Investigation of Broad Shear Modulus Profile Formed Across Glassy-Rubbery Interfaces of Polybutadiene and Polystyrene with Varying Molecular Weights

Dynamical gradients near interfaces leading to perturbations in glass transition temperature Tg and other related material properties like modulus have been an open question for decades. At the interface between different polymer domains, long range dynamical gradients in Tg and local modulus G(z) spanning hundreds of nanometers have been observed, despite a narrow compositional width of ~5 nm. Much of these observations were done with fluorescence using high molecular weight polymers. As no theoretical models at present can explain dynamical gradients beyond a few tens of nanometers, it has been suggested that perhaps these long range gradients are limited to some unidentified high molecular weight polymer phenomenon. In this study, we examine bilayer films of polybutadiene(PB) and polystyrene(PS) with varying PS molecular weights. Using a quartz crystal microbalance (QCM), we study the evolution of frequency and dissipation shifts upon annealing of the PB/PS interface, where a strong decrease in dissipation is observed and fit to a continuum mechanics model to determine the broad depth-dependent gradient in local modulus G(z) that emerges. Comparing samples made from 25 kg/mol and 400 kg/mol PS, we observe similar behavior suggesting that the long-range effects of polymer-polymer interfaces on dynamic properties are not limited to high molecular weight samples.