Enhanced Stiffness of Amorphous Polymer Surfaces under Confinement of Localized Contact Loads

Abstract

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...

Contact-resonance atomic force microscopy for viscoelasticity

“…These values are roughly consistent with those obtained in previous studies of PMMA films with various contact methods. 17,25,28 Several possible effects could explain any discrepancies between these and previously measured values. For instance, most of the previous work utilized tips with significantly larger radii and /or higher applied forces.…”

“…These values are consistent with those reported for PS in the literature. 17,25,26 It is important to note that the accuracy of our approach relies on accurate knowledge of the properties of the reference sample. For a diamond indenter tip, E = 1140 GPa and = 0.07 resulting in M t = 1145.6 GPa.…”

“…Furthermore, the contact loads applied with CRAVE can be limited to nanonewtons, thus reducing or eliminating sample damage and minimizing potential effects such as enhanced stiffness due to confinement ͑an effect still under investigation͒. 17 Figure 1 depicts an AFM cantilever in contact with a viscoelastic surface. Typically, an AFM cantilever is tilted at an angle ͑ 0 Ϸ 15°͒ relative to the sample surface.…”

Rate dependence of indentation size effects in filled silicone rubber

“…A phenomenological constitutive model incorporating both rate and size effects has been suggested in [Gudmundson 2006], for instance. The elasticity modulus has also been determined to vary with the indentation depth [McFarland and Colton 2005;Tweedie et al 2007]; this may be seen as a indication that a change in the dominant forces or mechanisms may be present at small indentation depths.…”

Probing polymer chain constraint and synergistic effects in nylon 6-clay nanocomposites and nylon 6-silica flake sub-micro composites with nanomechanics

“…This more commonly occurs when (i) the holding period is too short, (ii) and/or the loading rate is too high, (iii) and/or the indentation depth is lower than ~ 200 nm. 17,35,36 To avoid any of these artifacts influencing the measurements in this study the load vs. time protocol has been designed to (i) load sufficiently slowly (50 s), (ii) hold for a long time at peak load (120 s), and (iii) indent sufficiently deep (2-3 μm).…”