Unexpected Consequences
of Block Polydispersity on
the Self-Assembly of ABA Triblock Copolymers
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Abstract
Controlled/“living” polymerizations and
tandem polymerization
methodologies offer enticing opportunities to enchain a wide variety
of monomers into new, functional block copolymer materials with unusual
physical properties. However, the use of these synthetic methods often
introduces nontrivial molecular weight polydispersities, a type of
chain length heterogeneity, into one or more of the copolymer blocks.
While the self-assembly behavior of monodisperse AB diblock and ABA
triblock copolymers is both experimentally and theoretically well
understood, the effects of broadening the copolymer molecular weight
distribution on block copolymer phase behavior are less well-explored.
We report the melt-phase self-assembly behavior of SBS triblock copolymers
(S = poly(styrene) and B = poly(1,4-butadiene)) comprised of a broad
polydispersity B block (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> = 1.73–2.00) flanked by relatively narrow dispersity
S blocks (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub> = 1.09–1.36), in order to identify the effects of chain length
heterogeneity on block copolymer self-assembly. Based on synchrotron
small-angle X-ray scattering and transmission electron microscopy
analyses of seventeen SBS triblock copolymers with poly(1,4-butadiene)
volume fractions 0.27 ≤ <i>f</i><sub>B</sub> ≤
0.82, we demonstrate that polydisperse SBS triblock copolymers self-assemble
into periodic structures with unexpectedly enhanced stabilities that
greatly exceed those of equivalent monodisperse copolymers. The unprecedented
stabilities of these polydisperse microphase separated melts are discussed
in the context of a complete morphology diagram for this system, which
demonstrates that narrow dispersity copolymers are not required for
periodic nanoscale assembly