J. van Turnhout

Complex dynamics of hydrogen bonded self-assembling polymers

Supramolecular polymers, which are non-covalently bonded and formed by self association of di or trifunctional monomers exhibit, by virtue of quadruple hydrogen bonds, many of the properties of normal high molecular weight polymers, e.g. a dynamic rubber plateau. We focus on the molecular and cooperative dynamics of self-assembled linear polymers and networks, studied by broadband dielectric relaxation spectroscopy (DRS) in the frequency range from 10/sup -2/ to 10/sup 6/ Hz. The DRS analysis was backed up by dynamic mechanical and rheological experiments. In the high temperature region two loss processes (/spl alpha/ and /spl alpha/*) show up, the relaxation times of which obey the Vogel-Fulcher-Tammann (VFT) law. The dielectric /spl alpha/ process is related to the dynamic glass-rubber transition and is slightly faster than the corresponding mechanical /spl alpha/ process. A slower (high-temperature) /spl alpha/* relaxation is identified as chemical, corresponding to the mean lifetime of the hydrogen bonded linkages in the supramolecular chains. Its relaxation time /spl tau//sub /spl alpha/*/ was found to be 1 to 2 decades larger than the terminal flow relaxation time, indicating that the relaxation of an entire chain is dominated by the joint dynamics of many hydrogen bonds. The /spl beta/ relaxation, observed in both DRS and dynamic mechanical analysis (DMA) at temperatures below T/sub g/, arises from the local junction dynamics of hydrogen bonded units in the glassy state. Details of the temperature dependence and the shape of the loss peaks of the /spl alpha/ and /spl alpha/* relaxations will be discussed in terms of temporary physical networks and cooperativity.

Distribution and stability of charges in porous polypropylene films

The porous polypropylene films have an interesting morphology and peculiar charge distribution. Electric breakdown, which takes place in the voids during high voltage corona charging, is responsible for their internal distribution. The heterocharges created during this process are trapped around the voids. The homocharges injected into the outer layers during the corona charging initially outweigh the heterocharges. However, the homocharges are dissipated more rapidly during heating, and so the surface potential changes sign during charge-TSD. The complex charge distribution within porous propylene was calculated from the pyrocurrent spectra using an approximation suggested by Ploss (1994). This method is rarely used despite its simplicity.

A dielectric study on the relaxation and switching behaviour of liquid crystals confined within a colloidal network

The molecular ordering and dynamics of a liquid crystal (LC E7) in the presence of a three-dimensional network of submicron particles have been studied by dielectric relaxation spectroscopy. The field-dependent orientation of the LC was quantified by the director order parameter and modelled by use of a three-phase model. The influence of the colloidal network on the molecular dynamics was assessed from the dielectric spectra, e.g. from the position of relaxation peaks as well as from the strength of the two principal relaxations (α and γ). The spectra changed noticeably upon application of an increasing d.c. bias. A reduction of the threshold field was observed upon addition of colloidal particles to the LC. This was associated with a switching between two metastable states induced by anchoring on the filler particles. Modelled spectra were found to be in good agreement with the experimental data. The modelling showed that the confined LC phase is composed of two fractions, viz. an ordered and a disordered one with different molecular mobilities. Furthermore, switching experiments were conducted at various temperatures in order to evaluate the impact of the colloidal network on the (temperature-dependent) orientational behaviour of the LC molecules. For the colloid-filled LC higher conductivities were found, which gave rise to longer switch-off times.

Glass transition of one-dimensional molecular chains of p-nitroaniline confined in AlPO4-5 nanopores revealed by dielectric spectroscopy

The dynamics of molecular chains of hydrogen-bonded p-nitroaniline (pNA), confined in the one-dimensional pore system of AlPO4-5, was studied by broadband dielectric spectroscopy (10−2–106 Hz). Two relaxation processes were observed. A fast process (the α process) shows above 72 °C a Vogel–Fulcher–Tammann behavior typical of glass-forming materials, indicating cooperative dynamics of the “condensed” pNA chains. Below the glass temperature the α-relaxation peak broadens significantly and obeys an Arrhenius law. Both observations imply a change in length scale of cooperativity ξ from a temperature-controlled to a chain-length-dependent one. The second relaxation process is tentatively assigned to structural relaxations of the AlPO4-5 host lattice itself, since it is also present in calcined (empty) crystals.

Dielectric study on size effects in polymer laminates and blends

In this article we will focus on the dielectric properties of laminates and blends of a partially conducting (the liquid crystalline copolyesteramide Vectra B950) and an insulating (polypropylene or mica) phase. Dielectric spectroscopy was used as a tool to obtain information about the influence of the dimensions of the conducting phase in these laminates and blends. With decreasing thickness of the conducting layer in the laminates, the measured permittivities deviate more and more from the values predicted using conventional dielectric mixture models. From this discrepancy it is possible to calculate the thickness of the charge layer (=Debye length) in the conducting phase and the thickness of this phase itself, using a model derived by Trukhan. This model incorporates not only conduction, but also diffusion of the charges. Similar experiments were performed on a system of Vectra B950 particles in a polypropylene matrix. After the derivation of a new model, which combines the Trukhan model for space charges with the Bottcher equation for dielectric mixtures, we could make a distinction between samples containing large and small particles. For samples containing small particles, it is even possible to determine the variation in particle sizes. However, the use of a Debye length of 1.1 µm obtained from the laminates resulted in particle sizes that were two times higher than the actual values.

Orientational properties and dynamics of nematic liquid crystals mixed with dendrimers for electro-optical switches

Novel two-phase materials based on nematic liquid crystals (LC E7) filled with poly(propylene imine) dendrimers of 2–3 nm diameter were investigated for application in light-scattering, electro-optical switches. Polarizing optical microscopy, differential scanning calorimetry and rheological experiments all indicate the presence of a pronounced polydomain structure within these LC mixtures. The molecular ordering and dynamics of the dendrimer-filled nematics were studied in more detail by dielectric relaxation spectroscopy. The director order parameter S d was derived from the strength of the two principal molecular relaxations, α and δ, in the presence and absence of an electric field. In addition, a three-phase model was applied for determining the size of the different LC populations in the mixture. A substantial decrease in the fraction of randomly oriented LC was found upon application of a d.c. field. Over a broad temperature range the LC molecular dynamics were assessed from the variation in the strength of the dielectric relaxations as a function of the d.c. bias. A fast response was found at room temperature. Electro-optical switching experiments showed that the dendrimer-filled LCs can be switched rapidly from a light-scattering, opaque state to a more transparent state.

Electrochemical impedance study on the formation of biological iron phosphate layers

Phosphating is the most commonly applied pre-treatment process. However, some disadvantages exist: heavy metals and phosphoric acid are used in the process. Volkland et al. developed a process, in which these compounds can be left out, using bacteria to activate the phosphating process. This paper addresses the analysis of this phosphating process using electrochemical impedance spectroscopy. EIS-measurements are carried out during 150 h to understand the electrochemical processes taking place during the phosphate formation. A model is derived that can explain the processes. The process appears to be a two-step adsorption process. The first step is formation of a thin layer of bacterial products. Formation of a dense layer of iron phosphates as a second step takes place within 2 days.

Accumulation of charges in polycarbonate due to positron irradiation

Abstract Positron annihilation lifetime spectroscopy is a powerful technique for the study of free volume in polymers. The lifetime of ortho -positronium ( o -Ps), a bound state of an electron and a positron, can be used to assess the cavity size, while the intensity can be used to characterize the number of cavities. In the past, results have been published, which suffer from artifacts, whereby the drop in o -Ps intensity is not related to a decrease in the number of cavities. One of the possible artifacts is the build-up of an internal charge during long-term exposure to positron irradiation, which affects the o -Ps intensity. In this study, we focus on the charging created in polycarbonate by positron radiation, which we investigated by the laser intensity modulation method and by charge-decay experiments, both isothermally and with thermal stimulated discharge. From these measurements we conclude that: (1) the o -Ps intensity decrease during prolonged positron irradiation is due to the accumulation of a space charge; (2) in reverse, changes in the o -Ps intensity may be used to probe electric fields in dielectrics.

Dielectric water sorption analysis

The sorption of water vapor in Kapton® (polyimide) films has been investigated with a new method, dielectric sorption analysis (DSA). The technique is based on high-resolution time-resolved capacitance measurements performed during exposure of an organic coating to humidified nitrogen. The basis of the method is described together with experimental details related to the capacitance measurement, long-term stability, reproducibility, and other characteristics of the setup. The technique allows the determination of the sorption/desorption and diffusion characteristics of a coating on a time scale varying from seconds to days. Mass transport properties derived from complementary weight measurements are in good agreement with the DSA results. Real-time monitoring of the moisture sorption in organic coatings demonstrates the potential of the technique in the field of nondestructive coating inspection and testing.

Fibrous and Granular Filters with Electrically Enhanced Dust Capturing Efficiency

It is shown that electrification of a filter material greatly improves its capture efficiency. The electrification can be due to built-in charges (electret filter) or can be achieved by polarization in an external electric field. Various aspects of the two types of filters are discussed: design, choice of material, and filtration characteristics. In addition, the mathematical modeling of electret filters is discussed, in particular filtration with the fibers used in commercially available electret filters. Some applications of electrified filters are also briefly described (personal protection, vacuum cleaners, liquid filtration, etc.).