explain observations or characteristics of viscoelastic behaviour of polymer networks?

Answer 1

They are best explained in terms of changes of entropies in gases. Polymer networks are like cooked spaghetti, except stronger: the strands do not break unless really stressed. If the polymer network is completely relaxed, entropy is at a maximum, because order is a minmum. The spaghetti is collapsed in upon itself. But if one stresses the network, then the chains between the crosslinks straighten out, and the entropy decreases. When the stress eases, the tendency of entropy to increase causes the network to relax and contract. This is the origin of elasticity of rubber.