Structural Organization of Imidazolium-Based Ionic Liquids and Metal–Organic Frameworks within a Fluorinated Polymer Matrix and Its Influence on the Physical-Chemical Response

Ionic liquids (ILs) and metal-organic frameworks (MOFs) have been combined among themselves and with polymer matrices to obtain composites that are able to be applied in different fields, ranging from sensors and actuators to energy devices. Besides the great efforts devoted to the development and application of those composites, detailed information regarding the internal organization of IL and MOF within the composites is still needed, in order to further tune material properties toward applications. Thus, the present work reports on the development of polymer composites based on the MOF Basolite C300-HKUST-1 and different imidazolium-based ILs ([Bmim]-[FeCl₄], [Bmim]-[N-(CN)₂], and [Bmim]-[SCN]) with different types of anions incorporated into a fluorinated poly-(vinylidene fluoride) (PVDF) polymer matrix. Ternary MOF/IL/PVDF composites have been prepared by solvent casting incorporating 20 wt % of the different ILs together with 20 wt % of HKUST. The influence of MOF and ILs into the morphological, physical-chemical and electrical properties of the composites was evaluated. No significant morphological differences were observed upon the incorporation of the different ILs and MOF into the PVDF, all displaying compact microstructures. However, independently of the IL type, an increase in the electroactive β phase of the polymer was observed, with the highest amount in the ternary composite containing the IL [Bmim]-[N-(CN)₂], reaching approximately 74%. Furthermore, the degree of crystallinity of the composites increased with the IL incorporation. The electrical properties of the different ternary composites revealed an increase in the conductivity from 1.06 × 10^-13 S·cm^-1 for the pristine polymer to 7.48 × 10^-9 S·cm^-1, 7.52 × 10^-7 S·cm^-1, and 3.11 × 10^-9 S·cm^-1 for the ILs [Bmim]-[FeCl₄], [Bmim]-[N-(CN)₂], and [Bmim]-[SCN]-containing samples, respectively. The internal structure of the composite samples was evaluated, showing that the presence of [Bmim]-[SCN] and [Bmim]-[FeCl₄] in the ternary composites leads to a destruction of the MOF structure as well as to large-size inhomogeneities, indicating that IL accumulated with parts of the destroyed MOF as well as IL agglomeration through the matrix. These findings demonstrate the ability to tailor the electroactive phase content and conductivity through IL selection, which is highly relevant for functional applications. In particular, the developed composites hold strong potential for use in flexible sensors, electrochemical energy storage systems, and responsive devices where ionic conductivity and structural organization at the nanoscale are relevant.