, Y.; Tsui, O. K. C Macromolecules 2003, 36, 4084. (11) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Russell, T. P. Science 1996, 273, 931. (12) Kim, G.; Libera, M. Macromolecules 1998, 31, 2569. (13) Fukunaga, K.; Elbs, H.; Magerle, R.; Krausch, G. Macromolecules 2000, 33, 947. (14) Buck, E.; Fuhrmann, J. Macromolecules 2001, 34, 2172. (15) Elbs, H.; Drummer, C.; Abetz, V.; Krausch, G. Macromolecules 2002, 35, 5570. (16) Knoll, A.; Magerle, R.; Krausch, G. J. Chem. Phys. 2004, 120, 1105. (17) Chen, Y.; Huang, H.; Hu, Z.; He, T. Langmuir 2004, 20, 3805. (18) Huang, H.; Hu, Z.; Chen, Y.; Zhang, F.; Gong, Y.; He, T.; Wu, C. Macromolecules 2004, 37, 6523. (19) Gong, Y.; Hu, Z.; Chen, Y.; Huang, H.; He, T. Langmuir 2005, 21, 11870. (20) Gong, Y.; Huang, H.; Hu, Z.; Chen, Y.; Chen, D.; Wang, Z.; He, T. Macromolecules 2006, 39, 3369. (21) Cavicchi, K. A; Russell, T. P. Macromolecules 2007, 40, 1181. (22) Schmidt, K.; Schoberth, H. G.; Ruppel, M.; Zettl, H.; Hansel, H.; Weiss, T. M.; Urban, V.; Krausch, G.; Boker, A. Nat. Mater. 2008, 7, 142. (23) Kim, S. H.; Misner, M. J.; Russell, T. P. AdV. Mater. 2004, 16, 2119. (24) Perlich, J.; Schulz, L.; Kashem, M. M. A.; Cheng, Y. J.; Memesa, M.; Gutmann, J. S.; Roth, S. V.; Buschbaum, P. M. Langmuir 2007, 23, 10299. (25) Levicky, R.; Koneripalli, N.; Tirrell, M.; Ankner, J. F.; Kaiser, H.; Satija, S. K. Macromolecules 1998, 31, 4908. (26) Xuan, Y.; Peng, J.; Wang, H. F.; Li, B. Y.; Han, Y. C. Macromol. Rapid Commun. 2004, 25, 1181.

Guo et al. (27) Chen, Y.; Wang, Z.; Gong, Y.; Huang, H.; He, T. J. Phys. Chem. B 2006, 110, 1647. (28) Guo, R.; Huang, H.; Chen, Y.; Gong, Y.; Du, B.; He, T. Macromolecules 2008, 41, 890. (29) Koizumi, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1994, 27, 4371. (30) Court, F.; Hashimoto, T. Macromolecules 2001, 34, 2536. (31) Yamaguchi, D.; Hashimoto, T. Macromolecules 2001, 34, 6495. (32) Court, F.; Hashimoto, T. Macromolecules 2002, 35, 2566. (33) Hashimoto, T.; Yamauchi, K.; Yamaguchi, D.; Hasegawa, H. Macromol. Symp. 2003, 201, 65. (34) Chen, F.; Kondo, Y.; Hasimoto, T. Macromolecules 2007, 40, 3714. (35) Court, F.; Yamaguchi, D.; Hashimoto, T. Macromolecules 2008, 41, 4828. (36) Spontak, R. J.; Fung, J. C.; Braunfeld, M. B.; Sedat, J. W.; Agard, D. A.; Kane, L.; Smith, S. D.; Satkowski, M. M.; Ashraf, A.; Hajduk, D. A.; Gruner, S. M. Macromolecules 1996, 29, 4494. (37) Zoelen, W. V.; Asumaa, T.; Ruokolainen, J.; Ikkala, O.; Brinke, G. T. Macromolecules 2008, 41, 3199. (38) Peng, J.; Wei, Y.; Wang, H.; Li, B.; Han, Y. Macromol. Rapid Commun. 2005, 26, 738. (39) Luchnikov, V.; Kondyurin, A.; Formanek, P.; Lichte, H.; Stamm, M. Nano Lett. 2007, 7, 3628. (40) Huang, L.; He, X.; He, T.; Liang, H. J. Chem. Phys. 2003, 119, 12479. (41) Hasegawa, H.; Hashimoto, T. Polymer 1992, 33, 475. (42) Turner, M. S.; Rubinstein, M.; Marques, C. M. Macromolecules 1994, 27, 4986. (43) Suh, K. Y.; Kim, Y. S.; Lee, H. H. J. Chem. Phys. 1998, 108, 1253. (44) Szamel, G.; Muller, M. J. Chem. Phys. 2003, 118, 905. (45) Lyakhova, K. S.; Sevink, G. J. A.; Zvelindovsky, A. V.; Horvat, A.; Magerle, R. J. Chem. Phys. 2004, 120, 1127. (46) Shi, A.-C.; Noolandi, J. Macromolecules 1994, 27, 2936. (47) Shi, A.-C.; Noolandi, J. Macromolecules 1995, 28, 3103. (48) Hashimoto, T.; Yamaguchi, D.; Court, F. Macromol. Symp. 2003, 195, 191. (49) Tsarkova, L.; Knoll, A.; Krausch, G.; Magerle, R. Macromolecules 2006, 39, 3608. (50) Harrison, C.; Park, M.; Chaikin, P. M.; Register, R. A.; Adamson, D. H.; Yao, N. Polymer 1998, 39, 2733.

JP808551J

Solvent-Induced Morphology of the Binary Mixture of Diblock

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