Entropic Segregation of Polydisperse Polymers Near Obstacles

摘要

In deference to simplicity, discussions here are limited to systems containing linear, unbranched polymer molecules, although in principle branched systems can also be treated. It will be shown that longer molecules near an obstacle suffer a relatively greater conformational restriction than do shorter molecules. Hence in a system containing molecules of different lengths, there exists a thermodynamic preference for regions near impeding surfaces to be richer in shorter molecules than the bulk of the material. Such regions would display a lower degree of molecular entanglement which fact would lead to a corresponding decrease in cohesive strength for the interfacial region and thus a reduced strength of the adhesion. Here the thermodynamic limit of this mechanism is explored to determine how much enrichment of surface areas with short molecules can occur. The report may have application to a number of important problems involving the adhesion of polymers to various surfaces. The entropic 'force' described would be expected to cause a relatively weak surface layer at the boundary of an adhesive which might result in so-called 'adhesive failure.' Heretofore such failures have been assumed to result from some contamination or improper surface preparation. The work shows that the failure may well be caused by a weak boundary layer in the polymeric adhesive.