Ion Specific, Odd-Even Glass Transition Temperatures in Precise Network Polymerized Ionic Liquids

Relationships between T_g, fragility, nanostructure, electrostatics and conductivity are investigated in precise network polymerized ionic liquids (n-PILs). These n-PILs contain an exact number of carbon atoms (Cx) between charges in the backbone and can be exchanged to various anionic forms. The T_g exhibits an odd-even effect with the non-spherical, bulky bis(trifluoromethane sulfonamide) (TFSI) counter ion with a maximum jump of 45 K between C4 and C5 networks. In contrast, the same n-PIL networks with the smaller and spherical BF₄ anion show no odd-even T_g effect. Small angle X-ray scattering of TFSI networks suggests that ionic aggregation is not the primary cause of the odd-even effect. The amorphous halo exhibits a weak odd-even dependence indicating a minor role of backbone-backbone correlations and packing on the observed effects. However, odd-even effects appear to be largely dynamical in n-PILs with only minor variations in structure. Due to the large changes in T_g, the room temperature ionic conductivities of the TFSI n-PILs exhibit odd-even effects greater than an order of magnitude between adjacent Cx networks. Finally, dynamic fragility shows odd-even fluctuations with higher fragility corresponds to lower T_g, opposite to what is typically observed.