Surface Rheology and Structure of Model Triblock Copolymers at a Liquid-Vapor Interface: A Molecular Dynamics Study

The structure and surface rheology of two model symmetric triblock copolymers with different degrees of hydrophobicity but identical polymerization degree, spread at an explicit liquid/vapor interface, are investigated employing extensive equilibrium molecular dynamics and innovative nonequilibrium molecular dynamics simulations with semipermeable barriers in both the linear and nonlinear viscoelastic regimes. Results are obtained for interface microstructural and surface rheological quantities under dilatation and surface shear. Our results reveal that the more hydrophilic triblock copolymer (H21T8H21) imparts a higher surface pressure to the interface at a given surface concentration and takes on a conformation with a larger radius of gyration at the interface compared with H9T32H9, where H (hydrophilic) and T (hydrophobic) represent chemically different monomers. Increasing the surface concentration and/or decreasing the degree of hydrophobicity leads to an increase in both dilatational storage and loss moduli. Large amplitude oscillatory dilatation tests show that both interfaces exhibit strain softening at high strain amplitudes, while an intracycle nonlinearity analysis reveals an apparent strain hardening in extension. This paradox was already addressed for air-water interfaces stabilized by Pluronics in a preceding experimental work. Gyration tensor components parallel and normal to the interface as function of dilatational strain are used to characterize the microstructure; we demonstrate their close relationship to nonlinearity indices in both extension and compression. A structure-rheology relationship is obtained by means of the first harmonic analysis of the surface stress and the corresponding amplitude of the microstructure signal. In-plane oscillatory shear flow simulations are performed as well. The presented approach thus renders possible a test of theoretical frameworks, which link interfacial rheological data to the surface microstructure. It is furthermore shown to provide physical insights, which can be used for the interpretation of existing experimental surface rheological data.