Solid-state deuterium nuclear magnetic resonance of the methyl dynamics of poly(α-methylstyrene) and polymethylphenylsilane
Journal of Chemical Physics
The methyl-d 3 dynamics of two relatively similar polymers, poly͑␣-methylstyrene͒ (PAMS-d 3 ) and polymethylphenylsilane , 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 ϱ or M 0 to fit the relaxation data. Additionally, the three-site symmetric jump model
... shown to not have an observable azimuthal angular dependence for T 1 relaxation. The methyl dynamics are quantified with m , , and f which are the log-average correlation time, half-height full-width ͑base 10͒ 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 c '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-d 3 is at least 100 times slower than that of PMPS-d 3 . For both polymers, both m and decrease with increasing temperature, resulting in activation energies of 12 and 5 kJ/mol for PAMS-d 3 and PMPS-d 3 , 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-d 3 methyls have highly restrictive environments that may be closely coupled to phenyl-ring reorientation.