EXCHANGE KINETICS BETWEEN THE ADSORBED STATE AND FREE SOLUTION - POLY(METHYL METHACRYLATE) IN CARBON-TETRACHLORIDE

Adsorption and desorption rates at high surface coverage are reported for poly(methyl methacrylate) (PMMA) adsorbed from dilute carbon tetrachloride solution (1 mg-mL-1) onto a single silicon oxide substrate at 30.0 °C. The method of measurement was infrared spectroscopy in attenuated total reflection. The polymer fractions studied were deuterio (d-PMMA) and protio (h-PMMA) with matched degrees of polymerization Nw ≈ 570 and 640, respectively. No adsorption isotope effect was expected, nor noted experimentally. The rapid adsorption of d-PMMA onto the initially bare surface was followed for 1 h, at which time the ambient solution was changed either to a solution of h-PMMA or to pure CCl4. Desorption of d-PMMA after this time was linear in the elapsed time over 6 h, with rate on the order of 1% h-1, and this rate was the same into a solution of h-PMMA as into pure CCI4 at 30 and 40 °C. The infrared bound fraction, i.e., the fraction of segments in direct contact with the surface, was also measured. During adsorption onto initially bare surfaces, the average bound fraction took the constant level p ≈ 0.25 throughout the experiments. However, for h-PMMA penetrating the preadsorbed d-PMMA layer after this polymer was allowed to adsorb for 1 h, the average bound fraction was p ≈ 0.1 at elapsed times of minutes but rose to p ≈0.2 after 6 h. This indicates that incoming chains underwent spreading at the surface. Control experiments with stearic acid in CCI4 showed that adsorption-desorption kinetics in this system were more rapid than the experimental time scale. This suggests that despite slow rearrangements of the overall chains, rearrangements may have been rapid at the level of individual segments. The surface excess adsorbed amount in this PMMA system was dominated by sluggish relaxation and, by inference, by metastable nonequilibrium states. © 1990, American Chemical Society. All rights reserved.