How Local Structural Change Affects Polymer Segmental Dynamics: Crosslinks and Nanoparticles

Structural perturbations throughout a polymeric material can have a profound effect on its properties. In this work, we target two polymer families: one having an increasing presence of covalent crosslinks and the other having an increasing concentration of silica nanoparticles. We can predict and explain the different impacts of these local changes on the glass transition using thermodynamic analysis to inform our model of segmental dynamics. The presence of nanoparticles leads to a less efficiently packed, and therefore increased, close-packed volume of the polymer. There is, however, little change in available free volume and, therefore, little change in the glass transition. In contrast, crosslinking shrinks a polymer’s available free volume significantly, and this leads to a significant slowdown in the rate of segmental relaxation, thus to an increase in the glass transition temperature, Tg. The reduction in free volume upon crosslinking is quite large, yet the slow down in dynamics is not as strong as we would expect. This implies the sensitivity of system dynamics to changes in free volume is weakening with crosslinking. Our analysis demonstrates this in two ways: first, by comparing it to the broader trend connecting free volume and Tg observed for dozens of other polymeric materials. We find crosslinked systems exhibit a much weaker trend. Second, through quantitative characterization of the volume sensitivity of segmental relaxation using our model analysis of pressure dependent dynamics.