Impact of Adding an Impedance Matching Copolymer Layer on the Long-Ranged Coupling of Shear Modulus Across a Glassy-Rubbery Polymer Interface

Recent work proposed an acoustic wave coupling mechanism as a possible explanation for the long-ranged gradients in local glass transition temperature Tg(z) previously reported across glassy-rubbery polymer interfaces. The idea suggested that acoustic waves of wavelength ~5 nm associated with the boson peak could propagate across glassy-rubbery polymer interfaces which are sufficiently annealed, influencing alpha-relaxations over extended distances such that broad Tg(z) and modulus G(z) gradients emerge. We aim to test this picture by investigating how the addition of a ~5 nm impedance matching copolymer layer at the glassy-rubbery polymer interface alters the time-scale of G(z) gradient evolution in these systems as the interface is annealed. Our experiments build on the technique we recently established of using a transfer-matrix continuum mechanics model to determine the local depth-dependent shear modulus profile G(z) from quartz crystal microbalance (QCM) measurements of glassy-rubbery polymer bilayer films. Our previous findings demonstrated that a broad (~200 nm) and strongly asymmetric gradient in G(z) emerges as the interface between polystyrene (PS) and polybutadiene (PB) is annealed to equilibrium, forming an interfacial width of ~5 nm. Here, we compare the evolution of QCM resonances and associated G(z) profiles on progressive annealing of PS/PB bilayer films with and without the addition of PS/PB random copolymer layers of different thicknesses at the PS/PB interface.