Tetrablock Copolymers at the Symmetric Overall Volume Fraction.*

Block copolymers have been extensively investigated because of their various nanostructures. For a simple diblock copolymer, hexagonally packed cylindrical microdomains have been found with volume fractions of one block (f_A) having 0.2~0.35, while gyroid microdomains were observed at f_A having ~0.35. So, gyroid and cylindrical microdomain could not prepare pore volume larger than this value (~0.35) in diblock copolymers, because lamellar microdomains are expected. Here, we investigated morphology transitions of linear tetrablock copolymers of polystyrene-block-polyisoprene-block-polystyrene-block-polyisoprene (S₁I₁S₂I₂) by varying volume fraction of PI1 block (f_PI1), while maintaining the symmetric volume fraction of total PS blocks and PI blocks (f_PS1+f_PS2: f_PI1+f_PI2 = 1:1). An interesting sequence of morphology transitions was observed as f_PI1 was increased: lamellae→asymmetric lamellae→hexagonally packed PI-cylinders→double gyroid with PI-network domains→short-period lamellae. It is particularly interesting that cylindrical and gyroid morphologies were observed in linear block copolymers with symmetric overall volume fraction.