Nickel Metal-Organic Framework derived Hierarchically Mesoporous

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Nickel Metal-Organic Framework derived Hierarchically Mesoporous Nickel Phosphate towards Smoke Suppression and Mechanical Enhancement of Intumescent Flame Retardant Wood Fiber/Poly(lactic acid) Composites Lu Zhang, Siqi Chen, Ye-Tang Pan, Shuidong Zhang, Shibin Nie, Ping Wei, Xiuqin Zhang, Rui Wang, and De-Yi Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00174 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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ACS Sustainable Chemistry & Engineering

Nickel Metal-Organic Framework derived Hierarchically Mesoporous Nickel Phosphate towards Smoke Suppression and Mechanical Enhancement of Intumescent Flame Retardant Wood Fiber/Poly(lactic acid) Composites Lu Zhang a, Siqi Chen a,b, Ye-Tang Pan a, Shuidong Zhang c, Shibin Nie d, Ping Wei e, Xiuqin Zhang f, Rui Wang f and De-Yi Wang a* a.

IMDEA Materials Institute, Calle Eric Kandel, 2, 28906 Getafe, Madrid, Spain

b.

School of Material Science and Engineering, Tongji University, Cao’an Road 4800, Jiading District, 201804 Shanghai, P. R. China

c.

School of Mechanical and Automotive Engineering, South China University of Technology, Wushan Road 381, Tianhe District, 510640 Guangzhou, P. R. China

d.

School of Mining and Safety Engineering, Anhui University of Science and Technology, Taifeng Road 168, 232001 Huainan, P. R. China

e.

Nanning University, 53000 Nanning, Guangxi, P. R. China

f.

School of Materials Science and Technology, Beijing Institute of Fashion Technology, Yinghuayuan East Street A 2, Chaoyang District, 100096 Beijing, P. R. China

*Corresponding

author: De-Yi Wang, Tel.: 0034-917871888, E-mail: [email protected]

ABSTRACT: Over the past decade, the use of metal-organic framework derived materials has emerged as a novel direction to prepare high performance polymer composites. In this work, nickel based metal-organic framework (Ni-MOF) was synthesized via a costeffective and environmentally acceptable approach. Ni-MOF derived hierarchically mesoporous nickel phosphate was prepared via a facile hydrothermal method. Morphological evolution from Ni-MOF to nickel phosphate during the synthesis was clearly revealed. The micro-sized rod-like nickel phosphate was evaluated in both smoke suppression and mechanical enhancement of intumescent flame retardant wood fiber/poly(lactic acid) system. The cone calorimeter test showed a 43% reduction in total smoke production when 5 wt% ammonium polyphosphate was substituted by nickel phosphate. More importantly, both tensile and impact strength of the composites were 1

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improved with the addition of nickel phosphate. The as-synthesized nickel phosphate proved to be a promising substitution for ammonium polyphosphate in wood fiber/poly(lactic acid) composites. KEYWORDS: Metal-organic frameworks, Nickel phosphate, Polymer-matrix composites, Flame retardancy, Mechanical properties

INTRODUCTION Due to sustainability and environmental issues, the need for high-performance materials made from renewable resources is increasing worldwide. Wood fiber reinforced poly(lactic acid), a bio-composite containing biodegradable polymers reinforced with natural fibers, has been one of the most promising alternatives to the conventional petroleum based resins.1,2 As one of the very important parameters, flammability limits the application of biocomposites in fields such as electronics and transportations. Intumescent flame retardants (IFRs) have proved to be effective in improving flame retardancy of PLA,3 kenaf/PLA,4 flax/PLA5 and other polymer-matrix composites.6 However, a high loading of IFRs is needed to achieve good flame retardancy, which is always accompanied by the deterioration of mechanical properties.7 In some cases, the addition of IFRs also increases the amount of smoke production, which is one of the main fire hazard characteristics of polymer composites.8,9 Thus, many researchers hope to find strategies to improve the flame retardant efficiency, and at the same time maintain mechanical properties of polymer materials. Metallic compounds, like zeolite,10,11 montmorillonite,12 layered double hydroxide (LDH)13 and nickel phosphates14 have been used to improve the flame

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retardancy performance of IFRs in different polymer matrix composites. According to our previous work, the incorporation of nanoporous nickel phosphate and IFRs could strengthen the char layer formed during combustion and prevent the underlying polymer matrix from further burning.15 However, irregular large nickel phosphate particles made no contribution to the mechanical properties and the flame retardancy performance dropped continuously when the loading of nickel phosphate exceeded 1 wt%. To the best of our knowledge, porous micro-sized nickel phosphate might provide better synergistic effect with intumescent system to prepare high performance polymer composites. Due to the fast reaction between metal and phosphate anions, it is a great challenge to control the growth of mesoporous transition-metal phosphates. Until now, there are only a limited number of reports on formation of mesoporous transition metal phosphates.16,17 Metal-organic frameworks (MOFs) are an emerging class of materials constructed of metal ions or cluster and the organic linkers. With a high level of porosity with controlled pore size distribution, MOFs have been used in a variety of applications like drug delivery, gas storage or separation, catalysis and flame retardants in polymer composites.18-21 Besides the use of original MOFs, employing MOFs as the templates/precursors to construct functional materials such as metal oxides/sulfides/ phosphates have been extensively studied.22-25 Based on these studies, regular micro-sized metal phosphate could be derived from MOFs through substitution of organic linkers with phosphoric ions. Herein, a rod-like “green” Ni-MOF was synthesized in water-ethanol solvent under room temperature. The structure of Ni-MOF has a formulation of Ni(BTC)•12H2O, with a structure composed of 1-dimentional zigzag chains constructed from Ni(II) units and benzene-1,3,5-tricarboxylic acid (BTC) ligands.26,27 Interestingly, it is the first time found the morphology of Ni-MOF evolved from chrysanthemum-like to rod-like structures under 3



ultrasonic treatment. The mesoporous nickel phosphate (Ni-PO) was obtained through a facile MOFs-templated hydrothermal method. Burning behaviors and mechanical properties of the wood fiber/poly(lactic acid) composites containing APP and Ni-PO were evaluated through cone calorimeter, tensile and Charpy impact test, respectively.

EXPERIMENTAL SECTION Materials Nickel (II) nitrate hexahydrate [≥98%, Ni(NO3)2•6H2O], ammonium hydroxide (NH4OH) solution, ethanol (≥98%, absolute alcohol), Benzene-1,3,5-tricarboxylic acid (≥95%, BTC), Sodium phosphate (≥96%, Na3PO4) were all provided by Sigma-Aldrich Chemical Co. Deionized water was obtained from the water purification system by our laboratory. Poly (lactic acid) (PLA, 2003D) was purchased from Nature-Works LLC. This type of PLA had a D-isomer content of 4.3 wt%. Ammonia polyphosphate (APP, Exolit® AP 422, phase II, average particle size approx. 17 μm) was provided by Clarient. Steam exploded eucalyptus fiber was provided by South China University of Technology. The wood fiber had a diameter of 10-20 μm and length to diameter (L/D) ratio larger than 10. Synthesis of MOF-derived Ni-PO Ni-MOF was prepared from previously reported method with modification.25 2.5 g BTC was added into 400 ml ethanol water solution (volume ratio 1:1). The BTC solution was adjusted to pH 7 by dropwise addition of ammonia aqueous. 5 g Ni(NO3)2•6H2O was dissolved into 50 mL deionized water. The Ni2+ ion solution was transferred dropwise into BTC system under constant stirring. After 12 h reaction under room temperature, the light green colored precipitate was filtrated and then added into 500 mL ethanol water solution

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under ultrasonic treatment for 30 min. Finally, the Ni-MOF was collected by filtration and rinsed with ethanol and water repeatedly. The product was dried in a vacuum oven under 100 °C overnight. To synthesize Ni-PO, 0.98 g Na3PO4 was dissolved into 120 ml ethanol water solution (volume ratio 1:1), then 1 g prepared Ni-MOF was added into the solution. The mixture was continuously stirred for 15 min and followed by ultrasonic treatment for 30 min to prevent the aggregation. The resulting mixture was transferred to a 150 mL Teflon-lined stainless autoclave. The autoclave was held at 160 °C in the oven for 15 h. Finally, the light green Ni-PO powder was obtained through filtration and successive washing with deionized water and ethanol. Finally, Ni-PO powder was dried at 100 °C overnight in vacuum oven. Re-synthesis of Ni-MOF and Ni-PO After centrifugation, the solvent from hydrothermal synthesis was collected. Then 2 g Ni(NO3)2•6H2O dissolved in 20 mL deionized water was transferred dropwise into the recycled solution under constant stirring. Recycled Ni-MOF was obtained after the same washing and drying procedure. The re-synthesis of Ni-PO from recycled Ni-MOF followed the same procedure. Preparation of PLA Composites The preparation of W-PLA/APP/Ni-PO composites followed the traditional thermal plastic process by means of micro compounding and hot compression molding. Before the compounding, wood fiber, APP and PLA pellets were dried 6 h at 60 °C in a vacuum oven to remove moisture. The micro compounder (MC 15, Xplore) was used to mix wood fiber, APP, Ni-PO and PLA with a rotation speed of 100 rpm at 180 °C for 5 min. Then the composites were pressed into different specimens in a hot-plate machine (LabPro 400, Fontijne Presses) at 180 °C for 10 min under 30 MPa. The content of wood fiber was kept 5



at 25 wt%. The total content of APP plus Ni-PO in the composites was kept at 10 wt%. NiPO of different weight fractions (0, 1 wt%, 3 wt% and 5 wt%) were adopted to prepare PLA composites. For short, W-PLA-x represents x wt% Ni-PO and (10-x) wt% APP in the composites (W-PLA represents wood fiber reinforced PLA without flame retardant). Characterization Methods X-ray diffraction (XRD) patterns were collected on an X-ray diffractometer (Empyrean, PANalytical) using a Cu Kα radiation. The morphological features were characterized by scanning electron microscope (SEM, EVO MA15, Zeiss), high resolution transmission electron microscopy (HRTEM, Talos F200X, FEI), selected area electron diffraction (SAED) and energy dispersive X-ray spectrometry (EDS) mapping. Fourier transform infrared spectroscopy was obtained on a FTIR spectrometer (Nicolet iS50). Thermogravimetric analysis (TGA) was performed by the thermal gravimetric analyzer (TA Q50). X-ray photoelectron spectroscopy (XPS) was studied on VG ESCALAB MK II spectrometer with Al Kα excitation source under 10 kV and 10 mA. Limiting oxygen index (LOI) was performed on LOI instrument (FTT, UK) according to ASTM D 2863-2013. UL-94 vertical burning test was analyzed using the instrument (FTT, UK) according to ASTM D 3801- 2010. Cone calorimeter test (CCT) was carried out on FTT Cone Calorimeter following ISO 5660-1 under a heat flux of 35 kW/m2; the thickness of specimen was 3 mm and each sample was tested twice in the same conditions. Tensile test was performed on an universal electromechanical testing machine (INSTRON 3384, Instron) according to standard ASTM D638 at room temperature with a crosshead speed of 1 mm/min. The impact strength was measured using a Zorn Standal Instrumented Charpy Impact Tester (Germany). The samples were unnotched with a dimension of 50 mm × 6 mm × 4 mm. The impact velocity was 2.90 m/s (the corresponding 6


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drop height is 43 cm), and the energy of pendulum was 4.0 J, conforming to standard ISO 179.

RESULTS AND DISCUSSION Characterizations of Ni-MOF and Ni-PO Figure 1a showed the typical X-ray diffraction (XRD) patterns of Ni-MOF and NiPO. XRD pattern of Ni-MOF was in line with the previous report.26-27 The wide-angle XRD pattern of as-synthesized Ni-PO showed basically amorphous feature with several poor diffraction peaks corresponding to Ni(PO3)2 (JCPDS Card No. 28-0708).28 The wellresolved diffraction peak at small angel (1.44°) implied the mesostructured characteristic of Ni-PO.29 The N2 sorption measurements provided detailed information about the porosity of Ni-MOF and Ni-PO, as shown in Figure 1b (The enlarged nitrogen adsorptiondesorption isotherm curve of Ni-MOF was shown in Figure S1). The Ni-MOF had a low surface area of 6.1 m²/g. While the surface area of Ni-PO increased to 272.4 m2/g and the adsorption average pore width was 80.1 Å. The as-synthesized Ni-PO exhibited a combined I/IV type adsorption-desorption isotherm curve, indicating the existence of mesopores in Ni-PO. The FTIR spectra of Ni-MOF and Ni-PO were shown in Figure S2. The disappearing IR bands corresponding to BTC and emerging bands of PO43− confirmed the substitution reaction during the hydrothermal process. TGA curves of Ni-MOF and Ni-PO under nitrogen atmosphere in Figure S3 further proved that after the substitution of BTC by PO43−, Ni-PO was more thermally stable and no decomposition of organic ligands was observed during 400-500 °C.

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Figure 1. (a) XRD patterns of Ni-MOF and Ni-PO, (b) Nitrogen adsorption-desorption isotherm curves of Ni-MOF and Ni-PO (the insert was pore size distribution of Ni-PO). Morphologies of Ni-MOF template and MOF-derived Ni-PO were examined by SEM and TEM. Figure 2a presented the micro-sized rod-shaped morphology of Ni-MOF. Ni-PO inherited the rod-like morphology from Ni-MOF template (Figure 2b). Not like the smooth surface of Ni-MOF, Ni-PO was composed of loosely bonded microspheres, leading to numerous pores on the surface (Figure 2c). TEM image of the microsphere was shown in Figure 2d, clearly exhibiting its mesoporous structures. Insert of Figure 2d showed the diffuse rings in the selected area electron diffraction patterns (SAED), confirming the amorphous feature of Ni-PO. The microspheres with a radius of about 400 nm were formed during the hydrothermal synthesis through a recrystallization process. During the formation of Ni-PO, BTC ligands were substituted by PO43− units and dissolved in the water-ethanol mixed solution system. High angle annular dark field (HAADF) image and energy dispersive X-ray spectrometry (EDS) mapping results from an individual Ni-PO rod confirmed the uniform distribution of Ni, P and O elements throughout the rod-shaped frame (Figure 2e). The EDS spectra of Ni-PO (Figure S4) further validated the existence of principal Ni, P and O elements in the product.

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Figure 2. SEM morphology of (a) Ni-MOF, (b,c) Ni-PO, (d) TEM morphology of Ni-PO and (e) elemental mapping of an individual Ni-PO rod. The composition of Ni-MOF and Ni-PO was further studied by X-ray photoelectron spectroscopy (XPS). The survey spectrum (Figure 3a) revealed the presence of Ni, C, O in Ni-MOF and principal Ni, P, O in Ni-PO. Figure 3b showed the high-resolution Ni 2p XPS spectrum in Ni-PO. The peak of Ni 2p was deconvoluted into two spin-orbit doublets and two shake-up satellites, in which the two major peaks centered at 874.5 and 856.7 eV were indexed to Ni 2p1/2 and Ni 2p3/2, respectively, corresponding to characteristics of the Ni(II) state.30 As shown in Figure 3c, the asymmetric peak of P 2p spectrum in Ni-PO was split into two signals at 132.6 and 134.0 eV, which demonstrated that all P atoms were in the +5 state.30-31 Combined with the results from XRD and EDS, the as-synthesized Ni-PO was 9



further proved to be mainly nickel phosphate, containing a small amount of nickel metaphosphate.

Figure 3. (a) XPS spectra of Ni-MOF and Ni-PO, deconvoluted XPS survey of (b) Ni 2p and (c) P 2p in Ni-PO. In an attempt to understand the formation process of Ni-MOF rods, the intermediates were gathered after completing the dripping of Ni2+ solution at 0 h (MOF-0), 4 h (MOF-4), 12 h (MOF-12) and 12 h with 30 min ultrasonic treatment (MOF-12ut). Figure S5 showed the morphological evolution of Ni-MOF at different reaction times. In general, the morphology of the Ni-MOF was transformed from chrysanthemum-like structures into separated rods. The chrysanthemum-like MOF-0 structure was assembled by needle-like rods of varying sizes extending radically from center, even several new-born tiny (less than 10 μm) MOF-0 structures could be clearly observed. With increasing reaction time, the needles gradually grew up to a more regular rod-like shape for MOF-4 and MOF-12. Finally, separated Ni-MOF rods of average 1-2 μm width and 10-20 μm length were obtained after ultrasonic treatment. To our best knowledge, regular micro-sized rods might be better dispersed and caused less stress concentration in polymer matrix 10


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compared with chrysanthemum-like structures. The full morphological evolution process from Ni-MOF to Ni-PO was illustrated in Scheme 1.

Scheme 1. Schematic illustration of the morphological evolution from Ni-MOF to Ni-PO. Recycle of Ni-MOF template from the waste solvent after the hydrothermal synthesis of Ni-PO was performed. As shown in Figure S6, rod-like Ni-MOF precursor was again obtained, although with some impurities. Ni-PO prepared from the recycled NiMOF also showed similar morphology and dimension to the original one. According to these results, we concluded that the synthesis of Ni-PO was a sustainable and cost-effective method due to the recyclability of Ni-MOF template. Thermal Degradation Behaviors of PLA Composites The thermal stability of PLA composites was evaluated by thermogravimetric analysis (TGA) under nitrogen atmosphere. The detailed data from TGA curves was listed in Table S1. As shown in Figure 4a, W-PLA presented a one-stage main degradation process, with only 5.2 wt% residue left at 700 °C. In the presence of APP and Ni-PO, the thermal decomposition of PLA composites started at a temperature 40 °C lower than that of W-PLA. The weight loss took place in two steps for PLA composites containing flame retardant, as shown by the derivative thermogravimetric analysis (DTG) curves in Figure 4b. The emerging first step was mainly attributed to the earlier thermal decomposition of PLA triggered by the catalytic action of Ni-PO and condensed phosphoric acid formed by

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elimination of NH3 from APP.32 However, the maximum decomposition temperature of WPLA-x (1, 3, 5) was shifted to a higher temperature and more residue was obtained at 700 °C compared to W-PLA. These results proved that the incorporation of Ni-PO and APP into wood fiber reinforced PLA improved the thermal stability of the composites at higher temperature and promoted the formation of char residue. Furthermore, the highest char residue amount (16.7 wt%) was observed for W-PLA-1. When Ni-PO loading was further raised to 5 wt%, the residue weight of W-PLA-5 experienced a dramatic decrease from 21.6 wt% to 12.1 wt% between 400 °C and 700 °C. The weight decrease indicated the continued degradation of the char residue at higher temperature (beyond 400 °C). The increasing NiPO amount in the composite system did not further promote the charring process, which was due to the insufficient amount of condensed phosphoric acid produced from APP.

Figure 4. (a) TGA and (b) DTG curves of PLA composites. Fire Behaviors of PLA Composites LOI and UL-94 results of the composites were shown in Table S2. No rating in UL94 was observed for W-PLA. W-PLA-0 and W-PLA-1 reached a V-1 rating. Cotton was ignited by the dripping of W-PLA-3 and W-PLA-5 during UL-94 test, leading to a V-2

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rating for the materials. The LOI slightly decreased when APP was partly replaced by NiPO, which indicated the Ni-PO did not perform better than APP in fire inhibition under typical small-scale fire test conditions.33 Cone calorimetry is a quantitative test method that can effectively evaluate the combustion behaviors of polymer materials under real-world fire condition.34 Heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total smoke production (TSP) curves for W-PLA and its composites were displayed in Figure 5, respectively. The weight loss curves, CO2 and CO emission behaviors of the composites were displayed in Figure S7. Compared with W-PLA, W-PLA-0, W-PLA-1, W-PLA-3 and W-PLA-5 burned slowly and the peak heat release rate decreased drastically from 382.6 kW/m2 to 245.8 kW/m2, 245.2 kW/m2, 270.3 kW/m2 and 284.8 kW/m2 (Figure 5a), respectively. A similar trend was observed in THR curve of PLA composites (Figure 5b). The addition of APP and Ni-PO effectively reduced the THR of W-PLA, and the reduction was 25.8% (W-PLA-0), 26.3% (W-PLA-1), 25.7% (W-PLA-3) and 22.0% (W-PLA-5), respectively. It was worth noting that the HRR curve of W-PLA-0 and W-PLA-1 showed multi-peak phenomenon while only one peak was observed for W-PLA. The first main peak of HRR was assigned to the build-up of intumescent char layer. The intumescent char formed on the surface of the matrix acted as a thermal insulation layer and separated oxygen from underlying materials, leading to a drop of heat release rate. The second peak was due to the further degradation and breakage of the char structure at a higher temperature, which increased the burning area exposed to air. Thermal injury and smoke inhalation is the leading cause of death due to fires. And the production of toxic smoke in fires has become a major threat to victims of accidental fires in closed compartments. According to our previous research, pure PLA itself produced a very limited amount of 13



smoke.35 However, wood fiber with the main components as cellulose and lignin, could form smoke via different routes during the combustion.36 The addition of APP into W-PLA effectively decreased the heat release rate while at the same time endowed an increased smoke production of the composites. As shown in Figure 5c, the peak smoke production rate decreased with the increasing loading amount of Ni-PO. The smoke production rate of W-PLA-0 reached to its peak at 0.0124 m2/s during the early combustion stage, while only a low peak (around 0.0031 m2/s) for W-PLA-5 was observed. The TSP was reduced to 0.64 m2 (W-PLA-1), 0.55 m2 (W-PLA-3) and 0.41 m2 (W-PLA-5), which was 88.9%, 73% and 57% relative to that of W-PLA-0 (Figure 5d).

Figure 5. (a) HRR, (b) THR, (c) SPR and (d) TSP vs time curves of PLA composites.

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Considering both APP and Ni-PO worked mainly in the condensed phase of combustion, the morphology of char residues after the cone calorimeter test were studied to understand the flame retardant mechanism. Digital photos and SEM images of the char residues were shown in Figure 6. Only a small amount of wood residue was left for WPLA (Figure 6a), while intact char layers were preserved for W-PLA-0, W-PLA-1 and WPLA-3 (Figure 6b-d). However, the char layers cracked into several parts (Figure 6e) due to the increased rigidity with the addition of high loading Ni-PO. The rigid char layers could protect the underlying polymeric materials at the initial combustion stage, while it was not stable enough to support itself and broke into parts at the later stage of combustion, leaving the underlying polymer matrix directly exposed to air. The appearance of the char residue after the pressure of 500 g weight in Figure S8 validated the high rigidity of WPLA-5 char residue compared to W-PLA-0. This could be one important reason for the relatively increased THR of W-PLA-5 compared to W-PLA-0.

Figure 6. Digital photos and SEM images of residue (a,f) W-PLA, (b,g) W-PLA-0, (c,h) W-PLA-1, (d,i) W-PLA-3 and (e,j) W-PLA-5. The main sources of smoke were soot particles originated from the released gas products and micro residue pieces detached from the char layers.37,38 As shown in Figure 6f and g, the residue of W-PLA was composed of irregular small pieces, while loosely 15



bonded fiber-shaped residue with a clear outline was observed for W-PLA-0. After the addition of Ni-PO, the fiber-shaped residue tended to merge with each other, leading to more compact char layers (Figure 6h-j). The char residue was tightly held by Ni-PO, which greatly reduced the amount of detached residue pieces. During combustion, the hierarchically mesoporous Ni-PO provided countless nucleation sites for the formation and growth of soot particles, which effectively retained the smoke particles in the residue. To verify it, the morphology and EDS spectrum of Ni-PO after combustion of W-PLA-5 was studied. As shown in Figure S9, Ni-PO rods were covered with a thick layer of char and the volume of char covered Ni-PO greatly expanded compared to the pristine Ni-PO. Moreover, the formation of phosphorus-containing smoke was reduced with the decreasing amount of APP.9 For these reasons, compared with APP, Ni-PO exhibited obvious smoke suppression effect in the flame retardant PLA composites (illustrated in Scheme S1). Mechanical Properties of PLA Composites The tensile and Charpy impact test were accomplished to study the influence of NiPO on the mechanical properties of PLA composites. Tensile strength and Young’s modulus of the specimens were presented in Figure 7a. The tensile strength for W-PLA was 52.5±1.8 MPa, and the Young’s modulus was 2577±35 MPa. With the addition of 10 wt% APP, the tensile strength of W-PLA-0 declined significantly to 44.8±1.9 MPa. On the contrary, both the tensile strength and Young’s modulus experienced an increasing trend with the increasing loading of Ni-PO. When 5 wt% Ni-PO and 5 wt% APP were added into W-PLA, there were only a 2.5% decrement in tensile strength and a 14.2% increment in Young’s modulus compared to W-PLA. As shown in Figure 7b, the impact strength of WPLA-0 (9.5±1.3 kJ/m2) dropped to nearly half of W-PLA (16.6±1.2 kJ/m2). There was also

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an ascending trend in impact strength from low to high loadings of Ni-PO. The impact strength of W-PLA-5 increased to 12.4±1.6 kJ/m2.

Figure 7. (a) Tensile strength and Young’s modulus, (b) impact strength of PLA composites. To investigate the dispersion of Ni-PO and APP in PLA matrix, SEM images for freeze-fracture surface of PLA composites containing only Ni-PO or APP were observed. As shown in Figure S10, large APP particles could be well recognized under SEM, while Ni-PO particles with much smaller size were well dispersed in PLA matrix. To further investigate the dispersion, elemental analysis of the composite fracture surfaces were performed, as shown in Figure S11 and Figure S12. The results further revealed a good dispersion of Ni-PO particles in PLA matrix, even at a high concentration. To understand the mechanical failure mechanism, fracture surface of wood fiber/PLA composites after tensile test were observed by SEM, as shown in Figure 8. The difference in polarity caused a weak interfacial adhesion between APP and PLA matrix (marked by green arrow, Figure 8b and e).39,40 The EDS mapping of P element (Figure 8c) confirmed the existence of large APP particles in the composites. Besides, a weak interfacial adhesion between PLA and wood fibers was observed in W-PLA-1 (Figure 8d) compared with W-PLA (Figure 8a).

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There might be a negative effect of APP on the adhesion between PLA and wood fibers.4 The mesoporous Ni-PO owed a much higher surface volume compared with APP, which facilitated its interaction with PLA matrix.41 Compared to well-dispersed Ni-PO with a smaller particle size (1-2 μm width), the commercial APP with a size around 17 μm and poor adhesion to PLA matrix might act as failure initiation sites facilitating crack propagation, which greatly impaired both the tensile and impact strength.42

Figure 8. SEM images of fracture surface (a) W-PLA, (b) W-PLA-0, (c) EDS mapping of P element for W-PLA-0, (d) W-PLA-1, (e) W-PLA-3 and (f) W-PLA-5.

CONCLUSIONS To conclude, a rod-shaped Ni-MOF was designated as the template to prepare hierarchically mesoporous nickel phosphate through substitution of organic linkers with the phosphoric ions. The SEM results revealed the morphology of Ni-MOF was transformed from chrysanthemum-like structures into separated rods after the ultrasonic treatment. The as-synthesized Ni-PO was used to partly replace APP in wood fiber reinforced PLA 18


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composites. The results showed that the addition of Ni-PO effectively reduced the total smoke release amount of W-PLA. In addition, the mechanical properties were gradually improved with the increasing loading amount of Ni-PO. Though the peak heat release rate was not decreased, the total heat release of the flame retardant composites containing NiPO was kept at a low level compared to W-PLA. When partially substituted by Ni-PO, the insufficient amount of APP did not effectively promote the formation of intumescent char layers, leading to a weak protective barrier effect. However, it was believed that if combined with a higher proportion of APP, the Ni-PO could be more effective as a synergistic agent to improve the flame retardancy of PLA composites. Generally, the MOFderived Ni-PO showed a promising potential in the preparation of high performance polymer composites.

ASSOCIATED CONTENT Supporting Information Figure S1. Nitrogen adsorption-desorption isotherm curve of Ni-MOF. Figure S2. FTIR spectra of Ni-MOF and Ni-PO. Figure S3. TGA curves of Ni-MOF and Ni-PO. Figure S4. EDS spectra of MOF-derived Ni-PO. Figure S5. Morphological evolution of Ni-MOF at different reaction time. Figure S6. SEM images of (a) original Ni-MOF, (b) recycled Ni-MOF (c) re-synthesized Ni-PO. Figure S7. (a,b) Residue weight, (c) CO2 and (d) CO production vs time curves of PLA composites. Figure S8. Photos of CCT residues after pressure of 500 g weight (a) W-PLA-0, (b) W-PLA-5. Figure S9. SEM image and EDS spectrum of Ni-PO after cone calorimeter test of W-PLA-5. Figure S10. SEM images of fracture surface (a,b) PLA-7.7APP, (c,d) PLA-7.7NiPO. Figure S11. SEM images and

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EDS mapping of fracture surface for PLA-15.4APP. Figure S12. SEM images and EDS mapping of fracture surface for PLA-15.4NiPO. Scheme S1. The smoke suppression effet of Ni-PO in PLA composites. Table S1. TGA data of PLA composites. Table S2. LOI and UL-94 results of PLA composites.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This research is partly funded by Spanish Ministry of Economy and Competitiveness (MINECO) under Ramón y Cajal grant (RYC-2012-10737), Joint Research Fund for Overseas Chinese, Hong Kong and Macao Young Scholars (51628301), and the National Natural Science Foundation of China (51673003). Also, one of our authors (Mr. Lu Zhang) would thank the financial support from China Scholarship Council (ID: 201506370020).

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Synopsis Micro-sized Ni-PO rods via green synthesis towards smoke suppression and mechanical enhancement in bio-composites.


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Nickel Metal-Organic Framework derived Hierarchically Mesoporous

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