Surface Forces in Polymer Solutions and Melts

The presence of dissolved polymers in a dispersed system can drastically change the behavior of the dispersion or emulsion. The ancient Egyptians knew that one can keep soot particles dispersed in water when they were incubated with gum arabicum, an exudate from the stems of acacia trees, or egg white. In this way, ink was made. The reason for the stabilizing effect is the steric repulsion caused by adsorbed polymer. In the first case, the adsorbed polymers are polysaccharides and glycoproteins in the second case, it is mainly the protein albumin. 

Steric stabilization of dispersions is very important in many industrial applications. In particular in nonpolar solvents, the adsorption or grafting of a polymer onto the stuface of particles is the only effective way to establish dispersion stability and prevent flocculation caused by the attractive van der Waals forces because electrostatic interactions are virtually absent in nonpolar solvents. 

In many applications in mineral processing, papermaking, and waste water treatment, the opposite effect is desired. Then, polymers are used to induce flocculation. Usually, this is achieved by adding polymer, which can bridge two particles. To understand all these effects, we first need to introduce some fundamentals of the structure of polymers in solution and of polymer adsorption. Good introductions into polymer physics are Refs [1308-1310]. Polymer-induced forces are reviewed in Refs [1311 -1313]. 

---

Surface tension-driven breakup into droplets is rarely important in melt spinning, where the large viscous and elastic forces overwhelm the surface tension forces, ft is an important mechanism in the formation of the dispersed phase in polymer blends, and it is important in solution processing. The surface tension-driven breakup of a viscoelastic filament has been analyzed using both thin filament equations and a transient finite element analysis, but we will not pursue the topic here because it is not relevant to our present discussion. 

At a critical voltage, the repulsive force of the charged polymer overcomes the surface tension of the solution, and a charged jet erupts from the tip of the Taylor Cone, which moves toward the counter electrode and becomes narrower in the process. On the way to the counter electrode, the solvent evaporates (or the melt solidifies), and solid fibers with diameters ranging from micrometers to nanometers are often deposited as a randomly oriented, nonwoven mat see the inset of Fig. 1.4 for the SEM image of a typical sample [35,43,45]. With the use of this relatively simple and straightforward method, more than 100 different types of materials have already been processed as fibers with diameters ranging from several micrometers to a few nanometers. 

This book is solely concerned with polymers in the amorphous state, that is polymer molecules in solution, the melt or that are intrinsically amorphous in the solid state by virtue of their chemical structure. We discuss surfaces and interfaces involving pure polymeric phases and interfaces between simple liquids and solids or air that are modified by an accumulation of polymeric molecules. The situation is in one sense more complicated than that for materials composed of atoms or small molecules. For these systems, as hinted at above, there is a single length scale characterising the range of forces between molecules and this molecular length scale dictates the range over which the perturbation imposed by an interface persists. For polymers there are... 

---