Bound Layers “Cloak” Nanoparticles in Strongly Interacting Polymer Nanocomposites

Abstract

Polymer-nanoparticle (NP) interfacial interactions are expected to strongly influence the properties of nanocomposites, but surprisingly, experiments often report small or no changes in the glass transition temperature, T. To understand this paradoxical situation, we simulate nanocomposites over a broad range of polymer-NP interaction strengths, ε. When ε is stronger than the polymer-polymer interaction, a distinct relaxation that is slower than the main α-relaxation emerges, arising from an adsorbed "bound" polymer layer near the NP surface. This bound layer "cloaks" the NPs, so that the dynamics of the matrix polymer are largely unaffected. Consequently, T defined from the temperature dependence of the routinely measured thermodynamics or the polymer matrix relaxation is nearly independent of ε, in accord with many experiments. Apparently, quasi-thermodynamic measurements do not reliably reflect dynamical changes in the bound layer, which alter the overall composite dynamics. These findings clarify the relation between quasi-thermodynamic T measurements and nanocomposite dynamics, and should also apply to thin polymer films.

Translation of segment scale stiffening into macroscale reinforcement in polymer nanocomposites

“…According to previous simulation studies, polymer chains exhibit gradually prolonged relaxation times ( τ ) with decreasing distance from attractively interacting nanoparticle (as sketched in Fig. c) . Simulations also suggest that polymer chains can be spatially confined and chain R g is reduced as interparticle distance decreases as studied in fairly similar dimethacrylate polymer network (bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate blends) .…”

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

“…[202][203][204] The general trend that has emerged is that the polymer dynamics typically slows down if the NP-polymer interaction is attractive (T g increases) and speeds up if this interaction is strongly repulsive (T g decreases). 200,203,[205][206][207][208][209][210][211][212][213][214] The consequences of these changes on properties and their effects on dynamics and the NP dispersion are poorly understood.…”

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

“…Many experiments report little or no change of T g for strongly interacting NPs with diameter 10 nm. Simulations suggest that a bound interfacial layer dynamically decouples from the surrounding polymer matrix in these situations [21]; since the matrix polymer dominates the observed behavior, there is relatively little observed change in T g . In contrast, for small NP (≈ 1 nm), experiments imply that there are large T g shifts [22].…”