Segmental anisotropy in strained elastomers detected with a portable NMR scanner

Area of Science:

• Materials Science
• Polymer Physics
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Nuclear Magnetic Resonance (NMR) techniques are valuable for probing material properties.
• Segmental anisotropy in elastomers can be induced by external forces like uniaxial stretching.
• Understanding this anisotropy is crucial for predicting elastomer behavior under stress.

Purpose of the Study:

• To investigate proton transverse nuclear magnetic relaxation in elastomers using NMR.
• To quantify segmental anisotropy induced by uniaxial forces in natural rubber.
• To correlate NMR-MObile boX (NMR-MOUSE) measurements with material deformation.

Main Methods:

• Proton transverse nuclear magnetic relaxation was measured using the Hahn-echo decay.
• Measurements were performed on cross-linked natural rubber bands subjected to uniaxial stretching.
• The angular dependence of the Hahn-echo decay was analyzed in relation to the stretching direction and magnetic field gradients.

Main Results:

• The Hahn-echo decay showed a clear dependence on the angle between the stretching force and the NMR scanner's Z-axis.
• The observed anisotropy effect correlated with the extension ratio of the rubber bands.
• Segmental anisotropy was more pronounced in the liquid-like decay regime and less evident with multi-pulse sequences.

Conclusions:

• Single-side NMR, particularly with the NMR-MOUSE, is suitable for measuring segmental anisotropy in elastomers.
• An analytical theory was developed to explain the angular dependence of Hahn-echo decay in stretched elastomers.
• Numerical simulations using previous data confirmed the measured segmental anisotropy in inhomogeneous magnetic fields.