Transiently trapped entanglements in model polymer networks

Abstract

The relaxational dynamics of trapped entanglements in model silicone polymer networks is studied through the residual dipolar couplings (RDC) obtained by double quantum nuclear magnetic resonance (DQ NMR). These experiments were performed on model polymer networks containing linear pendant chains. The model networks where synthesized by end-linking a mixture of α,ω-divinyl poly(dimethylsiloxane) (B2) and ω-vinyl poly(dimethylsiloxane) (B1) with trifunctional (A3) or tetra-functional (A4) cross-linkers. At the time scale of the NMR experiments only a small fraction of the linear pendant chains B1 loses the memory of its early configuration. Then, the unrelaxed topological constrains involving pendant material render a nonzero average dipolar coupling that contributes to the solid-like behavior of the NMR response. Irrespective of the functionality of the cross-linkers, upon the presence in the network of pendant chains induced by the insertion of the B1 monofunctional poly(dimethylsiloxane) an important reduction in the RDC is observed as a consequence of the transiently trapped entanglements. It was also verified that, according to the viscoelastic response, the networks prepared with A4 cross-linkers show systematically higher values of the residual dipolar couplings than trifunctional cross-linked networks.