Michael Wübbenhorst

Analysis of complex dielectric spectra. I. One-dimensional derivative techniques and three-dimensional modelling

Several strategies for the analysis of isochronal and isothermal spectra as well as complete three-dimensional (3D) spectra are outlined and applied to data of (glass forming) liquids and polymers. Conduction-free loss spectra are calculated using a compact solution of the Kramers–Kronig transformation and an approximation based on oe 0 =o ln x. The usefulness of the dielectric modulus for data analysis is also evaluated. Apart from oe 0 =o ln x, other useful derivatives in the frequency and temperature domain are treated as well. Finally, we describe the evaluation of 3Drelaxation maps by means of 3D fitting. 2002 Elsevier Science B.V. All rights reserved.

Analysis of complex dielectric spectra. II: Evaluation of the activation energy landscape by differential sampling

Relaxation phenomena are controlled by thermally activated or by free-volume activated kinetics. Evaluation of the kinetic parameters across the whole experimental window is of prime importance. Techniques that provide information about the entire landscape of activation energies or fractional free volumes are discussed. Emphasis is given to a fine-structure analysis by bivariate differential sampling and its potential for studying the glass transition dynamics in equilibrium and non-equilibrium (physical aging) is shown.

A new organic nanoporous architecture: Dumb-bell-shaped molecules with guests in parallel channels

A new type of dumb-bell-shaped host molecule (6-8) has been synthesised, of which 1,8-bis((1)-adamantyl)-1,3,5,7-octatetrayne (8 = BAOT) forms an open porous architecture when cocrystallised with a number of typical solvent molecules. Adamantyl substituents attached to a tetraalkyne spacer build up the walls of parallel channels wherein guest molecules are aligned. Surprisingly, the tetraalkyne unit is significantly bent. Desolvation experiments provide evidence for a reversible inclusion of guests. In the case of the inclusion of 2-butanone, a partial substitution by symmetrical and asymmetrical long-chain chromophores during crystallisation was possible. Stained crystals showed optical frequency doubling. The crystal structure analysis revealed a centric space group, although considerable translational and orientational disorder was present. Application of scanning pyroelectric microscopy revealed that the growth of inclusion compounds with 2-butanone produced polar ordering of guest molecules, which were aligned in two macro-domains of opposing polarity. The resulting orientation of the carbonyl dipoles is in agreement with the theoretical prediction of a Markov model of spontaneous polarity formation based on molecular recognition processes on growing crystal faces. The present case represents a new example of a property-driven supramolecular synthesis.

Photocurrent generation of bi-functional carbazole containing polymers

A novel bi-functional photorefractive (PR) methacrylate polymer with carbazolyl fragments and chromophoric azophenyl sulfonyl group were synthesized and characterized. The generation of photocurrent on illumination and the photoconductive properties of the homopolymers and copolymers were studied. The addition of 2,4,7-trinitro-9-fluorenone (TNF) resulted in a considerable increase of the photoconductivity by about one order of magnitude.

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.

Statistically controlled self-assembly of polar molecular crystals

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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.

Dielectric relaxations in ultra-thin films of PMMA: assessing the length scale of cooperativity in the dynamic glass transition

The local and cooperative dynamics of supported ultra-thin films (L = 4-170 nm) of poly(methyl methacrylate) was investigated by dielectric spectroscopy at frequencies from 0.1 Hz to 10/sup 6/ Hz and temperatures from 250-423 K. To assess the effect of stereoregularity, three tacticities (i, a and s-PMMA) of similar molecular weight M/sub n/ = 120-140/spl times/10/sup 3/ g mol/sup -1/ were studied. A second M/sub n/ value for s-PMMA was also studied. For i-PMMA, a decrease of the glass transition temperature T/sub g/ by up to 10 K was observed for a film thickness L < 25 nm (which is comparable to R/sub EE/), indicated by a down-shift of the peak temperature T/sub /spl alpha// in the loss /spl epsiv/(T) at low and high frequencies and by a shift of the maximum in the apparent activation energy E/sub a/(T) of the /spl alpha/-process. Changes in the relaxation strength and the peak shape are apparent for L /spl les/ 50 nm. In addition, the /spl beta/-relaxation probing the local mobility was investigated for i-, a- and s- PMMA. Interestingly, all polymers revealed a systematic speedup in the local dynamics for L < 25 nm which is accompanied by systematic changes in the relaxation strength. For i-PMMA, a clear relation between the thickness dependence of /spl tau//sub /spl beta// and /spl tau//sub /spl alpha// was found indicating that thickness dependences in the cooperative dynamics are predominantly induced by changes in the local mobility. These findings are consistent with the idea of confinement induced changes in the orientation and conformation statistics for thicknesses below R/sub EE/.

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 72u200a°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.