Contrasting two different interpretations of the dynamics in binary glass forming mixtures

Area of Science:

• Materials Science
• Polymer Physics
• Physical Chemistry

Background:

• Previous studies on tripropyl phosphate (TPP) and polystyrene (PS) mixtures reported two distinct relaxations (α1 and α2) with different temperature dependencies.
• These mixtures exhibit a large difference in glass transition temperatures (ΔT_g ≈ 200 K) between the components.

Purpose of the Study:

• To re-examine existing data on the dynamics of TPP/PS binary mixtures.
• To reinterpret the nature of the observed α2-relaxation and its relationship with the α1-relaxation.

Main Methods:

• Re-analysis of previously published dynamic data for TPP/PS mixtures.
• Dielectric spectroscopy measurements at elevated pressures (P) and varying temperatures (T).
• Analysis of relaxation time ratios under different thermodynamic conditions.

Main Results:

• The faster α2-relaxation previously reported is identified as the Johari-Goldstein (JG) β-relaxation, characterized by Arrhenius temperature dependence.
• This JG relaxation is admixed with a true α2-relaxation exhibiting a stronger temperature dependence.
• The ratio of α1 to α2 relaxation times (τ_α1/τ_α2) remains invariant with temperature and pressure when τ_α1 is held constant, confirming the JG nature of α2.

Conclusions:

• The α2-relaxation is unequivocally identified as the JG β-relaxation, a precursor to the primary α1-relaxation.
• The true α2-relaxation for the TPP component is recovered, showing a stronger temperature dependence consistent with α-relaxation behavior.
• These findings align with studies on other binary polymer mixtures with large ΔT_g, supporting a universal understanding of relaxation dynamics in such systems.