Ola J. Karlsson

Phase diagrams come alive: understanding how to create, destroy or change ordered surfactant structures by polymerizing the counterions

Free radical polymerization of acrylate counterions to cationic alkyltrimethylammonium surfactant ions was performed in aqueous media. The results of the reactions were either formation, destruction or change of liquid crystalline surfactant structures, depending on the starting conditions such as the surfactant concentration, the content of inert counterions and the monomer-to-initiator ratio. The results were consistent with predictions inferred from equilibrium phase diagrams recently established for aqueous mixtures of the cationic surfactants alkyltrimethylammonium acetate or bromide (CnTAAc/Br; n = 12, 16) with the “complex salts” CnTAPAm, in which the counterions to the surfactant ions were polyacrylate polyions (PAs) of different degrees of polymerization (m = 25, 30 or 6000). Appropriate pathways, at constant water content, through the latter ternary phase diagrams show what happens, at equilibrium, when monomeric counterions to the surfactant ions are replaced by polymeric counterions. These pathways through the equilibrium phase diagrams thus “come alive” in the counterion polymerization processes performed here.

Spreading dynamics of a functionalized polymer latex.

Functionalized polymer nanoparticles are used as binders for inorganic materials in everyday technologies such as paper and coatings. However, the functionalization can give rise to two opposing effects: It can promote adhesion via specific interactions to the substrate, but a high degree of functionalization can also hamper spreading on substrates. Here, we studied the spreading kinetics of individual functionalized vinyl acetate-co-ethylene polymer nanoparticles on inorganic substrates by atomic force microscopy (AFM) imaging. We found that the kinetics underwent a transition from a fast initial regime to a slower regime. The transition was independent of functionalization of the particles but depended on the wettability of the substrate. Furthermore, the transition from the fast regime to the slow regime occurred at a size-dependent contact angle, leading to a h ∼ a(3/2) scaling dependence between the height (h) and the width (a) of the spreading particles. Thereafter, spreading continued on a slower time scale. In the slow regime, the kinetics was blocked by a high degree of functionalization. We interpret the observations in terms of a nanoscale stick-slip transition occurring at interface stress around 6 kPa. We develop models that describe the scaling relations between the particle height and width on different substrates.