ORDERED STRUCTURE IN MIXTURES OF A BLOCK COPOLYMER AND HOMOPOLYMERS .2. EFFECTS OF MOLECULAR-WEIGHTS OF HOMOPOLYMERS

Ordered microdomain structures for binary mixtures of a poly(styrene-b-isoprene) diblock copolymer (SI) with homopolystyrenes (HS) were investigated as a function of molecular weight of HS (Mhomo) under conditions where the majority of HS can be solubilized into the polystyrene microdomains. For a given fraction of HS, the interdomain distance D was found to increase with Mhomo, and a unique morphological transition from cylinders to lamellae was also found with increasing Mhomo. These results imply that HS is generally not miscible with the polystyrene (PS) block chains confined in the microdomain space, although they are chemically identical. The higher the value Mhomo, the lower the miscibility with the block PS chains. As a consequence, HS with higher Mhomo tends to cause the PS microdomains to swell less uniformly, resulting in segregation more toward the middle of the PS microdomains. Here the conformational entropy loss upon mixing plays a dominant role in the miscibility of HS and block PS and hence on solubilization of HS into PS microdomains. Thus the miscibility of the confined chains is distinctly different from that of the corresponding free chain. © 1990, American Chemical Society. All rights reserved.