In an attempt to study the specific influence of cross-linking on the alpha
relaxation in polymer networks, a series of model heterocyclic polymer net
works (HPN) with well-defined cross-link densities and constant concentrati
on of dipolar units were studied. Model HPN systems were prepared by simult
aneous trimerization of 1,6-hexamethylene diisocyanate (HMDI) and hexyl iso
cyanate (HI). These HPN systems were characterized by dielectric relaxation
spectroscopy in the 10(-1) Hz < F < 10(5) Hz frequency range and in the 12
3 K < T < 493 K temperature interval. The alpha relaxation in these systems
depends on network density and shifts toward higher temperatures as the cr
oss-link density increases for high HMDI/HI ratios. Discussion of the alpha
-relaxation shape in light of recent models indicates that segmental motion
s above the glass transition systematically experience a growing hindrance
with increasing degree of cross-linking. Description of the temperature dep
endence of relaxation times according to the strong-fragile scheme clearly
shows that fragility increases as polymer network develops. (C) 2000 Americ
an Institute of Physics. [S0021-9606(00)50724-7].