Kinetics of polymer adsorption, desorption and exchange

The aim of the study in this thesis was to gain more insight in the kinetics of polymer adsorption. To this end some well-characterised polymers have been systematically investigated.In the process of polymer adsorption one may distinguish three kinetic contributions: transport to the surface, attachment, and reconformation of the adsorbing and adsorbed chains. In order to assess the role of each of the three contributions it is necessary to measuri the adsorption kinetics under well-defined hydrodynamic conditions. For such measurements the transport (convection and diffusion) can be calculated and therefore it becomes possible to study unambiguously the interfacial processes, i.e., attachment and reconformation.For this study two experimental techniques were used that both fulfil the requirement that the adsorption occurs under well-defined hydrodynamic conditions: reflectometry in a stagnation point flow (chapters 2,3 and 5-7) and a streaming potential method (chapter 4). With both techniques it is possible to follow directly and continuously the buildup of an adsorbed layer. Reflectometry is a relatively new and simple optical technique for the measurement of adsorption on (optically flat) solid surfaces. In a reflectometer a linearly polarised light beam is reflected from the (adsorbing) surface, and the reflected beam is split into its parallel and perpendicular components. The intensity ratio between the two components is continuously measured. This ratio changes upon adsorption, and after calibration the adsorbed amount (mass/area) is obtained. For reflectometry there are only few restrictions on the choice of adsorbate, adsorbent and solvent.The applicability in this study of the streaming potential method is limited to adsorption of uncharged polymers from aqueous solution. For that case, the streaming potential can be related to the hydrodynamic layer thickness of the adsorbed polymer layer. This thickness is mainly determined by loose ends of adsorbed chains, and it is sensitive to very small changes in the adsorbed amount of long chains near saturation. Such small changes occur for desorption of long chains into solvent, so that the streaming potential method is especially suitable for the measurement of the desorption kinetics.In chapter I the aim and scope of this study of this study are explained, and a general introduction to adsorption of polymers is given.Chapter 2 deals with the measurement of adsorption by reflectometry. Using the results of an optical model we discuss the possibilities of the method for measuring the adsorption from dilute solution on a thin film on top of a silicon substrate. For a wide variety of solvents and film materials, a sensitivity can be obtained of the order of 1-2% change in reflectivity per mg/m 2adsorbed, which is quite enough for an accurate determination of the adsorbed amount. By choosing carefully the film thickness and angle of incidence of the light beam, it can be achieved that the reflected intensity varies proportionally with the adsorbed amount, independent of the concentration profile in the adsorbed layer. Under such conditions, the reflectometric signal can be simply converted into the adsorbed amount.In chapter 3 reflectometry is used to investigate the kinetics of adsorption of poly(ethylene oxide) (PEO) from water onto oxidised silicon. For the stagnation point flow the maximum rate of mass transfer of polymer to the surface is calculated. This rate is compared with the observed adsorption rate, and it is concluded that mass transfer is ratelimiting up to or nearly up to saturation, depending on the chain length. Only for long chains ( M >100 kg/mole) near saturation the adsorption rate is lowered by surface processes.In chapter 4 a model is discussed for the desorption rate of polymers into a flow of pure solvent. This model is based on the assumption that near the surface there is a rapid equilibration between free and adsorbed polymer, and that transport of free polymer away from the surface is ratelimiting for the desorption. Due to the shape of the (high affinity) isotherm, the equilibrium concentration of free chains even after a minute desorption is extremely low, so that the transport -and thus the desorptionproceeds slowly. Thus, in spite of the rapid local equilibration, the desorption is slow because of the slow mass transfer. For a logarithmic adsorption isotherm of the polymer (for which the adsorbed amount Γincreases linearly with the log of the concentration c in solution) an explicit expression for the adsorbed amount as a function of time is derived: the desorbed amount increases proportionally with log t. The model predicts that the absolute value of the slope of the (kinetic) desorption curve Γ(log t ) and the (static) adsorption isotherm Γ(log c ) are the same.Using the streaming potential method it is shown in chapter 4 that the above model gives an adequate description of the desorption kinetics in aqueous solutions of PEO on glass, even for high molar mass polymer (M = 847 kg/mole). Again, this shows that the equilibration of adsorbed layers of PEO is rapid as compared to the rate of mass transfer through solution.Chapter 5 describes the adsorption kinetics of polystyrene (PS) from decalin on oxidised silicon. On a bare surface the adsorption rate of PS is limited by mass transfer from solution, like for PEO. For PS, the adsorption rate decreases gradually with increasing coverage. This is due to a decreasing probability of attachment during a collision of a free chain with the (covered) surface. From experiments in which the chain length, the solvent quality and the adsorption energy were varied, the picture arises that the adsorption probability during a collision is the result of a balance between a gain in adsorption energy on the one hand, and repulsive interaction with the adsorbed layer on the other.Exchange between polymers that differ in chain length only is the subject of chapter 6. Displacement of adsorbed short chains of PEO by longer ones in solution is limited only by transport of long chains to the surface. The adsorbed layer is continuously in equilibrium with the solution near the surface. The same conclusion was drawn from the desorption kinetics of this polymer in a flow of pure solvent (chapter 4). For PS also surface processes play a role. During exchange of short by long chains of PS there is a temporary overshoot of short chains in the adsorbed layer. This overshoot may desorb either during adsorption of long chains, or by relaxation of the adsorbed layer. By interrupting the transport of long chains to the surface, this relaxation could also be directly observed. The higher chain stiffness of PS as compared to PEO possibly explains the slower equilibration of adsorbed PS.Finally, we present in chapter 7 some results on the exchange kinetics between three chemically different polymers: polystyrene (PS), poly(butyl methacrylate) (PBMA) and polytetrahydrofuran (PTHF). Displacement of adsorbed layers of the rather stiff polymers PS and PBMA by the very flexible PTHF is limited only by transport of the displacing polymer from the bulk solution. For mutual exchange between the two stiff polymers, surface processes play an important role: the displacer PBMA adsorbs quickly, whereas PS desorbs slowly. Possibly, the slow exchange kinetics is caused by the low mobility of the adsorbed polymers. The displacement rate of PS by PBMA increases considerably after addition of a displacer of low molar mass. The faster exchange kinetics is probably due to the lower binding strength and, consequently higher mobility of the adsorbed polymers.