Order to Order Transition from Cylindrical to Lamellar Microdomain Upon Heating with Upper Critical Ordering Transition Block Copolymer*

For upper critical ordering transition (UCOT) block copolymers 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 at the symmetric volume fraction of total PS and PI blocks but asymmetry of PI block (τ_PI = f_PI1 /(f_PI1+f_PI2) = 0.192), in which f_PI1 and f_PI2 are the volume fractions of PI in the second and forth blocks, respectively). 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.