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. 2022 Nov 1;38(45):13707–13719. doi: 10.1021/acs.langmuir.2c01788

Chain Movements at the Topmost Surface of Poly(methyl methacrylate) and Polystyrene Films Directly Evaluated by In Situ High-Temperature Atomic Force Microscopy

Kouki Koike 1, Jiro Kumaki 1,*
PMCID: PMC9671121  PMID: 36318939

Abstract

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The surfaces of polymeric materials are thermodynamically unstable, and the glass-transition temperature (Tg) is significantly lower than that in the bulk material. However, the mobility of the chains at the top of the surface has never been directly evaluated. In this study, the movements of the topmost chains of poly(methyl methacrylate) (PMMA) and polystyrene (PS) bulk films were observed in situ at high temperatures with atomic force microscopy in tapping mode. PMMA and PS chains started moving at ∼97 and ∼50 °C, respectively, which were slightly and significantly below the values of their bulk Tg (PMMA, 108 °C; PS, 104 °C), respectively. The activation energies of the apparent diffusion constants of PMMA and PS, derived by particle image velocimetry analysis, were 193 and 151 kJ mol–1, respectively, and reasonable for the glass transition. Movements of isolated PMMA chains deposited on a PMMA film by the Langmuir–Blodgett technique were also observed and confirmed to be essentially the same as those on the PMMA film surface.

1. Introduction

Since the invention of scanning probe microscopy, such as atomic force microscopy (AFM), polymer structures have been observed at the molecular level, and extensive studies have been reported.1−10 Currently, AFM is one of the most important research tools for studying polymer structures at the molecular level. We have been observing polymer structures mainly using two-dimensional (2D) films, especially Langmuir–Blodgett (LB) monolayers, which are suitable for observation by AFM at maximum resolution.5,9 We found that resolution close to or slightly less than 1 nm could be attained by conventional tapping-mode AFM and reported various molecular images: isolated chains,11 folded-chain crystal structures with tight and loose tie chains,12 in situ folded-chain crystallization,13 chain packing in amorphous monolayers,14 synthetic helical polymers,5,15 and a poly(methyl methacrylate) (PMMA) stereocomplex.16 Resolution higher than subnanometer was attained for the observation of poly(ethylene) crystals by torsional tapping mode17 and bimodal tapping mode18 by Hobbs et al. and Proksch et al., respectively. More recently, Korolkov et al. attained resolution close to the atomic level by the low-amplitude excitation of higher eigenmodes of a cantilever,19 and Heath et al. applied localization image reconstruction algorithms to peak positions in tapping-mode AFM images.20 However, to the best of our knowledge, the surfaces of conventional polymer films, such as polystyrene (PS) and PMMA, have never been observed at the molecular level. This is because (1) chains are closely condensed with separations much smaller than the resolution of usual AFM instruments; (2) unlike crystalline structures, it is difficult to use AFM to recognize irregular amorphous structures; and (3) the surfaces of bulk films are too rough to be observed at high magnification. In this article, we report that the information on chain movements of polymer surfaces was obtained by the direct in situ observation of bulk PS and PMMA films at high temperatures using conventional tapping-mode AFM (Figure 1).

Figure 1.

Figure 1

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Schematic representation of the evaluation of chain movement on the surfaces of PMMA and PS bulk films by AFM at high temperature.

Polymer surfaces are thermodynamically unstable, and the values of their glass-transition temperature (Tg) (surface Tg) are believed to be significantly lower than those of the bulk.21−26 In 1994, Keddie and co-workers studied the temperature dependence of the thickness of a PS ultrathin film on a silicone substrate by ellipsometry and found that the Tg was significantly reduced from the bulk value.27 Since then, extensive studies have been reported.21−26 Today, we believe that a thin film on a substrate has a three-layer structure: (1) a topmost layer at the air interface having a Tg several tens of degrees lower than that of the bulk; (2) beneath the topmost layer, a layer with a Tg comparable to the bulk value; and (3) a layer at the substrate interface with a Tg higher or lower than that of the bulk value, depending on the interaction with the substrate. The Tg values of ultrathin films have been studied by various methods, including the temperature dependence of film thickness measured by ellipsometry27 and X-ray and neutron reflectometry,28 the evaluation of molecular mobility using fluorescent probes,29,30 the temperature dependence of surface friction using lateral force microscopy,31 the near-edge X-ray adsorption fine structure analysis of a rubbed surface,32 the dielectric spectroscopy of thin films,33 and others. AFM has also been used to evaluate the decrease in Tg of thin film surfaces in a somewhat macroscopic manner by evaluating the penetration rate of gold nanoparticles deposited on a thin film34 and the fusion rate of nanoholes prepared by the deposition and removal of nanoparticles on a thin film.35

If the mobility of the topmost chains at the surfaces could be directly evaluated by AFM, then more direct information about the topmost surface Tg could be obtained. However, as mentioned above, due to the limited resolution of AFM, direct evaluation of the mobility of the topmost chains is difficult and has never been reported. In previous studies, to overcome this difficulty, we prepared a molecularly flat substrate by thermally imprinting a PMMA plate with an atomically stepped sapphire substrate or mica mold and deposited isolated PMMA chains by the Langmuir–Blodgett (LB) technique.36−38 The isolated chains deposited on the molecularly flat substrates were clearly observed by AFM, and the isolated chains and nanostructure at the flat substrate surface started moving at a temperature close to the bulk Tg.38 During the course of those studies, we considered possible ways to evaluate the movements of the topmost chains of films by using bulk PMMA films without nanoimprinting and the deposition of isolated PMMA chains.

In this article, we report that thin films of PMMA and PS were prepared by dip coating, and the movements of the topmost chains were directly evaluated by in situ real-time AFM observation of the surface at high temperatures (Figure 1). In addition to PMMA, we selected PS as an additional research sample because both of them are the most extensively studied polymers for the reduction of the surface Tg.26 We quantitatively analyzed the surface movements by image analysis and showed that the activation energies were reasonable for the glass transitions. The decrease in surface Tg from the bulk value depended on the polymer composition and was rather small, approximately 10 °C, for PMMA but ∼54 °C for PS. Furthermore, we showed that the movements of the PMMA surface were essentially equivalent to those of isolated PMMA chains deposited on the PMMA film by the LB technique. AFM observations of the PMMA and PS film surfaces were used successfully to evaluate the movements of the topmost chains at the film surfaces.

2. Experimental Section

2.1. Materials

PMMA with a number-average molecular weight (Mn) of 1.81 × 105 and a molecular weight distribution (Mw/Mn) of 1.84 and PS with a Mn of 1.40 × 105 and a Mw/Mn of 2.24 were purchased from Sigma–Aldrich. The molecular weight was measured by size exclusion chromatography (SEC) in tetrahydrofuran using PMMA and PS standards (Shodex, Tokyo, Japan) for calibration. The bulk Tg of PMMA and PS was measured with a Q-2000 differential scanning calorimetry (DSC) system (TA Instruments, New Castle, DE, USA) at a heating rate of 10 °C/min under a nitrogen atmosphere after the removal of the thermal history by heating to 150 °C (10 °C/min), and the samples were held at this temperature for 5 min and then cooled to 20 °C (10 °C/min) in advance of the measurements. The bulk Tg of PMMA and PS was 108 and 104 °C, respectively. A 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). Mica was purchased from Okensyoji Co. (Tokyo, Japan). Highly purified benzene (Infinity Pure, Fujifilm Wako Pure Chemical Corp., Osaka, Japan) was used as the solvent for the spreading solutions in the preparation of LB films. Water was purified using a Milli-Q system and used as the subphase for LB preparation.

2.2. Dip-Coated Film Preparation

Glass slides with a thickness of ∼1 mm were immersed in a PMMA or PS chloroform solution at a concentration of ∼4.4 wt % and withdrawn from the solution at a rate of 0.5 mm/s using a computer-controlled dipping machine (YN2-TKB, SAKIGAKE-Semiconductor Co., Ltd., Kyoto, Japan). The films were dried at room temperature in air for 24 h and then annealed at 150 °C for an additional 24 h in vacuo. The thickness of the dip-coated film was measured with an ellipsometer and was 430 nm for PMMA and 350 nm for PS. The interaction at the substrate/film interface is reported to propagate through the film and affect the dynamics at the topmost air surface by a long-range interaction; the effect extends as far as 180 nm39 or 250 nm.40 The PS and PMMA films are sufficiently thick that the effect of the film/substrate interface on the surface Tg is negligible.

2.3. Langmuir–Blodgett (LB) Film Deposition

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 (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 (Figure S1 in the Supporting Information). The PS-b-PMMA monolayer was deposited on a dip-coated PMMA film and mica for comparison by pulling them out of the water at a rate of 4.2 mm/min (the vertical dipping method). The monolayer was deposited at 0 mN/m over an area wider than the onset of the π–A isotherm with the moving barrier stopped to deposit individual PS-b-PMMA molecules prior to compression to form a condensed film. The details were shown in a previous report.38

2.4. AFM Observation and Image Analysis

The samples were observed with a commercial AFM (NanoScope IIIa and IIId/multimode AFM unit, Bruker, Santa Barbara, CA, USA) at room temperature or by high-temperature AFM (Agilent 5500) with a temperature-controlled sample plate enclosed in an environmental chamber for stable temperature control (Agilent Technologies, Inc., Santa Clara, CA, USA) at high temperatures with supersharp silicon cantilevers (model, SSS-NCH; nominal spring constant, 42 N/m; nominal tip radius, 4 nm; NanoWorld AG, Neuchâtel, Switzerland) in air in tapping mode. Preliminary experiments indicated that similar results were also obtained with standard silicon cantilevers (model, NCH; nominal spring constant, 40 N/m; nominal tip radius, 8 nm; Bruker). For high-temperature observations, the samples were first observed at a prescribed temperature and then heated stepwise to the next temperature at a heating rate of 5 °C/min. The temperature was allowed to stabilize until the drift became negligible for 30 min, at which point the AFM observation started and the sequence was repeated. The surface temperature of a dip-coated film on the heating plate was measured by a thermocouple sandwiched with a piece of mica, and the thermal loss due to the slide glass and the dip-coated film was corrected. The obtained AFM images are presented in this article without any image processing except flattening. Movies were constructed from sequences of AFM images by manually canceling the drifts of the images by ImageJ (public domain software from the National Institutes of Health) and further registered using the StackReg plug-in of ImageJ developed by Philippe Thévenaz to eliminate the small residual drift effect. The displacement of the surface structure was evaluated using the iterative particle image velocimetry (PIV) plug-in of ImageJ developed by Tseng Qingzong using two odd or even sequential images with the same slow scan direction to minimize the effect of deformation due to drift. The PIV plug-in was used in the template matching method by a normalized correlation coefficient algorithm. The plug-in searches identifying features that are present in the interrogation window in the first image and searches for them in the larger searching window for the next image. Then the plug-in repeats this procedure three times while reducing the interrogation and searching windows with the PIV information from the previous round as guidance for determining the correlation peak. The typical interrogation/searching window sets were 128/256, 64/128, and 32/64 pixels using cropped images from the original 750 pixel × 750 pixel (1 μm × 1 μm) AFM images, except where otherwise specified. The modification of the interrogation/searching window sizes and the magnification of the original AFM images slightly affected the absolute value of the displacement vectors; however, the effect on the temperature dependence of the displacement was negligible.

3. Results and Discussion

3.1. Chain Movements of a PMMA Film Surface

Figure 2 shows AFM height images of a PMMA film dip-coated on a slide glass observed at various surface temperatures. The root-mean-square (RMS) roughness was 0.28 nm at 25 °C and almost constant up to 119 °C. As shown in Figure 2, no meaningful structure or structure difference at various temperatures was found in this static AFM observation. However, in dynamic observations, we could recognize significant movements at higher temperatures. The sample was observed multiple times at each temperature, and a movie was constructed from 14 sequential images. Each image took 2.84 min, and the total movie took ∼40 min, as shown in the Supporting Information (Movie S1). The number of images in the movie was limited, and we recommend that readers view the movie in repeated replay mode in the viewer software. As shown in Movie S1, the surface structures were fixed at 25 °C, but vigorous movements of nanostructures less than a few tens of nanometers in size started at approximately 101 °C and became more significant with increasing temperature. Some alternating expansion and compression of images in the vertical direction were typically seen at 75–92 °C; these were distortions of images due to residual thermal drift that could not be removed by image processing. AFM evaluates a sample with a fast scan in a single horizontal direction but a slow scan in the vertical direction, alternatively changing the direction between downward and upward. Therefore, if a constant linear thermal drift was present during the observation, then the resultant images alternatively expanded and compressed mainly in the vertical direction with a minor distortion in the horizontal direction. Therefore, the alternative elongation and compression of entire images, except nanoscale movements, are not true movements but artifacts. Thus, the structure was fixed at 25–84 °C. If we assume that the thermal drift was linear at a constant rate and use sequential odd or even images with the same slow scan direction, then AFM always scans the same parallelogram shape. As a result, no alternative elongation or expansion of images is seen. Movie S2 was constructed from the third and fifth images of Movie S1, which were scanned in the same slow scan direction. Since most of the alternative expansion and compression of images were removed, we recognized more clearly that the movements at the nanoscale started at approximately 96 °C (shown in Movie S2 in repeating replay mode in the viewer software). In our typical observations, the thermal drift during the recording of each image was ca. 1% of the image size on average, and the error in the evaluation of a displacement described below was negligibly small. Figure 3a shows summed images of the third and fifth images in Movie S1, which were colored yellow and magenta, respectively. The time lapse between the two images was 5.67 min. Yellow and magenta are complementary color pairs. At 25 °C, no movements were observed, and as a result, the summation was a black and white image. With increasing movement between the two images, yellow and magenta areas were separately observed with increases in the size and area (101–119 °C). The structure of the moving units is not clear in the low-magnification movies. Movie S3 is a magnified version of Movie S1. The moving structures in condensed regions are not clear; however, in the dilute regions indicated by the arrows, chain-like structures 4 to 5 nm in width are clearly observed, which appeared to be moving, and the movements became more vigorous at high temperatures. The size of the structure moving in the condensed regions is quite similar, so it is reasonable to assume that the condensed regions are also composed of similar chain-like structures. Figure 3b shows a magnified AFM height image of the PMMA film at 110 °C with a height profile along the yellow line. As indicated, the height of the chain-like structures was 0.16–0.42 nm, which was comparable to the height of PMMA chains on mica (∼0.3 nm in Figure 8b). The width was ca. 4 to 5 nm, which should be broadened to a similar extent due to the tip radius (nominal diameter, 4 nm). Due to the broadening by the AFM tip, we could not precisely conclude, but we note that it could be a single chain or a bundle composed of a small number of PMMA chains. Figure 4 shows a vector plot for the displacement between the third and fifth images of Movie S1 obtained by PIV analysis. The displacement between two images is indicated by vectors. The size of the vector was enlarged by a factor of 5 for clarity. As shown, the surface structures apparently started moving at approximately 101 °C. The mean displacement at each temperature is plotted in Figure 5a as a blue filled triangle. The displacement started increasing at approximately 97 °C, which was only slightly lower than the bulk Tg (108 °C) of PMMA. No significant Tg decrease at the surface of the PMMA film occurred. The displacement obtained here was up to several nm/5.67 min. We derived the apparent diffusion constant by assuming that the surface movement obeyed the two-dimensional (2D) diffusion equation

3.1. 1

where λ is the displacement, D is the apparent diffusion constant, and t is the time. Figure S2a in the Supporting Information shows the λ2 plot as a function of time for the PMMA film at 101, 110, 115, and 119 °C. λ2 increased linearly with time, indicating that the movements of the nanostructure at the PMMA film surface obeyed the 2D diffusion equation in this time and temperature range. Figure 5b shows an Arrhenius plot of the apparent D estimated by eq 1 (blue filled circles). The activation energy obtained for the apparent D at temperatures higher than 96 °C was 193 kJ mol–1. The activation energies reported for the glass transition of PMMA in the literature are summarized in Figure 5c. The reported activation energies are scattered and include 400 kJ mol–1 by McCrum et al.,41 274 kJ mol–1 by Jud et al.,42 109 kJ mol–1 by Van Alsten et al.,43 and 371 kJ mol–1 by Shearmur et al.44 Our value is in the range of values reported for the glass transition and is at the somewhat lower end of that range. Some of the D values reported in the literature for PMMA systems with molecular weights similar to those in our study are shown in Figure 5b as blue open symbols. Liu et al. measured the interdiffusion of PMMA bilayers utilizing gold nanoparticles at the interface by X-ray reflectometry.45 In other studies, interdiffusion at the interface of deuterated and protonated PMMA was measured by neutron reflectometry (Lin et al.46), attenuated total reflectance infrared spectroscopy (ATR-IR) (Van Alsten et al.43), and nuclear reaction analysis (Shearmur et al.44). The D values reported by Van Alsten et al. were large and appeared to be consistent with our values, but most of the D values were shifted to higher temperatures and smaller than our values if compared at the same temperature. This is reasonable because high-molecular-weight polymer chains should diffuse in the entangled 3D melt by reptating, whereas at the surface, as shown in the movies, chains can move laterally and D should be larger. Although the D values in this study were generally larger than those in the 3D melt, the movement observed here had an activation energy that was reasonable for the glass transition. We concluded that the movements observed here were closely related to the glass transition.

Figure 2.

Figure 2

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AFM height images of a PMMA film at various surface temperatures. A movie constructed from sequential AFM images (14 images for 40 min) is available in Movie S1 in the Supporting Information. To observe the surface movements, we recommend viewing the movie in repeated replay mode in the viewing software. There appears to be no surface movement below 92 °C, and movement begins above 96 °C.

Figure 3.

Figure 3

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(a) Composite of the third and fifth AFM images of Movie S1 at 25, 101, 115, and 119 °C. Magnified images of the areas indicated by the yellow squares are also shown. The third and fifth images are colored yellow and magenta, respectively. When there is no movement at 25 °C, the composite image is black and white; with an increase in the surface movements, the yellow and magenta areas are separately observed. As shown in the magnified images, string-like structures with a width of ca. 4 nm are present. (b) Magnified AFM height image of a PMMA film at 110 °C with a height profile along the yellow line.

Figure 8.

Figure 8

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(a) Schematic representation of PS-b-PMMA on a water surface. AFM height images of PS-b-PMMA chains deposited on (b) mica and (c) a PMMA film from a water surface in a dilute state at 0 mN/m. The deposition direction was vertical. (d) AFM height images of PS-b-PMMA chains deposited on a PMMA film observed in situ at various surface temperatures. A movie constructed from sequential AFM images (12 images for 34 min) is available in Movie S6 in the Supporting Information. To observe the chain movements, we recommend viewing the movie in repeated replay mode in the viewer software. It appears that there was no movement of the PMMA chains of PS-b-PMMA below 101 °C, but the chains started moving above the bulk Tg at 106 °C. (b) Reproduced with permission from ref (36). Copyright 2019 American Chemical Society.

Figure 4.

Figure 4

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Vector plot of the displacement between the third and fifth images of Movie S1 obtained by PIV analysis. The vector is enlarged by a factor of 5 for clarity.

Figure 5.

Figure 5

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(a) Mean displacements of PMMA and PS films obtained by PIV analysis as functions of the surface temperature. The PMMA and PS surfaces started moving at approximately 97 and 50 °C, respectively. The PMMA and PS bulk Tg values are also indicated. (b) Arrhenius plot of apparent diffusion constants calculated from the mean displacement of a PMMA film (filled blue circles) and a PS film (filled red circles). The activation energy was approximately 193 kJ mol–1 for the PMMA film and 151 kJ mol–1 for the PS film. Diffusion constants reported in the literature are shown by blue open symbols for PMMA and red open symbols for PS. (c) The activation energy reported for the glass transition of PMMA and PS is summarized in comparison with the values obtained in this work. Molecular weights are indicated in parentheses. d- and h- indicate the molecular weights of the deuterated and hydrogenated polymers, respectively, used for interdiffusion measurements.

3.2. Chain Movements of a PS Film Surface

Figure 6 shows AFM height images of a PS film dip-coated on a slide glass observed at various surface temperatures. The RMS was 0.23 nm at 25 °C and almost constant up to 101 °C. Similar to the PMMA film, no meaningful structure or structural differences at various temperatures were observed in this static AFM observation. However, in dynamic observations, we could recognize significant movements at higher temperatures. A movie constructed from eight sequential images (total time: 22.7 min) is available in the Supporting Information (Movie S4). As shown in Movie S4, movements started at approximately 60 °C, which was much lower than that of the PMMA film and significantly lower than the Tg of bulk PS (104 °C), and they became more significant with increasing temperature. In the movie, chain-like structures several nanometers in width (indicated by the arrows in a magnified image of Figure 6b) appear to be moving, which was similar to the surface of the PMMA films (zoom in on Movie S4 to see the chain-like structures). Figure 7 shows a vector plot of the displacement between the fifth and seventh images with the same slow scan direction by PIV analysis. A movie constructed from the fifth and seventh images of Movie S4 is available in Movie S5 in the Supporting Information (we recommend using repeated replay mode). As shown in Figure 7, the movements started at approximately 60 °C, and the displacement increased with increasing temperature. The mean displacement at each temperature is plotted as red filled triangles in Figure 5a. The movements started at approximately 50 °C, which was 54 °C lower than the PS bulk Tg (104 °C). A significant decrease in Tg at the surface was observed for PS. The apparent D was estimated from the displacement using eq 1. A plot of λ2 as a function of time for the PS film at 84, 96, and 101 °C is shown in Figure S2b; λ2 increased linearly with time. The movements of the chain-like structure at the PS film surface obeyed the 2D diffusion equation in the observed time and temperature ranges. The estimated D values are plotted as red filled circles in Figure 5b. The values were somewhat scattered, but the activation energy derived from D values at temperatures higher than 50 °C was ∼151 kJ mol–1. The activation energies reported for the glass transition of PS in the literature are summarized in Figure 5c; they include 269 kJ mol–1 by Whitlow et al.,47 264 kJ mol–1 by Kim et al.,48 and 176 kJ mol–1 by Antonietti et al.49 The activation energy observed in our study was comparable to or slightly lower than the reported values. Some of the D values reported for PS systems with a molecular weight similar to that used in our experiment are shown as red open symbols in Figure 5b. Antonietti et al. studied the diffusion of fluorescein-labeled PS chains in a matrix,49 Whitlow et al. studied interdiffusion at the interface of deuterated and protonated PS by secondary ion mass spectrometry (SIMS),50 and Green et al. used Rutherford backscattering spectrometry to study the displacement of the interface marked by gold particles.51 The D values in 3D melts appeared to be shifted to higher temperatures. The D values in our study were roughly shifted by ∼30 °C below the literature value in 3D melts. The shift of the D value is smaller than the decrease of the surface Tg evaluated by the onset of the movements (∼54 °C). It is probably not appropriate to compare D values in this study with those obtained by different methods. We note that the apparent D value can be evaluated from AFM observations and that the activation energy was reasonable relative to those expected for glass transitions.

Figure 6.

Figure 6

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(a) AFM height images of a PS film at various surface temperatures. A movie constructed from sequential AFM images (eight images for 22.7 min) is available in Movie S4 in the Supporting Information. To view the surface movements, we recommend viewing the movie in repeated replay mode in the viewer software. It appears that there was no surface movement below 50 °C, and movement started above 60 °C. (b) Magnified AFM height image at 25 °C. Some string-like structures are indicated by yellow arrows. The significant movements of string-like structures are visible at higher temperatures in Movie S4.

Figure 7.

Figure 7

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Vector plot of the displacement between the fifth and seventh images of Movie S4 obtained by PIV analysis. The vector is enlarged by a factor of 3 for clarity.

As described above, the significant movements of chain-like structures several nanometers in width were observed at the surfaces of PMMA and PS films at high temperatures, and the activation energies of the movements were consistent with those expected for the glass transition. The movements observed here should be similar to the chain movements; however, the chain-like structures were several nanometers in width and observed to be broadened due to the tip diameter of the cantilever. We could not conclude that the chain movements were directly evaluated. If we could deposit isolated polymer chains on the same polymer film and compare the movements of the deposited isolated chains on the surface with the chain-like movements of the films observed here, then we could clarify how the chain-like movements relate to true chain movements at the surface.

Next, we compared the movements of isolated PMMA chains deposited on a PMMA film with the chain-like movements of the PMMA film surface observed here.

3.3. Deposition of Isolated PMMA Chains on a PMMA Film and AFM Observation of the Chain Movements

To use AFM to observe isolated polymer chains deposited on a substrate, we believe that the substrate should be sufficiently flat and the roughness should be smaller than the height of the isolated polymer chains. Thus, atomically flat substrates, such as mica and highly ordered pyrolytic graphite (HOPG), are used as substrates for the observation of deposits of isolated chains. We previously reported the preparation of molecularly flat PMMA substrates (RMA ≈ 0.20 and 0.15 nm) by heat pressing a commercial PMMA plate (2 mm thick, RMS ≈ 0.42 nm) with an atomically stepped sapphire substrate or a mica mold, using the LB technique to deposit isolated PMMA chains on the molecularly flat PMMA substrates, and using AFM to observe the isolated PMMA chains.34−36 Furthermore, we reported, but only qualitatively, that the deposited isolated PMMA chains and the nanostructures observed at the surface of the molecularly flat PMMA substrate started to move close to the Tg of bulk PMMA, and no significant decrease in Tg was observed.38 However, in this case, (1) the surface Tg of the PMMA plate may have increased by the thermal press process and (2) the PMMA plate was a commercially available cast plate obtained by the thermal polymerization of MMA and contained a small amount of release agent, which may have prevented the decrease in the surface Tg. As a result, the observed Tg may not have been apparently reduced from the bulk value.

In this study, we attempted to observe and evaluate the movements of isolated PMMA chains deposited on a dip-coated PMMA film without the effects of any additives or thermal press process. Is it possible to observe isolated PMMA chains deposited on the rough surface of a dip-coated film and use AFM to evaluate their movements?

To deposit PMMA as isolated chains by the LB method, a PMMA monolayer should be deposited in a dilute state at 0 mN/m before being compressed to form a condensed film. Since PMMA is deposited imhomogeneously on a substrate in the dilute state and it is expected to be difficult to find deposited PMMA chains, especially on the rough dip-coated film surface, we used a PS-b-PMMA block copolymer instead of a PMMA homopolymer. As shown in Figure 8a, PS without hydrophilic groups did not spread on the surface of water but formed particles, much thicker than monolayers, that were used as probes to find the PMMA chains emanating from them.11,36−38Figure 8b shows an AFM height image of PS-b-PMMA deposited on mica in a dilute state. PS particles with a thickness of ∼3.1 nm surrounded by a condensed PMMA monolayer with a thickness of ∼0.3 nm were observed. On the PMMA film, isolated PMMA chains emanating from PS particles were also clearly observed but extended in the dip direction. The observed structures were clearly different from those of the PMMA film surface without the deposition of PS-b-PMMA chains (Figure 2) and were PS-b-PMMA deposited on the PMMA film. Why were the PMMA chains deposited on the PMMA film elongated? Hydrophilic mica strongly adsorbed the PMMA monolayer, and a condensed PMMA monolayer formed on the water surface was transferred, maintaining its condensed structure. However, the hydrophobic PMMA film weakly adsorbed the PMMA monolayer, and PMMA chains were stretched in the dip direction during LB deposition.36−38 The widths of PMMA chains were distributed, but the width of the thinnest part was ∼4 to 5 nm, the nominal tip radius of the cantilever was 4 nm, and the PMMA chain should be observed to be broadened by the tip to a similar extent. The thinnest parts of the PMMA chains were similar to a single chain. The height of the thinnest part was ∼0.6 nm, which was slightly higher than that on mica (0.3 nm), probably due to the deposition of the chains on the rough surface of the dip-coated PMMA film.

Figure 8d shows the AFM height images observed at various temperatures. A movie constructed from 12 sequential images for 34 min observed at 25, 97, 101, 106, 110, 115, and 119 °C is available in the Supporting Information (Movie S6). A movie of the fifth and seventh images (Movie S7) and the vector diagram of the displacement evaluated by PIV analysis (Figure S3) are also available in the Supporting Information. Figure 9 shows the average displacement at each temperature in comparison with that of the PMMA film surface. The contrast of the images comes mostly from the deposited PMMA chains, indicating that the motion of the deposited PMMA isolated chains was mainly evaluated in the PIV analysis. At low temperatures, the deposited PMMA chains had slightly higher mobility than the PMMA film surface, and the temperature at which the movements become significant was ∼106 °C, which was slightly higher than the temperature of the PMMA film surface (∼97 °C). However, in the high-temperature region above 100 °C, the movements of the deposited PMMA chain and the surface of the PMMA film were almost the same, indicating that the movements observed for the PMMA film surface were similar to the movements of the PMMA chain deposited on the substrate. The chains at the surface of the PMMA film were expected to be partially exposed outside the surface, while the residual parts were embedded inside the film. However, the motion of the PMMA chains exposed outside the surface was essentially the same as that of the chains deposited two-dimensionally on the surface of the film. The motion we observed here is a significantly localized motion of the polymer chain, less than several nanometers, and the motion of chains exposed outside of the surface may not be affected by the chain part embedded inside the film. However, a large difference in dynamics between the surface chains of the film and the chains deposited on the surface would be expected in the translational motion of the chains over a long period of time.

Figure 9.

Figure 9

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Mean displacement of PS-b-PMMA chains on a PMMA film in comparison with that of a PMMA film obtained by PIV analysis as a function of surface temperature. PS-b-PMMA undergoes slightly higher movement at 25–101 °C but quite similar movements at higher temperatures, indicating the quite similar movements by chains at the surface of the PMMA film and PMMA chains deposited on the film. No significant difference was observed.

Since the motion of the PMMA chains deposited on the film surface by the LB method and that of the surface of the PMMA film were almost the same, we concluded that the motion of the dip-coated film observed by in situ AFM was closely correlated to the chain motion of the film surface.

We note that there is no significant difference between the surface Tg of the dip-coated pure PMMA film without the thermal press process and the surface Tg of the molecularly flat PMMA surface prepared with the thermal press process in a previous study,38 indicating that the thermal press process and additives such as the release agent did not significantly affect the surface Tg of PMMA and the decrease in the surface Tg of PMMA was limited.

The decrease in the surface Tg of the PMMA film was limited, but for the PS system, a significant decrease in Tg of as high as 54 °C was observed, indicating that the decrease in surface Tg was greatly influenced by the polymer composition. A considerable number of studies related to the surface Tg of PS and PMMA thin films have been reported. A number of studies are summarized in a recent comprehensive review, as shown in Table 2 for PS, Table 4 for PMMA, and Table 5 for a comparison between PS and PMMA in ref (26). A larger Tg decrease at the surface of PS compared to that of PMMA, similar to our study, was reported as follows. Torkelson and co-workers studied the surface Tg of PS and PMMA by a localized fluorescence method and found that the Tg of the topmost 12–14 nm layer of thick films (>250 nm) decreased 32 K for PS29 but decreased only 7 K for PMMA.52 The Tg of free-standing PS and PMMA films measured by ellipsometry indicated that the Tg at a thickness of 40 nm decreased 52 K for PS53 but only 15 K for PMMA54 for samples of similar molecular weight (∼7.7 × 105–7.9 × 105 g/mol). Paeng et al. evaluated the thickness of the surface mobile layer by measuring the reorientation of diluted fluorescent probes by the photobleaching technique and found that the thickness of the mobile layer was 7 nm for PS but 4 nm for PMMA at the bulk Tg.55,56 Inoue and co-workers studied the Tg distributions in PS and PMMA multilayer films composed of deuterated and hydrogenated polymers by neutron reflectivity and found that the Tg of the topmost surface (∼4 nm thick) decreased 20 K from the bulk value for PS, but no decrease was observed for PMMA. They interpreted these results to mean that the surface Tg was affected by the substrate interface due to the small total thickness of the multilayered film (∼90 nm).28,57 However, this may also indicate a small decrease in the surface Tg of PMMA.

4. Conclusions

The motion of PMMA and PS dip-coated film surfaces was observed in situ by high-temperature AFM. Vigorous movements of chain-like structures several nanometers in width were observed at higher temperatures. PIV analysis of the movements indicated that the movements started at ∼97 °C for PMMA, slightly below the bulk Tg (108 °C), but at ∼50 °C, significantly below the bulk Tg (104 °C), for PS. The apparent diffusion coefficient was derived from the displacement of the movements, and the activation energy of the diffusion coefficient was 193 kJ mol–1 for PMMA and 151 kJ mol–1 for PS, which was reasonable relative to the activation energy of the glass transition reported in the literature. The movements of isolated PMMA chains deposited on a PMMA film by the LB technique were evaluated and found to be essentially the same as the movements of the PMMA film surface. Thus, the chain-like movements observed at the film surface by AFM were similar to the chain movements of the film surface.

Previously, AFM was used to evaluate the surface Tg of film surfaces in somewhat macroscopic observations, such as the sedimentation of gold nanoparticles deposited on a film surface34 and the welding of nanoholes formed by nanoparticles on a film surface.35 The present results show that the chain movements of a film surface can be directly evaluated by dynamic AFM observations of the film surface.

Researchers generally believe that it is not possible to use AFM to obtain meaningful images of individual chains at film surfaces because the chains are condensed with an interchain separation much smaller than the resolution of AFM (usually ∼1 nm for tapping-mode AFM). Why is it possible to use AFM to evaluate surface motion similar to chain movements? As schematically shown in Figure 10, if the chains (gray chains) are uniformly condensed with an interchain separation smaller than the AFM resolution (typically ∼1 nm in tapping-mode AFM), then the single chains cannot be resolved with AFM. However, in reality, the surface of the film is rough. If a part of the chains exposed from the surface are deposited on the surface with a separation larger than the AFM resolution, then AFM could observe movements similar to single-chain movements. Researchers sometimes assume that the film surface is uniform, but in reality, it is not. If chains are discretely deposited on top of a film, then it is not surprising that AFM can evaluate motions similar to those of single chains. We propose that the chain-like motion we observed here was similar to the motion of single chains or bundles of small numbers of chains at the film surface.

Figure 10.

Figure 10

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Schematic representation of a film surface. If the film surface is flat and polymer chains are condensed with a chain–chain distance of less than the AFM resolution (gray chains), then it is not possible to observe movements close to single chains. However, the film surface is rough. If a part of the chain is exposed out of the surface and deposited on the surface discretely, as shown by the yellow chains, then movements close to single chains could be observed by AFM.

Acknowledgments

We thank associate professor Daisuke Yokoyama, Yamagata University, for the thickness measurements made with ellipsometry. We thank Daichi Ikejima, Yamagata University, for the Tg measurements made with DSC. This work was supported by JSPS KAKENHI grant numbers JP20H05201, JP21H01993, and JP21K18993.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.2c01788.

  • π–A isotherm of PS-b-PMMA; mean squared displacements of PMMA and PS films as functions of time; vector plot of the displacement of PMMA-b-PS deposited on the PMMA film; and a list of the supporting movies (PDF)

  • Movie constructed from sequential AFM height images of a PMMA film observed at various surface temperatures (14 images for 40 min) (Movie S1) (AVI)

  • Movie constructed from the third and fifth images of Movie S1 (Movie S2) (AVI)

  • Magnified version of Movie S1 (Movie S3) (AVI)

  • Movie constructed from sequential AFM height images of a PS film observed at various surface temperatures (8 images for 22.7 min) (Movie S4) (AVI)

  • Movie constructed from the fifth and seventh images of Movie S4 (Movie S5) (AVI)

  • Movie constructed from sequential AFM height images of PS-b-PMMA chains deposited on a PMMA film observed at various surface temperatures (12 images for 34 min) (Movie S6) (AVI)

  • Movie constructed from the fifth and seventh images of Movie S6 (Movie S7) (AVI)

The authors declare no competing financial interest.

Supplementary Material

la2c01788_si_001.pdf (737.1KB, pdf)
la2c01788_si_002.avi (13.2MB, avi)
la2c01788_si_003.avi (5.6MB, avi)
la2c01788_si_004.avi (3.7MB, avi)
la2c01788_si_005.avi (5.3MB, avi)
la2c01788_si_006.avi (4MB, avi)
la2c01788_si_007.avi (5.6MB, avi)
la2c01788_si_008.avi (2.8MB, avi)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

la2c01788_si_001.pdf (737.1KB, pdf)
la2c01788_si_002.avi (13.2MB, avi)
la2c01788_si_003.avi (5.6MB, avi)
la2c01788_si_004.avi (3.7MB, avi)
la2c01788_si_005.avi (5.3MB, avi)
la2c01788_si_006.avi (4MB, avi)
la2c01788_si_007.avi (5.6MB, avi)
la2c01788_si_008.avi (2.8MB, avi)

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