Self-assembly of oppositely charged polyelectrolyte block copolymers containing short thermoresponsive blocks.

The assembly of oppositely charged block copolymers, containing small thermoresponsive moieties, was investigated as a function of salt concentration and temperature. Aqueous solutions of poly-[N-isopropylacrylamide]-b-poly[dimethylaminoethyl methacrylate] (NIPAM44-b-DMAEMA216) and PNIPAM-b-poly[acrylic acid]-b-PNIPAM (NIPAM35-b-AA200-b-NIPAM35) were mixed in equal charge stoichiometry, and analysed by light scattering (LS), NMR spectroscopy and small angle X-ray scattering (SAXS). At room temperature, two different micelle morphologies were found at different salt concentrations. At NaCl concentrations below 0.75 M, complex coacervate core micelles (C3M) with a PNIPAM corona were formed as a result of interpolyelectrolyte complexation. At NaCl concentrations exceeding 0.75 M, the C3M micelles inverted into PNIPAM cored micelles (PCM), containing a water soluble polyelectrolyte corona. This behavior is ascribed to the salt concentration dependence of both the lower critical solution temperature (LCST) of PNIPAM, and the complex coacervation. Above 0.75 M NaCl, the PNIPAM blocks are insoluble in water at room temperature, while complexation between the polyelectrolytes is prevented because of charge screening by the salt. Upon increasing the temperature, both types of micelles display a cloud point temperature (T cp), despite the small thermoresponsive blocks, and aggregate into hydrogels. These hydrogels consist of a complexed polyelectrolyte matrix with microphase separated PNIPAM domains. Controlling the morphology and aggregation of temperature sensitive polyelectrolytes can be an important tool for drug delivery systems, or the application and hardening of underwater glues.

Thinning of wetting films formed from aqueous solutions of non-ionic surfactant.

We investigated the thinning of wetting films formed from aqueous solution of non-ionic triblock copolymer Pluronic F127 on the surface of silica using a home-made thin film balance and time-resolved ellipsometry. Imaging ellipsometry was used to visualize the film structures at subsequent stages of their development. The results unambiguously show that the time required for the formation of steady films strongly depends on the electrolyte concentration. When increasing the latter from 10(-4) to 0.1 M, this time typically increases with several orders of magnitude, from a few minutes to several hours. Moreover, for sufficiently large amounts of salt, two characteristic relaxation regimes can be clearly identified. After initial quick thinning, further thinning slows down enormously. These typical kinetic regimes are thought to result from the coupled dependencies of the bulk and interfacial properties of F127 on salt concentration. Possible explanations of the phenomenon are discussed.