Probing the subglass relaxation behavior in model heterocyclic polymer networks by dielectric spectroscopy.

Abstract

The subglass relaxation (beta) in model heterocyclic polymer networks (HPNs) with a controlled ratio of trimerized mono- and diisocyanates was characterized by dielectric spectroscopy in the frequency domain. The beta relaxation in the investigated HPNs follows the Arrhenius law with unusually low values of the preexponential factor (10(-17)<tau(beta0)<10(-15) s). However, little influence of the local environment, as characterized by the network density, on the apparent activation energies DeltaE(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 packed regions of a glassy quasilattice. Both the intensity and dielectric strength of the beta relaxation in HPNs increase with increasing apparent network density (i.e., with lower ratios of linear and network structures in the system, L/N). This effect was explained by a model assuming that the total, composition-invariant, free volume available was distributed between densely packed domains comprising linear, two-arm isocyanurate heterocycles (ISHs) and loosely packed, three-arm ISHs, which form continuous, three-dimensional network structures. The experimental data for HPNs confirm Ngai's correlation between the logarithm of the secondary beta-relaxation time and the Kohlrausch-Williams-Watts stretching exponent for the primary alpha relaxation. It is suggested that the absence of conjugated bonds within isocyanurate heterocycles makes them sufficiently flexible to allow for specific conformational transitions, like the "chair-boat-chair" transition in the structurally similar cyclohexyl ring.