Entangled and liquid-like chain discrimination on model polymer networks studied by Double Quantum CPMG based sequences.

RODOLFO H. ACOSTA; MARÍA BELÉN FRANZONI; GUSTAVO MONTI

Leipzig

Conferencia; 10th Bologna Conference on Magnetic Resonance in Porous Media (MRPM 10); 2010

NMR offers a great variety of techniques to assess information on elastomer microstructure. A multitude of experimental approaches can be used to estimate residual dipolar couplings or other anisotropic interactions, which in turn are related to the density of cross-links or macroscopic properties of the network. Residual interactions arise from fast segmental fluctuations that are anisotropic due to the existence of cross-links and other topological constraints and are directly proportional to a dynamic order parameter of the polymer backbone [3]. Model PDMS networks [1,2] are ideal systems not only to probe the validity of different models for the dynamics of the network but also offer the possibility to test the response of new pulse sequences. Double Quantum NMR [4] has been lately applied with great success for the measurement of residual dipolar couplings, which gives a direct measure of the elastic response of the system. Nevertheless, dynamic physical entanglements of the pendant chains also contribute in great extent to the elasticity of the networks and their discrimination is a priori not easily obtained by NMR methods [5]. In this work we use such systems to test the validity of the actual DQ approach for the determination of residual dipolar coupling. We test the different contributions of the NMR signal by inspection of the response to a CPMG sequence, which has been shown to give reliable information on the fraction of elastic and pendant material [6]. Additionally we show that the sequence performance can be extended to measure spin diffusion from elastic to pendant chains. Standard spin diffusion experiments rely on the generation of a magnetization gradient caused by the extremely different T2* of a sample with different components. In our networks the PDMS chains that conform the elastic and pendant fractions are spectroscopically almost identical and dynamically very similar, which renders the standard spin diffusion approach inefficient.