Activation free energy gradient controls interfacial mobility gradient in thin polymer films
WG Zhang and FW Starr and JF Douglas, JOURNAL OF CHEMICAL PHYSICS, 155, 174901 (2021).
We examine the mobility gradient in the interfacial region of substrate- supported polymer films using molecular dynamics simulations and interpret these gradients within the string model of glass-formation. No large gradients in the extent of collective motion exist in these simulated films, and an analysis of the mobility gradient on a layer-by- layer basis indicates that the string model provides a quantitative description of the relaxation time gradient. Consequently, the string model indicates that the interfacial mobility gradient derives mainly from a gradient in the high-temperature activation enthalpy & UDelta;H-0 and entropy & UDelta;S-0 as a function of depth z, an effect that exists even in the high-temperature Arrhenius relaxation regime far above the glass transition temperature. To gain insight into the interfacial mobility gradient, we examined various material properties suggested previously to influence & UDelta;H-0 in condensed materials, including density, potential and cohesive energy density, and a local measure of stiffness or u(2)(z)(-3/2), where u(2)(z) is the average mean squared particle displacement at a caging time (on the order of a ps). We find that changes in local stiffness best correlate with changes in & UDelta;H-0(z) and that & UDelta;S-0(z) also contributes significantly to the interfacial mobility gradient, so it must not be neglected.
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