SURFACE EXCHANGE KINETICS OF CHEMICALLY DIFFERENT POLYMERS

The kinetics of exchange between an adsorbed polymer and a chemically different, more strongly adsorbing polymeric displacer was investigate d by means of an optical reflectometry technique, using an impinging j et flow cell. Three polymer/displacer pairs were studied: polystyrene( PS)/poly(tetrahydrofuran) (PTHF), poly(butyl methacrylate) (PBMA)/PTHF , and PS/PBMA, all in decalin solutions. We find that both PS and fres hly adsorbed PBMA are displaced by PTHF at a rate which is entirely de termined by the supply of displacer through the solution. The surface processes in these cases are thus so rapid that they do not affect the overall exchange kinetics. A PBMA adsorbed layer which has been aged for 20 h is displaced slightly more slowly by PTHF than a fresh one, b ut the aging effect is minor. In contrast with the above, the displace ment of PS by PBMA is much slower and to a large extent determined by surface processes. The exchange of low molar mass adsorbed PS (M = 9 k g/mol) by PBMA in pure decalin proceeds in two steps which are separat ed by a distinct break in the kinetic curve: a rapid initial desorptio n of PS upon attachment of PBMA, followed by a much slower desorption of PS at constant PBMA coverage. For high molar mass PS the displaceme nt by PBMA in pure decalin was too slow to be measurable. However, upo n addition of low molar mass displacers like toluene or ethyl acetate, the displacement of PS becomes considerably faster. The kinetic curve s have again two distinct parts, related to fast desorption and slow r earrangements within the adsorbed layer, respectively. The second step (rearrangement process) could be shown to be a true surface process, that did not respond to changes in the free polymer concentration. We argue on the basis of these data that the segmental adsorption energy plays an important role in the kinetics but also that this factor cann ot explain the large difference between PBMA and PTHF as displacers. W e suggest that dynamic chain flexibility is another important factor.