Average molecular weight between crosslinks of dry polymer networks by NMR.

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

Congreso; III Taller de Resonancia Magnética ?NMR and EPR at the forefront of research?.; 2016

The purpose of this work is to contribute to the structural characterization of polymernetworks by Nuclear Magnetic Resonance (NMR) techniques. In particular, we pretend tostudy the possibility of determining the average molecular weight between crosslinks fromthe residual dipolar coupling constant obtained through NMR Double Quantum Coherenceexperiments.The residual dipolar coupling constant (Dres) is proportional to the dynamics chainorder parameter S and the crosslink density [1], and can be related to the molecular weightbetween 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 ofthe crosslinks arises. We evaluate both theories on a set of model polydimethylsiloxane(PDMS) networks prepared with varying functionality. Since transiently trappedentanglements contribute to the residual dipolar coupling, [4-5], we also made focus onhow defects present in the networks can affect the determination on Mc through Dres. Wecompared NMR results with Miller-Macosko Mean Field Calculations for Mc.Although proportionality between Dres and Mc has already been investigated andseveral theoretical approaches have been considered [6-8], quantitative discrepancieswere obtained, and contribution of defects to the determination of Dres is still a matter ofinterest.