Trapped entanglements in polymer networks and their influence on the stress-strain behavior up to large extensions

Abstract

The paper demonstrates how a characterization of unfilled, amorphous rubber networks can be evaluated from uniaxial stress-strain measurement data. A network model is proposed that includes finite chain extensibility, topological constraints, and a trapped entanglement constribution to the reduced stress, which does not vanish in the infinite strain limit. Beside their influence on topological constraints, trapped entanglements are assumed to act like additional junctions, which also reduce the limiting extensibility of the network. In this framework a relation between the crosslink contribution and the topological constraint contribution to the reduced stress is derived, which allows the determination of the trapping factor from experimental stress-strain measurement data. The experimental results found for NR-networks cured with thiuram (TMTD) and peroxide (DCP), respectively, confirm the proposed model. It becomes obvious that the low topological constraint contribution found for the DCP-cured networks is related to a lower trapping rate, which results from main-chain scission during the curing procedure.