Article pubs.acs.org/Langmuir
Rheological Changes and Kinetics of Water Uptake by Poly(ionic liquid)-Based Thin Films Tânia M. Benedetti* and Roberto M. Torresi* Instituto de Química, Universidade de São Paulo (USP), CP 26077, 05513-970, São Paulo-SP, Brazil S Supporting Information *
ABSTRACT: Water uptake by thin films composed of the poly(ionic liquid) poly[diallyldimethylammonium bis(trifluoromethanesulfonyl)imide] (PDDATf2N) and the ionic liquid N,N-butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1.4Tf2N) was studied with a quartz crystal microbalance with dissipation. The data obtained for films with different compositions during the passage of dry and wet N2 flow through the films were simulated with the Kevin−Voigt viscoelastic model for assessment of the mass of uptake water as well as the viscoelastic parameters. Our results show that the ionic liquid acts as a plasticizer, reducing the rigidity of the film and decreasing the capacity of water uptake. Introduction to a Li salt (LiTf2N) increases the water uptake capacity and also affects both elastic and viscous parameters due to aggregation among the ions from the ionic liquid and Li+. However, due to the preferable interaction of Li+ ions with water molecules, these aggregates are broken when the film is hydrated. In short, the presence of water in such films affects their mechanical properties, which can reflect in their performances as solid state electrolytes and ion-conducting membranes for electrochemical applications.
1. INTRODUCTION In recent years, there has been intense study of materials that combine desirable mechanical and ion transport properties for application as solid electrolytes and ion-conducting membranes. Examples of these materials include mixtures of polymers with ionic liquids (ILs), forming so-called ionogels,1−4 and block copolymers, in which one block provides structural stability while the other block guarantees ionic conductivity.5−7 Other promising materials are poly(ionic liquids) (POILs), which are polymers containing one or more ionic group from ILs associated with its monomeric unit.8,9 Although the reduced ionic conductivity when compared with the IL, caused by the immobilization of one of the ions in the polymeric chain, these are promising materials as many researchers have recently reported as solid electrolytes in Li-ion batteries12,13 and freestanding ion-conducting membranes5,14,15 that could be employed, for example, in fuel cells and Li−air batteries. Moreover, as observed with ILs, the properties of these compounds can be tuned by changing both the attached and mobile ionic groups.10 Recent contributions have demonstrated that properties such as ionic conductivity and viscosity of ILs are strongly affected by a small amount of water content,16 which is related to their performance as electrolytes. Moreover, the introduction of Li salt increases the capacity of water absorption.17 For polyelectrolytes, in addition to transport properties, water uptake can also affect their viscoelastic properties. In some applications, such as fuel-cell membranes, ion conduction must occur under humid conditions, and control of the amount of © 2013 American Chemical Society
water in the polymer is extremely important for maintaining structural stability and to tune the ionic conductivity of such materials.5,14 The amount of absorbed water can be controlled, for example, by changing the ratio of the membrane components. Because of its high resolution, versatility, and fast response, the quartz crystal microbalance with dissipation monitoring (QCM-D) has been preferably used among several other techniques to study the effect of the hydration of polymers on their properties, such as spectroscopic ellipsometry, atomic force microscopy, surface plasmon resonance, and optical waveguide lightmode spectroscopy.18 In the QCM-D technique, besides the oscillation frequency ( f), the dissipation energy (D) of the freely oscillating quartz crystal is also monitored, allowing for the accurate measurement of mass when the deposited film is not rigid as well as information about its viscoelastic properties by applying the proper viscoelastic models. The models are based on simulations with elastic springs and viscous dashpot circuits and include the Maxwell model, which is applied to predominantly liquid-like materials, and the Kevin−Voigt model for predominantly elastic materials. Some contributions report studies of ILs in a QCM-D; thin films of ILs deposited over a quartz crystal have been studied by several research groups as selective organic solvent vapor Received: October 8, 2013 Revised: November 29, 2013 Published: November 29, 2013 15589
dx.doi.org/10.1021/la4038809 | Langmuir 2013, 29, 15589−15595
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sensors.19−24 Very recently, a QCM-D study of water vapor absorption was also reported,25 where it was shown that the responses of f and D to water absorption can be predominantly driven by mass increase or viscosity decrease, depending on the IL properties. Concerning POILs, the ones based on vinylimidazolium with different anions have been deposited by casting over quartz crystal substrates and adsorption of CO2 was studied in a QCM (with no dissipation).26 Nevertheless, analyses in terms of f and D change were only accomplished qualitatively, with no assessment of the viscoelastic models to obtain the rheological parameters that could more accurately describe how these absorbed substances affect the material’s mechanical properties. In addition, the kinetics of the absorption process can be studied from the mass values given by modeling the QCM-D output data. In this work, a systematic study of moisture uptake by POILbased membranes was conducted in a QCM-D. A known POIL, poly[diallyldimethylammonium bis(trifluoromethanesulfonyl)imide] (PDDATf2N), previously described by Pont et al.27 and characterized together with an IL and a Li salt as a ternary polymer electrolyte for Li-ion systems,28 was chosen for preparation of thin films. The ion conduction properties of PDDATf2N were also very recently studied by Jeremias et al.,29 who employed a different approach to POIL obtention. For data treatment, the Kevin−Voigt viscoelastic model was accessed. The analysis of QCM-D data with viscoelastic models is important when the frequency shift is caused by changes in both mass and viscoelastic properties in the deposited film, making it possible to obtain separate quantitative values for both phenomena. Using that analysis, the rheology and kinetics of water uptake by the thin films were studied. The effects of their compositions as well as the relative humidity environment were also studied.
Table 1. Composition of the Suspensions Employed for Preparation of Thin Films mass/g suspension
PDDATf2N
Pyr1.4Tf2N
LiTf2N
100p0IL 67p33IL 60p40IL 50p50IL 50p50IL + 4.8%Li salt
0.1 0.067 0.060 0.050 0.050
0 0.033 0.040 0.050 0.050
0 0 0 0 0.005
flow. The obtained films homogeneities were evaluated by atomic force microscopy images taken with a PicoSPM-LE molecular imaging system with cantilevers operating in the intermittent contact mode (AAC mode), slightly below their resonance frequency of approximately 305 kHz in the air. 2.4. Membrane Preparation. For the membranes preparation, more concentrated mixtures were prepared by dissolving the components in 10 mL of acetone as follows: 50p50IL + 4.8%Li salt = 1 g of PDDATf2N + 1 g of Pyr1.4Tf2N + 0.1 g of LiTf2N; 50p50IL + 14.4%Li salt = 1 g of PDDATf2N + 1 g of Pyr1.4Tf2N + 0.3 g of LiTf2N; 60p40IL + 4.8%Li salt = 1.2 g of PDDATf2N + 0.8 g of Pyr1.4Tf2N + 0.1 g of LiTf2N. The solutions were placed over glass substrates previously treated with piranha solution to facilitate membrane detachment. The liquid was then left to dry at room temperature prior to the drying process under vacuum at 60 °C for 12 h. The previous drying at room temperature was necessary to avoid the formation of air vesicles in the resulting membrane. All components and obtained thin films and membranes were kept under an argon atmosphere in a glovebox with H2O and O2 content
