Dynamic behavior of poly(ethylene oxide) in dicationic ionic liquids with varying oligo(ethylene oxide) linker lengths

In this study, a series of dicationic ionic liquids ([EO n (mim) 2 ][TFSI] 2 ) with two charged imidazolium rings bridged by varying oligo(ethylene oxide) (EO) linker lengths and bis(trifluoromethylsulfonyl)imide as anions were successfully synthesized. The aggregation of [EO n (mim) 2 ][TFSI] 2 and their influence on the dynamic behavior of poly(ethylene oxide) (PEO) chain were systematically investigated by combining simulation and experimental methods. For [EO n (mim) 2 ][TFSI] 2 , there is a transition from unfolded to majority folded state depending on linker length, which could further affect the hydrogen-bonding (H-bonding) types between [EO n (mim) 2 ][TFSI] 2 and PEO. In the PEO/[EO n (mim) 2 ][TFSI] 2 mixtures, the introduction of [EO 1 (mim) 2 ][TFSI] 2 results in unexpectedly higher glass transition temperature ( T g ) and flow activation energy ( E a ) compared to neat PEO. As the linker length increases, both T g and E a exhibit an interesting decreasing trend followed by an increase, which is governed by the co-effects of [EO n (mim) 2 ][TFSI] 2 aggregation, H-bonding, and plasticization. [EO n (mim) 2 ][TFSI] 2 with shorter linker at unfolded state tend to form physical crosslinks among PEO chains through inter -chain H-bonding, causing higher T g and E a , while those with longer EO linker at folded state could more likely form intra -chain H-bonding. Besides, the PEO/[EO n (mim) 2 ][TFSI] 2 with longer EO linker ( n  > 3) exhibit higher heterogeneity order parameter (HOP) values, which implies larger [EO n (mim) 2 ][TFSI] 2 cluster and greater heterogeneity contributing to their higher crystallinity.