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

The methyl-d(3) dynamics of two relatively similar polymers, poly(alpha-met hylstyrene) (PAMS-d(3)) and polymethylphenylsilane (PMPS-d(3)), are investi gated via deuterium NMR relaxation experiments. Our analysis of the relaxat ion data uses the entire solid-echo spectra to maximize the precision of th e experiments with regard to the information available on the methyl dynami cs. The analysis is novel in that it does not use M-infinity or M-0 to fit the relaxation data. Additionally, the three-site symmetric jump model is s hown to not have an observable azimuthal angular dependence for T-1 relaxat ion. The methyl dynamics are quantified with tau(m), sigma, and f which are the log-average correlation time, half-height full-width (base 10) of a lo g-normal distribution of reorientation rates, and the anisotropy of the rel axation, respectively. The anisotropy parameter, f, is based on a serial co mbination of the rotational diffusion and symmetric three-site jump reorien tation of a methyl deuteron. This serial model coupled with a distribution of tau(c)'s has a minimal number of parameters that have physical meaning a nd quantify the observations of our relaxation data. Generally, at similar temperatures the methyl reorientation in PAMS-d(3) is at least 100 times sl ower than that of PMPS-d(3). For both polymers, both tau(m) and sigma decre ase 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 diffusi on is observed and quantified. This information, along with that of other s tudies, suggests that the PAMS-d(3) methyls have highly restrictive environ ments that may be closely coupled to phenyl-ring reorientation. (C) 2000 Am erican Institute of Physics. [S0021-9606(00)50416-4].