On the decoupling of relaxation modes in a molecular liquid caused by isothermal introduction of 2nm structural inhomogeneities

TL;DR

Introducing ~2 nm structural inhomogeneities into a fragile glassformer causes a decoupling of diffusion from viscosity, supporting the idea of a hidden polyamorphic transition related to dynamic heterogeneity near the glass transition.

Contribution

This study demonstrates that nanoscale inhomogeneities induce diffusion-viscosity decoupling, providing new insights into the origin of dynamic heterogeneity in fragile liquids approaching Tg.

Findings

Decoupling of diffusion and viscosity increases as temperature decreases.

Inhomogeneities are ~2 nm cresol-soluble POSS molecules.

Supports the polyamorphic transition hypothesis below Tg.

Abstract

To support a new interpretation of the origin of the dynamic heterogeneity observed pervasively in fragile liquids as they approach their glass transition temperatures Tg, we demonstrate that the introduction of ~2 nm structural inhomogeneities into a homogeneous glassformer leads to a decoupling of diffusion from viscosity similar to that observed during the cooling of orthoterphenyl OTP below TA, where Arrhenius behavior is lost. Further, the decoupling effect grows stronger as temperature decreases (and viscosity increases). The liquid is cresol and the ~2nm inhomogeneities are cresol-soluble asymmetric derivatized tetrasiloxy-based (POSS) molecules. The decoupling is the phenomenon predicted by Onsager in discussing the approach to a liquid-liquid phase separation with decreasing temperature. In the present case the observations support the notion of a polyamorphic transition in fragile liquids that is hidden below the glass transition. A similar decoupling can be expected as a globular protein is dissolved in dilute aqueous solutions or in protic ionic liquids.