Hierarchical Structures in Thin Films of Macrophase - American

Jun 27, 2012 - When the weight fraction of the lamellar DMPCS in the blend is 25%, the short ... structures in thin films of incompatible AB/AC BCP bl...
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Hierarchical Structures in Thin Films of Macrophase- and Microphase-Separated AB/AC Diblock Copolymer Blends Ling-Ying Shi, Yu Zhou, Zhihao Shen,* and Xing-He Fan Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Interesting hierarchical structures are generated in thin films of the AB/AC diblock copolymer (diBCP) blends of poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS, DMPCS) rod−coil diBCP and poly(dimethylsiloxane)-b-poly(methyl methacrylate) (PDMS-bPMMA, DMMA) coil−coil diBCP with the common block as the minor component in both diBCPs. The macrophase separation and microphase separation occur in the DMPCS/DMMA BCP blends in bulk, confirmed by small-angle X-ray scattering (SAXS) results. Moreover, the macrophase- and microphase-separated morphologies in thin films of the DMPCS/DMMA BCP blends are directly observed by transmission electron microscopy experiments owing to the different electron densities among the three different blocks. For the blends of DMPCS and DMMA, both of which have the nanostructures of hexagonally packed cylinders (HEX) (DMPCSHEX/DMMAHEX), when the blend contains 75 wt % of one diBCP, subordered macrophase-separated structures with ordered nanostructures in the macrodomains develop in the thin film. When the matrix of the macrophase is the coil−coil DMMAHEX diBCP which has the nanostructure of vertically oriented cylinders in the thin film, the macrophase-separated submicrometer structures become more ordered, and the interfaces of the macrodomains become more smooth. For the blends of the lamellar DMPCS and the HEX-structured DMMA having similar volume fractions of PDMS (DMPCSLAM/DMMAHEX), with 75 wt % of lamellar DMPCS in the blend, hamburger-like structures form in the DMPCSLAM macromatrix of the thin film, which is ascribed to the solubility of DMMA in the lamellar DMPCS on the segmental length scale. When the weight fraction of the lamellar DMPCS in the blend is 25%, the short DMPCS lamellae with a few layers are uniformly dispersed in the HEX-structured DMMA macromatrix.

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INTRODUCTION The various ordered nanostructures self-assembled from block copolymers (BCPs) have attracted significant scientific interest due to their great potential applications in nanotechnology.1,2 The final morphologies and the morphology tunability of BCPs are determined by the volume fraction f of the individual block and χN, where χ is the Flory−Huggins interaction parameter and N is the total degree of polymerization.3 Depending on these parameters, coil−coil diblock copolymers (diBCPs) can form lamellae (LAM), hexagonally packed cylinders (HEX), bicontinuous gyroid, and cubic arrays of spheres.4 The phase behavior of rod−coil diBCPs has some differences from that of coil−coil diBCPs, including the asymmetry of the phase diagram and the appearance of some new morphologies such as zigzag lamellae, wavy lamellae, and hexagonally perforated lamellae.5,6 Moreover, blending is another way of controlling the phaseseparated structures of block copolymers. The morphologies of various block copolymer blends have been explored from simulation to experiment such as the mixture of a diblock copolymer with a homopolymer (AB/A, AB/C)7−9 and the binary blend of two diblock copolymers (ABα/ABβ, AB/AC, AB/CD).10−15 Mixing a diBCP with a homopolymer is an © 2012 American Chemical Society

effective way to tune periodic size and/or induce morphology transition.16−18 Compared with the binary blends of diBCPs with homopolymers, the self-assembly of binary blends of two different diBCPs is much more complicated. Among various blends, the AB/AC BCP blends with a common block are especially interesting. In addition to three Flory−Huggins interaction parameters (χAB, χBC, and χAC),19 the molecular weight difference between the two block copolymers, the volume fractions of the common block ( fA1, fA2), and the composition (ϕAB, ϕAC) have great effects on the final morphologies of the AB/AC BCP blends. Depending on these factors, many different morphologies occur in the AB/AC (A, B, and C are incompatible) BCP blends such as the ABC triblock copolymer analogous morphologies,20,21 the microphase-separated nanostructures in the macrophase-separated domains,10 and the macrophase separation with partial ABC triblock copolymer analogous microphase separation in the macrophase-separated domains.22 In addition, one-to-one blend of AB and AC diBCPs can form Janus particles with two Received: December 7, 2011 Revised: June 19, 2012 Published: June 27, 2012 5530

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DMPCS) rod−coil diBCP and poly(dimethylsiloxane)-b-poly(methyl methacrylate) (PDMS-b-PMMA, DMMA) coil−coil diBCP. These two kinds of BCPs are strongly segregated BCPs with the common block, PDMS, as the minor component. The rod−coil DMPCS diBCP forms a HEX structure only when the volume fraction of PDMS ( f PDMS) decreases to ∼18% as reported previously.42 Therefore, when the DMPCS and DMMA have similar f PDMS values, they may self-assemble into different nanostructures. We studied the thin-film morphologies of the unsymmetrical blends of the HEXstructured DMPCS with the HEX-structured DMMA (DMPCSHEX/DMMAHEX) and those of the LAM-structured DMPCS with the HEX-structured DMMA having similar f PDMS values (DMPCSLAM/DMMAHEX). And we investigated the influence of the microphase-separated nanostructure on the order of the macrophase-separated structure and the confinement effect of macrophase morphologies on the orientation of the nanostructures in the macrodomains. Moreover, by comparing the thin-film morphologies of DMPCS HEX / DMMAHEX and DMPCSLAM/DMMAHEX BCP blends, we found that the different nanostructured DMPCS diBCPs had different solubility in the HEX-structured DMMA on the segmental length scale.

different domains of microphase separation by a simple solvent evaporation method.23 However, as far as we know, the arrangement of the macrophase- and microphase-separated structures in thin films of incompatible AB/AC BCP blends has not been reported. Block copolymer thin films with ordered nanostructures are useful in many fields such as nanolithography, nanoscale arrays, and separation membranes.24,25 How to control the orientation and order of the nanostructures in block copolymer thin films has always been a scientific problem which has been the target of many attempted methods,26,27 such as surface modification via random copolymers and solvent annealing for controlling orientation,27−30 graphoepitaxy, chemically patterned surface for directing order in the lateral direction,31−34 and so on. Furthermore, hierarchical patterns can be obtained in BCP thin films by combining microphase separation of BCPs with additional patterning or self-organization strategies.35 Such strategies include preparing BCP thin films over topographically or chemically patterned substrates,36−38 combining the ordered BCP nanostructure with liquid crystalline and semicrystalline ordering,2,39 and adding homopolymers to BCPs to form blends.35,40 Blending BCPs with homopolymers can produce micro- and nanostructured thin films depending on the molecular weights of the homopolymers and the compositions of the blends.35,40,41 Can the AB/AC BCP blends offer another way of producing hierarchical structures in BCP thin films? By combining the microphase and macrophase separations in the blends of strongly segregated AB and AC diblock copolymers with different microphase-separated nanostructures, hierarchical structures may be expected in thin films of AB/AC BCP blends. For example, when the AB and AC diBCPs have hexagonally packed cylinders of different periodic sizes formed by the common A block, the thin films of their blends with unsymmetrical blending compositions may form hierarchical structures as shown schematically in Figure 1. In this work, we present the hierarchical structures in thin films of unsymmetrical AB/AC binary BCP blends. The diblock copolymers used are poly(dimethylsiloxane)-b-poly{2,5-bis[(4methoxyphenyl)oxycarbonyl]styrene (PDMS-b-PMPCS,

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EXPERIMENTAL SECTION

Materials. Macroinitiator PDMS-Br was prepared as reported in our previous work.42 N,N,N′,N″,N″-Pentamethyldiethylenetriamine (PMDETA, TCI, 98%) was used as received. Methyl methacrylate (98%) and tetrahydrofuran (THF, Beijing Chemical Reagents Co., A.R.) were used after vacuum distillation and atmospheric distillation, respectively. Chlorobenzene was purified by washing with concentrated sulfuric acid to remove residual thiophenes, followed by washing with a 5% sodium carbonate solution and water, and then it was dried with anhydrous calcium chloride and finally distilled. CuBr (Beijing Chemical Reagents Co., A.R.) was purified by washing with acetic acid, followed by washing with methanol, and then it was dried for use. Thin-Film Preparation and Characterization. The pure diblock copolymers or their blends were dissolved in dry THF (with concentrations of 2.5−3 mg/mL) with stirring at ambient temperature for 12 h. Then the homogeneous solutions were cast on carbon membranes coated on Cu grids (T10044) in dry air, and the cast thin films were maintained at ambient temperature for 5 days for the solvent to evaporate completely, followed by annealing in vacuum at 180 °C for 24 h. Then the morphologies of the thin films were characterized by transmission electron microscopy (TEM). Characterization of Nanostructures. Small-angle X-ray scattering (SAXS) experiments were carried out on a Bruker Nanostar SAXS instrument using Cu Kα radiation at a wavelength of 0.154 nm. The working voltage and current were 40 kV and 40 mA, respectively. The scattering vector q is defined as q = 4π/λ sin θ, where the scattering angle is 2θ and the d-spacing (d) is given by 2π/q. TEM experiments were carried out on a Hitachi H-800 electron microscope operated at 100 kV. The fast Fourier transforms of the TEM micrographs were carried out on the Image J134 software.

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RESULTS AND DISCUSSION Phase Behavior of Pure DMPCS and DMMA DiBCPs and That of Their Blends in Bulk. The DMPCS and DMMA diblock copolymers were synthesized through atom transfer radical polymerization (ATRP) using the same PDMS macroinitiator as described in previously reported work42 and in the Supporting Information. The molecular weights, polydispersity indexes (PDIs), and the volume fractions of PDMS of the macroinitiator and the diBCPs are shown in Table 1. The synthesized DMPCS and DMMA have well-

Figure 1. Schematics of expected hierarchical structures in thin films of the unsymmetrical blends of AB and AC diblock copolymers with a common block A which forms hexagonally packed cylinders in both BCPs. 5531

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Table 1. Molecular Weights, Polydispersity Indexes, and f PDMS Values of the Macroinitiator and the Diblock Copolymers along with Nanostructure Characteristics of the Diblock Copolymers from SAXS Results42 sample

Mna (g/mol)

Mnb (g/mol)

PDIa

f PDMSb (%)

q*c (nm−1)

dc (nm)

nanostructurec

PDMS-Br PDMS58-b-PMPCS66 PDMS58-b-PMPCS54 PDMS58-b-PMMA200

4 300 16 400 13 800 20 000

4 500 31 100 26 100 24 500

1.07 1.07 1.07 1.09

100 17.5 20.7 21.5

0.223 0.240 0.302

28.2 26.2 20.8

HEX LAM HEX

a

Determined from gel permeation chromatography results using linear polystyrene standards. bDetermined from the absolute molecular weights of PDMS-Br and 1H NMR results of the block copolymers. cDetermined from small-angle X-ray scattering results.

neous solutions containing the two diBCPs with different mixing ratios in dry THF after stirring and dissolving for 12 h, followed by thermal annealing in vacuum at 180 °C for 24 h. The SAXS results of the two series of blends, DMPCSHEX/ DMMAHEX and DMPCSLAM/DMMAHEX, are shown in Figure 2. In Figure 2a, when the weight fraction of DMPCSHEX (ϕDMPCS) in the DMPCSHEX/DMMAHEX blend is 75%, the reflection peaks (pointed out by the black arrows) of the DMPCSHEX diBCP are visible, and the first reflection peak (green arrow) of the DMMAHEX diBCP is on the left of the second peak of DMPCSHEX which contributes to the intensity increase of the second peak of DMPCSHEX. When the ϕDMPCS is 25%, the reflection peaks of DMMAHEX dominate the scattering profile, while the weak reflection peaks of DMPCSHEX can still be distinguished. In order to clearly observe the reflection peaks of both diBCPs in the blends, the SAXS result of the blend with ϕDMPCS of 50% is also shown in Figure 2a in which all reflection peaks of both DMPCSHEX and DMMAHEX exist. In Figure 2b, the reflection peaks attributed to both DMPCSLAM (black arrows) and DMMAHEX (green arrows) also exist in the SAXS profiles of the DMPCSLAM/DMMAHEX BCP blends. From the above results, the scattering profiles of the blends include the reflection peaks of each pure diBCP, indicating that macrophase separation occurs in the DMPCS/DMMA blends.10 How will the macrophase- and microphase-separated structures of the blends arrange in the thin films, and can the macrophase-separated structures be controlled to be ordered? If the macrophase-separated structure is ordered and different nanostructures are packed in different macrophase domains, the hierarchical structures will be useful in certain specific applications. Moreover, the microphase-separated nanostructures will be confined in the ordered macrodomains, which may be beneficial for the orientation of the nanostructures in the macrodomains. Thus, we studied the morphologies of thin films of pure DMPCS and DMMA diBCPs as well as those of unsymmetrical DMPCS/DMMA BCP blends with ϕDMPCS of 75% or 25%. Thin-Film Morphologies of Pure DMPCS H E X , DMPCSLAM, and DMMAHEX DiBCPs. We prepared thin films by solution-casting on carbon membranes and characterized them using TEM. The thicknesses of the thin films can be controlled by the concentration and the volume of the solution used for casting a piece of thin film as described in the Supporting Information. In order to avoid the influence of the thin-film thickness on the morphology, we controlled the average thickness of the thin films of pure BCPs and their blends to be similar (100−105 nm). Moreover, PDMS is composed of Si, C, H, and O elements, while PMPCS and PMMA are composed of C, H, and O elements. Therefore, the electron density (ρe) of PDMS is the largest among these three polymers.44 Although PMPCS and PMMA are composed of the same elements, the repeat unit of PMPCS contains phenyl

defined chemical structures and narrow molecular weight distributions (PDIs < 1.10). The large differences of Hildebrand solubility parameters among PDMS, PMMA, and PMPCS (δPDMS = 14.9 J1/2 cm−3/2, δPMMA = 19.1 J1/2 cm−3/2, and δPMPCS = 21.8 J1/2 cm−3/2) indicate that the Flory−Huggins interaction parameters among these three polymers are large.42,43 Therefore, the two diBCPs are strongly segregated systems, and the different blocks in these two diBCPs are incompatible. The microphase-separated nanostructures of the pure DMPCS and DMMA diBCPs after thermal annealing in vacuum at 180 °C for 24 h were determined by SAXS. The peaks in the scattering profile of the pure PDMS58-b-PMPCS66 (DMPCSHEX) have a scattering vector ratio of 1:√3:√7:√13, and those in the profile of the pure PDMS58-b-PMMA200 (DMMAHEX) have a scattering vector ratio of 1:√3, indicating that they form hexagonally packed cylinders in bulk (Figure 2a). The primary reflections of DMPCSHEX and DMMAHEX are

Figure 2. SAXS profiles of DMPCSHEX, DMMAHEX, and the DMPCSHEX/DMMAHEX blends with different weight fractions of DMPCSHEX (a) and those of DMPCSLAM, DMMAHEX, and the DMPCSLAM/DMMAHEX blends with different weight fractions of DMPCSLAM (b). The measurements were performed after the samples were annealed in vacuum at 180 oC for 24 h. The black arrows indicate the reflection peaks of DMPCS, and the green ones indicate those of DMMA.

at q = 0.223 and 0.302 nm−1, corresponding to d-spacing values of 28.2 and 20.8 nm, respectively (Table 1).42 The peaks in the scattering profile of the pure PDMS 5 8 -b-PMPCS 5 4 (DMPCSLAM) have a scattering vector ratio of 1:2:3 (Figure 2b), which indicates that it forms a lamellar structure, and the d-spacing is 26.2 nm (Table 1).42 In order to confirm the microphase separation and macrophase separation of the DMPCS/DMMA BCP blends, we first explored the phase behavior of the BCP blends in bulk by SAXS. For SAXS experiments, the samples of the BCP blends were prepared by solution-casting from the homoge5532

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oriented morphologies. These factors contribute to the vertical orientation of the nanostructures in DMMA HEX and DMPCSLAM thin films and the hybrid orientation of the nanostructure in the DMPCSHEX thin film. However, the film surface of the DMMAHEX may be covered by the PDMS block, which is currently under investigation by atomic force microscopy (AFM). Finally, we were able to obtain the vertical orientation and partially vertical orientation of the nanostructures in the pure diBCP thin films under our experimental condition. And we used the same experimental condition to explore the morphologies of thin films of the DMPCS/DMMA blends. Thin-Film Morphologies of DMPCS HEX /DMMA HEX Blends. Then we studied the thin-film morphologies of the DMPCSHEX/DMMAHEX blends. Thin films of the blends were prepared, and the thicknesses were controlled to be similar to those of the pure BCP thin films as mentioned above. Figure 4

groups which have a relatively high electron density. In addition, the density of PMPCS (ρPMPCS) is larger than that of PMMA (ρPMPCS = 1.28 g/cm3 and ρPMMA = 1.18 g/cm3).42,45 Thus, the electron density (or the mass thickness which equals thickness × density) of PMPCS is larger than that of PMMA.44 Therefore, the relationship of the electron densities of PDMS, PMPCS, and PMMA is ρePDMS > ρePMPCS > ρePMMA, and we can distinguish the macrophase- and microphase-separated structures of the blends directly in the TEM micrographs. First, we studied the structures of thin films of the pure DMPCSHEX, DMPCSLAM, and DMMAHEX diblock copolymers as shown in Figure 3. In Figure 3a, the PDMS cylinders of the DMPCSHEX

Figure 3. TEM micrographs of thin films of pure DMPCSHEX (a), pure DMPCSLAM (b), and pure DMMAHEX (c) diBCPs. The inset in each micrograph is the corresponding fast Fourier transform of the selected region.

are oriented partially vertically and partially horizontally in the thin film. Figure 3b shows the vertically oriented lamellar structure of DMPCSLAM. And in Figure 3c, the PDMS cylinders of DMMAHEX are all vertically oriented in the PMMA matrix. The corresponding fast Fourier transforms (FFTs) of the selected regions of each TEM micrograph are indicated in the insets of Figure 3. The substrate surface, low-surface-energy carbon thin film,46 has no distinct preferential interaction with PDMS, PMPCS, or PMMA, and thus it is a relatively neutral surface for these diBCPs. For the top surface, the surface energies of PMMA, PMPCS, and PDMS are different.47 Therefore, these thin films are in asymmetric boundary conditions.41 Moreover, the thicknesses of the thin films have some effect on the orientation of the nanostructures.41 The thickness of the DMMAHEX thin film is almost the integral multiple of the periodic size of DMMAHEX (5d0 ∼ 104.0 nm), and that of the DMPCSLAM thin film is also almost the integral multiple of the periodic size of DMPCSLAM (4d0 ∼ 104.4 nm), while the thickness of the DMPCSHEX thin film is a little off from the integral multiple of the periodic size of DMPCSHEX (4d0 ∼ 112.8 nm). In addition, annealing temperature and time have some effects on the final morphologies.48 These thin films were annealed in vacuum at 180 °C for 24 h, which allowed for these BCP thin films to develop into suitable ordered and

Figure 4. TEM micrographs of thin films of a DMPCSHEX/DMMAHEX blend with 75 wt % of DMPCSHEX. (a) Low-magnification image. The darker region is the DMPCSHEX macro-matrix, and the lighter regions are the DMMAHEX macrodomains as indicated. (b) High-magnification image. The darker parts in both macrodomains are PDMS cylinders as indicated. (c) Histogram of the distribution of diameters of the macrophase-separated macrodomains (number fraction vs diameter in nanometers) in the TEM micrograph in part a.

shows the thin-film morphologies of the blend with ϕDMPCS of 75% (Figure S3a in Supporting Information shows the thin-film morphology in a large area). The brighter macro-region is the DMMAHEX domain, the darker macro-region is the DMPCSHEX domain, and the dark part of the microphase-separated nanostructure is the PDMS microdomain. The nanostructures of different macrodomains can be focused simultaneously in the TEM experiments, indicating that the thicknesses in different macrodomains are uniform. In the DMMA macrodomains, the hexagonally packed PDMS cylinders in PMMA matrix are ordered and vertically oriented. The PDMS cylinders in the DMPCS domains are still poorly arranged with both vertical and horizontal orientations. The macrodomains are almost in round shape (the interface of the round-shaped macrodomain is relatively small). Some of the horizontally oriented PDMS 5533

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statistically analyzed the macrodomain sizes of the thin film in Figure 5a, as shown in Figure 5c. The diameters of most DMPCSHEX macrodomains in the thin film of the DMPCSHEX/ DMMAHEX blend with 25 wt % of DMPCS are 130−160 nm. The results indicate that the regularity and ordering of the macrophase-separated structures are improved in the thin film of this blend. When the DMMAHEX is the major component of the blend, the improvement in the ordering of the macrophase-separated structure and the improvement of the vertical orientation of PDMS cylinders in the isolated DMPCSHEX macrodomains of the thin film can be ascribed to the inducing effect of the ordered and vertically oriented nanostructures of the DMMAHEX macro-matrix and the effect of space confinement of the macrophase-separated macrodomains, respectively. On one hand, in order to minimize the interfacial energy, the macrodomains incline to be round, similar to the case of the DMPCSHEX/DMMAHEX blend with 75 wt % of DMPCSHEX. Because of the low surface energy of PDMS, some of the curved PDMS microdomains are arranged around the macrodomains to form part of the macrodomain interfaces, which also minimizes the interfacial energy. The PDMS cylinders of the DMMAHEX macromatrix and the DMPCSHEX macrodomains are almost vertically orientated. Thus, the curved PDMS microdomains at the macrodomain interfaces should be contributed by the PDMS blocks of both DMMAHEX and DMPCSHEX. Furthermore, the arrangement of the curved PDMS microdomains at the interfaces reduces the interfacial energy, which contributes to the smoothness of the interfaces and the stability of the macrodomains. On the other hand, the subordered HEX-like macrophase-separated morphologies on the submicrometer length scale impose a space confinement which improves the vertical orientation of the nanostructures in the isolated DMPCSHEX macrodomains. Figure 5d shows the possible molecular arrangement in the region of the thin film near the interface of the macrodomain (indicated by the box in Figure 5b), which indicates that the PDMS domain at the interface may be contributed by the PDMS blocks of both DMPCSHEX and DMMAHEX. In addition, for the symmetrical DMPCSHEX/DMMAHEX blend with ϕDMPCS of 50%, the thin films do not exhibit any regular macrophase-separated morphology. Some DMPCSHEX macrodomains contain the DMMAHEX macrodomains, and some DMMAHEX macrodomains contain the DMPCSHEX macrodomains, as shown in Figure S4. Thin-Film Morphologies of DMPCS LAM /DMMA HEX Blends. In thin films of the DMPCSHEX/DMMAHEX blends described above, the HEX-structured DMPCS and the HEXstructured DMMA cannot dissolve each other on the segmental length scale to form triblock copolymer analogous structures in our experiments. Then we studied the morphologies of thin films of the DMPCSLAM /DMMA HEX blends in which DMPCSLAM and DMMAHEX diBCPs have similar f PDMS values. Parts a and b of Figure 6 show the morphologies of a thin film of a DMPCSLAM/DMMAHEX blend with 75 wt % of DMPCSLAM, which, as the major blending component, forms the macromatrix. However, the isolated DMMAHEX macrodomains are disordered. Interestingly, hamburger-like structures exist in microphase-separated structures of the lamellar DMPCS macromatrix. The DMMA is dissolved within the lamellar DMPCS on the segmental length scale to form ABC triBCP-like structure due to the same molecular weight and similar volume fraction of the common block, PDMS, in both

cylinders of DMPCSHEX and some curved PDMS microdomains that may be contributed by both DMPCSHEX and DMMAHEX are packed at the macrodomain interfaces. Moreover, the diameters of the DMMAHEX macrodomains in Figure 4a are statistically analyzed, as shown by the histogram (number fraction vs macrodomain diameter in nanometers) in Figure 4c, which indicates that the sizes of DMMAHEX macrodomains are mainly in the range 80−180 nm. We speculated that if DMMAHEX was the matrix of the macrophase, the vertically oriented and ordered microstructure of DMMAHEX might be beneficial for the ordered arrangement of the isolated DMPCSHEX macrodomains. Figure 5 shows the

Figure 5. TEM micrographs of a thin film of a DMPCSHEX/ DMMAHEX blend with 25 wt % of DMPCSHEX. (a) Low-magnification image. The darker regions are the DMPCSHEX macrodomains, and the lighter region is the DMMAHEX macro-matrix as indicated. The inset is the corresponding fast Fourier transform of the selected DMMAHEX region. (b) High-magnification image. The darker parts in both macroregions are PDMS cylinders, and the distorted PDMS cylinders are arranged around the macrodomain as indicated. (c) Histogram of the size distribution of diameters of the macrophase-separated macrodomains (number fraction vs diameter in nanometers) in the TEM micrograph in part a. (d) Possible molecular arrangement in the nearby region of the interface of the macrodomain as indicated by the box in part b.

morphologies of a thin film of the blend with DMMAHEX as a major component (ϕDMPCS is 25%) (Figure S3b in Supporting Information shows the thin-film morphology in a large area). The brighter macro-region matrix in the thin film is DMMAHEX diBCP in which the PDMS cylinders are highly ordered and vertically oriented as indicated by the corresponding FFT of the selected DMMAHEX region in the inset. The darker isolated macrodomains are subordered submicrometer HEX-like structures of DMPCSHEX with regular shapes and smooth interfaces. At the macrophase interface, some curved PDMS microdomains are arranged along the round macrodomain which is indicated by the dashed line in Figure 5b (it is the same case for the interface in the area indicated by the box in Figure 5b). Moreover, the PDMS cylinders in the isolated DMPCSHEX macrodomains are mostly vertically oriented. We 5534

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Figure 6. TEM micrographs of a thin film of a DMPCSLAM/ DMMAHEX blend with 75 wt % of DMPCSLAM. (a) Low-magnification image. The darker region is the DMPCSLAM macromatrix, and the lighter regions are the DMMAHEX macrodomains as indicated. (b) High-magnification image. The darker parts in DMMAHEX macroregions are PDMS cylinders, and there are hamburger-like structures in the DMPCSLAM macromatrix as indicated. (c) A simplified model of possible molecular arrangement of the hamburger-like structure.

Figure 7. TEM micrographs of a thin film of a DMPCSLAM/ DMMAHEX blend with 25 wt % of DMPCSLAM. (a) Low-magnification image. The darker regions are the DMPCSLAM macrodomains, and the lighter region is the DMMAHEX macromatrix as indicated. (b) Highmagnification image. The darker parts in both macrodomains are PDMS microdomains, and the PDMS lamellae are arranged parallel to a row of PDMS cylinders as indicated. (c) Model of the hierarchical structure formed in the thin film of this blend. (d) A possible molecular arrangement at the interface (the lamellae contributed by the PDMS blocks of both DMPCSLAM and DMMAHEX) of the DMPCSLAM macrodomain and the DMMAHEX macromatrix.

DMPCSLAM and DMMAHEX.22 Because the hamburger-like structures are partially dispersed in the lamellar DMPCS, the corresponding SAXS profile of the blend in bulk only shows the reflection peaks of DMPCSLAM and DMMAHEX. We can easily distinguish that the brighter core in the hamburger-like structure is PMMA which is sandwiched by the dark PDMS layers. Figure 6c shows the simplified model of possible molecular arrangement of the hamburger-like structure. The layers formed by the soft PDMS block separate PMMA and PMPCS to reduce the interfacial energy. However, the HEX nanostructures of the DMMA macrodomains are difficult to be disturbed. Therefore, the microstructures of the isolated DMMA macrodomains are still the PDMS cylinders in the PMMA matrix, which is consistent with the fact that there are no triblock copolymer analogous structures in thin films of DMPCSHEX/DMMAHEX blends. These results demonstrate that the lamellar DMPCS can dissolve DMMA, while the HEXstructured DMPCS cannot dissolve DMMA on the segmental length scale. This may be owing to two following reasons. First, the difference of composition between DMPCSHEX and DMMAHEX are larger than that between DMPCSLAM and DMMA HEX ( f PDMS ’s in DMPCS HEX , DMPCS LAM , and DMMAHEX are 17.5%, 20.7%, and 21.5%, respectively). Second, a PDMS cylinder of DMPCSHEX has a much smaller continuous PDMS microdomain area than a PDMS lamella of DMPCSLAM, which results in the difficulty for DMMA BCP chains to insert into a PDMS cylinder as the hamburger-like structure in the thin film of the DMPCSLAM/DMMAHEX blend with 75 wt % of DMPCSLAM. For the DMPCSLAM/DMMAHEX blends with 25 wt % of DMPCSLAM as shown in parts a and b of Figure 7 (and Figure S3c shows the thin-film morphology of this blending system in a large area), the macromatrix is the DMMAHEX domain, and

the isolated short DMPCS macro-lamellae composed of two PDMS lamellae on the two sides of a middle PMPCS lamella or three PDMS lamellae separated by two PMPCS interlayers are dispersed in the DMMAHEX macromatrix. The sizes of the isolated macrodomains of macrophase separation are still on the nanometer length scale. The PDMS lamellae are arranged at the interfaces of the two sides of DMPCSLAM macro-lamellae to shield PMPCS microdomains from PMMA microdomains. For DMPCSLAM, the number of PDMS lamellae should be the same as that of the PMPCS lamellae. Therefore, the two PDMS lamellae at the interface of a macrolayer should be contributed by PDMS blocks of both DMPCSLAM and DMMAHEX. Although the thin film of this blend does not show hamburger-like structures that appear in the thin film of the DMPCSLAM/DMMAHEX blend with 75 wt % of DMPCSLAM, the insertion of the DMMA polymer chains into the PDMS lamellae of DMPCSLAM on the segmental length scale still occurs. Because of the small area of the isolated macrolamellae, the PMMA segments of the DMMAHEX chains that provide PDMS segments to the PDMS lamellae still prefer to phaseseparate into the DMMAHEX macromatrix under thermal equilibrium. Moreover, the PDMS lamellae at the interfaces of the isolated DMPCSLAM domains are parallel to the rows of PDMS cylinders of DMMAHEX macromatrix as indicated by the dashed line in Figure 7b. Figure 7c shows the schematic view of the complex structure formed in the thin films of this blending system, while Figure 7d shows the possible molecular arrangement of the selected region in Figure 7c and indicates 5535

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macrophase separation and microphase separation can be controlled to be more ordered by selecting appropriate block copolymers and suitable thin-film preparation methods. Thus, various useful hierarchical structures may be expected.

that the PDMS lamellae at the interfaces of the macrodomains are contributed by the PDMS blocks of DMPCSLAM and DMMA HEX . The PDMS lamella-to-lamella distance of DMPCSLAM (26.2 nm) is similar to the PDMS cylinder-tocylinder distance of DMMAHEX (2d/√3 ≈ 24.0 nm), which may be beneficial for the short lamellar DMPCS macrodomains with a few PDMS and PMPCS layers to be packed along the rows of PDMS cylinders and to be uniformly dispersed in the macromatrix. Moreover, the arrangement of PDMS lamellae at the interfaces reduces the interfacial energy of the blend. Thus, hexagonally packed cylinders and lamellar structures harmoniously coexist in thin films of the DMPCSLAM/DMMAHEX blend. Finally, for the DMPCSLAM/DMMAHEX blend with 50 wt % of DMPCSLAM, the thin films do not show any regular macrophase-separated morphology. Both DMPCSLAM and DMMAHEX are continuous macrophases, as shown in Figure S5. The width of the DMPCSLAM macrodomain is relatively small, and there are no hamburger-like structures existing in the DMPCSLAM macrodomain, which is consistent with the results of the DMPCSLAM/DMMAHEX blend with 25 wt % of DMPCSLAM.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of PDMS-b-PMMA, the thinfilm thickness determination, the synthetic route of PDMS-bPMMA, the 1 H NMR spectrum and gel permeation chromatography curve of the PDMS-b-PMMA diblock copolymer, TEM micrographs of thin films in large areas of DMPCS/DMMA blends, and TEM micrographs of thin films of the DMPCSHEX/DMMAHEX blend with 50 wt % of DMPCSHEX and those of thin films of the DMPCSLAM/ DMMAHEX blend with 50 wt % of DMPCSLAM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected].

CONCLUSIONS In summary, we have demonstrated that thin films of unsymmetrical DMPCS/DMMA (AB/AC) BCP blends can form hierarchical structures owning to both macrophase separation and microphase separation, which occur in bulk, as confirmed by SAXS results. Different morphologies present in thin films of the DMPCSHEX/DMMAHEX and DMPCSLAM/ DMMAHEX blends. For the DMPCSHEX/DMMAHEX blends, when the blend contains 75 wt % of one diBCP, subordered macrophase-separated structures with ordered nanostructures in the macrodomains develop in the thin film. As the blending ratio is reversed, the macrophase-separated morphologies are also reversed. When the matrix of the macrophase is the coil− coil DMMAHEX diBCP which has the nanostructures of vertically oriented cylinders, the macrophase-separated morphology becomes more ordered, and the interfaces of the macrodomains become more smooth. Moreover, because of the space confinement of the isolated macrodomains, the PDMS cylinders in the isolated DMPCSHEX domains are vertically oriented. For thin films of DMPCSLAM/DMMAHEX blends, when the weight fraction of DMPCSLAM is 75%, hamburger-like structures form in the DMPCSLAM macromatrix, which can be ascribed to the solubility of DMMA in the lamellar DMPCS on the segmental length scale. When the weight fraction of DMPCSLAM is 25%, the short DMPCS lamellae with a few layers are dispersed in the HEX-structured DMMA macromatrix, and the PDMS lamellae in the isolated DMPCSLAM domains are parallel to the rows of PDMS cylinders. In thin films of both DMPCSHEX/DMMAHEX and DMPCSLAM/ DMMAHEX blends, the soft PDMS cylinders or lamellae are arranged at the interfaces of the macrodomains and hamburgerlike nanostructures to reduce the interfacial energy, which contributes to the stability of the final morphologies. The complex hierarchical structures on different length scales will be useful in specific fields. For example, for the DMPCSHEX/ DMMAHEX thin films with ordered macrophase- and microphase-separated structures, the PDMS cylinders can be selectively removed to obtain nanoporous templates with different pore sizes in different macrodomains of thin films, which will be useful as complex nanotemplates. Finally, the thin-film structures formed by AB/AC blends because of

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant 20874003) is gratefully acknowledged.

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REFERENCES

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