Flexible polymers at a solid-liquid interface

We undertook the present study in order to evaluate techniques which are devised to assess the comformations of adsorbed macromolecules. Since recent theories deal with these conformations, we also wanted to investigate to what extent these theories are supported by experimental data.

In chapter 1 we outline the scope of this study and we give reasons for using polyvinyl pyrrolidone / silica as the model system.

Chapter 2 deals with the general aspects of polymer adsorption. Some trends found from experimental work are mentioned, and the four main theories which have treated the problem of interacting chain molecules at a plane interface are briefly discussed. Arguments are presented which lead to the conclusion that, from a theoretical point of view, the best theory available at present is the one recently given by Scheutjens and Fleer (1979). However, it is shown that even this theory fails to take into account both the volume ratio between polymer and solvent and the flexibility of the polymer in a consistent way. We propose a simple way to modify the theory such that these properties are both incorporated; this requires the introduction of a new parameter into the theory: the segment-to-solvent volume ratio or bulkiness parameter b. A few calculations show how the results depend upon b. At the end of chapter 2 we also consider briefly the reversibility of polymer adsorption in connection with polydispersity effects.

Chapter 3 presents a selection of relevant properties of the polymer, the solvents and the adsorbent.

Adsorption experiments are discussed in chapter 4. Adsorbed amounts for two solvents (water and dioxane) are given as a function of chain length, properties of the adsorbent surface and polymer heterodispersity. It is found that PVP adsorbs strongly from both solvents, but in amounts that are substantially higher from dioxane than from water. Chapter 4 also deals with the desorption which occurs if suitable low molecular weight displacers are added to the solution. By means of a simple expression, the amount of displacer needed to just cause complete desorption is related to the strength with which the polymer is bound to the surface. With 5 different displacers an effective adsorption energy is estimated and its meaning is discussed. This type of measurements is entirely new in polymer adsorption studies. Although a detailed interpretation of the results is not straightforward, we obtain reasonable values for the adsorption energy parameter.

Measurements of conformation parameters for adsorbed PVP are described in chapter 5. Bound fractions are determined spectroscopically (infrared, nuclear magnetic resonance) and completed with data from the literature (electron spin resonance). Calorimetry is used as a means to study the surface occupancy. The discrepancies between results from the various methods are considerable, and it is concluded that the techniques do not measure the same property. Nevertheless, the results are useful in gaining a further understanding of the adsorption phenomenon.

In chapter 6 we compare in some detail experimental and theoretical results. The theory used is the aforementioned modification of the Scheutjens-Fleer theory and the experimental data comprise adsorbed amounts, bound fractions and interaction parameters. The adsorbed amounts, which can be fairly accurately measured, agree within 10 tot 20 % with the theoretically calculated values, over the whole molecular weight range. To our knowledge, this is the first time that quantitative agreement is obtained to such an extent. Bound fractions determined with ESR also agree fairly well with the theoretical values, and NMR gives a result in the right range. Infrared data lead to a serious underestimation of the bound fraction, which is easily explained on the basis of the chain structure. To a lesser extent, microcalorimetric results also tend to underestimate the bound fraction, although the results agree surprisingly well with enthalpies of wetting. The dependency on the solvent type is also correctly predicted.

The conclusion is drawn that the introduction of the bulkiness parameter is an essential step towards a complete theory and that the modified Scheutjens-Fleer theory is strongly supported by experiment. Finally, we picture in some detail the conformation of adsorbed PVP, as it emerges from this study..