IFEG   20353
INSTITUTO DE FISICA ENRIQUE GAVIOLA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Entangled and liquid-like chain discrimination on model polymer networks studied by Double Quantum – CPMG based sequences.
Autor/es:
R. H. ACOSTA; M.B. FRANZONI; G.A. MONTI
Lugar:
Leipzig, Alemania
Reunión:
Congreso; Magnetic Resonance in Porous Media 2010; 2010
Resumen:
<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:72.0pt 90.0pt 72.0pt 90.0pt; mso-header-margin:35.4pt; mso-footer-margin:35.4pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> A wide range of systems is characterized by having a superposition of solid and liquid-like behavior 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 the creation and evolution of multiple quantum coherences (MQC). By monitoring the creation and evolution of the different MQC a great deal of information ranging from cluster sizes, Hilbert space connectivity, and decoherence processes can be obtained. Nevertheless, the NMR signal attenuation arises not only from the system decoherence in terms of quantum dynamics, but also from molecular movements of the sample during the experimental time and from pulse sequence imperfections that are inherent to every experiment. In this work we present an experimental approach based on encoding the system by the creation of MQ coherences and the detection of the different contribution to the signals by a CPMG detection period. Model PDMS polymer networks are used to exemplify the performance of the proposed method.   Double-quantum (DQ) or, more generally, multiple quantum (MQ) spectroscopy has repeatedly been recognized as a uniquely selective tool for the determination of residual couplings in polymeric systems. From the viewpoint of NMR, cross-linked elastomers exhibit both liquidlike 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 and quadrupolar interactions are not completely averaged out and give rise to solid-like NMR properties. The success of the methodology hinges on the origin of the measured MQ signal coming only from actually coupled segments, while perturbing contributions from uncoupled segments, e.g., free dangling chains or sol in a network, are effectively suppressed. MQ data are commonly analyzed in terms of build-up curves of MQ coherences as a function of the excitation and reconversion time in the multiple-quantum experiment. The drawback in this experimental approach is that the influence of the liquid-like segments must be removed by data post processing. Aca falta algo mejor que decir, con el cpmg restando la cola tambien es data processing   The relaxation of 1H transverse magnetization is mainly determined by the dipole-dipole magnetic interaction between protons. This interaction is modulated at different extents by molecular motions, and therefore, it is sensitive to differences in the motion of the chains that form the polymer network. This technique has been shown to give a very precise measure of the amount of entangled polymer chains that form the elastic network and the pendant material. At the normal NMR time scales, the elastic chains behave as solid-like while the pendants chains behave as isotropic ones. Nevertheless, the extraction of dipolar couplings is subject to strong model assumptions and additionally is coupled to other parameters.   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.   Y ahora viene lo de spin diffusion monitoreando el cpmg     Additionally Average Hamiltonian theory breakdown conditions are investigated and an easy test parameter consisting of monitoring the collapse of MQC to ZQC is shown.