Segmental dynamics of polyethylene-alt-propylene studied by NMR spin echo techniques

Area of Science:

• Polymer Physics
• Materials Science
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Understanding polymer dynamics is crucial for predicting material properties.
• Segmental dynamics in entangled polymer melts are complex and challenging to probe experimentally.
• Existing methods have limitations in accessing the relevant time scales for polymer motion.

Purpose of the Study:

• To investigate the segmental dynamics of a highly entangled polyethylene-alt-propylene melt.
• To develop and apply a novel proton nuclear magnetic resonance (NMR) approach for probing dynamics over extended time ranges.
• To compare experimental findings with predictions from the tube-reptation model.

Main Methods:

• Utilized a novel proton nuclear magnetic resonance (NMR) approach based on 1H → 2H isotope dilution.
• Employed a solid-echo build-up function derived from Hahn echo and solid-echo pulse sequences.
• Combined the NMR technique with time-temperature superposition to cover 6 decades in time (10^-6 s to 1 s).

Main Results:

• Successfully extracted segmental mean-squared displacements in the millisecond time range.
• Observed three distinct power-law regimes in the time dependence of mean-squared displacement.
• Results at short times agree with existing fast-field cycling relaxometry and neutron spin echo data.
• Discrepancies were found between the experimental amplitude ratio of intermolecular to intramolecular dynamic dipole-dipole correlation functions and tube-reptation model predictions.

Conclusions:

• The novel NMR isotope dilution technique effectively probes segmental dynamics in entangled polymers over an extended time scale.
• The observed dynamic regimes are largely consistent with the tube-reptation model, supporting its applicability.
• Deviations in the ratio of intermolecular to intramolecular dynamics suggest limitations or refinements needed for the current tube-reptation model in describing specific aspects of polymer motion.