RELAXATION OF STRESS AND ORIENTATION IN THERMO-REVERSIBLE NETWORKS

4-(3,5-dioxo-1,2,4-triazolidin-4-yl) benzoic (U4A) or -isophthalic aci d (U35A) units randomly attached to a polybutadiene backbone form exte nded supramolecular hydrogen bond assemblies, which act as effective j unction zones in unusual thermoreversible elastomer networks. The rela xation behavior of stress and the segmental orientation induced after a sudden strain, has been investigated by means of step-strain experim ents and H-2-NMR spectroscopy. The stress relaxation behavior can be d escribed as a two-step process in which the first step is very sensiti ve to the stability of the supramolecular aggregates. About 20 s after reaching the initial strain, stress is reduced to a value comparable to the yield-stress in stress-strain cycles. The long-time relaxation behavior is interpreted as a reduction of the effective cross-link den sity due to a gradual breakdown of the hydrogen bond clusters. The rel axation data of the functionalized polybutadienes can be superimposed to give master curves, which are characteristic for the type of juncti on zone. The degree of aggregate degradation is estimated as a functio n of the initial stress. In the same type of step-strain experiment, t he H-2-NMR spectra show a decay of the line splitting as well as a cha nge in the complete lineshape. (C) 1997 Published by Elsevier Science Limited.