Lutz Hartmann

Molecular dynamics in grafted layers of poly(dimethylsiloxane)

Dielectric relaxation spectroscopy (10−1 Hz to 106 Hz) is employed to study the molecular dynamics of poly(dimethylsiloxane) (PDMS, Mw=1.7×105 g mol−1 and Mw=9.6×104 g mol−1) as grafted films with thicknesses d below and above the radius of gyration Rg. For the thinnest films the molecular dynamics becomes faster by up to two orders of magnitude with respect to the bulk resulting in a pronounced decrease of the Vogel temperature T0 and hence the calorimetric glass transition temperature Tg. For films with d=41 nm>Rg the molecular dynamics is comparable to that of the bulk melt. The results are interpreted in terms of a chain confinement effect and compared with findings for low molecular weight glass forming liquids contained in nanoporous materials. Crystallization effects—well known for PDMS—are observed for films of thicknesses above and below Rg.

Direct and inverse electromechanical effect in ferroelectric liquid crystalline elastomers

Ferroelectric liquid crystalline elastomers (FLCEs) form a novel class of materials, in which the properties of macroscopically oriented ferroelectric liquid crystals can be combined with those of elastomers. The (direct and the inverse) piezoelectric effect in these systems is measured and analyzed on a molecular level. Backed also by x-ray measurements, a model is suggested that interprets the observed piezoelectricity caused by electrically or mechanically induced motions of smectic layers which are inclined with respect to the sample surface. The strength of the observed electromechanical effects compares well with (or exceeds) that of classical piezoelectric materials like barium titanate, lead zirconate titanate or poled polymers like poly(vinylidene fluoride), making FLCEs an interesting candidate for applications in microsystems technology.

Piezoelectricity in ferroelectric liquid crystalline elastomers

The direct and inverse piezoelectricity in single crystal ferroelectric liquid crystalline elastomers was measured by means of purpose-made experimental setups. As expected the observed effects depended strongly on temperature and the strength of the applied alternating electric field; additionally they could be strongly enhanced by a superimposed direct current electric field. The latter resulted from a molecular amplification of the polarization vector of the single smectic layers by inducing a rotational bias of the lateral distribution of the polar groups of the mesogens. This resulted in a pronounced magnification of the (macroscopic) piezoelectric effect. Backed also by X-ray measurements, a model is suggested, which interprets the observed piezoelectricity as caused by a field-induced change of the inclination of the tilted smectic layers. The strength of the observed electromechanical effects compares well with or exceeds that of classical materials, such as barium titanate or lead–zirconate–titanate or the polymer polyvinylidene fluoride.

Molecular dynamics of grafted PBLG in the swollen and in the dried state

A novel technique to contact (ultra-) thin polymer layers is presented which enables to compare the molecular dynamics in grafted films of poly(y-benzyl-L-glutamate) (PBLG) to that of the bulk polymer by means of dielectric spectroscopy. Two relaxation processes are observed which are assigned to restricted fluctuations of the helical main chains and to the dynamic glass transition of the side chains. Furthermore, the swelling behavior of PBLG is studied.

Molecular Dynamics in Thin Polymer Films

The structural and dynamic properties of thin supported and freely standing polymer films are in the focus of scientific discussion [1–4]. Up to now, broadband dielectric spectroscopy has turned out to be a convenient experimental access which enables one to measure directly the molecular fluctuations of polar moieties in these systems in an extraordinary broad frequency and temperature range. A further benefit results from the fact that its sensitivity increases with decreasing sample thickness and hence with decreasing amount of sample material.

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).

Ultrathin Films from Fluorine Containing Polymers: Fabrication and Characterization

Monolayers and ultrathin films of some fluorine containing polymers were prepared by the Langmuir-Blodgett (LB) technique. The polymers were obtained using two main synthetic approaches: firstly, polyamide (PA-1) and polyimide (PI-1) were prepared from direct polycondensation of 4,4′-hexafluoroisopropylidenediphthalic anhydride and 4,4′-hexafluoroisopropylidenedianiline in N-methyl-2-pyrrolidone (NMP). The thermogravimetric analysis (TGA) for PI-1 yielded 580°C by 1% weight loss. They formed stable monolayers with a collapse pressure of 62 mN/m and a collapse area of 0.20 nm2 per repeat unit (R.U.) in the case of PA-1, for PI-1 the collapse pressure was 60 mN/m associated with a collapse area of 0.23 nm2 per R.U. Secondly, poly[(maleic acid perfluorooctylamide-imide)-co-ethylene] (PAPE) with fluorinated side chains was synthesized from poly[(maleic anhydride)-co-ethylene]. They formed stable monolayers too. Multilayer depositions onto various substrates were possible for all the synthesized polymers. LB films were characterized by ultra-violet/visible spectroscopy (UV-Vis), surface plasmon resonance (SPR), dielectric spectroscopy and atomic force microscopy (AFM). Gas phase polymerization of tetrafluoropropyl methacrylate (TFPM) was carried out in the presence of macroinitiator, poly[octadecene-co-(maleic anhydride)] modified with tert-butyl hydroperoxide. Film thickness could be controlled on different hydrophobic substrates varying the reaction time.

Structural Aspects and Host Effects of the Chirality Transfer by Mesogenic Substituted Aminoalcohols

ABSTRACT In continuation of the investigations on chiral dopants derived from the aminoalcohols ephedrine and pseudoephedrine as chiral bivalent structure elements, a series of new agents bearing very different substituents has been synthesized. The products turned out to be suited for preparation of short-pitch chiral liquid crystalline (LC) phases and were characterized by measuring their helical twisting power (HTP) in nematic host mixtures. In particular cases, structural features on the molecular level of dopants in relation to host constitution will be discussed. On the other hand, macroscopic properties, such as dynamic and dielectric data of achiral host systems, have been considered to find out practically useful guidelines for tailoring chiral LC guest–host systems. The studies include recording of nuclear magnetic resonance (NMR) spectra to study the dependence of helicity induction on the configuration of stereogenic center. Further, some photostability tests of cholesteric mixtures reflecting light in the visible range have been undertaken with regard to exploiting these dopants in electro-optic liquid crystal devices such as cholesteric bistable displays.

Simplified preparation of polymer-encapsulated LC layers by thermally induced phase separation

Stabilisation of functional layers in flexible liquid crystal displays (LCDs) is essential if they are to find use in advanced applications. Among the various methods for encapsulation of liquid crystals (LCs) by spatially controlled polymer wall formation within LCD cells, we have devoted particular attention to thermally induced phase separation (TIPS) in LC/polymer systems. A drawback of the standard TIPS procedure is the requirement to fill cells with the isotropic LC/polymer mixture at elevated temperatures. We have demonstrated that compartmentalisation can be simplified by precoating the substrate with an appropriate thermoplastic matrix polymer, for example poly(1-tetralyl methacrylate) (PTMA), before assembling and filling the cell with polymer-free LC at ambient temperature. The insular morphology originating from subsequent TIPS in different types of display cells prepared with nematic or chiral nematic mixtures has been examined by polarising microscopy and scanning electron microscopy. The electro-optical characteristics of test devices based on polymer-free LC films or on LC/polymer composites, prepared by both standard TIPS and our novel technique, are compared.

Chiral Dopants Derived from Ephedrine/Pseudoephedrine: Structure and Medium Effects on the Helical Twisting Power

Chiral dopants were obtained by acylation of enantiomerically pure ephedrine and pseudoephedrine with promesogenic carbonyl reagents. The products have been investigated with respect to their chiral transfer ability on nematic host matrices characterized by extreme differences of the dielectric anisotropy. It has been found that the medium dependence of the helicity induction nearly disappears at reduced temperatures. Based on variable temperature 1H NMR studies on monoacylated homologues, the estimated coalescence temperatures and free activation enthalpies for the hindered rotation around C‒N bonds could be correlated with the helical twisting power. Measurements by dielectric spectroscopy reveal the correlation between the molar mass of substituents linked to the chiral building block and the dynamic glass transition of corresponding chiral dopants. Furthermore, the effect of intramolecular and intermolecular hydrogen bonds has been studied by ATR-FTIR spectroscopy.