Mechanical Properties of Ultrathin Polystyrene-<i>b</i>-Polybutadiene-<i>b</i>-Polystyrene Block Copolymer Films: Film Thickness-Dependent Young’s Modulus
M. Saito, Kohzo Ito, Hideaki Yokoyama
Abstract
This study investigated the mechanical properties of ultrathin polystyrene-b-polybutadiene-b-polystyrene (SBS) block copolymer films with thicknesses ranging from 20 to 600 nm using a pseudo-free-standing tensile test carried out using an ultrathin film floating on water. The Young’s moduli of ultrathin SBS films increased drastically with decreasing thickness for films thinner than 100 nm, i.e., 2 or 3 times the domain spacing of the SBS. We analyzed the depth profiles of the polystyrene (PS) domain in the SBS thin films by dynamic secondary ion mass spectrometry and the surface and interface in-plane morphology using atomic force microscopy. The PS-rich continuous subsurface layer was observed to be a major factor for the drastic increase in Young’s modulus under ultrathin film conditions. Therefore, we propose a simple two-layer model consisting of a hard PS-rich layer and a soft bulk layer to explain the specific increase in Young’s modulus with decreasing film thickness.