Using Rheological and Dielectric Spectroscopy Measurements of Time-Temperature Superposition Breakdown to Validate Heterogeneous Rouse Model*

Time-Temperature Superposition (TTS) is a technique used widely to analyze materials dynamic and mechanical data. However, beginning with Plazek in the 1960’s, the breakdown of TTS was reported 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 the polymer chain and segmental dynamics, with viscosity generally exhibiting a weaker temperature dependence than the segmental dynamics. No settled consensus on the mechanistic origins of this breakdown has emerged over the last half century, despite its extensive utilization to predict and characterize polymer linear rheological response near the glass transition temperature (T_g).

Recently, our group introduced the Heterogeneous Rouse Model (HRM) to explain TTS breakdown. HRM is based on the existence of a distribution of relaxation times near T_g. Here, we report on extensions of this model to predict multiple linear-regime rheological behaviors, 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 T_g can be understood and quantitatively described via the HRM, providing a new basis for analysis of near-T_g linear rheological response.