Macromolecular structure: average molecular weight between crosslinks of dry polymer networks by NMR.

R. H. ACOSTA; D. A. VEGA; M. A. VILLAR; G. A. MONTI; F. CAMPISE; E. M. VALLES

Santa Fe

Taller; III Taller de Resonancia Magnética. NMR and EPR at the Forefront of Research; 2016

Universidad Nacional del Litoral

The purpose of this work is to contribute to the structural characterization of polymer networks by Nuclear Magnetic Resonance (NMR) techniques. In particular, we pretend to study the possibility of determining the average molecular weight between crosslinks from the residual dipolar coupling constant obtained through NMR Double Quantum Coherence experiments. The residual dipolar coupling constant (Dres) is proportional to the dynamics chain order parameter S and the crosslink density [1], and can be related to the molecular weight between crosslinks (Mc). The relation between Dres and Mc depends on the theoretical (rubber elasticity) basis considered. If the phantom model is used as theoretical basis [2], instead of the ?affine model? [3], then a great dependence on the average functionality of the crosslinks arises. We evaluate both theories on a set of model polydimethylsiloxane (PDMS) networks prepared with varying functionality. Since transiently trapped entanglements contribute to the residual dipolar coupling, [4-5], we also made focus on how defects present in the networks can affect the determination on Mc through Dres. We compared NMR results with Miller-Macosko Mean Field Calculations for Mc. Although proportionality between Dres and Mc has already been investigated and several theoretical approaches have been considered, quantitative discrepancies were obtained, and contribution of defects to the determination of Dres is still a matter of interest.