VISCOELASTIC PROPERTIES OF PHYSICALLY CROSS-LINKED NETWORKS - TRANSIENT NETWORK THEORY

A simple model is introduced to describe the dynamics of physically cross-linked networks in which junctions are sufficiently weak to break and recombine in thermal fluctuations. The time-evolution equation under arbitrary macrodeformation is derived for the creation and annihilation of the junctions-and hence for the number of elastically effective polymer chains with a fixed end-to-end vector. We focus our attention specifically on the unentangled networks in which the molecular weight M(x) between neighboring junctions is smaller than the entanglement molecular weight M(e), so that each chain obeys Rouse dynamics modified by sticky trapping centers rather than reptation. Stress-strain relation under shear and elongational deformation is detailed for a model network which is made up of polymers of uniform length with associating functional groups at their chain ends. On longer time scales than the junction breakage time, the total number of effective chains decreases with time-resulting in an intranetwork flow.