Communication: Proton NMR dipolar-correlation effect as a method for investigating segmental diffusion in polymer...

Area of Science:

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

Background:

• Investigating segmental diffusion in high molar mass polymer melts is crucial for understanding material properties.
• Traditional methods struggle to access segmental dynamics in the millisecond timescale.
• Magnetic dipole-dipole interactions provide information on molecular motion but require careful analysis.

Purpose of the Study:

• To present a simple, fast method for investigating segmental diffusion in high molar mass polymer melts.
• To develop a proton dipolar-correlation build-up function for analyzing inter- and intramolecular interactions.
• To enable the study of polymer segment displacements in the millisecond range.

Main Methods:

• Utilized a novel proton dipolar-correlation build-up function derived from Hahn Echo NMR signals at times t and t/2.
• Separated intermolecular and intramolecular magnetic dipole-dipole interactions using isotope dilution.
• Investigated protonated and deuterated polybutadiene melts (196,000 g/mol) at various temperatures.

Main Results:

• The proton dipolar-correlation build-up function's initial rise captures both inter- and intramolecular contributions.
• Segmental displacements were accessed in the millisecond range, a timescale difficult for other techniques.
• An observed power law exponent for segmental mean squared displacements (MSDs) was 0.32 ± 0.03.
• Intermolecular contributions to the NMR signal exceeded 50% and increased with time.

Conclusions:

• The presented method effectively probes segmental diffusion in polymer melts.
• Isotope dilution is key to separating different interaction contributions for accurate MSD analysis.
• The technique provides valuable insights into polymer dynamics at longer timescales.