Effects of substrate/polymer interfacial interactions on dynamics of thin polystyrene films

Abstract

The glass transition temperature ( T g) and viscoelasticity of thin polystyrene (PS) films supported on Si/Si-H and Si/SiO2 substrates were investigated using an ellipsometry and force-distance (F-D) measurements of atom force microscopy. It was found that T g of thin PS films deposited on either Si/Si-H or Si/SiO2 decrease with decreasing of film thickness. However, T g of PS film on Si/SiO2 substrate depressed more dramatically than that on Si/Si-H. The threshold thickness for T g reduction of thin PS film on Si/SiO2 (110 nm) is higher than that of film on Si/Si-H (40 nm). Meanwhile, thin PS film supported by Si/SiO2 exhibits the higher thermal expansivity compared with that on Si/Si-H substrate. Surface F-D measurements reveal the film on Si/Si-H substrate is stiffened, showing a higher elastic modulus. In order to understand the mechanism of the substrate effect, the interfacial energy and long-range van der Waals potential ( Φ vdw ) were investigated and a strong attractive interaction between PS and Si/Si-H substrate was found. Thus, the packing density was enhanced and the molecular mobility of PS chains at Si/Si-H/PS interface was suppressed. This strong interaction resulted in higher T g and elastic modulus of thin PS film compared with that on Si/SiO2 substrate.