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

Le résumé fourni par la source

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 M∞ 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 σ 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.

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