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Segmental Anisotropy in Strained Elastomers Detected with a Portable NMR Scanner

2002; Elsevier BV; Volume: 22; Issue: 2-3 Linguagem: Inglês

10.1006/snmr.2002.0067

1527-3326

Kidist Hailu, Radu Fechete, Dan E. Demco, Bernhard Blümich,

Elasticity and Material Modeling

Single-side NMR is particularly suitable for measurements of segmental anisotropy induced in elastomers by uniaxial forces or local strain. Proton transverse nuclear magnetic relaxation was investigated with the NMR-MOUSE® by recording the Hahn-echo decay in cross-linked natural rubber bands. This provided information on the dependence of the Hahn-echo decay on the angle between the direction of the uniaxial stretching force and the axis Ẑ defined direction perpendicular to the magnet pole faces of the NMR-scanner. The anisotropy effect on the Hahn-echo decay is correlated with the extension ratio, and it is more evident in the liquid-like regime of the decay. A weaker segmental anisotropy is detected by 1H solid- and Hahn-echo decays recorded by multi-pulse sequences. A qualitative understanding of the angular dependence is obtained by an analytical theory of the Hahn-echo decay adapted to the case of stretched elastomers and to strongly inhomogeneous magnetic fields. Using angular-dependent 1H residual second van Vleck moments and correlation times reported previously [P.T. Callaghan and E.T. Samulski, Macromolecules30, 113 (1997)] from stretched natural rubber bands the segmental anisotropy measured in inhomogeneous magnetic fields by the Hahn-echo decay was numerically simulated. As an example of a macroscopic distribution of local segmental anisotropy, 1H Hahn-echo decays were measured by the NMR-MOUSE® sensor in a stretched cross-linked natural rubber plate with a circular cut in the center.