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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/[NH[Formula: see text](CH[Formula: see text])[Formula: see text]NH[Formula: see text]][M[Formula: see text](HCOO)[Formula: see text]][Formula: see text] and PDLLA/[CH[Formula: see text]NH[Formula: see text]][M[Formula: see text](HCOO)[Formula: see text]][Formula: see text], in which the formic acid MOFs are with different structures and different metal ions as fillers, including [NH3(CH[Formula: see text]NH3][MII(HCOO)3]2 (namely MOF– Co (M [Formula: see text] Co), MOF– Mg(M [Formula: see text] Mg), MOF– Mn (M [Formula: see text] Mn), with 1, 4-butanediamine ion as guest) and [CH3NH3][M[Formula: see text](HCOO)3]2 (namely MOF– Co[Formula: see text] (M = Co), MOF– Ni[Formula: see text] (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[Formula: see text] (1 vol.%) and MOF– Ni[Formula: see text] (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.

Solvent vapor-driven soft actuators are promising devices for human-environment interaction and have gained considerable attention in recent years. To enhance the interaction with the solvents and deliver a fast response and superior sensitivity, polymer/MOF composites are promising candidates for the fabrication of solvent-driven actuators. This paper presents a multiple-solvent-driven intelligent MOF-polymer soft actuator, namely MIL-88B/PVDF asymmetric film, and compares it to analogs based on other MIL-88(Fe) series flexible MOFs. This comparison investigates the deformation-structure relationship and the solvent-driven mechanism. The deformation of the MIL-88(Fe)/PVDF composite films in response to various organic solvents has been studied, and the structure and composition of the MOF fillers and the composites were characterized by SEM, FT-IR, and XRD. Furthermore, the MIL-88B/PVDF composite, which shows ability to respond to multiple solvents and a fast deformation response to several organic solvents through the solvent swelling/adsorption of the MOF skeleton and the appropriate flexibility and group affinity from the terephthalic acid ligand, was used to fabricate an inverted opal gradient actuator composed of MIL-88B, carbon black (CB) and PVDF, namely MIL-88B/CB/PVDF. The MIL-88B/CB/PVDF film not only realizes excellent dual-responsive deformation and discoloration to methanol vapor with fast visible deformation within 1 s with a maximum crimp angle of up to 5581 (10 vol% concentration) and a wide color change range covering six viewing-angledependent high-saturation colors (red, orange, yellow, green, blue and purple) but also shows an approximate linear relationship between the maximum curl angle and the solvent concentration for three solvents (methanol, toluene and chloroform). In addition, several smart devices have also been fabricated, including methanol-driven bionic petals, a worm robot, an alertor and a LIG sensor. Therefore, such polymer/MOF soft actuators have great potential in bio-robots and this work provides a new design strategy for smart materials. [ABSTRACT FROM AUTHOR]

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/[NH 3 (CH 2 ) 4 NH 3 ][M I I (HCOO) 3 ] 2 and PDLLA/[CH 3 NH 3 ][M I I (HCOO) 3 ] 2 , in which the formic acid MOFs are with different structures and different metal ions as fillers, including [NH3(CH 2 ) 4 NH3][MII(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 I I (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. [ABSTRACT FROM AUTHOR]