Studies on Synthesis of Electrochemically Exfoliated Functionalized

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Studies on Synthesis of Electrochemical Exfoliated Functionalized Graphene and Polylactic Acid/f-GNS Nanocomposites as New Fire Hazard Suppression Materials Xiaming Feng, Xin Wang, Wei Cai, Shuilai Qiu, Yuan Hu, and Kim Meow Liew ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08373 • Publication Date (Web): 02 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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ACS Applied Materials & Interfaces

Studies on Synthesis of Electrochemical Exfoliated Functionalized Graphene and Polylactic Acid/f-GNS Nanocomposites as New Fire Hazard Suppression Materials Xiaming Feng,ab Xin Wang,*a Wei Cai,a Shuilai Qiu,ab Yuan Hu*ab and Kim Meow Liewbc a

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

Anhui 230026, P.R. China b

Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study,

University of Science and Technology of China, Suzhou, Jiangsu 215123, P.R. China c

Department of Architectural and Civil Engineering, City University of Hong Kong, T

at Chee Avenue, Kowloon, Hong Kong

Corresponding Author. *E-mail: [email protected]; Fax: +86-551-63601664; Tel: +86-551-63601664 *E-mail: [email protected]; Fax: +86-551-63601664; Tel: +86-551-63601664

Abstract Practical application of functionalized graphene in polymeric nanocomposites is hampered by the lack of cost-effective and eco-friendly methods for its production. Here, we reported a facile and green electrochemical approach for preparing ferric phytate functionalized graphene (f-GNS) by simultaneously utilizing bio-based phytic acid as electrolyte and modifier for the first time. Due to presence of phytic acid,

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electrochemical exfoliation leads to low oxidized graphene sheets (a C/O ratio of 14.8) with tens of micrometers large. Successful functionalization of graphene was confirmed by the appearance of phosphorus and iron peaks in the X-ray photoelectron spectrum. Further, high performance polylactic acid/f-GNS nanocomposites are readily fabricated by a convenient masterbatch strategy. Notably, inclusion of well-dispersed f-GNS resulted in dramatically suppression on fire hazards of polylactic acid in terms of reduced peak heat release rate (decreased by 40 %), low CO yield and formation of high graphitized protective char layer. Moreover, obviously improvements in crystallization rate, thermal conductivities of polylactic acid nanocomposites were observed, highlighting its promising potential in practical application. This novel strategy toward the simultaneous exfoliation and functionalization for graphene demonstrates a simple yet very effective approach for fabricating graphene based flame retardants.

Keywords Electrochemical exfoliation; Phytic acid; Functionalized graphene; Polylactic acid nanocomposites; Fire safety

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Introduction Graphene, which features superior electrical conductivity, thermal stability, mechanical properties and characteristic dimension effects, has emerged as a new-generation nanofillers for developing the polymeric nanocomposites.1-2 For example, with the incorporation of graphene, poly(vinyl alcohol) nanocomposites exhibited significantly improved mechanical properties,3-4 electrical conductivity5 and flame

retardancy.6

Moreover,

the

gas

barrier

performance

of

poly(vinyl

alcohol)/graphene nanocomposites is much superior to that of neat polymer duo to mass barrier action of graphene.7 However, there are still some challenges to be overcome in the practical application of polymer/graphene nanocomposites. The most important one is the production of graphene in scalable quantities because of the large consumption in fabrication of polymer/graphene nanocomposites. Over the past decade, several techniques have been exploited to synthesize monolayer graphene or delaminate graphene materials from original graphite.8-11 Chemical vapor deposition (CVD) is complicated to prepare high-quality graphene and is very limited by the low output. So far chemical oxidation-reduction of graphite is the most commonly used method to fabricate reduced graphene oxide and chemically functionalized graphene.12 However, because of the usage of several hazardous and toxic chemicals and a large amount of heat release, this approach is not suitable for manufacturing production. Direct exfoliation of graphite by sonochemical and shear methods offers a chance to fabricate graphene materials via a mild process. Unfortunately, the yields of few-layer graphene (～4 layers) obtained by these

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protocols is still insufficient for the demand of graphene in preparing nanocomposites. Recently, because of the simple and rapid characters, electrochemical exfoliation is supposed to be the very promising way to achieve the commercial application of graphene.13-16 Wang prepared high-quality graphene by the co-intercalation of Li/PC complex, which leads to the significant expansion and exfoliation of graphite.17 However, the employment of high-activity lithium salt and organic electrolyte cannot fully address the concerns of commercial production of graphene. Parvez et al. demonstrated the electrochemically exfoliated graphene by utilizing inorganic salt solution as electrolyte.18 However, it is difficult to achieve sufficiently good performance for polymer/graphene nanocomposites because of the bare surface nature of obtained graphene, which is terrifically incompatible with common polymer matrix.1, 19 It is well known that the proper surface modification for graphene plays a crucial impact on the properties enhancement of polymeric nanocomposites. For example, modification of graphene by polyphosphamide not only results in good compatibility of graphene, but also endows the functionalized graphene with high flame retardant efficiency within epoxy resin.20 Therefore, the development of a low-cost and eco-friendly method leading to uniform surface functionalized graphene is highly desirable. Polylactic acid (PLA) is now regarded as a valuable bio-sourced polymer alternative in long-term applications such as electronics and automotive.21 However, PLA suffers from some weaknesses such as high fire hazard and low rate of crystallization for such applications.22-24 There are a few of reports reveal the

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reinforcement of graphene on the mechanical properties and barrier performance of PLA materials,25-27 but the fire hazards of PLA/graphene nanocomposites have rarely been studied. Bao et al. only indicated the barrier effect of bare graphene on reducing heat release rate of PLA composites.28 Phytic acid, which is environmentally friendly, biocompatible, and consisting of six phosphate groups with high phosphorus content (28 wt%),29 has been performed as novel bio-based flame retardants for PLA.30 But the flame retardant effect of phytic acid and its derivatives is limited when used alone.31 To meet the demand for developing a completely green PLA/graphene nanocomposites with superior fire safety. In the present study, phytic acid was chosen as the green electrolyte and surface modifier to simultaneously fabricate high-quality flame retardant-functionalized graphene by electrochemical method for the first time. It is anticipated that this well-designed structure could effectively reinforce the fire safety of PLA materials.

Experimental 2.1 Materials Graphite foil (99 %) was provided by Xinda Material Co., Ltd., China. Phytic acid solution (70%) was offered by Aladdin Industrial Corporation. PLA (4032D) was provided by NatureWorks LLC. Iron nitrate nonahydrate (Fe(NO3)3·9H2O, AP) and tetrahydrofuran (THF) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. 2.2 Preparation of functionalized graphene Graphite foil, copper bar, and 0.1 M phytic acid solution were used as working 5



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electrode, counterelectrode, and electrolyte, respectively. The spacing between two electrodes was kept at approximately 1.0 cm. A constant 10 V voltage was performed for 2 h. After filtered under vacuum and washed by deionized water, the filter cakes were re-dispersed in deionized water by a mild sonication for 20min and then centrifuged at 1500 rpm for 10 min. The supernate was collected and named as phytic acid functionalized graphene (p-GNS). For further functionalization, the stable solution was mixed with excess Fe(NO3)3 solution to form the ferric phytate functionalized graphene (f-GNS). 2.3 Preparation of PLA/f-GNS nanocomposites Herein, we incorporated the f-GNS into PLA materials by a masterbatch premixing way. The wet f-GNS was flushed by THF to replace water and subsequently distributed in THF by sonication for 0.5 h. A certain amount of PLA was dissolved in above suspension and then dried to obtain the masterbatch. The calculated contents of masterbatch and PLA were processed by a twin roller mill (80 rpm) at 170 oC for 10 min to fabricate PLA/f-GNS nanocomposites (for short PLA/f-GNS-x). Finally, these samples were hot-pressed into sheet under 10 MPa at 180 oC. 2.4 Characterization Transmission electron microscopy (TEM) (JEM-2100F, Japan Electron Optics Laboratory Co., Ltd.) was used to study the micro-morphology and the accelerating voltage was 200 kV. Fourier transform infrared (FTIR) measurement were performed by a Nicolet 6700 spectrometer (Nicolet Instrument Corporation, U.S.) with KBr technique.

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X-ray photoelectron spectroscopy (XPS) was conducted by VGESCALB MK-II electron spectrometer (Al Kα excitation source at 1486.6 eV). X-ray diffraction (XRD) was monitored by Japan Rigaku D/Max-Ra rotating-anode X-ray diffractometer equipped with a Cu-Kα tube and a Ni filter (λ= 0.1542 nm). Morphologies of fractured surface were observed with a PHILIPS XL30E scanning electron microscope (SEM). The sample was cryogenically fractured in liquid nitrogen first and then sputter-coated with a conductive layer. Laser Raman spectroscopy (LRS) were performed by a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line. Differential scanning calorimetry (DSC) (Q2000 TA Instruments Inc., USA) was used to characterize crystallization performance. The thermal conductivity of samples were tested by a hot-disk thermal analyzer (TC 3000E, Xia Xi Technology, China) at 25 oC, adopting the transient plane source technique. Samples were tested by thermogravimetric analysis (TGA) (Q5000 thermoanalyzer instrument, TA Instruments Inc., U.S.) in air flow of 25 ml min-1 and heated (20 oC min-1) from 20 to 800 oC. Flammability of samples was evaluated by cone calorimeter test (CCT) (Fire Testing Technology, UK) according to ISO 5660 at a heat flux of 35 kW m-1. Thermogravimetric analysis-infrared spectrometry (TG-IR) was studied with TGA Q5000IR

thermogravimetric

analyzer

linked

to

a

7


Nicolet

6700

FTIR


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spectrophotometer from 20 to 700 oC at 20 oC min−1 (N2 atmosphere, flow rate of 30 ml min−1).

Results and discussion 3.1 Exfoliation and functionalization for graphene In this work, phytic acid was chosen as the electrolyte and modifier to fabricate the flame retardant-functionalized graphene. The presence of phosphate groups could facilitate the electrochemical exfoliation process and endow the graphene with high flame-retarding function. When an external voltage was performed, graphite was quickly enlarged and dissociated in electrolyte (Figure 1). After removing excess phytic acid, the products were mixed with Fe3+ solution to prepare the ferric phytate functionalized graphene, which could be well dispersed in THF (Figure 1d). The mechanism of graphite exfoliation in phytic acid solution is shown in Figure 1e. First, water was electrolyzed to produces hydroxyl (OH·) and oxygen radical (O·). Second, significant expansion and exfoliation of graphite would result from the formation of O2 and intercalation of phytic acid. Then phytic acid anions interact strongly with p-electron-rich graphene surfaces by the formation of hydrogen bonds, and resulting in the phytic acid functionalized graphene. For further improve the flame-retarding function of graphene, the adsorbed phytic acid was mixed with Fe3+ to form ferric phytate functionalized graphene.

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Figure 1. (a) Scheme for preparation of electrochemically exfoliated graphene, photos of graphite foils (b) before and (c) after exfoliation, (d) exfoliated graphitic material suspending in phytic acid aqueous solution, (e) dispersed f-GNS in THF, and (f) proposed mechanism for the functionalization of graphene. Figure 2a and b show the typical TEM images of exfoliated graphene, a transparent thin sheet-like structure with a lateral size of 5~20 µm can be clearly found, demonstrating the successful exfoliation of graphite assisting with phytic acid. The selected area electron diffraction (SAED) was collected and shown in Figure 2c. These diffraction spots with hexagonal pattern indicates hexagonal symmetry characteristic of exfoliated graphene with few layers in thickness. After mixed with Fe3+, the graphene sheets were obviously folded in several regions (Figure 2d), which is caused by the flexible nature of graphene sheets and the absorbed ferric phytate. The high-resolution TEM (HRTEM) image of the f-GNS indicates the formation of

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monolayer graphene by the single lattice fringe (marked with red line). Moreover, some unresolved lattice fringes can be observed on the surface of graphene layer, demonstrating the non-covalently adsorbed ferric phytate. For proving the attachment of ferric phytate, Figure 2f displays the undisputed signals corresponding to P and Fe elements of functionalized graphene in EDX spectrum.

Figure 2. Typical TEM images of (a, b) p-GNS and (d) f-GNS. (c) SAED pattern from the p-GNS. (e) HRTEM image and (f) the EDX result of f-GNS. For confirming the electrochemical exfoliation of graphite, Figure 3a reveals comparable XRD patterns of bare graphite, p-GNS and f-GNS. A sharp reflection and another weak reflection locate at 26.6 and 54.7 o can be observed in the XRD pattern of graphite foil, which are corresponding to (002) and (004) crystal planes of hexagonal graphite. In contrast, both of p-GNS and f-GNS exhibit a significantly reduced (002) diffraction peak, suggesting that this simple electrochemical exfoliation is successful to prepare graphene sheets. LRS is highly sensitive to electronic structures and is always perform to detect graphite and graphene materials. Figure 3b displays the Raman spectra of these three samples. The D band (1348 cm-1) and G

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band (1590 cm-1) for bare graphite is consistent with the previous report.32 After electrochemical exfoliation assisting with phytic acid, an obviously increase in ID/IG ratio for both of p-GNS and f-GNS can be observed, indicating the partial disorder of graphene sheets as well as a decrease in the size of in-plane sp2 domains. It can be attributed to the adsorption of phytic acid anion and other sp3 defects originating from electrolytic oxidation of graphene. It is well known that the reduction of graphene layer number could be further indicated by the intensity and position of 2D band. A red shift to 2708 cm-1 for the exfoliated graphene compared to the 2725 cm-1 for bulk graphite are observed, demonstrating that few-layer graphene sheets are successfully exfoliated. Interestingly, as the increase in modification level, the wider 2D band and higher I2D/IG can be obtained for functionalized graphene compare to those of bare graphite, suggesting the amorphization induced by functionalization. Furthermore, FTIR spectra were acquired to further identify the successful modification of graphene. Compared to bare graphite, the new characteristic absorptions at 1160 and 954 cm-1 are ascribed to P=O and P-O groups in phytic acid, respectively (Figure 3c). The characteristic band at 1086 cm-1 is derived from P-O bands in P-O-C groups. After forming ferric phytate complex, the peak positions of the P=O and P-O-C groups shift to lower wavenumber region and form a wide adsorption peak, indicating interactions between phytic acid and Fe3+ ions.33

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Figure 3. (a) XRD patterns, (b) Raman and (c) FTIR spectra of bare graphite, p-GNS and f-GNS. The graphene materials were studied by XPS to confirm the surface functionalization. XPS survey of bare graphite only shows the presence of carbon (96.46 at%) and oxygen (3.54 at%) in Figure 4a. However, XPS spectrum of p-GNS clearly shows the presence of phosphorus (0.84 at%) in addition to carbon (89.72 at%) and oxygen (9.44 at%). After deducting the oxygen content of adsorbed phytic acid, there is approximately 6.08 % of oxygen in the exfoliated graphene, which is caused by the electrochemical oxidation. A 14.8 C/O ratio was obtained, which is far more than conventional chemically reduced graphene oxide (∼3-10). XPS spectrum of C 1s of p-GNS further shows the presence of C-O (286.6 eV) functional group (Figure 4b). Importantly, the C=O group cannot be observed in the XPS spectrum of the C 1s, confirming the low oxidation degree of the obtained graphene. As depicted in Figure 4c, the P 2p survey of f-GNS was deconvoluted into two peaks at 133.4 and 134.2 eV, corresponding to P-O (P1) and P-O-Fe (P2). The new peak for P-O-Fe (531.4 eV) also can be seen in deconvoluted O 1s survey (Figure 4d). Furthermore, new peaks of Fe 2p can be observed in Figure 4e, directly indicating the iron element in the f-GNS.

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Figure 4. (a) XPS spectra of pristine graphite, p-GNS and f-GNS, (b) high-resolution XPS spectra of C 1s for p-GNS. Deconvoluted XPS survey of (c) P 2p, (d) O 1s and (e) Fe 2p for f-GNS. 3.2 Fractured surface characteristic of PLA/f-GNS nanocomposites The fractured surfaces of neat PLA and PLA/f-GNS nanocomposites were characterized by SEM. For neat PLA, the fractured surface is quite smooth and featureless in Figure 5a. After incorporating the f-GNS, there are much difference between the surfaces of neat PLA and PLA/f-GNS nanocomposites, both of them presented the rough and large-grained morphologies in Figure 5b and c. Obviously, the roughness of fractured surface increased as the f-GNS loadings get higher. It indicates that adding f-GNS has a great influence on the fractured surface characteristic of PLA/f-GNS nanocomposites. There are no aggregations observed for the PLA/f-GNS nanocomposites, demonstrating the well dispersed f-GNS in PLA

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matrix. Meanwhile, obvious pull-out of the f-GNS can be observed under high magnification, marked by red box, which indicates the strong interfacial adhesion and tightly embedding of the f-GNS within PLA matrix. As stated above, the organic modification instead of bare surface of graphene is the primary cause for the well dispersion and strong interfacial adhesion, which always results in the improvement for properties of polymeric nanocomposites.

Figure 5. SEM images of fractured sections of (a) neat PLA, (b) PLA/f-GNS-1.0 and (c) PLA/f-GNS-3.0 nanocomposites under different magnifications. 3.3 Properties of PLA/f-GNS nanocomposites Crystallization behavior The second melting curves of samples obtained from DSC are depicted in Figure 6a. Obviously, neat PLA exhibits a weak endothermic peak around 170 oC while 14


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composites exhibit the dramatically increased ∆Hm values, indicating the improved crystallinity. Notably, all of the PLA/f-GNS nanocomposites present the obvious cold crystallization process in contrast to the low crystalline ability of pure PLA. The cold crystallization temperature (Tcc) of PLA/f-GNS nanocomposites is considerably decreased as the f-GNS contents get increased. For investigating the crystallization kinetics of PLA/f-GNS nanocomposites, the relative crystallinity (Xc) curves versus time are plotted in Figure 6b and t0.5 values are summarized in Table 1 (the detail description can be seen in Supporting Information). Compared to neat PLA, an initially rapid decrease (from 29.0 to 4.5 min) in t0.5 value can be observed for PLA/f-GNS-0.1 sample. When further increasing the f-GNS loading to 3.0 wt%, there is a slight decrease in t0.5 value for PLA/f-GNS nanocomposites, which is consistent with previous report.34 These results confirmed the considerable heterogeneous nucleation action of f-GNS on the crystallization behavior of PLA materials.

Figure 6. (a) DSC melting curves and (b) relative crystallinity (Xc) versus time of PLA and its nanocomposites isothermally crystallized at 110 oC. Table 1 Crystallization characteristics of PLA and its nanocomposites with different f-GNS loadings. 15



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Tg (oC)

Tcc(oC)

Tm (oC)

∆Hm (J/g)

t0.5 (min)

PLA

63.0

--

167.6

3.4

29.0

PLA/f-GNS-0.1

62.2

109.4

170.3

33.3

4.5

PLA/f-GNS-0.5

62.4

108.5

170.4

32.6

4.3

PLA/f-GNS-1.0

62.1

108.0

169.9

32.7

3.7

PLA/f-GNS-2.0

61.1

105.9

169.4

36.1

2.9

PLA/f-GNS-3.0

61.0

104.2

168.3

35.0

2.7

Samples

Thermal stability and thermal conductivity The influence of f-GNS on thermal stability of PLA were characterized with TGA under air atmosphere, the corresponding TG curves are shown in Figure 7. All of the PLA/f-GNS nanocomposites follow a main degradation process (Figure 7a). As clearly shown in Figure 7b, when the loading of f-GNS increases, the initial decomposition temperature (2 wt% weight loss) of PLA/f-GNS nanocomposites decreased slightly. It is attributed to the early thermal degradation of ferric phytate and the superior heat conduction of f-GNS, by which heat from outside can be fast transfer to the inner of PLA matrix. However, the char residues of PLA/f-GNS nanocomposites at 600 oC obviously increased compared to that of neat PLA (hardly any char residue) (Figure 7c), indicating the enhanced thermal oxidative resistance of PLA composites. For clarifying the specific action of f-GNS, thermal conductivity of samples was measured and displayed in Figure 7d. In contrast to the 0.17 W mK-1 for virgin PLA, thermal conductivity of PLA/f-GNS-0.1 slightly increases to 0.19 W mK-1. The

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PLA/f-GNS-3.0 sample exhibits the as much as 76.5% improvement in thermal conductivity in contrast to that of virgin PLA. The significantly increased thermal conductivities confirm the early stage of thermal degradation of PLA/f-GNS nanocomposites when graphene is introduced. The well dispersed f-GNS favors to heat acoustic phonon scattering, minimize the interfacial phonon scattering and thus enhances heat conductivity of PLA composites.35

Figure 7. (a) TG curves, (b) TG curves at 300-350 oC, (c) TG curves at 400-600 oC. (d) Thermal conductivities of neat PLA and PLA/f-GNS nanocomposites. Fire hazards Cone calorimeter test (CCT) was performed to study the flammability of PLA/f-GNS nanocomposites. As shown in Figure 8a, the ignition times of 17



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PLA/f-GNS nanocomposites obviously decreased in contrast to that of neat PLA. As stated above, it can be ascribed to the heat transfer ability of graphene and consequently increased thermal conductivity of PLA/f-GNS nanocomposites, which leads to the formation of conductive network and fast heat diffusion within PLA matrix. Importantly, inclusion of as low as 0.1 wt% f-GNS resulted in an obvious reduction in peak heat release rate (PHRR) value (from 714.5 to 543.4 kW m-2), indicating the high flame retarding efficiency of the f-GNS. However, the PLA/f-GNS-0.1 has the similar total heat release (THR) to that of pure PLA in Figure 8b. As the f-GNS loading was up to 3.0 wt%, both of the PHRR and THR were significantly reduced (40% in PHRR, 16.4% in THR) in contrast to those of neat PLA. Such decreases in flammability of PLA/f-GNS nanocomposites are originated from the physical barrier action of graphene sheets and the protective char layer originated from synergistic charring effect of the f-GNS.

Figure 8. (a) HRR and (b) THR curves of PLA and PLA/f-GNS nanocomposites obtained from CCT. TG-IR method was used to monitor the pyrolysis gases during thermal degradation

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of PLA/f-GNS nanocomposites. Figure 9a and b present the 3D images of evolved gaseous products of neat PLA and PLA/f-GNS-3.0 separately during the TGA test. As depicted in Figure 9c, the main pyrolysis products of PLA were identified as lactide or cyclic oligomers (carbonyl compound, 1764 cm-1), hydrocarbon (2734 cm-1), CO (2177 cm-1) and CO2 (2358 cm-1), as well as the two peaks in the fingerprint range (1375 cm-1 for C−H, 1108 cm-1 for C−O).36 There is no obvious difference between the FTIR spectra of PLA and PLA/f-GNS-3.0, indicating that the incorporating f-GNS did not influence the decomposition products of PLA. However, there is an obvious reduction in peak value as compared to the 3D images for neat PLA and PLA/f-GNS-3.0. All absorbance peaks for neat PLA are stronger than those of PLA/f-GNS-3.0 sample, including hydrocarbons (Figure 9d), aromatic compounds (Figure 9e), and CO (Figure 9f). These dramatically reduction in fire hazards of PLA/f-GNS nanocomposites is mainly attributed to the physical barrier effect of graphene and charring ability of ferric phytate.

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Figure 9. 3D TG-IR spectra of (a) neat PLA and (b) PLA/f-GNS-3.0. (c) FTIR spectra of pyrolysis gases for neat PLA and PLA/f-GNS-3.0 at the maximum decomposition rate. Absorbance of pyrolysis products for PLA and PLA/f-GNS-3.0 vs time: (d) hydrocarbons; (e) CO; and (f) carbonyl compounds. Condensed phase flame retardation mechanism Figure 10 presents photos of chars after CCT of samples. Obviously, virgin PLA was completely burned and almost no char left in Figure 10a. As the content of f-GNS increasing, the content of char of PLA/f-GNS nanocomposites dramatically increased. As for PLA/f-GNS-3.0, far more than 3.0 wt% char residue was achieved, the cracks in char residue are caused by the cooling shrinkage effect (Figure 10f). As shown in Figure 10, for neat PLA, an amount of fragile and flaky char residue can be observed in Figure 10g. As for the PLA/f-GNS nanocomposites, there exhibits a higher dense

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whole char layer left as the contents of the f-GNS get increased (Figure 10h and i). Moreover, the flaky structures can be seen in the inset image of Figure 10i, indicating the retention of the incorporated graphene after combustion process. In Figure 10j, the graphitization degree of residue was indicated by ID/IG value.37-38 Notably, the ID/IG ratio significantly decreased as the loading of f-GNS get higher, from 3.78 for neat PLA to 0.35 for PLA/f-GNS-3.0, indicating the extremely high graphitized and thermal stable char residue of PLA/f-GNS-3.0. This abnormally high char residue can be attributed to synergistic effect between catalytic action of ferric phytate and physical effect of graphene sheets.

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Figure 10. Photos of the chars of (a) neat PLA and (b-f) PLA/f-GNS composites with increasing loadings of f-GNS after CCT. SEM images of the chars of (g) neat PLA, (h) PLA/f-GNS-1.0 and (i) PLA/f-GNS-3.0. Inset is the high resolution SEM image of the char of PLA/f-GNS-3.0 samples. (j) Raman spectra of the char of pure PLA, PLA/f-GNS-1.0 and PLA/f-GNS-3.0. XPS analysis was further performed to provide detail information of char residues. In Figure 11a, the relative content of carbon for PLA/f-GNS-3.0 is obviously higher

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than that of neat sample. For the C 1s spectrum in Figure 11b, a weak peak of C-O and absence of C=O are observed for the PLA/f-GNS-3.0 sample compared to those of neat PLA. As summarized in Figure 11c, a high C/O ratio (17.2) was achieved for PLA/f-GNS-3.0, which is far more than that of pure PLA, indicating the low oxidation defects in char residue. Furthermore, the peak areas of C-O and C-C were calculated to demonstrate the oxidation degree of char reside. The lower C-O area and higher C-C area are the indicative of high graphitized char residue. For O 1s spectrum, compared to only two peaks for neat PLA, a new peak ascribed to P-O-Fe (531.4 eV) for PLA/f-GNS-3.0 can be observed, indicating the formation of Fe-phosphate complexes.39-40 The P 2p survey has been attributed to pyrophosphate or polyphosphate compounds (133.9 eV) and Fe-phosphate complexes (133.3 eV),41-42 which demonstrates the phosphor carbonaceous char formed after combustion process. Moreover, XPS Fe 2p core level spectra shows Fe 2p3/2 peak and Fe 2p1/2 centered around 712.3 and 725.1 eV (Figure 11f), respectively, implying the action of iron in the burned char of PLA/f-GNS nanocomposites.

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Figure 11. (a) XPS spectra and (b) high-resolution spectra of C 1s regions of the char residues of neat PLA and PLA/f-GNS-3.0. (C) The characteristic data of XPS for neat PLA and PLA/f-GNS-3.0 sample. Relative distribution of prominent (d) O species and (e) P species for the different samples. (f) Fe 2p spectrum of the char residue of PLA/f-GNS-3.0. Based on the analysis aforementioned, a possible flame retardant mechanism of ferric phytate functionalized graphene in PLA matrix is postulated in Figure 12. First, during the earlier degradation, thermal stable graphene sheets act as a mass barrier to restrain the permeation of flammable gases. Meanwhile, the catalytic charring effect of absorbed phytate structure results in the generation of much phosphor carbonaceous char residue. Then high aspect ratio graphene can hold the char particles together to generate an adiathermic char shield on the inner materials. Moreover, Fe-P species in the char layer can exhibit the efficient catalyst towards the redox reaction during combustion process, confirmed by the obviously depressed reducing gaseous 24


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products (such as hydrocarbons and CO). It can be conclude that the tripartite cooperative mechanism of the f-GNS is the main source of the excellent flame retardancy of PLA nanocomposites.

Figure 12. Scheme of proposed flame retardant mechanism for f-GNS in PLA nanocomposites.

Conclusion In this work, we describe a facile route for the electrochemical preparation of ferric phytate functionalized graphene by utilizing phytic acid as green electrolyte and modifier,

simultaneously.

This

well-designed

graphene

materials

exhibited

multi-functional effect on improving properties of PLA nanocomposites. Compared to neat PLA, inclusion of the well-dispersed f-GNS resulted in the significantly increased crystallization rate due to the heterogeneous nucleation effect of graphene sheets. In addition, adding f-GNS (3 wt%) exhibited dramatically suppression on fire hazards of PLA in terms of reduced PHRR value (decreased by 40 %) and low CO

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yield, as well as formation of high graphitized protective char layer. These excellent enhancements on fire safety of PLA come from the tripartite cooperative mechanism (barrier action of graphene, catalytic charring performance of phytate and catalytic activity of Fe-P/C system) of the f-GNS. Moreover, the thermal conductivities of PLA nanocomposites also were obviously improved by adding f-GNS, highlighting its promising potential in actual industrial demand. In our view, this facile and green strategy represents a significant step forward in the design of multi-functionalized graphene for achieving their whole potential in polymer nanocomposites.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM images of exfoliated graphene, detail description of DSC measurement, DSC heat flow and enlarged relative crystallinity curves of PLA samples, XPS data of the residual chars (PDF)

Acknowledgements The work was financially supported by the National Basic Research Program of Ch ina (973 Program) (2012CB719701), the National Natural Science Foundation of Chi na (No. 21374111), and the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 9042047, CityU 11208914).

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Table of Contents Graphic

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Studies on Synthesis of Electrochemically Exfoliated Functionalized

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