Thermal behaviors and DC conductivity of linker spacer influence on polymerized ionic liquids with bis-imidazolium embedding in the backbone chain

A series of polymerized ionic liquids (PILs) with bis-imidazolium embedding into the backbone was designed to study the effect of linker spacers (C-2, C-4, C-6, and C-8) on their ionic conductivity and thermal behaviors. All PILs were prepared by step-growth polymerization using a modified Menshutkin reaction, where 1,1′-(1,4-butadiyl) bisimidazole (Bbim), was structurally modified by a series of bis-bromoacetates (1,1-(1,n-alkanediyl) bis(bromoacetate) (n = 2, 4, 6, and 8 correspond to 1,2-EBA, 1,4-BBA, 1,6-HBA, and 1,8-OBA, respectively). Interestingly, the density function theory (DFT)-derived complexation energy exhibits a correlation with spacer length, decreasing as the linker spacer increases. Moreover, shorter linker chains notably elevate the glass transition temperature (Tg). The room temperature dc-conductivity of C-8 is the highest (2.2 × 10–5 S/cm) among PILs with shorter linker chain lengths. These results indicate that PILs 1–4 have the potential to yield substantial ionic conductivity in a chain of flexible segments (low Tg), while exhibiting a decrease in complexation energy.