Fluorine-Containing Styrenic Block Copolymers toward High χ and

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Fluorine-Containing Styrenic Block Copolymers toward High χ and Perpendicular Lamellae in Thin Films Seongjun Jo,† Seungbae Jeon,† Taesuk Jun,† Cheolmin Park,‡ and Du Yeol Ryu*,† †

Department of Chemical and Biomolecular Engineering and ‡Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea

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S Supporting Information *

ABSTRACT: We propose a new approach to fluorinecontaining, high-χ styrenic block copolymers (BCPs) via side-chain modification in one block. Polystyrene-b-poly(2,2,2-trifluoroethyl acrylate)s (PS-b-PTFEAs) were synthesized by high-conversion transesterification in acrylate units of polystyrene-b-poly(tert-butyl acrylate)s (PS-b-PtBAs) with the narrow dispersity being unchanged. A simple modification from PtBA into PTFEA block effectuates a remarkable increase in Flory−Huggins interaction parameter (χ) between the two blocks, which measures χ = 30.86/T + 0.160, where T (K) is absolute temperature. The smallest half-pitch feature size of lamellae was evaluated to be 5 nm in 6.3 kg/mol PS-b-PTFEA. For versatility in thin film application, our results also offer a rapid perpendicular orientation of lamellar microdomains in PS-b-PTFEA films supported on a neutral homopolymer mat (cross-linked poly(4-trifluoromethylstyrene)), in which the lamellar spacing (L0) between the as-spun film and equilibrium bulk gets closer as the molecular weight of BCPs increases.

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INTRODUCTION Next-generation microelectronics has still required highperformance nanoscopic patterns miniaturized with ultrafine ordered arrays. The recent extreme-UV (EUV) lithography process approaches the 7 nm node level to overcome the topdown resolution limit of 193 nm immersion photolithography.1−3 Alternatively, block copolymer (BCP) self-assembly has become an attractive candidate for the bottom-up approach, since the BCP can form nanostructures spontaneously in long-range alignment over large area with smaller feature size.4−11 Moreover, it offers a good scalability and ease of pattern transfer, which stay competitive against conventional lithography.8,10−13 To achieve sub-10 nm half-pitch feature size in symmetric BCP self-assembly, a higher Flory−Huggins interaction parameter (χ) between the two blocks is essential to ensure an ordered state in lower degree of polymerization (N) because the lamellar spacing (L0) is proportional to N2/3χ1/6 in the strong-segregation limit.14,15 Since a finding of potential BCP lithography, polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) has been extensively studied for its facileness to form perpendicular orientation of nanoscopic arrays,16−19 while the minimum L0 of PS-b-PMMA reaches the limit of 17 nm due to the inherently low χ.20,21 Incessant efforts have been dedicated in designing high-χ BCPs that catch up with sub-10 nm half-pitch feature size.22−30 Kim and co-workers introduced a minimum L0 = 5.9 nm from HO-containing high-χ BCPs of polydihydroxystyrene-b-PS.31 Recently, Russell and co-workers achieved a minimum L0 = 5.4 nm from poly(glycerol monomethacrylate)-b-PS.32 However, © XXXX American Chemical Society

the above approaches were not able to identify perpendicular orientation of lamellar microdomains in thin film application because of large dissimilarity in the surface energies between the two blocks. As for the efforts on controlling interfacial interactions toward a desired orientation of nanoscopic lamellae and cylinders, Willson and co-workers elaborated a neutral top-coat strategy to obtain perpendicular orientation of lamellar microdomains using hybrid-type high-χ BCPs, approaching to L0 = 14 nm in poly(4-methoxystyrene)-b-poly(4-trimethylsilylstyrene).24,33 Hayakawa and co-workers utilized the similar surface-energy approach using hybrid-type high-χ BCPs of poly(polyhedral oligomeric silsesquioxane methacrylate)-bpoly(2,2,2-trifluoroethyl methacrylate) (PMAPOSS-b-PTFEMA).34 They demonstrated a minimum L0 = 11 nm by modulating perpendicular orientation of lamellar microdomains. The two approaches mentioned above turned out to be successful in not only mediating the chemical structures of BCP self-assembly but also preserving a perpendicular orientation of nanostructures, leaving a challenging homework behind to achieve smaller feature size (than 11 nm) with desired orientations. Fluorine-containing styrenic BCPs were also explored by Ober and co-workers to exploit high incompatibility between PS and fluorine-containing blocks.35 They synthesized PS-b-PTFEMA via anionic and/or atom Received: June 22, 2018 Revised: August 26, 2018

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DOI: 10.1021/acs.macromol.8b01325 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures and 1H NMR spectra for (a) PS-b-PtBA5.9, (b) partially substituted PS-b-P(tBA-r-TFEA), and (c) high-conversion (>95%) PS-b-PTFEA6.3.

transfer radical polymerizations. The BCP films exhibited a photoresist contrast for pattern transfer by the selective removal of PTFEMA, although the dispersity was relatively broad due to high reactivity of fluorine units. To resolve the synthetic drawbacks in fluoroacrylates, herein we suggest a simple and feasible approach to fluorinecontaining styrenic BCPs to enlarge incompatibility between PS and fluorine-containing blocks. Well-defined PS-b-poly(2,2,2-trifluoroethyl acrylate)s (PS-b-PTFEAs) were synthesized by high-conversion transesterification in acrylate units of PS-b-poly(tert-butyl acrylate)s (PS-b-PtBAs) with the narrow dispersity being unchanged. A remarkable increase in χ between the two blocks was evaluated after side-chain modification with fluoroalcohol. We also demonstrate a rapid perpendicular orientation of lamellar microdomains in PS-bPTFEA films supported on a neutral mat of cross-linked poly(4-trifluoromethylstyrene) (P4TFMS).

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PtBAs by transesterification in acrylate units. For the most salient procedure, an excess amount of degassed 2,2,2-trifluoroethanol (99+ %, Alfa Aesar) and polyphosphoric acid (115% H3PO4 basis, Aldrich) were first mixed in 250 mL flask at room temperature for 4 h, and then PS-b-PtBA powder was added to the mixture under Ar environment. The mixtures were further refluxed at 130 °C for 48 h to initiate transesterification. The products were precipitated in water solution saturated with sodium hydrogen carbonate (NaHCO3; 99+%, Junsei) to remove polyphosphoric acid. Additional precipitation in water was repeated to ensure the removal of remaining polyphosphoric acid and NaHCO3 thoroughly, and then the final products were dried under vacuum at 130 °C for 24 h. 1H NMR (CD2Cl2, 400 MHz): δ 0.50−1.92 (br, 4H, CH2 of PS and PTFEA backbone), 1.92−2.25 (br, H, CH of PS backbone), 2.25−2.67 (br, H, CH of PTFEA backbone), 4.10−4.70 (br, 2H, CH2CF3), 6.30− 7.35 (br, 5H, PS phenyl ring), as displayed in Figure 1. The molecular weights (Mn and Mw) and dispersity (Đ = Mw/Mn) were characterized by size-exclusion chromatography (SEC) for PS-b-PTFEAs. PS volume fraction (f PS) and the conversion of transesterification were calculated by 1H NMR in CD2Cl2 solution, in which the mass densities of each block are 1.05, 1.047, and 1.403 g/cm3 for PS, PtBA, and PTFEA, respectively. Sample characteristics of PS-b-PTFEAs are summarized in Table 1. Synthesis of Cross-Linkable P4TFMS. A cross-linkable P4TFMS (Mn = 19.8 kg/mol with Đ = 1.42) was synthesized by free radical polymerization with 4-trifluoromethylstyrene (4TFMS; 98%, Aldrich) and 2 mol % glycidyl methacrylate (GMA; 98%, Aldrich), where azobis(isobutyronitrile) (AIBN; 98%, Junsei) was used as an initiator in the similar manner with literature.36 A monomer of 4TFMS and cross-linking agent of GMA were degassed over CaH2 and vacuum-distilled before polymerization. The reaction was refluxed with THF at 90 °C for 24 h, and the solution was precipitated in cold methylene chloride/n-hexane (60/40 vol %) solution. The product was dried and stored under vacuum at room temperature. Thin Film Preparation. A cross-linkable P4TFMS mat was prepared on a standard Si wafer by spin-coating of polymer solution in (0.5 wt %) α,α,α-trifluorotoluene (TFT) at 3000 rpm, and a thin film was thermally annealed at 190 °C for 3 h under vacuum to develop a thin layer (∼7 nm) of cross-linked mat. PS-b-PTFEA films were also prepared on a cross-linked P4TFMS mat by spin-coating of BCP solutions, where the PS-b-PTFEAs were dissolved in a cosolvent composed of toluene and TFT for better solubility. The film thickness

EXPERIMENTAL SECTION

Synthesis of PS-b-PtBA. A series of PS-b-PtBAs were synthesized via anionic polymerization of styrene (99.5%, Junsei) and tert-butyl acrylate (98%, Aldrich). Styrene and tert-butyl acrylate were vacuumdistilled over dried dibutylmagnesium and trioctylaluminum, respectively, immediately after both monomers being degassed in the presence of calcium hydride (CaH2; 99.9%, Aldrich). A lithium chloride (LiCl; 99.9%, Alfa Aesar) was dissolved in tetrahydrofuran (THF) solvent at room temperature, followed by dispersion of secbutyllithium (1.3 M, Aldrich) as an initiator at constant −78 °C under Ar environment. Vacuum-distilled isopropyl alcohol was used as a terminator after polymerization, and the products were precipitated three times in cold methanol/water (80/20 vol %) solution. The polymer samples were dried under vacuum at room temperature to avoid cross-linking of tert-butyl acrylate units. 1H nuclear magnetic resonance (1H NMR) (CD2Cl2, 400 MHz): δ 0.50−1.69 (br, 4H, CH2 of PS and PtBA backbone), 1.44 (s, 9H, −C(CH3)3), 1.69−2.03 (br, H, CH of PS backbone), 2.03−2.35 (br, H, CH of PtBA backbone), 6.30−7.35 (br, 5H, PS phenyl ring), as displayed in Figure 1. Side-Chain Modification into PS-b-PTFEA. A series of symmetric PS-b-PTFEAs were obtained from the mother PS-bB

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Macromolecules Table 1. Sample Characteristics of PS-b-PTFEAsa BCPs (mother BCPs)

Mn (g/mol)

Đ (Mw/Mn)

f PS

conv (%)

PS-b-PTFEA5.5 (PS-b-PtBA4.8) PS-b-PTFEA6.3 (PS-b-PtBA5.9) PS-b-PTFEA8.2 (PS-b-PtBA7.3) PS-b-PTFEA12.6 (PS-b-PtBA11.8) PS-b-PTFEA23 (PSb-PtBA20.9) PS-b-PTFEA32 (PSb-PtBA29.3) PS-b-PTFEA47 (PSb-PtBA42.9)

5500

1.28

0.512

98.70

disorder

6300

1.15

0.510

95.09

8200

1.17

0.509

95.60

12600

1.15

0.511

97.02

ODT at 201.3 °C ODT at 262.5 °C order

23000

1.06

0.508

95.28

order

32000

1.07

0.497

97.14

order

47000

1.05

0.504

95.51

order

Appnano). Grazing incidence SAXS (GISAXS) experiments were used to analyze the film structures, which were performed at 9A beamline of PAL, Korea. The operating conditions were set at a wavelength of λ = 1.122 Å, a sample-to-detector distance of 2.5 m, and an exposure time of 20 s. 2D GISAXS patterns were recorded on the same 2D Mar CCD, where the incident angle (αi) was varied from 0.080° to 0.140° across a critical angle (0.109°) of PS-b-PTFEA films.

remark

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RESULTS AND DISCUSSION Scheme 1 depicts the synthesis of PS-b-PtBA via living anionic polymerization and subsequent transesterification in acrylate units to transform into PS-b-PTFEA. A series of PS-b-PtBAs, which are the mother BCPs, were first synthesized in tetrahydrofuran (THF) at −78 °C in the presence of small amount of LiCl (5 mol %), where sec-butyllithium was used as an initiator under Ar environment. The mother BCPs were controlled to be slightly asymmetric in PS volume fraction (f PS ∼ 0.46) on demand, and they were further modified into symmetric PS-b-PTFEAs with f PS ∼ 0.50 assuming the full conversion of transesterification. Sample characteristics of PSb-PTFEAs (and PS-b-PtBAs) are summarized in Table 1. Figure 1 shows the chemical structures and corresponding 1 H NMR data for two synthetic routes (Scheme 1) of transesterification in acrylate units using a mother PS-bPtBA5.9. Starting from a PS-b-PtBA5.9 (Figure 1a), the transesterification was performed at 130 °C for 48 h using 2,2,2-trifluoroethanol under two sorts of catalysts for comparison. A typical transesterification with p-toluenesulfonic acid (Figure 1b) produces a low conversion (95%) PS-b-PTFEA, which is evidenced by the entire generation of methylene units and the distinction of tert-butyl units. This synthetic route was successful even for high molecular weight mother BCP to produce PS-b-PTFEA47 with the narrow dispersity being unchanged (Figure S1), indicating that the transesterification with fluoroalcohol well occurs exceptionally under polyphosphoric acid due to strongly acidic proton of the catalyst.37 Figures 2a and 2b show the SAXS intensity profiles of PS-bPtBAs and PS-b-PTFEAs, respectively, measured at room temperature (25 °C) as a function of scattering vector (q), in which q = (4π/λ) sin θ, and 2θ and λ are scattering angle and

a

PS volume fraction (f PS) and conversion of transesterification were calculated by 1H NMR in CD2Cl2 solution, in which the mass densities of each block are 1.05, 1.047, and 1.403 g/cm3 for PS, PtBA, and PTFEA, respectively. was set at 1.5L0 by varying concentration of polymer solutions and/or spin rate, which was measured by a spectroscopic ellipsometry (SE MG-1000, Nanoview Co.) at an incident angle of 70.5°. Characterization and Measurements. For the bulk experiments, small-angle X-ray scattering (SAXS) experiments were performed at the 4C beamline of Pohang Accelerator Laboratory (PAL), Korea. The operating conditions were set at a wavelength of λ = 1.240 Å, sample-to-detector distance of 2−4 m, and exposure time of 1−10 s. The scattered intensities were collected on 2D Mar CCD (SX 165, Rayonix) at target temperatures. The specimens for heating experiments were sealed with high-temperature resistant sealant in a template, and the sample temperature was controlled at a heating rate of 0.7 °C/min under nitrogen flow to avoid thermal degradation of BCPs. Transmission electron microscopy (TEM; JEM-F200, JEOL) was also operated at an accelerating voltage of 120 kV to observe the bulk morphology of BCP. Ultrathin specimen (∼40 nm) was prepared using cryo-ultramicrotome (CRX-PTXL, RMC) at −60 °C. For higher electron density contrast, the PS block was selectively stained with RuO4 at room temperature for 15 min. The glass transition temperature (Tg) of each block was measured by differential scanning calorimetry (DSC; PerkinElmer Diamond) using 7 mg BCP samples at a heating rate of 20 °C/min from −40 to 200 °C under nitrogen flow (Figure S1). Tgs collected during the second heating run of PS, PtBA, and PTFEA were evaluated to be 106, 43, and 4 °C, respectively. For the film analyses, scanning force microscopy (SFM; Dimension 3100, Digital Instrument Co.) was used to observe the surface morphology of BCP films. The scanning experiments were operated at a tapping mode using a sub-5 nm tip-radius probe (ACTA-SS,

Scheme 1. Synthesis of PS-b-PtBA via Living Anionic Polymerization and Transesterification in Acrylate Units To Transform into PS-b-PTFEA

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Figure 2. SAXS intensity profiles of (a) PS-b-PtBAs and (b) PS-b-PTFEAs. The arrows indicate the q* and higher-order peaks. The intensity profiles are vertically shifted by a factor of 103 for clarity. An inset TEM image in (b) indicates the lamellar morphology of PS-b-PTFEA6.3. (c) Double-logarithmic plot of L0 as a function of N for PS-b-PTFEAs. The slope in the triangle indicates a relationship of L0 ∼ N2/3 (or N0.67). (d) Temperature dependence of χ for PS-b-PTFEA5.5. A linear-type χ is evaluated as χ = 30.86/T + 0.160 above 170 °C, indicating that the mean-field approximation is valid to assume intimately mixed phase at higher temperatures. An inset represents the best fit of the S(q) profile measured at 80 °C using eq 4.

weight PS-b-PtBAs, a PS-b-PTFEA5.5 (Figure 2b) displays a broad maximum arising from the correlation hole scattering of a disordered state due to an increase in electron density contrast between the two blocks. The intensity profiles of PS-bPTFEA6.3 and the other higher molecular weight PS-bPTFEAs exhibit a sharp primary peak and higher-order peaks at q/q* = 1:2:3:4:5:6:7:8∼, which correspond to a lamellar morphology. An inset TEM image also confirms the lamellar morphology of PS-b-PTFEA6.3. The L0 (consistent with dspacing, d = 2π/q*) is analyzed with increasing molecular weight from PS-b-PTFEA6.3 to PS-b-PTFEA47 to define their

wavelength of the incident X-ray beam, respectively. Prior to the measurements, the BCPs were thermally annealed at 130− 150 °C under vacuum for 12 h, which are above the glass transition temperature (Tg ∼ 100 °C) of the PS block. The intensity profiles of PS-b-PtBAs (Figure 2a) show no characteristic features up to PS-b-PtBA11.8 due to a disordered state with lower electron density contrast between the two blocks, whereas for PS-b-PtBA20.9, PS-b-PtBA29.3, and PS-bPtBA42.9, a primary peak (at q*) and higher-order peak at q/ q* = 1:3 indicate the self-assembly behavior into a lamellar morphology. Unlike no scattering features in lower molecular D

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Figure 3. (a) SAXS intensity profiles of PS-b-PTFEA6.3 measured during heating at a rate of 0.7 °C/min. The intensity profiles are vertically shifted by a factor of 103 for clarity. The ODT temperature (201.3 °C) of PS-b-PTFEA6.3 is clearly determined from the discontinuity in (b) 1000/ I(q*), full width at half-maximum (fwhm), and (c) d-spacing (d = 2π/q*).

feature sizes, as plotted in Figure 2c. The double-logarithmic plot of L0 as a function of N measures a linear relationship of L0 ∼ N0.68, indicating that even low-N PS-b-PTFEAs reside in the strong segregation regime with the relationship of L0 ∼ N2/3 (or N0.67).38 Notably, our results show the smallest L0 = 10.1 nm of PS-b-PTFEA6.3, approaching to 5 nm of half-pitch feature size. To evaluate χ between the two blocks, temperaturedependent SAXS intensity profiles were also measured with a disordered BCP of PS-b-PTFEA5.5 during heating. The SAXS profiles were collected at each temperature with interval of 10 °C from 60 to 240 °C, at which the temperature maintains for more than 15 min to presume the thermal equilibrium prior to the measurements. Over the entire temperature range, a broad maximum with no higher-order peaks corresponds to the correlation hole scattering of a disordered state in the length scale of Rg (radius of gyration) (Figure S2). The SAXS profiles are calibrated by a glassy carbon as the standard sample to correlate into the absolute intensity profiles (Iabs(q)).39 The absolute intensities are further applied to a dimensionless structure factor (S) by Iabs(q) =

d ∑ (q) KVref = (aS − a TFEA )2 S(q) dΩ NA

aS =

a TFEA =

F (x , f ) 1 = − 2χ S(q) N

NAρPTFEA Ze,TFEA [M]0,TFEA

re

(3)

(4)

where F is a Debye function with x = Rg2q2 and the volume fraction (f) of one block. An inset represents the best fit of the S(q) profile measured at 80 °C using eq 4. The χ decreases nonlinearly to 160 °C as 1/T decreases (or temperature increases) due to the effective thermal fluctuation.43 Above 170 °C determined as a mean-field temperature, a linear-type χ is evaluated as χ = 30.86/T + 0.160, indicating that the meanfield approximation is valid to assume intimately mixed phase at higher temperatures. For the symmetric PS-b-PTFEA system to account for a critical value of χN = 10.5 (or χ = 0.230) at 170 °C, consequently, the possible access to the minimum L0 in the proximity of microphase separation is estimated to be 8.8 nm (at N = 46 in Figure 2c or Mn = 5.8 kg/mol in Figure S3), which is smaller than the L0 = 10.1 nm of PS-b-PTFEA6.3. Particularly in PS-b-PTFEA6.3, an order-to-disorder transition (ODT) was identified during heating at a rate of 0.7 °C/ min since the χ between PS and PTFEA blocks decreases as temperature increases. Figure 3a shows the SAXS intensity

(1)

1/2

Vref

[M]0,S

re ,

where [M]0,i, ρi, and Ze,i are the molecular weight (g/mol) of monomers, mass density of polymers (g/cm3), and total electron number of monomers, respectively, and re = 2.82 × 10−13 cm as the radius of an electron.40,41 Figure 2d displays the χ between PS and PTFEA blocks as a function of inverse temperature (1/T), which is extracted from the Leibler fitting on the basis of incompressible random phase approximation (RPA)42 using

where K and NA represent the machine constant and Avogadro number, respectively. The reference molar volume (Vref) and scattering length (ai) of monomers are calculated by the following two equations: ij [M]0, S [M]0,TFEA yz zz = jjjj zz ρ ρ PTFEA { k PS

NAρPS Ze,S

(2) E

DOI: 10.1021/acs.macromol.8b01325 Macromolecules XXXX, XXX, XXX−XXX

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Figure 4. (a, b) SFM phase images of PS-b-PTFEA32 film supported on a standard Si wafer. (a) The film was thermally annealed at 150 °C under vacuum for 12 h. (b) As-spun film. Chemical structure of P4TFMS is described with the schematic film preparation. (c−f) SFM phase images and corresponding 2D GISAXS patterns for (c) PS-b-PTFEA12.6, (d) PS-b-PTFEA23, (e) PS-b-PTFEA32, and (f) PS-b-PTFEA47 films supported on a P4TFMS mat. (g) L0 of PS-b-PTFEA films to compare with that of the bulk BCPs.

Some distortions including hole/island formation occur during thermal treatment since the low surface energy character of fluorine-containing polymers overcomes an entropic loss to develop a perpendicular orientation of lamellar microdomains. However, the as-spun PS-b-PTFEA32 film on a Si wafer exhibits short-range stripe patterns of the bright PS blocks, which is represented in Figure 4b by a SFM phase image. Accordingly, a quick spin-coating process on a neutral substrate was designated to develop a desired lamellar orientation of BCP films, as the χN is high enough to order during rapid solvent evaporation while suppressing the unbalanced interfacial interactions from the air surface. A cross-linked mat of P4TFMS was utilized to provide a neutral moiety on the substrate. The 1.5L0 PS-b-PTFEA films were simply applied to the thin layer (∼7 nm) of P4TFMS mat by a quick spin-coating process, and the as-spun films were further treated by the same plasma etching to assess thin film morphology. A homopolymer of 20 kg/mol P4TFMS (containing 2 mol % GMA) was synthesized via free radical polymerization with AIBN as an initiator, where the epoxy units in GMA can form a cross-linked network.36 The chemical structure of P4TFMS is described in Figure 4 with the schematic film preparation. Note that we were not able to fabricate the flat film structure with the PS-b-PTFEA6.3 and PS-b-PTFEA8.2 films on a P4TFMS mat because the low molecular weight PS-b-PTFEA films rapidly dewet the substrate owing to the high mobility of the polymer chains and low surface energy character of fluorine-containing polymers. Figures 4c−4f show the SFM phase images and corresponding 2D GISAXS patterns of the as-spun films supported on a P4TFMS mat for PS-b-PTFEA12.6, PS-b-PTFEA23, PS-b-

profiles of PS-b-PTFEA6.3 measured at various temperatures during heating. The intensity profile measured at 140 °C exhibits a sharp primary peak (at q* = 0.641 nm−1) and higherorder peaks at q/q* = 1:2:3, corresponding to a lamellar morphology. The primary peaks remain sharp up to 200 °C as temperature increases, while the primary peaks significantly broaden across an ODT from a lamellar structure to a disordered state. An ODT was measured at 201.3 °C from the discontinuity in the inverse of the maximum intensity (1/ I(q*)) and full width at half-maximum (fwhm) (Figure 3b) as a function of 1/T. Interestingly, the shift in d-spacing (Figure 3c) at the ODT can be a consequence of stronger interchain interactions in a disordered state, but it is still questionable, which is presumably attributed to an increase in specific volume at the order−disorder coexistence region44 or large conformational asymmetry arising from the chain stiffness difference between the two blocks.45 As similarly identified in Figure 2b, the q* = 0.610 nm−1 (corresponding to d = 10.3 nm) of a disordered PS-b-PTFEA5.5 is slightly lower than q* = 0.622 nm−1 (d = 10.1 nm) of an ordered PS-b-PTFEA6.3, even though the molecular weight of PS-b-PTFEA5.5 is smaller than that of PS-b-PTFEA6.3. For thin film implementation, we first fabricated 1.5L0 PS-bPTFEA films onto a standard Si wafer by spin-coating of BCP solutions, where the PS-b-PTFEAs were dissolved in a cosolvent composed of toluene and TFT for better solubility. All the BCP films were further treated by the plasma etching with O2/Ar (5/1 in volume ratio) gas mixture; this process trims an ultrathin top layer (∼5 nm) to enhance the phase contrast between the two blocks. Figure 4a shows the scanning force microscopy (SFM) phase image of PS-b-PTFEA32 film that was thermally annealed at 150 °C under vacuum for 12 h. F

DOI: 10.1021/acs.macromol.8b01325 Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This research was supported by the Samsung Research Funding Center of Samsung Electronics under Project SRFC-MA1301-03.

PTFEA32, and PS-b-PTFEA47, respectively. The SFM images display the stripe patterns of the bright PS blocks, representing a perpendicular orientation of lamellar microdomains for all the PS-b-PTFEA films supported on a P4TFMS mat. The consistent film structures are confirmed by the corresponding GISAXS patterns measured at an incident angle (αi) of 0.120° because the out-of-plane Bragg rods at q* and higher-order peaks at q/q* = 1:2:3∼ arise from the lamellar microdomains oriented normal to the film surface. The L0 of PS-b-PTFEA films are analyzed from the in-plane intensity profile scanned along qxy-direction at αf = 0.121° from the GISAXS patterns (Figure S4), as shown in Figure 4g to compare with that of the bulk BCPs. The overall L0 increases as the number-average molecular weight (Mn) of PS-b-PTFEAs increases. Intriguingly, the L0 between the as-spun film and equilibrium bulk gets closer as the segregation power of χN increases, while the asspun films maintain a perpendicular orientation of lamellar microdomains. It should be pointed out that the stripe patterns from perpendicular lamellae become shorter when the film thickness increases ∼3L0 (Figure S5) because the interfacial interactions from P4TFMS mat dissipate with film thickness.

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CONCLUSIONS In summary, we designed a well-defined BCP system of fluorine-containing, high-χ PS-b-PTFEA via side-chain modification in one block as a candidate for the bottom-up selfassembly materials. The PS-b-PTFEAs were synthesized by high-conversion transesterification in acrylate units of PS-bPtBAs under the specific catalyst of polyphosphoric acid. The observed minimum L0 is 10.1 nm for PS-b-PTFEA6.3 (approaching to 5 nm of half-pitch feature size), while the possible access to the minimum L0 for the symmetric PS-bPTFEA system is estimated to be 8.8 nm at a critical value of χN = 10.5. Above all, a rapid perpendicular orientation of lamellar microdomains was achieved in the as-spun PS-bPTFEA films supported on a neutral mat of cross-linked P4TFMS. Our results indicate that the interfacial interactions are well balanced at the substrate even though the surface energy difference is still large between the two blocks, suggesting an approach to the extreme feature sizes of fluorine-containing BCPs. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01325. Figures S1−S5 (PDF)

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*E-mail: [email protected] (D.Y.R.). ORCID

Cheolmin Park: 0000-0002-6832-0284 Du Yeol Ryu: 0000-0002-0929-7934 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.macromol.8b01325 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.8b01325 Macromolecules XXXX, XXX, XXX−XXX