Effect of Composition and Chain Length on $\chi $ Parameter of Polyolefin Blends: A Molecular Dynamics Study

Polymer blends exhibit complex phase behavior which is governed by several factors including temperature, composition and molecular weight of components. The thermodynamics of polymer blends is commonly described using the $\chi $ parameter. While variety of experimental studies exist on identifying the factors affecting the $\chi $ parameter, a detailed molecular scale understanding of these is a topic of current research. We have studied the effect of blend composition and chain length on $\chi $ parameter values for two model polyolefin blends. The blends studied are: polyisobutylene (PIB)/polybutadiene (PBD) and polyethylene (PE)/atactic polypropylene (aPP). Molecular dynamics simulations in combination with the integral equation theory formalism proposed by Schweizer and Curro [Journal of Chemical Physics, 91, 5059 (1989)] are used to determine the $\chi $ parameter for these systems and thereby study the effect of blend composition and chain length. The resulting $\chi $ parameter values are explained in terms of the molecular structure of these polymeric systems.