Konstantinos Mpoukouvalas

The role of temperature and density on the glass-transition dynamics of glass formers

A correlation between the monomeric volume and the dynamic quantity E*(V)/H*, used to provide a quantitative measure of the role of temperature and density on the dynamics, is demonstrated for a series of polymers and glass-forming liquids. We show that monomeric volume and local packing play a key role in controlling the value of this ratio and thus the dynamics associated with the glass temperature.

Dissociation and Charge Transport in Salts of Dendronized Ions in Solvents of Low Polarity

The ion dissociation and transport properties of a series of tetrabutylammonium salts (TBA(+)) of rigidly dendronized anions with various sizes have been investigated in toluene, THF, and chloroform for a range of concentrations with dielectric spectroscopy and diffusion ordered spectroscopy (DOSY)-NMR. This is one of the first cases that one can study salts in low polarity solvents. The new synthetic approach increases the solubility and allows for investigation of both steric hindrance as well as electronic effects in producing weakly coordinating anions. We found that steric effects promote ion dissociation. In addition, fluorine substitution in the dendritic corona screens the electrostatic interactions and leads to increased dissociation. From the degree of dissociation and the measured diffusion coefficients, the free anion and cation diffusion coefficients were extracted and compared for the different dendrimer generations.

Conductivity of poly(pyrrole)-poly(styrene sulfonate) core-shell nanoparticles.

The dielectric properties of poly(styrene) nanoparticles decorated at their surfaces with poly(styrene sulfonate) [PSS] brushes and subsequently loaded with polypyrrole (PPy) were studied. These film-forming materials which may serve as hole-injection layers in organic light-emitting diodes, exhibit a core-shell-type morphology with a core of electrically insulating poly(styrene) and a shell consisting of a corona of PSS chains which form the matrix in which the electrically conducting complex of PPy and PSS is embedded. This conducting complex exists in form of domains of nanoscale dimensions. Thin compressed pellets of these nanoparticles were studied using mainly impedance spectroscopy. Measurements were carried out in the temperature range between 123 and 453 K and frequency range from 10(-1) to 10(6) Hz. While earlier studies were centered around the effect of polypyrrole volume fraction on the conductivity films and pellets composed of these nanoparticles, the present study reveals in which way the conductivity can be modified by exchange of the mobile inorganic counter ions of PSS. Besides the free-acid form (H(+)), the Li(+)-, Na(+)- and Cs(+)-salts of PSS were investigated. The PPy volume fraction was the same for all PPy/PSS core-shell nanoparticles. The distance for phonon-assisted hopping between next-neighbor polypyrrolium chains is influenced by the presence of these inorganic cations. For all samples containing PPy, a transition from insulating to conducting behavior in the range of 300-350 K was found. Using the fluctuation-induced tunneling model, the average tunneling distance, as well as the potential energy barrier separating neighboring conducting grains was estimated. Finally, a detailed analysis of the dielectric spectra suggests the localization length of the charge carriers to be about 0.33 nm.

Impedance spectroscopy investigation of conjugated polymer coated core-shell nanoparticles

Poly(styrene) nanoparticles decorated at their surface with poly(styrene sulfonate) brushes and subsequently loaded with polypyrrole have been prepared as film-forming materials to serve as hole injection layers in organic light-emitting diodes. Thin compressed pellets of these nanoparticles have been studied by impedance spectroscopy. Measurements were carried out in the temperature range between 123.15 and 453.15 K and frequency range from 10−1 to 106 Hz. The polypyrrole volume fraction φPPy was varied as well. The film-forming nanoparticles exhibit a core-shell-type morphology with a core of electrically insulating poly(styrene) and a shell consisting of a corona of poly(styrene sulfonate) chains, which form the matrix in which the electrically conducting complex of polypyrrole and poly(styrene sulfonate) is embedded. This conducting complex exists in forms of domains with nanoscale dimensions. It is demonstrated that the charge transport in samples with φPPy [for the calculation of the polypyrrole vol...