Spatially Distributed Rheological Properties in Confined Polymers by Noncontact Shear.

When geometrically confined to the nanometer length scale, a condition in which a large portion of the material is in the nanoscale vicinity of interfaces, polymers can show astonishing changes in physical properties. In this investigation, we employ a unique noncontact capillary nanoshearing method to directly probe nanoresolved gradients in the rheological response of ultrathin polymer films as a function of temperature and stress. Results show that ultrathin polymer films, in response to an applied shear stress, exhibit a gradient in molecular mobility and viscosity that originates at the interfaces. We demonstrate, via molecular dynamics simulations, that these gradients in molecular mobility reflect gradients in the average segmental relaxation time and the glass-transition temperature.

Combined Dependence of Nanoconfined Tg on Interfacial Energy and Softness of Confinement.

We employ molecular dynamics simulations of nanolayered polymers to systematically quantify the dependence of Tg nanoconfinement effects on interfacial energy and the "softness" of confinement. Results indicate that nanoconfined Tg depends linearly on interfacial adhesion energy, with a slope that scales exponentially with the ratio of the bulk Debye-Waller factors ⟨u2⟩ of the confined and confining materials. These trends, together with a convergence at low interfacial adhesion energy to the Tg of an equivalent freestanding film, are captured in a single functional form, with only three parameters explicitly referring to the confined state. The observed dependence on ⟨u2⟩ indicates that softness of nanoconfinement should be defined in terms of the relative high frequency shear moduli, rather than low frequency moduli or relaxation times, of the confined and confining materials.