Observation of phase‐separated polymeric LB films using scanning probe microscopy

Abstract

Technologies for manufacturing molecular assemblies have made rapid progress in the past decade. The Langmuir-Blodgett (LB) technique is one of the most hopeful candidates to assemble organic molecules, because it makes the fabrication of mono-molecular thin films possible. The molecules for LB films must be amphiphilic and are typically low molecular weight long-chained aliphatic acids (fatty acids). This class of LB films are seen as a standard and, consequently, have a long history. On the other hand, when these films are used in the fabrication of new devices, the shortcomings of LB films that arise from their inherent thermal and mechanical instabilities have been pointed out. Polymeric LB films were studied for two main reasons. One of them was to see if mono-molecular layers of polymers were possible and if so what the nature of such polymeric LB films was, and secondly to achieve an improvement of the physical shortcomings of low molecular weight LB films.We have investigated polyimide LB films.['] Polyimides are well known for their high degree of thermal stability and superior electrical properties. Since it is difficult to process polyimides because of their infusibility and insolubility in organic solvents, films are produced from the soluble precursor polymers (polyamic acids). The most interesting characteristic of polyimide LB films is that they do not have long alkyl chains in the structure, which makes them atypical for LB films.The preparation of the polyimide LB films is shown in Scheme 1. Polyamic acids 1 are mixed with long-alkyl amines 4 to introduce hydrophobic alkyl chains in order to form the amphiphilic structure 2. The LB films of the materials (2) are prepared by the usual LB technique, which consists of the preparation of a monolayer at the air-water interface and deposition of this layer onto a substrate. The LB films of 2 are dipped into a mixed solution of acetic anhydride and pyridine to convert the LB films into polyimide 3. Alternatively, the conversion can be achieved by heating the precursor LB films at 300 "C for a short period. The polyimide LB films have excellent thermal stability and solvent resistance as high as commercially available polyimide thick films. I 2 9 Scheme 1We have applied the polyimide LB films to devices such as liquid-crystalline optical cells where they were used as a liquid-crystalline aligning layer,r21 solar battery cells,[31 and photomemory devices working in the photon mode.[41 In the course of our studies concerning the formation of the microstructure of the polyimide LB films, we discovered phase separation behavior in the mixed LB films of two different p~lyimides.~~] The polyamic acids used in the present study are shown in Scheme 1. One of them has an aliphatic structure la, and the other consists of a wholly aromatic structure lb. These two polyamic acids were dissolved in the mixture of N,N-dimethylacetamide (DMAc) and benzene at the various ratios. The same molar amount of long-alkyl amine 4 (relative to the carboxylic function of the mixed...

Tip-Induced Structural Rearrangements of Alkanethiolate Self-Assembled Monolayers on Gold

“…FFM is, however, well documented to be sensitive to differences in functional groups in SAMs [52][53][54][55][56] and to give a higher image contrast compared to the normal force AFM signal in investigations of SAMs and Langmuir-Blodgett films. [57][58][59][60][61][62][63][64][65] It is not clear how the FFM data shown here reflect the placement of the alkanethiolates on the surface. It is expected that FFM is sensitive to changes in height; particularly the motion of the tip out of a surface depression induces a large tip bending and thus a large FFM signal.…”

Polymer Compatibility in Two Dimensions. Modeling of Phase Behavior of Mixed Polymethacrylate Langmuir Films

“…i i i b (7) where φ b is the volume fraction in the bulk (water) phase. This parameter represents the 2D distribution profile of the interfacial concentration of the polymer: its integration over the z-direction, normal to the interface, results in the adsorbed amount of component i per unit length in the y-direction θ i (x)…”

Phase-Separation Structure of a Monolayer of Binary Polymer Blend Studied by Fluorescence Scanning Near-Field Optical Microscopy

“…[16][17][18] However, little is known about that of polymers restricted in a two-dimensional plane. 19,20 We investigated a binary polymer blend, which consists of poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) (POD- MA), by a fluorescence SNOM with a resolution of ca. 100 nm using UV and visible lasers as the light source.…”

“…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. − However, little is known about that of polymers restricted in a two-dimensional plane. , We investigated a binary polymer blend, which consists of poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) ( PODMA), by a fluorescence SNOM with a resolution of ca. 100 nm using UV and visible lasers as the light source.…”