Miscibility and morphology of AB/C-type blends composed of block copolymer and homopolymer or random coploymer, 2. Blends with random copolymer effect

This work is an extension of the concept of miscibility window in homopolymer/random copolymer blends to blends containing block copolymers. The miscibility and morphology of THF-cast blends composed of the block copolymer polyisoprene-block-poly(methyl methacrylate) (Pl-b-PMMA) and the random copolymer poly(styrene-ran-acrylonitrile) (SAN) (22 wt.-% AN) has been studied by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). Most blends show miscibility between PMMA blocks and SAN, presenting a homogeneous microdomain structure. Coupling of macrophase separation occurs only when the ratio of molecular weights of SAN to the PMMA bock is very large and SAN becomes the dominant component. The excellent miscibility in the block copolymer/random copolymer blends can be understood by considering that the monomer unit composition of SAN is within the miscibility window in PMMA/SAN blends. The main driving force for the miscibility is repulsion between S and AN units rather than hydrogen bonding between SAN and PMMA, since a proton-acceptor type solvent does not show any effect on the miscibility in the blends. Apparent broadening of the glass transition of the ‘hard phase’ is observed and explained with the aid of the segment density gradient model.