Size of the cooperative rearranging regions vs. fragility in complex glassy systems: Influence of the structure and the molecular interactions

By using Donth's model, the size of the Cooperative Rearranging Regions (CRR) at the glass transition was determined from TM-DSC measurements on two glassy biopolymers with different molecular structures (amylose and amylopectin) as well as on starch, which is the mixture of amylose and amylopectin as it is obtained during its biosynthesis. For these materials, the ΔCp values at Tg and the values of the fragility index (m) were determined by conventional DSC. The fragility concept was then analyzed by taking into account the existence of two contributions: misoV (the so-called isochoric fragility, which is a pure thermal contribution) and mΔv (which incorporates the effects due to volume and density fluctuations). Whenever the molecular structure of a glass-forming liquid involves non-linear sequences of rigid and sterically hindered repeating units with high densities of hydrogen bonds, the fragility is found to be governed only by the temperature; the contribution of the volume (density and pressure contribution) has a quasi-nil effect