A MODEL OF ADAPTIVE LINKS FOR AGING VISCOELASTIC MEDIA

A new model is derived for the mechanical behavior of linear viscoelas tic materials subjected to physical aging. According to the model, a v iscoelastic medium is treated as a system of elastic springs (links be tween polymeric molecules) which replace each other. Two types of link s are distinguished: links arisen at the instant of quenching (type I) , and links emerging in the quenched material at a constant temperatur e (type ii). The mechanical behavior of an aging medium is determined by three material functions which characterize (i) the collapse of lin ks of type I, (ii) the breakage of links of type II, and (iii) the rat e of emergence of new links of type LI. We derive integral equations f or these functions and find their solutions using data of the standard relaxation tests. To verify the model, we calculate the material resp onse in the creep tests and compare results of numerical simulation wi th experimental data for an epoxy adhesive. The results obtained demon strate fair agreement between experimental observations and their pred iction. By using the model developed, we analyze numerically the behav ior of a viscoelastic medium under time-varying loads. We study elonga tion of a specimen with a constant rate of strain, its recovery after creep tests, and steady shear oscillations of a layer. In the latter c ase, numerical results are compared with experimental data for polypro pylene samples. (C) 1996 Society of Rheology.