IFEG   20353
INSTITUTO DE FISICA ENRIQUE GAVIOLA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
1H Solid State Nuclear Magnetic Resonance on Model Poly(dimethyl siloxane) Model Networks.
Autor/es:
G. A. MONTI; R. H. ACOSTA; F. CAMPISE; F. VACA CHAVEZ; M. A. VILLAR; E. M. VALLES; D. A. VEGA
Reunión:
Congreso; Vth Iberoamerican NMR Meeting; 2013
Institución organizadora:
AUREMN
Resumen:
We present studies on Poly(dimethyl siloxane) Model Networks concernig strucuture and dynamical properties. Silicone networks have been extensively studied in order to explain the influence of molecular structure on mechanical properties. Free chains (solubles) and pendant chains are the more common defects in polymer networks. The relative concentration and molecular structure of these defects have a strong influence on the viscoelastic properties of networks, changing considerably the spectrum of relaxation times. The studies were carried on model PDMS networks with controlled amounts of well-characterized pendant chains. The model networks where synthesized by end-linking a mixture of R,ω-divinyl poly (dimethylsiloxane) (B2) and ω-vinyl poly(dimethylsiloxane) (B1) with trifunctional (A3) or tetra-functional (A4) cross-linkers. Proton residual dipolar interactions measured by NMR provide information about the structure and molecular dynamics in soft solids like cross-linked elastomers above the glass transition temperature. We tested the sensitivity of DQ NMR to detect the influence of network defects on model polymer networks with a controlled amount of pendant chains. DQ NMR experimental data are widely and very well correlated with those data obtained by rheological techniques. The relaxational dynamics of trapped entanglements in model silicone polymer networks is studied also through the residual dipolar couplings (RDC) obtained by DQ NMR. 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.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. 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. ACKNOLEDGEMENTS: SECyT-UNC, CONICET, ANPCyT, MPI-Partner Group.