Morphology and Properties of Crystalline Polyelectrolytes: Poly(alkyleneimidazolium TFSI)s with Varying Alkylene...

The morphology and crystallization behavior of imidazolium ionenes as crystalline polyelectrolytes, poly(alkylene imidazolium bis(trifluoromethanesulfonimide)) (PC_mIm-TFSI) with varying methylene units (m), were systematically investigated. PC_mIm-TFSI, with m ranging from two to six, demonstrated thermoplastic properties and formed spherulites upon melt crystallization. The melting point (T_m) and radial growth rate of the spherulites (G) exhibited a marked dependence in the value of m, with T_m decreasing by approximately 40 °C, and the maximum values of G reducing to one-tenth or less as increased by one unit. An even-odd effect was observed in T_m, where the curve connecting T_m for odd m was lower than that for even m. Raman spectral imaging indicated that both the long axes of the polymer cations and the TFSI anions were tangentially aligned within the spherulites of all examined PC_mIm-TFSI. Blends of imidazolium ionenes with varying m or counteranions (TFSI^- or Br^-) exhibited diverse higher-order morphologies owing to segregation during crystallization. Immersion of a blended film composed of PC₃Im-TFSI and PC₃Im-Br in water led to the formation of pores owing to the differential water solubility of the segregated spherulites.