What Does the T-g of Thin Polymer Films Really Tell Us?

WG Zhang and JF Douglas and FW Starr, 9TH INTERNATIONAL CONFERENCE ON TIMES OF POLYMERS AND COMPOSITES: FROM AEROSPACE TO NANOTECHNOLOGY, 1981, 020083 (2018).

DOI: 10.1063/1.5045945

The disparate results for T-g shifts in ultra-thin polymer films with the same film thickness, substrate, and chemistry raises the question of whether T-g is a useful way to characterize thin films. Underlying many T-g measurements is the assumption that T-g estimates used for bulk polymer are helpful to infer the dynamical changes in thin films. We use molecular simulations of a coarse-grained polymer film supported on a substrate with variable interaction strength with polymer to test this assumption. As expected, the slowing of polymer dynamics near a highly attractive substrate forms a bound layer that effectively insulates the reminder of the film from strongly interacting substrate. As a result, thermodynamic estimates of T-g is relatively insensitive to the polymer- substrate interaction strength epsilon when the substrate interactions exceed those of the polymer-polymer interactions. Due to this insensitivity, thermodynamically defined measures of T-g decrease with decreasing film thickness, regardless of e. Dynamically defined measures of T-g also decreases with decreasing thickness when epsilon is relatively weak; however, the dynamic T-g increases when epsilon exceeds the polymer-polymer interaction strength. Our results show that these qualitatively different trends in T-g reflect differing sensitivities to the mobility gradient across the film We emphasize that using single measure of T-g to infer changes in the dynamics of polymer thin films is limited.

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