LETTERS Phase-Separation Structure of a Monolayer of Binary Polymer Blend Studied by Fluorescence Scanning Near-Field Optical Microscopy Hiroyuki Aoki, Yuji Sakurai, Shinzaburo Ito,* and Tatsuo Nakagawa † Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Sakyo, Kyoto 606-8501, Japan ReceiVed: June 29, 1999; In Final Form: October 13, 1999 A phase-separated monolayer of a binary mixture of pyrene-labeled poly(isobutyl methacrylate) (PiBMA- Py) and perylene-labeled poly(octadecyl methacrylate) (PODMA-Pe) was investigated by the fluorescence scanning near-field optical microscope (SNOM), which provided optical images of samples with a high resolution beyond the diffraction limit, by using evanescent field from an aperture smaller than the wavelength of light. Each phase of the PiBMA-Py/PODMA-Pe was selectively imaged as a bright area by choosing the excitation wavelength (325- or 442-nm line of a He-Cd laser). The fluorescence SNOM enabled mapping of the excitation energy transfer efficiency on the monolayer plane. The energy transfer measurement revealed that the phase separation of the PiBMA-Py/PODMA-Pe blend at the air/water interface was almost completed by annealing for 60 min at 40 °C and the phase boundary had a width of a few hundred nanometers. Introduction Recently, polymer monolayers, which are prepared at the air/ water interface, have attracted much attention because the sequential deposition of them by the Langmuir-Blodgett (LB) technique allows one to fabricate highly controlled molecular assemblies. Mobility and conformation of a polymer chain in a monolayer is thought to be extremely constrained, considering that the polymer chain is confined into a two-dimensional plane from a random-coil conformation in a three-dimensional space. 1-4 Thus, it is expected that the properties of polymer monolayers are different from those of polymers in bulk. Our research project is concerned with “polymer science in two dimensions”, and we have so far investigated the properties of polymer monolayers by various optical techniques, such as Brewster angle microscopy 5,6 and fluorescence spectroscopy. 7,8 However, the lateral resolution is not sufficient to observe microscopic structures due to the diffraction limit (λ/2) of conventional optical methods. Scanning near-field optical microscopy (SNOM), which has been developed recently, 9,10 allows one to obtain a fluorescence micrograph with a high resolution beyond the diffraction limit by monitoring fluores- cence from dye molecules excited by evanescent field of light from an aperture at the end of the raster-scanning optical fiber probe, whose diameter is smaller than the wavelength of light. Several studies have revealed that SNOM is a versatile tool for studying both the morphology and the spectroscopic properties of an organic thin film simultaneously. 11-15 In the current study, we concern with a two-dimensionally phase-separated polymer blend. The phase separation of polymer blends in three-dimensional space has been extensively studied by many workers. 16-18 However, little is known about that of polymers restricted in a two-dimensional plane. 19,20 We inves- tigated a binary polymer blend, which consists of poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) (POD- * Author to whom correspondence should be addressed. † Unisoku, Co. Ltd., Hirakata, Osaka 573-0131, Japan. Tel: +81-75- 753-5612. Fax: +81-75-753-5632. © Copyright 1999 by the American Chemical Society VOLUME 103, NUMBER 48, DECEMBER 2, 1999 10.1021/jp992228i CCC: $18.00 © 1999 American Chemical Society Published on Web 11/05/1999