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.


Background
Recently, ion/block copolymers (BCPs) hybrids have become highly attractive materials due to their flexibility, process stability, self-assembling ability and novel features of inorganic components such as electronic, magnetic and optical properties [1][2][3]. Spatz and co-workers created fused silica substrates with nanopillars on both sides with 99.8% transmittance and 0.02% reflectance, which was helpful for many laser applications [4]. Black et al. fabricated densely packed silicon nanotextures with feature sizes smaller than 50 nm by block copolymer self-assembly to enhance the broadband antireflection of solar cells [5]. Morris et al. fabricated Si nanowire array by self-assembly of block copolymer with LiCl, which showed the possible application in the area of photonics and photoluminescence [6].
The addition of salts into the BCPs is one of effective way to obtain ordered nanopatterns. Researchers have found that polyethylene oxide (PEO) [17][18][19], polymethyl methacrylate (PMMA) [20], poly(ε-caprolactone) (PCL) [21] or polyvinyl pyridine (PVP) [22,23] are iondissolving blocks, and polystyrene (PS) [24] is a nonconducting block. Wang and co-workers suggested that the selection of metal ions to blocks was primarily due to the large solvation energy when the lithium salts associate with the polar PEO domains, leading to a large increase in the effective segregation strength with lithium salt loading [25,26].
In previous experiments [6,17,27], the co-solvents for salts are frequently used because of the solubility of salts and the efficiency of coordination between salts and BCPs. Russell et al. continuously stirred after the mixture of LiCl in tetrahydrofuran (THF) and polystyreneblock-poly(methyl methacrylate) (PS-b-PMMA) toluene solution with moderate heating until most of THF was evaporated and the solutions became clear. And they spent a great deal of time (about 24 h) on stir and post-treatment (solvent vapor annealing and thermal annealing) to obtain ordered microphase-separated nanostructure [17,28].
Herein, we demonstrated a simple and convenient approach to generate various ordered nanopatterns of ion/triBCPs hybrids by spin-coating method without any further treatments. Morphological and structural variations of PS-b-P2VP-b-PEO thin films with different salt concentrations were examined by adjusting various processing parameters. This work indicated that the coordination between PS-b-P2VP-b-PEO and LiCl-toluene could be accelerated by ultrasonic treatment for fabricating ordered nanopattern.

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 solution was immediately added to the PS-b-P2VP-b-PEO micellar solutions. Those mixtures were treated by different ways to trigger complexation between Li + ions and polymer chains. The resultant solutions were spincoated 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 Sensors, Bulgaria Ltd.) was used to investigate the morphological features of PS-b-P2VP-b-PEO thin films. High-resolution transmission electron microscopy (HRTEM) measurement was carried out on a JEM-2100HR (JEOL, Japan) operated at 200 kV accelerating voltage. Film samples for TEM were prepared onto carbon-coated copper grids. Those samples were exposed to I 2 vapor for certain time period. Fourier transform infrared (FT-IR) spectra were recorded with a Nicolet 6700 (Thermo, USA) spectrophotometer in the range of 4000-400 cm −1 with KBr plates. Ultraviolet-visible (UV-vis) spectra were obtained on a UV-2450(Shimadzu, Japan) spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB 250 (Thermo, USA) with Al Ka excitation.

Morphology of Pure PS-b-P2VP-b-PEO Thin Film
When 0.1 wt% PS-b-P2VP-b-PEO toluene solution was stirred for 24 h and spin-coated on silicon wafer, nanoporous patterns could be observed in Fig. 1. The average size of nanopores was about 22 nm.

Dispersion of LiCl in Toluene
Dispersions of LiCl in toluene with various aging times are shown in Fig. 2. Toluene was not a good solvent for LiCl. So suspension with unstable status could be seen after ultrasonic treatment (Fig. 2a). It was noticeable that little sedimentation phenomenon was observed when the aging time was 5 min (Fig. 2d). Therefore, the prepared suspension should be used immediately after ultrasonic treatment.

Effect of Methods to Trigger the Coordination between LiCl and Polymer Chains
Generally, stir and post-treatment are required for polymer solution containing metal salts in order to trigger the coordination between salts and polymer chains for fabrication of ordered nanostructure, which takes a lot of time [22,28]. And the ultrasound is the simple way to accelerate the coordination between metal ions and block copolymer [29][30][31]. In order to demonstrate the advantage of ultrasonic treatment in this work, different methods were used after the mix of LiCl-toluene and triblock copolymer solution when the molar ratio of Li + ions to the total number of oxygen atoms (O) in PEO block and the nitrogen atoms (N) was 1:32.2([Li + ]:[O + N] = 1:32.2). When the mixed solution was stirred (1500 rpm) for 30 min at room temperature and then spin-coated onto substrate, no distinct ordered structure was observed in Fig. 3a. When the mixed solution was stirred at 1500 rpm for 30 min at 75°C and then spin-coated onto substrate, disordered cylindrical microdomains appeared in Fig. 3b. When the mixed solution was placed in ultrasonic cleaner for 30 min at room temperature, microphase-separated nanopattern with cylindrical microdomain was obtained obviously in Fig. 3c after spin-coating. The energy provided by sound waves was able to disrupt the larger aggregates of the micelles. And the sound waves could further increase the diffusion rate of metal ions in the solution, so the loading of Li + ions in micelles were expected to happen much faster than the conventional stirring method. This result indicated that ultrasonic treatment was a useful method to improve the efficiency of coordination between Li + ions and polymer chains.   placed in ultrasonic cleaners for various times before spin-coating. When the time was 7.5 min (Fig. 4a), the nanoporous morphology was similar to the film in Fig. 1. Compared with the film in Fig. 1, the number and the average size of nanopores decreased, which indicated that Li + ions began to load in PS-b-P2VP-b-PEO polymer chains after 7.5 min. The Li + ions loaded in polymer chains would increase with the time increasing. Parts of nanopores were connected when the time increased to 15 min (Fig. 4b).

Effect of Time Scale
When the time was 22.5 min, the nanopattern exhibited a coexistence of nanostripes and cylinders (Fig.  4c). When the time was prolonged to 30 min, microphase-separation with cylindrical microdomains occurred obviously (Fig. 3c). As the time extended to 37.5 min, the coexistence of nanostripes and cylindrical microdomains appeared again (Fig. 4d). From above results, when the time was less than 30 min, the complexation between Li + ions and PSb-P2VP-b-PEO was accelerated by ultrasonic treatment so that more and more Li + ions were coordinated with PS-b-P2VP-b-PEO, resulting in transition of nanopattern from nanoporous array to cylindrical array. When the time was more than 30 min, the energy provided by sound waves would break the coordination of Li + ions and polymer chains so that disordered nanopattern was found instead of the cylindrical array. Therefore, the time of ultrasonic treatment should be controlled in appropriate range to obtain obvious microphase-separated nanopattern.

Effect of LiCl Content in PS-b-P2VP-b-PEO Thin Films
The addition of LiCl has significant effects on morphology since Li + ions could be loaded in P2VP and PEO blocks [17-19, 22, 23]. And the molar ratio ([Li + ]:[O + N]) was varied in our work (Fig. 5).
When the molar ratio was 1:40.25, the nanopattern of stripes was obtained (Fig. 5a). When the molar ratio decreased to 1:32.2, nanopattern with cylindrical microdomains could be seen in Fig. 3c. As the molar ratio was 1:24.15, a lot of nanopores were connected to show the tendency from nanopores pattern transform to nanostripes (Fig. 5b). When the molar ratio was 1:16.1, disordered nanopores become the overall morphology (Fig. 5c). The average size of holes was larger than the film in Fig. 1. As the molar ratio further decreased to 1:8.05, a few of nanopaores was observed in Fig. 5d. The average diameter of these pores was more than 40 nm. From above results, an order-to-disorder transition was shown in Fig. 5 by

Microdomains Location of Three Blocks in PS-b-P2VP-b-PEO Thin Films
In order to explore the microdomain location of the three blocks in PS-b-P2VP-b-PEO thin film under different conditions, those samples were exposed to I 2 vapor for certain period before TEM measurement.
The PS-b-P2VP-b-PEO thin film without LiCl exhibited an array of dark rings after the selective staining of P2VP blocks, indicating that the periphery of the hole  corresponded to P2VP blocks (Fig. 6a). Thus, the rest of the hole should match with PEO blocks. The continuous matrix was PS blocks. The average outer diameter of the dark rings was about 21 nm and the average inner diameter of the dark rings was about 16 nm.
When the molar ratio ([Li + ]:[O + N]) was 1:40.25 after I 2 selective staining, the nanopattern of stripes was obtained (Fig. 6b). The bright regions of the spheres were depressed in striated structure. The bright regions were PEO blocks and the rest of stripes were P2VP microdomains. Hence, the continuous matrix was PS blocks. Distinct dark particulates (of LiOH presumably) were observed in P2VP domains [32]. The average diameter of PEO domains was about 17 nm, which was similar to the average domain size of PEO blocks in Fig. 6a. And the P2VP domains transformed from dark rings to stripes. This result indicated that most of Li + ions were preferentially coordinated with P2VP blocks when the molar ratio was 1:40.25.
When the molar ratio ([Li + ]:[O + N]) decreased to 1:32.2, an array of dark rings could also be seen (Fig. 6c) after I 2 selective staining. The dark rings were P2VP microdomains and the bright regions were PEO blocks. The continuous matrix was PS blocks. The average outer diameter of dark rings was about 32 nm, and the average inner diameter of dark ring was about 26 nm. It was demonstrated that the cylindrical domains in Fig. 3c were core-shell structure. The outer shell was P2VP blocks and the core was PEO blocks. Compared with the film in Fig. 6a, the PEO microdomains were obviously swelled and the P2VP domains slightly increased. Compared with the film in Fig. 6b, this result indicated that more Li + ions were coordinated with PEO blocks with more LiCl in PS-b-P2VP-b-PEO thin film.
The difference of Fig. 6b, c can be explained as shown in Fig. 7. Because of the selectivity of toluene for three blocks, the nanostructure of PS-b-P2VP-b-PEO micelles in toluene was core-shell structure. Considering the sequence of the three blocks in PS-b-P2VP-b-PEO, PS blocks were the outer shell. The inner shell was P2VP domain and the core was PEO blocks.  and the PEO blocks effectively increased due to the increase of LiCl-toluene, resulting in the obvious swelling in PEO domains [32][33][34][35][36].

Analysis of Competitive Interactions of Li + ions with PEO and P2VP Blocks
It is noteworthy that the competitive interactions of Li + ions with both the PEO and P2VP blocks exist in PS-b-P2VP-b-PEO/LiCl hybrids [3]. The interaction between the Li + ions and the PEO blocks was characterized by FT-IR (Fig. 8a). The parameter of I a /I f , which was the ratio of the peak intensity corresponding to the associated C-O-C to the peak intensity of free C-O-C, was used to evaluate the coordination between the Li + ions and the PEO blocks (Table 1)  The UV-vis spectra of various PS-b-P2VP-b-PEO thin films are illustrated in Fig. 8b, c. The absorption peak at 262 nm was assigned to pyridine groups and phenyl groups of PS-b-P2VP-b-PEO [39]. Based on the previous study [24], the obvious change of the intensity was attributed to the coordination between Li + ions and pyridine groups. The intensities of absorption peak at 262 nm with different samples were summarized in Table 2. The intensities of absorption peak at 262 nm for PS-b-P2VP-b-PEO thin films with LiCl (Fig. 8c) were weaker than the pure film (Fig. 8b). When the molar ratio ([Li + ]:[O + N]) was 1:40.25, 1:24.15, 1:16.1 and 1:8.05, the intensity of absorption peak at 262 nm decreased with the LiCl addition increasing (Fig. 8c), indicating 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.  Fig. 10a, b. The N1 s binding energy in PS-b-P2VP-b-PEO without LiCl was 398.88 eV, but the binding energy in the thin film with LiCl was 399.48 eV. The O1s binding energy in PS-b-P2VP-b-PEO without LiCl was 532.78 eV, but the binding energy in the thin film with LiCl was 533.08 eV. These shifts in binding energy were consequences of electron withdrawing effect caused by the coordination between Li + and PS-b-P2VP-b-PEO [40], validating the presence of Li element in the thin film after Li + ions were loaded. These results were essentially identical to the results in Fig. 8.