Although there is an increasing appreciation that physical properties of amorphous (glassy) polymer surfaces and interfaces can differ substantially from those of the bulk, the mechanisms and implications for mechanical performance of thin films, surfaces of bulk polymers, and nanocomposites are unclear. For example, several natural and synthetic nanocomposites exhibit markedly enhanced stiffness and strength that cannot be explained via two-phase composite rules-of-mixtures. Here we apply recent advances in contact deformation to determine the apparent elastic (or storage) moduli over 5 to 200 nanometers from the free surface of amorphous polystyrene, poly(methyl methacrylate), and polycarbonate. We observe that the apparent stiffness of the surface under contact can exceed that of the bulk by up to 200%, independent of processing scheme, macromolecular structural characteristics, and relative humidity. We attribute this enhanced apparent stiffness at the surface to the contact stress-induced formation of a mechanically confined phase at the probepolymer interface. These observations are consistent with the increased macromolecular mobility of glassy polymer free surfaces, and relate directly to the material physics of the interphase in synthetic and biological polymer nanocomposites.Most experimental investigations of amorphous polymer surfaces have focused on thermally activated behavior such as the glass transition temperature T g [1][2][3] and structural relaxation. [4,5] However, few overarching conclusions exist regarding surface and interface properties, [6] in large part because experimental and sample preparation capabilities have not yet been optimized for the nanometer-length scales over which these surface-specific phenomena are observed. There are two generally accepted conclusions regarding amorphous polymer surface behavior: that T g is a function of polymer film thickness t f for t f < 100 nm, and that the magnitude and direction of the T g shift depends on the polymer and/or substrate [7] . For example, the T g of amorphous polystyrene (PS) films has been found to be depressed by 35°C in spin-coated films of t f < 20 nm on Si substrates [1] and by 70°C for free standing films of t f < 30 nm, [2] while amorphous poly(2-vinylpyridine) has demonstrated a 35°C elevation in T g for t f = 10 nm that is attributed to secondary bonding with the Si substrate.[8]Here, we sought to consider the consequences of such a physical property variation on the resistance of amorphous polymer surfaces to localized contact deformation. Depression of T g in polymers such as PS and PMMA suggests that, over distances < 100 nm from the free surface of these socalled glassy polymers, the macromolecular chains are more mobile than those located within the bulk. This conceptualization is consistent with computational simulations of molecular mobility of free surfaces and confined volumes, [9][10][11] as well as recent experimental observations for PS thin films of t f < 40 nm, including broadened structural relaxation times...
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