Two series of semi-interpenetrating polymer networks (semi-IPN) based on the same linear polyurethane and two different heterocyclic polymer networks were characterized in terms of the complex dynamic modulus and the mechanical loss tangent, measured between 150 and 500 K at a frequency varying in the range between 0.3 and 30 Hz. The fragility and the deviations from exponentiality of the dynamics of the segmental motion of semi-IPNs in the glass transition interval have been obtained by modelling the mechanical αa-relaxation. It was observed that the incorporation of linear polyurethane softens the heterocyclic polymer networks, lowers their resistance to the temperature-induced structural degradation, and increases the complexity of the long-range segmental dynamics. There has also been found a correlation between the fragility and the anharmonicity of the system, the most fragile polymer being characterized by the largest degree of anharmonicity. By extending the analysis to linear polymers, it was established that a growing anharmonicity is predictive of an increasing fragility.
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