Double-network hydrogels improve pH-switchable adhesion

For environmentally-switchable adhesive systems to be reused repeatedly, the adhesive strength must not deteriorate after each adhesion cycle. An important criterion to achieve this goal is that the integrity of the interface must be retained after each adhesion cycle. Furthermore, in order to have practical benefits, reversing the adhesion must be a relatively rapid process. Here, a double-network hydrogel of poly(methacrylic acid) and poly[oligo(ethylene glycol)methyl ether methacrylate] is shown to undergo adhesive failure during pH-switchable adhesion with a grafted (brush) layer of polycationic poly[2-(diethyl amino)ethyl methacrylate], and can be reused at least seven times. The surfaces are attached at pH 6 and detached at pH 1. A single-network hydrogel of poly(methacrylic acid), also exhibits pH-switchable adhesion with poly[2-(diethyl amino)ethyl methacrylate] but cohesive failure leads to an accumulation of the hydrogel on the brush surface and the hydrogel can only be reused at different parts of that surface. Even without an environmental stimulus (i.e. attaching and detaching at pH 6), the double-network hydrogel can be used up to three times at the same point on the brush surface. The single-network hydrogel cannot be reused under such circumstances. Finally, the time taken for the reuse of the doublenetwork hydrogel is relatively rapid, taking no more than an hour to reverse the adhesion

Adhesion between oppositely-charged polyelectrolytes

The adhesion between a grafted polyelectrolyte layer (brush) and a gel of an oppositely charged polyelectrolyte has been measured as a function of applied pressure, and the interface has been traced using neutron reflectometry. The interface (in aqueous medium at pH 6) between the (polycationic) brush and the (polyanionic) gel has a limited pressure-dependence, with a small amount of deformation of the interface at the brush-gel contact. Brushes with a dry thickness of up to 13 nm exhibit weak adhesion (measured using a mechanical force tester) with an adhesive failure when the gel is detached. Thicker brushes result in the gel exhibiting cohesive failure. Reversing the geometry, whereby a polycationic brush is replaced with a polyanion and the polyanionic gel is replaced with a polycation reveals that the pH-dependence of the adhesion is moderately symmetric about pH 6, but that the maximum force required to separate the polycation gel from the polyanion brush over the range of pH is greater than that for the polycation brush and polyanion gel. The polyanion used is poly(methacrylic acid) (PMAA) and polycations of poly[2-(diethyl amino)ethyl methacrylate] (PDEAEMA) and poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) were used

Salt Dependence of the Tribological Properties of a Surface-Grafted Weak Polycation in Aqueous Solution

The nanoscopic adhesive and frictional behaviour of end-grafted poly[2-(dimethyl amino)ethyl methacrylate] (PDMAEMA) films (brushes) in contact with gold- or PDMAEMA-coated atomic force microscope tips in potassium halide solutions with different concentrations up to 300 mM is a strong function of salt concentration. The conformation of the polymers in the brush layer is sensitive to salt concentration, which leads to large changes in adhesive forces and the contact mechanics at the tip–sample contact, with swollen brushes (which occur at low salt concentrations) yielding large areas of contact and friction–load plots that fit JKR behaviour, while collapsed brushes (which occur at high salt concentrations) yield sliding dominated by ploughing, with conformations in between fitting DMT mechanics. The relative effect of the different anions follows the Hofmeister series, with I − collapsing the brushes more than Br − and Cl − for the same salt concentration

Adhesion of grafted-to polyelectrolyte brushes functionalized with calix[4]resorcinarene and deposited as a monolayer

Polyelectrolyte adhesives, either poly[2-(dimethylamino)ethyl methacrylate] or poly(methacrylic acid), functionalized with a surface-active calix[4]resorcinarene were grafted onto silicon wafers. Adhesion studies on these grafted-to brushes using polyelectrolyte hydrogels of opposite charge showed that it is the calix[4]resorcinarene, rather than adsorption of polyelectrolyte monomers, that adheres the brush to the silicon substrate. The adhesion measured was similar to that measured using polymers grafted from the surface, and was stronger than a control layer of poly(vinyl acetate) under the same test conditions. The limiting factor was determined to be adhesive failure at the hydrogel-brush interface, rather than the brush-silicon interface. Therefore, the adhesion has not been adversely affected by changing from a grafted-from to a grafted-to brush, demonstrating the possibility of a one-pot approach to creating switchable adhesives

Correlation between experimental and DFT calculations of photocatalytic and optical properties of La0.8Nd0.1Sr0.1MnO3 perovskite: degradation efficiency of methyl orange dye under visible-irradiated conditions

La0.8Nd0.1Sr0.1MnO3 perovskite was synthesized using the citrate gel method. The crystal structure was studied by X-ray diffraction. The Rietveld refinement reveals a single R-3C rhombohedral structure. The morphological properties were investigated by a scanning electron microscopy, connected to an energy-dispersive X-ray detector for mapping and elements composition, showing a sponge-like structure behavior of agglomerated particles. The band gap energy and the optical properties were studied theoretically by Density-Functional Theory (DFT) calculations. It showed that La0.8Nd0.1Sr0.1MnO3 exhibited a high photoabsorption property with band gap energy of ≈ 2.5\ua0eV. The methyl orange (MO) degradation properties were studied by measuring time-dependent UV–Visible absorption of the dye solution. The photocatalytic activity was evaluated at different times under visible light and ambient temperature conditions for La0.8Nd0.1Sr0.1MnO3 solution. A high degradation rate of 80% in 120\ua0min under UV irradiation conditions was observed. It indicated that La0.8Nd0.1Sr0.1MnO3 presents high photocatalytic efficiency and could be a good candidate for MO degradation