Diminishing Interfacial Effects with Decreasing Nanoparticle Size in Polymer-Nanoparticle Composites

Using molecular simulations on model polymer nanocomposites at fixed
filler loading we show that interfacial polymer dynamics are affected
less with decreasing nanoparticle (NP) size. However, the glass
transition temperature $T_g$ changes substantially more for extremely
small NP. The reason for this apparent contradiction is that the mean
NP spacing decreases with decreasing particle size. Thus, all polymers
are effectively interfacial for sufficiently small NP, resulting in
relatively large $T_g$ shifts. For larger NP, interfacial relaxations
are substantially slower than the matrix for favorable NP-polymer
interactions. The minority ``bound'' polymer dynamically decouples
from the polymer matrix, and we only find small changes in $T_g$
relative to that of the bulk polymer for large NP. These results are used
to organize a large body of relevant experimental data, and we propose an apparent
universal dependence on the ratio of the face-to-face distance between the NPs and
the chain radius of gyration.