o-Phthalaldehyde Covalent Organic Frameworks

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A novel melamine/o-phthalaldehyde covalent organic frameworks nanaosheets: Enhancement flame retardant and mechanical performances of thermoplastic polyurethanes Xiaowei Mu, Jing Zhan, Xiaming Feng, Bihe Yuan, Shuilai Qiu, Lei Song, and Yuan Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017

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ACS Applied Materials & Interfaces

A novel melamine/o-phthalaldehyde covalent organic frameworks nanaosheets: Enhancement flame retardant and mechanical performances of thermoplastic polyurethanes Xiaowei Mu1, Jing Zhan2, Xiaming Feng1, Bihe Yuan3, Shuilai Qiu1, Lei Song*1, Yuan Hu*1 1 State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China 2 School of Civil Engineering and Environmental Engineering, Anhui Xinhua University, Hefei, Anhui 230088, China 3 School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China * Correspondence to: Yuan Hu; Lei Song. Tel/fax: +86-551-63601664 E-mail: [email protected]; [email protected]

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Abstract Covalent organic frameworks (COFs) nanosheets prepared from condensation reaction between melamine and o-phthalaldehyde are firstly prepared through ball milling and then incorporated into thermoplastic polyurethanes (TPU) by solution mixing. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron

spectroscopy

(XPS)

and

Fourier

transform

infrared

(FTIR)

spectrometer are applied to characterize COFs nanosheets. It is observed apparently from TEM image that COFs nanosheets are obtained. Successful preparation of COFs nanosheets is proved further by vanishment of typical diffraction peak of COFs at around 23.5o in COFs nanosheets XRD pattern, appearance of quadrant and semicircle stretching of the s-triazine ring at 1568 and 1469 cm-1 in FTIR spectra and N=C bond at 389.5 eV in N1s high-resolution XPS spectra of COFs nanosheets. The thermal property, combustion behavior and mechanical performance of TPU naoncomposites are also investigated. Incorporation of COFs nanosheets into TPU contributes to char forming of TPU under nitrogen atmosphere and 14.3 % decrease of peak heat release rate of TPU. Besides, the elongation at break, Young’s modulus and fracture strength of TPU nanocomposites increase sharply compared with that of neat one. Keywords: COFs, TPU nanocomposites, ball milling, combustion behavior, mechanical performance.

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1 Introduction Covalent organic frameworks (COFs), as a new kind of two-dimensional or three-dimensional crystalline porous material, are built with organic moieties connected by strong covalent bonds.1-4 Recently, they have attracted huge attention around the world in gas storage and separation,5-6 catalysis,2, 7-8 chemical sensors,9 electrochemical and clean energy10-14 and nanochannels12,

15

due to their intrinsic

micro or mesoporous chemical structure. Actually, morphology of COFs is similar to that of graphite. Inspired by the fact that graphene is prepared from graphite, a large number of attempts have been done to achieve two-dimensional COFs nanosheets. There are two types of main methods to prepare COFs nanosheets. Firstly, “bottom-up” strategy is a measure that COFs nanosheets resulted from polymerization of organic monomers are usually templated by surface or interface of molecule.16-18 It has been reported that COFs nanosheets, featured with several atomic thick, grow on the surface of highly oriented pyrolytic graphite19 and flat faces of Au made on homemade single-crystal beads.20 Two-dimensional COFs with monolayer have also been synthesized at air/water21 and solid/vapor interface.22 However, preparation of COFs formed from “bottom-up” strategy is usually very expensive and inefficient. Recent works have shown that COFs nanosheets can be obtained from bulk COFs through simple ultrasonication or ball milling. This “top-down” approach provides a cheap and scalable way to prepare COFs nanosheets. The influence of ultrasonication time and specific constitutions of COFs on the size and thickness of COFs nanosheets obtained has been investigated.22 A series of COFs prepared from solvothermal 3



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aldehyde-amine Schiff base condensation reaction have also been exfoliated into COFs nanosheets via mechanochemical grinding method.23-24 However, raw materials applied in preparation of above COFs are expensive, leading to limitation of application of COFs. Schiff based COFs materials not only show better thermal stability but also display excellent stability in boiling water, acid and base compared with other types of COFs material.24-25 Thus, in this study, Schiff based COFs nanosheets prepared from condensation reaction between melamine and o-phthalaldehyde (OPA) are obtained through ball milling firstly. Then COFs nanosheets are incorporated into thermoplastic polyurethanes (TPU) by solution mixing. The thermal, mechanical and flame retardant properties of TPU nanocomposites have also been investigated. This work broadens the application of COFs into polymer matrix and provides cheaper method to synthesize COFs nanosheets. 2 Experimental 2.1 Materials TPU (85E85) was supplied by Bangtai Material Co., Ltd. (China). Melamine, acetic acid (98 wt%), N,N-Dimethylformamide (DMF), tetrahydrofuran (THF), methanol, acetone and Dimethyl sulfoxide (DMSO) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). OPA was obtained from Aladdin Chemistry Co., Ltd. (China). Deionized water was prepared in our laboratory. 2.2 Preparation of COFs and COFs nanosheets Firstly, melamine (2.02 g, 16 mmol) was dissolved in 100 ml DMSO in a single 4


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mouth bottle at 100 oC then OPA (3.22 g, 24 mmol) and 40 ml 3 M acetic acid were added into system in sequence. The whole mixture was heated at 135 oC for 72 h in a single mouth bottle which was evacuated totally. After cooling down to room temperature, the precipitated product was obtained through centrifugation at 3000 rpm for 4 mins and then purified by washing with THF, acetone and methanol orderly. The final off-white product was dried under oven at 80 oC for 12 h. COFs (1 g) were wet-ball milled with DMF (80 ml) as a medium at 225 rpm for 24 h and then dispersed by sonication for 30 mins and then centrifuged at 3000 rpm for 10 mins. The supernate with concentration of 0.41 wt% was collected and marked as COFs nanosheets. 2.3 Preparation of TPU/COFs nanocomposites TPU nanocomposite with different COFs nanosheets weight content (0 wt%, 0.4 wt%, 0.8 wt%, 1.6 wt% and 3.2 wt%) was prepared by solution mixing method. In brief, TPU was dissolved in supernate of COFs nanosheets with stirring at 100 oC for 1 h and then the hot mixture was poured into deionized water to precipitate TPU nanocomposite. The samples were dried under oven at 95 oC for 48 h. After that, they were melt-blended in a twin roller mill at 180 oC for 10 mins with a roller speed of 80 rpm to remove residual solvent in samples further. Finally, the resultant specimens were hot-pressed by a press vulcanizer. The detailed preparation process is illuminated in Fig. 1. 2.4 Characterization Fourier transform infrared (FTIR) spectra were obtained by a Nicolet 6700 5



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spectrometer (Nicolet Instrument Corp., US) with a wavenumber range of 4000 – 400 cm-1 to characterize chemical structure of samples. The samples were mixed with KBr before pressed into tablets. X-ray diffraction (XRD) results were performed on a Rigaku TTR-III X-ray diffractometer (Rigaku Co., Japan) equipped with a Cu Kɑ radiation (λ = 0.1542 nm). Thermogravimetric analysis (TGA) was measured from room temperature to 800 oC by a Q5000 thermoanalyzer instrument (TA Instruments Inc., US) with a linear heating rate of 20 oC min-1. X-ray photoelectron spectroscopy (XPS) was monitored on a VG ESCALAB 250 electron spectrometer (Thermo VG. Scientific Ltd., UK) with an Al Kα line (1486.6 eV). Transmission electron microscopy (TEM) tests were conducted on a JEM-2100F microscope (JEOL Co., Ltd., Japan) with an acceleration voltage of 200 kV to investigate the structure of samples. Morphologies of fractured surface were displayed on a FEI Sirion 200 scanning electron microscope (SEM) with an acceleration voltage of 5 kV (JEOL Co., Ltd., Japan). Young modulus, tensile strength and elongation at break of samples were tested on a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co., Ltd., China) at a crosshead speed of 100 mm min−1 according to ISO 527 standard. Combustion property was displayed on a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660 standard under a 35 kW m-2 heat flux. Raman spectra were tested on a LABRAM-HR laser confocal Raman spectroscope (Jobin Yvon Co., Ltd, France) with a 514.5 nm argon laser line. 3 Results and discussion 3.1 Characterization of COFs and COFs nanosheets 6


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As shown in Fig. 2, chemical constitution and morphology of samples are investigated by FTIR, XRD, TEM and XPS. Intensity of typical diffraction peak of COFs nonosheet at around 23.5o decreases sharply compared with that of COFs in Fig.2 (a).26 It can be explained that specific three-dimensional structure of COFs is destroyed by ball milling，leading to form of COFs nanosheets. In Fig. 2 (b), the wide absorption peak at 3401 cm-1 corresponds to stretching mode of NH groups in the aromatic ring.27 The peaks at 1568 and 1469 cm-1 are ascribed to the quadrant and semicircle stretching of the s-triazine ring, proving addition of melamine into COFs.26 The sharp characteristic peak at 1350 cm-1 belongs to -NH- stretching vibration.28 The bands centered at 1209 and 810 cm-1 are assigned to aromatic stretching vibration of C-N band. The peak at 1726 cm-1 is the typical band of unreacted aldehyde group.29 There is little change between two FTIR spectra in Fig. 2 (b) except decreased peak intensity of COFs nanosheets at 1726 and 1568 cm-1 compared with that of COFs. This may be explained that chemical structure of COFs is destroyed by ball milling at a certain degree. It is proved from FTIR spectra that COFs are synthesized successfully and COFs nanosheets contain similar chemical structure to that of COFs. It is observed from TEM image of COFs nanosheets in Fig. 2 (c) that multihole nanosheets with a lateral size of several hundred nanometers are prepared. The holes on COFs nanosheets are due to the intrinsic three-dimensional polyporous structure of COFs.30-32 Atom composition and bond state of the samples are investigated by XPS spectra in Fig. 2 (d - f). It is calculated from wide XPS scanning spectrum of COFs nanosheets that atomic percentage of carbon, nitrogen and oxygen is 64.86 %, 27.57 % 7



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and 7.62 %, respectively. The characteristic peaks of C1s in high-resolution XPS spectra are assigned to C-C (284.7 eV), C-N (285.4 eV), C=N (287.4 eV) and C=O (280.0 eV).26, 33-34 Two typical peaks are shown in N1s high-resolution XPS spectra, including N=C (389.5 eV) and N-H (399.9 eV).26 TGA and DTG curves of COFs and COFs nanosheets are displayed in Fig. 3 to investigate their thermal performance. It is obvious that both of COFs and their nanosheets have one main weight loss stage with a temperature at maximum weight loss rate (Tmax) of 416 oC and 353 oC under nitrogen, respectively. However, COFs nanosheets degrade slightly compared with that of COFs from 50 oC to 330 oC. This may be due to degradation of low molecular fragments resulting from ball milling of COFs. The pyrolysis process of COFs and their nanosheets under air and nitrogen atmosphere is same except for two weight loss stages under air. The first pyrolysis stage is corresponding to the decomposition of COFs and their nanosheets. The second weight loss stage under air is ascribed to the degradation of char formed in previous stages. 3.2 Thermal properties of TPU and its nanocomposites. Thermal stability of TPU and its nanocomposites under nitrogen and air atmosphere is studied in Fig. 4. The detailed TGA and DTG data are listed in Table 1 and Table 2. There are two TPU decomposition stages under nitrogen atmosphere. The first one is attributed to degradation of urethane bond of the TPU hard segments, resulting in formation of diisocyanate, diol and subsequent release of CO2.35-36 The second stage is assigned to thermal decomposition of soft segments of TPU, leading 8


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to formation of complex mixture of molecular species derived from the polyol segment fragmentation.35, 37-38 The temperature at 5 wt% weight loss is regarded as initial degradation temperature (T5 wt%). It is referred from Fig. 4 (a) that untreated TPU starts to degrade at 311 oC with Tmax of 352 oC and 418 oC. However, T5 wt% of TPU nanocomposites is lower than that of untreated TPU, due to decomposition of COFs nanosheets in advance. Besides, T5

wt%

of TPU nanocomposites decreases

slightly with increase in weight percent of COFs nanosheets. However, char residues of TPU nanocomposites under nitrogen increase with increased weight loading of COFs nanosheets. It is shown in Fig. 4 (c) that there are three weight loss stages of TPU under air. The third decomposition stage of TPU nanocomposites is corresponding to thermal oxidative degradation of char residues formed in previous stage. The weight percent of TPU nanocomposites is higher than that of untreated one at the end of second TPU pyrolysis stage, which is consistent with the TGA results of TPU and its nanocomposites under nitrogen. This may be concluded that COFs nanosheets contribute slightly to char forming of TPU, however, the formed char residues are prone to thermal oxidative degradation further. 3.3 Fractured surface characteristic, hydrogen bonding behavior and mechanical properties of TPU and its nanocomposites. The dispersion state and interfacial interaction of TPU nanocomposites are investigated by SEM in Fig. 5. It is obvious that neat TPU shows more smooth fractured surface compared with that of TPU with 3.2 wt% COFs nanosheets. This can be explained that COFs nanosheets, due to their intrinsic pure organic component 9



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and hydrogen bond interaction with TPU, displaying good compatibility in TPU matrix and strong interfacial adhesion between COFs nanosheets and TPU matrix. The small holes in SEM images are caused by vaporization of residual solvent in samples during hot-pressed process. The nanoscale sheets in marked areas in Fig. 5 (d) may be COFs nanosheets which are pulled out of matrix. There is no obvious agglomeration of COFs nannosheets found in fractured surface, indicating good dispersion of COFs nanosheets in TPU matrix. Thus, better mechanical properties of TPU nanocomposites are achieved. As shown in Fig. 6, FTIR tests of samples have been done to investigate the hydrogen bonding effect between COFs nanosheets and TPU further. The typical peak at 3433 cm-1 of untreated TPU is assigned to inter-molecular and intra-molecular hydrogen bonded urethane N-H groups.39-40 The bands of neat TPU centered at 1631 and 1381 cm-1 correspond to -NHC=O- and -CH3 stretching vibration, respectively.41 It is obvious that intensity of N-H groups of TPU composite is high than that of untreated TPU. Besides, the N-H peak of neat TPU shifts to lower wavenumber. These are due to hydrogen bonding interaction between COFs nanosheets and TPU.27-28 Mechanical performance is one of the most important properties of polymer materials. Here stress-strain curves of TPU and its nanocomposites are illuminated in Fig. 7. In Fig. 7 (a), Young’s modulus, elongation at break and fracture strength of TPU nanocomposites increase obviously compared with that of neat one. For example, the value of Young’s modulus, elongation at break and fracture strength of TPU with 10


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3.2 wt% COFs nanosheets increase by 45%, 3.5% and 64% compared with that of neat TPU, respectively. It is also shown in Fig. 7 (b) that elongation at break and fracture strength of TPU are enhanced with increase in weight percent of COFs nanosheets. These are due to enhancement effect of nanomaterials to polymer matrix and hydrogen bonding between TPU chains and COFs nanosheets.42-44 3.4 Combustion performance of TPU and its nanocomposites. Cone calorimetry, as standard combustion test equipment, is applied to characterize combustion properties of materials. Heat release rate (HRR) and total heat release (THR) curves of TPU and its nanocomposites are displayed in Fig 8 and the detailed data are listed in Table 3. The values of time to ignition (TTI) of TPU nanocomposites increase slightly compared with that of untreated TPU. It is clear that peak heat release rate (PHRR) of TPU nanocomposite decreases compared with that of untreated one. Besides, PHRR of TPU 4 and TPU 5 decreases by 14.3% and 13.6%, respectively. It is referred from Fig 8 (b) that THR of TPU nanocomposites achieves slight decrease. The digital images of TPU and its nanocomposties after cone calorimetry tests are shown in Fig. 9. There is almost no char residues of untreated TPU left after cone calorimetry test. A layer of yellow char residues resulted from COFs nanosheets are formed. The char residues of TPU nanocomposites increase obviously with increased weight loading of COFs nanosheets. Graphitization degree of char residues of TPU and its nanocomposites after cone calorimetry tests are studied by Raman spectroscopy in Fig. 10. The D band, centered at around 1365 cm-1, is due to the vibration of sp2-hybridized carbon atoms in graphite 11



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layer. The peak at around 1595 cm-1 is ascribed to G band which corresponds to an E2g mode of hexagonal graphite.45 The intensity ratios of D band to G band (ID/IG) of char residues of TPU nanocomposites decrease compared with that of neat one. It is suggested that graphitic degree of char residues of TPU increases due to the incorporation of COFs nanosheets. This may be illuminated that char residues with higher graphitic degree, working as a layered physical barrier, is formed due to the present of COFs nanosheets during combustion, leading to retard heat transfer and release of gaseous pyrolysis products. At the same time, the amount of char residues of TPU nanocomposites increased slightly according to images of TPU and its nanocomposites after cone calorimetry tests. Thus, flame retardant of TPU is enhanced. The feasible flame retardant mechanism of TPU/COFs nanosheets is displayed in Fig. 11. During combustion, not only do COFs nanosheets block heat radiation and release of gaseous pyrolysis products but also increase graphitic degree and weight amount of char residues. The higher-quality char residues contribute to flame retardant of TPU through physical barrier effect. 4 Conclusions Schiff based COFs nanosheets synthesized from condensation reaction between melamine and OPA are prepared successfully. TPU nanocomposites with different weight loading of COFs nanosheets have also been prepared. It is proved from TEM image that COFs nanosheets are achieved. Sharp decrease of intensity of COFs nanosheets in XRD patterns and emergence of the quadrant and semicircle stretching of the s-triazine ring at 1568 and 1469 cm-1 indicate preparation of COFs nanosheets. 12


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The char residues of TPU with 3.2 wt% COFs nanosheets increase by 88.8% from 3.6 wt% to 6.8 wt% under nitrogen atmosphere. The PHRR of TPU 4 deceases by 13.5 % compared with that of untreated TPU. A protective char is formed due to the present of COFs nanosheets and blocks radiant heat and release of gaseous pyrolysis products, leading to enhancement of flame retardant of TPU. The Young’s modulus, elongation at break and fracture strength of TPU/COFs nanosheets-3.2 wt% nanocomposite achieve increase of 45%, 3.5%, 64%, respectively. This work proves guidance for broadening the application of COFs materials into nanocomposites. Acknowledgements The work was financially supported by the National Basic Research Program of China (973 Program) (2014CB931804), National Natural Science Foundation of China (51473154) and Fundamental Research Funds for the Central Universities (WK2320000032).

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(bis

(phenoxy)


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phosphazene)/expandable graphite composite and its flame retardant mechanism. RSC Adv. 2015, 5, 76068-76078.

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Table captions Table 1 TGA data of TPU and its nanocomposites under nitrogen atmosphere. Table 2 TGA data of TPU and its nanocomposites under air atmosphere. Table 3 Cone calorimeter data of TPU and its nanocomposites.

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Table 1 TGA data of TPU and its nanocomposites under nitrogen atmosphere. Sample TPU 0 TPU 1 TPU 2 TPU 3 TPU 4

Formulation TPU TPU/NCOFs-0.4 wt% TPU/NCOFs-0.8 wt% TPU/NCOFs-1.6 wt% TPU/NCOFs-3.2 wt%

T5 wt% (oC) 311 308 304 300 299

Tmax (oC) 352, 418 365, 408 361, 408 350, 389 340, 389

NCOFs: COFs nanosheets.

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Residue (wt%) 3.6 4.0 5.2 5.4 6.8

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Table 2 TGA data of TPU and its nanocomposites under air atmosphere. Sample TPU 0 TPU 1 TPU 2 TPU 3 TPU 4

Formulation TPU TPU/NCOFs-0.4 wt% TPU/NCOFs-0.8 wt% TPU/NCOFs-1.6 wt% TPU/NCOFs-3.2 wt%

T5 wt% (oC) 313 309 307 303 303

Tmax (oC) 350, 399, 585 342, 399, 575 350, 387, 576 337, 383, 555 334, 380, 545

NCOFs: COFs nanosheets.

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Residue (wt%) 1.8 1.3 1.1 1.2 1.4


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Table 3 Cone calorimeter data of TPU and its nanocomposites. Sample TPU 0 TPU 1 TPU 2 TPU 3 TPU 4

TTI (s) 27 27 31 31 33

PHRR (kW/m2) 1245 1214 1193 1068 1076

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THR (MJ/m2) 92.8 92.8 93.6 90.7 92.7

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Figure captions Fig. 1 Schematic for (a) synthetic route of COFs nanosheets and (b) preparation process of TPU nanocomposites. Fig. 2 (a) XRD patterns and (b) FTIR spectra of COFs and COFs nanosheets; (c) TEM image, (d) wide XPS scanning spectrum and (e,f) high-resolution XPS spectra of COFs nanosheets. Fig. 3 (a,b) TGA and DTG curves of COFs and COFs nanosheets under nitrogen atmosphere; (c,d) TGA and DTG curves of COFs and COFs nanosheets under air atmosphere. Fig. 4 (a,b) TGA and DTG curves of TPU and its nanocomposites under nitrogen atmosphere; (c,d) TGA and DTG curves of TPU and its nanocomposites under air atmosphere. Fig. 5 SEM images of fractured surface of (a,b) neat TPU and (c,d) TPU 4 at different magnifications. Fig. 6 FTIR spectra of TPU and its nanocomposites. Fig. 7 (a) stress-strain curves of TPU and its nanocomposites; (b) curves of fracture strength and elongation at break of TPU and its nanocomposites versus weight percent of COFs nanosheets. Fig. 8 (a) HRR and (b) THR curves of TPU and its nanocomposites. Fig. 9 Digital images of (a) TPU 0, (b) TPU 1, (c) TPU 2, (d) TPU 3 and (e) TPU 4 after cone calorimetry tests. Fig. 10 Raman spectra of char residues of (a) TPU 0, (b) TPU 2 and (c) TPU 4. 25



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Fig. 11 Flame retardant mechanism of TPU nanocomposites.

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Fig. 1 Schematic for (a) synthetic route of COFs nanosheets and (b) preparation process of TPU nanocomposites.

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Fig. 2 (a) XRD patterns and (b) FTIR spectra of COFs and COFs nanosheets; (c) TEM image, (d) wide XPS scanning spectrum and (e,f) high-resolution XPS spectra of COFs nanosheets.

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Fig. 3 (a,b) TGA and DTG curves of COFs and COFs nanosheets under nitrogen atmosphere; (c,d) TGA and DTG curves of COFs and COFs nanosheets under air atmosphere.

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Fig. 4 (a,b) TGA and DTG curves of TPU and its nanocomposites under nitrogen atmosphere; (c,d) TGA and DTG curves of TPU and its nanocomposites under air atmosphere.

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Fig. 5 SEM images of fractured surface of (a,b) neat TPU and (c,d) TPU 4 at different magnifications.

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Fig. 6 FTIR spectra of TPU and its nanocomposites.

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Fig. 7 (a) stress-strain curves of TPU and its nanocomposites; (b) curves of fracture strength and elongation at break of TPU and its nanocomposites versus weight percent of COFs nanosheets.

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Fig. 8 (a) HRR and (b) THR curves of TPU and its nanocomposites.

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Fig. 9 Digital images of (a) TPU 0, (b) TPU 1, (c) TPU 2, (d) TPU 3 and (e) TPU 4 after cone calorimetry tests.

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Fig. 10 Raman spectra of char residues of (a) TPU 0, (b) TPU 2 and (c) TPU 4.

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Fig. 11 Flame retardant mechanism of TPU nanocomposites.

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Table of contents graphic

Schematic for (a) synthetic route of COFs nanosheets and (b) preparation process of TPU nanocomposites.

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o-Phthalaldehyde Covalent Organic Frameworks

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