Segmental dynamics of solid-state poly(methylphenylsilane) by 1D and 2D deuterium NMR

Abstract

The segmental dynamics of solid-state poly(methylphenylsilane) were probed with deuterium solid-echo and two-dimensional exchange (2D-X) NMR via a methyl-d3 label. Between 25 and 50 °C, the spectra indicated that the polymer consisted of two fractions - a fast fraction with correlation times (τC) below 10-5 s and one with τC's above 10 s. Above 50 °C, motion with τC's around 10-3 s was also detected. A minimization routine was developed to fit the 2D-X spectra to a model of isotropic rotational diffusion with a distribution of τC'S. The best fits were obtained with trimodal stretched-exponential distributions. The trimodal distributions consisted of a fast mode with τC's around 10-5 s, an intermediate mode with τC's between 10-4 and 0.3 s, and a slow mode with τC's generally above 10 s. As the temperature increased from 56 to 90 °C, the fast fraction steadily increased from 21% to 50% while its average τC remained around 10-5 s; the intermediate fraction remained relatively constant at 23% while its average τC decreased from 125 to 8 ms, and the rigid fraction decreased from 55% to 32% with an average τC around 40 s. The fast fraction was attributed to amorphous segments, the rigid fraction to crystalline segments, and the intermediate fraction to segments that formed an interphase between the two.