We measured the viscosity of poly(methyl methacrylate) (PMMA) films supported by silica, where the carbonyl group on the side chains of the polymer interacts strongly with the hydroxyl groups of the surface. The result shows that the viscosity increases with decreasing film thickness at temperatures above 110 °C, but displays an opposite trend at lower temperatures. A three-layer model, consisting of a mobile top layer, a bulk-like middle layer and an immobile bottom layer was found to fit the data well. A detailed breakdown of the layer contributions to the total mobility unveils that the mobility gain brought about by the top layer is balanced by the mobility loss by the bottom layer at 110 °C. When the temperature is lowered or raised, the balance is offset, in favor, of the top and bottom layer, respectively.

A novel approach for preparing waterborne corrosion protection polyaniline (PANI)-containing coatings was developed. First, conducting polyaniline/partially phosphorylated poly(vinyl alcohol) (PANI/P-PVA) spherical nanoparticles with significant dispersibility in aqueous media were prepared by the chemical oxidative dispersion polymerization in presence of partially phosphorylated poly(vinyl alcohol) (P-PVA). The PANI/P-PVA-containing coatings with different PANI/P-PVA contents were then prepared, employing waterborne epoxy resin as the matrix. The corrosion protection property of PANI/P-PVA-containing coatings on mild steel was investigated by salt spray test and electrochemical impedance spectroscopy (EIS) technique in 3.0 wt % NaCl aqueous solution. The results indicated that the waterborne PANI/P-PVA-containing coatings (PANI/P-PVA content, 2.5 wt %) could offer high protection because the impedance values remained at higher than 1 × 10(7) Ω cm(2) after 30 days of salt spray tests. All the results were compared with these of the waterborne coatings containing PANI nanoparticles in the emeraldine salt form (PANI ES), and the protection mechanism was also proposed with the evidence of scanning electron microscope (SEM) and X-ray photoelectron spectrometry (XPS).

Thermally activated flow dynamics of polystyrene films supported by silicon is studied for a wide range of film thickness (h 0 ) and molecular weights (M w ). At low M w , the effective viscosity of the nanometer thin films is smaller than the bulk and decreases with decreasing h 0 . This is due to enhancement of the total shear flow by the augmented mobility at the free surface. As M w increases, with h 0 becoming smaller than the polymer radius of gyration (R g ), the effective viscosity switches from being substrate-independent to substrate-dependent. We propose that interfacial slippage then dominates and leads to plug flow. The friction coefficient is found to increase with h 0 providing h 0 /R g < ∼1, demonstrating a surface-promoted confinement effect. ■ INTRODUCTIONMany recent experiments reveal that polymer under confinement in the nanometer range often exhibits dynamics distinguishably different from the bulk. 1−5 While the mechanisms underpinning the new properties are still controversial, most results suggest that they originate from surface effects. Usually, the chain dynamics near the free (air) surface is faster than the bulk 6−11 but that near the substrate is slower. 12,13 By embracing analogous effects in a layer model, variations in the dynamic properties, including the glass transition temperature, T g , 10,14,15 and effective viscosity, η eff , 8,12,16 with the average polymer thickness, h 0 , were explained. At high molecular weights, M w , where the unperturbed radius of gyration, R g , exceeds ∼h 0 , de Gennes further proposed that the highmobility chain segments at the free surface can bring about mobility enhancement to the whole chain through chain connectivity. 17 Here, we report a comprehensive study on the η eff of polystyrene supported by oxide-coated Si (PS-SiOx; SiOx thickness = 102 ± 5 nm) with 13 ≤ M w ≤ 2300 kg/mol (∼3 < R g < ∼41 nm (Supporting Information)), polydispersity index 1.01−1.1, and h 0 /R g from ∼0.2 to ∼10. A new dynamic regime dominated by plug flow and instigated by a surface-promoted confinement effect is found. ■ EXPERIMENTAL METHODSPolystyrene (PS), with a weight-average molecular weight, M w , of 13.7−2300 kg/mol and polydispersity index of 1.01−1.1, was purchased from Scientific Polymer Products (Ontario, NY). In this experiment, we use as-cast films made from spin-coating. Methods of sample preparation and characterization have been reported before. 8,18 In brief, the as-purchased polymer granules are first dissolved in toluene, and then the solution is filtered through a PTFE membrane filter with pore size 0.1 μm (Fisher Scientific Co.) before being spincoated onto a substrate to form the films. The substrates, purchased from Siltronix (France), are single crystal (001) silicon wafers covered by a 102 ± 5 nm thick thermal oxide. Before use, the wafers are cut into 1.5 cm × 1.5 cm slides and submerged in a piranha solution at 130°C for 10 min, followed by thorough rinsing in deionized water and then drying by 99.99% nitrogen. Afterward, the slides are furt...

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We have measured the effective viscosity of polystyrene films with a small (4 wt %) added amount of dioctyl phthalate (DOP) deposited on silica. A broad range of molecular weights, M w, from 13.7 to 2100 kg/mol was investigated. Our result shows that for the thin films with M w < ∼100 kg/mol the addition of DOP causes the effective viscosity to decrease by a factor of ∼4, independent of M w. But for the higher M w films, the effective viscosity of the DOP added films creeps toward that of the neat films with increasing M w. A model assuming the effective viscosity to be dominated by enhanced surface mobility for the lower M w films but surface-promoted interfacial slippage for the higher M w films is able to account for the experimental observations.