The effect of cross-link density on the structure of Poly-(N-isopropylacrylamide) microgel particles prepared by emulsion polymerization was investigated by static and dynamic light scattering measurements. According to these measurements the gel particles cannot be considered as homogeneous spheres. The structure of the gel particles strongly depends on the degree of cross-linking. At high cross-link density the particles can be described with a Gaussian segment density distribution. With decreasing cross-link density the structure of the microgel particles tends toward that of a highly branched coil.

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The binding isotherm of sodium dodecyl sulfate (SDS) on a hyperbranched polyethyleneimine (PEI) was determined by an equilibrium dialysis method. Dynamic light scattering, electrophoretic mobility, and coagulation kinetics measurements were also performed in order to monitor the changes in the charged nature and size of PEI/SDS complexes. The experimental binding isotherm shows that the SDS interacts with PEI in two different ways. In a first binding process, the dodecyl sulfate ions bind in monomer form to the protonated amine groups, which is accompanied by an increase of the pH. A quantitative model is presented to describe the relation between the surfactant binding and the pH change. Above a critical amount of the bound surfactant, the PEI/SDS complex molecules collapse and precipitate. After the collapse of the polyelectrolyte/surfactant molecules, the SDS adsorbs on the surface layer of the collapsed particles (causing a charge reversal). This means that the interaction of the SDS with PEI can be divided into different characteristic SDS concentration ranges. At low surfactant concentrations, the system is a thermodynamically stable solution of the polymer/surfactant complex molecules. Above this critical concentration, the system is an unstable colloid dispersion of the complex particles. At even higher surfactant concentrations, the system may be a kinetically stable dispersion of the PEI/SDS particles, depending on the method of preparation. It can be concluded that the observed mechanism of PEI-SDS interaction is different from the general characteristics of the oppositely charged linear polyelectrolytes and surfactants, where the precipitated complex dissolves in the excess surfactant due to a collective (micelle-like) polymersurfactant interaction.

The effect of different mixing protocols on the charged nature and size distribution of the aqueous complexes of hyperbranched poly(ethylene imine) (PEI) and sodium dodecyl sulfate (SDS) was investigated by electrophoretic mobility and dynamic light scattering measurements at different pH values, polyelectrolyte concentrations, and ionic strengths. It was found that at large excess of the surfactant a colloidal dispersion of individual PEI/SDS nanoparticles forms via an extremely rapid mixing of the components by means of a stop-flow apparatus. However, the application of a less efficient mixing method under the same experimental conditions might result in large clusters of the individual PEI/SDS particles as well as in a more extended precipitation regime compared with the results of stop-flow mixing protocol. The study revealed that the larger the charge density and concentration of the PEI, the more pronounced the effect of mixing becomes. It can be concluded that an efficient way to avoid precipitation in the solutions of oppositely charged polyelectrolytes and surfactants might be provided by extending the range of kinetically stable colloidal dispersion of polyelectrolyte/surfactant nanoparticles via the application of appropriate mixing protocols.

This work reports a unifying general physical description of the behavior of oppositely charged polyelectrolyte/surfactant mixtures at the air/water interface in terms of equilibrium vs nonequilibrium extremes. The poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate system with added NaCl at two different bulk polyelectrolyte concentrations and the poly(sodium styrenesulfonate)/dodecyltrimethylammonium bromide system have been systematically examined using a variety of bulk and surface techniques. Similarities in the general behavior are observed for all the investigated systems. Following the slow precipitation of aggregates in the equilibrium two-phase region, which can take several days or even weeks, depletion of surface-active material can result in a surface tension peak. The limiting time scale in the equilibration of the samples is discussed in terms of a balance between those of aggregate growth and settling. Bulk aggregates may spontaneously dissociate and spread material in the form of a kinetically trapped film if they interact with the interface, and a low surface tension then results out of equilibrium conditions. These interactions can occur prior to bulk equilibration while there remains a suspension of aggregates that can diffuse to the interface and following bulk equilibration if the settled precipitate is disturbed. Two clear differences in the behavior of the systems are the position in the isotherm of the surface tension peak and the time it takes to evolve. These features are both rationalized in terms of the nature of the bulk binding interactions.

Monodisperse microgel latex with homogeneous cross-link density distribution within the particles was prepared by feeding the monomer and cross-linker into the reaction mixture in a regulated way during the polymerization. To determine the appropriate monomer feeding parameters, the kinetics of the particle formation was investigated by HPLC. The swelling and optical characteristics of the prepared homogenously cross-linked microgel particles were compared to the properties of inhomogenously cross-linked microgels prepared by the normal precipitation polymerization method. The distribution of the cross-link density within the particles inserts a great influence on the characteristics of the system. The degree of swelling of the homogeneous particles is significantly higher than that of the heterogeneous microgel particles. Furthermore, at room temperature the pNIPAm latex containing the homogeneously cross-linked particles is transparent, while the heterogeneously cross-linked particles form a highly turbid system at the same 0.1 wt% concentration.

A novel method is presented for the design of robust, sustained nanochemomechanical oscillators. The approach is based on the switching of chemoresponsive nanogel beads between their collapsed and swollen state by coupling them to an appropriately chosen nonlinear reaction. The presented system utilizes a proton activated oscillatory reaction and pH-sensitive nanobeads of gel that provide more than an order of magnitude volume change. A key point of our approach is the control of the colloid stability of the nanobeads of gel in a wide range of experimental parameters (pH, ionic strength, temperature) without interfering with the swelling characteristics of the nanogel particles. This was achieved by utilizing the interaction of nanogels with ionic surfactants.

Polyethyleneimine (PEI) and Microfibrillated cellulose (MFC) have been used to buildup polyelectrolyte multilayers (PEM) on silicone oxide and silicone oxynitride surfaces at different pH values and with different electrolyte and polyelectrolyte/colloid concentrations of the components. Consecutive adsorption on these surfaces was studied by in situ dual-polarization interferometry (DPI) and quartz crystal microbalance measurements. The adsorption data obtained from both the techniques showed a steady buildup of multilayers. High pH and electrolyte concentration of the PEI solution was found to be beneficial for achieving a high adsorbed amount of PEI, and hence of MFC, during the buildup of the multilayer. On the other hand, an increase in the electrolyte concentration of the MFC dispersion was found to inhibit the adsorption of MFC onto PEI. The adsorbed amount of MFC was independent of the bulk MFC concentration in the investigated concentration range (15-250 mg/L). Atomic force microscopy measurements were used to image a MFC-treated silicone oxynitride chip from DPI measurements. The surface was found to be almost fully covered by randomly oriented microfibrils after the adsorption of only one bilayer of PEI/MFC. The surface roughness expressed as the rms-roughness over 1 microm2 was calculated to be 4.6 nm (1 bilayer). The adsorbed amount of PEI and MFC and the amount of water entrapped by the individual layers in the multilayer structures were estimated by combining results from the two analytical techniques using the de Feijter formula. These results indicate a total water content of ca. 41% in the PEM.