No abstract

We critically explore the role of particle dispersion on the melt state mechanical properties of nanocomposites formed by mixing polystyrene homopolymers with polystyrene grafted silica nanoparticles. We selected this system since we previously showed that nanoparticle spatial distribution can be controlled through judicious choices of the brush and matrix parameters. Here we focus on the temporal evolution of the nanoparticle self-assembly dispersion state and its effect on mechanical reinforcement using rheology, electron microscopy, and the measurement of nanoscale particle dynamics using X-ray photon correlation spectroscopy. Nanoscale and macroscopic experiments show that a composite with percolating sheets of particles displays "gel-like" or solid-like mechanical behavior at lower particle loadings than one with uniform particle dispersion. This conclusion allows us to conjecture that mechanical reinforcement is primarily controlled by interparticle interactions (including those facilitated by the grafted chains) and that the matrix plays a relatively minor role. This statement has far-reaching consequences on the design of polymer nanocomposites with desired properties.

The reductive dechlorination of pentachlorophenol (PCP) by Geobacter sulfurreducens in the presence of different biochars was investigated to understand how biochars affect the bioreduction of environmental contaminants. The results indicated that biochars significantly accelerate electron transfer from cells to PCP, thus enhancing reductive dechlorination. The promotion effects of biochar (as high as 24-fold) in this process depend on its electron exchange capacity (EEC) and electrical conductivity (EC). A kinetic model revealed that the surface redox-active moieties (RAMs) and EC of biochar (900 °C) contributed to 56% and 41% of the biodegradation rate, respectively. This work demonstrates that biochars are efficient electron mediators for the dechlorination of PCP and that both the EC and RAMs of biochars play important roles in the electron transfer process.