Jcm José Brokken-Zijp

Asymmetric block copolymers confined in a thin film

We have used a dynamic density functional theory (DDFT) for polymeric systems, to simulate the formation of micro phases in a melt of an asymmetric block copolymer, AnBm(fA=1/3), both in the bulk and in a thin film. In the DDFT model a polymer is represented as a chain of springs and beads. A spring mimics the stretching behavior of a chain fragment and the spring constant is calculated with the Gaussian chain approximation. Simulations were always started from a homogeneous system. We have mainly investigated the final morphology, adopted by the system. First, we have studied the bulk behavior. The diblock copolymer forms a hexagonal packed array of A-rich cylinders, embedded in a B-rich matrix. Film calculations have been done by confining a polymer melt in a slit. Both the slit width and surface-polymer interactions were varied. With the outcomes a phase diagram for confined films has been constructed. Various phases are predicted: parallel cylinders (C∥), perpendicular cylinders (C⊥), parallel lamella...

Systematic study of percolative network formation and effective electric response in low-concentration-carbon-black/polymer composites

We summarise the initial results of an experimental and theoretical study of the structure and effective electric response of carbon-black/polymer composites with a percolation threshold well below 2 wt% of carbon-black filler.

Antimony-doped tin oxide nanoparticles for conductive polymer nanocomposites

Nanoparticles of antimony-doped tin oxide (ATO) were characterized for 0–33.3% Sb doping, both in aqueous dispersion and as dried powder. Antimony is incorporated in the cassiterite SnO 2 structure of the ATO nanoparticles ( d ≈ 7 nm) up to the highest doping levels, mainly as Sb V , but with increasing Sb doping the Sb III content increases. We found adsorption of NH 3 at the particle surface and evidence for the incorporation of nitrogen in the crystal lattice of the particles. The total nitrogen content increases with increasing Sb doping of the particles. Compact powder conductivity measurements show an increase in conductivity of ATO powder up to 13% Sb and a small decrease for higher Sb contents. Furthermore, we show that these particles can be used to prepare highly transparent conductive cross-linked ATO/acrylate nanocomposites with a continuous fractal particle network through the polymer matrix and a very low percolation threshold (ϕ c ≈ 0.3 vol%).

Dielectric quantification of conductivity limitations due to nanofiller size in conductive powders and nanocomposites

Conducting submicron particles are well suited as filler particles in nonconducting polymer matrices to obtain a conducting composite with a low percolation threshold. Going to nanometer-sized filler particles imposes a restriction to the conductivity of the composite, due to the reduction of the density of states involved in the hopping process between the particles, compared to its value within the crystallites. We show how those microscopic parameters that govern the charge-transport processes across many decades of length scales can accurately and consistently be determined by a range of dielectric-spectroscopy techniques from a few hertz to infrared frequencies. The method, which is suited for a variety of systems with restricted geometries, is applied to densely packed 7-nm-sized tin oxide crystalline particles with various degree of antimony doping and the quantitative results unambiguously show the role of the nanocrystal charging energy in limiting the hopping process.

Physical properties of carbon-black/polymer compounds

The conductivity of carbon-black/polymer compounds with a low percolation threshold has been studied as a function of electrical-field strength, carbon-black concentration, temperature, pressure and frequency. In addition ESR experiments have been performed. The results are compared with predictions of classical-percolation and tunneling models.