Solid-state deuterium nuclear magnetic resonance of the methyl dynamics of poly(α-methylstyrene) and polymethylphenylsilane

Abstract

The methyl-d3 dynamics of two relatively similar polymers, poly(α-methylstyrene) (PAMS-d3) and polymethylphenylsilane (PMPS-d3), are investigated via deuterium NMR relaxation experiments. Our analysis of the relaxation data uses the entire solid-echo spectra to maximize the precision of the experiments with regard to the information available on the methyl dynamics. The analysis is novel in that it does not use Mx or M0 to fit the relaxation data. Additionally, the three-site symmetric jump model is shown to not have an observable azimuthal angular dependence for T1 relaxation. The methyl dynamics are quantified with τm, σ, and f which are the log-average correlation time, half-height full-width (base 10) of a log-normal distribution of reorientation rates, and the anisotropy of the relaxation, respectively. The anisotropy parameter, f, is based on a serial combination of the rotational diffusion and symmetric three-site jump reorientation of a methyl deuteron. This serial model coupled with a distribution of τc's has a minimal number of parameters that have physical meaning and quantify the observations of our relaxation data. Generally, at similar temperatures the methyl reorientation in PAMS-d3 is at least 100 times slower than that of PMPS-d3. For both polymers, both τm and a decrease with increasing temperature, resulting in activation energies of 12 and 5 kJ/mol for PAMS-d3 and PMPS-d3, respectively. Also, with increasing temperature a mechanistic change from three-site jump to rotational diffusion is observed and quantified. This information, along with that of other studies, suggests that the PAMS-d3 methyls have highly restrictive environments that may be closely coupled to phenyl-ring reorientation. © 2000 American Institute of Physics.