Influence of ionic liquids on the chain dynamics and enthalpy relaxation of poly(methyl methacrylate)

Abstract

Imidazolium ionic liquids (ILs) with various alkyl chain lengths on the cations ([C_nmim]⁺, n = 2, 4 and 8) and different combined anions ([TFSI]⁻ and [PF₆]⁻) were blended with poly(methyl methacrylate) (PMMA), and the effects of the IL structure on the chain dynamics of PMMA were experimentally investigated by rheology and DSC measurements combined with a simulation method. The results indicate that the interaction between PMMA and ILs becomes stronger as the alkyl chain length on the imidazolium ring increases or the anion changes from [PF₆]⁻ to [TFSI]⁻. As a result, a higher critical entanglement concentration and a larger entanglement molecular weight of PMMA were found in [C₈mim][TFSI] due to the stiffer conformation. Molecular dynamics (MD) simulations further demonstrated stronger interactions between PMMA and ILs with longer cationic alkyl chain lengths or [TFSI]⁻ anions, which showed smaller Flory–Huggins interaction parameters and larger radii of gyration, R_g. However, the larger size of alkyl chains or [TFSI]⁻ anions produced a larger free volume in the system as evidenced by positron annihilation lifetime spectroscopy (PALS), which competed with the molecular interaction and dominated the segmental motion. Therefore, a lower T_g and accelerated segmental relaxation were observed. Compared to alkyl chain length, the effect of anions on the interactions between ILs and PMMA is more prominent.