It has been shown in a previous article that melt blending of low levels of commercial poly(dimethyl siloxane) (PDMS) fluid with commercial thermoplastic polyurethanes has a significant positive impact on the coefficient of friction (CoF) and on the mechanical and wear properties of the polyurethanes. The improvements in CoF and wear resistance were expected due to surface modification of the polymer; however, the improvements in the mechanical properties were much more significant than expected. Evidence presented in the earlier publication suggests that the changes in the wear and mechanical properties are not due to surface modification alone, but are largely due to modification of the bulk by PDMS. In this article a model is presented that accounts for the observed relationship between PDMS content and the properties of the blends. It is proposed that the addition of PDMS facilitates an improved packing efficiency (antiplasticization) in the polyurethane soft domain, leading to improved material performance. Beyond an optimum PDMS concentration of 1.5-2.0%, phase separation of PDMS becomes significant, plasticization sets in, and mechanical properties then begin to diminish rapidly. This model has been rigorously investigated and has proven to be highly robust.