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 bends

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 °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 α-phase toward the electroactive and highly polar β-phase. On the contrary, blends containing [TFSI] as a common anion yield a mixture of α and β phases. Overall, the induced β-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 β-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.This work was supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UID/FIS/04650/2019. The authors thank FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the projects PTDC/BTM-MAT/28237/2017, PTDC/EEI-SII/5582/2014, and PTDC/FIS-MAC/28157/2017. D.M.C. and C.M.C. also acknowledge the FCT for grants SFRH/BPD/121526/2016 and SFRH/BPD/112547/2015, respectively. The financial supports from the Spanish Ministry of Economy and Competitiveness (MINECO) through the project MAT2016-76039-C4-(1 and 3)-R (AEI/FEDER, UE) (including the FEDER financial support) and from the Basque Government Industry and Education Departments under the ELKARTEK, HAZITEK, and PIBA (PIBA-2018-06) programs, respectively, are acknowledged. CIBER-BBN is an initiative funded by the VI National R&D&I Plan 2008–2011, Iniciativa Ingenio 2010, Consolider Program. CIBER Actions are financed by the Instituto de Salud Carlos III with assistance from the European Regional Development Fund