Preparation, Thermal Stability And Combustion Properties Of Typical Polymer Based Molybdenum Disulfide Nanocomposites

As a typical layered inorganic analogue of graphene, MoS2has gained intensive attention and became a research hotspot due to its unique two dimensional nanostructures and excellent properties. Meanwhile, many research work had been reported that the electrical, mechanical, thermal stability and combustion properties of polymer matrices could be improved remarkably by incorporating only a small amount of layered nanofillers. As a typical layered inorganic compound, MoS2had high thermal stability and low thermal conductivity, which could play a key role in improving the physical barrier effect of nanolayers during the pyrolysis and combustion process of polymer nanocomposites. At the same time, the transition metal element molybdenum, promoted the polymer matrix form dense char layer which could effectively inhibit the energy and mass transfer between the flame and matrix, and delay the degradation of inner polymer. In addition, the compounds containing molybdenum had been widely used as smoke suppression agents in various polymer matrices, which could reduce smoke density and CO production rate, resulting in improving the fire safety of materials. At present, MoS2has been incorporated into some polymer matrices, but these researches are mainly concentrated on their conductive and friction performance. Few of them have dealt with the thermal, mechanical and fire safety properties of the polymer composites.However, there are a lot of headaches in studying the MoS2-based polymer nanocomposites as follow as:(1) the low yield, high cost and time consuming exfoliated method of MoS2;(2) the exfoliated MoS2nanosheets are easy to restack in the preparation of polymer nanocomposites, the low compatibility and weak interaction between the polymer matrix and MoS2limits the dispersion of MoS2nanosheets in the polymer nanocomposites;(3) the low flame retardant efficiency of single MoS2nanosheets. This paper explores and designs efficient exfoliated methods for MoS2;Using some organic molecules to modify the exfoliated MoS2nanosheets which is conducive to improve its dispersion in the polymer matrix; Based on the molecular design, the organic flame retardants and inorganic metal compounds were grafted and loaded on the surface of MoS2nanosheets to obtain functionalized MoS2. And then combined with the nanocomposites technique to prepare various MoS2-based polymer nanocomposites, investigating the influence of functionalized modifications on the dispersion of MoS2nanosheets and thermal and fire safety performances of polymer/MoS2nanocomposites, the flame retardant mechanisms were discussed according to the analysis of gas phase product and structure, composition of char residues. The primary research work of this dissertation is illustrated as follows:1. Three different kinds of methods were selected to exfoliate the layered MoS2, indicating the single layer of MoS2could be obtained by n-Butyl Lithium intercalated-exfoliated method. The exfoliated single layer MoS2dispersed well in distilled water for long time stability. The exfoliated MoS2nanosheets were further selected to prepare polymer nanocomposites with typical water-soluble polymer matrix polyvinyl alcohol (PVA) by solution blending method. The exfoliated MoS2nanosheets is homogeneously dispersed and partially exfoliated in PVA matrix. The glass transition temperature (Tg), thermal stability, mechanical and flame retardancy properties of PVA nanocomposites were improved obviously by incorporating exfoliated MoS2nanosheets. The improvements are attributed to the good dispersion of MoS2, physical barrier effects of MoS2and strong interactions between PVA and MoS2.2. It is well known that there are no any active functional groups on the surface of MoS2nanosheets. The compatibility and interfacial interaction between the MoS2nanosheets and polymer matrix is poor and weak. At the same time, the exfoliated MoS2nanosheets produces by n-Butyl Lithium intercalated-exfoliated method are only dispersed well in solution, and difficult to disperse in hydrophobic organic solvents and polymer matrix. Organic modifiers are used to modify the exfoliated MoS2nanosheets firstly. The organic modified MoS2nanosheets disperse well in PS with a partial exfoliation structure. While the unmodified MoS2nanosheets appear in PS matrix with aggregation structure. The addition of organic modified MoS2nanosheets leads to more obvious improvements in thermal stability, fire resistance and smoke suppression properties which are mainly ascribed to good dispersion and compatibility of organic modified MoS2nanosheets, physical barrier effects, and catalytic char function of MoS2nanosheets.3. LDH-MoS2nanohybrids are firstly synthesized by self-assembly method through electrostatic interactions between the exfoliated MoS2nanosheets with negative charge and LDH nanosheets with positive charge and then the epoxy (EP) nanocomposites with the same content of LDH, MoS2and LDH-MoS2are prepared. The LDH-MoS2nanohybrids are well dispersed in the EP matrix. Meanwhile, the incorporation of LDH-MoS2nanohybrids obviously improves the char residues and decreases the thermal decomposition rate, PHRR, THR, AMLR and FIGRA of EP nanocomposites, indicating a better flame retardant efficiency than LDH or MoS2. In addition, the release volume of the flammable and toxic products for EP nanocomposites with LDH-MoS2nanohybrids is much lower than that of pure EP, exhibiting a better fire safety performance. The analysis of morphology, structure and composition of the char residues after cone test indicates the incorporation of LDH-MoS2nanohybrids lead to form char residues with compact and continuous structure, high graphitization degree. In addition, the transition metal oxide compounds migrate to the surface of char residues which are conducive to improve the thermal oxidative resistance of the char residues. The char residue with high thermal oxidative resistance and compact structure are good barriers to protect the underlying materials, reduces the diffusion of volatile combustible fragments and inhibit the exchange of combustible gases, degradation products and oxygen.4. Organic functional group (amino) decorated MoS2nanosheets are firstly synthesized based on the simultaneous exfoliation of bulk MoS2crystals and chemical conjugation of thiol ligands with L-cysteine. Hexachlorocyclotriphosphazene and ethylenediamine are further used to modify the MoS2nanosheets which can increase the number of amino groups on the surface of MoS2nanosheets. The amino groups on the surface of MoS2nanosheets can improve the interfacial interactions between and EP matrix and promote the dispersion of MoS2nanosheets in EP matrix through covalent bonding by the in-situ curing. The incorporation of organic flame retardants modified MoS2increases the Tg of the EP rianocomposites and exhibits better thermal stability with higher char residues and lower decomposition rate. The data obtained from Cone test and TG-IR show that the addition of organic flame retardants modified MoS2significantly improves the fire safety performance of EP nanocomposites, which are mainly attributed to the physical barrier effects, catalytic char function of MoS2nanosheets and flame retardant effect of grafted organic flame retardants.5. CeMnOx-MoS2and CeFeOx-MoS2hybrids are prepared by co-precipitation method and then the nanohybrids are incorporated into thermoplastic polyurethane (TPU) and EP matrix to prepare corresponding composites. The thermal stability, combustion performance and gas products during combustion of the composites are studied and the corresponding mechanism of reduced fire toxicity is discussed. XRD, XPS and TEM results suggest that CeMnOx-MoS2and CeFeOx-MoS2hybrids are synthesized successfully. The CeMnOx-MoS2and CeFeOx-MoS2hybrids were well dispersed in the TPU and EP matrix with no obvious aggregation structures. TGA results indicate that the composites with hybrids exhibited better thermal stability with higher char residues and lower decomposition rate, but the Toneset and Tmax of the composites decreased compared to pure TPU and EP. In addition, the incorporation of CeMnOx and CeFeOx-MoS2nanohybrids significantly improve the flame retardancy of the composites. The steady state tube furnace (SSTF) results indicate the addition of CeMnOx and CeFeOx-MoS2nanohybrids could reduce CO and smoke density, which is probably attributed to the synergism between the catalysis effect of metal oxide, physical barrier effect, smoke suppression and catalytic char function of MoS2nanosheets.