Quick Curing Mechanisms for All-Season Paints and Renders.

Paints and coatings are required to quickly cure under a broad variety of environmental conditions and deliver solid long-term performance. Achieving a balance during all seasons between quick curing of a coating film, i.e., early rain resistance, while maintaining sufficient workability and open time for an optimized aesthetic appearance is a challenge for the architectural coatings industry. This article describes how the colloidal physics differs between the current standard mechanism to achieve early rain resistance by inhibited coagulants in winter paints and a new mechanism that provides all-season paints. A combination of advanced physical characterization methods, such as electrophoretic mobility, dynamic light scattering and confocal laser scanning microscopy, in combination with application tests, is used to provide a comprehensive mechanism of the early rain resistance achieved by such paints. In addition, it is shown that this new system can be transferred to wood coatings and organic renders. The key finding of this article is that all-season paints combining early rain resistance at cold and damp conditions with open time at high temperatures and dry conditions rely on fast paint film formation with high early integrity rather than coagulants triggered by base evaporation.

Structure and dynamics of soft repulsive colloidal suspensions in the vicinity of the glass transition.

We use a combination of different scattering techniques and rheology to highlight the link between structure and dynamics of dense aqueous suspensions of soft repulsive colloids in the vicinity of a glass transition. Three different latex formulations with an increasing amount of the hydrophilic component resulting in either purely electrostatically or electrosterically stabilized suspensions are investigated. From the analysis of the static structure factor measured by small-angle X-ray scattering, we derive an effective volume fraction that includes contributions from interparticle interactions. We further investigate the dynamics of the suspensions using 3D cross-correlation dynamic light scattering (3DDLS) and rheology. We analyze the data using an effective hard sphere model and in particular compare the linear viscoelasticity and flow behavior to the predictions of mode coupling theory, which accounts for a purely kinetic glass transition determined by the equilibrium structure factor. We demonstrate that seemingly very different colloidal systems exhibit the same generic behavior when the effects from interparticle interactions are incorporated using an effective volume fraction description.

Molecular structure encodes nanoscale assemblies: understanding driving forces in electrostatic self-assembly.

Supramolecular nanoparticles represent a key field in recent research as their synthesis through self-assembly is straightforward and they often can respond to external triggers. A fundamental understanding of structure-directing factors is highly desirable for a targeted structure design. This contribution demonstrates a quantitative relation between the size of supramolecular self-assembled nanoparticles and the free energy of association. Nanoparticles are prepared by electrostatic self-assembly of cationic polyelectrolyte dendrimers as model macroions and oppositely charged di- and trivalent organic dye molecules relying on the combination of electrostatic and π-π-interactions. A systematic set of sulfonate-group carrying azo-dyes was synthesized. Light scattering and ζ-potential measurements on the resulting nanoparticles yield hydrodynamic radii between 20 nm < R(H) < 50 nm and positive ζ-potential values indicating a positive particle charge. Studies on dye self-aggregation and dendrimer-dye association by isothermal titration calorimetry (ITC) and UV-vis spectroscopy allow for the correlation of the thermodynamic parameters of dendrimer-dye association with the size of the particles, showing that at least a free energy gain of ΔG ≈ - 32 kJ mol(-1) is necessary to induce dendrimer interconnection. Structural features of the azo dyes causing these to favor or prevent nanoparticle formation have been identified. The dye-dye-interaction was found to be the key factor in particle size control. A simple model yields a quantitative relation between the free energy and the particle sizes, allowing for predicting the latter based on thermodynamic measurements. Hence, a set of different molecular "building bricks" can be defined where the choice of building block determines the resulting assembly size.

Effect of polyelectrolyte architecture and size on macroion-dye assemblies.

This study describes the influence of polyelectrolyte building block nature on the formation of supramolecular polyelectrolyte-dye nanoparticles self-assembled through electrostatic interactions of the oppositely charged building units and π-π interactions in-between dye molecules ("electrostatic self-assembly"). The anionic azo dye Acid Red 26 (Ar26) is combined with cationic polyamidoamine dendrimers of generations G0, G2, G4, G6, and G8 and linear polycations polylysine (PolyLys) and polyallylamine hydrochloride (PAH). Light scattering reveals defined supramolecular particles with hydrodynamic radii R(H) = 15 nm to R(H) = 50 nm for slight excess of G2-G8 dendrimers. G0 does not yield stable assemblies. Linear polyelectrolytes form assemblies with hydrodynamic radii from R(H) = 20 nm to R(H) = 200 nm. Thermodynamic measurements by isothermal titration calorimetry (ITC) show that all macroions bind dye molecules up to about charge stoichiometry, and assembly formation is predominantly enthalpically driven. Differences in dye-dye interactions are related to structural features by analyzing endothermic heats of aggregate disruption through excess macroion addition. A simple model connects thermodynamic parameters with the internal assembly structure.

Influencing particle size and stability of ionic dendrimer--dye assemblies.

This article focuses on the physical chemical aspects of the formation of supramolecular nanoparticles with defined size and varying shape through electrostatic self-assembly of macroions and multivalent aromatic counterions. For cationic poly(amidoamine) dendrimers and different di- and trivalent sulfonate groups carrying azo dyes, the onset of interdendrimer connection and assembly size (hydrodynamic radius 20 nm < R(h) < 150 nm) depend on counterion/macroion loading ratio. Centrifugation and dialysis experiments show assemblies coexisting with individual dye-loaded dendrimers with lower dye/dendrimer ratio at small loading ratio, while around charge stoichiometry only assemblies are present. Zeta-potential measurements reveal a positive charge for samples with excess dendrimer. For excess dye, overloading to negatively charged assemblies is possible for some dyes, which is consistent with concentration-dependent stability revealing a second mode of more loosely bound dye ions. Kinetic versus thermodynamic effects are discussed based on varying the preparation route. The interaction enthalpy is an important factor in determining assembly size. Solution structures are characterized by static and dynamic light scattering, while atomic force microscopy showed that assemblies can also be deposited on surfaces.

Structure and thermodynamics of ionic dendrimer-dye assemblies.

In this study, the association ("electrostatic self-assembly") of cationic generation 4 poly(amidoamine) dendrimer with a set of oppositely charged aromatic and aliphatic organic sulfonate counterions was investigated. Aggregate size and shape depend on the counterion structure. Formation of defined assemblies was possible with stiff, mutually interacting aromatic azo-dye counterions (acid red dyes Ar26, Ar44, Ar27, Ar18) but not with flexible or small aliphatic disulfonates and sulfate. Dynamic light scattering yielded sizes in the order of 100 nm. Particle shapes as investigated by small-angle neutron scattering were spherical, ellipsoidal, cylindrical, or of core-shell type. Results revealed that the molar charge ratio of the components is a crucial factor in the aggregation. UV-vis showed a cooperative binding process and yielded twist angles of dye molecules of 15 degrees to 30 degrees according to exciton theory. Isothermal titration calorimetry on dendrimer-dye assembly formation and comparison systems gave information on thermodynamics and contributing interaction energies. The main enthalpic contribution was provided by the mutual interaction of the dye molecules, while electrostatic interaction contributes a maximum of about 1/3 of the total interaction enthalpy.