A series of formic acid MOFs/polylactic acid blending composites with the improved dielectric performance in Co(II) systems

In order to improve the limited compatibility of existing polymer/ceramic dielectric composites and further enhance the energy storage density, MOF/polymer composite dielectrics have been explored, which exhibit good compatibility to the polymer matrix from abundant organic groups of the inorganic-organic hybrid metal-organic framework (MOF) fillers. However, they still lack a clear composition-structure-property rule, and the precise design of MOF fillers and polymer matrix becomes a prominent problem in these composites due to the diversity of the metal ions and the organic groups. Thus, in this paper, we present a series of formic acid MOFs/polylactic acid dielectric composites in which ferroelectric formic acid MOFs, namely PDLLA/[NH3(CH2)44NH(3)][M-II(HCOO)(3)](2) and PDLLA/[CH3NH3][M-II(HCOO)(3)](2), in which the formic acid MOFs are with different structures and different metal ions as fillers, including [NH3(CH2)(4))NH3][M-II(HCOO)(3)](2) (namely MOF-Co (M = Co), MOF-Mg(M == Mg), MOF-Mn (M == Mn), with 1, 4-butanediamine ion as guest) and [CH3NH3][M-II(HCOO)(3)](2) (namely MOF-Co-2(M = Co), MOF-Ni-2 (Ni), with methylamine ion as guest). The composition and morphology of composite films were characterized by XRD, IR, SEM, DSC and UV, respectively, while the dielectric characterizations of the composites including the dielectric permittivity, the dielectric loss, the breakdown field strength and the energy density were also performed. The composition-structure-property relationships were also investigated including the influence of MOF content and MOF category. With the introduction of MOFs, the dielectric constant of the polylactic acid substrate was improved slightly while the breakdown field strength can be improved in some systems. Interestingly, the Co(II)-containing formic acid MOF has advantages over other formic acid MOFs with similar structure for the enhancement of the dielectric constant and breakdown field strength. Also, in some composite films with methylamine ion guest MOF fillers and low-MOF content (MOF-Co-2 (1 vol.%) and MOF-Ni-2 (1 vol.%)), the breakdown electric field enhanced significantly and further led to improved energy storage density which was about 43% higher than that of the polylactic acid matrix. The possible reason is that in these composites, the orientation of C-H bonds of MOFs seems more beneficial to the formation of hydrogen bonds between the carboxyl group of formic acid and the polylactic acid matrix. These relationships obtained from formic acid MOFs/polylactic acid composites are valuable to the design of high-performance polymer/MOF energy storage composites and may be a new perspective to the practical use of ferroelectric MOFs.