(641e) Origins of Time-Temperature Superposition Breakdown Near the Glass Transition: Experimental Test of the Heterogeneous Rouse Model

Time-Temperature Superposition (TTS) is widely employed in analyzing dynamic and mechanical data. However, beginning with Plazek in the 1960’s, evidence points to a breakdown of TTS in diverse polymers near the glass transition. For example, rheological data on polystyrene show clear evidence of TTS breakdown between 105℃ and 130 ℃. This breakdown takes the form of a decoupling of polymer chain and segmental dynamics, with chain dynamics and viscosity generally exhibiting a weaker temperature dependence than does segmental dynamics. No settled consensus on the mechanistic origins of this breakdown has emerged over the last half century, despite its broad importance to the prediction and characterization of polymer linear rheological response near the glass transition temperature Tg.

Our group recently proposed the Heterogeneous Rouse Model (HRM), a generalization of the Rouse model that predicts TTS breakdown and a breakdown of Rouse scaling near Tg based on the presence of a distribution of relaxation times. Here, we report on extensions of this model to predict multiple linear-regime rheological properties such as complex modulus and creep, and we compare these predictions to new experimental data for TTS breakdown in multiple polymers, combining dielectric spectroscopy and rheological measurements. Results suggest that TTS breakdown near Tg can be understood and quantitatively described via the HRM, providing a new basis for analysis of near-Tg linear rheological response.

Acknowledgement is made to the donors of the American Chemical Society Petroleum Research Fund for support of this research.