Morphology Transitions of Linear A₁B₁A₂B₂ Tetrablock Copolymers at Symmetric Overall Volume Fraction

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 (L) → asymmetric lamellae (aL) → hexagonally packed PI-cylinders (C_PI) → double gyroid with PI-network domains (G_PI) → short-period lamellae (sL). The domain spacing of sL was nearly half that of L, while aL had asymmetric lamellar width of PS and PI microdomains. It is particularly interesting that cylindrical and gyroid morphologies were observed in linear block copolymers with symmetric overall volume fraction at intermediate segregation. The experimentally observed morphologies are in good agreement with the predicted equilibrium phases by self-consistent field theory (SCFT).