Low-Field NMR Investigations of Nanocomposites: Polymer Dynamics and Network Effects

TLDR

The dynamics of the polymer matrix in filled rubbers are altered by the presence of solid particles. The study used low‑field proton NMR to investigate model filled samples comprising grafted silica particles dispersed in an elastomeric matrix. Magic‑sandwich echo experiments were employed to quantify the fraction of polymer with slower dynamics and to correlate this fraction with the silica specific surface. Immobilized polymer is detected around silica particles regardless of covalent bonding, its fraction decreases with temperature or solvent, and covalent bonds locally increase cross‑link density—a novel observation attributed to good dispersion and covalent links.

Abstract

The dynamics of the polymer matrix in filled rubbers is modified by the presence of solid particles. We used low-field proton NMR to investigate model filled samples consisting of a dispersion of grafted silica particles into an elastomeric matrix. Exploiting magic-sandwich echo experiments, we were able to determine the fraction of polymer with slower dynamics and to correlate it to the silica specific surface. The presence of immobilized polymer—most probably due to a gradient of glass transition temperature around the solid particles—is detected whether there is a covalent bond between the filler and the matrix or not. Moreover, the fraction of immobilized polymer decreases in similar ways with either an increase of the temperature or the addition of solvent. In the case of covalent bonds between the silica and the polymer, multiple-quantum experiments reveal that the cross-link density of the elastomer matrix is locally increased in the vicinity of the particles. This is an observation that was not made in any conventional filled elastomer system and it can be attributed to the good particle dispersion and the covalent links in our model samples.