Abstract
An anomalous plasticizing effect was observed in polymer/ionic liquid (IL) solutions by applying broad range of rheological techniques. Poly(ethylene oxide)(PEO)/IL solutions exhibit stronger dynamic temperature dependence than pure PEO, which is in conflict with the knowledge that lower-Tg solvent increases the fractional free volume. For poly(methy methacrylate)(PMMA)/IL solutions, the subtle anomaly was detected from the fact that the effective glass transition temperature Tg,eff of PMMA in IL is higher than the prediction of the self-concentration model, while in conventional polymer solutions, Tg,eff follows the original Fox equation. Observations in both solutions reveal retarded segmental dynamics, consistent with a recent simulation result (Macromolecules, 2018, 51, 5336) that polymer chains wrap the IL cations by hydrogen bonding interactions and the segmental unwrapping delays their relaxation. Start-up shear and nonlinear stress relaxation tests of polymer/IL solutions follow a universal nonlinear rheological behavior as polymer melts and solutions, indicating that the segment-cation interaction is not strong enough to influence the nonlinear chain orientation and stretch. The present work may arouse the further theoretical, experimental, and simulation interests in interpreting the effect of complex polymer-IL interaction on the dynamics of polymer/IL solutions.
Highlights
Polymer/ionic liquid (IL) mixtures have received great attention as novel materials with versatile applications, such as plastic electronics [1,2], gas separation membranes [3,4,5,6,7,8], luminescent and electrochromic devices [9,10,11], high-capacitance gate dielectric [12,13], fuel cells [14], and solutions for cellulose spinning [15]
For Poly(ethylene oxide) (PEO)/IL mixtures, small-angle neutron scattering measurements were performed in a variety of ILs such as 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4 ]) [17,18], ethylammonium nitrate (EAN) [20,21], lithium tetraglyme bis(trifluoromethanesulfonyl)amide ([Li(G4)][Tf2 N]) [22], and propylammonium nitrate (PAN) [21]
Linear rheological measurement was conducted based on dynamic frequency sweeps, while nonlinear rheological measurements include both start-up shear test, which measures the transient viscosity upon applying a shear rate, and the step-strain test, which measures the stress relaxation after imposing a step strain deformation
Summary
Polymer/ionic liquid (IL) mixtures have received great attention as novel materials with versatile applications, such as plastic electronics [1,2], gas separation membranes [3,4,5,6,7,8], luminescent and electrochromic devices [9,10,11], high-capacitance gate dielectric [12,13], fuel cells [14], and solutions for cellulose spinning [15]. Representative illustration showing that chain segments (PEO) wrap around the cations of ionic liquids ([Bmim][PF6 ]) due to hydrogen bonding interaction. By analyzing the dynamic temperature dependence, the parameters, including the fractional free volume, the activation energy, and the effective glass transition temperature, can be extracted, which could quantitatively measure the influence of IL molecules on the segmental dynamics Another question underlying the polymer/IL solutions is whether the interaction could influence the nonlinear rheological behavior. The strong plasticizing effect makes it difficult to directly detect the subtle interaction effect through aT In this case, we resort to the concept of effective glass transition temperature Tg,eff , a measure of segmental dynamics in mixtures.
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