Long-Term Aging in Miscible Polymer Nanocomposites

Abstract

We find that the initial, solvent-cast state of nanoparticles (NPs) in a polymer matrix temporally evolves during thermal annealing such that, at steady state, NPs maximize their distance from each other subject to mass balance constraints. The observed timescales for this unexpected structural reorganization, as probed by small-angle X-ray scattering, are temperature-dependent and can be prohibitively large, especially at temperatures around and below 1.2T g. X-ray photon correlation spectroscopy measurements during reorganization reveal that the collective NP dynamics slow down with annealing at constant temperature; this is accompanied by changes in the low-frequency regime in macroscopic viscoelastic measurements in equilibrated materials. By ruling out other potential sources for these effects (i.e., electrostatic interactions, adsorbed layers), we attribute these results to a long-ranged repulsive force between the NPs caused by fluctuations in the polymer phase, i.e., the “anti-Casimir” effect proposed by Obhukhov and Semenov [Long-range interactions in polymer melts: The anti-Casimir effect038305Phys Rev Lett200595 Thus, our results highlight the important role of long-term, slow NP reorganization on the structure and, subsequently, the properties of polymer nanocomposites (PNCs), even in the case of nominally miscible polymer nanoparticle hybrids.

Depletion-Driven Assembly of Polymer-Coated Nanocrystals

“…The modularity of adding an exogenous molecular component to induce attractions provides flexibility in the design of new systems, and there are many opportunities for studying new, reconfigurable depletants within the widely studied field of responsive polymers. − While the present results establish the phase behavior of polymer-wrapped NCs with depletion attractions, the dynamics of assembly or reconfiguration in response to such perturbations are yet to be investigated. Recent work has used photon correlation imaging paired with dynamic light scattering, , and X-ray photon correlation spectroscopy , to investigate structural evolution with time toward assembly as well as the homogeneity (or heterogeneity) of the structures.…”

“…114−117 While the present results establish the phase behavior of polymer-wrapped NCs with depletion attractions, the dynamics of assembly or reconfiguration in response to such perturbations are yet to be investigated. Recent work has used photon correlation imaging paired with dynamic light scattering, 118,119 and X-ray photon correlation spectroscopy 72,120 to investigate structural evolution with time toward assembly as well as the homogeneity (or heterogeneity) of the structures. With the assembly of polymer-wrapped NCs via depletion attractions, we have introduced a model system with many "knobs" that may be tuned to deliberately assemble or disassemble the NCs and modify resulting properties.…”

Depletion-driven assembly of polymer-coated nanocrystals

“…Recent work has used photon correlation imaging paired with dynamic light scattering, 117,118 and X-ray photon correlation spectroscopy 72,119 to investigate structural evolution with time towards assembly, as well as the homogeneity (or heterogeneity) of the structures.…”

Relaxation dynamics of deformed polymer nanocomposites as revealed by small-angle scattering and rheology

“…Prior to loading, the PNC samples were molded into disks of 8 mm in diameter and 1 mm in thickness by a hydraulic press (Carver, Inc.) at 453 K, which is more than 1.2 times of the glass transition temperature, T g of PNCs. 44 The frequency sweep measurements in the range of 10 2 -10 À2 rad s À1 were performed between 383 K and 453 K with an interval of 10 K. The gap was adjusted accordingly during the measurements at different temperatures to ensure accurate measurements of modulus. The applied strain amplitude was in the linear response region varying from 0.1% to 1% depending on the testing temperature and NP loadings.…”