Symmetric Poly(ethylene oxide-b-styrene-b-isoprene) Triblock Copolymers: Synthesis, Characterization, and Self-Assembly in Bulk and Thin Film

The synthesis, characterization, and self-assembly of a series of linear poly(ethylene-b-styrene-b-isoprene) (PEO-PS-PI) triblock copolymers containing nearly equal volume fractions of PEO and PI (f(PEO) approximate to f(PI)) and various fractions of the middle PS block (70.8 vol % <= f(PS) <= 75.6 vol %) is reported. A range of azide-functionalized poly(ethylene oxide-b-styrene) diblock copolymers were prepared by atom transfer radical polymerization (ATRP) of styrene using the same batch of PEO-macroinitiator, followed by azide-functionalization. Monohydroxyl-terminated poly(cis-1,4-isoprene) was first alkyne-functionalized and then sequentially attached to azide-functionalized PEO-PS via copper(I)-catalyzed azide-alkyne cycloaddition reaction, producing PEO-PS-PI triblock copolymers with low dispersity. Bulk samples of each linear triblock copolymer reveal the formation of a binary microdomain structure in which a PS and PI domains mix to form a uniform matrix for spherical PEO microdomains, and the PRO microdomains are arranged in a BCC lattice. Additionally, the thin film ordering of these triblock copolymers was investigated using a high-humidity solvent annealing process, both with and without low concentrations of a lithium salt. The morphology was analyzed by atomic force microscopy and GISAXS, revealing layers of spherical PEO domains arranged with in-plane hexagonal symmetry. The inclusion of salt increased both the size and periodicity of PEO domains.