KINETICS OF ADSORPTION AND DESORPTION OF POLYSTYRENE ON SILICA FROM DECALIN

Measurements are made of the rate of adsorption and desorption of polystyrene (PS) from decalin onto silica. The adsorbed mass as a function of time is monitored by means of optical reflectometry, and mass transfer through the solution is controlled by means of an impinging jet geometry. It is found that the initial rate of adsorption is entirely controlled by mass transfer, attachment being a fast step on the time scale of the experiment. This result agrees with previous findings for the system poly(ethylene oxide)/water/silica. However, at higher coverage the rate of adsorption slows down gradually and for long chains becomes quite slow. By varying the concentration in solution, it could be shown that the rate of adsorption in this regime remains proportional to the bulk concentration. This can be understood in terms of a first-order attachment step; the corresponding rate coefficient is given as a function of coverage. For comparison, we include a few data for PS with a strongly adsorbing imine end group. These polymers initially adsorb very much like their unfunctionalized counterparts but at higher coverage show a slow continuous rise to considerably higher coverages than ordinary PS, which must be due to formation of a dense brush. As with PS, the overall kinetics is proportional to the bulk concentration which again points toward a rate-limiting attachment step. By means of variations in the chain length, in the segmental adsorption energy, and in the polymer/solvent interaction, it could be shown that the rate of attachment is not simply related to the coverage but rather to the amount of anchoring energy that an incoming chain can gain in its first encounter with the surface, i.e., to the degree of undersaturation. Deviations from fast equilibration were also observed for desorption of short PS chains into pure decalin.