Fabrication of Ordered Nanopattern by using ABC Triblock Copolymer with Salt in Toluene

Fabrication of Ordered Nanopattern by using ABC Triblock Copolymer with Salt in Toluene Ordered nanopatterns of triblock copolymer polystyrene-block-poly(2-vinylpyridine)-block- poly (ethylene oxide) (PS-b-P2VP-b-PEO) have been achieved by the addition of lithium chloride (LiCl). The morphological and structural evolution of PS-b-P2VP-b-PEO/LiCl thin films were systematically investigated by varying different experimental parameters, including the treatment for polymer solution after the addition of LiCl, the time scale of ultrasonic treatment and the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) in P2VP block. When toluene was used as the solvent for LiCl, ordered nanopattern with cylinders or nanostripes could be obtained after spin-coating. The mechanism of nanopattern transformation was related to the loading of LiCl in different microdomains. Keywords: Ultrasound, Microphase separation, Triblock copolymer, Lithium chloride Background and bicontinuous and tricontinuous ordered mesophases Recently, ion/block copolymers (BCPs) hybrids have be- [7–15]. However, ion/triBCPs hybrids are rarely reported come highly attractive materials due to their flexibility, [16]. To further explore the novel properties of ABC process stability, self-assembling ability and novel features triBCPs and develop more performance requirements, it of inorganic components such as electronic, magnetic and is necessary to study the ion/triBCPs hybrids. optical properties [1–3]. Spatz and co-workers created The addition of salts into the BCPs is one of effective fused silica substrates with nanopillars on both sides way to obtain ordered nanopatterns. Researchers have with 99.8% transmittance and 0.02% reflectance, which found that polyethylene oxide (PEO) [17–19], poly- was helpful for many laser applications [4]. Black et al. methyl methacrylate (PMMA) [20], poly(ε-caprolactone) fabricated densely packed silicon nanotextures with (PCL) [21] or polyvinyl pyridine (PVP) [22, 23] are ion- feature sizes smaller than 50 nm by block copolymer dissolving blocks, and polystyrene (PS) [24] is a non- self-assembly to enhance the broadband antireflection of conducting block. Wang and co-workers suggested that solar cells [5]. Morris et al. fabricated Si nanowire array the selection of metal ions to blocks was primarily due by self-assembly of block copolymer with LiCl, which to the large solvation energy when the lithium salts as- showed the possible application in the area of photonics sociate with the polar PEO domains, leading to a large and photoluminescence [6]. increase in the effective segregation strength with lith- Compared with diblock copolymers (diBCPs), ABC ium salt loading [25, 26]. triblock copolymer (triBCPs) can assemble into new In previous experiments [6, 17, 27], the co-solvents for morphologies such as periodic arrays of core/shell salts are frequently used because of the solubility of salts spheres and cylinders, tetragonal lattices of cylinders, and the efficiency of coordination between salts and BCPs. Russell et al. continuously stirred after the mix- ture of LiCl in tetrahydrofuran (THF) and polystyrene- * Correspondence: zxhong329@126.com; ygb702@163.com block-poly(methyl methacrylate) (PS-b-PMMA) toluene School of Chemical Engineering and Light Industry, Guangdong University of solution with moderate heating until most of THF was Technology, Guangzhou, Guangdong 510006, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Huang et al. Nanoscale Research Letters (2017) 12:491 Page 2 of 9 evaporated and the solutions became clear. And they resolution transmission electron microscopy (HRTEM) spent a great deal of time (about 24 h) on stir and measurement was carried out on a JEM-2100HR (JEOL, post-treatment (solvent vapor annealing and thermal Japan) operated at 200 kV accelerating voltage. Film annealing) to obtain ordered microphase-separated nano- samples for TEM were prepared onto carbon-coated structure [17, 28]. copper grids. Those samples were exposed to I vapor Herein, we demonstrated a simple and convenient for certain time period. Fourier transform infrared (FT- approach to generate various ordered nanopatterns of IR) spectra were recorded with a Nicolet 6700 (Thermo, −1 ion/triBCPs hybrids by spin-coating method without any USA) spectrophotometer in the range of 4000–400 cm further treatments. Morphological and structural varia- with KBr plates. Ultraviolet–visible (UV-vis) spectra tions of PS-b-P2VP-b-PEO thin films with different salt were obtained on a UV-2450(Shimadzu, Japan) spectro- concentrations were examined by adjusting various photometer. X-ray photoelectron spectroscopy (XPS) processing parameters. This work indicated that the co- measurements were performed on ESCALAB 250 ordination between PS-b-P2VP-b-PEO and LiCl-toluene (Thermo, USA) with Al Ka excitation. could be accelerated by ultrasonic treatment for fabricat- ing ordered nanopattern. Results and Discussion Morphology of Pure PS-b-P2VP-b-PEO Thin Film Methods When 0.1 wt% PS-b-P2VP-b-PEO toluene solution was Materials stirred for 24 h and spin-coated on silicon wafer, nano- Triblock copolymer polystyrene-block-poly(2-vinylpyri- porous patterns could be observed in Fig. 1. The average dine)-block-poly(ethylene oxide)(PS- b-P2VP-b-PEO, size of nanopores was about 22 nm. 45,000 g/mol, 16,000 g/mol, 8500 g/mol, M /M = 1.05) w n was purchased from Polymer Source Inc. and used without further purification in this study. Anhydrous Dispersion of LiCl in Toluene lithium chloride (LiCl, 95%+, AR) was purchased from DispersionsofLiClintoluene with variousaging timesare Tianjin Fuchen Chemical Reagents Factory. Toluene shown in Fig. 2. Toluene was not a good solvent for LiCl. (99 + %), ethanol and N,N-Dimethylform amide (DMF, So suspension with unstable status could be seen after analytical grade) were purchased from Tianjin Damao ultrasonic treatment (Fig. 2a). It was noticeable that little Chemical Co. Ltd. Silicon(Si) wafer was purchased from sedimentation phenomenon was observed when the aging No.46 Research Institute of China Electronics Technology time was 5 min (Fig. 2d). Therefore, the prepared suspen- Group Corporation (CETC). sion should be used immediately after ultrasonic treatment. Sample Preparation Si wafers were cleaned in DMF, ethanol and deionized water under ultrasonic for 30 min at room temperature, respectively. 0.1 wt% PS-b-P2VP-b-PEO toluene solution was stirred for 24 h at room temperature. And LiCl was dispersed in toluene by ultrasound for 30 min at room temperature. Then various volume of LiCl toluene solu- tion was immediately added to the PS-b-P2VP-b-PEO micellar solutions. Those mixtures were treated by dif- ferent ways to trigger complexation between Li ions and polymer chains. The resultant solutions were spin- coated immediately onto the substrate at 3000 rpm for 1 min after filtration. At last, the films were dried under nitrogen at room temperature to remove the residual solvent. Characterization Atomic force microscope (AFM) in SCANASYST- AIR mode (Nanoscope-V Multimode 8, Bruker Inc., Germany) by using a silicon cantilever (spring constant 5 N/m and resonant frequency ~ 150 kHz, Budget Sen- Fig. 1 AFM height images of PS-b-P2VP-b-PEO films spin-coated sors, Bulgaria Ltd.) was used to investigate the morpho- from 0.1 wt% PS-b-P2VP-b-PEO toluene solution logical features of PS-b-P2VP-b-PEO thin films. High- Huang et al. Nanoscale Research Letters (2017) 12:491 Page 3 of 9 Fig. 2 Dispersion of LiCl in toluene after ultrasonic treatment without and with different aging time: (a) without aging time, (b)1 min, (c)3 min, (d)5 min Effect of Methods to Trigger the Coordination between LiCl spin-coated onto substrate, disordered cylindrical mi- and Polymer Chains crodomains appeared in Fig. 3b. When the mixed so- Generally, stir and post-treatment are required for poly- lution was placed in ultrasonic cleaner for 30 min at mer solution containing metal salts in order to trigger the room temperature, microphase-separated nanopattern coordination between salts and polymer chains for fabri- with cylindrical microdomain was obtained obviously cation of ordered nanostructure, which takes a lot of time in Fig. 3c after spin-coating. The energy provided by [22, 28]. And the ultrasound is the simple way to acceler- sound waves was able to disrupt the larger aggregates ate the coordination between metal ions and block co- of the micelles. And the sound waves could further polymer [29–31]. In order to demonstrate the advantage increase the diffusion rate of metal ions in the solu- of ultrasonic treatment in this work, different methods tion, so the loading of Li ions in micelles were ex- were used after the mix of LiCl-toluene and triblock co- pected to happen much faster than the conventional polymer solution when the molar ratio of Li ions to the stirring method. This result indicated that ultrasonic total number of oxygen atoms (O) in PEO block and the treatment was a useful method to improve the effi- + + nitrogen atoms (N) was 1:32.2([Li ]:[O + N] = 1:32.2). ciency of coordination between Li ions and polymer When the mixed solution was stirred (1500 rpm) for chains. 30 min at room temperature and then spin-coated onto substrate, no distinct ordered structure was ob- Effect of Time Scale served in Fig. 3a. When the mixed solution was In order to investigate the time scale of ultrasonic treat- stirred at 1500 rpm for 30 min at 75 °C and then ment, the mixed solution ([Li ]:[O + N] = 1:32.2) was Fig. 3 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1 wt% toluene solution with different methods after the addition of LiCl-toluene when the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) is 1:32.2: (a) 1500 rpm stirring for 30 min at room temperature, (b) 1500 rpm stirring for 30 min at 75 °C, (c) ultrasonic treatment for 30 min at room temperature Huang et al. Nanoscale Research Letters (2017) 12:491 Page 4 of 9 placed in ultrasonic cleaners for various times before coordination of Li ions and polymer chains so that spin-coating. When the time was 7.5 min (Fig. 4a), disordered nanopattern was found instead of the cy- the nanoporous morphology was similar to the film lindrical array. Therefore, the time of ultrasonic in Fig. 1. Compared with the film in Fig. 1, the treatment should be controlled in appropriate range number and the average size of nanopores decreased, to obtain obvious microphase-separated nanopattern. which indicated that Li ions began to load in PS-b- P2VP-b-PEO polymer chains after 7.5 min. The Li Effect of LiCl Content in PS-b-P2VP-b-PEO Thin Films ions loaded in polymer chains would increase with The addition of LiCl has significant effects on the time increasing. Parts of nanopores were con- morphology since Li ions could be loaded in P2VP nected when the time increased to 15 min (Fig. 4b). and PEO blocks [17–19, 22, 23]. And the molar ratio When the time was 22.5 min, the nanopattern exhib- ([Li ]:[O + N]) was varied in our work (Fig. 5). ited a coexistence of nanostripes and cylinders (Fig. When the molar ratio was 1:40.25, the nanopattern 4c). When the time was prolonged to 30 min, of stripes was obtained (Fig. 5a). When the molar ra- microphase-separation with cylindrical microdomains tio decreased to 1:32.2, nanopattern with cylindrical occurred obviously (Fig. 3c). As the time extended microdomains could be seen in Fig. 3c. As the molar to 37.5 min, the coexistence of nanostripes and cy- ratio was 1:24.15, a lot of nanopores were connected lindrical microdomains appeared again (Fig. 4d). to show the tendency from nanopores pattern trans- From above results, when the time was less than form to nanostripes (Fig. 5b). When the molar ratio 30 min, the complexation between Li ions and PS- was 1:16.1, disordered nanopores become the overall b-P2VP-b-PEO was accelerated by ultrasonic treat- morphology (Fig. 5c). The average size of holes was ment so that more and more Li ions were coordi- larger than the film in Fig. 1. As the molar ratio fur- nated with PS-b-P2VP-b-PEO, resulting in transition ther decreased to 1:8.05, a few of nanopaores was of nanopattern from nanoporous array to cylindrical observed in Fig. 5d. The average diameter of these array. When the time was more than 30 min, the pores was more than 40 nm. From above results, an energy provided by sound waves would break the order-to-disorder transition was shown in Fig. 5 by Fig. 4 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1 wt% polymer-LiCl toluene solution with various time scale of ultrasonic treatment when the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) is 1:32.2: (a)7.5 min, (b)15min,(c) 22.5 min, (d) 37.5 min Huang et al. Nanoscale Research Letters (2017) 12:491 Page 5 of 9 Fig. 5 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1wt%polymer-LiCl toluene solution with various molar ratio of Li ions to ethylene oxide moieties and pyridine groups: (a) 1:40.25, (b) 1:24.15, (c) 1:16.1, (d) 1:8.05 controlling the addition of LiCl in ion/polymer hy- Microdomains Location of Three Blocks in PS-b-P2VP-b-PEO brids. The change of LiCl loaded in polymer chains Thin Films was the reason for the morphological transition. The In order to explore the microdomain location of the LiCl loading in polymer chains increased with de- three blocks in PS-b-P2VP-b-PEO thin film under differ- creasing molar ratio ([Li ]:[O + N]), leading to the ent conditions, those samples were exposed to I vapor different phase behaviors of the PS-b-P2VP-b-PEO/ for certain period before TEM measurement. LiCl hybrids. And the ordered arrangements of PS- The PS-b-P2VP-b-PEO thin film without LiCl exhib- b-P2VP-b-PEO/LiCl hybrids were formed with the ited an array of dark rings after the selective staining of critical amount of LiCl loaded. P2VP blocks, indicating that the periphery of the hole Fig. 6 TEM images of PS-b-P2VP-b-PEO film after I staining with and without LiCl: (a) without LiCl, (b) with LiCl-toluene and the molar ratio of LiCl to ethylene oxide moieties and pyridine groups was 1:40.25, (c) with LiCl-toluene and the molar ratio of LiCl to ethylene oxide moieties and pyridine groups was 1:32.2 Huang et al. Nanoscale Research Letters (2017) 12:491 Page 6 of 9 Fig. 7 Schematic illustration of the fabrication of PS-b-P2VP-b-PEO nanopattern with and without Li ions corresponded to P2VP blocks (Fig. 6a). Thus, the rest of diameter of dark rings was about 32 nm, and the average the hole should match with PEO blocks. The continuous inner diameter of dark ring was about 26 nm. It was matrix was PS blocks. The average outer diameter of the demonstrated that the cylindrical domains in Fig. 3c dark rings was about 21 nm and the average inner diam- were core-shell structure. The outer shell was P2VP eter of the dark rings was about 16 nm. blocks and the core was PEO blocks. Compared with When the molar ratio ([Li ]:[O + N]) was 1:40.25 after the film in Fig. 6a, the PEO microdomains were obvi- I selective staining, the nanopattern of stripes was ob- ously swelled and the P2VP domains slightly increased. tained (Fig. 6b). The bright regions of the spheres were de- Compared with the film in Fig. 6b, this result indicated pressed in striated structure. The bright regions were PEO that more Li ions were coordinated with PEO blocks blocks and the rest of stripes were P2VP microdomains. with more LiCl in PS-b-P2VP-b-PEO thin film. Hence, the continuous matrix was PS blocks. Distinct The difference of Fig. 6b, c can be explained as shown dark particulates (of LiOH presumably) were observed in in Fig. 7. Because of the selectivity of toluene for three P2VP domains [32]. The average diameter of PEO do- blocks, the nanostructure of PS-b-P2VP-b-PEO mi- mains was about 17 nm, which was similar to the average celles in toluene was core-shell structure. Considering domain size of PEO blocks in Fig. 6a. And the P2VP do- the sequence of the three blocks in PS-b-P2VP-b-PEO, mains transformed from dark rings to stripes. This result PS blocks were theouter shell. Theinnershell was indicated that most of Li ions were preferentially coordi- P2VP domain and the core was PEO blocks. When the + + nated with P2VP blocks when the molar ratio was 1:40.25. molar ratio ([Li ]:[O + N]) was 1:40.25, the Li ions were When the molar ratio ([Li ]:[O + N]) decreased to mainly focused on the inner shell of P2VP blocks because of 1:32.2, an array of dark rings could also be seen (Fig. 6c) the limited content of LiCl and the resistance of P2VP inner after I selective staining. The dark rings were P2VP mi- shell. As a result, only a few of Li ions were coordinated crodomains and the bright regions were PEO blocks. with PEO microdomains. When the molar ratio ([Li ]: The continuous matrix was PS blocks. The average outer [O + N]) was 1:32.2, the interaction parameter of Li ions Fig. 8 (a) FT-IR spectra of pure PS-b-P2VP-b-PEO thin film and the thin film with LiCl-toluene; (b) UV-vis spectra of pure PS-b-P2VP-b-PEO thin film; (c) UV-vis spectra of PS-b-P2VP-b-PEO thin film with LiCl-toluene Huang et al. Nanoscale Research Letters (2017) 12:491 Page 7 of 9 and the PEO blocks effectively increased due to the increase Table 2 The intensity of absorption peak at 262 nm with different PS-b-P2VP-b-PEO thin films of LiCl-toluene, resulting in the obvious swelling in PEO do- mains [32–36]. Samples Intensity at 262 nm (a.u.) PS-b-P2VP-b-PEO thin film 0.123 1:32.2 0.023 Analysis of Competitive Interactions of Li ions with PEO 1:40.25 0.015 and P2VP Blocks + 1:24.15 0.013 It is noteworthy that the competitive interactions of Li 1:16.1 0.011 ions with both the PEO and P2VP blocks exist in PS-b- P2VP-b-PEO/LiCl hybrids [3]. The interaction between 1:8.05 0.009 the Li ions and the PEO blocks was characterized by FT-IR (Fig. 8a). The parameter of I /I , which was the a f The PS-b-P2VP-b-PEO thin films without and with ratio of the peak intensity corresponding to the associ- LiCl-toluene ([Li ]:[O + N] = 1:32.2) were analyzed by X- ated C-O-C to the peak intensity of free C-O-C, was ray photoelectron spectroscopy (XPS) (Figs. 9 and 10). used to evaluate the coordination between the Li ions XPS survey spectra (Fig. 9) of PS-b-P2VP-b-PEO with LiCl and the PEO blocks (Table 1) [37, 38]. The C-O-C −1 confirmed the presence of C, O, N, Li and Cl. The C1s stretching vibration changed from 1124 to 1111 cm . binding energy in C-C bonds was 284.78 eV. The O1s The value of I /I increased with the doped LiCl increasing, a f binding energy of C-O-C in PEO block was 533.08 eV, and indicating that the loading of Li ions in PEO blocks in- N1 s binding energy based on the P2VP block was creased when the molar ratio ([Li ]:[O + N]) decreased 399.48 eV. The Cl2p appeared at 198.28 eV, and Li1s from 1:40.25 to 1:8.05. appeared in 55.88 eV. High resolution XPS spectra The UV-vis spectra of various PS-b-P2VP-b-PEO thin of N1 s binding energy and O1s binding energy in films are illustrated in Fig. 8b, c. The absorption peak at PS-b-P2VP-b-PEO with and without LiCl were 262 nm was assigned to pyridine groups and phenyl showninFig.10a,b.The N1 sbinding energy in groups of PS-b-P2VP-b-PEO [39]. Based on the previous PS-b-P2VP-b-PEO without LiCl was 398.88 eV, but study [24], the obvious change of the intensity was at- the binding energy in the thin film with LiCl was tributed to the coordination between Li ions and pyri- 399.48 eV. The O1s binding energy in PS-b-P2VP-b- dine groups. The intensities of absorption peak at PEO without LiCl was 532.78 eV, but the binding 262 nm with different samples were summarized in energy in the thin film with LiCl was 533.08 eV. Table 2. The intensities of absorption peak at 262 nm These shifts in binding energy were consequences of for PS-b-P2VP-b-PEO thin films with LiCl (Fig. 8c) were electron withdrawing effect caused by the coordin- weaker than the pure film (Fig. 8b). When the molar ra- ation between Li and PS-b-P2VP-b-PEO [40], valid- tio ([Li ]:[O + N]) was 1:40.25, 1:24.15, 1:16.1 and 1:8.05, ating the presence of Li element in the thin film the intensity of absorption peak at 262 nm decreased after Li ions were loaded. These results were essen- with the LiCl addition increasing (Fig. 8c), indicating tially identical to the results in Fig. 8. that more and more Li ions were coordinated with the P2VP blocks and PEO blocks. However, when the molar ratio ([Li ]:[O+N]) was 1:32.2, the absorption peak at 262 nm was stronger than the molar ratio 1:40.25. The reason should be that most of Li+ ions were loaded in PEO blocks but not P2VP blocks when the molar ratio was about 1:32.2, and the least LiCl was loaded in P2VP blocks at this molar ratio ([Li ]:[O + N] = 1:32.2) com- pared with other thin films with LiCl. Table 1 The data of I /I for different PS-b-P2VP-b-PEO/LiCl a f hybrids in LiCl-toluene [Li ]:[O + N] I /I a f 1:40.25 0.2522 1:32.2 0.3165 1:24.15 0.3670 1:16.1 0.4266 Fig. 9 XPS survey spectra of PS-b-P2VP-b-PEO thin film with LiCl-toluene 1:8.05 0.4908 Huang et al. Nanoscale Research Letters (2017) 12:491 Page 8 of 9 Fig. 10 High-resolution XPS spectra of (a) N1 s binding energy and (b) O1s binding energy in PS-b-P2VP-b-PEO with and without LiCl-toluene Conclusions of Guangdong province (Grant No. S2012040007725) and the Pearl River S&T Nova Program of Guangzhou (201710010144). In this study, we present a simple approach to fabri- cate ordered nanopatterns of ion/triblock copolymers Funding hybrids without post-processing. This work demon- National Natural Science Foundation of China (Grant No. 51273048 strated that toluene could be used as co-solvents for and 51,203,025). Natural Science Foundation of Guangdong province (Grant No. S2012040007725). LiCl in short time. An order-to-disorder transition Pearl River S&T Nova Program of Guangzhou (201710010144). was triggered by varying the addition of LiCl-toluene with ultrasonic treatment. And ordered microphase- Authors’ Contributions separated nanopatterns of cylindrical array and stripes HH, GY, and XZ designed the experiment and measurements. HH, BZ, MZ,and YZ executed theexperiments.HH, GY,XZ, BZ,HL, WL,MZ, were obtained. The mechanism of the morphological and YZ examined the written report. All authors read and approved transition was due to the LiCl loaded in different ion- the final manuscript. dissolving blocks. This rapid synthesis might boost future studies of ion/triblock copolymers hybrids be- Ethics Approval and Consent to Participate Not applicable. cause of the advantage of ultrasonic as compared to the conventional routes. Furthermore, this approach Consent for Publication has potential applications in developing ultra-small Not applicable. devices via techniques such as pattern transfer owing to its simplicity, effectiveness and low cost, especially Competing Interests The authors declare that they have no competing interests. regarding to fabrication time. Abbreviations Publisher’sNote + + [Li ]:[O + N]: the molar ratio of Li ions to the total number of oxygen atoms Springer Nature remains neutral with regard to jurisdictional claims in (O) in PEO block and the nitrogen atoms (N); AFM: Atomic force microscope; published maps and institutional affiliations. CETC: China Electronics Technology Group Corporation; diBCPs: diblock copolymers; DMF: N,N-Dimethylform amide; FT-IR: Fourier transform infrared; Received: 22 April 2017 Accepted: 1 August 2017 HRTEM: High resolution transmission electron microscopy; LiCl: lithium chloride; LiOH: Lithium hydroxide; PCL: poly(ε-caprolactone); PMMA: polymethyl methacrylate; PS: polystyrene; PS-b-P2VP-b-PEO: polystyrene-block-poly(2- References vinylpyridine)-block-poly(ethylene oxide); PVP: polyvinyl pyridine; triBCPs: triblock 1. Young WS, EppsIII TH (2009) Salt doping in PEO-containing block copolymers; UV-vis: Ultraviolet–visible; XPS: X-ray photoelectron spectroscopy copolymers: counterion and concentration effects. Macromolecules 42: 2672–2678 Acknowledgements 2. Ham SJ, Shin CH, Kim E, Ryu DY, Jeong U, Russell TP, Hawker CJ (2008) This work was supported by the National Natural Science Foundation of Microdomain orientation of PS-b-PMMA by controlled interfacial interactions. 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Fabrication of Ordered Nanopattern by using ABC Triblock Copolymer with Salt in Toluene

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Materials Science; Nanotechnology; Nanotechnology and Microengineering; Nanoscale Science and Technology; Nanochemistry; Molecular Medicine
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Abstract

Ordered nanopatterns of triblock copolymer polystyrene-block-poly(2-vinylpyridine)-block- poly (ethylene oxide) (PS-b-P2VP-b-PEO) have been achieved by the addition of lithium chloride (LiCl). The morphological and structural evolution of PS-b-P2VP-b-PEO/LiCl thin films were systematically investigated by varying different experimental parameters, including the treatment for polymer solution after the addition of LiCl, the time scale of ultrasonic treatment and the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) in P2VP block. When toluene was used as the solvent for LiCl, ordered nanopattern with cylinders or nanostripes could be obtained after spin-coating. The mechanism of nanopattern transformation was related to the loading of LiCl in different microdomains. Keywords: Ultrasound, Microphase separation, Triblock copolymer, Lithium chloride Background and bicontinuous and tricontinuous ordered mesophases Recently, ion/block copolymers (BCPs) hybrids have be- [7–15]. However, ion/triBCPs hybrids are rarely reported come highly attractive materials due to their flexibility, [16]. To further explore the novel properties of ABC process stability, self-assembling ability and novel features triBCPs and develop more performance requirements, it of inorganic components such as electronic, magnetic and is necessary to study the ion/triBCPs hybrids. optical properties [1–3]. Spatz and co-workers created The addition of salts into the BCPs is one of effective fused silica substrates with nanopillars on both sides way to obtain ordered nanopatterns. Researchers have with 99.8% transmittance and 0.02% reflectance, which found that polyethylene oxide (PEO) [17–19], poly- was helpful for many laser applications [4]. Black et al. methyl methacrylate (PMMA) [20], poly(ε-caprolactone) fabricated densely packed silicon nanotextures with (PCL) [21] or polyvinyl pyridine (PVP) [22, 23] are ion- feature sizes smaller than 50 nm by block copolymer dissolving blocks, and polystyrene (PS) [24] is a non- self-assembly to enhance the broadband antireflection of conducting block. Wang and co-workers suggested that solar cells [5]. Morris et al. fabricated Si nanowire array the selection of metal ions to blocks was primarily due by self-assembly of block copolymer with LiCl, which to the large solvation energy when the lithium salts as- showed the possible application in the area of photonics sociate with the polar PEO domains, leading to a large and photoluminescence [6]. increase in the effective segregation strength with lith- Compared with diblock copolymers (diBCPs), ABC ium salt loading [25, 26]. triblock copolymer (triBCPs) can assemble into new In previous experiments [6, 17, 27], the co-solvents for morphologies such as periodic arrays of core/shell salts are frequently used because of the solubility of salts spheres and cylinders, tetragonal lattices of cylinders, and the efficiency of coordination between salts and BCPs. Russell et al. continuously stirred after the mix- ture of LiCl in tetrahydrofuran (THF) and polystyrene- * Correspondence: zxhong329@126.com; ygb702@163.com block-poly(methyl methacrylate) (PS-b-PMMA) toluene School of Chemical Engineering and Light Industry, Guangdong University of solution with moderate heating until most of THF was Technology, Guangzhou, Guangdong 510006, People’s Republic of China © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Huang et al. Nanoscale Research Letters (2017) 12:491 Page 2 of 9 evaporated and the solutions became clear. And they resolution transmission electron microscopy (HRTEM) spent a great deal of time (about 24 h) on stir and measurement was carried out on a JEM-2100HR (JEOL, post-treatment (solvent vapor annealing and thermal Japan) operated at 200 kV accelerating voltage. Film annealing) to obtain ordered microphase-separated nano- samples for TEM were prepared onto carbon-coated structure [17, 28]. copper grids. Those samples were exposed to I vapor Herein, we demonstrated a simple and convenient for certain time period. Fourier transform infrared (FT- approach to generate various ordered nanopatterns of IR) spectra were recorded with a Nicolet 6700 (Thermo, −1 ion/triBCPs hybrids by spin-coating method without any USA) spectrophotometer in the range of 4000–400 cm further treatments. Morphological and structural varia- with KBr plates. Ultraviolet–visible (UV-vis) spectra tions of PS-b-P2VP-b-PEO thin films with different salt were obtained on a UV-2450(Shimadzu, Japan) spectro- concentrations were examined by adjusting various photometer. X-ray photoelectron spectroscopy (XPS) processing parameters. This work indicated that the co- measurements were performed on ESCALAB 250 ordination between PS-b-P2VP-b-PEO and LiCl-toluene (Thermo, USA) with Al Ka excitation. could be accelerated by ultrasonic treatment for fabricat- ing ordered nanopattern. Results and Discussion Morphology of Pure PS-b-P2VP-b-PEO Thin Film Methods When 0.1 wt% PS-b-P2VP-b-PEO toluene solution was Materials stirred for 24 h and spin-coated on silicon wafer, nano- Triblock copolymer polystyrene-block-poly(2-vinylpyri- porous patterns could be observed in Fig. 1. The average dine)-block-poly(ethylene oxide)(PS- b-P2VP-b-PEO, size of nanopores was about 22 nm. 45,000 g/mol, 16,000 g/mol, 8500 g/mol, M /M = 1.05) w n was purchased from Polymer Source Inc. and used without further purification in this study. Anhydrous Dispersion of LiCl in Toluene lithium chloride (LiCl, 95%+, AR) was purchased from DispersionsofLiClintoluene with variousaging timesare Tianjin Fuchen Chemical Reagents Factory. Toluene shown in Fig. 2. Toluene was not a good solvent for LiCl. (99 + %), ethanol and N,N-Dimethylform amide (DMF, So suspension with unstable status could be seen after analytical grade) were purchased from Tianjin Damao ultrasonic treatment (Fig. 2a). It was noticeable that little Chemical Co. Ltd. Silicon(Si) wafer was purchased from sedimentation phenomenon was observed when the aging No.46 Research Institute of China Electronics Technology time was 5 min (Fig. 2d). Therefore, the prepared suspen- Group Corporation (CETC). sion should be used immediately after ultrasonic treatment. Sample Preparation Si wafers were cleaned in DMF, ethanol and deionized water under ultrasonic for 30 min at room temperature, respectively. 0.1 wt% PS-b-P2VP-b-PEO toluene solution was stirred for 24 h at room temperature. And LiCl was dispersed in toluene by ultrasound for 30 min at room temperature. Then various volume of LiCl toluene solu- tion was immediately added to the PS-b-P2VP-b-PEO micellar solutions. Those mixtures were treated by dif- ferent ways to trigger complexation between Li ions and polymer chains. The resultant solutions were spin- coated immediately onto the substrate at 3000 rpm for 1 min after filtration. At last, the films were dried under nitrogen at room temperature to remove the residual solvent. Characterization Atomic force microscope (AFM) in SCANASYST- AIR mode (Nanoscope-V Multimode 8, Bruker Inc., Germany) by using a silicon cantilever (spring constant 5 N/m and resonant frequency ~ 150 kHz, Budget Sen- Fig. 1 AFM height images of PS-b-P2VP-b-PEO films spin-coated sors, Bulgaria Ltd.) was used to investigate the morpho- from 0.1 wt% PS-b-P2VP-b-PEO toluene solution logical features of PS-b-P2VP-b-PEO thin films. High- Huang et al. Nanoscale Research Letters (2017) 12:491 Page 3 of 9 Fig. 2 Dispersion of LiCl in toluene after ultrasonic treatment without and with different aging time: (a) without aging time, (b)1 min, (c)3 min, (d)5 min Effect of Methods to Trigger the Coordination between LiCl spin-coated onto substrate, disordered cylindrical mi- and Polymer Chains crodomains appeared in Fig. 3b. When the mixed so- Generally, stir and post-treatment are required for poly- lution was placed in ultrasonic cleaner for 30 min at mer solution containing metal salts in order to trigger the room temperature, microphase-separated nanopattern coordination between salts and polymer chains for fabri- with cylindrical microdomain was obtained obviously cation of ordered nanostructure, which takes a lot of time in Fig. 3c after spin-coating. The energy provided by [22, 28]. And the ultrasound is the simple way to acceler- sound waves was able to disrupt the larger aggregates ate the coordination between metal ions and block co- of the micelles. And the sound waves could further polymer [29–31]. In order to demonstrate the advantage increase the diffusion rate of metal ions in the solu- of ultrasonic treatment in this work, different methods tion, so the loading of Li ions in micelles were ex- were used after the mix of LiCl-toluene and triblock co- pected to happen much faster than the conventional polymer solution when the molar ratio of Li ions to the stirring method. This result indicated that ultrasonic total number of oxygen atoms (O) in PEO block and the treatment was a useful method to improve the effi- + + nitrogen atoms (N) was 1:32.2([Li ]:[O + N] = 1:32.2). ciency of coordination between Li ions and polymer When the mixed solution was stirred (1500 rpm) for chains. 30 min at room temperature and then spin-coated onto substrate, no distinct ordered structure was ob- Effect of Time Scale served in Fig. 3a. When the mixed solution was In order to investigate the time scale of ultrasonic treat- stirred at 1500 rpm for 30 min at 75 °C and then ment, the mixed solution ([Li ]:[O + N] = 1:32.2) was Fig. 3 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1 wt% toluene solution with different methods after the addition of LiCl-toluene when the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) is 1:32.2: (a) 1500 rpm stirring for 30 min at room temperature, (b) 1500 rpm stirring for 30 min at 75 °C, (c) ultrasonic treatment for 30 min at room temperature Huang et al. Nanoscale Research Letters (2017) 12:491 Page 4 of 9 placed in ultrasonic cleaners for various times before coordination of Li ions and polymer chains so that spin-coating. When the time was 7.5 min (Fig. 4a), disordered nanopattern was found instead of the cy- the nanoporous morphology was similar to the film lindrical array. Therefore, the time of ultrasonic in Fig. 1. Compared with the film in Fig. 1, the treatment should be controlled in appropriate range number and the average size of nanopores decreased, to obtain obvious microphase-separated nanopattern. which indicated that Li ions began to load in PS-b- P2VP-b-PEO polymer chains after 7.5 min. The Li Effect of LiCl Content in PS-b-P2VP-b-PEO Thin Films ions loaded in polymer chains would increase with The addition of LiCl has significant effects on the time increasing. Parts of nanopores were con- morphology since Li ions could be loaded in P2VP nected when the time increased to 15 min (Fig. 4b). and PEO blocks [17–19, 22, 23]. And the molar ratio When the time was 22.5 min, the nanopattern exhib- ([Li ]:[O + N]) was varied in our work (Fig. 5). ited a coexistence of nanostripes and cylinders (Fig. When the molar ratio was 1:40.25, the nanopattern 4c). When the time was prolonged to 30 min, of stripes was obtained (Fig. 5a). When the molar ra- microphase-separation with cylindrical microdomains tio decreased to 1:32.2, nanopattern with cylindrical occurred obviously (Fig. 3c). As the time extended microdomains could be seen in Fig. 3c. As the molar to 37.5 min, the coexistence of nanostripes and cy- ratio was 1:24.15, a lot of nanopores were connected lindrical microdomains appeared again (Fig. 4d). to show the tendency from nanopores pattern trans- From above results, when the time was less than form to nanostripes (Fig. 5b). When the molar ratio 30 min, the complexation between Li ions and PS- was 1:16.1, disordered nanopores become the overall b-P2VP-b-PEO was accelerated by ultrasonic treat- morphology (Fig. 5c). The average size of holes was ment so that more and more Li ions were coordi- larger than the film in Fig. 1. As the molar ratio fur- nated with PS-b-P2VP-b-PEO, resulting in transition ther decreased to 1:8.05, a few of nanopaores was of nanopattern from nanoporous array to cylindrical observed in Fig. 5d. The average diameter of these array. When the time was more than 30 min, the pores was more than 40 nm. From above results, an energy provided by sound waves would break the order-to-disorder transition was shown in Fig. 5 by Fig. 4 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1 wt% polymer-LiCl toluene solution with various time scale of ultrasonic treatment when the molar ratio of Li ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) is 1:32.2: (a)7.5 min, (b)15min,(c) 22.5 min, (d) 37.5 min Huang et al. Nanoscale Research Letters (2017) 12:491 Page 5 of 9 Fig. 5 AFM height images of PS-b-P2VP-b-PEO films spin-coated from 0.1wt%polymer-LiCl toluene solution with various molar ratio of Li ions to ethylene oxide moieties and pyridine groups: (a) 1:40.25, (b) 1:24.15, (c) 1:16.1, (d) 1:8.05 controlling the addition of LiCl in ion/polymer hy- Microdomains Location of Three Blocks in PS-b-P2VP-b-PEO brids. The change of LiCl loaded in polymer chains Thin Films was the reason for the morphological transition. The In order to explore the microdomain location of the LiCl loading in polymer chains increased with de- three blocks in PS-b-P2VP-b-PEO thin film under differ- creasing molar ratio ([Li ]:[O + N]), leading to the ent conditions, those samples were exposed to I vapor different phase behaviors of the PS-b-P2VP-b-PEO/ for certain period before TEM measurement. LiCl hybrids. And the ordered arrangements of PS- The PS-b-P2VP-b-PEO thin film without LiCl exhib- b-P2VP-b-PEO/LiCl hybrids were formed with the ited an array of dark rings after the selective staining of critical amount of LiCl loaded. P2VP blocks, indicating that the periphery of the hole Fig. 6 TEM images of PS-b-P2VP-b-PEO film after I staining with and without LiCl: (a) without LiCl, (b) with LiCl-toluene and the molar ratio of LiCl to ethylene oxide moieties and pyridine groups was 1:40.25, (c) with LiCl-toluene and the molar ratio of LiCl to ethylene oxide moieties and pyridine groups was 1:32.2 Huang et al. Nanoscale Research Letters (2017) 12:491 Page 6 of 9 Fig. 7 Schematic illustration of the fabrication of PS-b-P2VP-b-PEO nanopattern with and without Li ions corresponded to P2VP blocks (Fig. 6a). Thus, the rest of diameter of dark rings was about 32 nm, and the average the hole should match with PEO blocks. The continuous inner diameter of dark ring was about 26 nm. It was matrix was PS blocks. The average outer diameter of the demonstrated that the cylindrical domains in Fig. 3c dark rings was about 21 nm and the average inner diam- were core-shell structure. The outer shell was P2VP eter of the dark rings was about 16 nm. blocks and the core was PEO blocks. Compared with When the molar ratio ([Li ]:[O + N]) was 1:40.25 after the film in Fig. 6a, the PEO microdomains were obvi- I selective staining, the nanopattern of stripes was ob- ously swelled and the P2VP domains slightly increased. tained (Fig. 6b). The bright regions of the spheres were de- Compared with the film in Fig. 6b, this result indicated pressed in striated structure. The bright regions were PEO that more Li ions were coordinated with PEO blocks blocks and the rest of stripes were P2VP microdomains. with more LiCl in PS-b-P2VP-b-PEO thin film. Hence, the continuous matrix was PS blocks. Distinct The difference of Fig. 6b, c can be explained as shown dark particulates (of LiOH presumably) were observed in in Fig. 7. Because of the selectivity of toluene for three P2VP domains [32]. The average diameter of PEO do- blocks, the nanostructure of PS-b-P2VP-b-PEO mi- mains was about 17 nm, which was similar to the average celles in toluene was core-shell structure. Considering domain size of PEO blocks in Fig. 6a. And the P2VP do- the sequence of the three blocks in PS-b-P2VP-b-PEO, mains transformed from dark rings to stripes. This result PS blocks were theouter shell. Theinnershell was indicated that most of Li ions were preferentially coordi- P2VP domain and the core was PEO blocks. When the + + nated with P2VP blocks when the molar ratio was 1:40.25. molar ratio ([Li ]:[O + N]) was 1:40.25, the Li ions were When the molar ratio ([Li ]:[O + N]) decreased to mainly focused on the inner shell of P2VP blocks because of 1:32.2, an array of dark rings could also be seen (Fig. 6c) the limited content of LiCl and the resistance of P2VP inner after I selective staining. The dark rings were P2VP mi- shell. As a result, only a few of Li ions were coordinated crodomains and the bright regions were PEO blocks. with PEO microdomains. When the molar ratio ([Li ]: The continuous matrix was PS blocks. The average outer [O + N]) was 1:32.2, the interaction parameter of Li ions Fig. 8 (a) FT-IR spectra of pure PS-b-P2VP-b-PEO thin film and the thin film with LiCl-toluene; (b) UV-vis spectra of pure PS-b-P2VP-b-PEO thin film; (c) UV-vis spectra of PS-b-P2VP-b-PEO thin film with LiCl-toluene Huang et al. Nanoscale Research Letters (2017) 12:491 Page 7 of 9 and the PEO blocks effectively increased due to the increase Table 2 The intensity of absorption peak at 262 nm with different PS-b-P2VP-b-PEO thin films of LiCl-toluene, resulting in the obvious swelling in PEO do- mains [32–36]. Samples Intensity at 262 nm (a.u.) PS-b-P2VP-b-PEO thin film 0.123 1:32.2 0.023 Analysis of Competitive Interactions of Li ions with PEO 1:40.25 0.015 and P2VP Blocks + 1:24.15 0.013 It is noteworthy that the competitive interactions of Li 1:16.1 0.011 ions with both the PEO and P2VP blocks exist in PS-b- P2VP-b-PEO/LiCl hybrids [3]. The interaction between 1:8.05 0.009 the Li ions and the PEO blocks was characterized by FT-IR (Fig. 8a). The parameter of I /I , which was the a f The PS-b-P2VP-b-PEO thin films without and with ratio of the peak intensity corresponding to the associ- LiCl-toluene ([Li ]:[O + N] = 1:32.2) were analyzed by X- ated C-O-C to the peak intensity of free C-O-C, was ray photoelectron spectroscopy (XPS) (Figs. 9 and 10). used to evaluate the coordination between the Li ions XPS survey spectra (Fig. 9) of PS-b-P2VP-b-PEO with LiCl and the PEO blocks (Table 1) [37, 38]. The C-O-C −1 confirmed the presence of C, O, N, Li and Cl. The C1s stretching vibration changed from 1124 to 1111 cm . binding energy in C-C bonds was 284.78 eV. The O1s The value of I /I increased with the doped LiCl increasing, a f binding energy of C-O-C in PEO block was 533.08 eV, and indicating that the loading of Li ions in PEO blocks in- N1 s binding energy based on the P2VP block was creased when the molar ratio ([Li ]:[O + N]) decreased 399.48 eV. The Cl2p appeared at 198.28 eV, and Li1s from 1:40.25 to 1:8.05. appeared in 55.88 eV. High resolution XPS spectra The UV-vis spectra of various PS-b-P2VP-b-PEO thin of N1 s binding energy and O1s binding energy in films are illustrated in Fig. 8b, c. The absorption peak at PS-b-P2VP-b-PEO with and without LiCl were 262 nm was assigned to pyridine groups and phenyl showninFig.10a,b.The N1 sbinding energy in groups of PS-b-P2VP-b-PEO [39]. Based on the previous PS-b-P2VP-b-PEO without LiCl was 398.88 eV, but study [24], the obvious change of the intensity was at- the binding energy in the thin film with LiCl was tributed to the coordination between Li ions and pyri- 399.48 eV. The O1s binding energy in PS-b-P2VP-b- dine groups. The intensities of absorption peak at PEO without LiCl was 532.78 eV, but the binding 262 nm with different samples were summarized in energy in the thin film with LiCl was 533.08 eV. Table 2. The intensities of absorption peak at 262 nm These shifts in binding energy were consequences of for PS-b-P2VP-b-PEO thin films with LiCl (Fig. 8c) were electron withdrawing effect caused by the coordin- weaker than the pure film (Fig. 8b). When the molar ra- ation between Li and PS-b-P2VP-b-PEO [40], valid- tio ([Li ]:[O + N]) was 1:40.25, 1:24.15, 1:16.1 and 1:8.05, ating the presence of Li element in the thin film the intensity of absorption peak at 262 nm decreased after Li ions were loaded. These results were essen- with the LiCl addition increasing (Fig. 8c), indicating tially identical to the results in Fig. 8. that more and more Li ions were coordinated with the P2VP blocks and PEO blocks. However, when the molar ratio ([Li ]:[O+N]) was 1:32.2, the absorption peak at 262 nm was stronger than the molar ratio 1:40.25. The reason should be that most of Li+ ions were loaded in PEO blocks but not P2VP blocks when the molar ratio was about 1:32.2, and the least LiCl was loaded in P2VP blocks at this molar ratio ([Li ]:[O + N] = 1:32.2) com- pared with other thin films with LiCl. Table 1 The data of I /I for different PS-b-P2VP-b-PEO/LiCl a f hybrids in LiCl-toluene [Li ]:[O + N] I /I a f 1:40.25 0.2522 1:32.2 0.3165 1:24.15 0.3670 1:16.1 0.4266 Fig. 9 XPS survey spectra of PS-b-P2VP-b-PEO thin film with LiCl-toluene 1:8.05 0.4908 Huang et al. Nanoscale Research Letters (2017) 12:491 Page 8 of 9 Fig. 10 High-resolution XPS spectra of (a) N1 s binding energy and (b) O1s binding energy in PS-b-P2VP-b-PEO with and without LiCl-toluene Conclusions of Guangdong province (Grant No. S2012040007725) and the Pearl River S&T Nova Program of Guangzhou (201710010144). In this study, we present a simple approach to fabri- cate ordered nanopatterns of ion/triblock copolymers Funding hybrids without post-processing. This work demon- National Natural Science Foundation of China (Grant No. 51273048 strated that toluene could be used as co-solvents for and 51,203,025). Natural Science Foundation of Guangdong province (Grant No. S2012040007725). LiCl in short time. An order-to-disorder transition Pearl River S&T Nova Program of Guangzhou (201710010144). was triggered by varying the addition of LiCl-toluene with ultrasonic treatment. And ordered microphase- Authors’ Contributions separated nanopatterns of cylindrical array and stripes HH, GY, and XZ designed the experiment and measurements. HH, BZ, MZ,and YZ executed theexperiments.HH, GY,XZ, BZ,HL, WL,MZ, were obtained. The mechanism of the morphological and YZ examined the written report. All authors read and approved transition was due to the LiCl loaded in different ion- the final manuscript. dissolving blocks. This rapid synthesis might boost future studies of ion/triblock copolymers hybrids be- Ethics Approval and Consent to Participate Not applicable. cause of the advantage of ultrasonic as compared to the conventional routes. Furthermore, this approach Consent for Publication has potential applications in developing ultra-small Not applicable. devices via techniques such as pattern transfer owing to its simplicity, effectiveness and low cost, especially Competing Interests The authors declare that they have no competing interests. regarding to fabrication time. Abbreviations Publisher’sNote + + [Li ]:[O + N]: the molar ratio of Li ions to the total number of oxygen atoms Springer Nature remains neutral with regard to jurisdictional claims in (O) in PEO block and the nitrogen atoms (N); AFM: Atomic force microscope; published maps and institutional affiliations. CETC: China Electronics Technology Group Corporation; diBCPs: diblock copolymers; DMF: N,N-Dimethylform amide; FT-IR: Fourier transform infrared; Received: 22 April 2017 Accepted: 1 August 2017 HRTEM: High resolution transmission electron microscopy; LiCl: lithium chloride; LiOH: Lithium hydroxide; PCL: poly(ε-caprolactone); PMMA: polymethyl methacrylate; PS: polystyrene; PS-b-P2VP-b-PEO: polystyrene-block-poly(2- References vinylpyridine)-block-poly(ethylene oxide); PVP: polyvinyl pyridine; triBCPs: triblock 1. Young WS, EppsIII TH (2009) Salt doping in PEO-containing block copolymers; UV-vis: Ultraviolet–visible; XPS: X-ray photoelectron spectroscopy copolymers: counterion and concentration effects. Macromolecules 42: 2672–2678 Acknowledgements 2. Ham SJ, Shin CH, Kim E, Ryu DY, Jeong U, Russell TP, Hawker CJ (2008) This work was supported by the National Natural Science Foundation of Microdomain orientation of PS-b-PMMA by controlled interfacial interactions. 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Nanoscale Research LettersSpringer Journals

Published: Aug 15, 2017

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