FRAGILITY AND ANHARMONICITY IN INTERPENETRATING POLYMER NETWORKS

An intriguing aspect of the dynamic behaviour of polymers is the understanding of the microscopic mechanisms regulating the temperature dependence of cooperative segmental relaxations. Polymers exhibit a complex molecular structure characterized by different intra- and inter-molecular forces, so that the chain environment gives rise to significant deviations from linearity and exponentiality of the cooperative primary or ?-relaxation over the glass-rubber transition region. From this point of view the "fragility" of a glass-forming liquid, a concept introduced by C.A. Angell, is an important property which measures the departure from the Arrhenius behaviour of its viscosity ?. "Fragile" liquids, such as polymers, are substances with non-directional intermolecular bonds which permit drastic changes in the local order at the glass transition leading to pronounced deviations from the Arrhenius behaviour for the viscosity. Interpenetration of a polycyanurate network with linear polyurethane leads to the formation of semi-interpenetrating polymer networks (semi-IPNs) which exhibit an increasing dynamic fragility and a nearly constant thermodynamic fragility (measured by Cpl/Cpg, the ratio of the liquid to glassy specific heat capacities at Tg). These findings raise some doubts about the existence of a correlation between the cooperative molecular mobility and the variations of the configurational entropy at Tg characterizing the system, when it goes through the glass-rubber transition.