Composite materials consisting of polyaniline nanofibers decorated with noble-metal (Ag or Au) nanoparticles were synthesized with γ radiolysis. Aqueous solutions of aniline, a free-radical oxidant, and/or a metal salt were irradiated with γ rays. Exposure of the solutions to γ rays caused aniline to polymerize as very-thin fibers. Metal particles were also formed during irradiation and decorated the fibers. The shape and size of the metal particles could be changed from nanometer-sized spheres to micron-sized dendrites by varying the ratio of aniline to the metal precursor. Polyaniline-metal composites were characterized using UV-Vis spectroscopy, infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The electrical conductivity of the composites increased with the loading of metal in the nanocomposites and was up to 50 times greater than that of polyaniline fibers alone.

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We performed a combined calorimetric and molecular modeling investigation of poly(vinyl acetate) (PVAc) on silica to characterize the intermolecular interactions and the behavior of the adsorbed polymer. From temperature-modulated differential scanning calorimetry experiments, different regions of thermal activity suggested a gradient of mobility in the adsorbed polymer. Polymer segments in more direct contact with silica (tightly bound) showed a significantly elevated and broadened glass transition relative to the bulk polymer, while polymer further away (loosely bound) showed only a slightly elevated transition relative to the bulk polymer. A thermal transition for PVAc at the air interface (more-mobile) was also observed and was at lower temperatures than the bulk polymer. Density profiles from molecular dynamics studies suggested a structure of the adsorbed polymer similar to that experimentally observed. These studies were consistent with the presence of a motional gradient in the polymer segments, and concomitant glass transition changes from the silica to the air interfaces. These results also demonstrate that hydrogen-bonding interactions, at the PVAc/silica interface, are critical to the high-temperature shifts in the glass transition. ■ INTRODUCTIONAdsorbed polymer−substrate interactions usually lead to differences in properties of bulk and adsorbed polymers. 1−10 Interactions between adsorbed polymers and solid surfaces have been shown to provide advantageous physical, mechanical, and thermal properties, making these materials suitable as lubricants, adhesives, coatings, and corrosion-resistant agents. 11−17 These properties are closely related to those that determine the glass transition, which for small amounts of adsorbed polymers depend on film thickness, polymer molecular mass, intermolecular interactions, and the mobility of macromolecular chains. 18−20 For example, the T g will be elevated if the interactions between the polymers and the substrate are attractive and strong. 21 Strong attractive interactions, covalent or hydrogen bonding, between polymer segments and the substrate can potentially reduce the mobility of the adsorbed polymer segments. This reduction in mobility due to restrictions from attachment points has been proposed as the main reason for T g elevation. 22−24 Differential scanning calorimetry (DSC) is the most common technique used to investigate thermal characteristics of bulk polymers and composites. 25−27 Temperature-modulated DSC (TMDSC) is a variant of DSC that, in addition to providing the same information as conventional DSC, provides additional insight into the thermal behavior of materials by separating the heat flow data into reversing and nonreversing events. 28,29 TMDSC and its derivatives have been used to resolve both weak and multicomponent transitions that would be difficult to distinguish in a conventional DSC scan. 30−34 Molecular dynamics (MD) simulations have also been used to investigate the dynamics and thermodynamics of thin-film polymer coatings. 35−5...

Modulated differential scanning calorimetry has been used to quantify the glass transitions of small adsorbed amounts of poly(methyl methacrylate) (PMMA) on silica. While a relatively narrow, single glass transition was found for bulk PMMA, broader two-component transitions were found for the adsorbed polymer. A two-state model based on loosely bound polymer (glass transition similar to bulk) and more tightly bound polymer (glass transition centered around 156 degrees C) was used to interpret the thermograms. On the basis of this model, the amount of tightly bound polymer was found to be approximately 1.3 mg/m2, corresponding to a 1.1 nm thick layer. The change in heat capacity for the tightly bound polymer at the glass transition temperature was estimated to be about 16% of that of the bulk polymer.