Creation of Cylindrical Morphologies with Extremely Large Oblong

Feb 19, 2015 - We investigated the phase behavior of poly(isoprene-b-styrene-b-2-vinylpyridine) (ISP) triblock terpolymer binary blends with different...
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Creation of Cylindrical Morphologies with Extremely Large Oblong Unit Lattices from ABC Block Terpolymer Blends Yusuke Asai, Atsushi Takano, and Yushu Matsushita* Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: We investigated the phase behavior of poly(isoprene-b-styrene-b-2vinylpyridine) (ISP) triblock terpolymer binary blends with different chain lengths of the two end-blocks as a function of φP/φI. The two characteristic cylindrical morphologies with new tiling patterns have been found. There are two essential and common features of these morphologies: (1) domains with nonconstant sizes and shapes and (2) tiling patterns that consist of triangles and pentagons. Additionally, as φP/φI decreases, the shape of a unit lattice changes and its size is gradually increased. Eventually, the unit lattice shows an oblong shape, which could be typical of the pseudodecagonal phase. Thus, the present results are pointing toward a high possibility that a decagonal quasicrystal could be found in the vicinity of this study.

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polymer blends.26 Hayashida et al.22,23 and Bates et al.24,25 have found σ phase and dodecagonal quasicrystals from ABC star polymers and ABAC tetrablock terpolymers, respectively. Additionally, Izumi et al. reported a cylindrical structure with a new tiling pattern whose P phase has 5-neighbors using ISP/S homopolymer blend system.26 From their study, it has been also confirmed that φP/φI, which is the ratio of volume fractions of two end-blocks, is identified as an indicator of packing state. In this study, we demonstrate the creation of two cylindrical morphologies with new tiling patterns from binary blends of poly(isoprene-b-styrene-b-2-vinylpyridine) (ISP) triblock terpolymers with different chain lengths in the only two endblocks. Furthermore, φP/φI can be controlled by changing blend ratio of two ISP triblock terpolymers.

INTRODUCTION Block copolymers are composed of two or more chemically distinct polymers which are linked through covalent linkage. Because of chemical incompatibility and the existence of the covalent linkage, block copolymers form periodical microphaseseparated structures such as lamellae, bicontinuous gyroids, cylinders and spheres, depending on the volume fraction (φ) of each component and segregation strength (χN) where χ is Flory’s segmental interaction parameter and N is the total degree of polymerization.1,2 A large number of studies of monodisperse AB diblock copolymers have been performed both theoretically3−5 and experimentally.6−8 Lately, ABC triblock terpolymers have attracted much attention because they form complex and blended structures such as tetragonalpacked cylinder,9,10 core−shell cylinder,11,12 and others.13−17 This large variety in morphology is derived from three interaction parameters (χAB, χBC, χAC), volume fractions (φA, φB, φC,) and block sequences (ABC, ACB, BAC). The interface having constant mean-curvature (CMC) basically remains in ABC triblock terpolymer to minimize the free energy. However, some periodic structures having non-CMC have been seen with copolymer blends with interaction such as hydrogen bonding18,19 and with particular distributions such as molecular weight distribution20 and composition distribution.21 For example, Hawker et al. reported square-tetragonally coarrayed cylindrical microdomains from blends of two diblock copolymers with hydrogen bonding,18 and we developed rectangular cylinders with tetragonal packing with binary blends of poly(isoprene-b-styrene-b-2-vinylpyridine) (ISP) triblock terpolymers having distinctly different chain lengths in only the two end-blocks.21 Furthermore, some morphologies with characteristic domain orientations have been developed using multiple component systems with characteristic chain connection22−25 and polymer/ © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Two kinds of poly(isoprene-b-styrene-b-(2-vinylpyridine)) (ISP) triblock terpolymers (Figure 1), ISP-α and ISP-β, were synthesized via three-step anionic polymerizations.9 Table 1 summarizes the

Figure 1. Schematic representation of the two ISP triblock terpolymers in this study. Black, dashed, and gray chains represent I, S, and P, respectively. Received: December 22, 2014 Revised: February 5, 2015

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DOI: 10.1021/ma5025818 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Molecular Characteristics of the Poly(isoprene-bstyrene-b-(2-vinylpridine)) (ISP) Parent Triblock Terpolymers Used in This Study sample

Mn

φI

φS

φP

Mw/Mn

ISP-α ISP-β

122000 124000

0.06 0.39

0.62 0.56

0.32 0.05

1.02 1.06

volume fractions of the three components of the two ISP triblock terpolymers. These ISP triblocks have similar total molecular weights and midblock (S) molecular weights but differ in the molecular weights of their I and P end-blocks. Three blend samples with various compositions were prepared and identified as Blend(X/Y) as listed in Table 2, where X is the molar percentage of ISP-α and Y is that of ISPβ. Figure 2. Comparison of microphase-separated structures for the blend samples: (a) Blend(60/40), (b) Blend(48/52), and (c) Blend(40/60). All TEM images represent the cross sections of cylindrical morphology. (d), (e), and (f) indicate the corresponding tiling patterns. The red line connects each I domain to neighboring I domains to confirm the tiling pattern, and hence the imaginary polygons are imposed on the images. Tiling pattern of (e) consists of the assembly which has one triangle and three pentagons, while that of (f) consists of the two triangles and four pentagons. Scale bars all represent 100 nm.

Table 2. Molecular Characteristics of Blend Samples sample

φI:φS:φP

φP/φI

Nshort/Nlong

Blend(60/40) Blend(48/52) Blend(40/60)

0.20:0.60:0.20 0.24:0.59:0.17 0.27:0.58:0.15

1.03 0.72 0.57

0.67 1.08 1.50

The essential feature of these blend samples is that the φS is nearly constant and that φP/φI is gradually changed depending on the blending ratio. As mentioned above, φP/φI can be recognized as an indicator of packing manner of cylindrical structure.26 Furthermore, the ratio of the number of shorter P chains and longer P chains (Nshort/Nlong) is considered, which can be identified as an index of conformational entropy within P domains. Sample films for morphological observation were obtained by solvent casting from 5% THF solutions in Teflon beakers for over 2 weeks at room temperature. Subsequently, the films were dried under vacuum at room temperature for 1 day and were thermally treated at 150 °C for 5 days. For transmission electron microscopy (TEM) experiments, the samples were cut into ultrathin sections having a thickness of ca. 50 nm using an ultramicrotome. The ultrathin sections were stained with OsO4 and I2. The sections were imaged using a JEM-1400 (JEOL Ltd.) The TEM apparatus operated at an accelerating voltage of 120 kV. For microbeam-SAXS measurements, the annealed sample films were cut into thin sections having approximate thickness of ∼5−10 μm. The microbeam-SAXS measurements were performed using beamline BL-40XU at SPring-8 facility (Hyogo, Japan). The size of the X-ray microbeam was appropriately 5 μm × 5 μm (fwhm).

are nearly equivalent because φP/φI is 1.03, resulting in formation of tetragonal-packed state. As shown in Figure 2, there is a significant difference between the morphologies of the three blends. Interestingly, the morphologies of Blend(48/52) and Blend(40/60) have characteristic domain packing and domain shapes. The primary structure observed in Blend(48/52) is a cylindrical morphology based on Figure S3a, which shows the side view of a cylindrical structure. Note that the domain shapes found are not just simple circles, but anisotropic circles. That is, this blend sample also produces a surface with non-CMC. Furthermore, it is apparent from Figure 2b that the I domains have different sized domains arranged periodically. On the other hand, it can be seen that small P domain appears at the center surrounding from three I domains. In order to varify the orientation of P domains, we stained sample films with I2 to give strong contrast to only P phase. Surprisingly, two kinds of P domains were observed: one is a cylindrical structure, and the other is a spherical one that periodically aligns along cylindrical I domains (Figure S3b,c). In short, we have found a complex morphology based on the cylindrical structure which has domains with different sizes and shapes both in two phases composed of end blocks, i.e., I and P. The structure from Blend(40/60) shown in Figure 2c has some features similar to that of Blend(48/52): (1) the cross section of each cylinder is not just circular, (2) different sized I domains can be observed, (3) cylindrical morphology is formed, and (4) spherical P domains appear at the center surrounded from three larger I domains and aligned along the cylindrical I domains. However, the domain periodicities from the three blend samples are strongly different. They have been determined using a net (red solid line) that connects the nearest I domains together, as shown in Figures 2d−f. It is easily confirmed that the tiling pattern from Blend(60/40) consists of only squares (Figure 2d). Note that nets from Blend(48/52) and Blend(40/ 60) indicate that the tiling patterns consist of triangles and pentagons and that those polygons periodically line up. The

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RESULTS AND DISCUSSION The ISP-α possesses a cylinder-in-lamellae structure, while ISPβ gives a sphere-in-gyroid structure, as shown in TEM images in Figure S1. Since the samples were stained with OsO4 and I2 for the TEM observations, the black, white, and gray domains represent I, S, and P phases, respectively. Figures 2a−c compare the TEM images of microphaseseparated structures for the blend samples. All these morphologies could not be obtained from neat ABC triblock terpolymers with the same volume fractions as the present blend samples shown in Table 2. For example, we found that two ISP triblock terpolymers whose volume fractions are close to those of Blend(60/40) and Blend(48/52) showed gyroid structures.9 Rectangular-shaped tetragonal-packed cylinders were observed from Blend(60/40) as shown in Figure 2a. This structure is probably produced due to weak localization of the joint point of two parent triblock terpolymers along the domain interface, which releases the stress on chains, as previously reported.21 Moreover, the domain sizes of I and P B

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lines denote the real space lattice, which is consistent with the data obtained by X-ray experiments. Figure 3c exhibits a more simplified schematic illustration based on the TEM image of Blend(48/52), associated with electron density of components (I: 0.512; S: 0.565; P: 0.608 electron/cm3). Using the image in Figure 3c, a FT was performed, and the result is displayed in Figure 3d. The pattern reveals that the first ordered peaks (01, 10, 11̅, etc.) are weaker than the second ones (02, 20, 22,̅ etc.), and its feature is in good agreement with the measured pattern. Unit lattice data on the TEM image and the microbeamSAXS pattern of Blend(40/60) are shown in Figures 4a and 4b,

tiling pattern from Blend(48/52) is the same as our recent report.26 From this tiling pattern, we can fairly say that small P domains appear at the center of triangles that from larger I domains. However, there is a meaningful difference between tiling patterns of Blend(48/52) and Blend(40/60). That is, the unit of Blend(48/52) consists of a triangle and three pentagons(Figure 2e), whereas that of Blend(40/60) includes two triangles and four pentagons (Figure 2f). The two-dimensional SAXS diffraction pattern of Blend(60/ 40) is shown in Figure S2a. This indicates a tetragonal-packed cylindrical pattern represented by a √5 peak with a domain spacing of about 60 nm. The domain distance from this result is in good accordance with the real space image. Unit lattice data on the TEM image and the microbeamSAXS pattern of Blend(48/52) are shown in Figures 3a and 3b,

Figure 4. (a) Unit lattice arrangement of Blend(40/60), expressed with black solid line, estimated from microbeam-SAXS is superimposed on the TEM image. Scale bar represents 200 nm. (b) Microbeam-SAXS pattern. (c) Schematic illustration based on a TEM image. Black and white circles indicate P and I domain, respectively. (d) A FT pattern associated with the domain packing in (c). The radii of the dots represents the scattering intensities.

Figure 3. (a) Unit lattice data of Blend(48/52), expressed with black solid line, estimated from microbeam-SAXS is superimposed on the TEM image. Scale bar represents 100 nm. (b) Microbeam-SAXS pattern. (c) Schematic illustration based on a TEM image. Black and white circles indicate P and I domain, respectively, considering electron density. (d) A FT pattern corresponding to the domain packing in (c). The radii of the dots represents the scattering intensities.

respectively. Here one notices the pattern in Figure 4b is similar to that of hexagonal-packed state, though we can hardly find the hexagonal-packed morphology. Provided that the representative reflections indicate (20), (14), etc., as shown in Figure 4b, these give an oblong type unit cell whose lattice constants are 135 nm, 295 nm, and 88° for a1, a2, and α̅, respectively. These constants are in good accordance with the real space data, i.e., 131 nm, 279 nm, and 95° (α), respectively. Figure 4c exhibits the schematic illustration of domain packing considering electron densities, and the corresponding FT pattern is shown in Figure 4d, where the blue auxiliary lines linking the spots give an oblong-shaped unit lattice. One finds that the agreement in between Figure 4b and Figure 4d is reasonably satisfactorily good. We evaluated the periodicity of these complex morphologies by X-ray analysis. Surprisingly, the calculated size of unit lattice tends to increase rapidly as φP/φI decreases. Following the rectangular-shaped cylinders, we have found two additional characteristic morphologies from ISP triblock terpolymer blends. These two structures have two common

respectively. In Figure 3a, enclosed white arrows denote the real lattice vectors, a1 and a2. Figure 3b clearly exhibits hexagonal manner, where the white arrows show first-order scattering vectors. Spot-like reflections can be detected in the microbeam-SAXS pattern, indicating that the beam size of the X-ray is small enough to investigate a narrow range of grains where cylindrical structures order with regularity. The lattice constants, a1 and a2, were estimated to be 120 and 133 nm from the magnitudes of their corresponding scattering vectors q10 and q01, respectively. The angle α̅ between the two scattering vectors is 55°, its value being consistent with the real lattice data, i.e., α = 127°. These results signify that the structure of Blend(48/52), whose tiling pattern consists of triangles and pentagons, shows crystallographically a hexagonalpacked state. Specifically, the hexagonal shape of the unit lattice of Blend(48/52) is shown in Figure 3a, where the black solid C

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coordination structure has not been found from neat ISP triblock terpolymer having φP/φI with the value between tetragonal state and hexagonal state. This is due to the limitation of thermodynamical and geometrical terms. Otherwise, in this study, we have found two complex structures having a 5-coordinated packing with decreasing φP/φI. These characteristic packings are probably derived from synergetic effect between the asymmetric interface curvature and the chain localization. It should be stressed that systematically increasing the size of the unit lattice has been observed by only changing the blending ratio, not changing the parent polymers. The tiling pattern with triangles and pentagons has been developed, and an oblong unit lattice with a surprisingly large size was found. Changing the shape of the unit lattice from square to oblong indicates that the symmetry of the structure from these binary blends breaks. This phenomenon is similar to the one known in metal alloy system, in which pseudodecagonal approximants have been observed.27,28 Thus, we have found a 5-coordination structure that cannot be developed from classic crystallography. There is a high possibility that decagonal quasicrystal could be found in the vicinity of this study.

characteristic features: (1) domains with nonconstant sizes and shapes and (2) a tiling pattern that consists of imaginary triangles and pentagons. We first discuss feature (1). The possible chain orientation in domain structure is schematically drawn in Figure 5, which was

Figure 5. Schematic illustration of possible chain orientation in the domain structure of (a) Blend(48/52) and (b) Blend(40/60). Black, dashed, and gray chains represent I, S, and P, respectively.

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transferred from the real lattice image. It is mentioned above that a structure with non-CMC is formed by heterogeneous chain orientation within the domain. In this study, different sized chains are forced to reside within the domain and localize at the proper position of the domain boundary due to increased conformational entropy, resulting in the formation of characteristic-shaped cylinders. Furthermore, domains with different sizes and shapes have also been found in the unit lattice, as shown in Figures 2b,c. In particular, P phases show two kinds of domains: cylinder and sphere. The shorter P chains of ISP-β tend to form spherical domains, similar to the structure of the parent polymer. Furthermore, the diameters of smaller microdomains for Blend(48/52) and Blend(40/60) are about 4 nm, which is close to that of P microdomains from neat ISP-β based on TEM images. It is actually unnatural to fit larger P chains on ISP-α into these small domains, since the molecular weight of P block in ISP-α is 41 000 g/mol. Moreover, if we assume shorter chains are all distributed in smaller domains, the relative ratio of two P domains in the present TEM does not match the volume fractions of P chains in two samples. There is a high possibility that the spherical P domains of Blend(48/52) and Blend(40/60) were formed from only P chain of ISP-β because the number of shorter P chains is larger than the number of longer P chains in these blends. Alternatively, Nshort/ Nlong ratio is 0.67 in Blend(60/40), indicating that shorter chains may be easily miscible with an excess longer P chains, resulting in the formation of a homogeneous domain. When Nshort/Nlong is larger than unity, i.e., 1.08 for Blend(48/52) and 1.50 for Blend(40/60), conformational entropy loss within each domain is gradually increased along with heightened Nshort/ Nlong and cannot be recovered merely by chain localization. As a result, shorter P chains could be expelled from cylindrical P domains and form individual structures. That is, spherical structures possibly become stabilized by chain localization of shorter P chains in the unit lattice. Based on the assumption of the possible chain arrangement shown in Figure 5, it is reasonable that I domains next to spherical P domains become bigger. Moreover, forming the morphology with more spherical P domains observed in Blend(40/60) is consistent with the perspective of increased Nshort/Nlong. Next, we discuss feature (2). It was confirmed that the packing manner of cylindrical structure depends on φP/φI. A 5-

CONCLUSIONS We found two characteristic morphologies with binary blends of ISP triblock terpolymers. These morphologies have two specific features: (1) domains with different sizes and shapes, and (2) tiling patterns which consist of triangles and pentagons. The former is probably derived from weak localization of the junction point of the two parent triblock terpolymers within domains and that excess shorter chains form the individual structure to release conformational entropy loss within the unit lattice. The latter is due to the synergetic effect between asymmetric interfacial curvature of I and P domains and the introduction of composition distribution. One of the new structures is regarded as a pseudodecagonal phase; thus, the present work provides a new approach with respect to forming new morphologies, which could be used to develop decagonal quasicrystal structures that have not yet been observed in polymeric systems.

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ASSOCIATED CONTENT

S Supporting Information *

SAXS pattern and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Kakenhi (25248048), from MEXT; Y.M. is grateful for their support. Y.A. was financially supported by the Program for Leading Graduate Schools “Integrative Graduate Education and Research in Green Natural Sciences”, MEXT, Japan. We are grateful to Prof. T. Dotera of Kinki University for his kind suggestions and discussion on data analysis. Microbeam X-ray experiments were done using the machine time assigned for the Proposal No. 2014A1446 with the experimental assistance of Dr. N. Ohta. D

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Macromolecules The use of the synchrotron X-ray source was supported by Photon Factory, KEK, in Japan (No. 2013G737).

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DOI: 10.1021/ma5025818 Macromolecules XXXX, XXX, XXX−XXX