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Constructing 3D Polyphosphazene Nanotube@Mesoporous Silica@Bimetallic Phosphide Ternary Nanostructures via Layer-by-Layer Method: Synthesis and Applications Shuilai Qiu, Yongqian Shi, Bibo Wang, Xia Zhou, Junling Wang, Chengming Wang, Chandra Sekhar Reddy Gangireddy, Richard K.K. Yuen, and Yuan Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06440 • Publication Date (Web): 15 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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ACS Applied Materials & Interfaces

Constructing 3D Polyphosphazene Nanotube@Mesoporous Silica@Bimetallic Phosphide Ternary Nanostructures via Layer-by-Layer Method : Synthesis and Applications

Shuilai Qiu ad, Yongqian Shi b, Bibo Wang a,*, Xia Zhou a, Junling Wang a, Chengming Wang c, Chandra Sekhar Reddy Gangireddy a, Richard K. K. Yuen d and Yuan Hu a,*

a

State Key Laboratory of Fire Science, University of Science and Technology of

China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China b

College of Environment and Resources, Fuzhou University, 2 Xueyuan Road,

Fuzhou, Fujian 350002, P.R. China c

Hefei National Laboratory for Physical Sciences at the Microscale, University of

Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China d

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong

Corresponding Authors * Bibo Wang. Fax/Tel: +86-551-63602353. E-mail: [email protected] *Yuan Hu. Fax/Tel: +86-551-63601664. E-mail: [email protected].

ACS Paragon Plus Environment


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ABSTRACT: A novel ternary nanostructure polyphosphazene nanotube (PZS)@ mesoporous silica (M-SiO2)@bimetallic phosphide (CoCuP) was facilely fabricated, using PZS as the template, where large amount of cetyltrimethyl ammonium bromide molecules were anchored to PZS via a similar layer-by-layer assembly strategy, and then uniform M-SiO2 shells can be formed successfully by Hyeon’s coating method. Subsequently, the three-dimensional (3D) nanostructure on the basis of bimetallic phosphide (CoCuP) interconnected with PZS@M-SiO2 was synthesized via a convenient, mild hydrothermal route. It is noted that incorporating well-designed PZS@M-SiO2@CoCuP led to significant decrease on fire hazard of thermoplastic polyurethane (TPU), i.e., 58.2% and 19.4% reductions in peak heat release rate and total heat release, respectively, as well as lower toxic hydrogen cyanide and carbon monoxide yield accompanied by higher graphitized char layer. In the case of TPU/PZS@M-SiO2@CoCuP system, the storage modulus at −97 °C was dramatically improved by 62.6%, and glass transition temperature was shifted to higher value, compared to those of pure TPU. The enhanced fire safety and mechanical property for TPU composites can be ascribed to tripartite cooperative effect from respective parts (CoCuP and M-SiO2) plus the PZS.

KEYWORDS: PZS@M-SiO2@CoCuP; layer-by-layer; composites; flame retardant; mechanism


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1. INTRODUCTION Polyphosphazenes, a class of well-known organic–inorganic hybrid materials, which have been utilized as electrical and optical materials, biomaterials, hybrid materials, etc., due to their tremendous structural flexibility and outstanding thermal stability.1,2 As expected, polyphosphazenes are attractive candidates as flame retardants, and exhibit high limiting oxygen index (LOI). Usually, polyphosphazenes are categorized into three main types, such as linear polyphosphazenes and phosphazene polymers with cyclophosphazene units in backbone or side chains.3 Major disadvantages of linear polyphosphazenes, including lower output and higher cost, restrict their wider applications in material science.4,5 But on the other hand, another kind of polyphosphazenes, cyclotriphosphazenes, have attractive interest with a -P=N- backbone unit are utilizing to synthesize micro- or nano-scale polymer materials, such as polyphosphazene nanotubes (PZS), microspheres, nanofibers and nanochains by condensation polymerization.6-8 Considering the structural similarity of PZS with carbon nanotubes (CNTs), the PZS have attractive potentiality in mechanical enhancement and as flame-retardant additives.9 In addition, with the structure controllability of phosphazene units and presence of amino or hydroxyl active groups, PZS can be easily functionalized via covalent or noncovalent strategies. For instance, epoxy-groups functionalized PZS were incorporated into epoxy resins (EP) to reinforce the polymer matrix as CNTs effect.10 Our group reported that functionalized PZS were wrapped with a cross-linked DOPO-based flame retardants by one-step strategic syntheses and introduced into EP to improve its flame retardant property.11 Transition metal phosphides (TMPs), are a family of intrinsically metallic materials, which have been applied as efficient electrocatalysts with increasing research activities worldwide.12 The typical TMPs based on Co, Ni, Cu, Mo, Fe and W elements, have been reported, and these materials are the rising stars as electrocatalysts on the application of hydrogen evolution reactions. For example, CoP, Ni2P, FeP and MoP, etc. have been performed as superb catalytic systems.13-16 In addition to these monometallic phosphides, a smaller number of bimetallic



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phosphides, such as NiCoP, NiMoP, NiFeP and NiCoP, have also been reported.17-19 Monometallic phosphide can be synthesized by various methods, including temperature-programmed reduction, thermal-decomposing single-molecular precursor, solid-state metathesis, solvothermal method, etc.20-22 However, most of bimetallic phosphides are generally prepared by temperature-programmed reduction method, but the other methods being slightly reported for them. Therefore, it is an important aspect for adopting more simple and safe strategies to achieve superb materials under non-toxic and mild conditions. Based on this point, we use a mild, facile hydrothermal method to synthesize bimetallic phosphide in non-toxic water systems. Mesoporous silica (M-SiO2) has also received great attention as an versatile coating reagent due to its intrinsically unique characteristics including stable structures, high pore volume, large surface area, abundant mesopores, excellent biocompatibility, and easy surface modification, etc.23 It is of great significance to build a novel multifunctional nanoplatform by coating M-SiO2 on CNTs to overcome their limitations (e.g., aggregation or toxicity of CNTs).24,25 Owing to the structure of PZS similar to that of CNTs, surface encapsulation is an economically convenient route to achieve functionalized PZS and combine the merits of core shell components. Encapsulation of PZS in M-SiO2 shells not only avoids the undesirable aggregations of PZS and improves the compatibility between nanofillers and polymer matrices, but also expand their application ranges to separation, adsorption, catalysis and flame retardancy. These M-SiO2 coatings are usually synthesized via a sol-gel route in microemulsion media or in the aqueous or nonaqueous phase, and in virtue of the cationic surfactants like cetyltrimethyl ammonium bromide (CTAB) as the structure directing agent.26-28 The deposition of M-SiO2 on the nanoparticles in a thin intermediate layer, was found in the prior procedure of M-SiO2 growth.29,30 Moreover, The growth procedures are easily affected by reactant concentration, the temperature, encapsulated materials, pH and other external force.31,32 Thereby, increasing research attention is paid to developing a convenient and easy manageable procedure for the M-SiO2 coating. Based on the descriptions above, it is expected that the thermal stability and


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reinforcement of PZS and the flame retardant effect of M-SiO2 and catalytic effect of bimetallic phosphides may contribute to enhancing flame-retardant and mechanical performances of polymeric materials. Herein, we propose feasible route to prepare a novel nanostructure based on PZS nanotubes encapsulated with a layer of M-SiO2 coating and decorated with CoCuP crystalline phase. Firstly, the M-SiO2 coated on PZS nanotubes (PZS@M-SiO2) was prepared by a layer-by-layer (LBL) method, where PZS nanotubes were designed as the growth models. Then, the core-shell nanotubes interconnected with CoCuP were fabricated via a convenient hydrothermal route.

The

schematic

diagram

of

resulting

ternary

nanostructure

(PZS@M-SiO2@CoCuP) was illustrated in Scheme 1. A series of nanostructures then were introduced into the thermoplastic polyurethane (TPU) to manufacture TPU composites. The flame retardant and mechanical performances of the composites were investigated. In addition, the mechanisms for improved flame-retardant and toxicity-reduction were discussed. 2. EXPERIMENTAL SECTION 2.1 Materials Poly(sodium p-styrenesulfonate) (PSSF), hexachlorocyclotriphosphazene (HCCP) and 4,4’-sulfonyldiphenol (BPS) were provided by Aldrich Chemical Co. Ltd. (U.S.). Other chemicals, such as tetraethyl orthosilicate (TEOS), anhydrous ethanol, triethylamine (TEA), CTAB, sodium hydroxide, tetrahydrofuran (THF), copper chloride (CuCl2·2H2O) and cobalt chloride (CoCl2·6H2O) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). TPU (85E85) was purchased from Baoding Bangtai Chemical Industry Co., Ltd. (China). 2.2 Synthesis of PZS Nanotubes as Templates33 In a typical procedure, desired amount of BPS and TEA (4.16 g, 41.2 mmol) were dissolved in 250 mL of THF, and then poured into a 500 mL of three-necked flask. Follow by HCCP (2.4 g, 6.9 mmol) dissolved in 100 mL of THF and then added dropwise to the reaction system during 1 h. The mixture system was stirred under sonication for extra 6 h, and the reaction temperature was maintained at 40 oC. After the condensation polymerization of PZS completed, the solvent was removed by



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filtering and the white precipitate was washed with deionized water and anhydrous ethanol for three times, respectively. Finally, the obtained white powder was dried under vacuum at 65 oC. 2.3 Construction of PSSF/CTAB System Coated PZS Initially, PZS (500 mg) was dispersed in the PSSF solution (1 wt%, 150 mL) by bath sonication at room temperature for 20 min, follow by the centrifugal process of mixture at 9000 rpm for 10 min. The suspension was poured out completely to remove the redundant PSSF, meanwhile the precipitate was washed with deionized water by alternant centrifugation and sonication. Secondly, the PSSF-wrapped PZS was dispersed again in the CTAB solution (0.5 wt%, 150 mL), followed by bath sonication for 10 min at room temperature. Then, the redundant, CTAB, was removed by centrifugation at 9000 rpm, and simultaneously washed with deionized water under sonication, the resulting product, was denoted as PZS/PC1. Since, our experimental evidences the five times self-assembled results chosen as better conditions, and hence, the same procedure was adopted to repeatedly self-assemble the next layers of PSSF/CTAB onto PZS/PC1 to prepare PZS/PC5 meaning five layers of PSSF and CTAB wrapped on PZS. PZS/PC5 (500 mg) was dispersed in the CTAB solution (0.03 wt%, 500 mL) by bath sonication. During stirring, a small amount of NaOH solution (0.1 M, 10 mL) was added dropwise to adjust the pH value of system to 10-11. Afterward, 6 mL of TEOS solution were injected three times under assisted stirring at 20 min intervals, follow by the continue reaction of the mixture for 8 h. After that, the solvent was removed by centrifugation and obtain the white precipitate, named PZS@M-SiO2. It was washed with anhydrous ethanol for three times and dried in vacuum at 65 °C for 12 h to acquire fine product. 2.4 Synthesis of CoCuP Interconnected with PZS@M-SiO2 Nanostructures Red phosphorus powder (P) (200 mg) and copper chloride (CuCl2·2H2O) (200 mg) and cobalt chloride (CoCl2·6H2O) (135 mg) were dissolved in 70 mL of deionized water, followed by addition of PZS@M-SiO2. The above system was stirred for 30 min, and later transferred into a 100 mL Teflon stainless steel autoclave kept at


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200 oC for 48 h. Subsequently, the Teflon stainless steel autoclave was removed from the oven and then cooled to room temperature. The precipitate was obtained by centrifugation and then washed with deionized water and anhydrous ethanol three times, respectively. The as-obtained products, PZS@M-SiO2@CoCuP (Cu 10 wt%, Co 10 wt%) were collected and dried at 80 oC for 6 h. 2.5 Preparation of TPU/PZS@M-SiO2@CoCuP Composites The TPU solution was prepared by dissolving TPU in DMF at 80 oC. In an another reaction flask, the given content of PZS@M-SiO2@CoCuP nanohybrid was dispersed in DMF under sonication assisted agitation for 20 min, and then poured it into the above TPU solution. The resulted TPU solution mixture was further treated with assistant of sonication for 30 h. Subsequently, the solution was poured into ethanol in order to precipitating the TPU/PZS@M-SiO2@CoCuP master-batch. The resulting masterbatch was separated from ethanol by filtration, and then dried in an oven for overnight. Afterwards, the masterbatch was blended with neat TPU to fabricate

TPU/PZS@[email protected]

samples

containing

3

wt%

PZS@M-SiO2@CoCuP nanohybrids through melt blending method. For comparison, the similar process was utilized to prepared TPU/PZS2.0 and TPU/[email protected] and TPU/PZS@[email protected] composites when 2 wt% of PZS, PZS@M-SiO2 and PZS@M-SiO2@CoCuP were added, respectively. 2.6 Characterization Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd., Japan) was performed to study the morphologies of resulting nanomaterials. Fourier transform infrared (FTIR) spectra were employed on a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). X-Ray photoelectron spectroscopy (XPS) was monitored using a VG ESCALAB MK-II electron spectrometer (V.G. Scientific Ltd., UK). X-ray diffraction (XRD) was conducted by using an X-ray diffractometer (Rigaku Co., Japan), using Cu Kα radiation (λ= 0.15418 nm). Nitrogen adsorption-desorption isotherms were measured with a Micromeritics Coulter (USA) instrument. Morphologies of the fracture surface and residual char were observed by a PHILIPS XL30E scanning electron microscope (SEM). Thermogravimetric analysis



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(TGA) was carried out on a Q5000 thermo-analyzer instrument (TA Instruments Inc., USA) under nitrogen atmosphere, at a linear heating rate of 10 oC min-1 from 20 to 800 oC. Laser microRaman spectrometer (Jobin Yvon Co., Ltd., France) with an argon laser of 514.5 nm was employed to evaluated the structure components of the residual char. Cone calorimeter test (Fire Testing Technology, UK) was employed to investigated the fire performance of TPU and its composites according to the standard of ASTM E1354/ISO 5660. Every specimen was exposed horizontally under a heat flux of 35kW/m2. Thermogravimetric analysis/infrared spectrometry (TG-IR) was conducted with a TGA Q5000 thermogravimetric analyzer combined with a Nicolet 6700 FTIR spectrophotometer. The tensile strength data of TPU composites were obtained from an MTS CMT6104 universal testing machine (MTS Systems Co. Ltd., P.R. China) based on GB 13022−91. Each specimen was repeated for five times. Dynamic mechanical analysis (DMA) was conducted with the PerkinElmer Pyris Diamond DMA from -100 to 60 °C at a linear heating rate of 5 °C/min, at a frequency of 1 Hz in the tensile configuration.

Scheme 1. Synthetic route of ternary nanostructure PZS@M-SiO2@CoCuP. 3. RESULTS AND DISCUSSION 3.1 Characterization of PZS@M-SiO2@CoCuP Nanostructures Since CTAB has been reported as an suitable cationic surfactant to disperse and encapsulate the CNTs,34 it is believed that the Hyeon’s method35 can be used to form


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the M-SiO2 shells on the CTAB-wrapped PZS. For example, PZS was dispersed in 0.5 wt% CTAB solution under ultrasonic condition for 1 h to absorb the CTAB molecules. Then, the CTAB was anchored on PZS which was used as the model for the high-yield coating. Besides, the LBL self-assembly method has been widely utilized to synthesize polymer nanotubes, inorganic hollow spheres, core−shell nanostructures and multicomposites, according to the electrostatic attraction between charged species. LBL assembly method provides an approach that should anchored more CTAB onto the PZS though similar method. Herein, the PSSF can serve as primers to adsorb the cationic ions, which has a high density of negatively charged sulfonate groups. Based on this, we use CTAB molecule as the cation agent to form the electropositive layer around PSSF-wrapped PZS. It is expected that this CTAB-wrapped PZS can be successfully synthesized by using the similar LBL self-assembly method, based on which M-SiO2-coated PZS (PZS@M-SiO2) can be obtained by Hyeon’s method. The morphology of the obtained samples was observed by TEM. It can be clearly observed from Figure 1a that the surface of pristine PZS is smooth. After five bilayers of PSSF/CTAB are formed, the divisive M-SiO2 are deposited on the PZS (Figure 1b), and the M-SiO2 layers are uniform and smooth (Figure 1c and d), with a thickness of 30 nm. The obtained results indicate that the similar LBL method contributes to preparing the CTAB-wrapped PZS that can be encapsulated with uniform M-SiO2 by Hyeon’s method. After the CoCuP phase is successfully generated under hydrothermal condition, the crystalline bimetallic phosphide is formed on the surface of PZS@M-SiO2 (Figure 1e). However, the CoCuP component anchored on the PZS@M-SiO2 nanotubes shows poor dispersion (Figure 1f), which can be explained by the incoordination behavior between metal phosphide compounds and mesoporous silica components. As can be observed from Figure 1g, single bimetallic phosphide particle is indicative of good crystallinity of the CoCuP phase, with a lattice parameter of 0.210 nm, corresponding to the (211) diffraction plane of the bimetallic phosphide.36



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Figure 1. TEM images of the (a) PZS and (b, c and d) PZS@M-SiO2 and (e and f) PZS@M-SiO2@CoCuP; (g) HRTEM image of CoCuP. The

significant

structural

information

of

PZS,

PZS@M-SiO2

and

PZS@M-SiO2@CoCuP is provided by FTIR technique, as shown in `Figure 2a. In the case of PZS, two characteristic peaks at 1490 and 1589 cm−1 are ascribed to the aromatic C=C group absorptions of the phenol in BPS units. In addition, the apparent peaks at 883 and 1186 cm-1 are attributed to the stretching vibration of P–N and P=N groups, respectively. The absorption peaks at 1293 and 1153 cm -1 corresponds to the O=S=O group, and the distinct absorption of P–O–Ar band can be observed at 941 cm-1. From the IR spectra of PZS@M-SiO2 and PZS@M-SiO2@CoCuP, except for the characteristic peaks in PZS phase, the typical peaks at 800 and 1150 cm-1 are attributed to Si-O-Si bending and stretching vibrations; the sharp peak at 958 cm-1 is assigned to Si-OH stretching vibration, indicating the formation of M-SiO2 extra phase on the surface of PZS. Figure 2b presents the comparable XRD patterns of PZS, PZS@M-SiO2 and PZS@M-SiO2@CoCuP. It is found that there is a broad peak appears for PZS, indicating a reflection characteristic of amorphous phase.37 After wrapping with the M-SiO2, there is a sharp broad peak appears compared to pure PZS, and this similar broad peak corresponding to the amorphous silica. For PZS@M-SiO2@CoCuP, the characteristic peaks at 2θ = 23.2°, 28.6°, 36.9°, 39.4°, 41.8°, 45.3°, 46.5°, 49.1°, 52.4°, 54,4°, 59.8° and 68.3° can be assigned to the (002), (102), (112), (202), (211), (300), (113), (212), (302), (104), (222) and (214) reflection planes of copper phosphide,


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respectively. Furthermore, the peaks of 2θ = 27.2°, 31.0°, 35.4° and 43.4°, are indexed to the (110) (011) (111) and (201) lattice fringe diffractions of cobalt phosphide, indicating the formation of CoCuP crystalline phase on the surface of [email protected],39 These results are in accordance with the corresponding HRTEM image of CoCuP in Figure 1g. As shown in Figure 2c, PZS@M-SiO2@CoCuP nanohybrid shows nitrogen adsorption isotherm type IV curve, and the hysteresis loops reveal characteristics of mesoporous structure.45 The values of pore volume and surface area are calculated to be as high as approximately 0.09 cm3 g−1 and 44.0 m2 g−1. On the basis of the BJH equation, the detailed pore size distribution suggests that the PZS@M-SiO2@CoCuP has mesopores with an average pore size of around 3.8 nm (Figure 2d).

Figure 2. (a) FT-IR spectra; (b) wide-angle XRD patterns; (c) nitrogen adsorption– desorption isotherms and (d) pore size distribution curve of PZS@M-SiO2@CoCuP.



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Figure 3. (a) XPS survey spectra of PZS, PZS@M-SiO2 and PZS@M-SiO2@CoCuP; High-resolution XPS spectra of PZS@M-SiO2@CoCuP in the (b) Co 2p, (c) Cu 2p and (d) P 2p regions. The presence of CoCuP and PZS@M-SiO2 can be confirmed by EDX measurement, shown in Figure S1b. Furthermore, XPS provides crucial information with regard of chemical state and element composition for PZS, PZS@M-SiO2 and PZS@M-SiO2@CoCuP. From XPS survey spectra (Figure 3a), the surface elements of these samples are composed of C, O, P, N and S. Besides, the PZS@M-SiO2 exhibits an extra Si element and PZS@M-SiO2@CoCuP shows additional Co and Cu elements. Figure 3b and c plot the high-resolution XPS spectra of CoCuP in the Co 2p and Cu 2p regions. As can be observed from Figure 3b, two typical peaks located at 782.5 and 798.7 eV are ascribed to Co 2p3/2 and Co 2p1/2 of Co species in the bimetallic phosphide, respectively, which have partially positive from that of Co metal, along with two apparent satellite peaks at 803.8 and 786.3 eV, are attributed to the shakeup excitation of the high-spin Co2+ ions.40 From Figure 3c, the Cu 2p XPS spectrum exhibits two principal peaks located at 933.0 and 953.2 eV corresponding to


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Cu 2p3/2 and Cu 2p1/2 of Cu species, respectively, and the satellites on the high energy side of Cu 2p3/2 at 941.5 eV, are originated from the shakeup excitation of the high-spin Cu2+ ions. In addition, the characteristic peak at 130.0 eV can be assigned to P species of the CoCuP, indicating the emergence of trace CoCuP, while the other peaks at 135.4 and 133.2 eV are ascribed to the P–O and P–C of the polyphosphazene, respectively,41

revealing

that

the

CoCuP

phase

is

formed

in

the

PZS@M-SiO2@CoCuP nanostructure (Figure 3d).

Figure 4. TEM observations of the ultrathin sections obtained from (a) TPU/PZS@[email protected] and (b) TPU/PZS@[email protected] composites.

3.2 Thermal and Fire Properties of TPU Composites Well dispersion of nanoadditives in polymer matrices can remarkable enhance the mechanical related performances of nanocomposites. Therefore, it is necessary to study the dispersion state of PZS@M-SiO2@CoCuP in TPU matrix. The morphology of the TPU composites was examined by TEM. Figure 4a shows the TEM images of TPU/PZS@[email protected] composite, the additives are homogeneously dispersed in TPU matrix and there are no obvious agglomerations of the nanotubes observed, this indicates that the nanotubes are uniformly distributed within the matrix for TPU composites, due to the strong interfacial interaction and excellent compatibility between the PZS@M-SiO2@CoCuP and TPU matrix. However, slight re-aggregation and non-uniform dispersion of these PZS@M-SiO2@CoCuP hybrids are observed for TPU/PZS@[email protected] composite in Figure 4b. In order to investigate the effect of PZS@M-SiO2@CoCuP on the thermal



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stability of TPU, thermal properties of TPU composites were investigated by TGA analysis under N2. Upon the Figure 5a that TPU composites exhibit a two-stage decomposition process. The first step is originated from the rupture of principal TPU chains; and the second step is reasonable for the further thermal degradation of isocyanates and polyols. In contrast to those of pure TPU, the initial decomposition temperature of the TPU/PZS@M-SiO2@CoCuP sample increased slightly, however, the char residue of this sample at 800 °C apparently increased, which are ascribed to the high thermal stability and catalytic carbonization effect of these additives including CoCuP, M-SiO2 and PZS. Moreover, the maximum mass loss rates of TPU/PZS, TPU/PZS@M-SiO2 and TPU/PZS@M-SiO2@CoCuP composites are shifted to lower values, compared to that of pure TPU, as observed from Figure 5b, implying the enhanced thermal stability of the TPU composites.

Figure 5. (a) TGA and (b) DTG vs. temperature curves; (c) HRR and (d) THR vs. time curves of TPU and its composites. The fire performances of polymer composites were evaluated by cone calorimeter test which reflects real combustion situation of various materials. The heat release rate


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(HRR) and total heat release (THR) curves of TPU composites are portrayed in Figure 5c and d. The PHRR and THR values for the TPU containing 2 wt% of PZS@M-SiO2 are significantly decreased by 44.3% and 13.4%, respectively. Moreover, addition of 2 wt% of PZS@M-SiO2@CoCuP into TPU further dramatically reduces the PHRR and THR values. For example, the PHRR and THR of TPU/PZS@[email protected] are reduced by 51.7% and 15.2%, respectively, compared to those of pure TPU. Extraordinarily, the incorporation of 3 wt% PZS@M-SiO2@CoCuP into TPU leads to a 58.2% and 19.4% reduction in PHRR and THR, respectively, revealing the superior fire retardancy of the nanohybrid. The significant reduction in the fire hazard of TPU/PZS@M-SiO2@CoCuP can be ascribed to the gas phase and condensed phase action: the cooperative catalytic carbonization effect of the M-SiO2 and CoCuP retards the escape of pyrolysis volatile; on the other hand, the well-dispersed PZS networks act as physical barrier to hinder the heat and mass transfer and the release of degradation products. 3.3 Pyrolysis Products Analysis of TPU Composites To further study the thermal decomposition behavior of TPU composites, the pyrolysis gases generation released from TPU and TPU/PZS@M-SiO2@CoCuP was evaluated by TG-FTIR technique. In particular, 3D TG-FTIR and FTIR spectra which are available at the maximum evolution rate during the thermal pyrolysis of TPU composites are shown in Figure S3a-c. Several characteristic pyrolysis products are clearly identified by typical FTIR absorption peaks. The representative strong signals of the pyrolysis products are located in the ranges of 500-1000 cm-1, 1200–1500 cm-1, 1600–1900 cm-1, 2200–2500 cm-1, 2750–3200 cm-1 and 3500–3750 cm-1. It is easy to observe a distinct reduction in strong signals when pure TPU is compared with TPU/PZS@M-SiO2@CoCuP. Figure 6a and b present the FTIR spectra of the pyrolysis gases products for TPU and TPU/PZS@M-SiO2@CoCuP at different temperatures. Several characteristic peaks for the primary pyrolysis products of TPU composites are listed as follow: 2190 cm-1 (CO), 2360 cm-1 (CO2), 722 cm-1 (HCN), 2930 cm-1 (hydrocarbons), 1740 cm-1 (carbonyl compounds) and 1510 cm-1 (aromatic compounds).42 It is noted that the pyrolysis gases products are relatively late released



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for

TPU/PZS@M-SiO2@CoCuP

composite,

implying

Page 16 of 32

that

introduction

of

PZS@M-SiO2@CoCuP retards the escape of pyrolysis volatile due to its cooperative catalytic carbonization effect.

Figure 6. FTIR spectra of the pyrolysis products for (a) TPU and (b) TPU/PZS@[email protected] at different temperatures.

To detect the concentration evolution of pyrolysis products, the intensities of the characteristic evolved gaseous products vs. time for TPU composites are shown in Figure 7. With the addition of 3.0 wt% of PZS@M-SiO2@CoCuP, the maximum absorbance intensities of typical pyrolysis gases products, such as CO, CO2, HCN, carbonyl compounds, aromatic compounds and hydrocarbons are shifted to lower values than those of pure TPU. The reduction in concentration of flammable pyrolysis products including aromatic compounds and hydrocarbons contributes to decreased heat release and smoke. It is generally accepted that HCN and CO are recognized as the primary toxic substances during the combustion of TPU, and the decreases of intensities of HCN and CO are beneficial for reducing smoke toxicity, which is in favor of enhancing the fire safety.


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Figure 7. Absorbance of pyrolysis products for TPU and its composites vs. time: (a) CO; (b) CO2; (c) HCN; (d) hydrocarbons; (e) carbonyl and (f) aromatic compounds.

3.4 Condensed Phase Flame-Retardation Analysis To further assess the flame retardant properties of the TPU composites, the char residues of TPU composites obtained from cone tests were evaluated. As shown in Figure S4a, pure TPU is melted to warping and burning dramatically during the combustion, a part of cracked residual char left after the cone test. From the digital photo of the char residue in Figure S4d, it can be observed that a more compact and continuous char surface is formed after the TPU/PZS@M-SiO2@CoCuP combustion. The microstructures of external residues for TPU, TPU/PZS, TPU/PZS@M-SiO2 and TPU/PZS@M-SiO2@CoCuP composites are shown in Figure 8a-d. It can be observed that a part of flaky char layer with large opening holes occurs in pure TPU (Figure 8a). However, the TPU/PZS composite presents a more compact char with small particles scattered on the surface (Figure 8b). In addition, it is clearly seen that the continuous and high dense char layer surfaces are formed after combustion of the TPU/PZS@M-SiO2 and TPU/PZS@M-SiO2@CoCuP (Figures 8c and d). It is widely reported that a more cohesive and compact char layer is beneficial for retarding the mass and heat transfer and escape of pyrolysis volatile, thereby enhancing the fire safety. The char layer with higher graphitization degree acts as a barrier which can



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effectively inhibit the inner materials exposed to fire. Raman spectroscopy was used to study the microstructure components of the char residues. Figure 8e exhibits two characteristic peaks at 1365 and 1596 cm-1, which are denoted as D and G bands, respectively. The area ratio of D band to G band (ID/IG) is adopted to measure the graphitization degree of residual char, and lower ID/IG value reveals higher graphitization degree.43 In detail, the ID/IG value for neat TPU is 3.80, whereas the TPU/PZS and TPU/PZS@M-SiO2 composites present lower ID/IG values, i.e., 3.56 and 3.40, respectively. In particular, the TPU/PZS@M-SiO2@CoCuP composite exhibits lowest ID/IG value (2.82), indicating the highest graphitization degree. It could be inferred that the enhanced flame retardancy may be attributed to the cooperative catalyzing carbonization effect among CoCuP, M-SiO2 and PZS.

Figure 8. SEM images of the char residues from (a) TPU, (b) TPU/PZS2.0, (c) TPU/[email protected] and (d) TPU/PZS@[email protected]; Raman spectra of the char residues of (e) TPU, (f) TPU/PZS2.0, (g) TPU/[email protected] and (h) TPU/PZS@[email protected].

In order to confirm the inference, the structural composition of residual char of TPU composites is further investigated. Figure 9a and b show the FTIR spectra and XRD patterns of char residues after cone test, respectively. As can be seen from Figure 9a, TPU composites exhibit analogous char structure to pure TPU. In the case of TPU/[email protected] and TPU/PZS@[email protected], two weak peaks at 800 and 1135 cm-1, corresponding to Si-O-Si bending and stretching vibrations appear,


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implying the presence of silicate phase in char residue. The P=N and P–O–Ar groups in the polyphosphazene units can be observed at 1186 and 941 cm -1, respectively, revealing the generation of crosslinked phosphorus oxynitride.44 As shown in Figure 9b, TPU/PZS2.0 and TPU/[email protected] exhibit similar diffraction peak to that of pure

TPU,

which

depicts

a

broad

peak

at

around

25.2°,

For

TPU/PZS@[email protected] (Figure 9b), these diffraction peaks labeled are indexed to the lattice fringe diffractions of copper and cobalt phosphides, revealing the remaining CoCuP phase in the char residue of TPU composites. In the XPS survey spectrum of TPU composites (Figure 9c), it is evident that the surface of TPU/PZS@M-SiO2@CoCuP sample are composed of C, O, N, P, S, Si, Cu and Co elements, and the relative amount of C element in the TPU composite is higher than that of pure TPU, due to the catalytic charring effect of CoCuP and superior thermal stability of PZS. In Figure 9d, the characteristic peak at 130.0 eV is ascribed to P species of the CoCuP phase. Besides, the other peaks at 133.1 and 135.2 eV are ascribed to the P-C and P-O of the PZS, respectively, implying the formation of crosslinked phosphorus oxynitride.44 In particular, Figure 9e and f present the high-resolution XPS spectra of TPU/PZS@[email protected] in the Cu 2p and Co 2p regions, respectively. The Cu 2p BE of 953.2 eV is negatively shifted from that of Cu metal (954.3 eV), and the Co 2p BE of 798.7 eV is positively shifted from that of Co metal (797.8 eV), suggesting the electron density transfer between P and Cu or Co element. Therefore, CoCuP phase plays a crucial role in redox reaction during TPU composites combustion.



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Figure 9. (a) FTIR spectra; (b) XRD patterns and (c) XPS survey spectra for char residue of TPU and its composites after cone calorimeter tests; and high-resolution XPS spectra for char residue of TPU/PZS@M-SiO2@CoCuP in the (d) P 2p, (E) Cu 2p and (f) Co 2p regions

3.5 Flame-Retardant and Toxicity-Reduction Mechanisms Based on the mentioned analysis above, it is noted that incorporating


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PZS@M-SiO2@CoCuP led to significant decrease on fire hazard of TPU, i.e., significant reductions in PHRR and THR, accompanied by formation of higher graphitized char layer. The responsible mechanism for the fire retardancy and toxicity reduction of PZS@M-SiO2@CoCuP in TPU matrix is proposed in scheme 2. On one hand, random distributed PZS network structure acts as physical barrier to inhibit the escape of pyrolysis volatile with heat and mass transfer, and the formation of crosslinked phosphorus oxynitride and carbonized aromatic networks during combustion also act as a important role in condensed phase to enhance char formation,11 in accordance with formation of more compact and continuous char layer for TPU/PZS@M-SiO2@CoCuP, as shown in Figure S4. Furthermore, CoCuP acts as transition metal catalyst to accelerate formation of P-rich carbonaceous char owing to its catalytic carbonization effect, confirmed by XPS analysis in Figure 9. The previous literatures have reported that the efficient solid acid, silica has a large number of acid sites, which can easily catalyzed the carbonization of degradation products in the presence of metal oxides.45 As a results, M-SiO2 can promote the catalytic efficiency of CoCuP. On the other hand, the concentration of toxic HCN and CO and other typical pyrolysis products released from the TPU composites significantly decreased, we propose that CoCuP may participate in transform of CO to CO2 and HCN to oxynitride through a redox cycle. Hence, the stable M-SiO2, crosslinked phosphorus oxynitride

and

CoCuP

on

the

external

char

layer

render

TPU/PZS@[email protected] higher thermal stability and physical barrier effect to retard mass and heat transfer and escape of pyrolysis volatile. It is responsible to conclude that the tripartite cooperative mechanism of the PZS@M-SiO2@CoCuP is the primary contribution for the improved flame retardancy and toxicity reduction of TPU composites.



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Scheme 2. Schematic illustration of mechanism for flame retardancy and toxicity reduction of TPU/PZS@M-SiO2@CoCuP in flaming TPU composites.

3.6 Mechanical Properties of TPU Composites As stated, the microstructure of PZS is similar to that of CNTs. In addition, the prior works have reported that the CNTs or PZS significantly enhance mechanical properties of polymer materials by transferring load across the nanotube–polymer interfaces.10,46 In general, both good dispersion of additives and strong interfacial interaction between additives and polymer matrices should have a positive impact on the mechanical properties of polymer composites. Figure 10 presents the storage modulus (E′) and the tan δ curves as a function of temperature for TPU and its composites. Considering the E′ of TPU/PZS@M-SiO2@CoCuP composites, the E′ values are totally enhanced in the whole temperature regions, revealing the reinforcement effect of PZS@M-SiO2@CoCuP in TPU matrix. In particular, the E′ value at −97 °C of TPU/PZS@[email protected] is increased by over 62.6% compared with that of pure TPU. Moreover, the E′ value at −97 °C of the composites is gradually increased with increasing the loading of PZS@M-SiO2@CoCuP. This improvement can be attributed to explanations that excessive CoCuP particles on the


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surface of PZS@M-SiO2 restrain the movement of molecular chains, leading to the increased E′. Furthermore, it is found the peak of tan δ slightly shifts to the higher temperature, and a slight increase of 4 °C is achieved at loadings of 3 wt% PZS@M-SiO2@CoCuP. The increase in Tg values is ascribed to the strong interfacial adhesion between well-dispersed PZS@M-SiO2@CoCuP and TPU. These results imply that the single-tube morphology and surface functionalization of PZS@M-SiO2 improve its interfacial interaction with the TPU matrix, compared to untreated PZS, thereby enhancing the mechanical and thermal performances of TPU composites. The mechanical properties of TPU and its composites are further investigated by tensile tests, and the typical stress-strain curves and the related mechanical data as shown in Figure 10c and d, respectively. In Figure 10d, TPU/PZS2.0, TPU/[email protected], TPU/PZS@[email protected] and TPU/PZS@[email protected] are denoted as TPU-1, TPU-2, TPU-3 and TPU-4, respectively. It is easy to conclude that the reinforcing effect of PZS@M-SiO2@CoCuP is rather significant: 3 wt% PZS@M-SiO2@CoCuP in the TPU matrix leads to a 136% increase in the tensile strength of TPU, whereas the elongation at break values are slightly changed by analyzing the tensile data. The improvements in the tensile strength may be attributed to two main factors: the hydroxyl groups on the surface of PZS@M-SiO2 enhance the crosslink density; the better dispersion state and strong interfacial adhesion between the nanohybrid and TPU matrix result in efficient load transfer from the polymer interface to PZS@M-SiO2@CoCuP nanostructures.



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Page 24 of 32

Figure 10. (a) Storage modulus (E') curves and (b) Tanδ curves of the TPU and its composites as a function of temperature; (c) The typical stress-strain curves and (d) tensile stress data of the TPU and its composites;

4. CONCLUSIONS In

the

work,

CoCuP

interconnected

with

polyphosphazene

nanotube@mesoporous silica was fabricated via analogous layer-by-layer assembly method and hydrothermal strategy, where, PZS was served as model to form a layer of M-SiO2 shell, and then decorated with a novel bimetallic phosphide phase during hydrothermal

procedure

to

generate

hybrid

nanostructure

defined

as

PZS@M-SiO2@CoCuP. This well characterized hybrid nanostructure presented multi-functional effect of improving flame retardant and mechanical performance of TPU composites. The microstructural analysis revealed that the strong interfacial adhesion between nanostructures and TPU matrix was formed. Incorporation of the PZS@M-SiO2@CoCuP into TPU matrix increased the initial degradation temperature and char yield, implying the enhancement of thermal stability. In addition,


Page 25 of 32

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introduction of 3 wt% PZS@M-SiO2@CoCuP into TPU led to PHRR and THR increased by 58.2% and 19.4%, respectively, and the concentration of toxic HCN and CO and other typical pyrolysis products released from the TPU combustion significantly decreased, revealing enhanced fire safety. It is noted that the storage modulus at −97 oC was significantly improved by 62.6% and glass transition temperature was shifted to higher value. The apparent enhancement in the fire safety performance was principally assigned to the tripartite cooperative effect of PZS@M-SiO2@CoCuP (catalytic activity of CoCuP/C system combined with catalytic charring effect of M-SiO2 and barrier effect of random distributed PZS network). This work presents a novel strategy for the design of hybrid nanoarchitectures for achieving their potential applications in polymer composites.



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Page 26 of 32

Supporting Information TGA curves of PZS, PZS@M-SiO2 and PZS@M-SiO2@CoCuP; EDX result of PZS@M-SiO2@CoCuP; SEM images of the fractured sections for TPU composites under different magnifications; Three-dimensional (3D) TG-FTIR spectra and FTIR spectra of gasified pyrolysis products for TPU composites at the maximum evolution rate; Digital photos of the char residues of TPU composites; TGA data for TPU and its composites in nitrogen; Cone calorimeter data of TPU and its composites.

ACKNOWLEDGEMENTS The work was financially supported by the National Natural Science Foundation of China (No. 51303167), the Fundamental Research Funds for the Central Universities (No.

WK2320000027)

and

the

National

High-tech

R&D

program

(No.

2016YFB0302104), the Opening Project of State Key Laboratory of Fire Science of USTC (No. HZ2017-KF02), and the grants from the Research Grant Council of the Hong Kong Special Administrative Region (GRF Project No. CityU 11301015 and Theme-based Research Scheme Project No. T32-101/15-R).


Page 27 of 32

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Constructing 3D Polyphosphazene Nanotube ... - ACS Publications

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