“Reptational” Movements of Single Synthetic Polymer Chains on Substrate Observed by in-Situ Atomic Force Microscopy

In Situ Real-Time Atomic Force Microscopy Observation of the Surface Mobility on Each Domain of a Polystyrene-b-poly(methyl methacrylate) Film at High Temperatures

The surface chain movements within the microdomains of a polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) and corresponding homopolymer films were observed via in situ real-time atomic force microscopy (AFM) at high temperatures and analyzed quantitatively using particle image velocimetry (PIV). At low temperatures, mobility within the PS microdomains resembled that within the PS homopolymer film, but movements in the PMMA microdomains were notably accelerated compared to the PMMA homopolymer. Conversely, at high temperatures, mobility within both PS and PMMA microdomains was considerably suppressed compared to their respective homopolymer films, likely owing to the fixed linkage of the block chains at the microdomain interface. This combination of real-time AFM observation and PIV analysis is an effective method for quantitatively evaluating surface chain mobility in real space.

In situ AFM Observation of the Movements of Isolated Isotactic Poly(methyl methacrylate) Chains in a Precursor Film of an Oligo(methyl methacrylate) Droplet Spreading on Mica

Atomic force microscopy (AFM) is a powerful tool to observe polymer chains at the molecular level. In this study, we show that the movements of isolated linear polymer chains in a precursor film of a droplet of an oligomer spreading on a substrate could be visualized in situ at the molecular level by AFM for the first time. The system was an isotactic poly(methyl methacrylate) (it-PMMA) solubilized in an oligo(MMA) matrix (it-PMMA/oligo(MMA) = 1/10,000 w/w) spreading on mica under high humidity. Because of the limited resolution of the AFM instrument, condensed linear polymer chains could not be visualized, but a small amount of it-PMMA chains that were solubilized as isolated chains in the oligo(MMA) matrix could be visualized in the precursor film, the contrast of which came from a large difference in glass transition temperature (T_g) of it-PMMA and oligo(MMA). The it-PMMA chains in the precursor film spread in the radial direction of the droplet with vigorously changing chain conformations. The spreading rate of it-PMMA chains under 72% relative humidity was ∼1/30 of the spreading rate of the oligo(MMA) matrix, which was estimated based on the decrease in the volume of the macroscopic droplet. The spreading of the it-PMMA chains and droplet strongly depended on humidity and was suppressed with the decrease in humidity, most likely because of the increase in friction with the substrate. The difference in the spreading rate of it-PMMA and oligo(MMA) further increased under low humidity. The dynamic molecular information of a precursor film by AFM should help to elucidate the wetting dynamics on a substrate.

Poly(styrene-co-butadiene) random copolymer thin films and nanostructures on a mica surface: morphology and contact angles of nanodroplets

The self-assembly of poly(styrene-co-butadiene) random copolymers on mica surfaces was studied by varying solution concentrations and polymer molecular weights. Toluene solutions of the poly(styrene-co-butadiene) samples were spin coated onto a mica surface and the resulting polymer morphology was investigated by atomic force microscopy. At higher concentrations, thin films formed with varying thicknesses; some dewetting was observed which depended on the molecular weight. Total dewetting did not occur despite the polymer's low glass transition temperature. Instead, partial dewetting was observed suggesting that the polymer was in a metastable equilibrium state. At lower concentrations, spherical cap shaped nanodroplets formed with varying sizes from single polymer chains to aggregates containing millions of chains. As the molecular weight was increased, fewer aggregates were observed on the surface, albeit with larger sizes resulting from increased solution viscosities and more chain entanglements at higher molecular weights. The contact angles of the nanodroplets were shown to be size dependent. A minimum contact angle occurs for droplets with radii of 100-250 nm at each molecular weight. Droplets smaller than 100 nm showed a sharp increase in contact angle; attributed to an increase in the elastic modulus of the droplets, in addition, to a positive line tension value. Droplets larger than 250 nm also showed an increased contact angle due to surface heterogeneities which cannot be avoided for larger droplets. This increase in contact angle plateaus as the droplet size reaches the macroscopic scale.

Conformation and dynamics of single polymer chain studied by optical microscopy techniques beyond the diffraction limit

The origin of the unique properties of a polymer material is the large entropic term of a single molecule, which has a chain-like structure with a large molecular weight. From the viewpoint of understanding the fundamental polymer physics, conformation of the single polymer chain is one of the most important matters; however, it has been difficult to examine the behavior of a single chain because of the limitation of conventional experimental methods. Recent developments in optical microscopy allow the fluorescence imaging beyond the diffraction limit of light, and the author's group showed that the conformation and the dynamics of a single polymer chain can be examined by the high-resolution fluorescence imaging. This review presents the application of optical microscopy with nanometric spatial resolution to study the polymer materials at the single-chain level.

Nanoscale Study of Polymer Dynamics

The thermal motion of polymer chains in a crowded environment is anisotropic and highly confined. Whereas theoretical and experimental progress has been made, typically only indirect evidence of polymer dynamics is obtained either from scattering or mechanical response. Toward a complete understanding of the complicated polymer dynamics in crowded media such as biological cells, it is of great importance to unravel the role of heterogeneity and molecular individualism. In the present work, we investigate the dynamics of synthetic polymers and the tube-like motion of individual chains using time-resolved fluorescence microscopy. A single fluorescently labeled polymer molecule is observed in a sea of unlabeled polymers, giving access to not only the dynamics of the probe chain itself but also to that of the surrounding network. We demonstrate that it is possible to extract the characteristic time constants and length scales in one experiment, providing a detailed understanding of polymer dynamics at the single chain level. The quantitative agreement with bulk rheology measurements is promising for using local probes to study heterogeneity in complex, crowded systems.

Nanotemplate-directed DNA segmental thermal motion

Nanotemplate directed DNA segmental thermal motion on molecular nanotemplates on graphite was directly observed and characterized using AFM in a liquid.

Light-induced contraction and extension of single macromolecules on a modified graphite surface

Synthetic rigid-rod polymers incorporating multiple azobenzene photoswitches in the backbone were deposited from solution onto a monolayer of octadecylamine covering the basal plane of graphite. Large contractions and extensions of the single macromolecules on the surface were induced by irradiation with UV and visible light, respectively, as visualized by scanning force microscopy. Upon contraction, the single polymer chains form more compact nanostructures and also may move across the surface, resembling a crawling movement. We attribute the efficiency of these processes to the low mechanical and electronic coupling between the surface and polymers, the high density of azobenzenes in their backbones, and their rigidity, allowing for maximized photodeformations. The visualization of on-surface motions of single macromolecules directly induced by light, as reported herein, could help promote the development of optomechanical nanosystems.

Impact of Interaction Strength and Surface Heterogeneity on the Dynamics of Adsorbed Polymers

We present molecular dynamics simulations of bead-and-spring polymer chains on chemically heterogeneous, energetically disordered surfaces at near-monolayer coverages. The surfaces consist of random mixtures of weakly (W) and strongly (S) attractive sites. We explore systematically the effect of surface composition on the diffusive dynamics of the chains. The polymer diffusion coefficients have a near-Arrhenius temperature dependence, with activation energies which have a nonmonotonic dependence on the fraction of S sites. In other words, we see a nonmonotonic dependence of the interfacial polymer dynamics on its affinity with the surface, when the latter involves some heterogeneity. The maximum activation energy belongs to the surface containing 75% S and 25% W sites, which combines near-maximum average polymer-surface interactions with near-maximum spread or disorder in these interactions. Our results have interesting implications for polymer adhesion and friction and structure-property relationships in polymer nanocomposites.

Chain relaxation dynamics of DNA adsorbing at a solid-liquid interface

We have used scanning force microscopy (SFM) to elucidate the dynamic behavior of open (torsionally unconstrained) circular and long linear DNA molecules during the relaxation process following adsorption onto mica. We find that bending stress and excluded volume effects drive the conformational equilibration via segmental out-of-plane dynamics.

Temperature and concentration effects on the equilibrium and dynamic behavior of a Langmuir monolayer: from fluid to gel-like behavior

The equilibrium isotherms of monolayers of poly(4-hydroxystyrene) on the air/water interface have been studied in the 5-60 degrees C range. The results indicate that the interface is a poor solvent for the monolayers over the whole temperature range. For surface pressures within the semidilute regime, the plot of the area occupied by the polymer coils versus temperature at constant surface pressure shows a sharp change of slope near 30 degrees C. Also, the surface excess entropy shows a similar change of slope at the same temperature. The surface shear viscosity can be described by a power law of the surface concentration. Within the semidilute regime, the exponent of the power law changes. The temperature dependence of the viscosity points out a change from independent particle to collective dynamics.

Molecular weight dependence of the shear rheology of poly(methyl methacrylate) Langmuir films: a comparison between two different rheometry techniques

The surface shear rheology of Langmuir monolayers of poly(methyl methacrylate) (PMMA) has been studied as a function of polymer concentration (Gamma) and molecular weight (N). Two different rheology techniques were used, one based on free damped oscillations of a ring with a sharp edge and the other based on a forced oscillation of a biconical disk. Both instruments were used in the oscillatory mode at comparable oscillation frequency and amplitude, which gave access to the viscoelastic shear modulus (S). The two instruments, working in different viscosity ranges, provide complementary and mutually compatible data. The results obtained for four PMMA samples of molecular weight between 8x10(3) and 2.7x10(5) g.mol(-1) show powerlike behavior as S approximately Gamma10 and S approximately N4. These strong dependences suggest a structural scenario based on the 2D percolation of the polymer pancakes.

Solvent effects on isotactic poly(methyl methacrylate) crystallization and syndiotactic poly(methacrylic acid) incorporation in porous thin films prepared by stepwise stereocomplex assembly

The solvent effects on the crystallization of porous isotactic (it) poly(methyl methacrylate) (PMMA) thin films as well as the incorporation behavior of syndiotactic (st) poly(methacrylic acid) (PMAA) into the porous films were investigated. The porous it-PMMA thin films were prepared by the extraction of st-PMAA from a stepwise layer-by-layer (LbL) assembly composed of it-PMMA and st-PMAA. The X-ray diffraction pattern of the it-PMMA thin films after immersion in acetonitrile/water (4/6, v/v) showed two characteristic peaks of a crystalline it-PMMA double-stranded helix (2theta = 9 degrees and 14 degrees , d = 0.96 and 0.62 nm). This suggested that acetonitrile promoted the crystallization of the thin films, because water is a nonsolvent for PMMA. The surface structural change of the it-PMMA films was also analyzed by atomic force microscopy. The it-PMMA conformation was maintained after crystallization as observed by infrared spectroscopy. The incorporation percentages of st-PMAA into the porous it-PMMA thin films under various solvent conditions were estimated using a quartz crystal microbalance. The incorporation of st-PMAA decreased as the it-PMMA films crystallized or with growing thickness of the porous it-PMMA films. This suggested that the polymer-polymer interactions and the entanglement of the it-PMMA chains played an important role. The incorporation was promoted as the acetonitrile content in the st-PMAA solution increased, indicating that it was necessary for st-PMAA incorporation to solvate it-PMMA.

Phase behavior and dewetting for polymer blend films studied by in situ AFM and XPS: from thin to ultrathin films

In this work, the film thickness (l0) effect on the phase and dewetting behaviors of the blend film of poly(methyl methacrylate)/poly(styrene-ran-acrylonitrile) (PMMA/SAN) has been studied by in situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The thinner film shows the more compatibility of the blend, and the phase separation of the film occurs at l0>5Rg (radius of gyration). An initially time-independent q*, the characteristic wavenumber of the phase image, which is in good agreement of Cahn's linearized theory for the early stage of spinodal decomposition, has been obtained in real space and discussed in detail. For 5Rg>l0>3Rg, a "pseudo-dewetting/(phase separation+wetting)" behavior occurs, where the pseudo-wetting is driven by the concentration fluctuation mechanism. For l0<3Rg, a "real dewetting/(phase separation+wetting)" behavior occurs.

Adsorption of Polyelectrolyte versus Surface Charge: in Situ Single-Molecule Atomic Force Microscopy Experiments on Similarly, Oppositely, and Heterogeneously Charged Surfaces

We have studied the effect of the pH and surface charge of mica on the adsorption of the positively charged weak polyelectrolyte (PE) poly(2-vinylpyridine) (P2VP) using atomic force microscopy (AFM) single-molecule experiments. These AFM experiments were performed in situ directly under aqueous media. If the mica's surface and the PE are oppositely charged (pH > 3), the PE forms a flat adsorbed layer of two-dimensionally (2D) equilibrated self-avoiding random walk coils. The adsorbed layer's structure remains almost unchanged if the pH is decreased to pH 3 (the mica's surface is weakly charged). At pH 2 (the mica surface is decorated by spots of different electrical charges), the polyelectrolyte chains take the form of a 2D compressed coil. In this pH range, at an increased P2VP concentration in solution, the PE segments preferentially adsorb onto the top of previously adsorbed segments, rather than onto an unoccupied surface. We explain this behavior as being caused by the heterogeneous character of the charged surface and the competitive adsorption of hydronium ions. The further increase of polymer concentration results in a complete coverage of the mica substrate and the charge overcompensation by P2VP chains adsorbed on the similarly charged substrate, due to van der Waals forces.

Photochemically initiated single polymer immobilization

This Concept article surveys methods for attaching single polymer molecules on solid substrates. A general approach to single polymer immobilization based on the photochemistry of perfluorophenylazides is elaborated.

A scanning force microscopy study on the motion of single brush-like macromolecules on a silicon substrate induced by coadsorption of small molecules

Scanning force microscopy was applied to visualise the motion of single poly(butanoate-ethylmethacrylate)-graft-poly(n-butyl acrylate) molecules on silicon and SrTiO(3) substrates. Macromolecular mobility was induced by cyclic exposure of the wafers with the adsorbed brush-like macromolecules to water and alcohol vapours. Exposure to saturated alcohol vapour induced collapse of the adsorbed individual polymer chains while exposure to saturated water vapour promoted their extension. The characteristic times of both conformational changes were long enough that it was possible to visualise step-by-step the morphology transformation in situ by means of an environment-controlled scanning force microscope. Several successive collapse-decollapse cycles were recorded, and small diffusive shifts of the macromolecular position on the substrate were detected after each cycle. Manipulating and visualising single polymer molecules in situ and real time on a silicon substrate opens up new possibilities for the controlled structure formation in ultrathin polymer films. As shown on the sample of a faceted SrTiO(3) wafer, upon extension the brush-like molecules can crawl or extend along nanoscopic surface structures. Silicon can be structured both topographically and chemically at dimensions comparable to those of single polymer molecules with a variety of fabrication techniques ranging from well established conventional silicon micro- nano- machining to new tools constantly developed as dip-pen and nanoimprint lithography.

Functional polymers: scanning force microscopy insights

Scanning force microscopy (SFM) and related techniques make it possible to visualize polymer systems with a molecular resolution. Beyond imaging, they also enable the unveiling of a variety of (dynamic) physico-chemical properties of both isolated polymer chains and their supramolecular architectures, including structural, mechanical and electronic properties. This article reviews recent progress in the use of SFM on polymers, with a particular emphasis on the mechanical properties of copolymers and single polymer chains, as well as on the bottom-up fabrication of supramolecular polymeric (helical) nanostructures in particular based upon pi-conjugated macromolecules as building blocks for nanoelectronics. Through a detailed understanding of the polymer behavior, we propose solutions for the generation of organic functional (nano)systems.