Лако-красочные материалы — производство

Jean-Francois Joanny

Institut Charles Sadron, Strasbourg, France

G. INTRODUCTION

Polymers are involved in many practical adhesion problems. A polymer liquid can be present in the gap between the two media that adhere to one another in order to create strong attractive forces that strengthen the adhesion. In this context it is important to understand how polymer solutions interact with surfaces and how they create strong interactions between them [1]. The aim of this short review is to present rather qualita­tively our understanding of the equilibrium thermodynamic properties of polymer solu­tions close to surfaces. This is clearly one of the important factors in understanding the adhesion between two surfaces mediated by polymers, but one must keep in mind that adhesion is a nonequilibrium process where energy dissipation plays a major role. This aspect will not be considered in this chapter.

There are three main modes of interaction between a polymer solution and a solid surface. The first interaction mode is depletion [2,3]. If the monomers are repelled by the surface (or in other words if the attractive interaction between the solvent molecules and the surface is larger than the interaction between the monomers and the surface), the polymer concentration in solution decreases as the surface is approached and a region depleted in polymer exists in the vicinity of the surface. The size of this region is the size of the polymer chain if the solution is dilute and the size of the correlation length of the solution if the solution is semidilute (if the polymer chains overlap). When two surfaces are brought in close contact, the density in the gap between the surfaces is smaller than the bulk concentration and the osmotic pressure in the gap is smaller than the bulk osmotic pressure. This osmotic pressure difference induces an attraction between the surfaces. The depletion interaction is not specific to polymers and exists with any particle with a size in the colloidal range [4]. It has sometimes been used to induce adhesion between particles of mesoscopic size such as red blood cells. The only limitation to this qualitative description of the depletion force is that at equilibrium the polymer chains (or any other particles) must leave the gap as the surfaces get closer. There is no attractive depletion force if they remain trapped in the gap. We will not consider further the depletion interaction.

In the opposite case where the surface prefers to be in contact with the monomers, the polymer chains strongly adsorb on the surface [5]. The specific polymer effect is that the adsorbed layer is much thicker than a microscopic size; its thickness is of the order of the polymer chain radius of about 10 nm [6,7]. This means that polymer adsorption can induce interactions between surfaces over a range of the order of the size of the polymers. Adsorbed polymer layers are fluffy in the sense that they are not dense, the monomer concentration decreases from a high value on the surface to a very low value at the outer part of the layer. This is due to the fact that a polymer chain can form long loops on the surface and that the size of the loops can fluctuate from a microscopic size to the size of the polymer chain. In addition to these loops an adsorbed polymer chain has two long tails which are the end parts of the chain that do not fold back on the surface. The important feature for adhesion is that a polymer chain can bind to two surfaces and form bridges between them. In many instances, the adsorption is irreversible and bridging strongly enhances the adhesion.

A last interaction mode between polymer solutions and surfaces is obtained by grafting the polymers on the surface by one of the chain ends. The grafting can be covalent and thus fully irreversible or physical; physical grafting is achieved by anchoring the chains on the surface with strongly dipolar groups or by using diblock copolymers where one of the blocks strongly adsorbs on the surface. The experimental difficulty is that there is a strong potential barrier against grafting owing to the repulsive interactions with already grafted chains and that the graft density within reasonable experimental time scale is often rather low. Tricks have been found [8,9] to increase strongly the graft density such as growing the grafted chains from the surface or grafting the chains from a dense solution where the excluded volume interactions are screened and very dense grafted layers (one grafted polymer every 2 nm on the surface) can be obtained. The thickness of grafted layers is very large and can reach hundreds of nanometers. Adhesion between surfaces carrying grafted polymer layers can be achieved if the chains bind onto both surfaces and form bridges or if the grafted layers on the two surfaces undergo a chemical reaction that allows the formation of a bridge at contact. In the study of the adhesion of rubbers to a hard surface, the adhesion is strongly increased if the hard surface is covered by a grafted layer that can be interdigitated with the rubber [10,11].

In the remainder of this chapter, we discuss theoretically the properties of grafted polymer layers in Section II and of adsorbed polymer layers in Section III. In each case, we first consider the equilibrium properties of a single layer and then discuss briefly the interactions between two surfaces.