Gyroid Structures at Highly Asymmetric Volume Fractions by Blending

Nov 8, 2017 - Gyroid Structures at Highly Asymmetric Volume Fractions by Blending of ABC Triblock Terpolymer and AB Diblock Copolymer. Seonghyeon Ahn,...
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Gyroid Structures at Highly Asymmetric Volume Fractions by Blending of ABC Triblock Terpolymer and AB Diblock Copolymer Seonghyeon Ahn, Jongheon Kwak, Chungryong Choi, Yeseong Seo, and Jin Kon Kim* National Creative Research Initiative Center for Smart Block Copolymers, Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Republic of Korea

Byeongdu Lee X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Lemont, Illinois 60439, United States S Supporting Information *

ABSTRACT: We investigated, via small-angle X-ray scattering and transmission electron microscopy, the morphologies of binary blend of polyisoprene-b-polystyrene-b-poly(2-vinylpyridine) (ISP) triblock terpolymer and polyisoprene-b-polystyrene (IS) diblock copolymer. An asymmetric ISP with volume fractions (f) of 0.12, 0.75, and 0.13 for PI, PS, and P2VP blocks, respectively, showed a new morphology: coexistence of spheres and cylinders with tetragonal packing. Asymmetric IS with f I = 0.11 and f S = 0.89 showed conventional bodycentered cubic spherical microdomains. Very interestingly, a binary blend of ISP and IS with overall volume fractions of f I = 0.12, f S = 0.79, and f P = 0.09 exhibited core−shell double gyroid (CSG: Q230 space group), where PI consists of thin core and PS forms thick shell, while P2VP becomes thin matrix. It is very unusual to form CSG even at highly asymmetric volume fractions.

1. INTRODUCTION Block polymers self-assemble various nanostructures, such as lamellae, gyroids, cylinders, and spheres, depending on the volume fraction of one block, the degree of polymerization (N), and the Flory−Huggins interaction parameter (χ).1−4 ABC triblock terpolymers show diverse morphologies because of three volume fractions (fA, f B, f C = 1 − fA − f B) and three Flory−Huggins interaction parameters (χAB, χAC, χBC).5−18 ABC triblock terpolymers have shown more than 20 stable morphologies, for instance, two or three domain lamellae morphologies,13,19−22 core−shell versions of hexagonal and spherical morphologies,22−24 network phases (orthorhombic (O70 space group), alternating gyroid (AG: Q214 space group), core−shell gyroid (CSG: Q230 space group),8,9,11,13,20,25−27 and other mixed-phase morphologies.17,28−32 The experimentally observed morphologies of ABC triblock terpolymers are confirmed by theory and simulations.14−16,18 Among these morphologies, gyroid structures, which are cubic and 3-fold networks of different microdomains, allowing continuous pathways in three dimensions, have received much attention because of potential applications to solar cells,33 nanoporous materials,34−36 and optical devices.35,37 The gyroid morphology has been found in AB diblock copolymers at a narrow range of volume fraction (f) of a block (f is close to 0.35).1−4 Also, ABC triblock terpolymers and their © XXXX American Chemical Society

blends exhibit AG and CSG when the volume fraction of the largest block is 0.4−0.6. For instance, PI-b-PS-b-PEO (the volume fraction of each block: f I = 0.25, f S = 0.53, f O = 0.22),20 PI-b-PS-b-PMMA (f I = 0.27, f S = 0.5, f M = 0.23),38 and PI-bPS-b-P2VP ( f I = 0.26, f S = 0.48, f P = 0.26)39 showed AG, while PS-b-PB-b-PMMA (f S = 0.26, f B = 0.41, f M = 0.33),40 PB-b-PSb-PMMA (f B = 0.32, f B = 0.35, f M = 0.33),41 PI-b-PS-b-PEO ( f I = 0.50, f S = 0.36, f O = 0.14),20 and PI-b-PS-b-PMMA (f I = 0.32, f S = 0.54, f M = 0.14)42 exhibited CSG. Here, PI, PS, PEO, PMMA, P2VP, and PB denote polyisoprene, polystyrene, poly(ethylene oxide), poly(methyl methacrylate), poly(2-vinylpyridine), and polybutadiene, respectively. But, wide ranges of the channel width in the gyroids structure would be needed to design a new optical material such as metamaterial because bandgap and effective plasma wavelength are easily adjusted by the channel size of gyroid structure.43,44 To pursue this goal, one should synthesize gyroids even at the volume fraction of one block having larger than 0.8. However, when the largest volume fraction of a block in an AB and an ABC terpolymer is greater than 0.8, only spherical microdomains are formed.8,20,38 This indicates that gyroid morpholReceived: August 10, 2017 Revised: November 1, 2017

A

DOI: 10.1021/acs.macromol.7b01734 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Molecular Characteristics of Neat ISP and IS Used in This Study symbol

Mn,Totala (g/mol)

Mn,PIb (g/mol)

Mn,PSb (g/mol)

Mn,P2VPb (g/mol)

Mw/Mna

f I/S/Pc

ISP IS

84 000 77 000

8900 7800

63 000 69 200

12 100

1.14 1.05

0.120/0.748/0.132 0.113/0.887/0

a Determined by GPC based on PS standard. bCalculated by 1H NMR. cCalculated with known density at room temperature (ρI: 0.926; ρS: 1.05; ρP2VP: 1.14).48

ogy at highly asymmetric volume fractions having very thick (or thin) channels is very difficult to prepare. Blending of two (or more) block polymers can be used to create new morphologies that could not be prepared by neat block polymers. For example, we obtained highly asymmetric lamellar structure by blending asymmetric polystyrene-blockpoly(vinylpyridine) copolymer (as-PS-b-PVP) and asymmetric polystyrene-block-poly(4-hydroxystyrene) copolymer (as-PS-bPHS),45−47 although both neat as-PS-b-PVP and as-PS-b-PHS showed body-centered cubic (BCC) spherical microdomains. Recently, Asai et al. reported that binary blends of ISP triblock terpolymers with different volume fractions exhibited very interesting morphologies such as rectangular-shaped cylinders48 and cylindrical morphologies with nonuniform domain sizes and shape,49,50 which cannot be obtained from the neat ISP triblock terpolymers. Here, we reported CSG at highly asymmetric volume fractions obtained by blending a ISP triblock terpolymer and a IS diblock copolymer. Neat ISP with f I = 0.12, f S = 0.75, and f P = 0.13 showed a new morphologycoexistence of spheres and cylinders with tetragonal packingwhile neat IS with f I = 0.11 and f S = 0.89 showed conventional body-centered cubic spherical microdomains. Interestingly, a binary blend of ISP and IS with overall volume fraction of f I = 0.12, f S = 0.79, and f P = 0.09 exhibited CSG at highly asymmetric volume fraction, where P2VP having the smallest volume becomes a thin matrix.

Table 2. Molecular Characteristics of Binary Blend of ISP and IS blending ratio ISP/IS (mol/mol)

VolPI

VolPS

VolP2VP

ISP

100/0

0.120

0.748

0.132

ISP75 ISP67 ISP50 ISP33 ISP25 ISP17 IS

75/25 67/33 50/50 33/67 25/75 17/83 0/100

0.118 0.118 0.117 0.115 0.115 0.114 0.113

0.783 0.794 0.817 0.841 0.852 0.863 0.887

0.099 0.088 0.066 0.044 0.033 0.023 0

symbol

a

morphologya coexisting spheres and cylinders gyroids gyroids gyroids spheres spheres spheres spheres

Determined by SAXS and TEM images.

coupled detector (Mar USA, Inc.) was employed. The sample-todetector distance was 4 m. The thickness of the samples was 1.0 mm, and the exposure time was 100 s. Transmission Electron Microscopy (TEM). The samples were ultrasectioned by using a Leica Ultracut Microtome (EM UC6 Leica Ltd.) at room temperature with a thickness of 40 nm. Then, they were stained by exposure to OsO4 vapor for 2 days and I2 vapor for 1 h at room temperature. The PI, P2VP, and PS microdomains appear dark, gray, and white, respectively, in TEM images. To observe only P2VP microdomains, the samples were stained with only I2 vapor. The micrographs were taken at room temperature with bright-field TEM (S-7600 Hitachi Ltd.) at 80 kV. SAXS and TEM Simulations. For SAXS and TEM simulations, a gyroid model was generated with an assumption that the cocontinuous structure possesses the tricontinuous nature in which I/S and S/P interfaces are created by the pseudo-parallel shift of the threedimensional periodic minimal G surface.9 The model consisted of a unit cell of 100 × 100 × 100 voxels with three different electron densities (ρeI = 0.512 electrons/cm3, ρeS = 0.565 electrons/cm3, and ρeP2VP = 0.608 electrons/cm3),50 corresponding to I, S, and P blocks for SAXS. For TEM simulation, relative absorption coefficients were used instead of the electron densities. The integrated diffraction intensity for a given Miller index was computed from the model and corrected for multiplicity and Lorentz factors. The pseudo-Voigt function was used as a peak shape function of which breadth was calculated with appropriate Debye−Waller factor, domain size, and microstrain values that best fit the experimental data.51 Finally, diffuse scattering with a form of c/q3.5, were c is a scaling constant, was added to the sum of the peak functions for all diffraction peaks.52 Detailed information on SAXS and TEM simulations is given in section 5 of the Supporting Information.

2. EXPERIMENTAL SECTION Materials. ISP triblock terpolymer and IS diblock copolymer were synthesized by sequential anionic polymerization of isoprene, styrene, and 2-vinylpyridine in tetrahydrofuran under purified argon atmosphere using sec-butyllithium initiator. The polymerization temperature was 20 °C for isoprene and −78 °C for styrene and 2-vinylpyridine. Molecular Characterization. The number-average molecular weight (Mn) and polydispersity index (PDI) of ISP and IS were measured by size exclusion chromatography (SEC: Waters 2414 refractive index detector) based on PS standards. Two 300 mm (length) × 7.5 mm (inner diameter) columns including particle size of 5 μm (PLgel 5 μm MIXED-C: Polymer Laboratories) were used with THF as an eluent and a flow rate of 1 mL/min at 30 °C. The volume fractions of each block were determined by 1H nuclear magnetic resonance spectra (1H NMR: Bruker Avance III 400) with a solvent of chloroform-d (CDCl3) (see Figures S1 and S2 in the Supporting Information). The molecular characteristics of ISP and IS are summarized in Table 1. Sample Preparation. Neat samples and binary blends of ISP and IS were prepared by using THF and solution casted from 5 wt % THF solution and slowly evaporated over 1 week at room temperature. For the complete removal of THF, samples were thermally annealed at 240 °C for 3 h under high vacuum and quenched at room temperature. The molecular characteristics of all blend samples are summarized in Table 2. Small-Angle X-ray Scattering (SAXS). SAXS profiles [I(q) vs q (= (4π/λ) sin θ), where q and 2θ are the scattering vector and scattering angle, respectively, were obtained at the in-vacuum Undulator 20 beamline (4C SAXS II) of the Pohang Accelerator Laboratory (PAL) Korea. The wavelength and beam size were 0.675 Å and 0.2 (H) × 0.6 (W) mm2, respectively. A two-dimensional charge

3. RESULTS AND DISCUSSION Figure 1 shows SAXS profile and TEM images for neat ISP. The SAXS profile (Figure 1a) shows scattering peaks at position of 1:√2:2:√5 relative to q* (0.195 nm−1). Because of the absence of √3 peak and strong √5 peak, neat ISP shows tetragonally packed morphology.50 The domain spacing (2π/ q*) obtained from SAXS profile was 32 nm. In TEM images (Figures 1b and 1c) of the sample stained with both OsO4 and I2, dark, gray, and white regions correspond to PI, P2VP, and B

DOI: 10.1021/acs.macromol.7b01734 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. SAXS profile (a) and TEM images (b−d) of neat ISP. Top (b) and side (c) view TEM images of neat ISP stained with both OsO4 and I2. Dark, gray, and white regions depict PI, P2VP, and PS microdomains, respectively. Expanded TEM image (inset of (b)) clearly demonstrates that PI (black) and P2VP (gray) microdomains are tetragonal packing. (d) TEM image of neat ISP stained with only I2. The sample was cut along the cylinder axis of P2VP microdomains. The inset was TEM image when the sample was cut perpendicular to the cylinder axis. The black regions represent P2VP microdomains. (e) Schematic of the coexistence of spheres and cylinders with tetragonal packing and their molecular dispositions. Blue and green colors represent PI and P2VP, respectively. Scale bar is 30 nm.

Figure 3. Schematic of morphologies of neat ISP and IS (left panel). Five scenarios (cases 1−5) for the formation of double gyroids structures from binary blends of ISP and IS. Cases 1 and 2 are core− shell gyroid where all three constituted blocks are microphase separated, and cases 3−5 are conventional double gyroid where two blocks are mixed. Blue, red, and green colors represent PI, PS, and P2VP microdomains, respectively. Figure 2. SAXS profiles of neat and blend samples: (a) ISP, (b) ISP75, (c) ISP67, (d) ISP50, (e) ISP33, (f) ISP25, (g) ISP17, and (h) IS. SAXS peaks of (b, c, d) correspond to gyroid structures, while SAXS peaks of (e, f, g, h) correspond to BCC structures.

PS microdomains, respectively, due to the selective staining of PI by OsO4 and P2VP by I2.50 As shown in the top view TEM image (Figure 1b), PI and P2VP microdomains are tetragonally packed. From the side-view TEM image (Figure 1c), PI becomes spherical microdomains, while P2VP becomes C

DOI: 10.1021/acs.macromol.7b01734 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. SAXS profiles of ISP67 measured experimentally and simulated for five different cases. Experimentally observed SAXS peaks are shown as (○), while the peaks (×) do not appear in measured SAXS peaks.

cylindrical microdomains. As shown in Figure 1d, where the sample was stained with only I2, P2VP microdomains become tetragonal packed cylinders. The domain spacing between P2VP cylinders was 32 nm, which is consistent with SAXS result. Figure 1e is a schematic of neat ISP morphology where both spheres and cylinders are tetragonally packed. Mogi et al.8 reported that the boundary of spherical and cylindrical microdomains for neat ISP triblock terpolymers was observed at a volume fraction of the end block having ∼0.12. Since f I is 0.12 and f P is 0.13 in this study, these volume fractions are close to the boundary between spheres and cylinders. P2VP with slightly larger volume fraction than PI forms cylindrical microdomains, and PI becomes spherical microdomains. To accommodate a similar volume fraction of PI and P2VP, the diameter of PI spheres is larger than that of P2VP cylinders. Also, the spheres are located more densely along the cylinder axis compared with a conventional BCC structure. On the other hand, the neat IS diblock copolymer (f I = 0.11) shows conventional body-centered cubic (BCC) spherical morphology confirmed by SAXS profile and TEM image (Figure S3). To observe morphological change, we blended neat ISP and IS at various blending ratios. The numerical values in the sample notation represent the molar ratio of ISP to IS; for instance, ISP75 is 75/25 (mol/mol) ISP and IS. Interestingly, the SAXS profiles (Figure 2b,c) of ISP67 and ISP75 show scattering peaks at position of √6:√8:√16:√20:√22:√24:√32:√38:√40:√48:√54. These scattering peaks are consistent with Ia3d double-gyroid structure (Q230).20 Normally, AB diblock copolymers exhibit Q230 gyroid morphology when the volume fraction of one block is close to 0.35.1−4 Also, ABC triblock terpolymers and their blend systems exhibit AG (Q214) and CSG (Q230) only when

Figure 5. (a, b) TEM images of ISP67 stained with OsO 4 corresponding to [110] and [111] projections, respectively, where PI (black) becomes the core. (c) TEM image of ISP67 stained with I2 corresponding to [110] projection, where P2VP (black) becomes thin matrix. (d, e) TEM simulations of [110] and [111] projections, respectively, where black becomes the core. (f) TEM simulation of [110] projection where black is the thin matrix. The thickness employed for TEM simulations was 40 nm. Scale bar of all images is 100 nm.

the volume fraction of the largest block is 0.4−0.6.20,38,39,42 However, in this study, we found Q230 morphology at f I = 0.12, f S = 0.79, and f 2VP = 0.09 (ISP67), which is a highly asymmetric volume fraction. It is very interesting because spherical structures are found for neat ABC triblock terpolymer with the volume fraction of the largest block having ∼0.80.14−16,18 For example, Mogi et al.8 reported the neat ISP triblock terpolymer (f I = 0.13, f S = 0.78, and f P = 0.09), which has very similar volume fractions to ISP67, showed spherical morphology. ISP50 also showed CSG, although the √8 peak is weak and higher ordered peaks are not intense compared with ISP67. The SAXS profiles (Figure 2e−g) of ISP33, ISP25, and ISP17 show scattering peaks at position of 1:√3:√6 relative to q*, indicating body-centered cubic (BCC) spherical morphology. TEM images of the blend samples are given in Figures S4 and S5. There are several scenarios for the formation of double gyroids from binary blend of ISP and IS, as schematically drawn in Figure 3. One is a typical double gyroid observed for AB diblock copolymer, where PI and P2VP chains are mixed and form one channel, while PS becomes the matrix (case 3). However, this morphology would be difficult to form because χI,P (∼0.2) is higher than χI,S (∼0.06)53 and χS,P (∼0.1).47 D

DOI: 10.1021/acs.macromol.7b01734 Macromolecules XXXX, XXX, XXX−XXX

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4. CONCLUSION In conclusion, we have investigated the morphologies of neat ISP and IS and the binary blend. The morphology of neat ISP shows coexistence of PI spheres and P2VP cylinders with tetragonal packing in the matrix of PS. Interestingly a binary blend with f I = 0.12, f S = 0.79, and f P = 0.09 showed CSG morphology at highly asymmetric volume fraction, where even P2VP with the smallest volume fraction forms the matrix. The widening of the range of volume fractions for the formation of gyroids would allow one to design a new optical material by modifying effective plasma wavelength and nanoporous templates for a large volume of backfilling of metal or semiconducting materials.

Namely, PI and P2VP chains should be located far from each other. The χI,P was obtained from the absolute SAXS intensities of a low molecular weight and symmetric IP diblock copolymer exhibiting disordered states at the entire experimental temperatures (see Figure S6). Cases 4 and 5 are also difficult to occur. This is because χI,S ∼ 0.06, χS,P ∼ 0.1, and N ∼ 800. Thus, the value of χN for both IS and SP is large enough, which prohibits the mixing of PS and PI chains as well as PS and P2VP chains. Between two remaining CSG (cases 1 and 2), one should decide which block is located inside the core of CSG. Because of gyroid geometry, the width (or diameter) of channels connecting three arms together would be larger than that of remaining individual tripod. When P2VP is located inside the core, some P2VP chains at the connecting points should be much extended, causing a large entropy penalty (case 2). However, when PI chains exist inside the core, the entropy penalty would be much reduced. This is because of the existence of two different PI chains: one from ISP and the other from IS. Since PI chains of ISP are longer than those of IS, PI chains of IPS easily fill the channels at the connecting areas (case 1). The mechanism to form CSG by blending of ISP and IS might be explained as follows. When IS diblock copolymer is mixed with ISP triblock terpolymer, the PI chains of IS should be located near spherical microdomains made of PI chains of ISP. Then, the PI microdomains of IS may form a channel that connects the PI spheres of ISP. Eventually PI spheres of ISP may turn into 3-fold junctions of the gyroid structure. This structure formation mechanism is well corroborated by the chain entropies of PIs of IS and ISP. Namely, due to longer PI chains in ISP than those of IS, the former would prefer locating at the center parts, that is, 3-fold junctions of the gyroid channels. Since PS chains are directly connected to PI chains, PS microdomains should exist next to the PI microdomains; thus, PS chains form the shell part of gyroid channel. On the other hand, due to high repulsion between PI and P2VP, P2VP chains of ISP are located far away from PI chains. Thus, P2VP chains fill the remaining regions of core−shell gyroids and become the continuous matrix. To support CSG morphology of ISP 67, we obtained simulated SAXS profiles corresponding to all five cases given in Figure 3. Simulated SAXS profiles for cases 2, 4, and 5 show the √14 peak, which is not observed experimentally. Also, the √16 peak observed in the experimental data does not exist in the simulated SAXS profile for case 3. Therefore, among five simulated SAXS profiles, the SAXS profile for case 1 (Figure 3) is best matched with the experimentally measured one, suggesting that ISP 67 shows the CSG with PI core, PS shell, and P2VP matrix. To show direct visualization of ISP 67 morphologies, we obtained TEM images (left panels) as well as TEM simulation results (right panels) corresponding to two different projections ([110] and [111] planes), as shown in Figure 5. From Figures 5a and 5b, PI microdomains (black) form thin core, while PS and P2VP microdomains (both looking white) become the remaining part. To identify the location of P2VP microdomains, we stained the sample with only I2, which is a selective staining to P2VP. From Figure 5c, P2VP microdomains are clearly separated and form thin matrix. The experimentally obtained TEM images are well-consistent with TEM simulations. Thus, ISP 67 showed CSG with PI core, PS shell, and P2VP matrix at a highly asymmetric volume fraction.

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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.7b01734. GPC data, 1H NMR spectra, TEM images, SAXS profiles, χ I,P estimated from the absolute SAXS intensities, and detailed information on SAXS and TEM simulations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (J.K.K.). ORCID

Jin Kon Kim: 0000-0002-3872-2004 Byeongdu Lee: 0000-0003-2514-8805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (2013R1A3A2042196). SAXS experiments were conducted at 4C beamline of PAL (Korea). B.L. was supported by the U.S. Department of Energy (DOE) Office of Science under Contract DE-AC02-06CH11357.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01734 Macromolecules XXXX, XXX, XXX−XXX