Two-Dimensional Polymers Investigated by Scanning Near-Field Optical Microscopy: Phase Separation of Polymer Blend Monolayer

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.

Polymer compatibility in two dimensions. Modeling of phase behavior of mixed polymethacrylate Langmuir films

We analyze the possibility of polymer blends undergoing phase separation in two dimensions. To this end, we investigate a model system consisting of water-supported Langmuir monolayers, obtained from binary polyalkyl-methacrylate mixtures (PXMA, where X stands for any of the type of ester side groups used: M, methyl-; E, ethyl-; B, butyl-; H, hexyl-; O, octyl-; L, lauryl-methacrylate), by means of self consistent field (SCF) calculations. In particular, we address the conditions which determine demixing and phase separation in the two-dimensional system, showing that a sufficient chain length mismatch in the ester side group moieties is able to drive the polymer demixing. When the difference in length of the alkyl chain of the ester moieties on the two types of polymers is progressively reduced, from 11 carbon atoms (PMMA/PLMA) to 4 carbons only (POMA/PLMA), the demixing tendency is also reduced. The polymer/subphase interactions affect more the distribution of the polymer coils in the POMA/PLMA blend monolayer. Mixing of the two polymers is observed, but also a partial layering along the vertical direction. We also add, to a PMMA/PLMA blended monolayer, a third component, namely, a symmetrical diblock copolymer of the type PLMA-b-PMMA. We observe adsorption of the diblock copolymer exclusively at the contact line between the two homopolymer domains, and a concomitant lowering of the line tension. The line tension varies with the chemical potential of the diblock copolymer according to Gibbs' law, which demonstrates that PLMA-b-PMMA can act as a "lineactant" (the equivalent of a surfactant in two-dimensional systems) in the binary demixed PMMA/PLMA Langmuir monolayer.

Chain end distribution of block copolymer in two-dimensional microphase-separated structure studied by scanning near-field optical microscopy

The chain end distribution of a block copolymer in a two-dimensional microphase-separated structure was studied by scanning near-field optical microscopy (SNOM). In the monolayer of poly(octadecyl methacrylate)-block-poly(isobutyl methacrylate) (PODMA-b-PiBMA), the free end of the PiBMA subchain was directly observed by SNOM, and the spatial distributions of the whole block and the chain end are examined and compared with the convolution of the point spread function of the microscope and distribution function of the model structures. It was found that the chain end distribution of the block copolymer confined in two dimensions has a peak near the domain center, being concentrated in the narrower region, as compared with three-dimensional systems.

Microscopy, microstructure and displacement of proteins from interfaces: implications for food quality and digestion

The use of atomic force microscopy to visualise the molecular structure of protein-stabilised oil-water and air-water interfaces has provided new information on the microstructure of the interfaces. The images have shown the formation of heterogeneous protein networks and demonstrated how this heterogeneity affects the competitive displacement of proteins from interfaces by surfactants. The data collected have led to the discovery of a new 'orogenic' mechanism of protein displacement. This mechanism is generic and provides new insights into understanding and controlling the stability of food foams and emulsions. The present article describes the practical aspects of the orogenic model for enhancing food quality, discusses the potential implications for understanding the influence of interfacial structures in food emulsions on fat metabolism and also generic aspects of the displacement mechanism outside the food area.

Local Heterogeneity for a Eu3+-Doped Glass Evidenced by Time-Resolved Fluorescence Spectroscopy Coupled to Scanning Near-Field Optical Microscopy

Time-resolved fluorescence spectroscopy (TRFS) was applied to an aluminate glass sample doped with Eu3+ cation as a fluorescent probe of the chemical environment and local symmetry. Conventional far-field experiments revealed the presence of two different phases: an amorphous phase featured by a highly disordered environment surrounding the Eu3+ cation and a more ordered polycrystalline phase that exhibits a significant increase in the Eu3+ fluorescence decay time compared to that of the amorphous phase. Near-field fluorescence spectra and decay kinetics were recorded in the frontier region between the two phases using a home-built scanning near-field optical microscope. SNOM-TRFS experiments confirmed the presence of local heterogeneities in this part of the glass at a sub-micrometric spatial scale. Polycrystalline sites featured an important shear-force interaction with the probing fiber optic tip, a longer fluorescence decay time, and a higher Stark splitting of the 5D0 --> 7FJ (J = 1-4) electronic transitions of the Eu3+ cations.

Mapping the fluorescence decay lifetime of a conjugated polymer in a phase-separated blend using a scanning near-field optical microscope

We have studied a blend of the polymers poly(9,9-dioctylfluorene) (F8) dispersed in an inert matrix of polystyrene (PS) using time-resolved scanning near-field optical microscopy (SNOM). On spin-casting, phase separation occurs between the two polymers resulting in a thin film characterized by an F8-rich phase and a PS-rich phase. By spatially mapping the intensity of photoluminescence from the film, we find that there is a low concentration of F8 trapped within the PS-rich phase. We find that the fluorescence emission lifetime (measured at 440 nm) of F8 trapped within the PS-rich phase is significantly longer than that from the F8-rich phase (290 ps compared to 235 ps). Furthermore, spectral measurements indicate that the F8 emission from the PS-rich phase is characterized by a reduced fraction of emission from fluorenone defect states. Taken together, our measurements suggest that in the PS-rich phase interchain exciton diffusion between F8 molecules is suppressed significantly by the effect of dilution.

Morphology and oxygen sensor response of luminescent Ir-labeled poly(dimethylsiloxane)/polystyrene polymer blend films

Polymer films consisting of a linear poly(dimethylsiloxane) end-functionalized with a luminescent Ir(III) complex (Ir-PDMS), blended with polystyrene (PS), function as optical oxygen sensors. The sensor response arises by quenching of the luminescence from the Ir(III) chromophore by oxygen that permeates into the polymer film. The morphology and luminescence oxygen sensor properties of blend films consisting of Ir-PDMS and PS have been characterized by fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. The investigations demonstrate that microscale phase segregation occurs in the films. In blends that contain a relatively small amount of Ir-PDMS in PS (ca. 10 wt %), the Ir-PDMS exists as circular domains, with diameters ranging from 2 to 5 mum, surrounded by the majority PS phase. For larger weight fractions of Ir-PDMS in the blends, the film morphology becomes bicontinuous. A novel epifluorescence microscopy method is applied that allows the construction of Stern-Volmer quenching images that quantify the oxygen sensor response of the blend films with micrometer spatial resolution. These images provide a map of the oxygen permeability of the polymer blend films with a spatial resolution of ca. 1 mum. The results of this investigation show that the micrometer-sized Ir-PMDS domains display a 2-3-fold higher oxygen sensor response compared to the surrounding PS matrix. This result is consistent with the fact that PDMS is considerably more gas permeable compared to PS. The relationship of the microscale morphology of the blends to their performance as macroscale optical oxygen sensors is discussed.

Competitive adsorption of proteins and low-molecular-weight surfactants: computer simulation and microscopic imaging

Proteins and low-molecular-weight (LMW) surfactants are used in the food industry as emulsifying (and foaming) ingredients and as stabilizers. These attributes are related to their ability to adsorb at fluid-fluid (and gas-fluid) interfaces lowering the interfacial (and surface) tension of liquids. Hence, the study of the properties of adsorbed layers of these molecules can be expected to lead to a better understanding of their effect on food products. Direct proof of the validity of mesoscopic models of systems of proteins and LMW surfactants can only be achieved by quantitative theoretical predictions being tested against both macroscopic and mesoscopic experiments. Computer simulation constitutes one of the few available tools to predict mathematically the behaviour of models of realistic complexity. Furthermore, experimental techniques such as atomic force microscopy (AFM) now allow high resolution imaging of these systems, providing the mesoscopic scale measurements to compare with the simulations. In this review, we bring together a number of related findings that have been generated at this mesoscopic level over the past few years. A useful simple model consisting of spherical particles interacting via bonded and unbonded forces is described, and the derived computer simulation results are compared against those from the imaging experiments. Special attention is paid to the adsorption of binary mixtures of proteins, mixtures of LMW surfactants, and also protein+surfactant mixed systems. We believe that further development of these mathematically well-defined physical models is necessary in order to achieve a proper understanding of the key physico-chemical processes involved.