Combined Dependence of Nanoconfined T_g on Interfacial Energy and Softness of Confinement

We employ molecular dynamics simulations of nanolayered polymers to systematically quantify the dependence of T_g nanoconfinement effects on interfacial energy and the "softness" of confinement. Results indicate that nanoconfined T_g 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 T_g 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.