Probing the subglass relaxation behavior in model heterocyclic polymer networks by dielectric spectroscopy - art. no. 051802

The subglass relaxation (beta) in model heterocyclic polymer networks (HPNs ) with a controlled ratio of trimerized mono- and diisocyanates was charact erized by dielectric spectroscopy in the frequency domain. The beta relaxat ion in the investigated HPNs follows the Arrhenius law with unusually low v alues of the preexponential factor (10(-17)<<tau>(beta0)<10(-15) s). Howeve r, little influence of the local environment, as characterized by the netwo rk density, on the apparent activation energies <Delta>E-beta is observed. This fact, combined with their fairly low absolute values (50.4-58.3 kJ/mol ), were considered as typical of a noncooperative relaxation in loosely pac ked regions of a glassy quasilattice. Both the intensity and dielectric str ength of the beta relaxation in HPNs increase with increasing apparent netw ork density (i.e., with lower ratios of linear and network structures in th e system, L/N). This effect was explained by a model assuming that the tota l, composition-in variant. free volume available was distributed between de nsely packed domains comprising linear, two-arm isocyanurate heterocycles ( ISHs) and loosely packed, three-arm ISHs, which form continuous, three-dime nsional network structures. The experimental data for HPNs confirm Ngai's c orrelation between the logarithm of the secondary beta -relaxation time and the Kohlrausch-Williams-Watts stretching exponent for the primary alpha re laxation, It is suggested that the absence of conjugated bonds within isocy anurate heterocycles makes them sufficiently flexible to allow for specific conformational transitions, like the "chair-boat-chair" transition in the structurally similar cyclohexyl ring.