Slow polymer dynamics in poly(3-hexylthiophene) probed by muon spin relaxation

Abstract

The molecular dynamics of regioregulated poly(3-hexylthiophene) P3HT is investigated using muon spin relaxation (μSR). The response of the μSR spectra to a longitudinal magnetic field (BLF, parallel to the initial muon spin direction) indicates that the implanted muons form both muonated radicals localized on the thiophene ring and diamagnetic states with comparable yields. Moreover, the unpaired electron in the radical undergoes hyperfine interactions with muon bound to thiophene and with neighboring protons, whose fluctuations can serve as a measure of the molecular dynamics. The BLF dependence of the longitudinal muon spin relaxation rate (1/T1μ) measured in detail at several temperatures is found to be well reproduced by the spectral density function J(ω) derived from the local susceptibility that incorporates the Havriliak–Negami (H–N) function used in the analysis of dielectric relaxation, χ(ω)∝1/[1−i(ω/ν~)δ]γ (where ν~ is the mean fluctuation rate and 0<γ,δ≤1). The magnitude of ν~ and its temperature dependence deduced from the analysis of 1/T1μ are found to be consistent with the motion of hexyl chains and thiophene rings suggested by 13C-NMR. The present result marks a methodological milestone in the application of μSR to the dynamics of complex systems with coexisting fluctuations over a wide range of time scales, such as polymers.