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

R. H. ACOSTA; M. B. FRANZONI; G. A. MONTI

Congreso; Magnetic Resonance in Porous Media 2010; 2010

A variety of systems is characterized by having both solid-like and liquid-like behaviors that can be often individualized by relaxation experiments, as for example by using a CPMG sequence. An alternative method that is commonly used is the measurement of creation and evolution of multiple quantum coherences (MQC). By monitoring the build up and evolution of the different MQC a great deal of information ranging from cluster sizes, connectivity, and decoherence processes can be obtained [1-2]. In this work we present an approach based on an encoding period with MQC and a detection one with a CPMG pulse sequence. Very well characterized model PDMS polymer networks are used to test the performance of the proposed method. Double-quantum (DQ) spectroscopy has repeatedly been recognized as a uniquely selective tool for the determination of residual couplings in polymeric systems. Cross-linked elastomers exhibit both liquid-like and solid-like features. At temperatures well above the glass transition temperature, the time scales of molecular motion are liquid-like. However, the presence of permanent cross links and topological constraints prevents the chain motion from being isotropic. Thus, anisotropic spin interactions such as dipolar interactions are not completely averaged out and give rise to solid-like NMR properties [3-4]. On the other hand, relaxation of transverse magnetization is mainly determined by the dipole-dipole magnetic interaction between protons. This interaction is modulated at different extents by molecular motions and is sensitive to differences in the motion of the chains that form the polymer network. This technique is very precise to measure the ratio between the entangled chains that form the network and the pendant material [5]. In this work we combine both methods in order to obtain the very reliable information of dipolar couplings obtained by DQ-NMR while the influence of liquid-like material is removed by direct inspection of the signal detected by a CPMG pulse sequence. If an additional DQ encoding period with a duration fixed at the maximum of the DQ build up curve, a filtering of the signals that comes from the liquid like segments is produced. Monitoring the evolution of the signals with the CPMG sequence can render information on spin-diffusion from entangled chains to pendant material. 1) R. R. Ernst, G. Bodenhausen, and A. Wokaun, Principles of Nuclear Magnetic Resonance in One and Two Dimensions; Oxford University Press, Oxford, 1987. 2) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: London, 1996. 3) Saalwachter, K. Prog. NMR Spectrosc. 2007, 51, 1-35. 4) Acosta, R.H.; Monti, G.A. ; Villar, M.A.; Vallés, E.M.; Vega, D.A.; Macromolecules 2009, 42, 4674-4680. 5) Vega, D. A.; Villar, M. A.; Valles, E. M.; Steren, C. A.; Monti, G.A. Macromolecules 2001, 34, 283–288.