Initial Solvent-Driven Nonequilibrium Effect on Structure, Properties, and Dynamics of Polymer Nanocomposites

Abstract

Unusual structures and dynamic properties found in polymer nanocomposites (PNCs) are often attributed to immobilized (adsorbed) polymers at nanoparticle/polymer interfaces, which are responsible for reducing the intrinsic incompatibility between nanoparticles and polymers in PNCs. Although tremendous efforts have been made to characterize the presence of immobilized polymers, systematic understanding of the structure and dynamics under different processing conditions is still lacking. Here, we report that the initial dispersing solvent, which is not present after producing PNCs, drives these non-equilibrium effects on polymer chain dynamics at interfaces. Employing extensive small angle scattering, proton NMR spectroscopy, and rheometry experiments, we found that the thickness of immobilized layer can be dependent on initial solvent, changing the structure and the properties of the PNC significantly. In addition, we show that the outcome of the initial solvent effect becomes more effective at particle volume fractions where the immobile layer begins to interact. The incorporation of nanoparticles into a polymer matrix, thus creating polymer nanocomposites (PNCs), is regarded as a general strategy to enhance the physical properties of neat polymers …[1-4]. However, the intrinsic incompatibility between nanoparticles and polymers requires the effective control of polymer–nanoparticle interactions at the interface. Polymers can be chemically grafted or physically adsorbed onto the particle surface, creating an immobilized layer, which is believed to control the resulting structures and properties of PNCs …[5-9]. Many attempts have been made, therefore, to develop a stable immobilized layer by changing the chemical structure of particles/polymers, and to characterize governing parameters such as grafting/adsorption density, the sizes of the polymers/particles, and their compositions …[10-19]. Few studies, however, have reported on non-equilibrium effects present during the processing of PNCs …[4,20-24]. While PNC production involves complicated yet dynamic processes such as initial dispersion in solvents, mixing with polymers, solvent evaporation, and drying, the relaxation time of polymers in the presence of nanoparticles may significantly increase, suggesting that the polymers and particles may not reach their equilibrium structures in experimentally accessible processing times, becoming kinetically trapped …[25,26]. In this letter, we report that when the initial dispersing solvent is varied, PNCs may not reach their equilibrium state, resulting in a dramatic change in particle dispersion, polymer dynamics, and rheological properties. We composed PNCs with poly(ethylene glycol) (PEG) and silica nanoparticles using either ethanol or water as casting solvents. Employing extensive small angle X-ray scattering (SAXS), NMR free induction decay (FID), double-quantum (DQ), and rheometry experiments, we found that the initial solvent influences (i) the initial/final particle microstructure, (ii) the dynamics of the immobilized layers, and (iii) the resulting physical properties of the PNCs, even though the solvent was thoroughly evaporated and thus not present in the final state of the PNCs.