Viscosity and Surface-Promoted Slippage of Thin Polymer Films Supported by a Solid Substrate

Abstract

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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