Influence of Cation and Anion Type on the Formation of the Electroactive β-Phase and Thermal and Dynamic Mechanical Properties of Poly(vinylidene fluoride)/Ionic Liquids Blends

Films based on poly(vinylidene fluoride) (PVDF) blended with ionic liquids (ILs) comprising different cations and anions were developed to investigate the IL influence on the resulting PVDF crystalline phase. Blends with 25 wt % IL content were produced by solvent casting followed by solvent evaporation at 210 degrees C in an air oven. Five different ILs containing the same cation 1-ethyl-3-methylimidazolium [Emim] and five ILs containing the same anion bis(trifluoromethylsulfonyl)imide [TFSI] were selected. The formation of the different phases and the resulting thermal and dynamic mechanical properties were studied by Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis. The incorporation of [Emim]-based ILs successfully directs the PVDF crystallization from the nonpolar alpha-phase toward the electroactive and highly polar beta-phase. On the contrary, blends containing [TFSI] as a common anion yield a mixture of alpha and beta phases. Overall, the induced beta-phase ranges between 14 and 95% depending on the incorporated IL type. Interestingly, 1-ethyl-3-methylimidazolium chloride [Emim] [Cl] is the most effective IL among the studied ones to significantly enhance the beta-phase content, showing also a marked nucleating effect. The results suggest that ions favorably interact with PVDF chains and are located occupying the amorphous interlamellar PVDF regions. Furthermore, the studied ILs act as plasticizers, yielding lower glass-transition temperatures of the amorphous phase and decreasing mechanical storage modulus at room temperature.