Secondary relaxations in heterocyclic polymer networks: a study of mechanical spectroscopy

Experiments of differential scanning calorimetry and dynamic mechanical spectroscopy on linear and cross-linked heterocyclic polymer networks (HPNs) have been performed over the temperature range between 120 and 400K. The temperature dependences of the storage modulus E and the internal friction Q−1 of HPNs, investigated by mechanical waves of frequencies ranging between 0.3 and 30 Hz, reveal anelastic behaviors which are mainly governed by the secondary beta- and primary alpha-relaxations. The addition of cross-links in an apparently linear polymer network was accompanied by the shift of the glass transition to higher temperatures. Differently from the alpha-relaxation whose strength decreases with cross-linking, the beta-relaxation strength increases. This observation implies the lack of any dynamical hierarchy between the local and cooperative segmental dynamics in these systems. The beta-relaxation follows the Arrhenius law with values of the characteristic frequency ω0 and the activation energy Eact ranging between 1015 and 1016 s−1 and 43.7 and 48.7 kJ/mole, respectively. In both the HPNs the beta-loss peaks have been quantitatively analyzed by the symmetric double-well potential (SDWP) model.