Rational Design of a Block Copolymer with a High Interaction
Parameter
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Abstract
A series of poly(4-<i>tert</i>-butylstyrene-<i>block</i>-2-vinylpyridine) [P(tBuSt-<i>b</i>-2VP)] block copolymers
(BCPs) with varying volume fractions, molecular weights, and narrow
dispersities were synthesized from the commercially available monomers
by sequential living anionic polymerization. The copolymers were thoroughly
characterized by <sup>1</sup>H NMR spectroscopy, size exclusion chromatography,
thermal gravimetric analysis, and differential scanning calorimetry
(DSC). To examine the effect of the <i>tert</i>-butyl group
on the effective interaction parameter (χ<sub>eff</sub>) relative
to poly(styrene-<i>block</i>-2-vinylpyridine) [(P(S-<i>b</i>-2VP)], the self-assembly of symmetric copolymers was studied
by small-angle X-ray scattering (SAXS) and transmission electron microscopy.
Order-to-disorder transitions (ODTs) were identified by both DSC and
SAXS on five copolymers, to define the equation χ<sub>eff</sub>(<i>T</i>) = (67.9 ± 1.3)/<i>T</i> –
(0.0502 ± 0.0029), which shows a higher enthalpic contribution
to χ<sub>eff</sub> than P(S-<i>b</i>-2VP) and approximately
1.5 times larger χ<sub>eff</sub>. This enables a minimum full
pitch of 9.6 nm for the symmetric copolymers. Asymmetric copolymers
were also examined for bulk self-assembly by SAXS and TEM, exploring
both P2VP and PtBuSt cylindrical phases with diameters as small as
6 nm. Feasibility of thin film assembly by thermal annealing was demonstrated
for a P2VP cylinder forming BCPs to yield parallel cylinders that
were seeded with Pt ions and etched to yield Pt nanowires with diameters
as small as 5.8 nm
