Fabrication of a Polymer Molecularly Flat Substrate by Thermal

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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Fabrication of a Polymer Molecularly Flat Substrate by Thermal Nanoimprinting and AFM Observation of Polymer Chains Deposited on It Ryota Umetsu and Jiro Kumaki* Department of Organic Materials Science, Graduate School of Organic Materials Science, Yamagata University, Yonezawa, Yamagata 992-8510, Japan Downloaded via MACQUARIE UNIV on August 29, 2019 at 16:17:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The surface of polymers is believed to have high mobility, and their glass transition temperature decreases significantly from the bulk value. In this study, we prepared a molecularly flat poly(methyl methacrylate) (PMMA) substrate by thermal nanoimprinting a PMMA plate with an atomically stepped sapphire substrate. The imprinted PMMA surface with a step height of ca. 0.2 nm was stable for months in air at room temperature and was retained even after immersion in water for 10 h. PMMA chains and an isotactic PMMA crystalline monolayer deposited on the imprinted PMMA substrate by the Langmuir−Blodgett technique were observed by atomic force microscopy at the molecular level, and the molecular structures were stable for months in air at room temperature. These significant stabilities of the surface of the imprinted PMMA substrate and the monolayers deposited are unexpected and differ from the results expected based on our present understanding of the polymer surfaces. The results indicate that the properties of the thermally pressed polymer surface may be different from those of the previously studied solvent-cast polymer surfaces. flat PMMA surface be observed by AFM? Is the chain stable, or does it easily merge and become buried in the substrate? These experiments will obtain new insight into understanding the polymer surface. Tan, Yoshimoto, and co-workers reported that an atomically stepped structure of a sapphire substrate (α-Al2O3 single crystal) can be imprinted on a PMMA plate by thermally pressing the sapphire substrate onto the PMMA plate.11,12 The atomically stepped structure of a sapphire substrate was formed by annealing a sapphire single crystal at a temperature between 1000 and 1400 °C in air.13 The imprinted PMMA surface exhibited regularly arranged steps with a step height of ca. 0.26 nm that was comparable to the step height of the used (101̅2) sapphire template (0.31 nm) and had rather flat terraces with a root-mean-square (RMS) roughness of 0.10 ± 0.01 nm. To observe a polymer chain on a substrate by AFM, the substrate should be atomically flat, so that mica, highly oriented pyrolytic graphite (HOPG), and the atomically stepped sapphire are used. The roughness of the terraces of the PMMA substrate appears to be sufficiently smaller than the thickness of a PMMA chain (ca. 0.3 nm)14,15 so that if we could deposit a PMMA chain on the imprinted PMMA surface, in principle, it should be possible to observe the chain by AFM. To deposit a PMMA chain on the imprinted PMMA, spincasting of a dilute PMMA solution on the substrate cannot be used because the spin-casting solvent also dissolves the PMMA

1. INTRODUCTION The physical properties of a polymer at a surface are different from those in bulk.1 The surface is unstable mainly due to the excess surface free energy as well as the perturbation of the chain conformation and the enrichment of the chain ends at the surface, so that the glass transition temperature, Tg, at the surface is believed to generally decrease by up to several ten degrees relative to Tg of the bulk.2−10 The surface effect should be most enhanced at the topmost surface, particularly for the topmost chains at the surface. If we could observe a polymer chain deposited on a surface of the same polymer, how would it behave? As shown in Figure 1 (left), in this study, we attempted to deposit a poly(methyl methacrylate) (PMMA) chain on a molecularly flat PMMA substrate and observe the chain by atomic force microscopy (AFM) at the molecular level. This is a very peculiar situation. Our questions are the following: Can the PMMA chain deposited on a molecularly

Received: June 21, 2019 Revised: July 18, 2019

Figure 1. Schematic representation of an isolated chain and polymer crystals deposited on an atomically flat polymer substrate. © XXXX American Chemical Society

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DOI: 10.1021/acs.macromol.9b01280 Macromolecules XXXX, XXX, XXX−XXX

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effective moving barrier length of 10 cm (3-22YG3, USI, Fukuoka, Japan). The surface pressure−area (π−A) isotherms were measured at a constant compression rate with a moving barrier speed of 0.5 mm/s. For deposition, the monolayer was compressed with the same speed, and a single monolayer was deposited on a nanoimprinted PMMA plate and mica for comparison by pulling them out of the water at a rate of 4.2 mm/min (the vertical dipping method). An it-PMMA crystalline monolayer was deposited at a π of 12 mN/m, higher than the crystalline transition of the it-PMMA (ca. 9 mN/m), while a PS-bPMMA was deposited at 0 mN/m at an area wider than the onset of the π−A isotherm with the moving barrier stopping to deposit individual PS-b-PMMA molecules prior to the compression to be a condensed film. 2.4. AFM Observations. The samples were observed using a commercial AFM (NanoScope IIIa, IIId, and V/multimode AFM unit, Bruker, Santa Barbara, CA) with standard silicon cantilevers (model: NCHV; nominal spring constant: 40 N/m; nominal tip radius: 8 nm; Bruker) in air at room temperature in the tapping mode. The typical settings of the AFM observations were as follows: an amplitude of 1.0−1.5 V, a set point of 0.67−1.3 V, and a scan rate of 2.0−3.0 Hz. The tapping force during the observations typically corresponded to ca. 0.5 nN. The penetration depth of an AFM tip into the PMMA plate during the observations was evaluated based on the simple contact mechanics with the Hertzian model20,21 using Poisson’s ratio (0.33) and Young’s modulus (3.3 GPa)22 of PMMA, the nominal AFM tip radius (8 nm), and the tapping force estimate above (0.5 nN). The contribution from the tip was ignored because the tip is much stiffer than the PMMA plate, and the contribution is negligible.20,21 The evaluated penetration depth of an AFM tip into the PMMA plate in this study was 0.11 nm. The AFM images obtained are presented without any image processing except flattening.

plate. Instead, in this study, we deposited a PMMA chain on the substrate by the Langmuir−Blodgett (LB) technique.14,15 During the deposition of LB films, the substrate was immersed in water, and we first needed to confirm the stability of the imprinted PMMA plate immersed in water. Previously, we observed an isotactic (it) PMMA monolayer that was compressed to crystallize on the water surface and deposited on mica and then visualized the it-PMMA folded chain crystals at the molecular level.16,17 In addition to the PMMA chains, as shown in Figure 1 (right), we also deposited a single it-PMMA crystalline monolayer on the imprinted PMMA plate and attempted to observe the crystals at the molecular level. Here, we sought to determine whether the it-PMMA crystal deposited on the PMMA plate could be observed at the molecular level and was stable on the substrate. As will be described below, surprisingly, a single PMMA chain and an it-PMMA crystal deposited on an imprinted PMMA substrate were observed by AFM at the molecular level, and they were even stable for several months in air at room temperature. This surprising stability raises new questions regarding the nature of the polymer surfaces and the imprinted polymer surface that will be discussed below.

2. EXPERIMENTAL SECTION 2.1. Materials. Two millimeter thick PMMA plates with different molecular weights (Acrylite S and L) were obtained from Mitsubishi Chemical Corporation, Tokyo, Japan. The molecular weights of these PMMA were measured by size exclusion chromatography (SEC) in tetrahydrofuran using PMMA standards (Shodex, Tokyo, Japan) for calibration. The number-averaged molecular weight (Mn) and molecular weight distribution (Mw/Mn) were 2.48 × 106 and 1.95 for Acrylite S and 1.79 × 105 and 2.98 for L, and the substrates were denoted as PMMA plates (2480k) and (179k) in this study. Sapphire (Al2O3) single-crystal substrates (0.55 mm thick) with (0001) surface atomically flat steps with a step height of 0.21 nm13 were purchased from Shinkosha Co., Ltd. (Yokohama, Japan), and the width of the terraces between the steps of the substrate varied depending on the cutoff angle of the original sapphire single crystals. An it-PMMA with a Mn of 2.90 × 105, a Mw/Mn of 1.13, and an mm content of 98% was prepared by the isotactic-specific anionic living polymerization of tertbutyl methacrylate in toluene at −78 °C with (1,1-diphenyl-3methylpentyl)lithium, followed by hydrolysis and methylation with diazomethane.18,19 A polystyrene (PS)-b-PMMA diblock copolymer with a PS block Mn of 1.40 × 105, a PMMA block Mn of 6.56 × 105, and a Mw/Mn of 1.32 was purchased from Polymer Source, Inc. (Montreal, Canada). Highly purified chloroform and benzene (Infinity Pure, Wako Chemicals, Osaka, Japan) were used as the solvents for the spreading solutions for the preparation of LB films. Water was purified by using a Milli-Q system and was used as the subphase for the LB preparation. 2.2. Nanoimprinting. A sapphire mold was ultrasonicated in acetone and next in ethanol and then was rinsed with Milli-Q water and purged by nitrogen. A PMMA plate was rinsed with ethanol prior to the imprinting. A lower and an upper heating plate of a pressing machine (IMC-183D, Imoto Machinery Co., Ltd., Kyoto, Japan) were heated at the prescribed temperatures. An aluminum foil, a sapphire mold ((1.0−7.0) × 1.0 cm2) with an atomically stepped surface upward, a PMMA plate ((1.0−7.0) × 1.0 cm2), and an aluminum foil were placed sequentially on the lower heating plate and then were pressed with the upper plate at the prescribed pressure for 300 s; then, the sample was removed and allowed to cool at room temperature. For optimal conditions, the mold was easily released from the PMMA plate. 2.3. Langmuir−Blodgett (LB) Film Deposition. An it-PMMA chloroform solution (3.0 × 10−5 g/mL) or a PS-b-PMMA benzene solution (2.0 × 10−5 g/mL) was spread on a water surface at 23 °C in a commercial LB trough with an area of 37.5 × 10 cm2 and an

3. RESULTS AND DISCUSSION 3.1. Nanoimprinting Conditions. Figure 2 shows asobtained AFM height images of an atomically stepped sapphire substrate and a PMMA plate (2480k). The sapphire plate had an atomically flat surface with steps of 0.21 nm, while the PMMA plate originally had a rough surface. Figure 3 shows AFM height images of the PMMA plate (2480k) after nanoimprinting with the sapphire mold at a pressure of 5 MPa for 300 s. The temperatures of the upper and lower plates of the pressing machine are indicated in the images. Although the data were somewhat scattered, with an increase of the temperature of the lower heating plate up to 140 °C, the transferred step structure became clearer, and the RMS of the roughness decreased. However, if the temperature of the upper plate was raised to 100 °C with the lower plate at 140 °C, the mold strongly adhered to the PMMA plate, and as the result, the surface of the PMMA plate was completely destroyed. On the basis of this result, we chose 140 and 80 °C for the lower and upper plates, respectively. Figure 4a shows the dependence of the results on the pressure at these temperature settings. The best result was obtained at 10 MPa. At 30 MPa, the surface was destroyed due to the strong adhesion to the mold. The resultant surface obtained at 10 MPa was comparable to that of the atomically flat sapphire mold (Figure 4b) and was sufficiently smooth to observe the polymer chains deposited on the surface. The step height of the PMMA plate was slightly smaller than that of the mold, most likely due to thermal shrinkage. As mentioned in the Experimental Section, the mold was only cleaned by ultrasonication in the organic solvent. We attempted to clean it by irradiation with a 172 nm ultraviolet light using a Xe dielectric-barrier discharge lamp (UER20-172, Ushio Inc., Japan) for the complete removal of organic contaminations, but the mold was strongly adhered to a B

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during the nanoimprinting, with 0.2 MPa for Tan et al. and 10 MPa in our work. In any case, we confirmed that the manual pressing machine we used still worked well to obtain the nanoimprinted PMMA plates. 3.2. Stability of the Nanoimprinted PMMA Substrate in Air and Water. To use the nanoimprinted PMMA plate as a substrate for AFM observation of polymer chains deposited on the plate, it is necessary to examine its stability. Figure 5 shows a nanoimprinted PMMA plate (2480k) after storage in an autodesiccator under ca. 30%RH at room temperature for 1 month. No significant change of the surface was observed, and the step height and surface roughness were maintained after 1 month, indicating that the nanoimprinted surface was stable in air. We wanted to observe the PMMA chains deposited on the nanoimprinted PMMA surface. The simplest way to do this is to deposit isolated PMMA chains by spin-casting a dilute solution on the substrate. However, this approach is not possible because the solvents for PMMA also dissolve the surface of the PMMA substrate. Therefore, we planned to deposit a PMMA Langmuir monolayer on the PMMA substrate. In this case, the nanoimprinted PMMA substrate should be stable against immersion of the substrate into water. Figure 6 shows the AFM height images of the nanoimprinted PMMA plate (2480k) after immersion in water for 1−10 h (Figure 6a) and the step height and the RMS of the substrate as a function of the immersion time (Figure 6b). The step height was almost constant, and the RMS roughness increased slightly but still remained small even after the immersion in the water for 10 h, indicating that the nanoimprinted PMMA substrate was not significantly affected by the water immersion and a PMMA monolayer could be deposited on the substrate by the LB technique without deterioration of the atomically flat surface. 3.3. Deposition of an it-PMMA Crystalline Monolayer on the Imprinted PMMA Substrate and AFM Observation. First, we deposited an it-PMMA crystalline monolayer on the imprinted PMMA substrate and tried to observe the chain packing in the it-PMMA crystal at the molecular level. As observed from the π−A isotherm presented in the Supporting Information (Figure S1), it-PMMA showed a crystalline transition at ca. 9 mN/m. A monolayer was compressed at 12 mN/m beyond the transition, and a single crystalline monolayer was deposited on mica and on the imprinted PMMA substrate. Figure 7 shows the AFM low-magnification height (upper) and high-magnification phase (lower) images of the it-PMMA monolayer deposited on mica. The it-PMMA formed folded-chain lamella crystals with a lamella width of ca. 13 nm, and at high magnification, the chain−chain packing inside the lamella was clearly observed and was ca. 1.27 nm (yellow lines in the lower image). Kusanagi and Tadokoro studied the crystalline structure of uniaxially stretched itPMMA strands by X-ray scattering and analyzed it as a crystal composed of double-stranded helices of it-PMMA chains.23,24 In their model, two 10/1 helices of it-PMMA chains are intertwined to form a double-stranded helix that packs in an orthorhombic crystal with a helix−helix distance of ca. 1.22 nm. The observed structure was in good agreement with the model, indicating that the crystalline structure of the it-PMMA on mica was observed at the molecular level.16 Figure 8a shows the AFM height and phase images of an it-PMMA monolayer deposited on the imprinted PMMA (2480k) substrate. In the low-magnification height image (upper), similar lamella crystals deposited on the step structure of the PMMA

Figure 2. AFM height images of (a) an atomically stepped sapphire mold and (b) a PMMA (2480k) plate.

PMMA plate and the steps were not transferred. A small amount of contaminants at the surface may act as a release agent. We used the PMMA plate imprinted at 140 °C for the lower plate and 80 °C for the upper plate under a pressure of 10 MPa for 300 s for further experiments. The actual temperature of the PMMA surface attached to the mold during the imprinting was measured by a thermocouple sandwiched between the PMMA plate and the mold in the same experimental conditions. The temperature reached a constant value ca. 137 °C within 20 s. The temperature settings were quite similar to the peak temperatures of the heating plates used previously by Tan and co-workers.11 They used a fully automated nanoimprinting equipment, and the temperatures of the lower and upper plates were raised to 140 °C at 13 °C/min and 80 °C at 4 °C/min, respectively, and then the plates were cooled to 30 °C at the rates of ca. 10 and 6 °C/min, respectively, prior to the removal of the sample. However, different pressures were applied to the substrate C

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Figure 3. AFM height images of a PMMA plate (2480k) nanoimprinted with an atomically stepped sapphire mold. The temperatures of the upper and lower plate of the pressing machine and the RMS of the resultant PMMA surface are indicated in the figure. The PMMA plates were pressed at 5 MPa for 300 s.

improve the stability of the monolayer on the substrate? Therefore, next, we deposited an amorphous atactic-PMMA chain on the same atactic-PMMA substrate and tried to observe it. 3.4. Deposition of a PMMA Chain on an Imprinted PMMA Substrate and AFM Observation. It was not clear whether a PMMA chain deposited on the imprinted PMMA substrate can be observed by AFM. The PMMA chain has a small height; it may not be visible on a somewhat rough surface of the PMMA substrate by AFM, or alternatively the PMMA chain may be merged and buried in the substrate. Therefore, we used a PS-b-PMMA diblock copolymer instead of a PMMA homopolymer. Because PS does not have any hydrophilic groups in the repeating unit, it does not spread on the water surface as a monolayer but forms a particle that is much thicker than a monolayer. By contrast, PMMA contains a hydrophilic ester group and spreads on water as a monolayer (Figure 9a).14,15 Thus, PS particles can be used as a probe to detect the PMMA chain that grows from it. Figure 9b shows the AFM height image of the PS-b-PMMA deposited on mica at 0 mN/m at an area wider than the onset of the π−A isotherm. The concentration of the spreading solution was not sufficiently dilute (2.0 × 10−5 g/mL), and unlike in the previous studies (4.0 × 10−6 g/mL),14,15 the PS particle was not a single PS block particle but was composed of several PS blocks from which several PMMA block chains grew as a monolayer. The heights of the PS particle and the PMMA monolayer were ca. 3 and 0.3 nm, respectively. The PMMA showed a condensed π−A isotherm and existed as a condensed monolayer on the water surface, so that it was transferred as a condensed monolayer around the PS particles (Figure 9b). However, at the circumference of the PMMA monolayer, loops and tails of the PMMA chains were visible. Figure 9c (upper) shows the AFM height image of a PS-b-PMMA monolayer

substrate are observed, while the distribution of the height from the substrate (0.6−1.0 nm) was larger than that on mica (ca. 0.6 nm), most likely due to the increase in the roughness of the substrate. In the high-magnification image (lower), the chain−chain packing in the lamella was again clearly observed, indicating that the it-PMMA crystal on the PMMA plate can be imaged at the molecular level with a resolution of ≈1 nm. The chain−chain distance (1.18 nm) varied slightly due to the deformation of the image by a small drift during the highmagnification AFM observation. Figure 8b shows the highmagnification AFM images of the monolayer obtained after keeping the sample in an autodesiccator for 3 months. Surprisingly, the chain−chain distance in the lamella crystal could be observed even after keeping the sample for 3 months in air, indicating that the crystalline monolayer on the PMMA plate was sufficiently stable and remained unchanged for a long time in air at the molecular level. The quality of the AFM images at high magnification varied due to the difficulty of high-magnification observation that is sensitive to parameters such as the quality of the scanning tip. The image obtained after 3 months was somewhat less clear than that observed immediately after the deposition, but this does not directly imply the deterioration of the monolayer. We ascribe this difference mainly to the variation of the AFM observations. The important finding is that the chain−chain packing was observed even after keeping the sample in air for long time. Strictly speaking, the stereoregularities of the monolayer and the substrate were different, but it was still surprising that the PMMA crystalline monolayer on the PMMA substrate was stable at room temperature for a long time. The above-described results raise several questions: Does the difference between the stereoregularities of the monolayer and the substrate affect the stability of the monolayer on the substrate, and does the crystalline structure exceptionally D

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Figure 5. AFM height image of a nanoimprinted PMMA plate (2480k) after storage under ca. 30% RH at room temperature for 1 month.

chain deposited on the same at-PMMA substrate could be observed by AFM. Figure 9c (lower) shows an AFM height image of the PS-b-PMMA deposited on a PMMA substrate observed after storage in an autodesiccator for 1 month. Surprisingly, again almost the same structure was retained with no recognizable changes, indicating that the PMMA chains deposited on the PMMA substrate were stable for long time in air. We note that the stereoregularities of the PMMA chain and the substrate were the same, and isolated PMMA chains comparable to a single chain were deposited as amorphous chains; nevertheless, this chain was still stable for a long time in air. Recent reports of ultrathin polymer films on substrates indicate that the glass transition (Tg) temperature of the surface of the films decreased from that of the bulk by several ten degrees. The decrease in the Tg depends on the molecular weight of the film and is smaller in high-molecular-weight polymers. The molecular weight of the PMMA plate (2480k) used was quite large. Is this the main reason for this unexpected stabilities of the substrate surface and the monolayer deposited on the surface? To answer this question, we studied a low-molecular-weight substrate as described in the next section. 3.5. Deposition of an it-PMMA Crystalline Monolayer and a PMMA Chain on the Imprinted Low-MolecularWeight PMMA Substrate (179k) and AFM Observation. We used a PMMA plate (179k) for which the molecular weight was ca. 14 times smaller than that of the PMMA plate (2480k) and was reasonably small. First, we optimized the nanoimprinting conditions as shown in Figure S2. Similar to the PMMA plate (2480k), the PMMA plate (179k) also well transferred the atomically stepped structure of the sapphire substrate. The conditions were almost the same, and only the temperature of the upper heating plate was slightly reduced from 80 to 75 °C (see Figure S2). The conditions selected for the PMMA plate (179k) were as follows: lower plate

Figure 4. (a) AFM height images of PMMA plate (2480k) nanoimprinted with an atomically flat sapphire mold under various pressures (indicated in the figure) for 300 s. (b) AFM height image of an atomically stepped sapphire mold. (c) Schematic representation of the nanoimprint conditions.

deposited on an imprinted PMMA substrate with the dipping direction along the vertical axis. PS particles with a height of ca. 3 nm (blue arrow) from which PMMA chains were elongated in the vertical dipping direction were observed on the step structure of the PMMA substrate. The different structures of the PMMA monolayer on mica and the PMMA substrate were most likely due to the hydrophilic and hydrophobic natures of the substrates, respectively. The elongated PMMA chains formed bundles, but the thickness of the thinnest part was ≈0.6 nm (pink arrow), which was slightly higher than that on mica, most likely because the chain was deposited on a rough surface and the thickness was overestimated or because the chain may actually be composed of a few chains. Importantly, it was found that at-PMMA chains comparable to a single E

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Figure 6. (a) AFM height images of a nanoimprinted PMMA plate (2480k) after immersion in water for 1−10 h. The images were obtained in air after the samples were dried in vacuo. (b) Step height and the RMS of the surface of the nanoimprinted PMMA plate (2480k) as a function of the immersion time in water.

Figure 7. AFM low-magnification height and high-magnification phase images of an it-PMMA(290k) crystalline monolayer deposited at 12 mN/m on mica. The monolayer was observed immediately after deposition. The chain−chain packing in the it-PMMA lamella crystals is clearly visualized in the magnified images. The dipping direction was vertical.

temperature of 140 °C, upper plate temperature of 75 °C, pressure of 10 MPa, and pressing time of 300 s. The actual temperature of the PMMA surface attached to the mold during the imprinting in these conditions was measured by a thermocouple sandwiched between the PMMA plate and the mold and reached a constant value of ca. 132 °C within 15 s. The surface structure of the nanoimprinted PMMA (179k) was again stable after an immersion in water for 10 h, as shown in Figure S3. Figure 10 shows the AFM low-magnification height and high-magnification phase images of an it-PMMA (290k) crystalline monolayer deposited on the imprinted PMMA plate (179k) observed immediately after the deposition (Figure 10a) and after storage for three months in an autodesiccator (Figure 10b). As shown in the magnified

images, the chain packing in the lamella crystal was clearly visible for the both samples, indicating that the monolayer deposited on the PMMA plate (179k) was again stable for a long time at the molecular level. The quality of the AFM image after storage for 3 months was clearly better than that of the image immediately after the deposition, showing a variation of the image quality for the high-magnification imaging and not a true structure change during the storage; thus, the monolayer did not deteriorate on this time scale. The long-term stability observed for the PMMA crystalline monolayer on the PMMA F

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at 12 mN/m on a nanoimprinted PMMA plate (2480k) (a, b). The monolayer was observed immediately after deposition (a) and 3 months after deposition (b). The chain−chain packing in the itPMMA lamella crystals is clearly visualized in the magnified images. The dipping direction was vertical.

plate (2480k) was not due to the large molecular weight of the PMMA plate. Figure 11 shows the AFM height image of PS-b-PMMA deposited on the imprinted PMMA (179k). PS particles (ca. 3 nm thick) from which the growing PMMA chains were observed. The thickness of the thinnest part of the PMMA chains (0.355 nm thick) was comparable to that of the single PMMA chain on mica, indicating that the single PMMA chain deposited on the PMMA plate was clearly observed even on the low-molecular-weight PMMA plate (179k). Again, the PMMA chains were found to be stable on the PMMA plate for a month in air, and it is clear that the molecular weight of the substrate did not affect and reduce the stability of the chains. 3.6. Discussion on the Significant Stability of the Imprinted PMMA Plate and PMMA Chains Deposited on It. As mentioned above, the surface of the PMMA plate nanoimprinted with the atomic steps of a sapphire substrate was stable in air for months, and furthermore it remained even after the immersion in water for 10 h. The PMMA chains and it-PMMA crystals deposited on the imprinted PMMA plates were also stable for months in air after the deposition. This significant stability was not due to the exceptionally large molecular weight of the PMMA substrate (2480k) used; the lower molecular weight PMMA substrate (179k) was also stable, and the PMMA chains and it-PMMA crystals deposited on the plate showed similar stability. The observed significant stabilities are very different from our recent understanding that the polymer surface is unstable. In this section, we discuss the unexpected stability of the PMMA surface. For more than two decades, polymer surfaces have been a subject of intense studies and have been thought to be unstable in comparison with the bulk. Keddie and co-workers evaluated the temperature dependence of the thickness of a PS ultrathin film on a substrate by ellipsometry and first reported that the Tg of the ultrathin film was significantly reduced with decreasing thickness of the film.2 They also showed that the Tg of an ultrathin PMMA significantly decreased on a substrate that weakly adsorbed the PMMA and, by contrast, increased on a strongly adsorbing surface.3 After these reports, a significant number of studies were performed.4−10 Currently, we believe that there is a Tg distribution in a polymer ultrathin film on a substrate; the Tg at the surface decreases mainly due to the excess of the surface free energy, and the Tg at the interface to the substrate decreases on a weakly adsorbing substrate or increases on a strongly adsorbing substrate.5,7,9,10 Generally, a decrease in the Tg of an ultrathin film with a thickness of a few nanometers on a weakly adsorbing substrate becomes as large as several ten degrees. The decrease in the Tg is even more significant for free-standing ultrathin films without a supporting substrate.4,8 The Tg measurement for an entire ultrathin film cannot be used in a straightforward manner for understanding the properties of the polymer surface. Tanaka, Takahara, and Kajiyama measured the viscoelastic behavior of polymer surfaces as a function of temperature using lateral force

Figure 8. AFM low-magnification height and high-magnification phase images of an it-PMMA(290k) crystalline monolayer deposited G

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deposition (b, c (upper)) and 1 month after deposition (c (lower)). The dipping direction was vertical.

microscopy (LFM, also called as friction force microscopy) and evaluated the Tg of the polymer surfaces directly. They reported that the Tg of the PS films surface strongly depended on the molecular weight of the PS, and the Tg decreased to ca. 77 °C for a PS with a Mn of 2 × 106, to ca. 62 °C for a Mn of 2 × 105, and even lower to ca. 12 °C for a Mn of ca. 2 × 104. The molecular weight dependence is moderate at the higher molecular weight region (Mn > 105) but is significant for the lower molecular weight region (Mn ∼ 104), so that the surface of the lower Mn was in a rubbery state at room temperature.25 The significant Tg decrease for the low-molecular-weight case was generally ascribed due to the enrichment of the chain ends at the surface and the resulting increase in the free volume. They also reported that the Tg of the surface of a PMMA film (Mn = 1.58 × 106) decreased to ca. 47 °C in air,26 and it further decreased to ca. 21 °C in water due to the absorption of the water.21 The penetration depth of the AFM tip into the surface in the LFM measurement was estimated to 4.5 nm; therefore, the Tg of the PMMA at the topmost surface around a single-chain thickness (ca. 0.3 nm) should show a greater decrease. Tanaka and co-workers also studied the density profile of a deuterated PMMA film immersed in water along the film thickness direction by specular neutron refractivity (NR) and found that the surface of the PMMA film was swollen by the water for a depth of ≈20 nm from the surface.27 The density of the PMMA film decreased by as much as 15% of the bulk at a depth of 4.5 nm, so that a significant Tg decrease in water was expected. Considering the significant increase in the mobility of the PMMA surface in both air and water, we could not understand the exceptional stabilities of the nanoimprinted PMMA plate and the PMMA monolayers deposited on it. The obtained results suggest that the Tg of the PMMA plate, even for a highMn PMMA (Mn = 1.58 × 106), decreases to ca. 47 °C. The molecular weight dependence of the surface Tg of the PMMA is unclear, but if we assume that it is similar to that of the PS, the Tg of the PMMA (179k) should decrease to ≈32 °C, slightly higher than the room temperature. However, the LFM measured the region with a depth of ca. 4.5 nm, and the step height of the imprinted PMMA was ca. 0.2 nm, which is comparable to the thickness of a single chain, so that the Tg should be significantly below the room temperature. If the PMMA plate is immersed in water, the surface should be swollen with water and the Tg should be significantly reduced. The stability of the PMMA chains and it-PMMA crystals deposited on the unstable PMMA surface could also not be explained based on our current knowledge about the polymer surface. Why is the nanoimprinted PMMA surface so stable? In the previous studies on polymer surfaces, polymer films prepared by solvent casting were usually used. The nanoimprinted PMMA plates used in this study were thermally pressed with a mold. The pressure applied during the nanoimprinting process was ≈10 MPa, which was quite normal and not too high for a thermal press. Is it possible that the pressure applied to the surface increases the density and modifies the conformation of the topmost chains, giving rise to a significant increase in the surface Tg of the PMMA plate? The thermal property of the surface of the thermally pressed

Figure 9. (a) Schematic representation of PS-b-PMMA on water surface. (b, c) AFM height images of a PS-b-PMMA monolayer deposited at 0 mN/m on mica (b) and a nanoimprinted PMMA plate (2480k) (c). The monolayers were observed immediately after H

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Figure 11. AFM height image of PS-b-PMMA deposited at 0 mN/m on a nanoimprinted low-molecular-weight PMMA plate (179k). The monolayer was observed immediately after deposition (a) and 1 month after deposition (b). The dipping direction was vertical.

polymer films has not been well studied so far and should be studied in future work. Tan and co-workers heated a nanoimprinted PMMA stepwise to 115, 122, 128, and 135 °C with a rather fast temperature jump rate of ≈5 min/step and found that the step started melting and the surface became rougher at ≈128 °C, higher than the bulk Tg (105 °C). The Mn of the PMMA was quite high (Mn ∼ 106) and the polymer was

Figure 10. AFM height and high-magnification phase images of an itPMMA (290k) crystalline monolayer deposited at 12 mN/m on a nanoimprinted low-molecular-weight PMMA plate (179k). The monolayer was observed immediately after deposition (a) and 3 months after deposition (b). The dipping direction was vertical. I

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the PMMA plates. This work was supported by JSPS KAKENHI Grants 15H03861, 17K19147, and 18H02025.

very viscous, necessitating a careful reevaluation of the melting point of the surface, but their results indicate a possible increase in the melting temperature for the imprinted surface. The evaluation of the thermal stabilities of the nanoimprinted PMMA surface and the PMMA monolayers deposited on it should be investigated in future work. The PMMA plates used in this study were an industrial product and were prepared by thermal polymerization of an MMA monomer with an initiator and did not contain any cross-link agent but contained a small amount of a release agent, ultraviolet and thermal stabilizers. The small amount of the release agent enriched at the surface may affect the properties of the PMMA surface, but even if this is the case, the significant increase of the thermal stability may not be rationalized. However, the effect of additives on the surface properties also should be studied to understand the properties of industrial materials.

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(1) Jones, R. A. L., Richards, R. W., Eds.; Polymers at Surfaces and Interfaces; Cambridge University Press: New York, 1999. (2) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-Dependent Depression of the Glass Transition Temperature in Polymer Films. Europhys. Lett. 1994, 27, 59−64. (3) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Interface and Surface Effects on the Glass-Transition Temperature in Thin Polymer Films. Faraday Discuss. 1994, 98, 219−230. (4) Forrest, J. A.; Dalnoki-Veress, K. The Grass Transition in Thin Polymer Films. Adv. Colloid Interface Sci. 2001, 94, 167−196. (5) Ellison, C. J.; Torkelson, J. M. The Distribution of GlassTransition Temperatures in Nanoscopically Confined Glass Formers. Nat. Mater. 2003, 2, 695−700. (6) Fujii, Y.; Akabori, K.; Tanaka, K.; Nagamura, T. Chain Conformation Effects on Molecular Motions at the Surface of Poly(methyl methacrylate) Films. Polym. J. 2007, 39, 928−934. (7) Inoue, R.; Kawashima, K.; Matsui, K.; Kanaya, T.; Nishida, K.; Matsuba, G.; Hino, M. Distributions of Glass-Transition Temperature and Thermal Expansivity in Multilayered Polystyrene Thin Films Studies by Neutron Reflectivity. Phys. Rev. E 2011, 83, 021801. (8) Paeng, K.; Ediger, M. D. Molecular Motion in Free-Standing Thin Films of Poly(methyl methacrylate), Poly(4-tert-butylstyrene), Poly(α-methylstyrene), and Poly(2-vinylpyridine). Macromolecules 2011, 44, 7034−7042. (9) Zuo, B.; Liu, Y.; Wang, L.; Zhu, Y.; Wang, Y.; Wang, X. Depth Profile of the Segment Dynamics at a Poly(methyl methacrylate) Film Surface. Soft Matter 2013, 9, 9376−93. (10) Kanaya, T., Ed.; Glass Transition, Dynamics and Heterogeneity of Polymer Thin Films. In Advances in Polymer Science; Springer: Heidelberg, 2013. (11) Tan, G.; Inoue, N.; Funabasama, T.; Mita, M.; Okuda, N.; Mori, J.; Koyama, K.; Kaneko, S.; Nakagawa, M.; Matsuda, A.; Yoshimoto, M. Formation of 0.3-nm-High Stepped Polymer Surface by Thermal Nanoimprinting. Appl. Phys. Express 2014, 7, 055202. (12) Tan, G.; Nozawa, Y.; Funabasama, T.; Koyama, K.; Mita, M.; Kaneko, S.; Komura, M.; Matsuda, A.; Yoshimoto, M. Atomic-Scale Thermal Behavior of Nanoimprinted 0.3-nm-High Step Patterns on PMMA Polymer Sheets. Polym. J. 2016, 48, 225−227. (13) Yoshimoto, M.; Maeda, T.; Ohnishi, T.; Koinuma, H.; Ishiyama, O.; Shinohara, M.; Kubo, M.; Miura, R.; Miyamoto, A. Atomic-Scale Formation of Ultrasmooth Surfaces on Sapphire Substrates for High-Quality Thin-Film Fabrication. Appl. Phys. Lett. 1995, 67, 2615−2617. (14) Kumaki, J.; Nishikawa, Y.; Hashimoto, T. Visualization of Single-Chain Conformations of a Synthetic Polymer with Atomic Force Microscopy. J. Am. Chem. Soc. 1996, 118, 3321−3322. (15) Kumaki, J.; Hashimoto, T. Conformational Change in an Isolated Single Synthetic Polymer Chain on a Mica Surface Observed by Atomic Force Microscopy. J. Am. Chem. Soc. 2003, 125, 4907− 4917. (16) Kumaki, J.; Kawauchi, T.; Yashima, E. Two-Dimensional Folded Chain Crystals of a Synthetic Polymer in a Langmuir-Blodgett Film. J. Am. Chem. Soc. 2005, 127, 5788−5789. (17) Kumaki, J. Observation of Polymer Chain Structures in TwoDimensional Films by Atomic Force Microscopy. Polym. J. 2016, 48, 3−14. (18) Varshney, S. K.; Gao, Z.; Zhong, X. F.; Eisenberg, A. Effect of Lithium Chloride on the “Living” Polymerization of tert-Butyl Methacrylate and Polymer Microstructure Using Monofunctional Initiators. Macromolecules 1994, 27, 1076−1082. (19) Kawauchi, T.; Kawauchi, M.; Takeichi, T. Facile Synthesis of Highly Syndiotactic and Isotactic Polymethacrylates via Esterification of Stereoregular Poly(methacrylic acid)s. Macromolecules 2011, 44, 1066−1071.

4. CONCLUDING REMARKS In summary, a molecularly flat PMMA substrate was prepared by thermal nanoimprinting with an atomically stepped sapphire substrate. The imprinted PMMA surface was stable for months in air and was retained after immersion in water for 10 h. The PMMA chains and it-PMMA crystals were deposited on the imprinted PMMA plate and were observed at the molecular level by AFM. The deposited monolayers were stable in air for several months. The significant stabilities of the surface of PMMA plate and the monolayers deposited on the PMMA plate were unexpected and differ strongly from the results expected based on our present understanding of polymer surfaces. The properties of the thermally pressed polymer surface should be reevaluated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b01280. π−A isotherms of the it-PMMA and PS-b-PMMA, AFM images of a PMMA plate (179k) nanoimprinted with a sapphire template at various temperature conditions, and stability of an nanoimprinted PMMA plate (179k) in water (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Tel/Fax +81-238-263071. ORCID

Jiro Kumaki: 0000-0001-9552-3303 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely appreciate Dr. Goon Tan, Kobe University, for instructions on the nanoimprinting technique. We also appreciate Prof. Keiji Tanaka, Kyushu University, for valuable discussions. We thank Associate Professor Takehiro Kawauchi, Ryukoku University, for the polymerization of the it-PMMA. We also thank Dr. Yoshihiro Uozu and Mr. Kazuhiko Nakagawa, Mitsubishi Chemical Corporation, for providing J

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Macromolecules (20) Butt, H.-J.; Cappella, B.; Kappl, M. Force Measurements with the Atomic Force Microscope: Technique, Interpretation and Applications. Surf. Sci. Rep. 2005, 59, 1−152. Table 2 on p 40. (21) Fujii, Y.; Nagamura, T.; Tanaka, K. Relaxation Behavior of Poly(methyl methacrylate) at a Water Interface. J. Phys. Chem. B 2010, 114, 3457−3460. (22) Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Polymer Handbook, 4th ed.; Wiley-Interscience: New York, 1999; p V88. (23) Kusanagi, H.; Tadokoro, H.; Chatani, Y. Double Strand Helix of Isotactic Poly(methyl methacrylate). Macromolecules 1976, 9, 531− 532. (24) Kusanagi, H.; Chatani, Y.; Tadokoro, H. The Crystal Structure of Isotactic Poly(methyl methacrylate): Packing-Mode of Double Stranded Helices. Polymer 1994, 35, 2028−2039. (25) Tanaka, K.; Takahara, A.; Kajiyama, T. Rheological Analysis of Surface Relaxation Process of Monodisperse Polystyrene Films. Macromolecules 2000, 33, 7588−7593. (26) Fujii, Y.; Akabori, K.-i.; Tanaka, K.; Nagamura, T. Chain Conformation Effects on Molecular Motions at the Surface of Poly(methyl methcrylate) Films. Polym. J. 2007, 39, 928−934. (27) Tanaka, K.; Fujii, Y.; Atarashi, H.; Akabori, K.; Hino, M.; Nagamura, T. Nonsolvents Cause Swelling at the Interface with Poly(methyl methacrylate) Films. Langmuir 2008, 24, 296−301.

K

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