Design Of Transition Metal-Organic Frameworks And Investigation On Properties Of Their Flame Retardant Polymer Composites

Organic-inorganic hybrid material not only has good compatibility with the polymer matrix,but also has high thermostability and structural adjustability,and is often used to improve the flame retardancy and mechanical properties of the polymers.Metal organic frameworks(MOFs)are composed by organic and inorganic components(metal ions or clusters)and are a class of organic-inorganic hybrid materials.There are few studies on the using of MOFs and their derivatives to enhance the thermostability,flame retardancy and mechanical properties of polymers.The structure of MOFs is the basis of its performance.Flame retardant units and excellent thermal stability are the key to the preparation of high performance polymer/MOFs composites.Therefore,the development of MOFs(or derivatives)with above advantages is essential for improving the flame retardancy of the composite.This paper firstly studied the effects of the types and components of MOFs(organic ligands and metal units)on the combustion behaviors of polymers.Secondly,two modification methods were used to design flame-retardant elements containing MOFs to improve their flame retardant efficiency.Finally,two kinds of layered MOFs derivatives were prepared under the guidance of flame retardant mechanism,and studied their effects on the combustion behavior of the polymers.The research progress achieved is shown as follows:1.This work firstly investigated the effect of MOFs on the thermostability and flame retardancy of polystyrene(PS).MIL-53(Fe)and ZIF-67 were selected as the research objects.The degradation behaviors of MOFs and the degradation products in gaseous and condensed phases were studied.The effects of degradation products on the properties of PS composites were analyzed.This work also studied the dispersion state of MOFs and their compatibility with PS matrix.The degradation and combustion behaviors of PS/MOFs showed that the addition of MOFs could significantly improve the thermostability and flame retardancy of PS composites.The peak heat release rate(PHRR)of PS/MIL-53 and PS/ZIF-67 was visibly lower than that of PS.More importantly,the release of toxic CO gas in the PS combustion products was also significantly suppressed with the addition of MIL-53(Fe)and ZIF-67.This work also analyzed of the flame-retardant mechanisms of MOFs.The results showed that MOFs can be used to enhance the thermostability and flame retardancy of polymers.2.In order to further understand the influence of structural units of MOFs on the thermostability and flame retardancy of polymers,three organic ligands(terephthalic acid,trimesic acid,2-methylimidazole)and various transition metal(aluminum,cadmium,cobalt,zinc,iron,copper)salts were selected to prepare ten different MOFs and added them into different polymers(epoxy:EP;polystyrene:PS).The results showed that MOFs were well dispersed in the polymer matrixes,and the addition of MOFs can significantly improve the thermal stability of the composites.By comparatively analyzing the combustion behaviors of PS/MOFs and EP/MOFs composites,it can be found that the metal part played a decisive role in improving the flame retardancy of the polymers rather than organic ligands.Meanehile,cobalt-based MOFs exhibited superior flame retardancy compared with other transition metal element-based MOFs.3.The flame retardant efficiency of MOFs still needed to be improved.Compounding is one of the most effective methods to improve the performance of materials.In order to improve the flame-retardant efficiency of MOFs,Ni-MOF/GO(GOF)hybrids were prepared by combining nickel-based MOF(Ni-MOF)with graphite oxide(GO).GOF exhibited the most excellent dispersibility in the matrix compared to the single components.Compared with the single component,GOF also decreased PHRR and total heat release(THR)values more pronounced.Compared with PS,the PHRR and THR of PS/GOF decreased by 33%and 16.7%,respectively,indicating that GOF significantly improved the flame retardancy of PS.The effect of GOF was more prominent in suppressing the release of CO during combustion.The total CO release of PS/GOF was reduced by 52.3%compared with PS.This can be sescribed to barrier effect and catalytic oxidation of GOF and its degradation products(NiO/rGO).The synergistic effect between layered GO and porous Ni-MOF improves the thermostability and flame retardancy of PS composites.This work provided a viable way to design efficient MOFs-based flame retardants.4.Flame-retardant elements can be introduced into MOFs by pre-preparing flame-retardant organic ligands or flame retardants modified MOFs with active sites.This method can further increase the possibility of MOFs as flame retardants.In this work,two transition metal(cobalt)-based MOFs were prepared by using different molecular design methods described above.Two MOFs prepared in this work have a two-dimensional structure and show good compatability with polymer matrix.The cobalt-based MOF prepared by the first method can effectively promoted the char forming behaviord in the degradation process of EP.DOPO-modified cobalt-based MOF can improve the thermostability,flame retardancy and mechanical properties of polylactic acid(PLA).Molecular design is an effective method to obtain MOFs with excellent properties.The introduction of flame-retardant elements into the structure of MOFs not only preserved the compatibility of MOFs with polymer matrix,but also significantly improved the efficiency of MOFs as flame retardants.5.In order to improve the possibility of MOFs as flame retardants,based on the principle of flame retardant and reasonable structural design,two specific derivatives of MOFs were prepared:layered Co-Ni layered double hydroxide(Co-Ni LDHs)and carbon nitride(C2N)nanosheets.As prepared derivatives were separately added into unsaturated polyester resin(UPR)and PLA.The thermostability,flame retardancy and mechanical properties of the composites were analyzed.Co-Ni LDHs prepared by using MOF as a Co source,which not only had porous structure composed by stacked sheets,but also uniformly dispersed on the surface of the carbon materials.The preparation of C2N by using MOF as a carbon source not only can be prepared rapidly,but also had good compatibility with PLA.The results showed that these two-dimensional nanomaterials have high flame-retardant efficiency.In addition,Co-Ni LDHs had a good smoke and attenuation effect,while C2N can enhance the strength and toughness of PLA.