Molecular dynamics of $cis$-polybutadiene across the glass transition revealed by muonated-radical spin relaxation

TL;DR

This study uses muonated-radical spin relaxation to investigate local molecular motions in cis-polybutadiene across its glass transition, revealing Arrhenius behavior consistent with neutron scattering results and demonstrating a new method for atomic-scale dynamics analysis.

Contribution

It introduces muonated-radical spin relaxation as a novel technique to directly probe local molecular motions in polymers at the atomic scale.

Findings

Relaxation rate follows Arrhenius temperature dependence.

Activation energy matches neutron scattering results.

Method effectively captures sub-nanosecond molecular dynamics.

Abstract

The local molecular motion of $cis$-polybutadiene, a typical polymeric material exhibiting a glass transition ($T_{\rm g}=168$ K), is investigated by the spin relaxation of muonated radicals, where the relaxation is induced by the fluctuation of hyperfine (HF) fields exerted from an unpaired electron to a nearby muon and surrounding protons. The relaxation rate $1/T_\mu$ measured under various longitudinal magnetic fields is analyzed using the recently developed theory of spin relaxation to consider the coexistence of quasistatic and fluctuating HF fields, where the fluctuation frequency for the latter $\nu$ is evaluated over a temperature $T$ range of 5-320 K. The obtained $\nu(T)$ is found to be well reproduced by the Arrhenius relation, and the activation energy and preexponential factor are in good agreement with those for the "elemental process" revealed by quasielastic neutron scattering and attributed to a fluctuation across three carbon-carbon bonds. This result demonstrates that muonated-radical spin relaxation is a promising approach for direct access to local molecular motions in the sub-nanosecond range and for their detailed modeling at the atomic scale.