Dieter Geschke

Molecular Dynamics in Poly(N-vinylpyrrolidone)-Poly(ethylene glycol) Blends Investigated by the Pulsed-Field Gradient NMR Method: Effects of Aging, Hydration and PEG Chain Length

Full Paper: The self-diffusion in PVP-PEG blends has been studied as a function of PEG molecular weight, water content and temperature. The blend corresponding to a stoichiometric PVP-PEG complex with 36 wt.-% PEG400 results from hydrogen bonding of the PEG terminal hydroxyl groups to the carbonyls of the PVP repeating units. Changes in diffusion parameters during the time of storage of the samples up to 1.5-2 months after preparation have been established. The attainment of an equilibrium value is characterized by a redistribution of the PEG molecules bonded to the comparatively long PVP chains, resulting in the formation of a supramolecular network. The spin echo attenuation, S inc (q, t d ), has been found to depend on the temperature, PEG chain length and water content. Water acts as a mediator in the complex formation, accelerating the PVP-PEG complexation. The PVP blend with PEG200 retains about 21 wt.-% of water after evacuation at 378 K. This amount of water is believed to be incorporated into the network. The longer the PEG chain length, i.e. the larger the flexibility of the chain, and the lower the OH group concentration in the blend, the smaller the number of PEG molecules bound to PVP. This has been established by comparing diffusion parameters of PVP blends containing PEG400 and PEG600, respectively.

Spatial distribution of space charge in conjugated polymers

We demonstrate the pyroelectric effect in a conjugated polymer, poly(2,5-pyridinediyl) (PPY), and we use the laser intensity modulation method (LIMM) to resolve the spatial distribution of electric field and space charges inside a 5 μm Au/PPY/Au sandwich device. The pyroelectric signal shows hysteresis behavior with respect to the applied bias indicating permanent storage of injected charges. From the analysis of the LIMM spectra we conclude that application of a bias leads to the accumulation of space charges near the electrodes, while a zone of opposite space charge may establish in a distance of about 1 μm from it. The charged state retains after removing the bias and can lead to an internal electric field that is opposite to the external poling field in the bulk of the polymer film.

Decay of space charge in conjugated polymers measured using pyroelectric current transients

The decay of space charge in conjugated polymers due to detrapping from deep traps after the turn-off of an external bias has been investigated. We present an experiment for measuring time-resolved laser intensity modulation method spectrum with a resolution of about 1 s. For this pyroelectric current transients have been recorded at different temperatures from 220 to 360 K. The data have been analyzed, assuming detrapping of charge carriers from single energy trap levels to a Gaussian distribution of transport levels to be the predominating process of the space-charge decay. In poly[2-methoxy, 5-(2’-ethyl-hexyloxy)-p-phenylene-vinylene], we find hole trapping with a trap depth of Et=0.6 eV and a trap density Nt>2×1021 m−3. In poly(2,5-pyridinediyl) both electron and hole trapping are observed, and the analysis of the decays yield Et=0.55 eV and Nt>1021 m−3. No deep trapping could be observed in poly(9,9-dioctylfluorene), confirming the high chemical purity of this polymer.

Electrically induced dynamic processes in nematic liquid crystals: 1H nuclear magnetic resonance investigations

The director reorientation of some nematic liquid crystals in the presence of both magnetic and electric field is described by a modified from of the Leslie equation. Proton nuclear magnetic resonance experiments observing static director orientations in different angles between the director and the magnetic field are described together with reorientation experiments driven by various electric fields. A new technique to measure Δχ/Δe is presented which gives direct access to the anisotropies without any influence of elastic properties. Using electric fields of different strength and in different angles with respect to the magnetic field the dynamic processes in liquid crystals can be investigated in a very flexible way. A number of experiments investigating the homogeneous director reorientation in the electric field is presented. As one result the rotational viscosity was determined. As a surprising result we found a homogeneous reorientation in the electric field and an inhomogeneous reorientation back ...

Peculiarities of self-diffusion studies on polymer systems by the NMR pulsed field gradient technique

Abstract The measurement of polymer self-diffusion coefficients in solution and melt with the NMR pulsed field gradient technique demands special care because of the slow and generally complicated diffusion process. The echo attenuations are derived and discussed for the following special cases: (i) the macromolecules have a distribution of molar masses, (ii) the species undergo diffusion with two mechanisms, (iii) the diffusion is hindered. The effects of chemical exchange and inner gradients are also discussed. In all these cases deviations from an exponential echo attenuation with a time-dependent self-diffusion coefficient are to be expected, leading to a complication of the interpretation of the experimental data. Based on the mathematical expressions thus obtained and on measurements with a highly sophisticated NMR pulsed field gradient spectrometer, it is shown that the NMR self-diffusion data of polymers in solution and melt so far reported are most likely the effect of mere center-of-mass motions of the macromolecules.

Piezoelectric and Pyroelectric Investigations on Microtomized Sections of Single-Crystalline Ferroelectric Liquid Crystalline Elastomers (SC-FLCE)

Abstract Microtomized sections of single-crystalline ferroelectric liquid crystalline elastomers (SC-FLCE) have been prepared. The piezoelectric (and electrostrictive) responses in these sections have been measured by means of an interferometric setup and the pyroeletric properties have been determined using the laser intensity modulation method (LIMM).

Temperature dependence of the space-charge distribution in injection limited conjugated polymer structures

Temperature dependent pyroelectric measurements have been carried out on high barrier devices of conjugated polymers. At room temperature we find significant buildup of space charge at the metal/polymer interface as well as in the bulk despite the very low device current. At lower temperature (180 K) the charge profile is “frozen in” and shows nearly no bias dependence, while above room temperature the space charge seems to become more mobile and leads to broadening of the charge profile. By applying an injection model that accounts for charge backflow via interface recombination we demonstrate that the high amount of space charge detected in our films indicates the existence of deep traps in the surface region that can effectively collect charge from the electrode. In the bulk of the film we find space charge of different sign, which is assumed to be due to separation in the field of charge from intrinsic defect states. These bulk charges are also immobile at low temperatures, indicating that they corres...

A magic-angle spinning system for high-temperature NMR eExperiments

Abstract A magic-angle spinner is described which permits spinning at 4 kHz of samples sealed in glass tubes. The use of the device is shown with experiments on polyethylene melts at 425 K.

Nematic reorientation in electric and magnetic fields

Homogeneous reorientation processes of two nematic liquid crystals in electric and magnetic fields have been observed using proton nuclear magnetic resonance spectroscopy (NMR). Using a recently developed experimental set-up, it is possible to study reorientation processes in liquid crystals by means of NMR experiments in a very flexible way. The time constant τ describing these processes has been determined as a function of the applied electric field. It emerges that the electric field cannot only be used to increase the reorientation time but also to slow the director reorientation by approximately one order of magnitude. Experimental data for 5CB and a fluorinated liquid crystal (BCH-5 FFF) are presented. The reorientation time measured as a function of the electric field can be used to calculate the rotational viscosity γ 1. By repeating these experiments at different temperatures it was possible to investigate the temperature behaviour of γ 1.

Study of fast switching processes due to electric and magnetic fields--an NMR approach.

Solid state NMR techniques have been developed to investigate dynamic molecular effects (e.g., molecular reorientations) due to simultaneously applied external electric fields on electrically sensitive materials such as liquid crystals (LC), liquid crystalline polymers (LCP) and polymeric electrets. Such effects can be observed only on relatively thin systems (10-200 microm). That means that many scans are necessary to achieve a sufficiently high signal-to-noise-ratio in the spectra (500-1000 scans). If the material is also magnetically sensitive, the electric field can be used to orient molecules in a starting orientational state and by switching-off the voltage to access fast reorientation processes in the magnetic field B0. Until now, the behaviour of orientable molecular systems under the influence of electric fields has been investigated by means of a more or less quasistatic approach (LCP: 100 V, electrets: 1 kV) in equilibrium states. The achievable time resolution depends on the desired signal-to-noise-ratio. For the case of proton NMR this means a time resolution of about 10 min. However, very often switching processes occur on a much shorter time scale. Using conventional techniques it is impossible to observe fast (ca. 100 micros) electrically or magnetically induced reorientation processes. In this work, we present a concept to overcome the problems outlined above and to extend the area of our current in situ NMR investigations on thin electrically-switched or poled polymeric layers. The basic idea is to include synchronized electric pulses during the NMR experiment using the preparation and/or mixing periods of a 1D or 2D pulse sequence for the application of an orienting field (electric or magnetic) and to use the reversibility of the molecular switching phenomenon to achieve a sufficient signal-to-noise-ratio. The techniques extend the range of possible investigations from about 100 micros to approximately T1 for correlated spectra (and to longer times of applied fields for uncorrelated spectra). Results are shown for a nematic LC and a nematic polymer having a similar side chain.