Kinetic Traps in Polymer Adsorption. 2. Polystyrene Displaced by Polyisoprene at 50 °C

We analyze adsorption-desorption kinetics at significantly higher temperatures than in the accompanying study (Schneider, H. M.; Granick, S.; Smith, S. preceding paper in this issue) so as to contrast enthalpic, detachment-limited desorption with the diffusion-limited regime. The experiments involve measurements of protiopolystyrene (PS) and deuterio-cis-polyisoprene (PI) adsorbed sequentially onto oxidized silicon from a dilute solution in carbon tetrachloride at 48.5 °C. The time dependence of the displacement of PS by PI is analyzed as it depends on the PS molecular weight (M_PS). In contrast to behavior at low temperature, if M_PS is sufficiently low (<10⁵), the PS desorption is a simple exponential in elapsed time, as expected for desorption that is rate-limited by segmental surface detachment. Larger M_PS chains, however, even at this elevated temperature, display kinetics that is strongly nonexponential in time. When fit to a stretched exponential as suggested by a simple kinetic model, the power of time is β≅0.6, near the β = 0.5 limit expected for diffusion-limited desorption. In contrast to behavior at low temperature, a prominent overshoot of PI surface excess occurred for adsorption onto PS of low molecular weight. This overshoot, which is also seen for PI adsorption onto a bare surface, is tentatively attributed to initially rapid unequilibrated adsorption, followed by conformational rearrangement of adsorbed PI.