No abstract

Nanomaterials derived from metal–organic frameworks (MOFs) are highly promising as future flame retardants for polymeric materials. The precise control of the interface for polymer nanocomposites is taking scientific research by storm, whereas such investigations for MOF-based nanofillers are rare. Herein, a novel yolk-double shell nanostructure (ZIF-67@layered double hydroxides@polyphophazenes, ZIF@LDH@PZS) was subtly designed and introduced into epoxy resin (EP) as a flame retardant to fill the vacancy of yolk/shell construction in the field. Meanwhile, the interface of the polymer nanocomposites can be further accurately tailored by the outermost layer of the nanofillers from PZS to Ni­(OH)2 (NH), by which hollow nanocages with treble shells (LDH@PZS@NH) were obtained. It is remarkably interesting that LDH@PZS@NH endows the EP with the lowest peak of heat release rate in the cone calorimeter test, but the total heat and smoke releases (THR and TSP) of the nanocomposites are even higher than those of the neat polymer. In contrast, EP blended with ZIF@LDH@PZS shows outstanding comprehensive performance: with 2 wt.%, the limiting oxygen index is increased to 29.5%, and the peak heat release rate is reduced by 26.0%. The impact and flexural strengths are slightly lowered, while the storage modulus is enhanced remarkably compared with that for neat EP. The flame retardant mechanism is systematically explored focusing on the interfacial interactions of different hybrids within the epoxy matrix, ushering in a new stage of study of nanostructural design-guided interface manipulation in MOF-based polymer nanocomposites.

No abstract

The direct coordination between polyhedral oligomeric silsesquioxane (POSS) and Co forms an assembly of nanoparticles with low specific surface area and leads to a poor dispersion state in the epoxy resin matrix, resulting in unsatisfactory flame-retardant efficiency. Metal–organic frameworks (MOFs), for instance, ZIF-67, provide not only the cobalt element but also the porous framework that endows the nanocomposite of MOFs and POSS with high specific surface area and abundant Co sites in the silica skeleton. Herein, ZIF-67 is hybridized with octacarboxyl POSS, resulting in the removal of the alkaline ligand to form novel metal POSS–organic frameworks (MPOFs). The size differences for organic groups and silica nanocages of POSS vs. micropores of ZIF-67 gave rise to a reverse click reaction, reforming octavinyl POSS isolated on the outer surface of the Co complex, which could be further modified by a phosphorous flame retardant using an addition reaction. The obtained MPOFs-P with 2 wt % loading in epoxy resin could improve the limiting oxygen index value of the composites to 27.0% and pass the V-0 rating in the UL-94 test. Meanwhile, the peaks of the heat release rate and especially the total smoke production were reduced by 46.6 and 25.2%, respectively. The robust char layer reduces the emission of toxic gas CO by 39.8%. The above epoxy product with promising flame retardancy also improved mechanical properties, thanks to the filler with a unique nanostructure. The ingenious work offers enlightenment for the hybridization method of MOFs and POSS to fabricate a multielement flame-retardant system for epoxy resin with high efficacy.

Boron nitride (BN) has great potential to improve the thermal conductivity of polymers, and polyhedral oligomeric silsesquioxane (POSS) revealed the ability to prepare polymers with a low dielectric constant. Combining the two abovementioned materials, an iron-containing polyhedral oligomeric silsesquioxane assembly supported on hexagonal boron nitride (FePOSS@hbno) was prepared via two steps involving hydroxylation and selfaggregation. The morphology of nanohybrid FePOSS@hbno indicated that FePOSS successfully aggregated on the surfaces and edges, which was evidenced by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. The nanohybrid was also confirmed by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The well-characterized FePOSS@hbno was incorporated into epoxy resins, and the thermal conductivity of epoxy composites increased by 104.3% with 20 wt % FePOSS@hbno. The dielectric constant of epoxy composites containing 3.6 wt % FePOSS@hbno clearly decreased in the frequency range of 10 3 −10 6 Hz. Significantly, 3.6 wt % FePOSS@hbno reduced the peak of the heat release rate by 49.3% and increased the LOI value to 31.2%. Moreover, the condensed and gaseous phases were investigated in detail. The enhanced flame retardancy originated from the nanohybrid structure as well as the elemental merits.