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:
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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.