Phase Behavior of Mixtures of Block Copolymer and Homopolymers in Thin Films and Bulk

Mixtures of polystyrene-block-poly(methyl methacrylate) (PS−PMMA) having PMMA cylindrical microdomains with homopolymers of PMMA and poly(ethylene oxide) (PEO) were used to study the effect of the interaction between homopolymers confined to the cylindrical microdomains and the PS matrix on the microdomain spacing of the mixtures. The microdomains in all thin films are oriented perpendicular to the film surface. It was found that the miscibility between PMMA homopolymer and PMMA block in thin film was enhanced compared with that in bulk at a given molecular weight of PMMA homopolymer. Also, the PMMA homopolymer chains in thin films were more localized to the center of PMMA microdomains than in the bulk, which results in a larger increase of the lattice spacing (D) in the former. D increased rapidly at low volume fractions of PMMA homopolymer, but it approached a saturated value as the content of PMMA increased. The increase of D with increasing molecular weight of PMMA homopolymer was saturated at a certain molecular weight. For mixtures of PS−PMMA with PEO, changes in D were similar to those seen with PMMA, except that the saturation of D was reached at a much lower molecular weight compared with PMMA. This is attributed to the strong repulsion between PEO and PS that forces a stronger localization of PEO chains to the center of the PMMA microdomains. The degree of localization of the homopolymer inside the cylindrical microdomains for all mixtures is discussed in term of increase of D.