KINETIC TRAPS IN POLYMER ADSORPTION .1. POLYSTYRENE DISPLACED BY POLYISOPRENE AT 12-DEGREES-C

Sequential adsorption experiments were performed to determine the kine tics of polymer displacement. Protiopolystyrene (PS) and deuterio-cis- polyisoprene (PI) were allowed to absorb sequentially at 12-degrees-C, from dilute solution in carbon tetrachoride, onto a single surface of oxidized silicon. Whereas the net surface excess equilibrated rapidly , the individual populations of PS and PI equilibrated far more slowly . The adsorption of PI was enthalpically favored, but PS arrived first at the surface, so initially-adsorbed PS was displaced. This displace ment was strongly nonexponential in time. When desorption kinetics wer e fitted to the stretched-exponential function as suggested by theoret ical considerations, the power of time, beta, increased from beta cong ruent-to 0.2 for the PS chains of lowest molecular weight (M(PS)) to a limiting value, beta congruent-to 1/2, for the highest M(PS). The tim e constant for desorption, tau(off), depended only weakly on M(PS). Th e minimal molecular weight dependence of tau(off) in this PS-PI system contrasts with our findings in the PS-PMMA [poly(methyl methacrylate) ] system and is tentatively attributed to the fact that less PI adsorb ed at any instant in time, the higher the M(PS). However, the pronounc ed nonexponential desorption confirms previous experience, as well as expectation based on a simple kinetic model. This gives corroborative evidence that beta congruent-to 1/2 is the limiting value to be expect ed of simple diffusion-limited desorption.