Pendant Groups in the Majority Domain of Aliphatic Multiblock Copolymers Prohibit the Double Gyroid Morphology

We report the self-assembly behavior of strictly alternating multiblock copolymers with a sulfonated polar block and various alkyl blocks. The alkyl blocks are linear with x carbons (PESMx), a mixture of linear and sterically hindered fatty acid dimer blocks (PESNax-20%Pripol), or sterically hindered with an isopropyl group (PESMx-iPr). These new multiblock copolymers have blocks with ≤ 20 backbone bonds and a large interaction parameter. Polymers with 20% Pripol blocks exhibit reduced crystallinity relative to the multiblock copolymers with linear alkyl blocks, while polymers with 100% iPr groups are fully amorphous. While PESMx polymers form the double gyroid morphology above the melting temperature over a broad range of polar volume fractions, the new PESNax-20%Pripol and PESMx-iPr multiblock copolymers exclusively form hexagonal morphologies in the melt. We attribute the absence of a stable double gyroid morphology to prohibitively high degrees of chain stretching of the alkyl blocks. This study highlights the importance of packing frustration in the majority domain on the self-assembly behavior in high-χ, low-N multiblock copolymers.