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Neutron reflectivity measurements on deuterated polystyrene (dPS) thin films reveal a strong dependence of the coefficient of thermal expansion (CTE) on both the thickness of the film and the specific confinement boundaries. A direct comparison is made between films on low energy, fluorinated polyimide (fPI) substrates with a free surface (bilayer) and the same films after capping the bilayer with another fPI layer (trilayer). For bilayers, a significant decrease in αr, the CTE above the bulk glass transition temperature, is observed in dPS with thicknesses d < 35 nm. For trilayers, a decrease of αr is observed at larger film thicknesses, d < 60 nm. Replacing the fPI substrate with an SiOx substrate, αr remains bulklike, showing that the length scale for deviations from bulk behavior is sensitive to the specific confining boundaries.

Thermal Expansion of Supported Thin Polymer Films: A Direct Comparison of Free Surface vs Total Confinement