R. Pelster

Dielectric studies of glass transition in confined propylene glycol

The dynamical behaviour of the glass transition of propylene glycol confined in droplets in butyl rubber (three-dimensional confinement, mean droplet diameter d = 8-11 nm) and in pores in controlled porous glasses (two-dimensional confinement, mean pore diameter d = 2.5-7.5 nm) has been studied in detail by means of broadband dielectric spectroscopy (5 Hz-2 GHz) and of thermally stimulated depolarization current measurements. Effective medium theory corrections of the data are discussed. The results indicate the existence of a relatively immobile interfacial layer close to the wall. For the volume liquid the dynamics of the glass transition becomes faster and the glass transition temperature decreases compared to the bulk liquid. The shifts increase with decreasing d, are larger in butyl rubber than in controlled porous glasses (three-dimensional versus two-dimensional confinement) and vanish for nm. These results are discussed in relation to those obtained with polymers confined in thin polymeric films (one-dimensional confinement) and in semicrystalline polymeric samples and are explained on the basis of the cooperativity concept and the model of Adam and Gibbs. The cooperativity length at is determined to be nm in both butyl rubber and controlled porous glasses. Interesting effects of confinement are observed on the shape of the dielectric response of the process associated with the glass transition.

A novel analytic method for the broadband determination of electromagnetic impedances and material parameters

A simple analytic technique is described that allows the determination of complex impedances from quasistatic frequencies (some Hz) up to 2 GHz with one setup and a single sweep. Samples are placed in a shielded capacitor-like measurement cell that is inserted into a transmission line. The transmission coefficient of the setup is measured for one signal direction, i.e., only two receiver channels are needed. A complete calibration is achieved with only two standards. Dielectric and, with restrictions in the frequency range, magnetic material parameters can be determined via these impedance measurements. Temperature-dependent calibration and measurement are possible and even low losses down to tan /spl delta/=5/spl middot/10/sup -4/ and small impedances Z/spl ges/0.05 /spl Omega/ can be determined. The applicability of the method is experimentally verified over a frequency range of nearly nine decades from 5 Hz to 2 GHz. >

Glass transition in 2- and 3-dimensionally confined liquids

Abstract Broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents (TSDC) measurements were employed to investigate effects of confinement on the glass transition of the hydrogen bonded liquid propylene glycol (PG) and the non-associating liquid N-methyl-e-caprolactam (NMEC). The liquids were confined 2-dimensionally in the pores of porous glasses with mean pore diameter 2.5⩽d⩽20.0 nm and 3-dimensionally confined in butyl rubber with mean droplet diameter 7⩽d⩽11 nm. The data provide evidence for both the cooperativity concept and the existence of two states (interfacial layer and liquid). With decreasing d the α relaxation associated with the glass transition of the liquid becomes faster and broader and the glass transition temperature decreases. These effects are larger for 3- than for 2-dimensional confinement. The cooperativity length, ξ, at Tg is determined to ξ⩽6 nm for PG and ξ⩽12 nm for NMEC.

Influence of the Laplace pressure on the elasticity of argon in nanopores

At the beginning of an isothermal desorption process with the adsorbate argon, the nanopores of the porous glass sample remain virtually completely filled over a certain pressure range. A reduction of the external pressure p below the bulk vapour pressure of argon, p0, results, however, in the formation of concave menisci at the pore ends. The related decrease of the radius of curvature causes an increase of the negative Laplace pressure. This occurance is known to provoke a contraction of porous samples. Here we show that the Laplace pressure also influences the elastic properties of the filled porous sample. A decrease of the radius of curvature at the pore ends becomes noticeable in a decrease of the effective longitudinal modulus. The analysis of our ultrasonic measurements reveals that this decrease of the effective longitudinal modulus originates in a reduction of the longitudinal modulus of the adsorbate.

Unexpected sorption-induced deformation of nanoporous glass: evidence for spatial rearrangement of adsorbed argon.

Sorption of substances in pores generally results in a deformation of the porous matrix. The clarification of this effect is of particular importance for the recovery of methane and the geological storage of CO2. As a model system, we study the macroscopic deformation of nanoporous Vycor glass during the sorption of argon using capacitative measurements of the length change of the sample. Upon desorption we observe an unpredicted sharp contraction and re-expansion peak, which contains information on the draining mechanism of the porous sample. We have modified the theoretical model by Gor and Neimark1 to predict the sorption-induced deformation of (partly) filled porous samples. In this analysis, the contraction is attributed to a metastable or nonequilibrium configuration where a thin surface layer on the pore walls coexists with capillary bridges. Alternatively, pore blocking and cavitation during the draining of the polydisperse pore network can be at the origin of the deformation peak. The results are a substantial step toward a correlation between the spatial configuration of adsorbate, its interaction with the host material, and the resulting deformation.

Microstructure and effective properties of nanocomposites: ferrofluids as tunable model systems

We have studied the correlation between microstructure and effective material properties using colloidal dispersions of magnetic nanoparticles in a carrier liquid (ferrofluids). Their microstructure can be altered in a continuous and reversible way via an external magnetic field. Two-dimensional small angle x-ray scattering and Monte Carlo simulations show that field-induced structural anisotropy develops due to the formation of anisometric particle clusters having a preferred orientation parallel to the field. In this polydisperse system particles of all sizes take part in cluster formation. The structural data are compared with results of dielectric measurements in the frequency range from 5 Hz to 1 GHz. We show that dielectric anisotropy is correlated with the shape anisometry of oriented clusters.

Phase transitions and molecular dynamics of n-hexadecanol confined in silicon nanochannels

We present a combined x-ray diffraction and infrared spectroscopy study on the phase behavior and molecular dynamics of n-hexadecanol in its bulk state and confined in an array of aligned nanochannels of 8 nm diameter in mesoporous silicon. Under confinement the transition temperatures between the liquid, the rotator RII and the crystalline C phase are lowered by approximately 20K. While bulk n-hexadecanol exhibits at low temperatures a polycrystalline mixture of orthorhombic beta- and monoclinic gamma-forms, geometrical confinement favors the more simple beta-form: only crystallites are formed, where the chain axis are parallel to the layer normal. However, the gamma-form, in which the chain axis are tilted with respect to the layer normal, is entirely suppressed. The beta-crystallites form bi-layers, that are not randomly orientated in the pores. The molecules are arranged with their long axis perpendicular to the long channel axis. With regard to the molecular dynamics, we were able to show that confinement does not affect the inner-molecular dynamics of the CH_2 scissor vibration and to evaluate the inter-molecular force constants in the C phase.

Analysing dielectric interphases in composites containing nano- and micro-particles

We have investigated a molecular relaxation process in a solid polymer filled with dispersed magnetite particles (Fe3O4 in epoxy resin). In particular, we compare systems containing nano-particles with diameters between 20 and 30 nm and micro-particles with diameters between 0.5 and 5 µm. Temperature-dependent broadband dielectric spectroscopy in a frequency range between 50 Hz and 1 GHz reveals that the presence of nano- or micro-particles does not affect the molecular dynamics, i.e. frequency, shape and thermal activation of the relaxation process. However, there is a marked difference in the polymers relaxation strength, reflecting both the polarizability and the number of relaxing units. This quantity is evaluated from the measured effective data using the spectral representation, i.e. in spite of the complex microstructure we are able to separate unambiguously the contribution of the polarized conductive particles. While in the micro-composites the polymer matrix behaves bulk-like, its relaxation strength increases in the nano-composites, the deviation from the bulk value being proportional to the volume fraction of particles. We discuss the results in terms of interphases of thickness δ around particles and agglomerates, the volume fraction of which increases with increasing particle concentration and decreasing particle size.

Size-limited conductivity in submicrometre metal particles. Similarities with conducting polymers?☆

Abstract We report on the conductivity of submicrometre metal particles, which experience a size-induced metal-insulator transition (SIMIT). Also, their frequency behaviour and their dependence on temperature are quite different from bulk metal properties. Investigations of the conductivity of thin polyaniline layers have revealed that the two systems have some prominent properties in common; for instance both systems exhibit a pronounced relaxation process. A comparison and discussion of the similarities are presented.

CHARGE TRANSPORT IN SILICON CARBIDE : ATOMIC AND MICROSCOPIC EFFECTS

It is shown that charge transport in SiC ceramics includes atomic mechanisms as well as phenomena which depend on the microstructure of the material. Both aspects are revealed by the analysis of temperature‐dependent dc and ac measurements. The complex dielectric function (DF) of boron‐doped SiC ceramics with various additives has been measured at frequencies from 5 Hz to 2 GHz and at temperatures between 100 and 330 K. In addition, the dc conductivity was measured between 40 and 220 K. A transport mechanism on an atomic scale determines the temperature dependence of the dc conductivity. At low temperatures 3D variable range hopping between boron impurity states or point defects takes place whereas at higher temperatures Arrhenius‐like carrier activation becomes dominant. The ac behavior depends on the dc conductivity, but it reflects phenomena on a larger microscopic scale as well. The real part of the DF has huge values of up to 104. Two polarization processes have been identified. The low‐frequency pro...