Segmental orientation of a single polymer chain in LangmuirBlodgett films studied by defocus fluorescence imaging

INTODUCTION Amphiphilic polymers, such as poly(alkyl methacrylate)s and poly(alkyl acrylamide)s, are known to form stable monolayers on a water surface by the Langmuir–Blodgett (LB) method.1,2 The polymer monolayer has an ultimate thinness of the size of the monomer unit, and it has few defects in the film plane. Therefore, the monolayer of an amphiphilic polymer has attracted much interest as an ideal building block to fabricate well-designed layered structures at a nanometric scale. In a monolayer of a polymer consisting of amphiphilic monomers such as alkyl ester and amide groups, each monomer unit is strongly adsorbed onto a water surface. Consequently, the main chain is restricted in a two-dimensional plane, resulting in a greatly reduced degree of freedom compared with a threedimensional bulk state. The macroscopic properties of the polymeric materials are closely related to the conformation of the polymer chain; the polymer monolayer shows different properties from the threedimensional bulk system.3–8 Therefore, it is important to understand the conformation and orientation of the polymer main chain in a monolayer at the single molecule level. The chain structures in LB films have been extensively studied by various methods such as infrared and optical spectroscopy.9–12 The polarization dichroism of the absorption spectra provides information on the average orientation of functional groups in the polymer chain. However, because of the weak signal intensity from the ultrathin films, the observation has been limited to the chemical moieties with a strong absorption coefficient, such as a carbonyl group in infrared spectroscopy. The fluorescence labeling technique is useful for probing the local structure of polymer monolayers because of the high sensitivity in detecting fluorescence emissions.13 In addition to high sensitivity, the appropriate molecular design to introduce the fluorescent moiety allows the selective measurement of a specific site in the system. Highly sensitive optical imaging devices have enabled the direct observation of a fluorescent molecule; therefore, fluorescence microscopy would be a powerful tool to study the chain structure in an LB film at the single chain level. In a conventional fluorescence image, each molecule is observed as a circular spot. However, slight deviation of the objective of the microscope from the focal condition can change the observed pattern of a single molecule, which is strongly dependent on the orientation of the transition dipole.14 Consequently, the image analysis of the defocus pattern provides information on the orientation of a single dye molecule. This technique determines both the polar and azimuthal angles (y, j) of an individual molecule from the fluorescence image. Such defocus imaging techniques have been used to discuss the molecular orientation of dye molecules doped in thin films and molecular motor systems.15–19 The current study focused on the orientation of the polymer main chain in an LB film. The conformation of the main chain is closely related to its macroscopic physical properties; however, the direct evaluation of the orientation of the backbone segment has been difficult. We previously reported the synthesis of the poly(methacrylate)s labeled by a perylene diimide (PDI) molecule at the center of the chain, where the transition dipole of PDI was incorporated into the backbone of the polymer chain.15 The defocus imaging of the center-labeled polymer chain enabled the discussion of the segmental orientation of a single polymer chain. In this paper, the direct evaluation of the orientation of the single backbone segment of the poly(methyl methacrylate) (PMMA) chain in an LB film was demonstrated. The molecular orientation of the center segment of the PMMA chain was quantitatively evaluated by the defocus fluorescence imaging technique, and the effect of the deposition method is discussed.