Phase behavior of polyisoprene-poly(butylene oxide) and poly(ethylene-alt-propylene)-poly(butylene oxide) block copolymers

We prepared two sets of model polyisoprene-poly(butylene oxide) (PI-b-PBO) and poly(ethylene-alt-propylene)-PBO (PEP-b-PBO) diblock copolymers (molecular weights: 20000 ∼ 60000 g/mol and polydispersity indices <1.1) through anionic polymerization and catalytic hydrogenation. The phase behavior was investigated by rheology and small angle X-ray scattering. A relatively high-molecular-weight (60000 g/mol) PI-b-PBO is still disordered at room temperature. Four ordered morphologies, i.e., lamellae (LAM), hexagonally packed cylinders (HEX), gyroid and hexagonally perforated layers (HPL), were found in the PEP-b-PBO copolymers at PBO volume fractions of 0.49-0.62. Notably, a highly ordered HPL structure was observed in a PEP-b-PBO sample without being subjected to any shear alignment. Very long periods of annealing experiments (up to 16 months) were performed at room temperature to study the complex phase transitions among gyroid, HPL and HEX/LAM in a PEP-b-PBO copolymer.