A. Hauser

Application of atomic force microscopy and ultrasonic resonator technology on nanoscale: Distinction of nanoemulsions from nanocapsules

Oily core nanocapsules were prepared by sequential addition of positively and negatively charged polyelectrolytes based on a nanoemulsion and transformation thereof into a core-shell structure. The capsules were well characterized by photon correlation spectroscopy, laser diffraction, zeta-potential and transmission electron microscopy and feature an average size of 150nm and a negative surface charge. The aim of the current study was to improve the dispersion stability and mechanic rigidity of the capsule wall by depositing an increasing number of up to five layers. Therefore, atomic force microscopy (AFM) and ultrasonic resonator technology (URT) were applied to investigate the shell of the nanoemulsion, the intermediate and final nanocapsules in more detail. AFM was performed to investigate the shape, morphology and mechanic properties of the emulsion and capsule shell. It proved to be a feasible technique to distinguish nanoemulsions from nanocapsules by stiffness analysis. URT was utilized in order to observe the ultrasound velocity and could confirm the AFM results. Both techniques demonstrated that the shell around an oil droplet solidified with increasing number of polyelectrolyte layers. Since a solid wall might have the potential of a strong diffusion barrier, nanocapsules might present a feasible prolonged release drug delivery system in contrast to nanoemulsions.

Order Parameter and Molecular Polarizabilities of Liquid Crystals with Nematic and Smectic Phases

Abstract In four substances exhibiting nematic and smectic phases the refractive indices ne, no , the density and the order parameter have been measured as a function of the temperature. From the experimental data the molecular polarizability anisotropy was calculated using different internal field models. The results were compared with the data obtained by an extrapolation procedure and by the addition of tabulated bond polarizabilities.

Fabrication and characterization of a biomimetic composite scaffold for bone defect repair.

For successful bone tissue engineering, scaffolds with tailored properties are a basic requirement. The combination of different available materials not only appears to be desirable but also very challenging. In this study, a composite material consisting of hydroxyapatite and collagen was produced by a biomimetic precipitation method and characterized by X-ray diffraction (XRD) and thermogravimetry (TGA). Subsequently, a suspension-quick-freezing and lyophilization method was used to incorporate the hydroxyapatite into a polymeric matrix consisting of collagen and chitosan. Before physicochemical characterization, the highly porous scaffolds were consolidated by a dehydrothermal treatment (DHT). The main attention was focused on the particle size of hydroxyapatite, which should be in the nanometer range. This is relevant to achieve a homogeneous resorption of the material by osteoclasts. Small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), and environmental scanning electron microscopy (ESEM) were used to evaluate the outcome. The results suggest a successful polymeric embedding of nanoscaled hydroxyapatite particles into the matrix of the spongy construct. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Characterization of the B4 phase by dielectric and AFM measurements

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B2 and B4 phases. As usual, two ranges of relaxation were detected in the B2 phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B4 phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B2 modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B4 phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B4 phase; then do not appear in the crystalline state. For this reason the B4 phase is regarded as different from a classical crystalline phase.

Collective Dynamics in Some Phases Consisting of Banana-Shaped Molecules

Abstract Dielectric measurements on two samples of banana-shaped molecules showing a B2 and a B7 phase were carried out. The low frequency dispersion in the B2 phase is interpreted as a collective motion of ferroelectric clusters and the high frequency one as the reorientation of single molecules with a strong positive dipole correlation. In the B7 phase a, soft mode-like“ behavior was detected pointing to an additional phase transition. AFM measurements support the existence of helical super-structures which probably appear in B7 and in the low temperature phase Bx.

AFM investigations of a sample showing the B2 phase

Atomic force microscopy (AFM) images reveal that the liquid crystalline B2 phase has no helical superstructure, in contrast to the B7 phase. This is demonstrated on comparing the AFM images of focal-conic domains in the smectic A, cholesteric, B2 and B7 phases. The B2 and B7 phases are phases formed by banana-shaped molecules. The results point to the existence of ferroelectric clusters with a random size distribution which also cause the low frequency dielectric relaxation.

Synthesis and liquid-crystalline properties of compounds incorporating the 2,6,7-trioxa-bicyclo(2.2.2)octane

Abstract We have prepared twenty six new derivatives of 2,6,7-trioxa-bicyclo(2.2.2)-octane with the general molecular strutures, All the compounds are mesomorphic the great majority of which exhibit exclusively smectic B phases. In order to characterize this new structural class we measured refractive indices, transition enthalpies and polarizability as well as their viscosity properties for some selected examples.

Dielectric characterization of the B7 and a BX phase

The first clear experimental evidence of a dielectric relaxation in the B7 phase is given. The associated molecular process is probably the reorientation of the bent-shaped molecules about the long molecular axis. It is also shown that the dielectric constant strongly increases at the phase transition I-B7 pointing to a co-operative motion of ferroelectric clusters. Furthermore, a phase transition B7-BX is observed. The values of about 10 for the high frequency limit of the dielectric constants of the B7 and BX phases and the static dielectric constants of the isotropic phase are in agreement. Thus, in all the phases the main dipoles can reorient faster than 10−8 s, i.e. the experimental limit. Only in the supercooled BX phase were the dynamics of the transversal dipoles measured. Using atomic force microscopy, focal-conic domains modulated by parallel lines have been observed at room temperature.

New liquid-crystalline heteroalicyclic compounds

Abstract A wide variety of new sulphur-containing heteroalicyclic liquid-crystalline compounds have been synthesized. Special attention has thus been focussed on the influence of the heteroatoms on the mesomorphic behaviour. 2-Cyclohexyl substituted 1,3-dithianes and 1,3-oxathianes have been found to be superior to those with 2-phenyl substituents. The clearing temperatures of the 2-phenyl-1,3-dithianes depend on the electron donating or electron accepting properties of the substituents attached to the phenyl group. The mesophase stability of the 2-cyclohexyl substituted (hetero)alicycles increases in the order: 1,3-dioxane < 1,3-oxathiane < cyclohexane < 1,3-dithiane. This order is partly reversed for the cyclohexane annulated compounds: 1-oxa-3-thiadecalin < 1,3-dioxadecalin < decalin < 1,3-dithiadecalin. Selected values of the birefrigence and some melting enthalpies have been measured.

Dielectric response of the B2 phase

A compound consisting of banana-shaped molecules was synthesized and well purified. The sample shows the B2 phase which was characterized by dielectric spectroscopy. Two relaxation ranges were detected. The high frequency one, related to the reorientation about the long molecular axis, was observed in the isotropic phase, too. The increments point to a strong positive dipole correlation in the B2 state. There are arguments that low frequency mechanism can not be connected with the reorientation about the short molecular axis. AFM-investigations give no hints to a super- structure. Therefore, this absorption was related to a collective response of ferroelectric clusters.