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 methac rylate), PnHMA, was investigated by heat capacity, dielectric, and shear sp ectroscopy. Two distinct parts of the dynamic glass transition were observe d 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 tem perature process a, above the saddle, is characterized by nearly temperatur e independent Delta c(p) values and a very small but constant cooperativity . Its calorimetric relaxation strength dies off above the saddle. The conve ntional cooperative a relaxation sets on about one frequency decade below. Its calorimetric relaxation strength Delta c(p) and cooperativity N-alpha s trongly increases with decreasing temperature. The calorimetric onset of th e a relaxation in the crossover region is confirmed by an additive analysis of dielectric data using two Havriliak-Negami functions. The results are d iscussed with respect to cooperativity as a function of temperature.