SPREADING OF POLYMER-MOLECULES ON SOLID-SURFACES

When a homopolymer adsorbs from dilute solution onto a solid surface i t first attaches and then maximises its number of contacts with the su bstrate by means of a spreading process. Evidence for this spreading p rocess can be obtained from experiments on the adsorption kinetics. We report on a case where the adsorption kinetics depend on the rate at which the polymer was supplied to the surface (a protein adsorbing ont o silica). Also, we discuss competitive adsorption experiments in whic h one kind of chain molecule attempts to displace another one from the surface. In these experiments, the desorption rate of the displaced s pecies reflects the spreading rate of the displacer. When this rate is slower than the supply of displacer molecules, oversaturated layers r esult that spontaneously eject polymer. We have measured the rate of t hese displacement-driven desorption processes in various cases and con clude that it depends strongly on the energy of the segment-surface bo nd. A model involving the diffusion of defects over the surface may ac count for this finding.