Polydispersity effects in poly(isoprene-b-styrene-b-ethylene oxide) triblock terpolymers
Abstract
Four hydroxyl-terminated poly (isoprene-b-styrene) diblock copolymers with comparable molecular
weights and compositions (equivalent volume fractions of polyisoprene and polystyrene) but
different polystyrene block polydispersity indices (Mw/Mn=1.06, 1.16, 1.31, 1.44) were synthesized
by anionic polymerization using either sec-butyllithium or the functional organolithium
3-triisopropylsilyloxy-1-propyllithium. Poly (ethylene oxide) (PEO) blocks were grown from the
end of each of these parent diblocks to yield four series of poly(isoprene-b-styrene-b-ethylene
oxide) (ISO) triblock terpolymers that were used to interrogate the effects of varying the
polydispersity of the middle bridged polystyrene block. In addition to the neat triblock samples, 13
multicomponent blends were prepared at four different compositions from the ISO materials
containing a polystyrene segment with Mw/Mn=1.06; these blends were used to probe the effects of
increasing the polydispersity of the terminal PEO block. The melt-phase behavior of all samples was
characterized using small-angle X-ray scattering and dynamic mechanical spectroscopy. Numerous
polydispersity-driven morphological transitions are reported, including transitions from lamellae to
core-shell gyroid, from core-shell gyroid to hexagonally packed cylinders, and from network
morphologies [either O70 (the orthorhombic Fddd network) or core-shell gyroid] to lamellae.
Domain periodicities and order-disorder transition temperatures also vary with block
polydispersities. Self-consistent field theory calculations were performed to supplement the
experimental investigations and help elucidate the molecular factors underlying the polydispersity
effects. The consequences of varying the polydispersity of the terminal PEO block are comparable
to the polydispersity effects previously reported in AB diblock copolymers. Namely, domain
periodicities increase with increasing polydispersity and domain interfaces tend to curve toward
polydisperse blocks. The changes in phase behavior that are associated with variations in the
polydispersity of the middle bridged polystyrene block, however, are not analogous to those
reported in AB diblock copolymers, as increases in this middle block polydispersity are not always
accompanied by (i) increased domain periodicities and (ii) a tendency for domain interfaces to curve
toward the polydisperse domain. These results highlight the utility of polydispersity as a tool to tune
the phase behavior of ABC block terpolymers.
Department
Description
At time of publication all authors were at the University of Minnesota. Christopher Ellison is currently Asst. Professor at the University of Texas at Austin.