Phase Behavior of Binary Blend Consisting of Asymmetric Polystyrene

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Phase Behavior of Binary Blend Consisting of Asymmetric Polystyrene-block-poly(2-vinylpyridine) Copolymer and Asymmetric Deuterated Polystyrene-block-poly(4-hydroxystyrene) Copolymer Jongheon Kwak, Sung Hyun Han, Hong Chul Moon, 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

Jaseung Koo and Jeong-Soo Lee Division of Neutron Science, Korea Atomic Energy Research Institute (KAERI), 989-111 Daedeok-daero, Yuseong-gu, Daejeon, 305-353, South Korea

Victor Pryamitsyn and Venkat Ganesan Department of Chemical Engineering, University of Texas, Austin, Texas 78712, United States S Supporting Information *

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INTRODUCTION Block copolymers have been extensively investigated due to their various self-assembled nanoscale structures such as lamellae, cylinders, gyroids, and spheres depending on the volume fraction (f) of one block, the degree of polymerization (N), and the Flory−Huggins segmental interaction parameter (χ).1−4 For easy tuning of nanostructures and finding unusual microdomains that cannot be obtained by neat block copolymers, many research groups have employed binary blends of two block copolymers: for instance, (A−B)1/(A−B)2 or (A−B)/(A−C) and (A−B)/(C−D).5−16 A blending of two or more block copolymers is also an attractive way for adjusting physical properties and expanding the phase boundary of conventional block copolymers.17−23 Recently, we reported the phase behavior of binary blends consisting of polystyrene-block-poly(2-vinylpyridine) copolymer (PS-b-P2VP) and polystyrene-block-poly(4-hydroxystyrene) copolymer (PS-b-PHS), which has a strong favorable interaction between P2VP and PHS by hydrogen bonding. The binary blends of high molecular weight and symmetric PS-b-P2VP and low molecular weight and symmetric PS-b-PHS (the volume fraction of PS (f PS) in both PS-b-P2VP and PS-b-PHS was ∼0.5 and exhibited lamellar microdomains in neat block copolymers) showed the phase transformation from lamellar microdomains to hexagonally packed (HEX) cylindrical microdomains and bodycentered cubic (BCC) spherical microdomains with the increase of the amount of PS-b-PHS in the blends.24 Interestingly, the binary blends of high molecular weight of asymmetric PS-bP2VP (as-PS-b-P2VP) and low molecular weight of asymmetric PS-b-PHS (as-PS-b-PHS), which have asymmetric volume fraction (f PS ∼ 0.8) and exhibit spherical microdomains, showed the phase transformation from BCC spherical microdomains to highly asymmetric lamellar microdomains,25 which cannot be straightforwardly achieved by conventional block copolymer selfassembly. We also demonstrated the potential of lithographic applications that the line width can be reduced to the sub-10 nm © XXXX American Chemical Society

scale and its periodicity can be precisely tuned by fabricating a thin film having vertically oriented asymmetric lamellar microdomains.25 We consider possible morphologies of a binary blend of high molecular weight of as-PS-b-P2VP and lower molecular weight of as-PS-b-PHS based on the conjecture. The first one is that as-PS-b-PHS chains are located in either PS microdomains or P2VP microdomains of as-PS-b-P2VP (Figure 1a−c). Because the Flory−Huggins segmental interaction parameter (χ) of PS and PHS is quite large (0.68,26 which is about 6 times larger than that (0.1) of PS/P2VP;27 χ of P2VP/PHS should be negative because of a miscible blend resulting from hydrogen bonding), PS-b-PHS chains could form as micelles. The micelle core would consist of PHS chains when PS-b-PHS chains are located inside the PS microdomains, while that consisting of PS chains is expected when PS-b-PHS chains are located inside the P2VP microdomain. For the former case, the effective PS volume should increase; thus, the expected morphology should be spherical morphology (Figure 1a). This is inconsistent with experimental results. Although PS-b-PHS chains in the P2VP microdomains would form micelles with PS core to reduce the repulsion interaction between P2VP and PS (Figure 1b), the micelle formation is not easy due to the much longer PS chain compared to the PHS chain. An alternative situation is that PS-b-PHS chains are more and less uniformly dispersed in the P2VP microdomain by the favorable hydrogen-bonding interaction between P2VP and PHS blocks (Figure 1c). However, a large unfavorable interaction exists between PS chains in PS-b-PHS and P2VP chains.27 Furthermore, for the cases corresponding to Figure 1b,c, the effective volume of P2VP microdomains should be increased, leading to more symmetric morphology, not highly asymmetric morphology. This is also Received: October 28, 2014 Revised: January 14, 2015

A

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asymmetric microdomains.25,28 However, the above argument and the exact distribution of short PS-b-PHS chains could not be confirmed experimentally because the location of PS-b-PHS chains cannot be verified with small-angle X-ray scattering (SAXS) or transmission electron microscopy (TEM). Since neutron reflectivity (NR) is sensitive to scattering length density (SLD) gradients normal to the substrate of a film, the segment density profiles can be easily determined by selective deuterium labeling of one block.29−31 In this study, we synthesized asymmetric deuterated polystyrene-block-poly(4hydroxystyrene) copolymer (as-dPS-b-PHS) to investigate, by NR, the exact location of short as-dPS-b-PHS diblock chains in binary blend of as-PS-b-P2VP and as-dPS-b-PHS. We confirmed by SAXS and TEM that the binary blend also formed asymmetric lamellar microdomains in bulk state. For NR experiment, we prepared a thin film having parallel orientation of asymmetric lamellae along a substrate by adjusting the film thickness. SLD profile fitted from NR shows that most of the dPS chains are located on both sides of P2VP microdomain, and there are few dPS chains at the middle of P2VP and PS microdomain. This experimental result suggests that almost the whole junction of dPS-b-PHS chains is located at the interface of PS and P2VP microdomains, which is consistent with the assumptions of the strong stretching theory.28

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Figure 1. Schematics for expected morphologies of a binary blend of two block copolymers with hydrogen bonding focusing on to the location of short diblock chains. (a) PS-b-PHS micelles with PHS core in the PS microdomain, (b) PS-b-PHS micelles with PS core in the P2VP microdomain, (c) PS-b-PHS chains are dispersed in the P2VP microdomain by the favorable hydrogen bonding interaction between PHS and P2VP chains, (d) all PS-b-PHS chains are located at the interface of PS and P2VP microdomains, (e) some PS-b-PHS chains are located at the interface of PS and P2VP microdomains, but other chains are located inside both PS and P2VP microdomains as micelles, and (f) some PS-b-PHS chains are located at the interface of PS and P2VP microdomains. Other chains are located in the PS microdomain as micelles with PHS core, while the remaining chains are dispersed in the P2VP microdomain.

EXPERIMENTAL SECTION

Materials. as-PS-b-P2VP was purchased from Polymer Source. asdPS-b-PHS was prepared by the hydrolysis of deuterated polystyreneblock-poly(4-tert-butoxystyrene) copolymer (dPS-b-PtBOS) which was synthesized by sequential anionic polymerization in tetrahydrofuran (THF) at −78 °C under an argon environment using sec-butyllithium (s-BuLi) as an initiator32 (see Figure S1 in the Supporting Information). A binary blend of 60:40 (w/w) as-PS-b-P2VP and asdPS-b-PHS was prepared using a cosolvent of dimethylformamide (DMF). A bulk sample was prepared by solution casting (10 wt % DMF solution), while the thin film was prepared by spin coating of 2 wt % DMF solution at 2000 rpm for 60 s on a silicon wafer. All samples were thermally annealed at 180 °C for 5 days under vacuum and quenched at room temperature. Molecular Characterization. The number-average molecular weight (Mn) and polydispersity index (PDI) of as-dPS-b-PHS 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 weight fraction of each block was determined from the molecular weight of deuterated PS precursor and as-dPS-b-PHS. The hydrolysis reaction of dPS-b-PtBOS was confirmed by 1H nuclear magnetic resonance spectra (1H NMR: Bruker Avance III 400) with a solvent of

inconsistent with experimental results. Thus, the only possible scenario for the formation of highly asymmetric lamellar is that all (or some) junction points of short PS-b-PHS chains should be located at the interface of PS and P2VP microdomains (Figure 1d−f). Once all of PS-b-PHS chains are not located at the interface of PS and P2VP microdomains, some chains are located inside both PS and P2VP microdomains as micelles (Figure 1e). Or, other chains are located in the PS microdomain as micelles with PHS core, while remaining chains are dispersed in the P2VP microdomain (Figure 1f). When the junction points of short PS-b-PHS chains are located at the interface of PS and P2VP microdomains, the long P2VP chains should be contracted, while short PHS chains should be stretched to increase the hydrogen-bonding interactions between P2VP and PHS chains. When long P2VP chains are contracted greatly in spherical geometry, the spherical microdomains are no longer maintained; thus, they should be transformed to less-curved morphology (such as cylinder or lamellae). Namely, the lamellar phase can easily overlap the P2VP and PHS chains by changing the interface curvature, which results in asymmetric lamellar microdomains. We previously developed a strong stretching theory (SST) based on the Semenov’s theory to explain the phase transformation from BCC spherical microdomains to highly

Table 1. Molecular Characteristics of Polymers Employed in This Study sample as-PS-b-P2VP as-dPS-b-PHS blendc

Mna (g/mol)

Mw/Mn

106000 21000

1.07 1.08

f PS (or fdPS)b

morphologyd

0.83 0.79 0.81

spheres spheres asymmetric lamellae

a

Mn and Mw are the number- and weight-average molecular weights measured by GPC using PS standards. bfdPS is calculated with the Mn of dPS precursor and as-dPS-b-PHS and known density at room temperature (dPS = 1.15 g/cm3;33 PHS = 1.16 g/cm3 24). cBlend ratio of as-PS-b-P2VP and as-dPS-b-PHS is 60/40 (w/w). dMorphology of neat as-PS-b-P2VP and as-dPS-b-PHS was determined by SAXS and TEM images (Figure S3 in the Supporting Information). B

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Figure 2. (a) SAXS profile and (b) TEM image for 60/40 (w/w) asPS-b-P2VP/as-dPS-b-PHS blend. (c) Scattering length density profile in the direction perpendicular to the lamellar plane where each lamellar microdomain has different thickness. ρ is scattering length density; ai and bi are the ith lamellar microdomain thickness. (d) Curve-fitted SAXS profile by using the variable lamellar thickness structure model. The open circles are the experimentally measured SAXS intensity, and the solid line is the fitted SAXS intensity profile. chloroform-d (CDCl3) for dPS-b-PtBOS and acetone-d ((CD)3CO) for as-dPS-b-PHS (see Figure S2 in the Supporting Information). The molecular characteristics for the samples are summarized in Table 1. 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 chargecoupled detector (Mar USA, Inc.) was employed. The sample-todetector distance was 4 m. The sample was annealed at 180 °C for 5 days in a vacuum, followed by quenching at room temperature. The thickness of the sample was 1.0 mm, and the exposure time was 100 s. Transmission Electron Microscopy (TEM). The bulk sample was ultrasectioned by using a Leica Ultracut Microtome (EM UC6 Leica Ltd.) at room temperature with a thickness of ∼40 nm. To see the cross-sectional TEM image of thin film, the film was first cross-linked with 1,4-dibromobutane vapor to protect the film damage during the detachment from the silicon substrate in HF solution. Then, it was coated with carbon and embedded in an epoxy resin and then ultrasectioned at room temperature with a thickness of ∼40 nm. Finally, both samples were stained by exposure to iodine vapor for 1 h at room temperature. The P2VP and PHS microdomains look dark in TEM image. The micrographs were taken at room temperature by bright-field TEM (S-7600 Hitachi Ltd.) at 80 kV.

Figure 3. (a) Cross-sectional TEM image for a thin film of 60/40 (w/w) as-PS-b-P2VP/as-dPS-b-PHS blend having 1.5L0 thickness. (b) Neutron reflectivity (Rq4) as a function of qz (Å−1) for 1.5L0 thin film. (c) Scattering length density (SLD) profile normal to the film surface. Neutron Reflectivity (NR). A thin film with 1.5L0 thickness (54 nm, here L0 is the lamellar domain spacing in bulk) was prepared by spin-coating of 2 wt % DMF solution and thermally annealed at 180 °C for 5 days under vacuum. A specular neutron reflectivity experiment was conducted with the REF-V reflectometer at the Cold Neutron Laboratory building of the HANARO at the Korea Atomic Energy Research Institute (Daejeon, Korea) with a wavelength (λ) of 4.75 Å and ΔQ/Q = 0.02−0.06. Neutron reflectivity curves were analyzed with the Parratt’s recursive method,34 where the films are divided into sublayers with various thicknesses, interfacial roughness and SLD. Initial trial values of these parameters were chosen based on Figure 1d and TEM image of the film. Then, the algorithm is used for the least-squares fitting to minimize the errors between the measured and predicted reflectivity. C

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Figure 4. (a) Volume fraction profile of dPS segments within one lamellar spacing and (b) schematic for block copolymer chains. X-ray Photoelectron Spectroscopy (XPS). To investigate the elemental analysis of the bottom of the film, thin film was first crosslinked with 1,4-dibromobutane vapor and then detached from silicon substrate in HF solution and flipped over on another silicon substrate. XPS spectra were obtained on beamline 4D at the Pohang Light Source (Korea).

reflected scattering intensity as a function of the momentum transfer normal to the substrate, qz = 4π sin θ/λ, where θ is the grazing incidence angle and λ is the neutron wavelength, respectively. The open circles in Figure 3b are the experimentally measured reflectivity of the thin film, and the solid line is the calculated reflectivity profile using the scattering length density (SLD) profile shown in Figure 3c. The calculated reflectivity (Rq4) profile agrees well with the experimental data. In the SLD profile, the air/polymer interface is located at z = 0 and SLD of air is zero. The values of SLD of silicon substrate and native oxide layer are 2.07 × 10−6 and 3.47 × 10−6 Å−2, respectively. In Figure 3c, there are three SLD peaks (∼4.50 × 10−6 Å−2), which arise from higher SLD value of dPS (6.58 × 10−6 Å−2). The values of SLD of PS, P2VP, and PHS are 1.41 × 10−6, 1.94 × 10−6, and 1.69 × 10−6 Å−2, respectively. From the locations of the first and second SLD peaks, most dPS chains are not located in the middle of P2VP lamellar microdomains, suggesting that most of as-dPS-b-PHS chains should be located at the interface of PS and P2VP microdomain. The low value of SLD (∼1.4 × 10−6 Å−2) near air suggests that although dPS has lower surface energy than PS,35 the short dPS chains could be located near the interface between PS and P2VP microdomain due to favorable interaction between P2VP and PHS chains via hydrogen bonding. Interestingly, there exists another SLD peak near the substrate. This is because dPS chains could be located near the substrate by the favorable interaction (or even covalent bonding) between the hydroxy group of PHS and SiOx during thermal annealing at 180 °C. To analyze the distribution of dPS segments quantitatively, we calculated the volume fraction of dPS near the P2VP microdomain within one lamellar spacing excluding near the substrate region (Figure 4a). We assumed that there exist only PS and dPS segments in the PS microdomain, while P2VP, PHS, and dPS segments coexist in the P2VP microdomain (Figure 4b). Also, SLD value in each microdomain is calculated by the weight-average of SLD of all segments. It is seen that dPS volume fraction near P2VP microdomain can reach ∼0.6, much higher than average dPS volume fraction (∼0.3) in the blend. Interestingly, almost no dPS segments are located in the middle of P2VP and PS microdomain. This means that as-dPSb-PHS chains could not be uniformly distributed in the P2VP microdomain because of a large enthalpic penalty between dPS and P2VP, which overwhelms the translational entropy effects, and also they did not form the micelle in PS microdomain.

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RESULTS AND DISCUSSION Figures 2a and 2b give SAXS profile and the TEM image for the 60/40 (w/w) as-PS-b-P2VP/as-dPS-b-PHS blend. It shows the scattering peaks at the positions 1:2:3:4 relative q* (0.1615 nm−1), suggesting that binary blend of as-PS-b-P2VP/as-dPS-b-PHS shows lamellar microdomains. The domain spacing (2π/q*) obtained from SAXS profile was 39 nm, consistent with TEM result. The bright and dark areas in the TEM image (Figure 2b) correspond to PS and P2VP (plus PHS) microdomains, respectively, because P2VP and PHS were selectively stained by iodine. Also, we analyzed the obtained SAXS profile by fitting with a variable lamellar thickness structure model (Figure 2c,d). The details are described in the Supporting Information. The estimated volume fraction of PS microdomain in the blend is 0.8, which is in a good agreement with f PS and fdPS of as-PS-bP2VP and as-dPS-b-PHS. Furthermore, the predicted lamellar width ratio of PS to P2VP lamellar microdomain is 4, which is consistent with TEM image in Figure 2b. Based on these results, the binary blend of as-dPS-b-PHS and as-PS-b-P2VP formed asymmetric lamellar microdomains, similar to the blend of as-PS-b-PHS and as-PS-b-P2VP. Figure 3a shows a cross-sectional TEM image of a thin film of 60/40 (w/w) as-PS-b-P2VP/as-dPS-b-PHS blend. Since the thickness of the film was 54 nm, which is 1.5L0, the parallel lamellae should be formed because P2VP and PHS prefer to be located at the silicon substrate due to the favorable interaction with SiOx, while PS prefers to be located at the air side due to the smaller surface tension than P2VP (or PHS). We confirmed via XPS measurement that most PHS chains were located at the bottom of the film, although small amounts of P2VP chains were present at the bottom of the film (Figure S6 in the Supporting Information). This might be attributed that PHS chains has more favorable interaction of SiO2 layer on the silicon substrate compared with P2VP chains. Figure 3b shows the neutron specular reflectivity profile for a thin film of 60/40 (w/w) as-PS-b-P2VP/as-dPS-b-PHS blend. We plotted the D

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By being located at the interface of PS and P2VP microdomains, the volume fraction ratio of each microdomain is maintained as that of neat block copolymers. Therefore, we can conclude that the microdomain transformation of binary blend of as-PS-b-P2VP/as-dPS-b-PHS from spherical microdomain to asymmetric lamellar microdomain is caused by change of the interface curvature, which results from stretched PHS chains and contracted P2VP chains by hydrogen bonding interaction between P2VP and PHS chains. In summary, we synthesized as-dPS-b-PHS copolymer via sequential anionic polymerization and hydrolysis reaction to investigate the mechanism of the microdomain transformation of as-PS-b-P2VP/as-dPS-b-PHS binary blend from spherical microdomain to asymmetric lamellar microdomain by hydrogen bonding between P2VP block and PHS block. Through neutron reflectivity experiment, the exact location of the short dPS-b-PHS chains was determined. The microdomain transformation from spherical to asymmetric lamellar microdomains is due to the interface curvature change by hydrogen bonding interaction between P2VP block and PHS block.

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ASSOCIATED CONTENT

* Supporting Information S

Synthetic scheme, SEC chromatogram, NMR spectra, and rheological data for as-dPS-b-PHS; SAXS profiles and TEM micrographs for neat as-PS-b-P2VP and as-dPS-b-PHS; fitting data of SAXS profile with a variable lamellar thickness structure model; XPS spectra for the bottom side of thin film of as-PS-bP2VP/as-dPS-b-PHS blend; cartoon of as-PS-b-P2VP/as-dPSb-PHS blend film for the entire thickness. 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] (J.K.K.). 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) and Nuclear Energy Research Infrastructure Program supported by the National Research Foundation of Korea (2013M2B2A4041167). SAXS experiment and XPS experiments were done at 4C beamline and 4D beamline, respectively, of PAL (Korea). The NR experiment was carried out at HANARO at the Korea Atomic Energy Research Institute.

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REFERENCES

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