Crossover Region of Dynamic Glass Transition in Poly(n-hexyl methacrylate) by Heat Capacity Spectroscopy

The crossover region of the dynamic glass transition in poly( n -hexyl methacrylate), PnHMA, was investigated by heat capacity, dielectric, and shear spectroscopy. Two distinct parts of the dynamic glass transition were observed in the calorimetric data, surprisingly separated by a saddle in the contour map at log( f /Hz) = 0.33 and T = 2 °C. The high temperature process a, above the saddle, is characterized by nearly temperature independent Δ c p values and a very small but constant cooperativity. Its calorimetric relaxation strength dies off above the saddle. The conventional cooperative α relaxation sets on about one frequency decade below. Its calorimetric relaxation strength Δ c p and cooperativity N α strongly increases with decreasing temperature. The calorimetric onset of the α relaxation in the crossover region is confirmed by an additive analysis of dielectric data using two Havriliak−Negami functions. The results are discussed with respect to cooperativity as a function of temperature.