Functionalized Carbon Nanotubes with Phosphorus- and Nitrogen

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Functionalized Carbon Nanotubes with Phosphorus- and NitrogenContaining Agents: Effective Reinforcer for Thermal, Mechanical, and Flame-Retardant Properties of Polystyrene Nanocomposites Weiyi Xing,† Wei Yang,*,†,‡ Wenjie Yang,‡ Qihang Hu,‡ Jingyu Si,‡ Hongdian Lu,‡ Benhong Yang,‡ Lei Song,† Yuan Hu,† and Richard K. K. Yuen§ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ Department of Chemical and Materials Engineering, Hefei University, 99 Jinxiu Road, Hefei, Anhui 230601, People’s Republic of China § Department of Architecture and Civil Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong People’s Republic of China S Supporting Information *

ABSTRACT: Aminated multiwalled carbon nanotubes (AMWCNT) were reacted with diphenylphosphinic chloride (DPP-Cl) to prepare the functionalized MWCNT (DPPAMWCNT). A-MWCNT and DPPA-MWCNT were respectively mixed with polystyrene (PS) to obtain composites through the melt compounding method. SEM observations demonstrated that the DPPA-MWCNT nanofillers were more uniformly distributed within the PS matrix than A-MWCNT. PS/DPPAMWCNT showed improved thermal stability, glass transition temperature, and tensile strength in comparison with PS/AMWCNT, resulting from good dispersion and interfacial interactions between DPPA-MWCNT and PS matrix. The incorporation of DPPA-MWCNT to PS significantly reduced peak heat release rate, smoke production rate, and carbon monoxide and carbon dioxide release in cone calorimeter tests. The enhanced fire-retardant properties should be ascribed to the barrier effect of carbon nanotubes, which could provide enough time for DPPA-MWCNT and its functionalized groups to trap the degrading polymer radicals to catalyze char formation. The char layer served as an efficient insulating barrier to reduce the exposure of polymer matrix to an external heat source as well as retarding the flammable gases from feeding the flame. KEYWORDS: carbon nanotubes, nanocomposites, particle reinforcement, thermal properties, flame retardant

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carbon nanotubes were found to be a highly efficient fire retardant for ethylene−vinyl acetate to inhibit the production of fire hazards. Effective improvement in the properties of nanocomposites was fundamentally dependent on good dispersion of nanofillers and strong interfacial interaction between carbon nanotubes and polymers. Nowadays, functionalized carbon nanotubes have been regarded as an effective solution to enhance the dispersibility of carbon nanotubes in polymers and the interfacial interaction between these two phases, leading to enhancement of mechanical, thermal, and flame-retardant properties.12−16 In

INTRODUCTION

Polymer-based nanocomposites have attracted considerable interest, since the inclusion of a few nanoparticles can significantly reinforce thermal, mechanical, electrical, and fireretardant performance.1−4 Carbon nanotube-filled polymer nanocomposites are of great significance and importance in polymer-based nanocomposites owing to their extraordinary physicochemical behaviors.5,6 Incorporation of carbon nanotubes can remarkably enhance thermal and mechanical properties in various polymers.7−9 Because of their highly elongated shape, carbon nanotubes are considered as useful additives to reduce the fire hazards of polymers. Kashiwagi et al.10 reported that introduction of carbon nanotubes could significantly enhance the fire-retardant properties of poly(methyl methacrylate). In a study by Beyer,11 multiwalled © XXXX American Chemical Society

Received: June 8, 2016 Accepted: September 14, 2016

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DOI: 10.1021/acsami.6b06864 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

°C for 24 h under dry nitrogen atmosphere. The suspension was filtered, and the filtrate was washed with DMF and dried in a vacuum oven at 80 °C to constant weight. The functionalized multiwalled carbon nanotube product DPPA-MWCNT was used in the current work. The yield of DPPA-MWCNT was about 80%. The preparation route of DPPA-MWCNT is shown in Figure S1. PS, A-MWCNT, and DPPA-MWCNT were continuously dried at 80 °C for 12 h. In a typical melt blending experiment, 0.5 g of AMWCNT or DPPA-MWCNT was blended with 49.5 g of PS to prepare the composites by use of an XK-160 twin-roll mill (Jiangsu, China) at 190 °C at constant rotation speed of 100 rpm for about 10 min. They were then molded through a hot press or microinjection molding machine at 195 °C to obtain samples with different sizes for further measurements. Other samples were fabricated by the same procedure. The prepared nanocomposites were designated as PS/AMWCNT and PS/DPPA-MWCNT with the same concentration (1 wt %). Characterization. The chemical structures of A-MWCNT and DPPA-MWCNT were characterized by Fourier transform infrared (FTIR) and Raman spectrocopy and by thermogravimetric analysis (TGA). FTIR spectra were recorded on a FTIR spectrophotometer (Nicolet 6700). Raman spectra were obtained from a DXR Smart Raman spectrometer (Thermo Fisher Scientific Inc.) in the wavenumber range from 500 to 2000 cm−1. Thermal decomposition under nitrogen atmosphere was performed on a Q5000 IR TGA (TA Instruments). The specimens were heated from 25 to 700 °C in N2 atmosphere. The heating rate was 20 °C/min, and sample mass was in the range 5−10 mg. Dispersion and morphology were measured by scanning electron microscopy (SEM). The samples were immersed into liquid nitrogen for 10 min. The fractured surface was observed on a Hitachi SU8010 SEM. A conductive gold layer was coated on the fractured surface prior to SEM observations. Glass transition behaviors were investigated on a Q2000 differential scanning calorimeter (DSC; TA Instruments). The experimental procedure was as follows: samples were heated from 50 to 150 °C under nitrogen and remained at 150 °C for 5 min, cooled to 50 °C and kept at 50 °C for 5 min, and finally heated to 150 °C. The heating and cooling rates used were both 10 °C/min in this work. Thermal decomposition and thermo-oxidation degradation behaviors were measured on a Q5000 IR TGA (TA Instruments). All specimens were heated from 25 to 700 °C under N2 and air conditions. The heating rate was 20 °C/min, and sample mass was in the range 5−10 mg. Tensile behaviors were evaluated by a WD-20D universal testing machine. Experiments were conducted on the basis of ASTM D-638. The width of specimens was 4.0 ± 0.1 mm, and the thickness was 2.0 ± 0.1 mm. The cross-head speed was 50 mm/min. Five runs for each sample were measured and the mean value was reported. Flammability properties were analyzed on a FTT cone calorimeter on the basis of standard ISO 5660-1. Sample size was 100 mm × 100 mm × 3.0 mm. All samples were wrapped by a layer of aluminum foil. They were then horizontally irradiated under a heat flux of 35 kW/m2.

the work of Bikiaris et al.,12 the influence of acid-treated carbon nanotubes on the mechanical performance of polypropylene was investigated. With increased treatment time, the length of carbon nanotubes was gradually decreased, whereas the mechanical properties of the nanocomposites were enhanced. Hu et al.13 prepared functionalized multiwalled carbon nanotubes (MWCNT) with tri(1-hydroxyethyl-3-methylimidazolium chloride) phosphate (IP). They found that the presence of IP-MWCNT improved the thermal stability and fireretardant properties of polylactide. Ma et al.14 used an intumescent flame retardant, poly(diaminodiphenylmethane spirocyclic pentaerythritol bisphosphonate) (PDSPB), to modify the acidified MWCNT. The results indicated that the functionalized MWCNT could promote the distribution of nanofillers in acrylonitrile−butadiene−styrene (ABS) matrix, resulting in enhanced fire-retardant properties. In the study of Du and Fang,15 MWCNT was wrapped by layered double hydroxide. The functionalized MWCNT could further reduce the fire hazards of polypropylene through a good barrier effect. Zhou et al.16 prepared functionalized MWCNT wrapped with MoS2 nanolayers. The addition of MoS2−CNT hybrids reduced the peak heat release rate (PHRR) as well as carbon monoxide and smoke production. In conclusion, functionalized CNTs with various agents effectively enhanced the overall properties of polymer nanocomposites. In the current study, a facile functionalization method has been used to prepare modified CNTs with phosphorus- and nitrogen-containing agents, which is different than reported multistep treatments.13−16 Aminated multiwalled carbon nanotubes (A-MWCNT) have first been reacted with diphenylphosphinic chloride (DPP-Cl) to prepare functionalized multiwalled carbon nanotubes (DPPA-MWCNT). The two different functionalized MWCNTs have been used as nanoadditives for polymers. Because of its outstanding performance, including heat and electric insulation, transparency, rigidity, and chemical resistance, polystyrene (PS) has acquired wide application in the fields of vehicles, heat insulating materials, electronic and electrical sectors.17−19 However, PS is flammable, as are many synthetic polymers. When ignited, it burns with many fire hazards, such as ease of ignition, fast propagation of flame, heat release, smoke production, and obscuration.20,21 These thermal and nonthermal hazards pose a great potential threat to people’s life and property safety, which has received substantial attention from governments and society. The aim of this work is to prepare PS nanocomposites filled with functionalized CNTs, study the distribution of functionalized CNTs within PS, and evaluate the influence of functionalized CNTs on the mechanical, thermal, and flameretardant performances of the resultant PS nanocomposites.

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RESULTS AND DISCUSSION Characterization of Functionalized Multiwalled Carbon Nanotubes. Figure 1 exhibits the FTIR spectra of AMWCNT and DPPA-MWCNT. Bands at 3425 and 1630 cm−1 correspond with the N−H stretching and bending vibrations of amino groups, respectively. The peak at 1378 cm−1 is ascribed to stretching vibrations of C−N. A strong characteristic peak at 1077 cm−1 appears in the FTIR spectrum of DPPA-MWCNT, corresponding to P−N stretching vibration.14,22 The results show that DPP-Cl reacted with amino groups in A-MWCNT through nucleophilic substitution, leading to the graft modification of phosphorus−nitrogen-containing compound onto MWCNT.

EXPERIMENTAL SECTION

Materials. Polystyrene (PS, 158K) was provided by BASF-YPC Co. Ltd.. The carbon nanotube used was aminated multiwalled carbon nanotube (A-MWCNT, -NH2 content 0.45 wt %, purity >95%, length 10−50 μm, diameter 8−15 nm), which was purchased from Chengdu Organic Chemicals Co. Ltd.. Diphenylphosphinic chloride (DPP-Cl, 98%), triethylamine (TEA, 99.5%), and N,N-dimethylformamide (DMF, 99.5%) were purchased from Aladdin Reagent Co. Ltd. Preparation. A-MWCNT (100 mg), DMF (50 mL), and 2 drops of TEA (catalyst) were charged into a three-necked flask. DPP-Cl (80 mg) was dissolved in DMF (10 mL) and then added dropwise into the suspension containing A-MWCNT and TEA. The compounds were stirred with a magnetic stirrer in an ice−water bath for 1 h under dry nitrogen conditions. The mixture was then continuously reacted at 80 B


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Figure 3. Mass loss curves of A-MWCNT and DPPA-MWCNT under nitrogen conditions.

Figure 1. FTIR spectra of A-MWCNT and DPPA-MWCNT.

Figure 2 presents the Raman spectra of A-MWCNT and DPPA-MWCNT. The two dominant bands observed at 1340

phosphorus−nitrogen-containing compound has been successfully grafted onto MWCNT. The functionalized MWCNTs usually show much higher solubility or better dispersion in solvents. In order to compare the dispersion of functionalized MWCNTs, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) were utilized to prepare suspensions of MWCNTs (0.5 mg/mL). The solutions have been ultrasonically treated for 5 min. Figure 4 shows

Figure 2. Raman spectra of A-MWCNT and DPPA-MWCNT.

and 1571 cm−1 are attributed to the D-band and G-band, respectively.23 The G-band is attributed to first-order scattering of the sp2 carbon atoms of CNTs, while the D-band corresponds to disorder-induced or sp3 carbon atoms of CNTs.24−26 A new band, D′, at higher wavenumber close to Gband corresponds to the functionalized CNTs.26,27 The G-band in the Raman spectrum of DPPA-MWCNT show a spectral shift in comparison with A-MWCNT, implying side-wall or end-cap modification.27 The relative intensity ratios of G-band to D-band (IG/ID), which can monitor the purity and functionalization of MWCNTs, are 0.80 for A-MWCNT and 0.74 for DPPA-MWCNT. The results demonstrate the successful grafting of MWCNT with the phosphorus−nitrogen-containing compound. TGA profiles of A-MWCNT and DPPA-MWCNT under nitrogen atmosphere are given in Figure 3. A-MWCNT shows no fundamental mass loss in the whole temperature range studied. In comparison with A-MWCNT, DPPA-MWCNT exhibits about 14% mass loss in the temperature range 488− 700 °C, which is ascribed to decomposition of functionalized species on the carbon nanotubes. It indicates that the

Figure 4. Observation of functionalized MWCNT dispersions in THF and DMF after ultrasonication for 5 min and waiting for 1 week: (a) DPPA-MWCNT in THF; (b) A-MWCNT in THF; (c) DPPAMWCNT in DMF; (d) A-MWCNT in DMF.

photos of the dispersion state of A-MWCNT and DPPAMWCNT suspensions after a week of standing. The dispersion of A-MWCNT is stable in THF, resulting from good hydrogenbonding ability between -NH2 of A-MWCNT and oxygen in THF. However, A-MWCNT is poorly dispersed in DMF with a solid precipitate. In contrast, DPPA-MWCNT generates a stable suspension in DMF with a deeply black solution but an unstable dispersion in THF. Because the polarity of DMF is higher than that of THF, the strongly polar reagent DPP-Cl can be preferentially dissolved in DMF. The reaction between AMWCNT and DPP-Cl results in improved polarity of functionalized CNTs. Therefore, DPPA-MWCNT shows better C


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ACS Applied Materials & Interfaces dispersibility in DMF. These results agree well with the reported literature.14 Morphological Analysis. The cross sections of PS/AMWCNT and PS/DPPA-MWCNT composites were characterized by SEM, as shown in Figure 5. It is observed that A-

Figure 6. DSC curves of neat PS, PS/A-MWCNT, and PS/DPPAMWCNT under nitrogen conditions at a heating rate of 10 °C/min.

macromolecular chains probably wrap the DPPA-MWCNT nanoparticles, resulting in noncovalent cross-linking, which can strongly restrict the segmental motion of PS chains.28−32 The glass transition temperature is thus increased with the addition of DPPA-MWCNT. Thermal Decomposition Behavior. TGA profiles of PS and its nanocomposites are presented in Figure 7, and the corresponding data are listed in Table 1. Thermal degradation behavior of each sample exhibits a single stage under nitrogen atmosphere, as shown in Figure 7a. In the case of virgin PS, more than 99% of the mass is lost, with only 0.7 wt % residue left at 700 °C. Incorporation of A-MWCNT and DPPAMWCNT leads to improvement in thermal stability and char yields of PS. The T−1% value is increased from 340 °C for virgin PS to 344 °C for PS/A-MWCNT and 347 °C for PS/DPPAMWCNT. Based on TGA data of neat PS and the two functionalized CNTs, the calculated residual contents of both PS/A-MWCNT and PS/DPPA-MWCNT at 700 °C are 0.9 wt %. In experimental results shown in Figure 7a and Table 1, PS/ DPPA-MWCNT has a high residual weight (3.1 wt %), more than that of PS/A-MWCNT (2.4 wt %), implying that the incorporated functionalized CNTs effectively promote char formation. From Figure 7b, it is seen that addition of AMWCNT and DPPA-MWCNT nanoparticles slightly reduces the maximum mass loss rates (MMLR). All three samples have the same Tmax value (temperature at MMLR), which shows that both CNTs enhance the thermal stability of PS. The improved effect is more notable in PS/DPPA-MWCNT. The CNTs play an important role in retarding the escape of volatile decomposition products from the PS nanocomposites.10,28 Additionally, good dispersion and strong interfacial interaction between DPPA-MWCNT and PS effectively retard the thermal motion of PS macromolecular chains, resulting in further enhancement in the thermal stability of PS/DPPAMWCNT.28,35 Thermo-oxidation Decomposition Performance. Thermo-oxidation decomposition performance of virgin PS and its nanocomposites was measured by TGA under air conditions. Figure 8 shows the thermo-oxidation degradation curves of each sample, and the corresponding data are listed in Table 1. Thermo-oxidation degradation profiles of PS and its nanocomposites are different than their thermal degradation curves

Figure 5. SEM images of composite fracture surfaces: (a, b) PS/AMWCNT and (c, d) PS/DPPA-MWCNT.

MWCNT is poorly distributed within the PS matrix. Agglomerated CNT nanoparticles can be found. This indicates that there is no interface interaction between A-MWCNT and PS molecular chains. In contrast, DPPA-MWCNT nanoparticles are uniformly distributed within the PS matrix, which can be observed in Figure 5c,d. The nanoparticles are completely integrated into PS molecular chains, leading to disappearance of the interface between DPPA-MWCNT and PS. A higher-magnification SEM image of PS/DPPA-MWCNT is displayed in Figure S2. It shows that the PS macromolecular chains probably wrap the DPPA-MWCNT nanoparticles, resulting in noncovalent cross-linking.28−32 The good dispersion is attributed to two reasons. First, the functional groups on the surface of DPPA-MWCNT enhance the repulsion forces among DPPA-MWCNT nanoparticles, restricting the formation of agglomerates.33 On the other hand, DPPA-MWCNT has many benzene groups, which are the ingredients of PS. Similar structure leads to good combination of the two different materials. Glass Transition Behavior. Figure 6 shows the DSC curves of neat PS and its nanocomposites. The neat PS used in the current work is an atactic polystyrene. Its glass transition temperature (Tg) is 105.3 °C, which is consistent with the reported value.29,34 There is a slight increase in Tg for PS/AMWCNT composite, resulting from the weak interaction between PS and A-MWCNT. The poor interaction can be verified by morphological analysis of PS/A-MWCNT composite. The dispersion of A-MWCNT in PS matrix is inhomogeneous. Many aggregates do not induce an obvious impact on the segmental motion of PS chains.29 Addition of DPPA-MWCNT into PS leads to an increase of Tg value from 105.3 to 109.4 °C. This result suggests that there is good interaction between PS chains and DPPA-MWCNT surface. From morphological analysis of PS/DPPA-MWCNT, it is observed that the distribution of DPPA-MWCNT within the PS matrix is very good. The nanoparticles are completely integrated into PS molecular chains, leading to the disappearance of interface between DPPA-MWCNT and PS. The PS D


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Figure 7. Thermal decomposition curves of neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT under nitrogen condition: (a) mass loss and (b) mass loss rate.

asymmetric temperature distribution.37,38 The PS chains close to the agglomerates first begin thermo-oxidation degradation because of the higher thermal radiation from the aggregated CNTs. Although the T−1% value of PS/A-MWCNT is slightly reduced, the Tmax value is increased with lower maximum mass loss rate. This indicates that incorporation of A-MWCNT inhibits thermo-oxidation degradation of PS. In comparison with neat PS and PS/A-MWCNT, PS/DPPA-MWCNT shows better thermo-oxidative stability during thermo-oxidation degradation. The T−1% and Tmax values respectively increase from 274 and 352 °C for neat PS to 281 and 377 °C for PS/ DPPA-MWCNT nanocomposite. Addition of either AMWCNT or DPPA-MWCNT nanoparticles reduced the maximum mass loss rates, as shown in Figure 8b, indicating that incorporation of either of the two functionalized CNTs inhibits the thermo-oxidative decomposition of PS composites. The enhanced thermo-oxidative stability of PS/DPPAMWCNT is attributed to the good dispersion and interface compatibility between DPPA-MWCNT and PS matrix. Meanwhile, MWCNTs can serve as high-temperature stabilizers during the thermo-oxidative decomposition reactions, which is similar to fullerenes.39,40 The good physical barrier effect of

Table 1. TGA Data for Each Sample under Nitrogen and Air Conditionsa N2 atmosphere sample error neat PS PS/AMWCNT PS/DPPAMWCNT a

air atmosphere

T−1% (°C)

Tmax (°C)

residue (wt %)

T−1% (°C)

Tmax (°C)

residue (wt %)

±1 340 344

±1 397 397

±0.5 0.7 2.4

±1 274 271

±1 352 369

±0.5 0.7 1.5

347

397

3.1

281

377

2.6

20 °C/min, 5−10 mg; residue obtained at 700 °C.

in nitrogen. The thermal stability of PS is reduced, resulting from oxidation, which can accelerate the decomposition of PS macromolecules.36 The initial thermo-oxidation decomposition temperature of PS/A-MWCNT is reduced in comparison with virgin PS. Morphological analysis of PS/A-MWCNT shows that there are agglomerates of CNTs as small distinct domains in PS matrix. Lots of heat focus on the domains due to the CNTs’ excellent thermal conductivity resulted in the

Figure 8. Thermo-oxidation degradation curves of neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT under air conditions: (a) mass loss and (b) mass loss rate. E


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Fire Hazards. Cone calorimeter is a widely used bench-scale instrument to study the fire hazards for polymers, which can give many important fire-related parameters, including heat release rate (HRR) and total heat release (THR).42−45 The HRR and THR profiles of neat PS and its nanocomposites under a heat flux of 35 kW/m2 are presented in Figure 10, and related data are listed in Table 2. Neat PS burns rapidly after ignition with a high PHRR value, and a HRR cone peak appears at 185 s (Tp value). Addition of either A-MWCNT or DPPAMWCNT to PS brings a reduction in PHRR with increasing Tp values. The HRR curves also become much flatter. PHRR value decreases from 907 kW/m2 for neat PS to 734 kW/m2 for PS/ A-MWCNT and 625 kW/m2 for PS/DPPA-MWCNT, a reduction of 19% and 31%, respectively. Tp is increased from 185 s for virgin PS to 200 s for PS/A-MWCNT and 215 s for PS/DPPA-MWCNT. THR is reduced after introduction of either of the two functionalized CNTs, as shown in Figure 10b. The lowest THR value is observed for PS/DPPA-MWCNT, which is decreased by 7% in contrast to virgin PS. PS is one of the pendant aromatic group-containing polymers. There are lots of smoke and toxic gases released during the combustion process of PS, which can directly put people to death by poisoning and suffocation.21,46,47 Therefore, it is necessary to study the emission of smoke and toxic gases. Figure 11 shows the smoke production rate (SPR) and total smoke production (TSP) profiles of virgin PS and its nanocomposites. The corresponding data are listed in Table 2. Neat PS shows high peak SPR (PSPR) and TSP values. With addition of A-MWCNT, the two values slightly decrease. Incorporation of DPPA-MWCNT to PS significantly reduces the PSPR value from 0.478 m2/s for neat PS to 0.307 m2/s for PS/DPPA-MWCNT, a reduction of 36%. PS/DPPA-MWCNT also shows the lowest TSP. The results indicate that introduction of DPPA-MWCNT effectively inhibits smoke production during the combustion process of PS composites. In the toxic gases produced from burning polymers, CO and CO2 are the main culprits, represented by their asphyxiant properties causing fatality in a real fire. CO and CO2 production profiles of virgin PS and its nanocomposites are shown in Figure 12, and the corresponding parameters are listed in Table 2. Addition of either A-MWCNT or DPPAMWCNT to PS brings a reduction in PCOP and PCO2P. PS/ DPPA-MWCNT shows a more significant inhibition effect on

MWCNTs can provide enough time to MWCNTs and surface phosphorus-containing compounds to trap the degrading polymer radicals to inhibit thermo-oxidative degradation.10,28,40,41 Therefore, there is a high residual yield from 420 to 700 °C for PS/DPPA-MWCNT. Tensile Behavior. Figure 9 shows the tensile behavior of neat PS and its nanocomposites. The related data are

Figure 9. Tensile stress−strain curves of neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT.

summarized in Table S1. Incorporation of A-MWCNT and DPPA-MWCNT into PS improves the tensile strength, due to the nanoreinforcing effect of CNTs with ultrahigh aspect surface area.28 The improvement is more significant in the case of PS/DPPA-MWCNT. The tensile strength of PS/DPPAMWCNT is improved by 12%, from 65.3 to 73.4 MPa, which is ascribed to good dispersion and strong adhesion between DPPA-MWCNT and the PS matrix. Meanwhile, the addition of A-MWCNT or DPPA-MWCNT leads to reduced elongation at break of PS nanocomposites. This indicates that the PS composites are brittle in comparison with neat PS, resulting from the enhanced stiffness of PS nanocomposites and the formation of microvoids around the MWCNTs during the tensile measurement.28

Figure 10. (a) HRR and (b) THR curves of neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT in cone calorimeter testing at 35 kW/m2. F


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ACS Applied Materials & Interfaces Table 2. Cone Calorimeter Data for Each Sample at 35 kW/m2 a sample

TTI (s)

Tp (s)

PHRR (kW/m2)

THR (MJ/m2)

PSPR (m2/s)

TSP (m2/kg)

PCOP (g/s)

PCO2P (g/s)

residue (wt %)

error neat PS PS/A-MWCNT PS/DPPA-MWCNT

±5 52 30 33

±5 185 200 215

±15 907 734 625

±0.5 104.0 98.6 96.3

±0.01 0.478 0.391 0.307

±20 1229 1215 1161

±0.005 0.0371 0.0276 0.0249

±0.02 0.848 0.694 0.639

±0.2 0.8 2.5 4.7

a

TTI, time to ignition; Tp. time to peak heat release rate; PHRR, peak heat release rate; THR, total heat release; PSPR, peak smoke production rate; TSP, total smoke production; PCOP, peak CO production; PCO2P, peak CO2 production.

Figure 11. (a) SPR and (b) TSP curves of neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT in cone calorimeter testing at 35 kW/m2.

Figure 12. (a) CO and (b) CO2 production as a function of burning time for neat PS, PS/A-MWCNT, and PS/DPPA-MWCNT in cone calorimeter testing at 35 kW/m2.

release of CO and CO2. In comparison with neat PS, the PCOP and PCO2P values for PS/DPPA-MWCNT are reduced by nearly 33% and 25%. respectively. Reduction of CO and CO2 greatly contributes to reduced toxicity of volatile products in the burning process, which may be favorable to fire rescue.21 The improved flame-retardant properties of PS/DPPAMWCNT nanocomposites are probably attributed to the good carbonization effect. From the TGA results, we have concluded that these two functionalized CNTs can improve char formation and inhibit the degradation of PS. PS/DPPAMWCNT shows a more significant carbonization effect, which is in good agreement with its higher residual yield (4.7 wt %) in cone calorimeter testing. From Figure 13, it is obvious that higher char yields are left in the cone calorimeter testing of PS/

Figure 13. Digital photographs of residues after cone calorimeter testing: (a) neat PS; (b) PS/A-MWCNT; (c) PS/DPPA-MWCNT.

DPPA-MWCNT. The good physical barrier effect of MWCNTs can provide enough time for MWCNTs and functionalized phosphorus-containing compounds to trap the G


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degrading polymer radicals, inhibiting the thermal degradation leading to formation of char residues.10,28,40,41 The char layer resulting from the carbonization reaction can function as an insulating barrier to reduce the exposure of PS matrix in the composites to an external heat source.48−50 Therefore, the flame retardancy is remarkably enhanced.

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CONCLUSIONS A-MWCNT was reacted with diphenylphosphinic chloride (DPP-Cl) to prepare functionalized multiwalled carbon nanotubes (DPPA-MWCNT). Structural characterizations showed that the phosphorus- and nitrogen-containing agents were successfully grafted on the surface of MWCNT. The functionalized MWCNTs, A-MWCNT and DPPA-MWCNT, were separately melt-compounded with PS to obtain the hybrid composites. SEM observations revealed that DPPA-MWCNT was more uniformly distributed within the PS matrix than AMWCNT. In comparison with PS/A-MWCNT, PS/DPPAMWCNT showed higher glass transition temperature, thermal stability, and tensile strength, resulting from the uniform distribution and strong interfacial interactions between DPPAMWCNT and PS matrix. Incorporation of DPPA-MWCNT into PS significantly reduced PHRR, SPR, and CO and CO2 production in cone calorimeter tests. The improved flameretardant properties should be ascribed to the barrier effect of carbon nanotubes that could provide enough time to DPPAMWCNT and its functionalized groups to trap the degrading polymer radicals to improve char formation. The char layer served as an insulating barrier to reduce the exposure of PS matrix in the composites to an external heat source as well as retarding the flammable gases from feeding the flame.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06864. One table and two figures showing tensile properties, preparation route, and SEM image of composite fracture surface with high magnification (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone +86-551-62158394; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge projects from National Natural Science Foundation of China (51403048 and 51276054), Anhui Provincial Natural Science Foundation (1508085QE111), Natural Science Foundation in University of Anhui Province (KJ2016A606), Natural Science Projects in Research Development Foundation of Hefei University (16ZR09ZDA), Fundamental Research Funds for the Central Universities (WK2320000024), China Postdoctoral Science Foundation (2013M531524), and Program for Excellent Young Talents in University of Anhui Province.

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Functionalized Carbon Nanotubes with Phosphorus- and Nitrogen

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