Preparation of MetalâOrganic Frameworks and Their Application as

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Preparation of metal-organic frameworks and their application as flame retardants for polystyrene Yanbei Hou, Weizhao Hu, Zhou Gui, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04920 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 8, 2017

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Industrial & Engineering Chemistry Research

Preparation of metal-organic frameworks and their application as flame retardants for polystyrene Yanbei Hou, a Weizhao Hu, a Zhou Gui, a∗ Yuan Hua∗ a

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Abstract: In this work, iron-based and cobalt-based metal-organic frameworks (MOFs) were successfully synthesized by a facile solvothermal method. The obtained MOFs were added into polystyrene (PS) as flame retardants for the first time. The results of thermal gravity analysis and cone calorimeter indicated the addition of MOFs significantly enhanced the thermostability and flame retardancy of the PS composites. Compared with neat PS, more than 14% and 28% decrease in the peak heat release rate were observed for PS/Fe-MOF and PS/Co-MOF, respectively, suggesting a flame retardant effect of MOFs. Based on thermogravimetric analysis-infrared spectrometry (TG-IR) results and analysis of combustion residues, the possible mechanism of the enhanced thermostability and flame retardancy of the PS composites was proposed as the combination of thermal barrier effect and catalytic effect of MOFs, which would provide promising application in the development of fire safety polymer materials. Key words: metal-organic framework; flame retardancy; polystyrene.

∗

Corresponding author. Tel/Fax: +86 551 63601669 (Z. Gui) +86 551 63601664 (Y. Hu). E-mail address: [email protected](Z. Gui) [email protected] (Y. Hu).

ACS Paragon Plus Environment


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1. Introduction Metal-organic frameworks (MOFs) attract broad interests that arise from viewing MOFs as gas storage and separation, catalysis, sensing, and drug delivery

1-4

. These

materials are constructed by joining metal-containing units with organic linkers, using strong bonds to create open crystalline frameworks with permanent porosity. Tunable functionalities and convenient synthesis increase their attraction for scientific researches. Despite

these

interesting

features,

some

MOFs

exhibit

weak

thermostability that has hindered their deployment. Due to the slow release of guest molecules and unreacted species, the weight loss of metal–organic framework begins with heating 5. Zeolitic imidazolate frameworks (ZIFs) are subclass of MOFs, typically comprised by divalent metal cations and imidazolate bridging ligand. Compared to most of MOFs, ZIFs exhibit better thermal, hydrothermal and chemical stability 6, which is contributed to closer interaction between metal cations and the nitrogen atoms of imidazolate ligand than that of carboxybenzene. Inorganic-organic hybrid nature of MOFs usually results in improved compatibility with polymers. Ferraris et al. have developed a novel approach for compatibilizing immiscible polymer blends using ZIF-8 7. The selective location of ZIF-8 on two phase boundary reduced the interfacial energy of immiscible polymers, contributing to better compatibility. MOF/polyvinylidene fluoride membranes were prepared by Zhang’s group 8. As synthesized membranes exhibit hollow fiber structure and enhanced structural stability, and therefore hold excellent H2 permselectivity. Walter and coworkers presented recent development of porphyrin-based polymers/MOF with


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various optoelectronic applications 9. As the variety and diversity of new polymers coordination systems grow, it is excepted than these platforms will provide many novel directions for integrating polymers and MOFs into various functional applications. Recently, polymeric composites (PCs) have drawn widespread attention for their outstanding performance and facile preparation process

10, 11

. However, the inherent

flammability limits many potential applications for safety considerations. The development of new PCs with enhanced thermal stability and fire resistance properties has been an active area of research to improve public safety

12

. To our

knowledge, the application of MOFs in polymers as flame retardants (FRs) has not been previously reported. With excellent thermostability, MOFs can enhance thermal stability of polymers in theory, which is similar with inorganic FRs

13

. Organic

ligands not only contribute to superior compatibility but provide ignition-proof elements or groups, such as nitrogen-containing groups and aromatic derivative. Incorporating MOFs with polymers to construct functional composites theoretically can reduce the fire risk of complex materials. To verify the feasibility of the hypothesis, two kinds of MOFs, iron-based metal-organic framework (Fe-MOF) and cobalt-based metal-organic framework (Co-NOF), were prepared by hydrothermal reaction method and applied as FRs for polystyrene (PS) in this work. The thermostability and combustion behaviors were characterized by thermal gravimetric analyzer (TGA) and cone calorimeter. Meanwhile, feasible flame-retardant mechanisms for two different MOFs were



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provided. 2. Experimental sections 2.1 Materials Iron nitrate nonahydrate (Fe(NO3)3·9H2O), 1,4-benzenedicarboxylate acid (H2BDC), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 2-methylimidazole and methanol were provided by Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Polystyrene (PS) was supplied by Yangzi petrochemical co., Ltd. (Nanjing, China). 4,4-Diaminodiphenylmethane (DDM) was provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water was used for all experiments unless otherwise stated. Chemical reagents used for the preparation of Fe-MOF and Co-MOF nanocomposites were all of analytical grade purity and were used without further purification. 2.2 Preparation of MOFs MOFs were prepared by the solvothermal methodology. In typical way, 2.5 mmol Fe(NO3)3·9H2O was mixed into 10 mL H2BDC aqueous dispersion (0.54 M), followed by magnetic stirring for 0.5 h. Then the homogeneous dispersion was transformed into an autoclave (80 mL) and held temperature at 200 oC for 4 h. The product was filtered, washed with deionized water, and then dried at 80 oC in a vacuum oven. The preparation process of Co-MOF was similar with that of Fe-MOF, 2.82 mmol Co(NO3)2·6H2O was dissolved into 2-methylimidazole solution (44 mmol) of methanol. The mixture was performed the same thermal treatment process as Fe-MOF


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and collected in the same way. 2.3 Preparation of PS/MOF composites PS/MOFs composites were prepared using a facile solution blending method. Briefly, the preparation of PS composites with 2 wt% MOFs were performed as follow: as-prepared MOFs (0.8 g) were added into 200 mL N,N-dimethylformamide (DMF) and sonicated until completely dispersed. The dispersion solution was heated to 80 oC and 39.2 g PS was added into the system with vigorous stirring for 2 h. The obtained mixtures were kept in a drying oven at 80 oC 12 h to remove excess solvent. Internal mixer was applied to exclude remained solvent in composite at 185 °C. Pure PS underwent the same process to ensure preciseness of experiments. 2.4 Characterization The morphology and structure of MOFs were studied by transmission electron microscopy (TEM, JEM-2100F, Japan Electron Optics Laboratory Co., Ltd, Japan) with an accelerating voltage of 200 kV and an FEI Sirion 200 scanning electron microscope (SEM) at an acceleration voltage of 5 kV. The fracture appearance and residues of PS composites were also observed by SEM. X-ray diffraction (XRD) measurements were performed on a Japan Rigaku D Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Kα tube and a Ni filter (λ=0.1542 nm). The scanning rate was 4°·min-1 and the range was 5-65°. X-ray photoelectron spectroscopy (XPS) was recorded using a Kratos Axis Ultra DLD spectrometer employing a monochromatic Al Kα X-ray source (hν = 1486.6 eV), hybrid (magnetic/electrostatic) optics and a multi-channel plate and delay line



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detector. The thermal behaviour of the PS composites was investigated by using a Q2000 differential

scanning

calorimeter

(DSC)

(TA

Instruments

Inc.,

USA).

Thermogravimetric analysis (TGA) was performed on a TGA Q5000IR (TA Instruments, USA) thermo-analyzer instrument from room temperature to 800 °C at a heating rate of 20 °C·min-1 under an air flow of 60 mL·min-1. Samples of about 5.0 mg were measured in an alumina crucible. Flammability of the samples was characterized using a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660. Square specimens (100×100×3 mm3) were irradiated at a heat flux of 35 kW·m-2, corresponding to a mild fire scenario. All the measurements were repeated three times and the results were averaged. Thermogravimetric analysis-infrared spectrometry (TG-IR) was conducted using a TGA Q5000IR thermogravimetric analyzer that was linked to a Nicolet 6700 FTIR spectrophotometer. About 5-10 mg of the sample was put in an alumina crucible and heated from 30 to 700 °C. The heating rate was 20 °C·min-1 (nitrogen atmosphere, flow rate of 30 mL·min-1). Laser Raman spectroscopy (LRS) measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line. 3. Results and discussion 3.1 Characterizations of MOFs


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Fig. 1 SEM (a, b) and TEM (c, d) images of as-synthesized MOFs As prepared reddish-brown Fe-MOF and aubergine Co-MOF were powder states. The representative SEM and TEM images of exfoliated MOFs are depicted in Fig. 1. It is obvious that Fe-MOF particles show nano-scale size, while Co-MOF exhibits better crystallization shape and bigger size. From SEM images (a, b) of as-synthesized MOFs, separate presence particles illustrate weaker interaction of Co-MOF than that of Fe-MOF, implying better dispersion in matrix. The crystallographic structure and phase purity were examined by X-ray diffraction (XRD) measurement. Fig. 2a shows the XRD patterns of Fe-MOF and Co-MOF, in which all peaks are in agreement with previous reports

14, 15

confirmed the successful synthesis of the metal organic

frameworks. Reaction temperature in this work is slightly higher than that of previous method, while the reaction time is shorter. The characteristic XRD patterns of the MOFs indicate that the change of reaction conditions had little influence in the formation of MOFs. X-ray photoelectron spectroscopy (XPS) technology was applied to determine the composition and chemical state of the atoms in the synthesized



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MOFs. The wide XPS spectrum in Fig. 2b lists the binding energy at 284.8 eV, 399.0 eV, 532.7 eV, 711.4 eV and 781.3 eV, respectively corresponding to C1s, N1s, O1s, Fe2p, Co2p

16-18

. There is no impurity elements detected both in Fe-MOF and Co-MOF,

indicating that the chemical compositions of MOFs were obtained as design. To further understand the structure of the composites, XPS curves of Fe2p, Co2p are presented in Fig. 2c-d. Iron and cobalt severally exhibit +Ⅲ and +Ⅱ formal oxidation number. The Fe2p and Co2p spectrums of the target Fe-MOF and Co-MOF shows two main components and satellite peaks. These values, as show in figure, are in agreement with the previously reported data 18, 19. All above results illustrate that Fe3+, Co2+ with organic ligands composed the synthesized MOFs with typical crystallographic structure.

Fig. 2 XRD patterns (a) and XPS (b-d) spectra of Fe-MOF and Co-MOF 3.2 Characterizations of PS composites The XRD technique was used to characterize the crystallographic structure of


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fillers in matrix, to verify the influence of machining process on MOFs. XRD curves were presented in Fig. 3a. Compared with neat PS, there are some spikes were plotted in XRD curves of PS/MOFs composites, which are corresponding to characteristic peaks of MOFs respectively, as confirmed by Fig. 2a, demonstrating the crystal integrity of fillers. DSC was utilized to characterize the glass transition behaviour of PS composites. DSC curves of PS and its composites marked with glass transition temperature (Tg) are shown in Fig. 3b. Adding MOFs individually increases the Tg value of PS/MOFs composites, as a result of the hindrance of molecular mobility in the vicinity of MOFs. The organic ligands from MOFs enhanced homodisperse of fillers in the PS matrix, which can lead to a high density of confinement effect. In case of PS/Fe-MOF, the increase in Tg is slightly higher than PS/Co-MOF. It results from smaller particle-size and benzene-containing structure of Fe-MOF, which provide superior compatibility of filler in matrix. The dispersion and interfacial interaction between fillers and the matrix display the key factors to influence the properties of polymeric composites. To evaluate the dispersion state of MOFs hybrids in the PS matrix, the morphologies of fractured surfaces were observed by SEM (Fig. 3c-e). Pure PS has a smooth fracture surface with few cracks, indicating the typical brittle rupture. Different from neat PS, fractured surface of PS composites exhibited distinctly different morphologies. The PS/MOFs composites present rough fractographic features with obvious pull-out structure. Energy dispersive spectrometer (EDS) was applied to confirm the identity of MOFs in PS matrix. The accurately appeared element peaks verified the existence



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of Fe-MOF and Co-MOF. From Fig. 3d, individual Co-MOFs are well dispersed and embedded in the PS matrix without aggregation, indicating the strong interfacial interaction between Co-MOF particles and matrix. The blurry boundary of surface can be observed between Fe-MOF particles and matrix (Fig. 3e) which is probably attributed to the excellent compatibility of organic ligands from Fe-MOF and PS.

Fig. 3 XRD patterns (a), DSC curves (b) and SEM images of the fractured surface of PS (c), PS/Fe-MOF (d), and PS/Co-MOF (e) composites. 3.3 Thermostability of MOFs and PS/MOFs composites Thermostability of Fe-MOF and Co-MOF was tested by TGA under air condition. As show in Fig. 4a, compared with Fe-MOF, the mass loss of Co-MOF appeared at much higher temperature and showed a drastic decrease at 400 °C. The slightly weight gain may be contributed to the oxidation of cobalt complex. DTG curves demonstrate the degree of degradation rate. As shown in Fig. 4b, it is obvious that the degradation temperature of Fe-MOF is lower than that of Co-MOF, while the degradation rate of it is lower. The TG curves of MOFs under nitrogen are showed in Fig. S1a. Both


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Fe-MOF and Co-MOF still exhibited weight loss at 700 oC, indicating the excellent thermal stability of MOFs under oxygen-free amorphous. The TG and DTG curves of PS and its composites were presented in Fig. 4c-d. The decomposition temperature of PS/MOFs, especially PS/Co-MOF, is higher than that of pure PS, which is contributed to excellent thermal stability of MOFs. The temperatures corresponding to 5, 10 and 50 wt% weight loss (T-5%, T-10%, and T-50%), which are used to evaluate the decomposition of PS on the onset stage and half period temperature, are listed in Table 1. It is summarized that the T-5% and T-10% of the two composites are higher than that of pure PS, which indicates that the presence of MOFs defers the initial thermal degradation of PS. As shown in Table 1, the above characteristic temperatures are significantly enhanced after the addition of the MOFs. From DTG curves, temperatures of initial decomposition (Ti) and the maximum degradation rate (Tmax) are remarkably delayed, demonstrating that the pyrolysis processes of composites are retarded. These results suggest that incorporating MOFs into PS will hinder the thermal degradation of PS matrix and markedly enhance thermostability of PS. In addition, char yields of composites are higher than that of PS, implying the inclusion of MOFs promotes char formation of PS/MOFs during the decomposition process. The decomposition of MOFs needs great heat, which decreases the heat absorption of matrix. Meanwhile, MOFs and their degradation products block the heat transfer, and thus the thermostability of composites is improved.



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Fig. 4 TG and DTG curves of MOFs (a, b) and PS/MOFs composites (c, d) Table 1 TG curves of PS and its composites Sample code T-5% T-10% T-50%

Ti

Tmax

Residues at 800 oC (%)

PS

293

306

352

260

365

0.77

PS/Fe-MOF

298

311

363

267

380

2.36

PS/Co-MOF

347

357

389

300

392

2.67

3.4 Pyrolysis gaseous product characterization To investigate the adsorption and catalytic oxidation behaviour of MOFs, the TG-IR technique was performed to detect the gaseous products during the thermal decomposition of PS composites and the results were plotted in Fig. 5. Wilkie et al. demonstrated that pure PS primarily decomposes into oligomers of phenyl alkenyl 20. FT-IR spectra obtained at the maximum evolution rate during the thermal


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decomposition of PS and its composites are shown in Fig. S1b. The typical thermal decomposition products of the composite were similar with pure PS. These peaks at 1496, 772 and 698 cm-1 are associated with aromatic compounds, while the peak at 1596 cm-1 is correlated with alkenyl compounds

21, 22

. All the assembled systems

exhibit a similar pyrolysis gas evolution as PS. To further highlight the difference between PS and its nanocomposites, the principal pyrolysis products as a function of temperature at different wavenumbers are plotted in Fig. 5b-d. It can be clearly seen that addition of MOFs into PS matrix can reduce evolution of the aromatic compounds, implying the decrease of toxic organic volatiles. The decreased amount of the organic volatiles further results in the suppression of smoke, since the aromatic compounds may be aggregated to form smoke, indicated superior fire safety of PS composites. Between the two composites, Co-MOF exhibited superior catalytic effect in reducing the release of aromatic compounds, which may attributed to the char-forming effect of cobalt compounds during combustion 23, and thus decrease the emission amount of organic compounds.



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Fig. 5 Comparison of pyrolysis gaseous products of PS and its composites 3.5 Flame retardancy assessment of MOFs and PS/MOFs composites Cone calorimeter is one of the most effective methods for investigating the fire resistance properties of polymers. The total heat release rate (HRR), in particular the peak heat release rate (pHRR) and total heat release (THR) value, proves to be one of the most important parameters to evaluate fire safety 24. The HRR and THR plots for pure PS and its composites are shown in Fig. 6a-b. Compared with pure PS, the HRR curves of PS/MOFs are accompanied by prolongation of burning time and relative flat lines. The pHRR values of PS/Fe-MOF and PS/Co-MOF composites are 14.4% and 28.0% lower than that of pure PS, while THR values are 4.0% and 17.6% decrease. It is evident that the addition of MOFs improves the fire resistance of the composites. Time to pHRR is delayed more than 40 and 50 second for PS/Fe-MOF and PS/Co-MOF composites respectively, comparing with neat PS, which confirmed that


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the fire safety of composites is enhanced by adding MOFs. It can be speculated that the barrier of MOFs suppressed the heat and fire spreading during burning, which is beneficial for the decrease of pHRR and THR. Meanwhile, the catalytic effect of MOFs reduced the emission amount of decomposition products, which directly contribute to retarding the combustion of matrix. Mass loss and mass loss rate are another important data obtained from cone test. The mass loss-combustion time curves are plotted in Fig. 6c. There are 2.5 and 2.7 wt% of residues left corresponding to the PS/Fe-MOF and PS/Co-MOF respectively, while nearly no residue left for neat PS. The curves showed that the inclusion of MOFs improved the char formation processes of composites. The addition of the additives prolonged the burning time and decreased the mass loss rate markedly, which is accord with the heat release plots. Carbon monoxide (CO), generated during incomplete combustion, is the most lethal factor in the fire. Thus, CO suppression is an inevitable issue for the flame-retardant polymers. It has been confirmed that transition metal compounds can catalyze oxidation of CO

23, 25

. Fig. 6d clearly

presented the CO emission behaviors of PS and its composites during decomposition. It is obvious that the release rate of CO of composites was significantly restricted, which is ascribed to the addition of transition metal-containing MOFs. The CO release peaks of composites were significantly delayed, which should be very helpful for the people to get away from the fire. In addition, total CO productions of composites were 3.2% and 8.3% lower than that of neat PS, respectively for PS/Fe-MOF and PS/Co-MOF, indicating the fire safety of composites was enhanced.



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Fig. 6 HRR (a), THR (b), weight loss (c), and CO (d) curves of PS and its composites obtained by cone calorimeter 3.5 Speculation of Flame-retardant mechanisms To further understand the flame-retardant mechanism of MOFs, gaseous products of MOFs were collected and identified during the thermal decomposition. FT-IR spectra obtained at the maximum evolution rate during thermal decomposition of MOFs are shown in Fig. 7. The intensive absorbance at 2360 cm-1 origination from the generation of CO2 during pyrolysis of Fe-MOF is very conspicuous, indicating that the main gaseous product of Fe-MOF is nontoxic and noncombustible. The released CO2 dilute concentration of fuel in the gaseous phase, and then retard the combustion of matrix. Products generated from Co-MOF are mainly composed by –C=C- (1680 cm-1), -C-H (1530 cm-1), -NH (680 cm-1) and secondary amine (3300 cm-1)

26, 27

. Intense decomposition of Co-MOF occurs at 540 oC under nitrogen

atmosphere. Free groups were released during pyrolysis process, including -C-H and


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-N-H, which can neutralize free radical released by matrix in gaseous phase and free oxygen in air.

Fig. 7 FT-IR spectrum of pyrolysis products for Fe-MOF (a) and Co-MOF (b) at the maximum decomposition rate The morphologies and components of combustion residues were investigated by SEM, XRD and EDS. It is obvious to observe open-framework structures with micro-scale diameter composed by particles, both in residue of PS/Fe-MOF (Fig. 8a) and PS/Co-MOF (Fig. 8b). The porous structures can retard the diffusion of fuel released by PS matrix from inside to combustion area, which contributes to the suppression of combustion. Typical XRD curves of iron sesquioxide and cobaltosic oxide were exhibited in Fig. 8c. The change of valence state implied oxidation reaction happened during pyrolysis of PS composites. Metal oxide generated during decomposition showed high specific heat capacity, which contributed to excellent heat absorption 28, 29. The metal oxides acted as heat barrier during combustion, results in the inhibition of fire spread. Meanwhile, it has confirmed that some transitional



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metal compounds (including iron, cobalt and nickel) can effectively improve thermal stability of char residues through catalytic reactions in solid phase 30-32. Different EDS curves of neat MOFs (Fig. S1c), carbon became main element in the constitution of residue. It could be speculated that during the thermal decomposition process of the PS composites, the MOFs degrade into metallic oxides with porous structures, which results in the absorption of the decomposition products originated from the PS matrix. The adsorbed compounds transfer into carbon by catalytic action in solid phase.

Fig. 8 SEM images (a, b), XRD patterns (c) and EDS curves (d) of residue of PS/MOFs composites As shown in Fig. 9a-c, nearly no residues left for the neat PS after cone test, while the char residues of composites can be easily collected. To further understand the formation of the carbonaceous char and its graphitization degree, LRS was utilized and the results were displayed in Fig. 9d. There are two peaks at 1350 and 1590 cm-1, which are corresponding to D and G band respectively


33

. As D band is

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generated from the disordered carbon atoms and G band originated from graphite and glassy carbons, the ratio of the integrated intensity of D and G bands (ID/IG) can be used to evaluate the graphitization degree of char

34

. The ID/IG ratio of PS (2.66) is

larger than that of PS/Fe-MOF (2.46) and PS/Co-MOF (2.33), indicating the char of PS/MOFs has a higher graphitization degree and thermal stability, which is attributed to the catalyzing carbonization of MOFs in the degradation process.

Fig. 9 Digital images (a-c) and Raman spectra (d) of PS and its composites after combustion Based on the analysis above, it is interesting to observe that the addition of MOFs obviously improves the thermal stability and fire retardancy of PS. The proposed mechanisms for the improved thermal and flame retardant behaviors of the materials are illustrated in Fig. 10. Due to different ligands, the two MOFs showed different gaseous products. Nevertheless, the gaseous products from MOFs both contributed to reduce the concentration of combustion-supporters in gaseous phase, including the dilution effect of CO2 (Fe-MOF) and neutralization of active groups (Co-MOF). The release of oligomers originated from the pyrolysis of PS matrix was



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inhibited by cooperation MOFs with PS, implying the reduced combustion-supporters of the volatiles. Therefore, the combustion process of PS composites was retarded. In solid phase, thermostable MOFs worked as heat barrier, and their decompositions need a lot of heat. The porous metallic oxides, generated during combustion process, not only retard the heat spread but also suppresses the release of fuel from solid phase to combustion area. Therefore, this combined action contributes to the superior thermostability and flame retardancy of PS/MOFs composite than that of neat PS. The mechanisms of MOFs are similar with inorganic compounds. However, not only the Ti, Tmax, but also THR values obtained in this manuscript are much higher than that of reported date with additive amount

35

, which confirms the potential application of

MOFs as flame retardants.

Fig. 10 Proposed mechanisms for illustration of the thermal stability flame retardancy of PS and its composites Conclusion In this work, two MOFs were synthesized by solvothermal method and added into polymeric materials as flame retardants for the first time. XRD and electronic microscope analysis showed that the typical crystalline compounds were successfully


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prepared. TGA and cone calorimeter results indicated the addition of MOFs observably enhanced the thermostability and flame retardancy of the PS composites. More than 14% and 28% decrease in PHRR at 2 wt% content confirmed the MOFs can be used as flame retardants. In addition, the release of toxic styrene oligomers was restrained, implying the reduced toxicity of the volatiles. To further understand the flame-retardant mechanisms of MOFs, the gaseous and solid products was detected and analyzed. Based on the results, it can be speculated that during the pyrolysis process of PS matrix, the gaseous products of MOFs reduce the concentration of degradation products and reactive oxygen, while the porous metallic oxides acted as thermal and fuel delivery barrier. This combined action contributes to the superior thermostability and flame retardancy of PS/MOFs composite. The approach described herein could provide a promising solution in the development of a novel and efficient flame retardants for polymers. Acknowledgements The work was financially supported by the National Natural Science Foundation of China (51403196), the Natural Science Foundation of Jiangsu Province (BK20130369) and the Fundamental Research Funds for the Central Universities (WK2320000032). Supporting Information TG curves of MOFs under N2 atmosphere (a), FT-IR spectrum of pyrolysis products for PS and its composites (b), EDS curves of MOFs after annealing at 650 °C for 30 min in air.



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Author information E-mail: [email protected](Z. Gui) [email protected] (Y. Hu). ORCID: Yuan Hu 0000-0003-0753-5430 Notes: the authors declare no competing financial interest.


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Preparation of MetalâOrganic Frameworks and Their Application as

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