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

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

Intermediate motions as studied by solid-state separated local field NMR experiments

In this report, the application of a class of separated local field NMR experiments named dipolar chemical shift correlation (DIPSHIFT) for probing motions in the intermediate regime is discussed. Simple analytical procedures based on the Anderson-Weiss (AW) approximation are presented. In order to establish limits of validity of the AW based formulas, a comparison with spin dynamics simulations based on the solution of the stochastic Liouville-von-Neumann equation is presented. It is shown that at short evolution times (less than 30% of the rotor period), the AW based formulas are suitable for fitting the DIPSHIFT curves and extracting kinetic parameters even in the case of jumplike motions. However, full spin dynamics simulations provide a more reliable treatment and extend the frequency range of the molecular motions accessible by DIPSHIFT experiments. As an experimental test, molecular jumps of imidazol methyl sulfonate and trimethylsulfoxonium iodide, as well as the side-chain motions in the photoluminescent polymer poly[2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene], were characterized. Possible extensions are also discussed. (c) 2008 American Institute of Physics