Adsorption dynamics of hydrophobically modified polymers at an air-water interface

The adsorption dynamics of a series of hydrophobically modified polymers, PAAαCn, at the air-water interface is studied by measuring the dynamic surface tension. The PAAαCn are composed of a poly(acrylic acid) backbone grafted with a percentage α of C8 or C12 alkyl moieties, at pH conditions where the PAA backbone is not charged. The observed adsorption dynamics is very slow and follows a logarithmic behavior at long times indicating the building of an energy barrier which grows over time. After comparison of our experimental results to models from the literature, a new model which accounts for both the deformation of the incoming polymer coils as well as the deformation of the adsorbed pseudo-brush is described. This model enables to fit very well the experimental data. The two fitting parameters give expected values for the monomer size and for the area per adsorbed polymer chain. Introduction Polymers are widely used as interface stabilizers, emulsifiers or suspension dispersants. Adsorbed at the liquid/liquid or solid/liquid interfaces, polymer layers form loops, which provide a steric protection for droplets or particles against coalescence or flocculation respectively. Amphiphilic polymers, such as statistical and block copolymers or polymers grafted with hydrophobic anchors adsorb more strongly to 2 interfaces than homopolymers. The structure of adsorbed amphiphilic polymer layers has been the object of theoretical and experimental research in the 90s [1]. For block copolymers, it was predicted that the hydrophobic parts form 2D coils while the hydrophilic parts form 3D coils swollen in the bulk solution [2, 3]. At high surface polymer concentration, the hydrophilic parts stretch perpendicularly to the interface thus forming a pseudo-brush [1-7], the term of brush being usually reserved for covalently end-grafted polymers at a solid interface. Pseudo-brushes are also expected for other amphiphilic polymers such as telechelic [8], end-functionnalized [9] or hydrophobically modified polymers [10]. The adsorption dynamics of amphiphilic polymers has attracted much less attention than their structure [11-18], even though it is relevant in foaming or emulsification processes as well as for foam and emulsion stability [19, 20]. A fine understanding of the influence of the polymer architecture on the adsorption dynamics is however required for desired applications. Polymer molecules adsorb slowly at an interface and their surface concentration usually never reaches equilibrium [5, 10]. Millet et al. [10] measured a logarithmic adsorption dynamics at long time scales in the case of charged polyelectrolytes statistically grafted with hydrophobic anchors. Several theoretical studies [5, 9, 21] also predicted logarithmic adsorption regimes at long times for surfactants and polymers due to an energy barrier which grows over time. In 1952, Ward and Tordai [21] described theoretically the adsorption dynamics of surfactant molecules. They predicted an energy barrier which scales linearly with the surface pressure and the area per surfactant molecule. It corresponds to the work required to clear a section of the interface in order to allow for the adsorption of an incoming surfactant molecule. Later, Johner and Joanny [5] and Ligoure and Leibler [9] developed theories to describe the adsorption dynamics of block copolymers and of the grafting dynamics of end-functionalized polymers at an interface, respectively. In both their models, the adsorbed polymer chains form a pseudo-brush at the interface and the incoming polymer chains have to stretch and undergo a diffusive motion inside the pseudo-brush in order to reach the interface. In that case, the energy barrier arises from the stretching of the adsorbing chains but the adsorbed pseudo-brush remains undeformed. Although some attempts have been made to fit these models to experimental data [10], there is still no clear experimental validation. In this article, we investigate the adsorption dynamics of a series of hydrophobically modified polymer molecules: PAA, poly (acrylic acid), grafted with hydrophobic alkyl anchors of varying length and grafting degree. We observe logarithmic adsorption dynamics at long times, which are compared to the models described above. We show that these models cannot satisfactorily be used to describe our data. We suggest a 3 new model accounting for both the deformation of the coils as they adsorb at the interface and the deformation of the pseudo-brush which has to clear a portion of the interface to enable the adsorption of a deformed coil. This new model, which involves only two fitting parameters, the area per adsorbed polymer chain and the monomer size, is in agreement with the experimental data. From the fits, we recover the expected value for the monomer size. Moreover we find that the area of an adsorbed coil is much lower that the area of an undeformed coil and does not vary significantly with the number of grafts. Our study paves the way towards the rational design of polymer molecules for interfacial stabilization.