Cylindrical to Lamellar Microdomain Order to Order Transition upon Heating for Upper Critical Ordering Transition Block Copolymer*

For block copolymers with upper critical ordering transition (UCOT) where χ decreases with increasing temperature, the sequence of order to order transition (OOT) upon heating of AB diblock and A₁BA₂ triblock copolymers with UCOT is as follows: lamellae to gyroids to cylinders to spheres before order to disorder transition. However, the opposite direction OOT is impossible in UCOT block copolymers.
Here, we investigated the OOT of polystyrene-block-polyisoprene-block-polystyrene-block-polyisoprene (S₁I₁S₂I₂) linear tetrablock copolymer. Very interestingly, this block copolymer showed cylindrical to lamellar microdomains transition upon heating. This behaviour is the exactly opposite to commonly observed OOT for linear AB diblock and A₁BA₂ triblock copolymers with upper critical ordering transition, where lamellar microdomains have been transformed to cylindrical microdomains upon heating. This interesting transition sequence is due to the chain architecture of linear tetrablock copolymer with two PS and two PI blocks.