Effect of the alkyl chain length of the ionic liquid anion on polymer electrolytes properties

New polymer electrolytes (PEs) based on chitosan and three ionic liquid (IL) families ([C2mim][CnSO3], [C2mim][CnSO4] and [C2mim][diCnPO4]) were synthesized by the solvent casting method. The effect of the length of the alkyl chain of the IL anion on the thermal, morphological and electrochemical properties of the PEs was studied. The solid polymer electrolytes (SPE) membranes were analyzed by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), polarized optical microscopy (POM), atomic force microscopy (AFM), complex impedance spectroscopy (ionic conductivity) and cyclic voltammetry (CV). The obtained results evidenced an influence of the alkyl chain length of the IL anion on the temperature of degradation, birefringence, surface roughness and ionic conductivity of the membranes. The DSC, XRD and CV results showed independency from the length of the IL-anion-alkyl chain. The PEs displayed an predominantly amorphous morphology, a minimum temperature of degradation of 135 °C, a room temperature (T = 25 °C) ionic conductivity of 7.78 × 10−4 S cm−1 and a wide electrochemical window of ∼ 4.0 V.

This work was supported by FEDER through the COMPETE Program and by Fundação para a Ciência e a Tecnologia (FCT) in the framework of the Strategic Projects PEST-C/QUI/UI0686/2013, PEst-OE/SAU/UI0709/2014 and UID/Multi/00709/2013, UID/Multi/04551/2013, projects PTDC/QUI/117340/2010 and COST Action MP1202 “Rational design of hybrids organic inorganic interfaces”, grants SRFH/BD/90366/2012 (R.L.) and SFRH/BPD/63152/2009 (S.C. Nunes) and FCT Investigator contract (J.M.S.S.E.). The authors are also indebted to CNPq, FAPESP and CAPES for the financial support given to this research. M.M. Silva acknowledges Fundação Luso- Americana (FLAD) and CNPq (PVE grant 406617/2013-9), for the mobility grant provided by these institutions.