Orientation control of sub-10-nm lamellar structures in amphiphilic copolymer films via water annealing

Abstract

The fabrication of polymer lamellar films with sub-10 nm periodicity and perpendicularly aligned structures is essential for applications in lithography and separation membranes. In this study, annealing an amphiphilic statistical copolymer, poly(N-octadecyl acrylamide-stat-N-(2-hydroxyethyl acrylamide)), in water induces the formation of perpendicularly oriented lamellae, where the lamellar planes align perpendicular to the substrate. The statistical copolymer with a 1 : 1 comonomer composition is synthesized via free-radical polymerization, and thin films are prepared by spin coating. The nanoscale phase-separated structures formed by annealing in water are analyzed. Structural analysis using two-dimensional grazing-incidence small-angle X-ray scattering reveals that the annealed films exhibit both parallel and perpendicular lamellae with the periodicity of 4.2–5.7 nm, and the formation of perpendicular lamellae was strongly influenced by direct contact with water. Furthermore, depth-dependent XRD studies using low-energy grazing-incidence small-angle X-ray scattering show that parallel lamellae form at the topmost film surface, while perpendicular lamellae develop in the interior. Water contact angle and Fourier-transform infrared spectra measurements further confirm this structure. Exposure to water induces alignment of the hydrophobic octadecyl side chains parallel to the substrate, promoting the formation of perpendicular lamellae in the film interior. In addition, experiments with thin films sandwiched between hydrophilic or hydrophobic glass substrates demonstrate that the nature of the interface plays a crucial role in determining the lamellar alignment. These findings emphasize the importance of interface modification in controlling thin-film structures and provide insights for designing functional materials with tailored properties.

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Introduction

Nanoscale periodic structures formed by the self-assembly of polymers have potential applications in lithography,^1–4 separation membranes,⁵ biomedical materials,^6–8 and polymer electrolytes.^9–13 The fabrication of these structures relies on the microphase separation of AB diblock copolymers, which consist of two immiscible homopolymers, A and B, connected by a covalent bond.^14–17 This separation leads to well-ordered microphase-separated structures, whose formation depends on the Flory–Huggins segment interaction parameter (χ), degree of polymerization (N), and volume fraction of one block (φ_A or φ_B). Microphase separation occurs when χN is sufficiently large, with the morphology transitioning from random to spherical, cylindrical, and finally lamellar structures as the φ_A increases from 0% to 50%. The domain size of the self-assembled structure is determined by N^2/3χ^1/6.¹⁸ Therefore, the synthesis of diblock copolymers with a low N and high χ is essential for achieving nanoscale periodic structures.

To facilitate the formation of nanoscale periodic structures, strategies leveraging phase separation between the main and side chains in homopolymers^3,19–21 and/or immiscible side chains in random/statistical copolymers,^22–29 and brush copolymers^30–32 have been developed. For example, random/statistical copolymers can form nanoscale structures with periodicities of several nanometers through nanophase separation between hydrophobic alkyl chains and hydrophilic groups.^{22–25,27–29,33,34} This approach offers the advantage that the domain size is less affected by the molecular weight and molecular weight distribution, while nanoscale control can be achieved by adjusting the side chain length.^20,35 In addition, random/statistical copolymers are easier to synthesize and provide a broader selection of monomers compared with diblock copolymers.

Recently, a unique structural transition has been observed in the thin films of such statistical copolymers. Thin films of the statistical copolymer [p(ODA50-stat-HEAm50)] (Fig. 1), composed of N-octadecyl acrylamide (ODA) and N-(2-hydroxyethyl acrylamide) (HEAm), form two types of self assembled lamellae depending on the thermal annealing temperature.²⁸ Specifically, annealing at 50 °C above the glass transition temperature (T_g) results in the formation of “side-chain-segregated lamellae”, while annealing at 10 °C above the T_g leads to “side-chain-mixed lamellae”. In particular, side-chain-segregated lamellae, consisting of alternating layers of hydrophilic and hydrophobic side chains, are promising for applications in patterned resists for silicon technology and ion separation membranes. However, these copolymers predominantly form “parallel lamellae”, where the lamellar planes align parallel to the substrate (Fig. 2a).^{25,27–29,36,37} In contrast, applications such as lithography and electrolyte membranes often require “perpendicular lamellae”, where the lamellar planes are oriented perpendicularly to the substrate (Fig. 2b).

Fig. 1 Chemical structure of the statistical copolymer p(ODA50-stat-HEAm50).

Fig. 2 Schematic representation of parallel (a) and perpendicular lamellae (b). Red and gray lines denote the octadecyl sides-chain and lamellar plane composed of main chain, respectively, while light-red and blue regions indicate the hydrophobic and hydrophilic layers, respectively.

In block copolymers, techniques such as substrate surface treatment^38,39 and solvent vapor annealing^40–43 have been developed to align microphase-separated structures. However, orientation control methods for the self-assembled structures of homopolymers and random/statistical copolymers remain limited. Seki et al. reported that a side-chain liquid crystalline polymer with mesogens exhibiting homeotropic alignment (perpendicular to the substrate) in its native state could be modified to align parallel to the substrate by coating its surface with a free-surface-active polymer.^44–48 Furthermore, they employed two-dimensional grazing-incidence small-angle X-ray scattering (2D GI-SAXS) with low-energy X-rays (tender X-rays) to investigate the orientation of mesogens in the depth direction of the film. Tender X-rays (1–4 keV) allow for controlled penetration depth from a few to a hundred nanometers by varying the incident angle.^49,50 Using tender X-rays, Seki et al. revealed that parallel-stacked lamellae formed at the air–polymer interface, while perpendicularly stacked lamellae developed within the film near the solid–polymer interface, enabling the elucidation of depth-dependent structures in thin films. Nevertheless, the aforementioned orientation control relies on mesogen alignment in side-chain liquid crystal polymers, underscoring the need for alternative strategies to control the orientation of periodic structures formed through nanophase separation in random/statistical copolymers composed of non-mesogenic polymers.

The statistical copolymer p(ODA50-stat-HEAm50) forms parallel lamellae in thin films because of differences in interfacial energy compared with the bulk states. In this study, thin films of p(ODA50-stat-HEAm50) were found to form perpendicular lamellae upon annealing in water. Structural analysis using 2D GI-SAXS, 2D tender X-ray GI-SAXS, and Fourier-transform infrared spectroscopy reveals that modifying the polymer–air surface to a polymer/hydrophilic liquid surface induces the alignment of alkyl side chains parallel to the substrate, resulting in the formation of perpendicular lamellae.

Materials and method

Materials

Octadecyl amine (>85%, Tokyo Chemical Industry Co., Ltd. (TCI)), acryloyl chloride (>98.0%, TCI), triethylamine (>99.0%, TCI), trichloro(octyl)silane (97%, Sigma-Aldrich Co. LLC), 2-hydroxyethyl acrylamide (HEAm, >98.0% TCI), chloroform (guaranteed reagent grade, Kanto Chemical Co., Inc.), toluene (super dehydrated grade, FUJIFILM Wako Pure Chemical Corp.), N,N-Dimethylformamide (super dehydrated grade, FUJIFILM Wako Pure Chemical Corp.), acetonitrile (guaranteed reagent grade, Nacalai Tesque Inc.), acetone (guaranteed reagent grade, Nacalai Tesque Inc.), and isopropyl alcohol (guaranteed reagent grade, Nacalai Tesque Inc.), were used as received. 2,2′-Azobisisobutyronitrile (AIBN, FUJIFILM Wako Pure Chemical Corp.) was recrystallized from ethanol. n-Tetradecane (>99.0%, Kanto Chemical Co., Inc.)

Synthesis

Monomers and statistical copolymers were synthesized following previously reported methods.²⁸ Briefly, ODA was prepared by reacting octadecylamine with acryloyl chloride, followed by recrystallization from methanol. The statistical copolymer [p(ODA50-stat-HEAm50)] was synthesized via free-radical copolymerization. ODA and HEAm were dissolved in a mixture of toluene and N,N-dimethylformamide (volume ratio of 4 : 1) at a molar ratio of 1 : 1. AIBN, corresponding to 1 mol% of the total monomers, was added to this solution, resulting in a final concentration of 0.2 M. All mixing procedures were conducted in a nitrogen atmosphere within a glovebox. The polymerization reaction proceeded at 60 °C for 12 h. The copolymer was purified by reprecipitation from chloroform into acetonitrile three times, yielding a white polymer as the final product.

The molar ratio of ODA to HEAm was determined from the integral ratio of the methyl protons of ODA (approximately 0.8 ppm) to the ethylene protons of HEAm (2.7–4.2 ppm) in the ¹H NMR spectrum (ECA-500, JEOL). The number-average molecular weight (M_n) and polydispersity index (M_w/M_n) of the copolymers were determined by gel permeation chromatography (Shodex GPC-101, Showa Denko K.K.) using three columns (TSKgel SuperHZ2000, TSKgel SuperHZ3000, and TSKgel SuperHZ4000, Tosoh Corp.) and a differential refractometer (Shodex RI-71S, Showa Denko K.K.). Measurements were performed at 40 °C using tetrahydrofuran as the eluent at a flow rate of 0.5 mL min⁻¹. The thermal properties of the copolymers were evaluated using differential scanning calorimetry (DSC8231, Rigaku Corp.). Heating and cooling rates were set to 10 and −10 °C min⁻¹, respectively, within a temperature range of −40 to 200 °C.

Thin-film preparation

Silicon and glass substrates were ultrasonically cleaned with acetone and isopropyl alcohol, with the cleaning process repeated twice. Then, the substrates were treated with UV–O₃ (PL16-110, SEN Lights Corp.) to ensure a clean surface. To prepare hydrophobic substrates, the UV–O₃ treated substrates were immersed in a trichloro(octyl)silane/chloroform solution (approximately 1 × 10⁻⁵ M) at 19–22 °C for 1 d. Thin films were prepared using the spin-coating method with a chloroform polymer solution at a concentration of 1 or 5 wt%. The spin-coating process involved an initial spin at 1000 rpm for 5 s, followed by a second spin at 2500 rpm for 60 s. Subsequently, the substrates were immersed in water and annealed at 60 or 90 °C, with these temperatures selected based on a previous report.²⁹

To investigate the polymer orientation at the solid–polymer interface, thin films were prepared on hydrophobic glass substrates. The film surface was then covered with either a hydrophobic or hydrophilic glass substrate and secured with clips. The water contact angles of the hydrophobically treated silicon and glass substrates were 100° and 105°, respectively. In contrast, the hydrophilic glass substrate showed a contact angle of 22°.

Measurement

Surface profiler

The film thickness was determined using a surface profilometer (Dektak-XT, Bruker Corp.) under ambient conditions (20 °C, 60% relative humidity). Measurements were conducted with a stylus force of 0.03 mg and scan speed of 0.10 mm s⁻¹. The thickness was determined as the average of three different locations on the thin film.

Two-dimensional grazing-incidence small-angle X-ray scattering

2D GI-SAXS measurements of polymer thin films on a solid substrate were performed using a small-angle X-ray scattering instrument (Nanoviewer, Rigaku corp.) with a Cu Kα (λ = 0.1542 nm) X-ray source and 2D detector (PILATUS 100 K, Dectrics AG). The incidence angle was approximately 0.15°, and the sample-to-detector distance was 0.37 m. Scattering angles were calibrated using silver behenate as a reference standard.

Two-dimensional tender X-ray grazing-incidence small-angle X-ray scattering

2D GI-SAXS measurements using tender X-rays were conducted at BL-15A2 of the Photon Factory, High Energy Accelerator Research Organization (KEK) in Tsukuba. The X-ray energy was set to 2.40 keV (0.517 nm) and 2.47 keV (0.502 nm), and the sample-to-detector distance was 0.831 m. The X-ray incidence angles ranged from 0.40° to 0.70°. A PILATUS3 2M detector was employed to acquire the 2D scattering patterns.

Contact angle measurement

The water contact angles of the thin films fabricated on silicon or glass substrates were measured using a contact angle meter (NICK, LSE-MEI). A 1.0-μl water droplet was placed on the thin film, and the contact angle was measured 10 s after deposition. Measurements were performed at 19 °C and 56% relative humidity.

Atomic force microscopy

Surface observations of the static copolymer thin films were conducted using atomic force microscopy (AFM; AFM100plus, HITACHI Corp.). A cantilever with a spring constant of 26 N m⁻¹ and resonance frequency of 300 kHz (OMCL-AC160TS-R3, OLYMPUS Corp.) was employed. The measurements were performed with a scan range of 1000 μm and scan frequency of 1.1 Hz.

Second-generation of multiple-angle incidence resolution spectrometry⁵¹

The orientation of the polymer films was evaluated using second-generation multiple-angle incidence resolution spectrometry (MAIRS2) measurements. These measurements were performed by attaching an automatic analysis electric stage (TN10-3001, Thermo Fisher Scientific Inc.) to an infrared spectrometer (Nicolet iS50-FT-IR, Thermo Fisher Scientific Inc.), which was equipped with a mercury cadmium–telluride (MCT) detector and infrared polarizer. The polarization angle was automatically varied in steps of 15° from 0° to 90° (s- to p-polarization). During the measurement, the incident angle was maintained at 45° with respect to the substrate plane. All measurements were conducted under dry air conditions using an AT-20H instrument (Airtech Corp.). The MAIRS2 spectra in both the in-plane (IP) and out-of-plane (OP) directions were automatically analyzed using pMAIRS II automatic analysis measurement software (TN60-2000, Thermo Fisher Scientific Inc.).

Results and discussion

Characterization of the p(ODA50-stat-HEAm50) thin film

The synthesized copolymers were obtained as white powders. The comonomer composition of ODA and HEAm was determined to be 1 : 1 (mol) with M_n = 6600 and M_w/M_n= 2.15. It has been previously reported that this copolymer exhibited a melting temperature (T_M) for the crystallized octadecyl side chains at 22 °C and T_g at 104 °C. Furthermore, the T_g decreased to approximately 50 °C in the hydrated state.^28,29 The obtained copolymer formed a uniform thin film on a solid substrate. The film thicknesses were determined to be 910 ± 68 and 130 ± 19 nm when prepared from 5 and 1 wt% solution, respectively.

Structural analysis of annealed films in water using two-dimensional grazing-incidence small-angle X-ray scattering

It has been previously reported that thermal or humid annealing induces nanophase separation in p(ODA50-stat-HEAm50), leading to the formation of a lamellar structure.^28,29 This phase separation occurred between the hydrophilic main and hydrophobic octadecyl side chains or between the hydrophilic and hydrophobic side chains. The resulting lamellar structure, known as a “parallel lamellae”, featured main chains aligned parallel to the substrate, with side chains oriented perpendicular to the main chains. This orientation arose as the hydrophobic octadecyl side chains aligned perpendicularly to the air to reduce the excluded volume.⁵² Therefore, replacing the solid–air interface with a solid–liquid and/or solid–solid interface allowed control over the alignment of the alkyl side chains through the excluded volume effect. In fact, Seki et al. demonstrated that mesogens, which typically adopt a homeotropic alignment at free surfaces, exhibited a random planar orientation owing to the excluded volume effect when a polymer with a low surface free energy was introduced.^44,45,47,48 Therefore, we annealed at the solid–liquid interface by immersing the thin film in water, and the nanophase-separated structure formed under these conditions was subsequently analyzed.

Fig. 3 shows the 2D GI-SAXS images of p(ODA50-stat-HEAm50) thin films annealed in water at 60 and 90 °C for 3 h, along with the one-dimensional (1D) profiles extracted in the OP and IP directions. The 2D images of the thin films annealed in water at both 60 and 90 °C exhibited spots in both the OP and IP directions around at q = 1.1 nm⁻¹ (Fig. 3a and c). The analysis of the 1D profiles revealed that the thin film annealed in water at 60 °C showed a broad scattering peak at q_z = 1.2 nm⁻¹ in the OP direction (Fig. 3b). This q_z value matched with that of the side-chain segregated lamellae, where hydrophobic ODA and hydrophilic HEAm chains aligned in opposite directions through the polymer main chain, as reported previously.^28,29 Furthermore, a sharp scattering peak was observed at q_y = 1.1 nm⁻¹ in the IP direction, which was close to the q_z value of parallel lamellae stacked in the OP direction (Fig. 3b). These results suggested that both parallel and perpendicular lamellae were formed within the thin film. The periodicity (d) of the lamellae was 5.2 nm (q_z = 1.2 nm⁻¹) and 5.7 nm (q_y = 1.1 nm⁻¹) using d = 2π/q. The OP scattering was quite broad in the OP direction compared with the IP peak, but sharp in the azimuthal direction (no arcs). Using the Laue function, the number of lamellar layers (repeating units) was estimated to be 2 to 3 based on the broadening of the OP direction (Fig. S1, ESI†).⁵³ This indicate that the OP scattering units are very thin and only a few molecular layers, or are formed in minor domains. The IP scatterings are observed in a spot-like figure with a vertical arc, suggesting that adequate scattering structures were formed in the IP direction with some fluctuations. Therefore, the perpendicular lamellae were preferentially induced in the thin film annealed in water at 60 °C. On the other hand, the thin film annealed in water at 90 °C exhibited scattering peaks at q_z = 1.3 nm⁻¹ (d = 4.8 nm) in the OP direction and q_y = 1.2 nm⁻¹ (d = 5.2 nm) in the IP direction, indicating that both parallel and perpendicular lamellae were also formed (Fig. 3d). Unlike at 60 °C, the scattering in the OP direction was observed strongly and in spots. This indicates that the scattering structure in the OP direction is formed with sufficient thickness and regularity. Notably, IP scattering was not observed in the thin films annealed under humid conditions,²⁹ indicating the absence of perpendicular lamellae. These results suggested that complete contact between the thin-film surface and liquid water was crucial for the formation of perpendicular lamellae. This phenomenon was similar to that reported by Seki et al.^44,45,47,48 and was attributed to the excluded volume effect of octadecyl side chains at the solid–liquid interface (free interface).⁵² In order to evaluate the surface orientation structure in these films, depth analysis was performed.

Fig. 3 2D GI-SAXS images [(a) and (c)], and 1D profiles extracted from the out-of-plane and in-plane directions of the 2D images [(b) and (d)] for p(ODA50-stat-HEAm50) thin films on silicon substrate subjected to water annealing at 60 °C or 90 °C for 3 h.

Analysis of depth-dependent self-assembled structures using two-dimensional tender X-rays grazing-incidence small-angle X-ray scattering

The depth-dependent structures of the thin films were evaluated using 2D tender X-ray GI-SAXS. The X-ray penetration depth into the sample (Λ) is given by:⁴⁹

where α_i and α_c represent the incident and critical angles of total reflection, respectively, and β is the imaginary part of the complex refractive index. The α_c was calculated from and estimated to be 0.51° (2.40 keV) and 0.50° (2.47 keV), where δ is the real part of the complex refractive index.

The δ and β of the refractive index are given by:

where r_e is the classical electron radius, N_A is Avogadro's number, ρ_M is the mass density, w_z is the fraction of element Z, A_Z is the relative atomic mass, f_0z is nonresonant term of the atomic scattering factor corresponding to the atomic number, and and represent the real and imaginary parts of the anomalous dispersion for the incident X-ray energy E, respectively.

At 2.40 keV, the δ and β were calculated to be 4.1 × 10⁻⁵ and 7.8 × 10⁻⁷, respectively. At 2.47 keV, the δ and β were calculated to be 3.8 × 10⁻⁵ and 7.4 × 10⁻⁷, respectively. Fig. 4 shows the 2D scattering images of thin films annealed in water at 60 and 90 °C for 3 h, with measurement performed at an α_i ranging from 0.40° to 0.70°. Fig. 5 presents the 1D profiles extracted from the IP directions of the 2D images. For the thin film annealed in water at 60 °C, a spot-like scattering corresponding to parallel and perpendicular lamellae was observed in the OP and IP direction at α_i = 0.40°, corresponding to Λ = 7.3 nm (Fig. 4a and Fig. S2a, ESI†). This indicates that the broad and weak scattering in the OP direction (Fig. 3a and b) occurs in the surface layer of several tens of nanometers, and that the parallel lamellae form at the topmost surface. As the α_i increased to 0.70° (Λ = 440 nm), spot-like scattering intensity in IP direction became stronger (Fig. 4d), indicating the formation of perpendicular lamellae inside the film. Furthermore, the 1D profiles in the IP direction extracted from the 2D images revealed that the scattering intensity at q_y = 1.3 nm⁻¹ (d = 4.8 nm) in the IP direction increased with an increase in Λ (Fig. 5a). Similarly, the 1D profiles obtained by azimuthal integration also showed that the in-plane scattering intensity increased with increasing α_i (Fig. S3, ESI†). These results suggest that parallel lamellae were formed in only a few layers at the topmost film surface, while perpendicular lamellae dominated the film interior. For the thin film annealed in water at 90 °C, spot-like scattering was observed only in the OP direction at α_i = 0.40° (Λ = 7.6 nm), and arc-shaped scattering appeared as the α_i increased (Fig. 4e–h and Fig. S2b, ESI†). The arc width is narrow in the scattering vector direction, indicating that sufficient scattering repeating units are present even in the surface region of several tens of nanometers. The Deby-Sherrer ring in the GI-SAXS images is much apparent in the film prepared by annealing at 90 °C than 60 °C. This is because the former film was kept in ambient condition longer than the reorientation time (see below), whereas the latter was measured just after being taking out from water. On the other hand, the presence of the scatterings indicate that the lamellar formation was independent of the annealing temperature (60 or 90 °C). Based on the 2D tender X-ray GI-SAXS measurement results, we proposed the following mechanism for the lamellar orientation (Fig. 6): when the thin films were in contact with water, the hydrophobic octadecyl side chains aligned parallel to the substrate, exposing the hydrophilic amide group, which promoted the formation of perpendicular lamellae (Fig. 6a). Upon exposure to air after taking out from water, water desorbed from the polymer chain at the thin-film surface, rendering the film–air interface hydrophobic. Consequently, the hydrophobic octadecyl side chains at the mobile topmost surface regions⁵⁴ were relaxed and reoriented perpendicular to the substrate to reduce the excluded volume,⁵² leading to the formation of parallel lamellae on the film topmost surface (Fig. 6b). We have reported that water desorbs from the film within a few minutes.²⁹ We characterized the reorientation dynamics by measuring the time dependency of water contact angle. The contact angle measured immediately after water annealing was 60°. Then the value increased with the duration of exposure of the film to ambient conditions at room temperature, reaching a saturated value of approximately 80° after 10 hours (Fig. S4, ESI†). The result suggests the reorientation occurred just after desorption of water. It should be noted that, as seen in the two-dimensional images, in addition to the spots in the OP and IP directions, Debye–Sherrer ring is also observed. This suggests that not only the parallel and perpendicular lamellae, but also lamellae with planes tilted in random directions are also present within the film. However, the OP and IP peak intensity are stronger than Debye–Scherrer ring, we believe that the film was dominated by the parallel and perpendicular lamellar structures. A careful observation of 1D intensity profiles (Fig. S3, ESI†) showed that OP intensity increased with increase α_i = 0.40° to 0.50° and then almost unchanged. Therefore, we concluded the depth of reoriented layer present within Λ = 7.6 nm to 20 nm at the surface, and perpendicular lamellae were preferentially formed inside the film under both annealing conditions. This depth match with previous report from Seki et al.,⁴⁶ and the domain size of 10 to 15 nm (Fig. S1, ESI†).

Fig. 4 2D GI-SAXS images of p(ODA50-stat-HEAm50) thin films annealed in water at 60 (a)–(d) and 90 °C (e)–(h) for 3 h. The measurements were performed at incidence angles ranging from 0.40° to 0.70° using tender X-rays (wavelength = 0.52 or 0.50 nm). A square feature observed at q_y = −1.1 nm⁻¹ in the 2D images is due to a defect in the module of the 2D detector.

Fig. 5 1D intensity profiles in the in-plane direction, extracted from the 2D images of p(ODA50-stat-HEAm50) thin films annealed in water at 60 °C (a) and 90 °C (b) for 3 hours.

Fig. 6 Schematic illustration of the mechanism for the formation of perpendicular and parallel lamellae. Red and gray lines represent the octadecyl side-chain and lamellar plane composed of main chain, respectively. Light-red and blue regions denote the hydrophobic and hydrophilic layers, respectively.

Focusing on the IP scattering in the 2D images of the thin film annealed at 90 °C, two scattering peaks were observed (Fig. 4e–h). At the shallowest α_i = 0.40° (Λ = 7.6 nm), a scattering peak was observed at q_y = 1.5 nm⁻¹ (d = 4.2 nm) (Fig. 4e). As the α_i increased to 0.50° (Λ = 40 nm), another scattering peak appeared at q_y = 1.3 nm⁻¹ (d = 4.8 nm), alongside the peak at q_y = 1.5 nm⁻¹ (Fig. 4h). However, at the deepest α_I = 0.70° (Λ = 460 nm), the peak intensity at q_y = 1.5 nm⁻¹ decreased, and q_y = 1.3 nm⁻¹ became more prominent (Fig. 4h and 5b). The scattering at 1.5 nm⁻¹ match with that observed in the previously reported side-chain mixed lamellae (q_mix), while the scattering at 1.3 nm⁻¹ coincidence with the side-chain-segregated lamellae (q_seg).^28,29 Therefore, at the film surface region, the copolymer exclusively formed side-chain-mixed lamellae, whereas the film interior was dominated by side-chain-segregated lamellae (Fig. S5, ESI†). It has been reported that when a large amount of water adsorbs onto polymer chains, the formation of side-chain-segregated lamellae becomes more favorable, whereas the desorption of water results in the formation of side-chain-mixed lamellae. This is because the adsorption/desorption of water onto the hydrophilic groups increases/decreases the phase-separation force within the polymer chains, leading to the formation of two types of lamellar structures with different phase-separation forces.²⁹ Therefore, at the interface where water is desorbed more easily, side-chain-mixed lamellae are formed. This result is consistent with the formation of parallel lamellae near the interface. Similar results were observed in the OP scattering analysis, where an unknown broad peak appeared at q_z = 1.9 nm⁻¹ (Fig. S6, ESI†).

Water contact angle of the thin film surface

The surface properties of thin films annealed under humid conditions and in water at 60 and 90 °C for 3 h were evaluated by measuring their water contact angles. The water contact angle was 103° for the thin films treated by humid annealing at 60 °C and 101° at 90 °C (Fig. 7a and b). As previously reported, p(ODA50-stat-HEAm50) forms parallel lamellae, with hydrophobic octadecyl side chains aligned perpendicular to the substrate to reduce the excluded volume, resulting in a large water contact angle.²⁹ In contrast, the water contact angles of the thin films annealed in water at 60 and 90 °C were 82° and 87°, respectively (Fig. 7c and d). The decrease in the contact angles of the water-annealed films compared with those of the humidity-annealed films suggested the exposure of hydrophilic amide groups at the surface of the water-annealed films, supporting the proposed mechanism for perpendicular lamellae formation (Fig. S7, ESI†). Indeed, the contact angle of tetradecane was 20° on the surface of the film annealed in water at 60 °C for 3 h, whereas it showed a larger value of 33° on the film annealed under humid condition for 3 h.

Fig. 7 Water contact angles of p(ODA50-stat-HEAm50) thin films annealed at 60 (a) and 90 °C (b) under humid conditions, and thin films annealed in water at 60 (c) and 90 °C (d).

Atomic force microscopy observation

In this section, we focus on the thin film annealed at 60 °C in water or humid condition, as it predominantly forms side-chain-segregated lamellae. The surface morphologies of p(ODA50-stat-HEAm50) thin films annealed in water and under humid conditions were examined using AFM. Fig. 8 shows the height images of the thin films annealed at 60 °C for 3 h in water or under humid conditions. The surface of the thin film annealed under humid conditions exhibited angular domains of approximately 100 nm in size (Fig. 8a), while the thin film annealed in water showed smaller domains, a few tens of nanometers in size (Fig. 8b). Upon humid annealing, the copolymer thin films formed highly ordered parallel lamellae, resulting in the densely packed domains of octadecyl side chains oriented perpendicular to the substrate on the film surface (Fig. S8a, ESI†). In contrast, water-annealing caused the octadecyl side chains to align parallel to the substrate because of their interactions with water. Subsequent exposure to air induced the reorientation of these side chains perpendicular to the substrate. As a result, the packing density of the octadecyl side chains on the film surface was lower for the water-annealed thin films than for those annealed under humid conditions (Fig. S8b, ESI†). Consequently, the water-annealed film surfaces exhibited smaller domains compared with those of the films annealed under humid conditions. The presence of densely packed alkyl side chains on the surface of the humid-annealed film is consistent with the larger water contact angle observed for the humid-annealed film compared to the water-annealed film.

Fig. 8 AFM height images of p(ODA50-stat-HEAm50) thin films annealed at 60 °C for 3 h under humid conditions (a) and in water (b). The surface roughness (RMS) of the films were 5.2 nm (a) and 5.0 nm (b), respectively.

Second-generation of multiple-angle incidence resolution spectrometry measurements

The structure of the self-assembled lamellae was studied using MAIRS2. The molecular orientation was determined from the band positions in the MAIRS2 spectrum, as well as the relative intensities of the IP and OP vibrational modes.⁵¹ Generally, the strongest absorbance occurred when the electric field vector of the infrared light was aligned with the orientation of the molecular vibration. Fig. 9 shows the MAIRS2 spectra for the OP and IP directions of copolymer thin films annealed under humid conditions and in water at 60 °C for 3 h. The CH₂ vibrations at approximately 2917 (v_s) and 2850 cm⁻¹ (v_as) were particularly useful for characterizing the orientation of the alkyl side chains.⁵⁵ For example, when the alkyl chains were oriented perpendicular to the substrate, the absorption in the IP direction (2A_IP) became stronger than that in the OP direction (A_OP), and vice versa. The thin film annealed under humid conditions at 60 °C showed absorption in both the IP and OP directions (Fig. 9a). The calculated ratio of IP absorption to total absorption, r_AIP= 2A_IP/(2A_IP + A_OP), based on the absorption intensities in both the OP and IP directions, was 0.83, supporting the perpendicular orientation of the alkyl side chains relative to the substrate. On the other hand, r_AIP decreased to 0.69 for the thin film annealed at 60 °C in water (Fig. 9b). This decrease suggested an increase in the number of parallel alkyl side chains in the water-annealed films. These results aligned with the findings from tender X-ray GI-SAXS, which indicated the formation of perpendicular lamellae, with the alkyl side chains aligning parallel to the substrate.

Fig. 9 MAIRS spectra of p(ODA50-stat-HEAm50) thin films annealed under humid condition (a) and in water (b) at 60 °C for 3 h, focusing on the alkyl regions. The red and blue lines represent the in-plane and out-of-plane spectra, respectively.

Thin films covered with hydrophilic or hydrophobic glass substrates

To confirm that the formation of perpendicular lamellae was due to the contact of the polymer surface with a hydrophilic surface, polymer thin films were prepared on glass substrates and sandwiched between hydrophobic or hydrophilic glass substrates, then annealed in water at 60 °C for 3 h.

Fig. 10 shows the 2D images and 1D GI-SAXS profiles of the polymer thin films sandwiched between hydrophobic and hydrophilic glass substrates. The polymer thin film sandwiched between hydrophobic glass substrates exhibited strong scattering in the OP direction, suggesting the formation of parallel lamellae (Fig. 10a and b). In contrast, films covered with a hydrophilic substrate showed stronger scattering in the IP direction than in the OP direction, similar to the results for films in direct contact with water (Fig. 10c and d). These results confirmed that changing the air–film interface with a hydrophilic (liquid or solid)–film interface was crucial for inducing perpendicular lamellae. It should be mentioned that because the thin film is not perfectly adhered to the glass substrate, water vapor can penetrate through the gap (creating a humid-annealing condition), which likely disrupts the perpendicular orientation.

Fig. 10 2D GI-SAXS images [(a) and (c)], and 1D profiles extracted from the out-of-plane and in-plane directions of the 2D images [(b) and (d)] for p(ODA50-stat-HEAm50) thin films prepared on glass substrates covered with hydrophobic [(a) and (b)], or hydrophilic [(c) and (d)] glass substrates and annealed in water at 60 °C for 3 h. The broad peak observed around 3.2 nm⁻¹ was attributed to the crystallization of octadecyl side chains.²⁰

Conclusion

We demonstrated that annealing the p(ODA50-stat-HEAm50) thin film in water induced the formation of perpendicular lamella. Depth profiling of the thin-film structure using 2D tender X-ray GI-SAXS revealed that parallel lamellae preferentially formed at the topmost surface, which was formed by relaxation of oriented lamellar after being taken out from water. MAIRS2 results indicated that the alkyl side chains were oriented parallel to the substrate plane. It was proposed that the contact of the film with water caused the octadecyl side chain to reorient, exposing the hydrophilic amide groups, which led to the formation of perpendicular lamellae. Exposure of the thin film to air caused the hydrophobic octadecyl side chains to orient toward the air interface, thereby reducing the excluded volume. In addition, experiments with thin films covered with hydrophilic or hydrophobic glass substrates confirmed that the nature of the interface in contact with the film significantly influenced the alignment of the lamellar structure. Specifically, when the film was in contact with a liquid or hydrophilic solid surface, perpendicular lamellar structures preferentially formed. These results highlight the importance of interface design in controlling the structure of thin films and offer new guidance for the development and application of functional materials.

Author contributions

M. K. carried out most of the experiments, analysis and wrote the first draft of the paper. H. H. and K. Y. carried out Tender X-rays diffraction measurements and analyzed results. S. N. carried out 2D GI-SAXS. S. N. analyzed X-ray scattering results. H. E. support the whole analysis of the experimental results. J. M. directed the whole project and wrote the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported by KAKENHI grant 24K21669, 24K01292, Partnerships for International Research and Education (PIRE) grant number JPJSJRP20221201, Core-to-Core Program (A. Advanced Research Networks) Number JPJSCCA20220006 from the Japan Society for the Promotion of Science (JSPS), JST, CREST Grant Number JPMJCR21B5, Kato Foundation for Promotion of Science and the Research Program “Network Joint Research Center for Materials and Devices”. 2D Tender X-ray GI-SAXS measurements were performed at the Photon Factory of High Energy Accelerator Research Organization (approval 2024G012 to K. Y.). We thank Prof. Togashi for his help with contact angle measurements.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sm00313j

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