Hybrid κ-carrageenan-based polymers showing "schizophrenic" lower and upper critical solution temperatures and potassium responsiveness.

Novel multiresponsive hybrid biocompatible systems of κ-carrageenan-graft-poly(2-isopropyl-2-oxazoline-co-2-butyl-2-oxazoline)s with unique combination of responsiveness to external stimuli were synthesized and studied. The polymer thermoresponsive behavior proved the existence of both lower and upper critical solution temperatures in aqueous milieu, forming gel at lower temperature, a solution at room temperature and cloudy nanophase-separated dispersion at elevated temperature. The limit temperatures can easily be adjusted by the polyoxazoline graft length and grafting density. Moreover, the polymer behavior is additionally dependent on the concentration of potassium ions. The polymers behave similarly as the original κ-carrageenan, and thus, the poly(2-alkyl-2-oxazoline) grafts do not decrease the ability of the κ-carrageenan to form the self-assembled structures. Molecular principles beyond this multistimuli-responsive behavior were elucidated with the use of dynamic light scattering, magnetic resonance and fluorescence measurements as well as atomic force microscopy. These polymers could be used in a wide range of biological applications demanding thermo- and potassium-responsiveness.

The filler-rubber interface in styrene butadiene nanocomposites with anisotropic silica particles: morphology and dynamic properties.

Silica-styrene butadiene rubber (SBR) nanocomposites were prepared by using shape-controlled spherical and rod-like silica nanoparticles (NPs) with different aspect ratios (AR = 1-5), obtained by a sol-gel route assisted by a structure directing agent. The nanocomposites were used as models to study the influence of the particle shape on the formation of nanoscale immobilized rubber at the silica-rubber interface and its effect on the dynamic-mechanical behavior. TEM and AFM tapping mode analyses of nanocomposites demonstrated that the silica particles are surrounded by a rubber layer immobilized at the particle surface. The spherical filler showed small contact zones between neighboring particles in contact with thin rubber layers, while anisotropic particles (AR > 2) formed domains of rods preferentially aligned along the main axis. A detailed analysis of the polymer chain mobility by different time domain nuclear magnetic resonance (TD-NMR) techniques evidenced a population of rigid rubber chains surrounding particles, whose amount increases with the particle anisotropy, even in the absence of significant differences in terms of chemical crosslinking. Dynamic measurements demonstrate that rod-like particles induce stronger reinforcement of rubber, increasing with the AR. This was related to the self-alignment of the anisotropic silica particles in domains able to immobilize rubber.

Transport Phenomena and Electrode Reactions Generated by an Electric Field in Colloidal Silica.

The kinetics of transport phenomena generated by an electric field and leading to the formation of density gradients in suspensions of charged colloidal silica were studied by using various electrodes. The rate of approach to a steady-state density gradient was found to be much higher when using metallic electrodes (Cu, Fe, and Pt) in comparison with graphite (C) electrodes. Nevertheless, the initial rate with C electrodes was substantially increased by the addition of hydroquinone-quinone because the redox reactions, necessary for electrode-electrolyte current transfer, occur at lower potential compared with the electrolysis of water. On the other hand, the products of oxidation of hydroquinone which accumulate in the system bring about an important decrease of the zeta potential of silica particles and progressive deceleration of their electrophoretic mobility. A detailed study was carried out, by using thin-layer isoperichoric focusing, UV-vis spectrophotometry, and voltamperometry, to explain the observed phenomena which can interfere in electric polarization or focusing field-flow fractionation. Copyright 2000 Academic Press.

Effect of Ionic and Nonionic Solutes on the Transient and Steady States in the Settling of Charged Colloidal Particles.

Centrifugation was applied to study the transport phenomena in various suspensions of charged colloidal silica particles. Isopycnic focusing of colored density marker beads was used to view the evolution of the density gradient and to determine its steady-state shape from the measured positions of the focused zones. The effect of the ionic and nonionic solutes on the transient and steady states was studied. The experimental results are discussed in relation with the published theoretical dependence of the transport coefficients on the volume fraction of the settling particles. The understanding of the transport phenomena and of the interfering particle interactions governing the formation of the density gradient and the subsequent isopycnic focusing under model static conditions is necessary for the optimization of the isoperichoric focusing field-flow fractionation dynamic experiments, but it should allow a more accurate description of the relaxation phenomena and retention in polarization field-flow fractionation as well. Some previous experimental results concerning the study of the transport phenomena of the charged silica particles in an electric field are exploited here to demonstrate the general character of the observed behavior, independent of the field nature. Copyright 2000 Academic Press.

Isopycnic Focusing Study of the Transient State in the Settling of Colloidal Particles

Various effective field forces can generate the concentrating flux of the colloidal particles. Centrifugal field forces were used in this work to study the kinetics of the transport phenomena and the effect of interparticle interactions of the model silica suspension. Isopycnic focusing of the colored density marker beads was used to visualize the evolution and final quasi-equilibrium shape of the density gradient formed in silica suspension. Many operational parameters and experimental conditions (thickness of the liquid layer in measuring cells, intensity of the centrifugal forces, average concentration of the silica particles, and geometrical shape of the measuring cells) were tested to evidence their influence on the concentration and density gradient formation. The main objective was to apprehend the processes governing isoperichoric focusing in thin layers under model static conditions which is necessary for the optimization of isoperichoric focusing field-flow fractionation dynamic experiments carried out under conditions of hydrodynamical flow as an active factor of the separation by focusing effect. The experimental results were compared with the hypothetical model of noninteracting particles to demonstrate the effect of the actual interparticle interactions.