Study on Flame Retardant Properties of Functionalized Two-Dimensional Black Phosphorus/Polymer Nanocomposites


Student thesis: Doctoral Thesis

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Awarding Institution
Award date1 Aug 2019


In recent years, black phosphorus (BP) has attracted extensive attention. Studies have shown that black phosphorus has shown good application prospects in energy storage, catalysis, photoelectronics, biomedicine and other fields. Black phosphorus has a honeycomb-like folded lamellar structure and is thermodynamically stable. As a new two-dimensional (2D) material, black phosphorus can be peeled into nanosheet. Similar to 2D graphene, black phosphorene has the potential to be used as a nano-additive to enhance the mechanical properties, thermal stability and flame retardancy of polymer materials, due to its two-dimensional morphology, inherent high strength and outstanding thermal properties. The addition of low content of 2D nano-filler can significantly improve the mechanical and thermal properties of polymer materials. Compared with the allotrope of black phosphorus, the traditional flame retardant red phosphorus, BP may show higher flame retardant efficiency due to its special geometric characteristics. In the process of research, we need to solve several fundamental problems. How to prepare the 2D black phosphorene in large scale to meet the requirements of composite preparation, and improve its compatibility and dispersion with the polymer matrix, as well as ensure the air stability of black phosphorene, is the key to obtain high-performance 2D black phosphorene/polymer nanocomposites. Therefore, it is of great importance to develop methods suitable for the preparation of 2D black phosphorene/polymer nanocomposites and to systematically study the mechanical, thermal and flame retardant properties of the polymer composites.

In this thesis, we first consider the simple preparation of polymer-based nanocomposites, and considering the air stability of the black phosphorene itself, simultaneously, 2D black phosphorene and its surface functionalized materials were prepared by several different methods. We prepared high quality black phosphorus crystal by modified gas phase transfer method. Then, by means of surface coating of polyphosphazene, the air stability of black phosphorene was improved and the dispersion of black phosphorene in epoxy resin and the phase interfacial effect of the nanocomposite system were improved. The black phosphorene functionalized by polyphosphazene can be dispersed uniformly in the polymer matrix and show obvious physical barrier effect. Secondly, the exfoliation and functionalization of black phosphorene were realized simultaneously by means of electrochemical method. The addition of modified black phosphorene with a low amount obviously endowed the polymer materials with excellent mechanical and thermal properties. Finally, black phosphorus was exfoliated and surface functionalized simultaneously by small molecule assisted ball milling method. The hydroxylated and aminated black phosphorene were obtained for further functionalization and application. The surface functionalized black phosphorene can improve the interfacial interaction with the polymer matrix, and contribute to the further surface reaction and give it new properties. Through these designs, these functionalized black phosphorene can give the polymer materials better mechanical and flame retardant properties, thus reducing the fire risk of the polymer composite system. The main research work is as follows.

1. The exploration of BP-polymer nanocomposites is rare due to the BP tends to be oxidized in the atmosphere. We developed a cross-linked polyphosphazene functionalized BP with abundant -NH2 groups via a one-pot polycondensation of 4,4′-diaminodiphenyl ether (ODA) and hexachlorocyclotriphosphazene (HCCP) on the surface of BP nanosheets. Whereafter, the resulted polyphosphazene functionalized black phosphorus (BP-PZN) was incorporated into epoxy resin (EP). Due to the surface functionalization, the 2D black phosphorene in epoxy composites presents a completely delaminated and uniform dispersion state. Compared with the addition of pure black phosphorene, the thermal stability, mechanical and flame retardant properties of polymer composites with functionalized black phosphorene have been significantly enhanced. Strong interfacial interaction (covalent bonding) and comprehensive flame retardant effect (instinct flame retardant effect of black phosphorene and polyphosphazene, physical barrier effect of BP nanosheet) are the key to enhance the combustion performance of polymer composites. Meanwhile, the EP/BP-PZN nanocomposites exhibit air stability after exposure to ambient conditions for four month. The air stability of the BP nanosheets in EP matrix is assigned to surface wrapping by polyphosphazene and embedding in the polymer matrix as dual protection.

2. The crucial step towards practical application of BP is the scalable preparation of single- or few-layer BP nanosheets. We utilized a facile, green and scalable electrochemical strategy for generating cobaltous phytate functionalized BP nanosheets (BP-EC-Exf) where the BP crystal used as the cathode, phytic acid served as modifier and electrolyte simultaneously. Moreover, high performance polyurethane acrylate/BP-EC-Exf (PUA/BP-EC) nanocomposites are easily prepared by a convenient UV-curable strategy for the first time. By means of electrochemical method, the exfoliation and functionalization of black phosphorene were realized simultaneously. Significantly, conclusion of introducing BP-EC-Exf into PUA matrix resulted in enhancements in mechanical properties of PUA in terms of the tensile strength and tensile fracture strain; the distinct suppression on flame retardant of PUA in terms of decreased peak heat release rate and total heat release, lower intensities of pyrolysis products including toxic CO. Moreover, the PUA/BP-EC nanocomposites present air stability after exposure to ambient conditions for four months. This modified electrochemical method toward the simultaneous exfoliation and functionalization of BP nanosheets provides an efficient approach for fabricating BP-polymer based nanocomposites.

3. Natural nacre offers an optimized guideline for assembling 2D nanosheets into high performance nanocomposites with light weight, high strength and excellent mechanical properties. Inspired by the "brick-and-mortar" layered structure of nacre, a multifunctional bioinspired nanocomposites of few layer hydroxyl functionalized black phosphorus (BP-OH) with nanofibrillar cellulose (NFC) were fabricated via a vacuum-assisted filtration self-assembly procedure. Owing to the interfacial interaction between 2D BP-OH and one-dimensional (1D) NFC, the effective synergistic strengthening effect of the novel nacre-like BP-OHx/NFC composite film has been successfully achieved, resulting in maximum tensile strength (214.0 MPa) and tensile fracture strain (23.8%). Moreover, these nacre-like composite films show high thermal stability and good fire resistance. The nacre-inspired approach in this work demonstrates a promising strategy for the design of the high performance and flexible BP based composite films.

4. In order to further improve the phase interaction of 2D black phosphorene in the polymer matrix, and utilize the phosphorus-nitrogen synergistic effect to improve the flame retardant efficiency of the polymer composites. The triazine based covalent organic framework/BP-NH2 (BP-NH-TOF) nanohybrid was synthesized via in situ condensation polymerization method using BP-NH2 as a template. The sandwich inorganic-organic hybrid flame retardant was added to the EP to improve the thermal and flame retardant properties of the composites. Through this design, the nanohybrids not only have high specific surface area of 2D materials, but also show synergistic flame retardant effect in polymer nanocomposites. Compared with the addition of pure BP-NH2, the thermal stability, flame retardancy, and smoke suppression and toxicity reduction of EP/BP-NH-TOF nanocomposites have been significantly enhanced. The significant reduction of the fire hazard was primarily due to the synergistic action between the catalytic effect of BP-NH2 and physical barrier effect for the both of BP-NH2 and TOF nanosheets.

5. The flame retardant properties of polymer composites were improved by the synergistic effect of 2D black phosphorene and traditional flame retardants. MCA supramolecular/BP-NH2 (BP-NH-MCA) nanohybrid was prepared via in situ self-assembly method using BP-NH2 nanosheet as template. Due to the presence of amino group, BP-NH2 nanosheet and MCA supramolecular are combined by hydrogen bond interaction. This kind of inorganic-organic hybrid flame retardant can maintain the high specific surface area of the original 2D morphology of BP, and show good flame retardant and smoke suppression and toxicity reduction effects. The properties of epoxy composites with BP-NH-MCA hybrid nano-flame retardant were enhanced mainly due to the interface interaction of hydrogen bond and non-covalent bond, as well as the inherent flame retardant effect and physical barrier effect of MCA and BP-NH2 nanosheets.

    Research areas

  • Two-dimensional BP, Polymer nanocomposites, Mechanical properties, Thermal stability, Flame retardancy