Relaxation of the methylene blue monomer–dimer equilibrium in supercooled glycerol near the glass transition

The relaxation of the methylene blue monomer–dimer equilibrium in supercooled glycerol near the glass transition temperature, Tg, was studied by monitoring the change in the optical absorbance associated with dimer formation during continuous cool/heat cycles. A hysteresis is observed in the absorbance at ≈Tg+25 K. Based on an analysis of the data using the Tool–Narayanaswamy–Moynihan model, relaxation in the hysteresis region is exponential. Exponential relaxation and the occurrence of the hysteresis at temperatures above the calorimetric Tg are rationalized in terms of the length scale for monomer diffusion versus the length scale of the dynamic heterogeneity in glycerol.