Long-Time Diffusion in Polymer Melts Revealed by ¹H NMR Relaxometry

Area of Science:

• Polymer Physics
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Materials Science

Background:

• Understanding polymer dynamics, particularly translational diffusion, is crucial for predicting material properties.
• Traditional methods for measuring diffusion coefficients can be complex and time-consuming.
• Nuclear Magnetic Resonance (NMR) relaxometry is a powerful tool for probing molecular motion.

Purpose of the Study:

• To establish field-cycling ¹H NMR relaxometry as a straightforward method for determining translational diffusion coefficients in polymers.
• To investigate the molecular mass dependence of the diffusion coefficient (D) in polybutadiene.
• To compare NMR relaxometry results with established techniques like field gradient NMR.

Main Methods:

• Utilized field-cycling ¹H NMR relaxometry to measure spin-lattice relaxation dispersion (R₁(ω)).
• Studied polybutadiene samples with varying molecular masses (M) across a wide temperature range.
• Analyzed the intermolecular contribution to relaxation rates in the low-frequency regime.

Main Results:

• Determined translational diffusion coefficients (D) from the frequency dependence of relaxation rates.
• Observed a molecular mass dependence of D following two distinct power laws: D ∝ M^-1.3±0.1 and D ∝ M^-2.3±0.1.
• Identified a crossover in the D(M) dependence near the entanglement molecular mass (M_e) of polybutadiene.

Conclusions:

• Field-cycling ¹H NMR relaxometry is a viable and simple technique for measuring polymer diffusion.
• The observed molecular mass dependence of diffusion aligns with predictions from the tube-reptation model.
• This method provides accurate diffusion coefficients comparable to field gradient NMR.