Experimental Realization of Unequal-Coordination Hexagonal Cylinder Packing in ABCBA Pentablock Terpolymers.

A series of poly(4-vinylpyridine)-b-polystyrene-b-polydimethylsiloxane-b-polystyrene-b-poly(4-vinylpyridine) (P4VP-b-PS-b-PDMS-b-PS-b-P4VP, denoted as VxS104D69S104Vx) pentablock terpolymers with different length x (31, 46, 99, 145) of P4VP blocks were synthesized via two-step reversible addition-fragmentation chain transfer (RAFT) polymerization. Their bulk self-assembly behavior was investigated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). With increasing P4VP block length, the equilibrium morphology evolved successively from tetragonally packed alternating P4VP/PDMS cylinders in the PS matrix (TETVD) to hexagonally packed alternating P4VP/PDMS cylinders with unequal coordination numbers (HEXVD), then to lamellae containing discontinuous PDMS subdomains (LAM), and finally to coexisting LAM and PDMS/PS core-shell hexagonally packed cylinders (CSHSD). Notably, the HEXVD phase represents, to our knowledge, the first experimental realization of a hexagonal cylinder phase with unequal coordination numbers in an ABCBA pentablock terpolymer system. The TETVD to HEXVD transition is governed by the compositional asymmetry and the topological constraint imposed by the doubly tethered middle PDMS block. These results demonstrate that tuning the terminal-block length in ABCBA pentablock terpolymers provides an effective route to unconventional three-phase nanostructures.