We probe the mechanical shear and compression properties of hydrogen-bonded polymer multilayers directly assembled at the oil−water interface using interfacial rheology techniques. We show that the polymer multilayers behave mechanically like a transient network, with elastic moduli that can be varied over 2 orders of magnitude by controlling the type and strength of physical interactions involved in the multilayers, which are controlled by the pH and the hydrophobicity of the polymer. Indeed, the interplay of hydrogen and hydrophobic interactions enables one to obtain a tighter and stronger network at the interface. Moreover, we show how a simple LBL process applied directly on emulsion droplets leads to encapsulation of a model oil, dodecane, as well as perfume molecules. P olymer capsules are used in a wide range of applications such as cosmetics, pesticides, food and detergency, where protection, transport and delivery of active species are needed. 1 Capsules prepared by the layer-by-layer assembly (LBL) 2 of polymer multilayers onto a colloidal template is a powerful method to produce polymer membranes with controlled thickness, porosity, and permeability. 3−6 Such capsules are a good alternative to other technologies based on cross-linking reactions at the interface of oil droplets 7 that may raise concerns about potential residuals in suspensions of capsules. However, in this LBL process, the core has to be dissolved to obtain a hollow capsule that then needs to be filled with active species. Recently, to circumvent these issues, stable O/W emulsions were prepared using a polymer surfactant and then the LBL method was used to build in situ a polymer membrane on the oil droplets. 8−10 In addition to stability and encapsulation properties, most applications require precise control of the mechanical properties of the capsules to resist environmental stresses and achieve suitable encapsulation/ release properties with respect to the desired application. For example, in detergency applications, fragrance capsules need to remain stable during storage in end-use products like liquid detergents, but the perfume molecules should be delivered to clothes at some key stages of the whole washing/drying process. This example can be extended to other fields of capsule application and shows the importance of an appropriate control of the mechanical properties of the capsules membrane. To the best of our knowledge there are no rules to guide the capsule design, mainly because of a lack of experimental methods available to easily measure the mechanical properties of capsules. 11−14 In this letter, we probe the shear and compression mechanical properties of hydrogen-bonded polymer multilayers directly assembled on a single millimetric oil droplet in water and at a flat oil−water interface. We show that the elastic moduli of the multilayers can be varied over orders of magnitude by using an interplay of hydrogen and hydrophobic interactions between the layers. Moreover, we show how a simple LBL process allows the assembly o...

The process for making melamine formaldehyde microcapsules containing fragrant oil is well-known. Recently, this technology has been used to enhance the olfactory performance on fabrics. However keeping the fragrance in the capsule during storage, improving the olfactory benefit and releasing a low amount of formaldehyde is highly challenging. To answer these challenges, Givaudan has developed its own melamine formaldehyde microcapsule, called Mechacaps, which is described in this article.

We study the 2D rheological properties of hydrogen-bonded polymer multilayers assembled directly at dodecane-water and air-water interfaces using pendant drop/bubble dilation and the double-wall ring method for interfacial shear. We use poly(vinylpyrrolidone) (PVP) as a proton acceptor and a series of polyacrylic acids as proton donors. The PAA series of chains with varying hydrophobicity was fashioned from poly(acrylic acid), (PAA), polymethacrylic acid (PMAA), and a homemade hydrophobically modified polymer. The latter consisted of a PAA backbone covalently grafted with C12 moieties at 1% mol (referred to as PAA-1C12). Replacing PAA with the more hydrophobic PMAA provides a route for combining hydrogen bonding and hydrophobic interactions to increase the strength and/or the number of links connecting the polyacid chains to PVP. This systematic replacement allows for control of the ability of the monomer units inside the absorbed polymer layer to reorganize as the interface is sheared or compressed. Consequently, the interplay of hydrogen bonding and hydrophobic interactions leads to control of the resistance of the polymer multilayers to both shear and dilation. Using PAA-1C12 as the first layer improves the anchoring energy of a few monomers of the chain without changing the strength of the monomer-monomer contact in the complex layer. In this way, the layer does not resist shear but resists compression. This strategy provides the means for using hydrophobicity to control the interfacial dynamics of the complexes adsorbed at the interface of the bubbles and droplets that either elongate or buckle upon compression. Moreover, we demonstrate the pH responsiveness of these interfacial multilayers by adding aliquots of NaOH to the acidic water subphase surrounding the bubbles and droplets. Subsequent pH changes can eventually break the polymer complex, providing opportunities for encapsulation/release applications.