FREE SURFACES OF POLYMER BLENDS .2. EFFECTS OF MOLECULAR-WEIGHT AND APPLICATIONS TO ASYMMETRIC POLYMER BLENDS

The composition of the free surface of a binary polymer blend has been investigated in this work as a function of molecular weights, energetic parameters, and composition. The approach involved the use of a compressible mean-field lattice model that was developed in the spirit of the Scheutjens and Fleer theory of polymer solutions [J. Chem. Phys. 98, 6516 (1993)]. For symmetric polymer blends it was found that the surface segregation was driven by the degree of incompatibility of the blend, with the segregation increasing monotonically with the quantity chi1,2 r. These results are in qualitative agreement with conclusions obtained from an incompressible model for polymer blends near a surface, and suggests that the inclusion of compressibility effects do not change the predicted surface segregation significantly in these cases. In contrast, the behavior of isotopic polymer blends of disparate molecular weights, which could not be reproduced in the case of incompressible models, can be captured by the compressible model [J. Chem. Phys. 98, 4163 (1993)]. This fact therefore stresses the importance of compressibility effects in the context of segregation to free surfaces, especially for blends of disparate molecular weight polymers, and suggests that such models have to be utilized to incorporate all of the physics in these situations.