Rational Design of a Block Copolymer with a High Interaction Parameter

A series of poly(4-tert-butylstyrene-block-2-vinylpyridine) [P(tBuSt-b-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 1H NMR spectroscopy, size exclusion chromatography, thermal gravimetric analysis, and differential scanning calorimetry (DSC). To examine the effect of the tert-butyl group on the effective interaction parameter (χeff) relative to poly(styrene-block-2-vinylpyridine) [(P(S-b-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 χeff(T) = (67.9 ± 1.3)/T – (0.0502 ± 0.0029), which shows a higher enthalpic contribution to χeff than P(S-b-2VP) and approximately 1.5 times larger χeff. 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.