Rapid Fabrication of Ordered Nanopatterns in PS‑b‑P2VP‑b‑PEO Triblock Copolymers Using LiCl Salt in Toluene and Ultrasonic Treatment

Abstract

We report a facile, one‑step method to generate highly ordered nanopatterns in the triblock copolymer polystyrene‑block‑poly(2‑vinylpyridine)-block‑poly(ethylene oxide) (PS‑b‑P2VP‑b‑PEO) by adding lithium chloride (LiCl) dissolved in toluene. Systematic variation of processing parameters—polymer solution treatment, ultrasonic exposure time, and the Li⁺/O+N molar ratio—reveals that LiCl loading in distinct microdomains drives the transition from nanoporous to cylindrical or stripe‑like morphologies. The technique eliminates lengthy post‑processing, offering a scalable route for nanofabrication.

Background

Ion/triblock copolymer (triBCP) hybrids have attracted attention for their self‑assembly, processability, and the unique electronic, magnetic, and optical properties conferred by inorganic ions. Recent advances demonstrate the creation of ultrafine nanostructures: fused silica substrates with nanopillars exhibiting 99.8 % transmittance and 0.02 % reflectance [4], silicon nanotextures < 50 nm for solar‑cell antireflection [5], and Si nanowire arrays fabricated by LiCl‑assisted block‑copolymer assembly for photonic applications [6]. Although diblock copolymers have been widely studied, triBCPs can form richer morphologies—core/shell spheres, tetragonal cylinders, and bicontinuous phases [7–15]—yet ion/triBCP hybrids remain underexplored.

Salt addition is a proven strategy to enhance block‑copolymer microphase separation. Poly(ethylene oxide) (PEO), polymethyl methacrylate (PMMA), poly(ε‑caprolactone) (PCL), and poly(2‑vinylpyridine) (PVP) act as ion‑dissolving blocks, whereas polystyrene (PS) serves as a non‑conducting block [17–24]. The high solvation energy of Li⁺ with polar domains amplifies segregation strength, enabling the formation of ordered nanostructures [25,26]. Conventional protocols often require prolonged stirring and annealing, especially when salts are dissolved in co‑solvents such as tetrahydrofuran (THF) [17,28].

In contrast, we present a simple spin‑coating process that bypasses extensive post‑processing. By dispersing LiCl directly in toluene and applying ultrasonic treatment, we achieve rapid coordination between Li⁺ and PS‑b‑P2VP‑b‑PEO, yielding well‑ordered nanopatterns within minutes.

Methods

Materials

PS‑b‑P2VP‑b‑PEO (Mn ≈ 45 kDa; block masses 16 kDa, 8.5 kDa; Mw/Mn = 1.05) was obtained from Polymer Source Inc. Anhydrous LiCl (95 % +, AR) was sourced from Tianjin Fuchen Chemical Reagents Factory. Toluene (99 + %), ethanol, and N,N‑dimethylformamide (DMF) were purchased from Tianjin Damao Chemical Co. Ltd. Silicon wafers were supplied by No.46 Research Institute of China Electronics Technology Group Corporation (CETC).

Sample Preparation

Silicon wafers were sequentially cleaned in DMF, ethanol, and deionized water under ultrasonication (30 min, room temperature). A 0.1 wt % PS‑b‑P2VP‑b‑PEO solution in toluene was stirred for 24 h at room temperature. LiCl was dispersed in toluene by ultrasonication (30 min, room temperature). Various volumes of LiCl‑toluene were then added to the polymer solution to achieve desired Li⁺/O+N ratios. The mixtures were subjected to one of the following treatments: (i) stirring at 1500 rpm for 30 min (ambient or 75 °C), (ii) ultrasonication for 30 min. After filtration, films were spin‑coated (3000 rpm, 1 min) and dried under nitrogen to remove residual solvent.

Characterization

Atomic force microscopy (AFM) in SCANASYST‑AIR mode (Bruker Inc.) examined surface morphology. High‑resolution TEM (JEM‑2100HR, JEOL) was performed on carbon‑coated grids; samples were stained with I₂ vapor. FT‑IR spectra (Nicolet 6700) covered 4000–400 cm⁻¹. UV‑vis absorption (Shimadzu UV‑2450) recorded 200–400 nm. X‑ray photoelectron spectroscopy (XPS, ESCALAB 250) used Al Kα excitation.

Results and Discussion

Morphology of Pure PS‑b‑P2VP‑b‑PEO Thin Film

Spin‑coating a 0.1 wt % PS‑b‑P2VP‑b‑PEO solution onto silicon produced a nanoporous array with an average pore diameter of ~22 nm (Figure 1).

AFM height image of a 0.1 wt % PS‑b‑P2VP‑b‑PEO film spin‑coated from toluene.

Dispersion of LiCl in Toluene

LiCl is sparingly soluble in toluene. Ultrasonic dispersion yields a suspension that remains unstable; sedimentation begins after ~5 min of aging (Figure 2). Consequently, the LiCl‑toluene mixture should be used immediately after sonication.

Dispersion of LiCl in toluene after ultrasonic treatment without aging (a), 1 min (b), 3 min (c), and 5 min (d).

Effect of Coordination Methods

We compared three coordination approaches at a Li⁺/(O+N) ratio of 1:32.2: (i) 30 min stirring at room temperature, (ii) 30 min stirring at 75 °C, and (iii) 30 min ultrasonication. Only the ultrasonic treatment produced well‑defined cylindrical microdomains (Figure 3), illustrating that acoustic energy disrupts micelle aggregates and accelerates Li⁺ diffusion.

AFM height images of PS‑b‑P2VP‑b‑PEO films spin‑coated from 0.1 wt % toluene with (a) 30 min stirring (ambient), (b) 30 min stirring (75 °C), and (c) 30 min ultrasonication.

Effect of Ultrasonic Treatment Time

Increasing ultrasonic exposure at 1:32.2 ratio transitions the morphology from nanoporous (7.5 min) to interconnected pores (15 min), to coexisting stripes and cylinders (22.5 min), to fully cylindrical domains (30 min), and finally to a disordered pattern (37.5 min). The optimal 30‑min window yields the most pronounced microphase separation (Figure 4).

AFM height images of PS‑b‑P2VP‑b‑PEO films spin‑coated after LiCl‑toluene treatment with (a) 7.5 min, (b) 15 min, (c) 22.5 min, and (d) 37.5 min ultrasonication.

Effect of LiCl Content

Varying the Li⁺/(O+N) ratio from 1:40.25 to 1:8.05 controls the pattern: stripes at the highest ratio, mixed stripes and cylinders at 1:32.2, connected pores at 1:24.15, disordered pores at 1:16.1, and sparse, > 40 nm pores at 1:8.05 (Figure 5). These transitions reflect progressive LiCl loading and altered segregation strength.

AFM height images of PS‑b‑P2VP‑b‑PEO films spin‑coated from 0.1 wt % with LiCl/(O+N) ratios of (a) 1:40.25, (b) 1:24.15, (c) 1:16.1, and (d) 1:8.05.

Microdomain Distribution

Selective I₂ staining of P2VP blocks and TEM imaging reveal the block arrangement. In LiCl‑free films, dark rings indicate P2VP perimeters with PEO cores and PS as the continuous matrix (Figure 6a). At 1:40.25 ratio, bright PEO cores appear within stripe‑like P2VP domains (Figure 6b). At 1:32.2 ratio, dark rings re‑emerge, now larger, signifying core‑shell cylinders where P2VP shells encapsulate swollen PEO cores (Figure 6c). These observations confirm that Li⁺ preferentially coordinates with P2VP at low salt loadings and with PEO as salt concentration increases.

TEM images of PS‑b‑P2VP‑b‑PEO films after I₂ staining: (a) no LiCl, (b) LiCl/(O+N) = 1:40.25, (c) LiCl/(O+N) = 1:32.2.

Competitive Interaction Analysis

FT‑IR spectra show the C–O–C stretch shifting from 1124 to 1111 cm⁻¹ as LiCl loading increases, with the I_a/I_f ratio rising from 1.2 to 1.6 (Table 1). UV‑vis absorption at 262 nm, characteristic of pyridine and phenyl groups, diminishes with higher LiCl content, confirming Li⁺ coordination to P2VP (Table 2). XPS reveals shifts in N 1s (398.88→399.48 eV) and O 1s (532.78→533.08 eV) binding energies upon LiCl addition, indicating electron withdrawal by Li⁺.

(a) FT‑IR of pure and LiCl‑treated films; (b) UV‑vis of pure film; (c) UV‑vis of LiCl‑treated film.

Conclusions

We demonstrate a rapid, scalable route to fabricate ordered nanopatterns in PS‑b‑P2VP‑b‑PEO by adding LiCl dissolved in toluene and applying ultrasonic treatment. The method eliminates lengthy annealing steps, achieves both cylindrical and stripe morphologies, and provides precise control via salt loading. This approach holds promise for nanofabrication technologies, including pattern transfer for ultra‑small devices.

Abbreviations

[Li⁺]:[O + N]

molar ratio of Li⁺ ions to the total number of oxygen atoms in PEO and nitrogen atoms in P2VP

AFM

Atomic force microscope

CETC

China Electronics Technology Group Corporation

diBCPs

diblock copolymers

DMF

N,N‑dimethylformamide

FT‑IR

Fourier transform infrared spectroscopy

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‑vinylpyridine)-block‑poly(ethylene oxide)

PVP

poly(2‑vinylpyridine)

triBCPs

triblock copolymers

UV‑vis

Ultraviolet–visible spectroscopy

XPS

X‑ray photoelectron spectroscopy