Introduction

Stereoregular poly(methyl methacrylate)s (PMMA) have attracted much attention because of their unique crystalline structure, morphology, and other particular properties since the first report by Stroupe and coworkers.1 Isotactic (it) PMMA and syndiotactic (st) PMMA can self-aggregate either by themselves or as stereocomplexes. it-PMMA crystals form a double-stranded helical structure,2 whereas st-PMMA does not crystallize, but includes specific organic solvents in a cavity within its single-stranded helix.3 The stereoregular PMMA stereocomplex is a complementary helical structure formed on the basis of structural fitting between two chains of it-PMMA and st-PMMA.4, 5, 6 Recently, the discussion has arisen on the basic structure of the PMMA stereocomplex by using X-ray7 and atomic force microscopy (AFM),8, 9 although it was concluded that the structure of the stereocomplex is a helix with it-PMMA on the inside surrounded by st-PMMA.

Interfacial rearrangements of it-PMMA chains are expected to occur more easily than st-PMMA or atactic-PMMA chains, because the glass transition temperature of it-PMMA (∼40 °C) is significantly lower than that of st-PMMA or atactic-PMMA (around 105–135 °C).10 The crystallization of it-PMMA films has been researched by annealing at elevated temperatures for more than a few days, or under compression.11, 12, 13, 14, 15, 16, 17, 18, 19 Most of these studies focused mainly on structural analyses of the crystals using infrared (IR) spectroscopy and X-ray diffraction (XRD), but the morphology of crystallized it-PMMA films has also been studied for a long time.11, 12, 16, 18 The film-formation methods used were either solvent evaporation from solution-cast films or the Langmuir–Blodgett technique as described below. Over 30 years ago, Klement and Geil11 studied the lamellar growth of it-PMMA cast films annealed at a narrow temperature range (55–65 °C) with electron microscopy, whereas Challa and coworkers12 observed the macroscopic hexagonal structures of crystallized it-PMMA from casts at 90–130 °C using a polarization microscope. Later, Brinkhuis and Schouten16 demonstrated the epitaxial crystallization in it-PMMA cast films covered by Langmuir–Blodgett overlayers. Recently, Yashima and coworkers18 directly observed two-dimensional folded-chain crystals of it-PMMA monolayers obtained by Langmuir–Blodgett deposition using AFM.

In contrast, macromolecularly porous it-PMMA thin films have been studied as a scientifically important model, as they have strictly controlled nanospaces and were utilized for specific molecular recognition or precise reaction fields by forming stereocomplexes on substrates.20, 21, 22, 23, 24, 25 Porous it-PMMA thin films were prepared on quartz crystal microbalance (QCM), silica or glass substrates through layer-by-layer (LbL) assembly26 of it-PMMA/st-poly(methacrylic acid) (PMAA)27, 28, 29 and the selective extraction of st-PMAA from it-PMMA/st-PMAA stereocomplex films, taking advantage of the different solubilities of each component polymer. The it-PMMA/st-PMAA stereocomplex also forms a double-stranded helix similar to the aforementioned it-PMMA/st-PMMA stereocomplex.30 In addition, porous films prepared by LbL assembly have been compared with spin-coated it-PMMA films. The spin-coated films showed neither the specific recognition of st-polymethacrylate20 nor the template polymerization of methacrylic acid.22 Thus, it is concluded that these phenomena are particular to the macromolecularly porous films.

More recently, we observed partial it-PMMA crystallization, as well as st-PMAA incorporation, into porous films by XRD analyses.31, 32 It was revealed that crystallized it-PMMA chains in films formed double-stranded helices.2, 33 The crystallization of it-PMMA occurred merely by immersing the porous films in a mixed solvent of acetonitrile/water (4/6, v/v) at room temperature, which is a different procedure and milder than conventional conditions.11, 12, 13, 14, 15, 16, 17, 18, 19 Furthermore, st-PMAA incorporation into porous it-PMMA films was analyzed at the molecular level by QCM. Porous it-PMMA films incorporated st-PMAA with increasing acetonitrile content in the st-PMAA solution. Therefore, it was concluded that acetonitrile was important for it-PMMA chain motion in porous films. st-PMAA was also incorporated into it-PMMA films driven by stereocomplex formation, even after it-PMMA crystallized partially, although the amount of st-PMAA incorporated into crystallized films was reduced as compared with the incorporation into porous films.32 However, both the changes in the morphology or properties of the films and the mechanisms responsible for these phenomena remained unclear.

In this study, we focused on the morphological changes of porous thin films during it-PMMA crystallization and the subsequent st-PMAA incorporation. Films were characterized by scanning electron microscopy (SEM), AFM, X-ray photoelectron spectra and static contact angles. We also investigated whether the morphological changes of spin-coated films occur after immersion into a mixed solvent of acetonitrile/water in order to study the mechanisms of these interfacial rearrangements.

Experimental procedure

Materials

it-PMMA34 (Mn=22 900, Mw/Mn=1.21, mm:mr:rr=96:2:2) and st-PMAA35 (Mn=33 700, Mw/Mn=1.45, mm:mr:rr=1:5:94) were synthesized by conventional anionic polymerization. The number average molecular weights and distributions were measured by size exclusion chromatography using PMMA standards with a tetrahydrofuran eluant at 40 °C and a flow rate of 0.6 ml min−1. Tacticities were analyzed from α-methyl proton signals using 400-MHz nuclear magnetic resonance (nitrobenzene-d5, 110 °C). Characterization of st-PMAA was achieved after the carboxyl group was methylated. Acetonitrile was purchased from Wako Pure Chemical Industries (Osaka, Japan). Ultrapure distilled water was provided by the MILLI-Q laboratory system (Millipore, Billerica, MA, USA).

Film preparation

An AT-cut QCM with a parent frequency of 9 MHz was obtained from USI (Fukuoka, Japan) and used as the substrate. Frequency was monitored by an Iwatsu frequency counter (Model 53131A: Iwatsu Test Instruments Corp., Tokyo, Japan). The quartz crystal (9 mm diameter) was coated on both sides with mirror-like polished gold electrodes (4.5 mm in diameter). At first, the QCM electrode was cleaned three times with a piranha solution, a mixed aqueous solution of H2SO4/40% H2O2 (3/1, v/v) for 1 min, followed by rinsing with ultrapure water and drying with N2 gas. LbL films were prepared as follows: the substrate was alternatively immersed into an it-PMMA acetonitrile solution and an st-PMAA acetonitrile/water (4/6, v/v) solution at a concentration of 0.017 unitM for 5 min at 25 °C each. The substrate was rinsed with each solvent, and dried with N2 gas whenever it was taken out from each solution. This alternative deposition step was repeated 16 times to fabricate stereocomplexes of it-PMMA/st-PMAA on the substrates. Porous it-PMMA thin films were prepared by immersion into a 10 mM NaOH aqueous solution. The porous films were immersed in mixed solvents of acetonitrile/water (4/6, v/v) for 10 h, and then immersed in st-PMAA solutions of mixed acetonitrile/water (4/6, v/v) solvents for 115 min.32 Spin-coated it-PMMA films were prepared on the QCM at 1500 r.p.m. for 1 min with 0.051 unitM it-PMMA chloroform solutions. Acetonitrile was not used because it was a poor solvent for PMMA10 and could not dissolve it-PMMA completely at more than 0.017 unitM. Thus, chloroform, a good solvent for PMMA, was used to prepare the spin-coated films. The amount of it-PMMA adsorbed onto the substrate was approximately −1600 Hz per QCM. Spin-coated films were then immersed in acetonitrile/water (4/6, v/v) for 10 h.

Measurements

SEM images were obtained with a JSM-6701F (JEOL, Akishima, Japan) at an acceleration voltage of 5 kV after osmium tetraoxide was spattered onto the surfaces of films at a thickness of approximately 5 nm. AFM images were obtained with a JSPM-5400 (JEOL) that was operated in tapping mode in air at 25 °C. Scanning was performed using silicon cantilevers (NSC35, μ-masch; resonance frequency: around 150 kHz; spring constant: 4.5 N m−1) within an area of 5 × 5 μm2 with a 512 scan line and a scan speed of 5.0 μm s−1. We did not perform any image processing other than the flat leveling. The mean square roughness (Ra) in the observed areas was estimated from the following equation, where F(x,y) is the surface relative to the center plane, which is a flat plane parallel to the mean plane, and Lx and Ly are the dimensions of the surface.

X-ray photoelectron spectroscopy (XPS) was obtained with a spectrometer (AXIS 165, Shimadzu-Kratos, Kyoto, Japan) using MgKα radiation. The typical operating conditions were as follows: X-ray gun, 12 kV, 10 mA; takeoff angle, 90 °C; pressure in the source chamber, ∼10−9 torr. The static contact angles of the thin films were measured by dropping ultrapure water on the films at 25 °C with a DropMaster 500 (Kyowa InterFACE Science, Niiza, Japan). Contact angles were determined 10 s after applying the drop. The volume of water in the drop was 0.5 μl. All reported values represent the average of at least six measurements taken at different locations on the film surface.

Results and Discussion

We used QCM analysis to calculate the amount of it-PMMA assembled on the gold substrate by Sauerbrey’s equation.36 A typical QCM analysis is shown in Figure 1, and the time evolution means the period of immersion time in each solution or solvent. The stepwise stereocomplex assembly of it-PMMA/st-PMAA and the selective extraction of st-PMAA from assembled films were confirmed in previous papers.20, 23, 25, 32 Interestingly, the elution of porous it-PMMA thin films was barely observed after the films were immersed into acetonitrile/water (4/6, v/v) for 10 h, although the crystallized it-PMMA peaks were observed by XRD.31, 32 These results indicate that the interfacial rearrangements of it-PMMA chains that occurred in films were driven by the mixed solvent, whereas it-PMMA crystallization was driven by annealing at high temperature or under compression in previous studies.11, 12, 13, 14, 15, 16, 17, 18, 19 The crystallized it-PMMA thin films can incorporate st-PMAA, most likely because further interfacial rearrangements also occurred in these films. Therefore, the morphological changes of the porous thin films during it-PMMA crystallization and the subsequent st-PMAA incorporation were initially analyzed by SEM and AFM. The same crystallized it-PMMA films, which corresponded to approximately −1400 Hz per QCM, and the st-PMAA incorporated films were used in SEM and AFM observation.

Figure 1
figure 1

Typical QCM analysis of the layer-by-layer assembly, st-PMAA extraction, isotactic it-PMMA crystallization and subsequent st-PMAA incorporation. it-PMMA (white circles) in acetonitrile and st-PMAA (black circles) in acetonitrile/water (4/6, v/v) were alternately assembled on a QCM substrate at 0.017 unitM at 25 °C. Porous it-PMMA films were prepared using 10 mM NaOH(aq) (white square). The following it-PMMA crystallization occurred in a mixed solvent of acetonitrile/water (4/6, v/v) (white triangle). The last st-PMAA incorporation was observed at 0.017 unitM at 25 °C (black squares).

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We have observed several hemispherical outshoots on the surface of crystallized it-PMMA films by AFM,32 which implies that the morphological changes of the films occurred in a wide area as compared with the scanning area (1 × 1 μm) used in previous studies.20, 29, 32 Thus, SEM was used to observe the macroscopic deformation of porous thin films during it-PMMA crystallization and the subsequent st-PMAA incorporation. Figure 2 shows SEM images of crystallized it-PMMA films and st-PMAA incorporated films after 10 h of immersion in acetonitrile/water (4/6, v/v). Dotted aggregates of crystallized it-PMMA and networks of the it-PMMA/st-PMAA mixed assembly broadened evenly at the submillimeter scale (Figures 2a and b). The patterns were not observed on the surface of a bare QCM substrate and porous it-PMMA films (Supplementary Figure S1). In the magnified view (tilted at 40-degree angles), some dots were connected to form big outshoots on the crystallized it-PMMA films (Figure 2c), and some isolated outshoots were also observed on st-PMAA incorporated films after it-PMMA crystallization (Figure 2d). However, most regions of the crystallized it-PMMA films and st-PMAA-incorporated films formed dots and networks, respectively, although the configurations of these assemblies cannot be controlled yet. This morphological change from dots to networks likely resulted from an increased polymer density of films during the incorporation of st-PMAA.

Figure 2
figure 2

SEM images of crystallized it-PMAA films (a, c) and st-PMAA incorporated films (b, d) after 10 h of immersion in acetonitrile/water (4/6, v/v). (a, b) Top view. (c, d) Tilted view.

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AFM analysis was used to calculate both the height of the assemblies and the surface roughness of films, which was difficult to measure by SEM. Figure 3 shows AFM images of crystallized it-PMMA films and st-PMAA incorporated films after it PMMA crystallization. Hemispherical outshoots of crystallized it PMMA, which were several hundred nanometers high, were arranged in a dot pattern on films (Figure 3a), whereas porous films were relatively flat (Supplementary Figure S2). The mean square roughness (Ra) of the crystallized it-PMMA films was 53 nm, although the Ra of the smooth parts (0.3 × 0.3 μm) of films was 3.3±0.2 nm (n=3). This value was more similar to that of a bare QCM substrate (3.1 nm) than that of porous it-PMMA films (17 nm) (Supplementary Figure S2), suggesting that the bare gold surface is exposed. In addition, the outshoots of crystallized it-PMMA films, which corresponded to approximately −700 Hz per QCM (Supplementary Figure S2), were smaller and more than that of the crystallized films in Figure 3a (−1400 Hz per QCM), which implies that film thickness is one of the important parameters for morphological changes of it-PMMA films. Furthermore, a similar dot pattern was also observed on crystallized it-PMMA (−700 Hz per QCM) after 40 h of immersion in acetonitrile/water (4/6, v/v) (Supplementary Figure S2). On the other hand, networks of agglomerates composed of crystallized it-PMMA and stereocomplexes were observed on the surface of st-PMAA incorporated films (Figure 3b), and the heights of these assemblies were not significantly different from that of crystallized it-PMMA films. Therefore, the polymer chains in films likely spread in a horizontal direction during st-PMAA incorporation. Indeed, the Ra of st-PMAA-incorporated films was 46 nm, but the Ra of the smooth parts (0.3 × 0.3 μm) was 3.8±0.5 nm (n=3). This morphological change in films indicated that some noncrystalline parts of it-PMMA could incorporate st-PMAA, and that a rearrangement of the partially crystallized it-PMMA might occur on the film surface.

Figure 3
figure 3

AFM images of crystallized it-PMAA films (a) and st-PMAA incorporated films (b) after 10 h of immersion in acetonitrile/water (4/6, v/v).

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Next, we were interested in the changes in properties of porous it-PMMA films. The deformation of films during it-PMMA crystallization would dynamically change the film thickness. XPS analysis is a common tool to confirm chemical bonds on a surface at ∼10 nm of the measured depth. The XPS spectra of a bare QCM substrate, porous it-PMMA films and crystallized it-PMMA films are shown in Figure 4. Distinct Au 4f peaks at around 84.1 and 87.7 eV were observed on the QCM substrate (Figure 4a). In contrast, these Au peaks were not observed on the surface of porous films (Figure 4b), showing that the thickness of porous films was more than 10 nm, and that the substrate was evenly covered with film. In a previous paper,20 the thickness of porous it-PMMA films was determined to be 44 nm by AFM scratching mode, which corresponded to approximately −800 Hz per QCM. Therefore, it-PMMA film thickness was calculated as 83 nm (−1500 Hz per QCM) in this study (Figure 1), according to a constant rate of 0.055 nm/Hz. Note that the intensity of Au 4f peaks on the surface of crystallized it-PMMA films was about two-thirds of that of the QCM substrate (Figure 4c), and that the heights of the outshoots on the crystallized it-PMMA films reached several hundred nanometers (Figure 3a), where Au peaks should not be detected. Thus, the film thickness of the smooth parts became less than 10 nm after it-PMMA films were immersed in acetonitrile/water (4/6, v/v). These results confirmed that the homogeneously broadened it-PMMA in porous films became localized because of its crystallization, which brought about differences in the polymer density of films.

Figure 4
figure 4

XPS in the region of the gold element of a bare quartz crystal microbalance substrate (a), porous it-PMMA films (b) and crystallized it-PMMA films (c) after 10 h of immersion in acetonitrile/water (4/6, v/v).

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Furthermore, film characteristics were analyzed on the millimeter scale with static contact angles, which have been used to confirm stepwise stereocomplex formation during LbL assembly.29 We thought that the contact angles of each film would change according to its surface components or shapes. Figure 5 shows the dependence of static contact angles on various film surfaces. Surface components and polymer conformations were analyzed by attenuated total reflection-IR (Supplementary Figure S3).20 The mean angles of porous it-PMMA films as prepared and after 10 h of immersion in water were 49.2±1.9° and 51.7±1.0°, respectively (Figures 5a and b). This is reasonable because XRD analyses revealed that no structural changes of porous it-PMMA films occurred in water.32 In contrast, the angles of crystallized it-PMMA films after 10 h immersion in acetonitrile/water (4/6, v/v) was 36.5±1.6° (Figure 5c). These differences likely resulted from a surface morphology change of the thin films, because both films were composed only of it-PMMA and the two porous films were smooth compared with crystallized films (Figure 3a, Supplementary Figure S2). We should also consider that the film thickness of the smooth parts in crystallized it-PMMA films was less than 10 nm and the gold surface of QCM is exposed, according to XPS results (Figure 4c). Although the mean angle of a bare QCM substrate was 69.2±1.4°, Burton and Bhushan37 reported that increasing roughness on hydrophilic surfaces, such as the patterned PMMA surface, decreased the contact angle, which was consistent with these results.

Figure 5
figure 5

Static contact angles of porous it-PMMA films as prepared (a) and after 10 h of immersion in water (b), crystallized it-PMMA films after 10 h of immersion in acetonitrile/water (4/6, v/v) (c), stereocomplex films (d) and st-PMAA incorporated films just after st-PMAA extraction (e) and after 10 h of immersion in water (f) and acetonitrile/water (4/6, v/v) (g).

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This tendency was also observed in the case of stereocomplex films. The contact angle of the original LbL films was 30.2±2.5°, and the angles of st-PMAA incorporated films after st-PMAA extraction and a subsequent 10 h of immersion in water were 26.4±1.3° and 21.4±0.8°, respectively (Figures 5d—f). The mean values of stereocomplex films were smaller than those of porous it-PMMA films,29 because the hydrophobic it-PMMA is buried in the helical structure of hydrophilic st-PMAA in the stereocomplex model.30 st-PMAA incorporated films remained flat (Supplementary Figure S2), although the slow crystallization of it-PMMA also occurred during the incorporation of st-PMAA.31 Therefore, the fast st-PMMA incorporation by stereocomplexation might prevent film deformation because of it-PMMA crystallization. On the other hand, the angle of st-PMAA incorporated films after it-PMMA crystallization was 13.6±0.7° (Figure 5g). It is indicated that the surface of films was covered with stereocomplexes (Figure 6), although the it-PMMA inside the aggregates is densely packed and loses the ability to incorporate st-PMAA. It also remains unclear whether st-PMAA adsorbed both on the crystallized it-PMMA outshoots and on the smooth domains of films. These tendencies of static contact angles also corresponded to the relatively regular surface profile at the submillimeter scale, which was observed by SEM (Figures 2a and b).

Figure 6
figure 6

Schematic illustration of the morphological changes of macromolecularly porous thin films in it-PMMA crystallization and st-PMAA incorporation.

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We were also interested in whether ordinary it-PMMA films would form outshoots after immersion into a mixed solvent of acetonitrile/water. Spin-coated it-PMMA films were prepared on the QCM substrate from the chloroform solution as described in the experimental procedure section. Figure 7 shows the AFM and SEM images of spin-coated it-PMMA films as prepared and after immersed in acetonitrile/water (4/6, v/v) for 10 h. The Ra of spin-coated films was 8.4 nm and the film surface was relatively smooth (Figure 7a). A few big craters were formed on the films, most likely because of the evaporation of chloroform (Figure 7c). The QCM frequency shift was a few hertz after 10 h of immersion in acetonitrile/water (4/6, v/v), which implies that the spin-coated films did not dissolve in the mixed solvent either. Semispherical outshoots were also observed on films after immersion (Figure 7b), and dotted aggregates of it-PMMA broadened evenly at the submillimeter scale (Figure 7d). The same tendency of crystallized it-PMMA films was obtained by LbL assembly, even though the Ra of films (82 nm) was much greater than that of it-PMMA films from LbL assembly (32 nm). The surface roughness derived by the crystallization of it-PMMA is dependent on the initial thickness, roughness or morphology of the films, such as the cratered surface. These results indicate that the formation of these hemispherical outshoots would occur by rearrangements of it-PMMA chains in thin films.

Figure 7
figure 7

AFM and SEM images of spin-coated it-PMMA films prepared from a chloroform solution (a, c) and after 10 h of immersion in acetonitrile/water (4/6, v/v) (b, d).

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Conclusions

We investigated the morphological changes of porous thin films on a QCM substrate during it-PMMA crystallization and the subsequent st-PMAA incorporation. Dotted aggregates of crystallized it-PMMA appeared on films on SEM and AFM images, although the films were not dissolved in a mixed solvent of acetonitrile/water. Gold substrate peaks were observed on crystallized it-PMMA films by XPS. Therefore, this indicated that the adsorbed it-PMMA spontaneously localized in films to reform the surface profile. These dotted it-PMMA aggregates were also observed when spin-coated films were immersed in the mixed solvent, suggesting that this immersion method is a simple and different approach to film deformation, as compared with previous studies. On the other hand, networks of crystallized it-PMMA and the stereocomplex appeared on st-PMAA-incorporated films. In this way, the design and restructuring of self-assembled films at the nano or micrometer scale were achieved through self-organization and stereocomplex formation of stereoregular polymethacrylates.