Synthesis of a Fluoromethacrylate Hydroxystyrene Block Copolymer

Letter Cite This: ACS Macro Lett. 2019, 8, 368−373

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Synthesis of a Fluoromethacrylate Hydroxystyrene Block Copolymer Capable of Rapidly Forming Sub‑5 nm Domains at Low Temperatures Chenxu Wang, Xuemiao Li, and Hai Deng* Department of Macromolecular Science and State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China

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S Supporting Information *

ABSTRACT: A series of poly(pentadecafluorooctyl methacrylate)-block-polyhydroxystyrene (PPDFMA-b-PHS) block copolymers (BCPs) were synthesized via reversible addition− fragmentation chain-transfer polymerization and subsequent deprotection. Because of the high incompatibility between hydroxyl groups and fluoro groups, the interaction parameter (χ) of these BCPs, determined by temperature-resolved smallangle X-ray scattering (SAXS), was extremely high. The χ value of PPDFMA-b-PHS was 0.48 at 150 °C, 16× larger than the χ of polystyrene-block-poly(methyl methacrylate). The microphase behavior of PPDFMA-b-PHS with various volume fractions of PHS block was determined by SAXS, yielding ordered lamellar morphologies with different sizes of domain spacing (d), and further confirmed by transmission electron microscopy. The minimum d obtained was 9.8 nm annealed at a mild temperature for a short time (80 °C for 1 min) by SAXS analysis, indicating the width of each lamellar domains was 10.5). Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), is one of the most widely used conventional BCPs for selfassembly, and draws ongoing attention.10,11 However, a significant limitation to PS-b-PMMA is its relatively low χ, which essentially limits its minimum feature size to ∼22 nm pitch for a lamellar morphology, while sub-10 nm resolution is generally demanded for next generation lithography.12,13 Therefore, in order to form well-ordered nanostructures with sub-10 nm resolution, a series of high-χ BCP systems have been developed.14−40 Hillmyer et al. investigated poly(cyclohexylethylene)-block-poly(methyl methacrylate) (PCHE-b-PMMA), which had d < 10 nm.15 Gopalan et al. synthesized poly(4-tert-butylstyrene)-block-poly(2-vinylpyridine) (PtBuSt-b-P2VP) BCPs, which showed a minimum d of 9.6 nm for the lamellar morphology.16 Silicon-containing © XXXX American Chemical Society

Received: March 8, 2019 Accepted: March 15, 2019

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DOI: 10.1021/acsmacrolett.9b00178 ACS Macro Lett. 2019, 8, 368−373

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ACS Macro Letters Scheme 1. Synthetic Routes of PPDFMA-b-PHS via Reversible Addition−Fragmentation Chain-Transfer (RAFT) Polymerization and Subsequent Deprotection

Table 1. Characteristics of PPDFMA-b-PHS Copolymers sample D1 D2 D3 D4 D5 D6 D7 D8 D9

(3.7k−5.8k) (3.7k−5.5k) (3.3k−4.6k) (3.3k−4.4k) (2.8k−3.0k) (2.8k−2.4k) (2.8k−1.5k) (2.8k−1.2k) (1.4k−0.8k)

Mn,overalla (kg mol−1)

PDIb

Nc

f PHSd

morphologye

d-spacinge (nm)

9.5 9.2 7.9 7.7 5.8 5.2 4.3 4.0 2.2

1.11 1.12 1.11 1.11 1.11 1.09 1.10 1.09 1.08

100.3 96.9 82.7 80.8 59.5 51.7 41.2 37.7 20.5

0.69 0.68 0.67 0.66 0.61 0.55 0.44 0.40 0.44

LAM LAM LAM LAM LAM LAM LAM LAM DIS

19.5 18.7 17.5 16.6 12.8 11.5 10.0 9.8

a Molecular weight and compositions were measured by quantitative 1H NMR. bPolydispersity index (PDI) was determined by GPC using a PS standard. cN was calculated using a reference volume of 118 Å3, based on the densities of PPDFMA and PHS being 1.73 and 1.16 g/cm3, respectively. df PHS represented the volume fraction of PHS domains. eMorphologies and domain spacings were obtained by SAXS. LAM and DIS indicates lamellar and disordered morphologies, respectively.

(P3HS-b-PDMS) in order to improve resolution as well as etch selectivity. 26 Sub-3 nm BCPs were also obtained by introducing two hydroxyl groups. Kim et al. determined that the χ value of poly(dihydroxystyrene)-block-polystyrene (PDHS-b-PS) is about 6× larger than that of P4HS-b-PS, though the only difference is one additional hydroxyl group, resulting in the formation of lamellar microdomains of PDHSb-PS with d = 5.9 nm after annealing at 170 °C.27 Russell et al. achieved d = 5.4 nm in a lamellar morphology with poly(glycerol monomethacrylate)-block-polystyrene (PGM-bPS), which is the smallest domain spacing for lamellar features in BCPs reported to date.28 Additionally, after further modifying and doping, this series of hydroxyl group-containing polymers showed potential for use in a range of applications, such as photoresists for deep ultraviolet (DUV) or EUV lithography41,42 and cross-linkable polymers.43 However, based on the previous theoretical research, the extremely high χ of BCPs will lead to kinetic barriers for pattern formation because of the slow interdiffusion of the BCP’s polymer chains.44−46 To overcome these issues and achieve highly ordered microdomains, high temperatures and long times are generally used for annealing high χ hydroxyl group-based BCPs.24,25,27 However, as the production of integrated circuits requires high throughput, the desirable time for a thermal annealing process should be less than several minutes.17 Therefore, a much higher temperature above the Tg is required to accelerate the mobility of chains and drive BCPs to complete self-assembly within a few minutes. For example, a temperature of 120 °C above Tg is required for PS-b-PMMA to self-assemble completely within a short time.47 However, for hydroxyl group-based BCPs, annealing at such high temperatures is very difficult because side reactions like self-crosslinking and degradation could occur simultaneously with the process of self-assembly, resulting in many more defects in morphologies.24,48−50

Our previous studies have demonstrated that the perfluoroalkyl-containing methacrylic block can shorten the thermal annealing time and lower the annealing temperature.33,34 Additionally, the greater chemical incompatibility between the fluorine-containing block and the hydroxyl block shows promising potential to achieve sub-5 nm microdomains. Hence, in this work, we designed a novel, high-χ BCP, poly(pentadecafluorooctyl methacrylate)-block-polyhydroxystyrene (PPDFMA-b-PHS), which we synthesized via reversible addition−fragmentation chain-transfer (RAFT) polymerization and subsequent deprotection. The χ value of PPDFMA-b-PHS obtained (0.48 at 150 °C) was approximately 16× larger than χ of PS-b-PMMA (∼0.03). Sub-5 nm domains in ordered lamellar morphologies were achieved after thermal annealing in mild conditions, indicating great potential in sub-5 nm nanolithography. A series of PPDFMA-b-PHS BCPs were synthesized via RAFT polymerization and subsequent deprotection to achieve precise control of the volume fraction of PHS domains (f PHS), as shown in Scheme 1. For RAFT polymerization, the first block should be a good leaving group.51 Therefore, the PPDFMA homopolymer based on PMMA was synthesized first to serve as a macro-chain transfer agent for the RAFT polymerization of styrene-type monomers. In order to avoid side reactions from the free hydroxyl group during living free radical polymerization, a tetrahydropyran (THP) group was used to protect the PHS block, and the deprotection conditions were mild enough to prevent cleavage of the PPDFMA block. The synthesis of the 3-(2tetrahydropyranyloxy)styrene (THPOS) monomer,52 polymerization process, subsequent deprotection, and analytical methods are fully described in the Supporting Information. All of the structures were fully characterized by 1H and 19F NMR spectroscopy (Supporting Information, Figures S1, S2, S3, and S5) and gel permeation chromatography (GPC, Figure S4). By changing the ratio between monomers and macro-chain 369

DOI: 10.1021/acsmacrolett.9b00178 ACS Macro Lett. 2019, 8, 368−373

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ACS Macro Letters

macro-chain transfer agent and the removal of the THP groups (Figure 1c). In addition to chemical deprotection, thermal deprotection, which can be done simultaneously with thermal annealing, is also an approach to transform PPDFMA-bPTHPOS to PPDFMA-b-PHS, as described in the Supporting Information. 1H NMR was used to monitor the conversion of PPDFMA-b-PTHPOS throughout the entire annealing process at 160 °C under a nitrogen atmosphere (Figure S8). The extent of deprotection was calculated by integrating the characteristic peaks of THP at ∼5.3 ppm and the phenolic group at ∼7.8 ppm. The conversion rate was 53% after 0.5 h and 78% after 1 h. PPDFMA-b-PTHPOS was completely converted to PPDFMA-b-PHS after 2 h. The morphologies and domain spacings of PPDFMA-b-PHS obtained by chemical deprotection from PPDFMA-bPTHPOS were analyzed by SAXS. All of the BCPs displayed SAXS peaks positioned at q/q* ratios of 1:2:3:4 (Figure 2a),

transfer agents, PPDFMA-b-PHS BCPs with different compositions were obtained and the characterization data are shown in Table 1. The morphologies of the PPDFMA-b-PHS BCPs were studied by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) and thermal gravity analysis (TGA) were used to characterize the thermal properties of PPDFMA-b-PTHPOS and PPDFMA-b-PHS. All of the synthesized PPDFMA-b-PTHPOS BCPs only showed a singular glass transition temperature (Tg) after the first cycle of DSC, which was similar to our previous work,33,34 and BCPs studied by other groups.15 After deprotection, a higher Tg appeared, corresponding to the Tg of the hydroxyl block (Figure S6). TGA curves showed that PPDFMA-bPTHPOS had a two-stage weight loss, corresponding to the thermal deprotection of the THP moiety at ∼250 °C and the backbone degradation at ∼350 °C. After deprotection, TGA curves of PPDFMA-b-PHS showed only one-stage weight loss, corresponding to backbone degradation at ∼350 °C, indicating the high thermal stability of the PPDFMA-b-PHS BCPs (Figure S7). The use of THP is one of the most prevalent methods in organic synthesis for protecting hydroxyl groups, as it can be deprotected in mild conditions. After treatment with 1 M HCl in THF/methanol (3/1 v/v), all PPDFMA-b-PTHPOS BCPs were fully deprotected at room temperature within 10 min (Figure 1a). 1H NMR spectra in Figure 1b shows the

Figure 2. (a) SAXS intensity profiles for PPDFMA-b-PHS BCPs after thermal annealing at 160 °C for 24 h. The size of microdomains are determined by d = 2π/q*, where q* is the position of the primary scattering peak. Data are shifted vertically for clarity. (b) Scaling results of PPDFMA-b-PHS between the domain spacing (d) and the total degree of polymerization (N), which was calculated using a reference volume of 118 Å3, as shown in Table 1. The slope from a linear fit denotes the scaling relationship: d ∼ N0.74. (c) TEM images of the bulk samples of D7 (left) and D8 (right). The bright and dark parts correspond to PHS and PPDFMA microdomains, respectively.

indicating lamellar morphologies. As the degree of polymerization decreased, the domain spacing ranged from 19.5 to 9.8 nm (d = 2π/q*), which were calculated from the primary peak position (q*) and are summarized in Table 1. In the theoretical predictions, the molecular weight depends on d as the power law d ∝ Nδ.15,26,36 In order to better understand how d is affected by N and identify the scaling exponent (δ) of PPDFMA-b-PHS BCPs, a linear fit was found for the log−log graph of d values of lamellar samples versus N in Figure 2b. The slope from the linear fit was δ = 0.74 (R2 = 0.99), which was greater than the 2/3 value predicted by strong segregation theory.7,8 This phenomenon has also been reported by many other groups, indicating these BCP chains adopt a relatively stretched conformation.36,53,54 TEM images of the lamellar morphologies formed after microphase separation of D7 and D8 at 160 °C for 24 h are shown in Figure 2c. The dark and bright parts correspond to PPDFMA and PHS domains, respectively. Consistent with the SAXS results, the d values observed were 7.6 nm for D7 and

Figure 1. Chemical deprotection of PPDFMA-b-PTHPOS (P8) at room temperature. (a) Acid-catalyzed reaction of PPDFMA-bPTHPOS (P8) to PPDFMA-b-PHS (D8) within 10 min. (b) 1H NMR spectra of P8 (top) in chloroform-d before deprotection reaction and D8 (bottom) in THF-d8 after deprotection reaction. (c) GPC curves for samples showing PPDFMA (red), PPDFMA-bPTHPOS (blue), and PPDFMA-b-PHS (yellow).

disappearance of peaks at 3.0−4.0 ppm and ∼5.3 ppm completely, which are the initial peaks assigned for the THP group, demonstrating the completion of the deprotection reaction. The peak of the phenolic proton was assigned at ∼7.8 ppm. The ratio of integral areas between the aromatic units from the PHS block and the methylene units from the PPDFMA block remained consistent after the reaction, verifying that PPDFMA-b-PHS was synthesized successfully without any cleavage of the PPDFMA block. GPC curves also exhibited the successful initiation of the second block by the 370

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indicating that the microphase separation had reached thermal equilibrium, as shown in Figure 4a. For D8 (Mn = 4.0 kg

7.2 nm for D8, exhibiting the sub-5 nm feature width of the lamellar domains. According to the mean-field theory, χ of PPDFMA-b-PHS must have been large enough such that the PPDFMA-b-PHS with the smallest N (D8) had χN > 10.5 and was able to selfassemble into ordered, lamellar microdomains. In order to determine the χ value experimentally, temperature-resolved SAXS was used to measure the disordered PPDFMA-b-PHS (D9), and a best fit was obtained for the results to Leibler’s mean-field theory, which incorporates the effects of dispersity and segmental volume asymmetry.55,61,62 The detailed description of the calculation method and data analysis are summarized in the Supporting Information. The result of the best fits at different temperatures are presented in Figure 3a. A

Figure 4. SAXS intensity profiles for block copolymers D4 (Mn = 7.7 kg mol−1, f F = 15 wt %) and D8 (Mn = 4.0 kg mol−1, f F = 35 wt %) at room temperature and after thermal annealing at 50 °C for 1 min, 80 °C for 1 min, and 160 °C for 1 min, 1 h, and 24 h, respectively, where f F is defined as the mass fraction of fluorine atoms in BCPs.

mol−1, f F = 35 wt %), all q* values stayed at 0.644 nm−1 with a similar sharpness and fwhm (full width at half-maximum) of both primary (fwhm = 0.049 nm−1) and secondary scattering peaks (fwhm = 0.06 nm−1) at 80 °C for 1 min and 160 °C for 1 min, 1 h, and 24 h. The similar sharpness of the SAXS peaks indicated that microphase separation may have been complete, even at 80 °C in only 1 min (Figure 4b). The ordered lamellar morphology can also be observed by TEM, as shown in Figure S10. The d spacing observed in the TEM image matched the value determined from the SAXS result. The features detected by SAXS at room temperature showed that the BCPs had undergone some self-assembly during the formation of the films, perhaps due to the PPDFMA-b-PHS chains having extra mobility as the solvent evaporated from the film. D5 (Mn = 5.8 kg mol−1, f F = 21 wt %), D6 (Mn = 5.2 kg mol−1, f F = 25 wt %), and D7 (Mn = 4.3 kg mol−1, f F = 31 wt %) also were thermally annealed under different conditions, as shown in Figure S9. By comparing PS-b-PMMA47 or hydroxylgroup-containing BCPs previously reported24,25,27 with our BCPs, it can be seen that the existence of fluoro blocks in PPDFMA-b-PHS may accelerate microphrase separation and overcome the slow interdiffusion of polymer chains.33,34,56−60 According to SAXS results, with Mn decreasing and fluorine content increasing (from 15 wt % to 35 wt %), the short chains with a high level of fluorine (>30 wt %) possess high mobility and low coefficient of friction properties. They may further facilitate the assembling process and make it possible to complete microphase separation within several minutes, even at low temperatures that can minimize side reactions and meet the requirements of industrial production. In conclusion, a series of PPDFMA-b-PTHPOS BCPs were synthesized via RAFT polymerization, and PPDFMA-b-PHS could be further obtained by both thermal and acid-catalyzed deprotection. The χ parameter between PPDFMA and PHS estimated by the Leibler mean-field theory was χ = 4.4/T + 0.4742, corresponding to a value of 0.48 at 150 °C. Such a high χ value allowed PPDFMA-b-PHS to form a 9.8 nm lamellar morphology, as determined by SAXS analysis and further

Figure 3. (a) SAXS intensity profiles of disordered PPDFMA-b-PHS (D9) at different temperatures: 140 °C (red triangle), 170 °C (blue circle), 200 °C (yellow square), and 230 °C (green diamond); and the best fitted-lines were obtained from Leibler mean-field theory. (b) Linear dependence of χ as a function of inverse T.

linear fit of the calculated χ values as a function of 1/T yielded the relationship χ = 4.4/T + 0.4742 (Figure 3b). These results show that the temperature dependence of χ was weak and that the entropic contribution of the χ value of PPDFMA-b-PHS was much greater than the enthalpic contribution. The molar volume difference between PPDFMA and PHS was quite large (Table S2), which may be responsible for such a large entropic contribution.63 The value of χ is 0.48 at 150 °C, more than 16× larger than the χ of PS-b-PMMA (∼0.03). We also compared the χ value to other high-χ BCPs reported in the literature that were determined using the same testing method. At the reference temperature of 150 °C, P4HS-b-PS has a χ value of 0.12.24 By changing the position of the phenol group and introducing PDMS as the second block, the χ value of P3HS-b-PDMS increased to 0.39.26 Compared to these PHSbased BCPs, the χ value for PPDFMA-b-PHS is quite large, indicating a significant incompatibility between its two blocks. A solubility parameter analysis and surface energy measurements confirmed the high level of incompatibility between the PPDFMA and the PHS blocks, as described in the Supporting Information. In order to optimize the annealing conditions, PPDFMA-bPHS BCPs were thermally annealed over a range of conditions. For D4 (Mn = 7.7 kg mol−1, f F = 15 wt %), weak SAXS peaks at q* and 2q* were observed, showing it was not completely disordered, even at 50 °C. After annealing at 80 °C for 1 min, the scattering peaks sharpened noticeably and there was a slight shift of the primary scattering peak. When the temperature reached 160 °C, the primary scattering peak stayed at q* = 0.380 nm−1, and a higher ordered peak at 4q* appeared. This morphology could be maintained for 24 h, 371

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(12) Wan, L.; Ruiz, R.; Gao, H.; Patel, K. C.; Albrecht, T. R.; Yin, J.; Kim, J.; Cao, Y.; Lin, G. The Limits of Lamellae-Forming PS-bPMMA Block Copolymers for Lithography. ACS Nano 2015, 9, 7506−7514. (13) Zhao, Y.; Sivaniah, E.; Hashimoto, T. SAXS Analysis of the Order-Disorder Transition and the Interaction Parameter of Polystyrene-block-Poly(methyl methacrylate). Macromolecules 2008, 41, 9948−9951. (14) Sinturel, C.; Bates, F. S.; Hillmyer, M. A. High χ−Low N Block Polymers: How Far Can We Go? ACS Macro Lett. 2015, 4, 1044− 1050. (15) Kennemur, J. G.; Yao, L.; Bates, F. S.; Hillmyer, M. A. Sub-5 nm Domains in Ordered Poly(cyclohexylethylene)-block-poly(methyl methacrylate) Block Polymers for Lithography. Macromolecules 2014, 47, 1411−1418. (16) Sweat, D. P.; Kim, M.; Larson, S. R.; Choi, J. W.; Choo, Y.; Osuji, C. O.; Gopalan, P. Rational Design of a Block Copolymer with a High Interaction Parameter. Macromolecules 2014, 47, 6687−6696. (17) Lane, A. P.; Yang, X. M.; Maher, M. J.; Blachut, G.; Asano, Y.; Someya, Y.; Mallavarapu, A.; Sirard, S. M.; Ellison, C. J.; Willson, C. G. Directed Self-Assembly and Pattern Transfer of Five Nanometer Block Copolymer Lamellae. ACS Nano 2017, 11, 7656−7665. (18) Son, J. G.; Gwyther, J.; Chang, J. B.; Berggren, K. K.; Manners, I.; Ross, C. A. Highly Ordered Square Arrays from a Templated ABC Triblock Terpolymer. Nano Lett. 2011, 11, 2849−2855. (19) Kim, E. J.; Kim, W. J.; Lee, K. H.; Ross, C. A.; Son, J. G. A Top Coat. with Solvent Annealing Enables Perpendicular Orientation of Sub-10 nm Microdomains in Si-Containing Block Copolymer Thin Films. Adv. Funct. Mater. 2014, 24, 6981−6988. (20) Cushen, J. D.; Bates, C. M.; Rausch, E. L.; Dean, L. M.; Zhou, S. X.; Willson, C. G.; Ellison, C. J. Thin Film Self-Assembly of Poly(trimethylsilylstyrene-b-d,l-lactide) with Sub-10 nm Domains. Macromolecules 2012, 45, 8722−8728. (21) Nakatani, R.; Takano, H.; Chandra, A.; Yoshimura, Y.; Wang, L.; Suzuki, Y.; Tanaka, Y.; Maeda, R.; Kihara, N.; Minegishi, S.; Miyagi, K.; Kasahara, Y.; Sato, H.; Seino, Y.; Azuma, T.; Yokoyama, H.; Ober, C. K.; Hayakawa, T. Perpendicular Orientation Control without Interfacial Treatment of RAFT-Synthesized High-χ Block Copolymer Thin Films with Sub-10 nm Features Prepared via Thermal Annealing. ACS Appl. Mater. Interfaces 2017, 9, 31266− 31278. (22) Hirai, T.; Leolukman, M.; Jin, S.; Liu, C. C.; Han, E.; Kim, Y. J.; Ishida, Y.; Hayakawa, T.; Kakimoto, M.; Nealey, P. F.; Gopalan, P. One-Step Direct-Patterning Template Utilizing Self-Assembly of POSS-Containing Block Copolymers. Adv. Mater. 2009, 21, 4334− 4338. (23) Lo, T.-Y.; Krishnan, M. R.; Lu, K.-Y.; Ho, R.-M. Silicon containing block copolymers for lithographic applications. Prog. Polym. Sci. 2018, 77, 19−68. (24) Sweat, D. P.; Kim, M.; Schmitt, A. K.; Perroni, D. V.; Fry, C. G.; Mahanthappa, M. K.; Gopalan, P. Phase Behavior of Poly(4hydroxystyrene-block-styrene) Synthesized by Living Anionic Polymerization of an Acetal Protected Monomer. Macromolecules 2014, 47, 6302−6310. (25) Kanimozhi, C.; Kim, M.; Larson, S. R.; Choi, J. W.; Choo, Y.; Sweat, D. P.; Osuji, C. O.; Gopalan, P. Isomeric Effect Enabled Thermally Driven Self-Assembly of Hydroxystyrene-Based Block Copolymers. ACS Macro Lett. 2016, 5, 833−838. (26) Azuma, K.; Sun, J.; Choo, Y.; Rokhlenko, Y.; Dwyer, J. H.; Schweitzer, B.; Hayakawa, T.; Osuji, C. O.; Gopalan, P. Self-Assembly of an Ultrahigh-χ Block Copolymer with Versatile Etch Selectivity. Macromolecules 2018, 51, 6460−6467. (27) Kwak, J.; Mishra, A. K.; Lee, J.; Lee, K. S.; Choi, C.; Maiti, S.; Kim, M.; Kim, J. K. Fabrication of Sub-3 nm Feature Size Based on Block Copolymer Self-Assembly for Next-Generation Nanolithography. Macromolecules 2017, 50, 6813−6818. (28) Jeong, G.; Yu, D. M.; Mapas, J. K. D.; Sun, Z. W.; Rzayev, J.; Russell, T. P. Realizing 5.4 nm Full Pitch Lamellar Microdomains by a Solid-State Transformation. Macromolecules 2017, 50, 7148−7154.

confirmed by TEM. Meanwhile, SAXS revealed that the annealing of PPDFMA-b-PHS was completed at a low temperature (80 °C) and in a short time (1 min). Due to the combination of its high resolution and fast microphase separation, PPDFMA-b-PHS is a promising candidate for use as a lithography material with sub-5 nm resolution.

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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/acsmacrolett.9b00178.

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Experimental section and Figures S1−S10 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chenxu Wang: 0000-0002-1542-5971 Xuemiao Li: 0000-0002-2086-2172 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by Fudan University (IDH1717041), the Ministry of Science and Technology of China (2016YFA0203302), and Science and Technology Commission of Shanghai Municipality (No. 18511104900). The authors also acknowledge experimental support from the State Key Laboratory of Molecular Engineering of Polymers and the Nano-fabrication Laboratory of Fudan University.

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REFERENCES

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Letter

ACS Macro Letters

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DOI: 10.1021/acsmacrolett.9b00178 ACS Macro Lett. 2019, 8, 368−373