Microphase Separation Behavior of Poly(4‐Vinylpyridine)‐ <i>b</i> ‐Poly(Styrene)‐ <i>b</i> ‐Poly(Ethylene Oxide)‐ <i>b</i> ‐Poly(Styrene)‐ <i>b</i> ‐Poly(4‐Vinylpyridine) Pentablock Terpolymers in Bulk

ABSTRACT Complex multiblock structures offer unexplored avenues for tailoring self‐assembly of block copolymers (BCPs). In this study, the microphase separation behavior of a series of symmetric ABCBA‐type pentablock terpolymers, poly(4‐vinylpyridine)‐ b ‐polystyrene‐ b ‐poly(ethylene oxide)‐ b ‐polystyrene‐ b ‐poly(4‐vinylpyridine) (P4VP‐ b ‐PS‐ b ‐PEO‐ b ‐PS‐ b ‐P4VP, or V y S x O 90 S x V y for short) with various lengths of PS ( x ) and P4VP ( y ) blocks in bulk were investigated. Small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM) analyses revealed that the microphase separation structures of V y S x O 90 S x V y pentablock terpolymers changed from disordered (DIS) to hexagonally packed P4VP cylinders (CYR V ) to lamellae (LAM) and finally to hexagonally packed S x O 90 S x cylinders (CYR OS ) with increasing the volume fraction of P4VP ( f P4VP ). While the corresponding S x O 90 S x and V y O 90 V y triblock copolymers exhibited DIS structures. For V y S x O 90 S x V y pentablock terpolymers, the miscibility of PS and PEO blocks made the middle S x O 90 S x triblock behave as a “one‐block” mixing unit. Due to the large χ value between P4VP and PS blocks, the effective Flory–Huggins interaction parameter ( χ eff ) between P4VP and the S x O 90 S x “one‐block” increased with the length ( x ) of PS block. With x = 6, the LAM phase can be observed in V 26 S 6 O 90 S 6 V 26 . Furthermore, the microphase separation structures of V y S x O 90 S x V y pentablock terpolymers exhibited good thermal stability up to 180°C.