SWELLING A LAMELLAR DIBLOCK COPOLYMER WITH HOMOPOLYMER - INFLUENCES OF HOMOPOLYMER CONCENTRATION AND MOLECULAR-WEIGHT

The lamellar morphology in 14 binary blends of a lamellar diblock copolymer and various homopolymers was investigated as a function of homopolymer concentration and homopolymer molecular weight. The diblock copolymer was a poly(styrene-b-isoprene) diblock copolymer with a total molecular weight of 48 700 and a polystyrene block molecular weight of 26 600. The homopolystyrenes ranged in molecular weight from 2600 to 36 700 and were blended with the diblock copolymer up to a concentration of 24 wt %. A constant glass transition temperature for the PI microdomains established the presence of pure microdomains. Samples were characterized by small-angle X-ray scattering and transmission electron microscopy. As the homopolymer concentration increased, the PS layer thickness increased, the PI layer thickness decreased, and the area per junction increased. Increasing the homopolymer molecular weight at a fixed concentration increased the PS and PI layer thicknesses and decreased the area per junction. The symmetry of swelling in the PS layer was quantitatively evaluated by the ratio of the axial expansion to the lateral expansion. The swelling in the PS layer became dramatically asymmetric with an increase in the homopolymer concentration and/or the homopolymer molecular weight. The onset of asymmetric swelling is consistent with a proposed homopolymer concentration profile with a maximum in the center of the PS layer.