Modulus, Confinement, and Temperature Effects on Surface Capillary Wave Dynamics in Bilayer Polymer Films Near the Glass Transition

Abstract

We report relaxation times (τ) for surface capillary waves on 27-127 nm polystyrene (PS) top layers in bilayer films using x-ray photon correlation spectroscopy. At ∼10 °C above the PS glass transition temperature (T(g)), τ tracks with underlayer modulus, being significantly smaller on softer substrates at low in-plane scattering wave vector. Relative to capillary wave theory, we also report stiffening behavior upon nanoconfinement of the PS layers. At PS T(g)+40 °C, both effects become negligible. We demonstrate how neighboring polymer domains impact dynamics over substantial length scales.

Size-dependent mechanical behavior of free-standing glassy polymer thin films

“…. Forrest et al 47 32,48,49 have shown that the observed changes in rheological properties of polymer films with varying film thicknesses could not be well correlated with the ! measurement.…”

“…On the other hand, recent experimental evidences show surface stiffening as observed in polymer thin films in the rubbery regime above ! [30][31][32] , in which the underlying physics of mechanical behaviors could be fundamentally different from that below ! .…”

X‐ray Photon Correlation Spectroscopy Studies of Surfaces and Thin Films

“…[6b], More recently, Sikorski et al and Chushkin et al measured the capillary wave relaxation rates of super cooled organic glass forming liquids of dibutyl phthalate and polypropylene glycol, respectively, and discovered strong evidence of surface induced elasticity . In another study, Evans et al by measuring the top layer capillary wave relaxations, reported that the stiffening of the polystyrene film depends on the modulus of the underlying substrates near the glass transition temperature (T g + 9 °C) even for very thick films, implying a strong effect of the substrate on the film over a substantial length scale even exceeding both the cooperative length of segmental mobility and the radius of gyration. In this review, we will briefly go through some historical but noteworthy experimental studies using surface XPCS and discuss recent progress on the dynamical study of surfaces of a variety of systems including bulk liquids, liquid crystals, thin films and polymer/nanoparticle composites.…”

Thickness dependence of structural relaxation in spin‐cast polynorbornene films with high glass transition temperatures (>613 K)

“…Glassy polymers confined to sub‐100 nm dimensions tend to exhibit altered physical properties, including their glass transition temperature ( T g ), mechanical properties, polymer and segmental diffusivity, viscosity, and aging . Thin films provide a simple route to examine confinement, which has led to decades of work, predominately focused on the T g of polystyrene thin films .…”