Bulk and Thin Film Morphological Behavior of Broad Dispersity Poly(styrene-<i>b-</i>methyl methacrylate) Diblock
Copolymers
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Abstract
We
describe the morphological implications of broad molecular weight
dispersity on the bulk and thin film self-assembly behavior of seven
model poly(styrene-<i>block</i>-methyl methacrylate) (SM)
diblock copolymers. Derived from sequential nitroxide-mediated polymerizations,
these unimodal diblock copolymers are comprised of narrow dispersity
S blocks (<i>Đ</i> ≤ 1.14) and broad dispersity
M blocks (<i>Đ</i> ∼ 1.7) with total molecular
weights <i>M</i><sub>n,total</sub> = 29.2–42.9 kg/mol
and M volume fractions <i>f</i><sub>M</sub> = 0.35–0.63.
Small-angle X-ray scattering (SAXS) and transmission electron microscopy
(TEM) analyses demonstrate that these diblock copolymers microphase
separate into lamellar and cylindrical morphologies with substantially
larger microdomain spacings at lower overall molecular weights as
compared to their narrow dispersity analogues. The observed microphase-separated
melt stabilization is also accompanied by a substantial shift in the
lamellar phase composition window to higher values of <i>f</i><sub>M</sub>. In thin films, these polydisperse copolymers form perpendicularly
oriented morphologies with modest degrees of lateral order on substrates
functionalized with P(S-<i>ran</i>-MMA) neutral polymer
brush layers