Walter Richtering

Gel point in physical gels: rheology and light scattering from thermoreversibly gelling schizophyllan

Abstract The β-1.3/1.6 glucan schizophyllan is a fungal polysaccharide that is known to suppress tumor growth. Thermoreversible, optically transparent gels of schizophyllan were obtained on addition of sorbitol to aqueous solutions of schizophyllan. Gel temperatures were independent of the history of sample preparation. The gelation process was followed by low-amplitude oscillatory shear and by static and dynamic light scattering. Power law behavior of storage modulus G′(ω) and loss modulus G″(ω) was found at the gel temperature with the exponent n=0.5. The static LS measurements showed a strong irregularity at the gel temperature. The intensity time correlation functions (TCFs) as measured by dynamic LS during the gelation process were transformed into the electric field TCFs, taking into account the heterodyne contributions. At the gel temperature which was estimated by rheology, DSC and static LS, no power law was found for the TCF. The angular dependence of the intensity TCF was measured and converted to the field TCF by use of a non-ergodicity treatment and by the heterodyne approximation, respectively. Both methods result in mean relaxation times that agree quite well. Two relaxation modes were found in the whole temperature range (above and below the gel temperature) indicating the existence of inhomogeneities in the gel.

Progress in thick-film pad printing technique for solar cells

The aim of this work was to study the suitability of pad printing in connection with fine-line printing on solar cells. Pad printing is a kind of gravure offset printing technique that offers the possibility of a simple, economic and high throughput production of fine lines up to 32 μm even on uneven surfaces, which is not possible with traditional screen printing (Hahne et al., Proceedings of the Second World Conference on Photovoltaic Solar Energy Conversion, Vienna, 1998, p. 1646). The different inks and silicone rubber pads have been characterised by several methods like thermal analysis, rheological, hardness and surface tension measurement. Simple solar cells on multicrystalline wafers with rapid thermal sintering show efficiencies up to 13.4%.

Linear and nonlinear rheology of micellar solutions in the isotropic, cubic and hexagonal phase probed by rheo-small-angle light scattering

Aqueous solutions of a branched nonionic surfactant were studied in the isotropic, cubic and hexagonal phase by means of rheological and small-angle light scattering (SALS) experiments. The isotropic phase behaved like a Newtonian liquid. An increase of activation energy of viscous flow was found near the overlap concentration of spherical micelles, but no shear thinning was observed. The viscosity of low concentrated samples increased slightly when the lower critical solution temperature was approached. This increase of viscosity was much smaller compared to common nonionic surfactants. The cubic phases behaved as elastic solids with a high plateau modulus, and shear melting occurred at high shear stresses. The hexagonal phase showed complex behavior. Shear orientation could be achieved by large amplitude oscillatory shear and was proved by rheo-small-angle light scattering. Two orientations were observed, at first perpendicular to the flow direction, i.e., log-rolling state and, secondly, an in-shear-plane orientation parallel to the flow direction. The linear viscoelastic region of the hexagonal phase was extremely small and was detected by simultaneous rheo-small angle light scattering. Shear alignment lead to a decrease of the moduli.

Rheology and diffusion of concentrated monodisperse and bidisperse polymer latices

Abstract Three monodisperse, electrostatically stabilized latices of different sizes (306, 164 and 64 nm) have been synthesized by emulsion polymerization of styrene and acrylic acid, and purified by ion-exchange. Particle size and size distribution were determined by transmission electron microscopy as well as static and dynamic light scattering from dilute dispersions in various aqueous media. Electrostatic properties were determined through titration and ξ-potential measurements. Concentrated colloidal suspensions at different volume fractions up to 50%, and various pH and ionic strengths have been studied with regard to their microscopic dynamics by fiber-optic quasielastic light scattering (FOQELS). At high ionic strength, the concentration dependence of the diffusion coefficient was in agreement with theoretical predictions for the self-diffusion of hard spheres. At low ionic strength, significant deviation from hard-sphere behavior was observed and FOQELS provided an optical technique for determining the presence of a yield stress. The same samples have been studied in various rheometrical tests. At low ionic strength elastic properties dominated; at high ionic strength, samples were predominantly viscous. Intermediate viscoelasticity, at well balanced strength of interaction, could be detected. The transition from viscous to elastic character occurred over a narrow regime of ionic strength or concentration. Curves of storage modulus, G′ versus frequency, ω, at different particle contents in the viscoelastic region could be shifted to a mastercurve by scaling ω with the longest relaxation time. Bimodal mixtures of small and large particles were also investigated by means of viscometry and FOQELS. When the number concentration of large spheres was small, FOQELS could be used to measure a probe-diffusion that showed a concentration dependence similar to that of viscosity and which resembled the behavior of long-time self-diffusion.

Use of poly(styrene)-block-poly(ethyleneoxide) as emulsifier in emulsion polymerization

SummaryPoly(styrene)-block-poly(ethyleneoxide), abbreviated as (PS-b-PEO) were used as emulsifiers in emulsion polymerization of styrene and methyl methacrylate. The block copolymers had a poly(styrene) block with Mn=1000 g/mol and a poly(ethyleneoxide) block with Mn=1000, 3000 or 5000 g/mol, respectively. Stable dispersions were obtained when the PEO block molecular weight was higher than 1000 g/mol. Also the amphiphilic properties of the copolymers depended on the PEO chain length. Block copolymer micelles with hydrodynamic radii between 11 and 17nm were observed. Emulsion polymerization was performed at different block copolymer concentration at 60 and 80°C. Particle size varied between 50 and 300nm and decreased with increasing copolymer concentration. The particle size was larger at higher temperature, but the size distribution was narrower. Polymerization of methyl methacrylate gave smaller particles when compared to styrene. The dispersions were very stable towards high electrolyte concentration, but flocculation occurred at elevated temperatures. Both observations indicate that the dispersions are sterically stabilized.

Rheology and diffusion in concentrated sterically stabilized polymer dispersions

Abstract Two monodisperse latices of different size (226 and 350 nm) were synthesized by the emulsion polymerization of styrene. Subsequent absorption of poly(vinyl alcohol) (PVA) gave two sterically stabilized latices of different size and thickness of the adsorbed layer, but the same PVA concentration in similar volume fractions. Particle size and size distribution were determined by transmission electron microscopy and dynamic light scattering from dilute dispersions. Polymer layer thickness was determined by capillary viscometry and by the difference of hydrodynamic radii from quasielastic light scattering (QELS) in dilute solution. Concentrated dispersions up to 30% ( w w ) were studied with regard to their microscopic dynamics with fiber-optical quasielastic light scattering (FO-QELS). The concentration dependence of the diffusion coefficients of the covered latices showed deviations from theoretical predictions for the self-diffusion of hard spheres. Moderate deviation from theory was also observed for the uncovered latices. Various rheometrical tests were performed on the sample. The viscoelastic transition region extended over a relatively broad concentration range. Curves for G′ at different particle compositions in the viscoelastic region were incorporated into a mastercurve by including the longest relaxation time. The concentration dependence of the diffusion processes, as derived from FO-QELS, was different from that of the rheological phenomena.

Shear orientation of a lamellar lyotropic liquid crystal

Shear orientation of a lyotropic lamellar liquid crystalline phase of tetra ethyleneglycol mono dodecylether, C12A4 in water was studied by combined rheo-small-angle light scattering. Shear thinning was observed with a sample of 500 μm thickness. The scattering patterns showed that domains were aligned and stretched in flow direction. Shear thinning was also observed with a thinner sample, but caused a strong change in light-scattering pattern. A maximum of scattering intensity was observed at finite scattering vector and shifted to a higher scattering vector with increasing shear stress. This observation can be explained by a fragmentation of domains at high shear stress and is supported by a mosaic texture observed in optical microscopy.

Emulsion polymerization of styrene in the presence of carbohydrate-based amphiphiles

SummaryNovel amphiphilic N-alkyl-, N-decenyl-, and N-alkyl-N-methyl-semicarbazones of maltose and maltotriose with CnH2n+1 alkyl chains and n=8,9,10,11,12,16 were applied as emulsifiers in styrene emulsion polymerization. Surfactant properties, emulsion polymerization, and rheology were examined as a function of amphiphile molecular architectures. Both critical micelle concentration and average polystyrene particle size increased with decreasing alkyl chain length. Using N-methyl-N-[11-(acryloylamino)-undecanoyl]-glucamine comonomer in a starved feed emulsion polymerization process, it was possible to prepare uniform polystyrene latex with glucamide surface.

Small-angle neutron scattering from a hexagonal phase under shear

SummaryThe shear orientation of a micellar hexagonal liquid crystalline phase was investigated by small-angle neutron scattering. The hexagonal phase in the quiescent state showed a symmetrical scattering pattern typical of a polydomain structure. Enhanced scattering along the flow direction was observed during shear and the anisotropy of scattering intensity became stronger with increasing shear rate. The anisotropic scattering pattern corresponds to an orientation perpendicular to the flow direction and can be interpreted as a log-rolling state. The oriented sample did not relax after cessation of shear. The results from small-angle neutron scattering confirm data obtained previously from rheo-small angle light scattering measurements and are discussed in comparison to shear alignment of lyotropic liquid crystalline polymer solutions.

High Frequency Viscoelasticity of Concentrated Colloidal Dispersions

The high-frequency viscoelastic properties of concentrated colloidal dispersions are discussed. ‘Highfrequency’ here means, that times probed by the experiment are much shorter than the timescale of diffusive colloidal motion, but still longer than momentum relaxation and vorticity propagation times. For typical liquid-like, industrial dispersions (particle radius a = 100 nm) this high frequency limit is around 10 kHz. The viscosity η’∞ is mainly a hydrodynamic quantity, while direct interactions among the colloidal particles determine the modulus G’∞. The concept to extract information about particle interactions from shear modulus measurements is not new, but here we present a method to determine the effective colloidal charge Q in concentrated (φ > 0.3), but liquid dispersions [1]. For electrostatically stabilized dispersions η’∞ is investigated for the first time here and is compared to short time self diffusion coefficients Ds from fiberoptical quasi-elastic light scattering (FOQELS) [2].