Bicontinuous Double-Diamond Structures Formed in Ternary Blends

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Bicontinuous Double-Diamond Structures Formed in Ternary Blends of AB Diblock Copolymers with Block Chains of Different Lengths Wataru Takagi,† Jiro Suzuki,‡,§ Yoshitaka Aoyama,∥ Tomohiro Mihira,∥ Atsushi Takano,† and Yushu Matsushita*,†

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†

Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ‡ Computing Research Center, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan § Information System Section, J-PARC Center, 2-4 Shirakatashirane, Tokai, Ibaraki 319-1195, Japan ∥ JEOL Limited, 1-2 Musashino, 3-Chome Akishima, Tokyo 196-8558, Japan S Supporting Information *

ABSTRACT: We report on the formation of a bicontinuous double diamond (DD) structure in ternary blends of poly(styrene-b-isoprene) (SI) diblock copolymers with chains of different lengths merely in a polyisoprene block. Certainly, the materials revealing the DD structure with mesoscopic length scale have strongly been expected due to high potential for applications; however, it is difficult to form owing to thermodynamically evident disadvantages such as wider surface area and larger domain thickness variation, particularly for monodisperse copolymers. The DD structure was successfully formed for ternary blends composed of three parent diblock copolymers; SI-L (M = 190k, φs = 0.46), SI-G (M = 151k, φs = 0.62), and SI-C (M = 124k, φs = 0.73), resulting in covering the overall composition range 0.57 ≤ φs ≤ 0.60, where φss denotes volume fractions of polystyrene blocks. Four-branched double network structure with space group symmetry of Pn3̅m was clearly proved by transmission electron microscopy (TEM) observation aided by computer simulation, combined with diffracted data by small angle X-ray scattering. In addition, TEM tomography gave direct information concerning interwoven double networks and four-branching nature of the diamond framework.

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INTRODUCTION Morphologies of block copolymers have long been studied by experimental approaches1−4 and theories5−8 extensively. One important aspect of research activities consists of the morphological transition with relative composition of components, that is, spherical, cylindrical, bicontinuous, and lamellar structures, are known to appear if composition A for AB diblock copolymer increases. Among these, bicontinuous structures have been highly focused because those possess high potentials for attractive applications such as photonic crystals9 or solar batteries10 and so on. A periodic bicontinuous structure was first reported on two component poly(styrene-b-isoprene) (SI) star-shaped copolymers in 1986; their morphologies were assigned as the ordered bicontinuous double diamond (DD) structures.11,12 In the same era, a four-branched tetrapod-type double network structure was found for simple SI diblock copolymers.13 In addition, tricontinuous double network structures with two different networks were also reported for poly(isoprene-bstyrene-b-2-vinylpyridine) (ISP) triblock terpolymers.14,15 Meanwhile, a new double network structure, double gyroid (DG) structure was found and reported for SI star (or diblock) copolymers by Hajduk et al. in 1994, with the support of © XXXX American Chemical Society

analytical evidence that a part of DD was transformed into DG by careful thermal treatment,16,17 by pointing out the partial misidentification of structure analyses in previous reports.11,12 The formation of DD was simultaneously reported on styreneb-2-vinylpyridine diblock copolymers by Schulz et al.18 These experimental results were supported by self-consistent field theory calculation for AB diblock copolymers by Matsen and Schick.19 Successively, tricontinuous DG structure was also discovered for ISP triblock terpolymers in 1998.20,21 The DG structure is composed of two interwoven networks, each of which is a periodic sequence of “three-branched” nodes connected with struts having Ia3̅d spatial symmetry,16 whereas the DD structure reveals “four-branched” nodes giving Pn3̅m symmetry.22 These two resemble each other, however, they are quite different in the geometric sense. Surface geometry problems were dealt in detail by Hyde et al. and the characteristics of DG and DD surfaces together with Schoen G-surface and Schwarz D-surface have been clarified and introduced.23,24 Received: April 10, 2019 Revised: August 9, 2019

A

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were able to create stable DD structures under thermal equilibrium conditions for the present ternary system.

DD is conceived to possess disadvantages to form in comparison with DG because of the following reasons. (1) Surface area per unit volume is possibly larger for 4-branched DD than 3-branched DG, and (2) the increments of variation of interfacial curvature and domain thickness for the former is larger. The second reason insists for the longer chains to fill in a thinner region and for the shorter chains to fill in thicker nodes; both of these tend to lead to definite disadvantages in forming the DD structure over DG, and hence DD is hard to form in order to save conformational entropy.25,26 To overcome these difficulties, several molecular designs were adopted in recent years. Takagi et al.27 reported on the formation of a DD structure by using an SI diblock copolymer/ I homopolymer blend, where the added homopolymer plays an important role to fill the thicker portion of nodes selectively, whereas Chu et al.28,29 succeeded in finding a DD structure from poly(styrene-b-syndiotacticpolypropyrene) diblock copolymer (S-sP), where helical nature of sP induced the DD structure. On the other hand, Asai et al.30 reported on a triplyperiodic DD structure by using a binary blend of ISP triblock terpolymers with different chain lengths in two end blocks, that is, I and P. This molecular design can cause weak localization of I and P block chains at different locations in a unit piece, composed of nodes and struts, so as to escape from conformational entropy loss or packing frustration. In this study, we adopted the idea of Asai et al. and the method was applied to the diblock copolymer system, although the situation for ISP terpolymer is largely different from the diblock system because the former has a center S block chain which has to have “bridge” conformation, in contrast to the tail conformation for the latter. Here, further attention must be paid on the fact that it is very hard for a monodisperse diblock copolymer to form a bicontinuous structure, and this is a distinct negative feature of diblock systems,31,32 and hence the present trial is a big challenge in polymer morphology so as to create a new structure with higher application potential. Most of the structure analysis on polymer morphology has been conducted by combining transmission electron microscopy (TEM) observation with small angle X-ray scattering (SAXS) measurements; however, these two methods are not sufficient to elucidate three-dimensional (3D) features of bicontinuous structures, as clarification of the connection manners of nodes is essentially important to clarify the structural features in the present case. To realize this requirement, 3D TEM tomography (TEMT) is a powerful method to utilize, and hence, TEMT was also carried out in addition to 2D-TEM. In the pursuing experiments, parent three SI block terpolymers with chains of different lengths merely in the I component (red in Figure 1), by keeping the lengths of S constant, were prepared by anionic polymerizations and the thermally stable bulk structures of binary and ternary blends were investigated in this study. In fact, finally we

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EXPERIMENTAL SECTION

Three polystyrene-polyisoprene (SI) diblock copolymers were synthesized by living anionic polymerizations in tetrahydrofuran (THF) at −78 °C using cumyl potassium as an initiator.33,34 Characteristics of these copolymers are summarized in Table 1. The

Table 1. Molecular Characteristics of SI Diblock Copolymers 10−3 Mn

composition

sample codes

Sa

Ib

Mw/Mnc

S/I (vol)d

microstructure of Id (1,2):(1,4):(3,4)

SI-L SI-G SI-C

93.4 99.0 93.4

96.4 51.6 30.8

1.04 1.03 1.03

0.46:0.54 0.62:0.38 0.73:0.27

0.38:0.06:0.56 0.39:0.04:0.57 0.39:0.04:0.57

a c

Measured by SEC and MALS. bEstimated by 1H NMR and Mn.S. Determined by SEC. dDetermined by 1H NMR.

number average molecular weights (Mn), polydispersity indices (Mw/ Mn), and volume fractions of S and I were determined by combining size exclusion chromatography (SEC), multi-angle light scattering (MALS), and proton nuclear magnetic resonance (1H NMR) in the same manner as reported previously.35 SEC profiles of diblock copolymers together with S-precursors are shown in Figure S1 in the Supporting Information. These three SI diblock copolymers have almost the same molecular weights for the S block chains, ca. 100 kg/ mol, whereas the molecular weights of I block chains are different, that is, they are 96, 52, and 31 kg/mol, respectively. Microstructures of polyisoprene blocks were determined by 1H NMR; their fractions were added in Table 1. These three copolymers were coded as SI-L, SI-G, and SI-C because it has been confirmed that they show lamellar (L), DG (G), and cylindrical (C) structures, as will be reported later. Binary blend samples consisting of SI-L and SI-C are coded as LC(x/z), where x and z denote the weight percentages of SI-L and SIC in the blend samples. The weight ratio of SI-L and SI-C in the binary blend series was varied from 10/90 to 90/10 with 10% step. Ternary blends were coded as LGC(x/y/z), where x, y, and z denote the weight percentages of SI-L, SI-G, and SI-C, respectively. In these ternary blends, the weight ratio of SI-L to SI-C was fixed at 70:30 for the reason evidenced in binary blends as will be described in the next section. In this article, therefore, the results for the binary LC blends are partly displayed, and the results for ternary blends are the main focus. All samples for morphological observation were prepared by the solvent casting procedure using oxidant-free THF, as reported previously.27 The amounts of parent SI diblock copolymers for the blends were controlled by preparing mother solutions, where they were dissolved in THF to attain 5 wt %, followed by stirring for 1 day at room temperature. Then, the blend samples were prepared by measuring the volume of the mother solutions accurately using a micropipette. The homogenous solutions were transferred into a Teflon beaker, and the solvent was evaporated slowly for over 2 weeks at 25 °C. Subsequently, the films were dried under vacuum at room temperature for 1 day, followed by thermal annealing at 150 °C for 3 days. The microphase separated structures were observed by TEM, SAXS, and TEMT. For TEM observations, annealed bulk films were embedded in an epoxy resin, then they were cut into ultrathin sections with 80−100 nm thickness by an ultramicrotome of Leica, EM UC7. The ultrathin sections were stained with osmium tetroxide (OsO4) vapor for 40 min at 70 °C. This staining procedure ensures that the I phase appears dark, whereas the S phase appears bright. TEM experiments were performed with a JEM-1400 (JEOL Co. Ltd., Japan) operated at acceleration voltage of 120 kV.

Figure 1. Molecular designs of (a) binary and (b) ternary blends of AB diblock copolymers having the same A-component chain length but with different B-component chain length. B

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Macromolecules All SAXS data shown in the present work were basically collected using the beamline BL40B2 at the SPring-8 synchrotron radiation source (Hyogo, Japan). The wavelength of X-ray used was 0.15 nm; the sample to detector distance was 2 m. A charge-coupled device camera was used as a detector for some of the binary LC blend samples, whereas a PILATUS3S-2M with a size of 300 mm × 300 mm was used for all other samples. This geometry covers the q range of 0.05 < q < 1.0 (nm−1), where q (=4π sin θ/λ) is the scattering vector with 2θ and λ being the scattering angle and wavelength of X-ray used, respectively. To investigate the 3D structures, a TEMT experiment was carried out using a JEM-2100Plus (JEOL Co. Ltd.) with acceleration voltage of 200 kV. Bulk films for the TEMT experiments were cut into a small strip, which was stained with 3% OsO4 aqueous solution for 1 h at 70 °C. Using the stained strip, a rod-shaped specimen with a diameter of approximately 400 nm was fabricated by a focused ion beam system of JIB-4000Plus (JEOL Co. Ltd.), which was used for precise 3D reconstruction of the sample without the missing wedge. A series of 180 TEM images was collected with tilt angles ranging from −86° to 93° in 1° increment. The series of TEM images were aligned by the fiducial marker method using Au nanoparticles as the markers. The three-dimensionally reconstructed-image was obtained by using a reconstruction software, TEMT (JEOL Co. Ltd.).36

from these diffracted patterns, whereas the corresponding values evaluated from TEM images was 85, 135,37 and 65 nm, respectively. These values are in good agreement with each other. Binary LC Blends. Volume fractions of the S phase, φss, for nine LC binary blend samples prepared from SI-L and SI-C are summarized in Table 2. Their morphologies confirmed by Table 2. Ratios of Two Parent Copolymers and Average Polystyrene Fractions, φs, of Binary LC Blends and Their Associated Morphology

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RESULTS AND DISCUSSION Parent SI Diblock Copolymers. Figure 2 shows TEM images and SAXS profiles of parent copolymers (SI-L, SI-G, SI-

sample code

φs

morphologya

LC(90/10) LC(80/20) LC(70/30) LC(60/40) LC(50/50) LC(40/60) LC(30/70) LC(20/80) LC(10/90)

0.484 0.510 0.537 0.563 0.590 0.617 0.643 0.671 0.698

L L DG(DD) DG(DD) C C C C C

a

L, DG, DD, and C denote lamellar, double gyroid, double diamond, and cylindrical structures, respectively.

TEM and SAXS are also included in this table, although most of the observation data are displayed in the Supporting Information, Figures S2 and S3. Summarizing the morphological change first, the blends show lamellar structure when the volume fraction of the S phase, φs, is around 0.5, for LC(90/10) and LC(80/20), and it transforms into bicontinuous structure if φs increases up to 0.54 for LC(70/30) and 0.56 for LC(60/40). Furthermore, when φs increases beyond 0.56, the morphology transits to a cylindrical structure at LC(50/50), and it is maintained with further increase of the fraction of SI-C. In this work, we merely focus on LC(70/30) and LC(60/40) because they show bicontinuous structures. Figure 3 compares TEM images of LC(70/30) and LC(60/40), together with computer-generated black and white images. Simulated TEM images were obtained by setting the following parameters: S/I contrast, volume fraction of the two components, observation plane, and thickness of the sample, under the assumption that the S/I interfaces of the DG and DD structures locate on a pair of level surfaces created by shifting surfaces in the opposite sides of the “Schoen G surface” and the “Schwarz D surface”, respectively.38,39 Figure 3a−c show three different TEM images for LC(70/ 30) blend, whereas Figure 3d,e are the simulated images for (112) and (011) planes of DG, and Figure 3f is (122) plane of DD. It is evident that TEM images in Figure 3a−c are similar to the images in Figure 3d−f. Along the same line, TEM images in Figure 3g,h for LC(60/40) blend are similar to those simulated for (111) and (112) planes of DG as shown in Figure 3j,k, whereas the image in Figure 3i is equivalent to the generated image for (112) plane of DD. These facts indicate that DD has been created partly in the LC(70/30) and LC(60/40) blends in addition to the DG structure. In general, scattering experiments can give the structural information for much wider area than TEM experiments. Figure 4 compares SAXS diffraction patterns for these two binary blends. It is evident that the relative scattering peak locations along the horizontal axis for LC(70/30) and LC(60/

Figure 2. TEM images and circular-averaged SAXS profiles for three SI diblock copolymers, SI-L, SI-G, and SI-C. Scale bars in TEM images indicate 100 nm. The sequence of peaks in SAXS profiles is pointed out by inverted triangles with the relative magnitudes of scattering vectors at peak positions. The insets in SAXS profiles are the original 2D SAXS patterns.

C). TEM images clearly represent that SI-L, SI-G, and SI-C give lamellar, bicontinuous and cylindrical structures, respectively; their morphology assignments were well supported by SAXS observations. That is, the relative scattering peak series for SI-L is integer order, that for SI-G is 6 : 8 : 16 : 22 with respect to q, which corresponds to the sequence for the DG structure, and that for SI-C is 1: 3 : 7 , which is a typical series for hexagonal packing of cylinders in matrix in the SAXS profiles. The lamellar repeating distance for SI-L was estimated to be 87 nm, characteristic unit cell length for SI-G was obtained as 150 nm,37 and interdomain distance for cylinders was 67 nm C

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the Pn3̅m symmetry should give the diffraction peak series of 2 : 3 : 4 : 6 : 10 , whereas the DG structure having I a 3̅ d s y m m e t r y d o e s t h e s e r i e s o f 6 : 8 : 14 : 16 : 22 : 24 . Referring to these experimental results, binary blends adopted in this study are concluded to possess the DG structure dominantly. Figure 5 summarizes the morphological phase diagram of binary blends clarified in this work compared with that of

Figure 5. Morphological phase diagrams of (a) LC binary blends and (b) monodisperse SI diblock copolymers.13 LAM, BC, CYL, and SPH denote lamellar, bicontinuous, cylindrical, and spherical structures, respectively.

monodisperse SI diblock copolymers.13 In binary blends, it was revealed that the BC structure appears in smaller φs region than the monodisperse SI series due to the “cosurfactant effect” occurring because of chain length difference in merely the I block.40 In short, the longer chains could fill the space that the smaller chains cannot fill due to incompressibility requirement, resulting in increasing the curvatures of interfaces. Therefore, BC structures appeared in the volume fraction range where monodisperse SI diblock copolymers give lamellar structures. Ternary LGC Blends. In the previous subsection, it has been found that a DD structure was partly formed in binary blends of lamella-forming SI-L and cylinder-forming SI-C; however, it also has been found that the DD structure was not quite stable at the same time. Here, we should take into account the fact that diblock copolymer system is not likely to give bicontinuous structures. Then, we changed the start point, that is, gyroid-forming copolymer component was decided to be adopted as the principal parent sample. Characteristics of ternary blend samples prepared by blending three SI diblock copolymers, SI-L, -G, -C are listed in Table 3. In particular, the series of ternary LGC blends were

Figure 3. Three different TEM images of LC(70/30) (a−c) and LC(60/40) (g−i) each with the scale bars of 100 nm. The images in (d−f) are simulated ones for G(112) (d), G(011) (e), and D(122) (f), whereas (j−l) are for G(111) (j), G(112) (k), and D(112) (l), where G and D express gyroid and diamond, and the three-digit numbers as Miller indices represent the planes of the simulated projection.

Table 3. Characteristics of Ternary LGC Blends and Their Resulting Morphology sample code

φs

morphology

LGC(7/90/3) LGC(14/80/6) LGC(28/60/12) LGC(42/40/18) LGC(56/20/24)

0.612 0.604 0.587 0.570 0.553

DG DD DD DD DG

prepared by changing the ratio of SI-G from 90% down to 20 wt % while keeping the SI-L/SI-C ratio at 70/30, as this 70/30 blend was proved to give a DD structure in a previous section. TEM images of five ternary LGC blends are compared in Figure 6 it is evident that all ternary blend samples reveal BC structures. Among these samples, the LGC(14/80/6) blend was picked up as a representative example and its three different TEM images are displayed in Figure 7, together with the simulated images. Consequently, experimentally observed

Figure 4. Circularly averaged SAXS profiles of LC(70/30) and LC(60/40) and their original 2D patterns. The diffraction peaks are pointed out by inverted triangles with the relative magnitudes of scattering vectors at their peak positions.

40) are 6 : 8 : 16 : 22 . According to the previous articles on experimental studies,27−29 the DD structure having D

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Figure 6. TEM images of (a) LGC(7/90/3), (b) LGC(14/80/6), (c) LGC(28/60/12), (d) LGC(42/40/18), and (e) LGC(56/20/24). Scale bars indicate 100 nm.

Figure 7. Three typical TEM images for LGC(14/80/6) (a−c) and the corresponding simulated images of the DD structure for three projection planes: (d) D(112), (e) D(122), and (f) D(233).

In general, diffraction patterns for BC structures of microphase-separated block copolymers are not always evident because X-rays hit many grains with different orientation. Therefore, merely in the case where the degree of orientation is high, the sample gives certain diffracted spots as the case in the present experiment. In fact, we notice many high intensity spots in Figure 9a, indicating that the grains are wellorientated; then the treatment of the previous work was adopted in this study. Suzuki et al. found that the (110) plane of the BC structure from a ISP triblock terpolymer is directed to the film surface; therefore, we assumed the same orientation, although the present study is for the SI diblock copolymer system.41 As a result, most of the high-intensity spots were able to be assigned as the possible diffraction plains for a DD

TEM images in Figure 7a−c were well expressed with the simulated D(112), D(122), and D(233) images as shown in Figure 7d−f, respectively. That is, TEM results can anticipate that the dominant structure of LGC(14/80/6) could be a DD structure. On the other hand, Figure 8 shows circularly averaged SAXS profiles of five ternary LGC blends. By analyzing these data carefully, two blends are seen, that is, LGC(7/90/3) at the top and LGC(56/20/24) at the bottom that possess the diffraction peak series, 6 : 8 : 14 : 16 : 22 which is associated with the DG structure, whereas three blends in the middle, LGC(14/80/6), LGC(28/60/12), and LGC(42/40/18) are giving a different series, 2 : 3 : 4 : 6 , which corresponds to the DD structure. Moreover, Figure 9 shows a 2D-SAXS pattern of LGC(14/ 80/6), where X-ray was irradiated from the direction facing a cross section of the sample film, that is, “edge” observation view. Figure 9a shows the data covering all detector areas, whereas Figure 9b is an enlargement of the second quadrant.

structure as designated in Figure 9b using the h2 + k 2 + l 2 values, where h, k, and l denote the Miller indices. Furthermore, Figure 10 represents the circularly averaged SAXS profile of LGC(14/80/6) in comparison with the E

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calculated intensities for the DD structure, where magnitude of

h2 + k 2 + l 2 , was used as the horizontal axis.42 The intensities, I(q) were calculated according to the following equation I(q) =

∫unit cell (ρ(q)exp(−iqr )dr )2

(1)

where r and q are the position vector defined in a unit cell and the scattering vector, respectively. ρ(r) is essentially the electron density of polymer component, but it is simplified in this case to express the scattering contrast and its magnitude is binarized as ρ(q) = 1 for S = 0 for I

(2)

To perform the calculation, level surfaces generated from the Schwarz D-surface are expressed as the following approximate equation using Cartesian coordinates Figure 8. Circularly averaged edge-view SAXS profiles of ternary LGC blends with various blending compositions. The filled inverted triangles indicate magnitudes of the scattering vector at the relative peak positions with 6 : 8 : 14 : 16 : 22 , which correspond to the series for the DG structures, whereas the open inverted triangles indicate those with 2 : 3 : 4 : 6 , which correspond to the series for DD structures.

cos(X ) × cos(Y ) × cos(Z) + cos(X ) × sin(Y ) × sin(Z) + sin(X ) × cos(Y ) × sin(Z) + sin(X ) × sin(Y ) × cos(Z) = 0

(3)

We adopted ±0.73 as the right side values in eq 3 for the level surfaces considering the volume fractions of the present binary blend, that is, φs/φI = 0.604/0.396. It is evident that sequence of the intensity peak series obtained in the experiment, that is, 2 (110), 3 (111), 4 (200), 6 (211), 10 (310), 12 (222), 17 (322), and so on, are in good agreement with the calculated ones. From these detailed SAXS data analyses, the space group for the present complex structure was determined to be Pn3̅m. Combining this result with the real space observation by TEM (Figures 6 and 7), we can safely conclude that the BC structure created from the ternary blend, LGC(14/80/6), is a DD structure. In addition, Figure 11 compares the snapshots of 3Dreconstructed TEMT images for LGC(14/80/6). The movie of the whole images is shown in the Supporting Information. Here, the microdomain of the I component, which appeared bright, is selected and provided. First, a double network structure given by the I component is confirmed in Figure 11a. Figure 11b shows a double network of LGC(14/80/6), which looks like a tetragonal lattice structure, which is conceived to be the (001) plane of the DD structure. Second, Figure 11c, which was obtained by magnifying one branch point of the double network in Figure 11a, clearly reveals a tetrahedral connection. Thus, it has been totally proved that the DD structure was constructed in LGC(14/80/6) by combination of TEM, TEMT, and SAXS, and also has been confirmed that the other ternary blends, that is, LGC(28/60/12) and LGC(42/ 40/18) also reveal DD structures so as to cover a wide composition range, although the data were not shown in this article. From the above, it has been clarified that the present “ternary blend” of diblock copolymers with chain length difference in minor component is a novel molecular design to create the DD structures. Finally, the reason for the appearance of the DD structures in the ternary blends should be discussed. At first, LC binary blends could not construct a stable DD structure, where long

Figure 9. 2D-SAXS patterns of LGC(14/80/6) from edge view for (a) full detector area and (b) second quadrant. All diffracted spots are assigned as characteristic projection planes with the corresponding Miller indices as displayed.

Figure 10. Edge view SAXS profile for LGC(14/80/6) and the calculated intensities for the DD structure with φs of 0.607. F

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chain conformational entropy. Here, a DD structure has larger interfacial area and higher incremental variation in interfacial curvature than a DG structure, and hence the interfacial enthalpy is higher for the former; in addition, the localization of SI diblock copolymers loses placement entropy compared to homogeneous mixing. In reality, however, the present blend samples were successfully able to overcome probable disadvantages of interfacial enthalpy and the placement entropy. Thus, we succeeded in creating DD structures in ternary diblock copolymer blends, where it is hard for the component copolymer to form even a periodic continuous structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00724. SEC profiles of three parent SI diblock copolymers and those of their precursors, TEM images of nine binary blends, and circularly-averaged SAXS profiles of nine binary blends (PDF)

Figure 11. 3D-reconstructed images of LGC(14/80/6): (a) double network, (b) its 100 projection, and (c) piece of tetrahedral connection within a single network.

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and short block chains of SI-L and SI-C are not sufficient to fill network domain, whose thickness varies dramatically in a DD structure as schematically shown in Figure 12a. Therefore,

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-52-7894604. Fax: +81-52-789-3210. ORCID

Atsushi Takano: 0000-0002-5188-5166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Kakenhi (Grant-in-Aid for Scientific Research (A), no. project #16H02292) and Y.M. is grateful for its support, and also by Kakenhi (Grant-in-Aid for Specially Promoted Research, no. 18H05209) and A.T. is grateful to the support. All SAXS measurements were carried out using beam times permitted by SPring-8 facility, the beam line BL40B2, their project numbers are 2016A1078, 2016B1073, and 2018A1474, we are grateful to these programs.

Figure 12. Schematic illustrations of possible chain packing manners within a 4-fold diamond network for (a) binary and (b) ternary blends.

binary blends can construct predominantly DG structures, whose variation of domain thickness and interfacial area is smaller than DD structures. In contrast, LGC ternary blends, composed of SI-G whose I chains have the intermediate length between SI-C and SI-L, can form DD structures in several samples, where SI-L chains selectively fill the thick connections of networks, SI-C chains locally fill narrow channels, and SI-G chains do the middle range smoothly as shown in Figure 12b. As a result, three block chains can play individual roles to save conformational entropy, resulting in forming the DD structures. In conclusion, we investigated the morphology of binary and ternary blends composed of SI diblock copolymers with chains of different lengths merely in the I component in this work. The DD structures were created for ternary blends of SI diblock copolymers; their morphologies are lamellar, DG, and hexagonal cylindrical structures. Structure analyses were conducted in detail by the combination of TEM, SAXS, and TEMT. In these blends, weak localization of I blocks possibly occurs, that is, long I chains in SI-L fill predominantly the fourfold nodes, short I chains in SI-C fill the thin channels, whereas middle I chains in SI-G do the intermediate parts. The present striking results of forming the DD structures are apparently due to the molecular design adopted in this work so as to earn

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REFERENCES

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DOI: 10.1021/acs.macromol.9b00724 Macromolecules XXXX, XXX, XXX−XXX