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

The morphology and crystallization behavior of imidazolium ionenes as crystalline polyelectrolytes, poly(alkylene imidazolium bis(trifluoromethanesulfonimide)) (PC_<i>m</i>Im-TFSI) with varying methylene units (<i>m</i>), were systematically investigated. PC_<i>m</i>Im-TFSI, with <i>m</i> ranging from two to six, demonstrated thermoplastic properties and formed spherulites upon melt crystallization. The melting point (<i>T</i>_m) and radial growth rate of the spherulites (<i>G</i>) exhibited a marked dependence in the value of <i>m</i>, with <i>T</i>_m decreasing by approximately 40 °C, and the maximum values of <i>G</i> reducing to one-tenth or less as increased by one unit. An even-odd effect was observed in <i>T</i>_m, where the curve connecting <i>T</i>_m for odd <i>m</i> was lower than that for even <i>m</i>. 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_<i>m</i>Im-TFSI. Blends of imidazolium ionenes with varying <i>m</i> 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.