Displacement of adsorbed polymers : a systematic study of segment - surface interactions

This thesis describes a method to determine the thermodynamic (reversible) adhesion strength of polymers on inorganic solids.
This adhesion strength of polymers is an important factor in many applications. Examples are the quality and properties of glass fibre reinforced composites, coatings, and adhesives.

The key idea of the method is that the thermodynamic adhesion strength can be obtained from polymer displacement experiments. Polymers adsorbed from solution on an inorganic adsorbent can be desorbed by adding a more strongly adsorbing solvent component (a so-called displacer ). At a certain critical displacer concentration (the critical point) the polymer is entirely desorbed by the displacer. Cohen Stuart et al. showed that this critical solvent composition can be related to the effective adsorption energy per segment. Using an apolar solvent, the thermodynamic work of adhesion of a polymer can be estimated from this effective adsorption energy, the segmental cross- section, the solvent surface tension, and the dispersion contribution of the surface free energy of the substrate.

In this study, critical points have been measured for various systems by 5 different methods. Some methods do not only give the critical point, but also specific information about the conformational state of adsorbed polymers. The following procedures were used:
- The adsorbed amount of polymer as a function of the displacer concentration was determined indirectly by measuring the free polymer concentration in solution (see chapters 2, 4, and 5). From the mass balance the adsorbed amount can then be calculated. This method is, however, very time consuming and often difficult to carry out because of analytical problems.
- Thin-Layer Chromatography was used to measure the interfacial residence time of polymer on the substrate as a function of the eluent composition. The critical point is found from the (sharp) transition between full retention (where the polymer is immobile on the thin layer) and no retention (where the polymer moves with the eluent front). In adsorption chromatography studies, the solvent strength model of Snyder ⁽²⁾is frequently used. When this model is applied to polymer adsorption, it turns out to be similar to the model of Cohen Stuart et al. ⁽¹⁾Solvent strength data, which are available in the chromatographic literature, can also serve as a useful source of information for polymer adsorption and adhesion studies. The most important advantage is that the cumbersome procedure to determine separately the displacer energy can now be avoided. Chromatographic experiments are described in chapters 2 and 6.
- Attenuated Total Reflection Infrared Spectroscopy was used to determine critical points and the kinetics of polymer desorption (chapter 3). The advantages of this technique are that the measurements can be done in situ , and that different species on the surface can be detected simultaneously. The rate of polymer desorption by a low molecular weight displacer is much more rapid (i.e., within a few minutes) than the rate of desorption by a displacing polymer. The latter process may have time scales of the order of weeks. Polymer desorption by a more strongly adsorbing polymer seems to occur segment-by-segment.
- Dynamic Light Scattering was used to measure the hydrodynamic thickness of adsorbed polymer layers (chapter 4). This thickness is dominated by the free chain ends ( tails ). The results show that the hydrodynamic layer thickness δ _h  is constant or increases slightly with increasing amount of displacer up to the critical point and then drops sharply to zero. The adsorbed amount decreases much more gradually as a function of the amount of displacer added. All data agree with most earlier measurements on different substrates, and corroborate the theoretical result that the segmental adsorption energy has no effect on δ _h until very close to full desorption.
- Chapter 5 describes Proton Magnetic Relaxation measurements of silica dispersions carried out as a function of polymer coverage, solution pH, and amount of displacer added. We find that the spinlattice relaxation rate of the solvent is enhanced as a result of polymer adsorption and that, with proper calibration, this enhancement can be used to obtain the amount of polymer segments directly bound to the surface (the so-called trains ). It turned out that the number of train segments is affected much more strongly by addition of displacer than the tail segment density, as measured by dynamic light, scattering.

By means of the methods described above, we determined adhesion strengths for 5 different polymers on silica and alumina. In order to study the effects of the functional group and the chain structure of the polymer on the adhesion strength we choose the following polymers: polystyrene, poly(butyl methacrylate), poly(tetrahydrofuran), poly(methyl methacrylate), and poly(ethylene oxide). The given order of these polymers corresponds to an increasing segmental adsorption energy on both substrates. We find that polymers with more methylene groups per segment in the main chain or with larger alkyl side groups have a smaller adsorption strength. The adsorption energy for each individual polymer is higher on silica than on alumina.

The displacer concept may also be used to determine the adsorption energy of solvents but this time polymers are used as a standard. Doing so, we obtained the same trends for the adsorption energy of solvents as for polymers with respect to the functional group and the size of the monomer unit.

In chapter 6 we estimate the reversible work of adhesion for polymers in vacuum from the segmental adsorption energy. For pure polymers on silica, this work is of the order of 100-200 mJ/m². It turns out that the contribution of specific interactions to the total work of adhesion ranges from about 10% for polystyrene to about 50% for poly(ethylene oxide). The largest contribution to the work of adhesion on silica with respect to vacuum is thus due to nonspecific, dispersive interactions.