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).

DOI: 10.1063/5.0064866

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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