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

The double gyroid morphology (DG) is associated with improved performance such as enhanced mechanical properties and ionic conductivities. We previously observed this morphology in strictly alternating multiblock copolymers comprised of a short sodium-sulfonated polar block containing 4 carbons and a linear alkyl block of precisely 12 to 23 carbons (PES4Nax). These materials crystallize into layers at room temperature and transition to the DG upon melting, suggesting that the crystallization of the alkyl blocks prevents the DG from forming. In this work, multiblock copolymers (PES4Nax-R) were synthesized to disrupt crystallinity by either (1) adding an isopropyl group to the center of each alkyl block or (2) replacing 20% of the linear alkyl blocks with branched alkyls. These modifications to the alkyl block successfully reduce or prevent crystallization. In PES4Nax, the DG was observed above T_m when the volume fraction of the polar block ranged from 0.27 to 0.41. Although the PES4Nax-R materials are in the same composition range (0.29 to 0.39), X-ray scattering experiments show the DG is absent. We interpret this result by considering the amount of chain stretching required to form the DG using a medial packing model. Compared to PES4Nax, PES4Nax-R significantly increases the degree of chain stretching required to form the DG. These results highlight the importance of chain stretching in the majority domain on the stability of the DG in multiblock copolymers.