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 c(p)" (logf,T) contour map at log(f/Hz) = 0.33 and T = 2 degrees C. The high temperature process a, above the saddle, is characterized by nearly temperature independent Delta c(p) values and a very small but constant cooperativity. Its calorimetric relaxation strength dies off above the saddle. The conventional cooperative a relaxation sets on about one frequency decade below. Its calorimetric relaxation strength Delta c(p) and cooperativity N-alpha strongly increases with decreasing temperature. The calorimetric onset of the a 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.