Development of broad modulus profile upon polymer–polymer interface formation between immiscible glassy–rubbery domains

Abstract

Interfaces of glassy materials such as thin films, blends, and composites create strong unidirectional gradients to the local heterogeneous dynamics that can be used to elucidate the length scales and mechanisms associated with the dynamic heterogeneity of glasses. We focus on bilayer films of two different polymers with very different glass transition temperatures ( T g ) where previous work has demonstrated a long-range (∼200 nm) profile in local T g ( z ) is established between immiscible glassy and rubbery polymer domains when the polymer–polymer interface is formed to equilibrium. Here, we demonstrate that an equally long-ranged gradient in local modulus G ~ ( z ) is established when the polymer–polymer interface ( ≈ 5 nm) is formed between domains of glassy polystyrene (PS) and rubbery poly(butadiene) (PB), consistent with previous reports of a broad T g ( z ) profile in this system. A continuum physics model for the shear wave propagation caused by a quartz crystal microbalance across a PB/PS bilayer film is used to measure the viscoelastic properties of the bilayer during the evolution of the PB/PS interface showing the development of a broad gradient in local modulus G ~ ( z ) spanning ≈ 180 nm between the glassy and rubbery domains of PS and PB. We suggest these broad profiles in T g ( z ) and G ~ ( z ) arise from a coupling of the spectrum of vibrational modes across the polymer–polymer interface as a result of acoustic impedance matching of sound waves with λ ∼ 5 nm during interface broadening that can then trigger density fluctuations in the neighboring domain.