Flame-Retardant Epoxy Resin Nanocomposites ... - ACS Publications

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Flame-Retardant Epoxy Resin Nanocomposites Reinforced with Polyaniline-Stabilized Silica Nanoparticles Hongbo Gu,†,‡ Jiang Guo,† Qingliang He,† Sruthi Tadakamalla,†,# Xi Zhang,†,# Xingru Yan,† Yudong Huang,‡ Henry A. Colorado,ζ Suying Wei,*,# and Zhanhu Guo*,† †

Integrated Composites Lab (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710, United States ‡ School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China # Department of Chemistry and Biochemistry, Lamar University, Beaumont, Texas 77710, United States ζ Materials Science and Engineering, University of California - Los Angeles, Los Angeles, California 90066, United States S Supporting Information *

ABSTRACT: Epoxy resin nanocomposites reinforced with silica nanoparticles have been prepared at different nanoparticle loading levels. The surface functionality of the silica nanoparticles is manipulated by the phosphoric acid (H3PO4)-doped conductive polyaniline (PANI) via a surface initiated polymerization (SIP) method. The improved glass transition temperature (Tg) and enhanced mechanical properties of the cured epoxy resin nanocomposites filled with the functionalized silica nanoparticles are observed compared with those of the cured pure epoxy resin. The flammability and thermal stability behaviors of these nanocomposites are evaluated using microscale combustion calorimeter (MCC) and thermogravimetric analysis (TGA). The heat release rate (HRR) peak of the epoxy filled with functionalized silica nanoparticles is observed to decrease dramatically with increasing functionalized silica particle loadings, indicating a flame-retardant performance from the phosphoric acid-doped PANI. nanthrene-10-oxide (DOPO).16 Zhang et al.17 have studied the effect of microsized and nanosized Mg(OH)2 particles on the flame-retardant properties of the ethylene−propylene−diene monomer rubber (EPDM) composites and confirmed that the nanosized nanoparticles can efficiently improve the flameretardant properties. Normally, phosphorus-, nitrogen-, and silicon-based flame retardants8 (also called green products) are preferred in improving the flame retardancy of the materials because they are friendly to the environment compared with the halogenated compounds.1,7 Recently, the epoxy resin nanocomposites reinforced with silica nanoparticels have gained more and more attentions due to the high stability of silica at high temperatures and in strong acid/base environments18 and the strong adhesion between silica and epoxy matrix.19 Liu et al.14 have prepared silicacontaining phosphorus epoxy resin nanocomposites from nanoscale colloidal silica. The enhanced thermal stability was obtained by adding silica nanoparticles. However, the effectively synergistic effects of the colloidal nanosilica on the char formation and flame retardancy with phosphorus have not been observed, and the depressed glass transition temperature (Tg) has been obtained owing to the plasticizing effect of the colloidal silica. Suzuki et al.20 have focused on the obtained low thermal expansion property of the silica/epoxy resin nanocomposites with bimodal filler system consisting of mesoporous

1. INTRODUCTION Epoxy resin is one of the most important engineering polymers with wide industry applications, such as packaging,1 adhesives,2 encapsulating materials for electronics,3 coatings,4 and aerospace and marine systems5 because of its good chemical resistance, excellent thermal and electrical insulating properties, and high tensile strength.6 However, untreated epoxy is highly inflammable, which significantly limits its applications. Therefore, the modification of epoxy in order to improve its flame retardancy is an important issue and needs be addressed.7 Generally, there are three approaches to reduce the polymer flammability, i.e., (1) to use the inherently flame-retarded polymers such as polyimide, poly(p-phenylene-2,6-benzobisoxazole) (PBO), and poly(p-phenylene-2,6-benzobisthiazole) (PBZT); (2) to chemically modify the existing polymers (copolymerization of flame-retardant monomers into the polymer chains); and (3) to incorporate flame retardants into the hosting polymer matrix.8 All these methods could be used for enhancing the flame retardancy of epoxy resin, such as polyimide-modified epoxy system,9 epoxy resin containing phosphorus (phosphorus has been incorporated within the backbone of the epoxy resin), 7 polyhedral oligomeric silsesquioxanes (POSS) reinforced epoxy composites.10 For the third method, the inorganic nanoparticles are often used to enhance the flame retardancy performance of the polymer matrix. The commonly used inorganic nanoparticles include magnesium hydroxide (Mg(OH)2),11 antimony trioxide (Sb 2 O 3 ), 12 aluminiumoxide trihydrate (ATH), 13 silica (SiO2),14 nanoclay,15 and newly developed phosphoruscontaining flame retardants 9,10-dihydro-9-oxa-10-phosphaphe© XXXX American Chemical Society

Received: January 24, 2013 Revised: May 1, 2013 Accepted: May 17, 2013

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dx.doi.org/10.1021/ie400275n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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silica and fine silica nanoparticles. Chen et al.21 have investigated the thermal and mechanical properties of the silica/epoxy resin nanocomposites with different silica loadings. Silica loading is observed to have different effects on the Tg, tensile modulus and fracture toughness, and the addition of silica into the epoxy matrix is a promising method to enhance the properties of epoxy resin. Sun et al.22 have studied the thermal properties, moisture absorption, and dielectric properties of the silica/epoxy composites with micro- and nanosized silica for their potential applications in the electronic packaging devices. Polyaniline (PANI), consisting of alternative amine and imine groups in its backbone, is one of the important conjugated polymers. Recently, Gu et al.5,6 have reported that the PANI can serve as the coupling agent to improve the dispersion quality of the nanoparticles within the epoxy matrix and to enhance the interfacial interaction between the nanoparticles and epoxy matrix. The exploration of the nanocomposite formation mechanism has demonstrated that PANI can serve as a bridge to connect the nanoparticles and the hosting epoxy matrix. However, the introduction of the phosphoric acid-doped PANI-functionalized silica into epoxy resin to enhance the flame-retardant properties of epoxy has not been reported yet. In the present work, the epoxy resin nanocomposites reinforced with silica nanoparticles were successfully prepared. The surface functionality of the silica nanoparticles was achieved by the H3PO4-doped polyaniline (PANI) using the surface initiated polymerization (SIP) method. The mechanical properties including dynamic mechanical analysis (DMA) and tensile tests of these cured epoxy resin nanocomposites were studied. The flame-retardant behaviors of these nanocomposites were investigated using microscale combustion calorimetry (MCC). The fracture surface of the cured epoxy resin nanocomposites was observed by a scanning electron microscope (SEM). The nanocomposites filled with the as-received silica nanoparticles were also prepared for comparison. The interaction between the as-received silica (or the functionalized silica) nanoparticles and epoxy matrix was elaborated on the basis of the DSC test.

2.2. Surface Functionalization of Silica Nanoparticles. The silica nanoparticles were functionalized with PANI using a facile SIP method.23 Briefly, the silica nanoparticles (60−70 nm, 2.511 g), H3PO4 (15 mmol), and APS (9 mmol) were added into 100 mL of deionized water in an ice−water bath for 1-h mechanical stirring (SCILOGEX OS20-Pro LCD Digital Overhead Stirrer, 300 rpm) combined with sonication (Branson 8510). Then the aniline solution (18 mmol in 25 mL deionized water) was mixed with the above silica nanoparticle suspension and then mechanically stirred under ultrasonication for an additional hour in an ice−water bath for further polymerization. The product was vacuum filtered and washed with deionized water until the pH value was about neutral. The precipitant was further washed with methanol to remove any possible oligomers. The final black-green powders (the green color means the doped form of PANI, which confirms the presence of H3PO4 in the PANI. PANI has different colors with different oxidation states.24) were dried at 50 °C overnight. Pure PANI was also synthesized following the aforementioned procedures without adding nanoparticles for comparison. 2.3. Preparation of Silica/Epoxy Resin Nanosuspensions and Cured Nanocomposites. The Epon resin nanosuspensions containing 1.0, 3.0, 5.0, and 7.0 wt % of the as-received (untreated) (60−70 nm) silica (u-silica) and functionalized silica (f-silica) nanoparticles were prepared. Both u-silica and f-silica nanoparticles were immersed with epoxy resin overnight without any disturbance so that the resin could wet the nanoparticles completely; the solution was then mechanically stirred for one hour (600 rpm, Heidolph, RZR 2041). All the procedures were carried out at room temperature. The curing agent EpiCure W was added into the Epon monomers or the above prepared silica nanoparticle epoxy resin nanosuspensions with a monomer/curing agent weight ratio of 100/26.5 following the recommendation from the MillerStephenson Chemical Company for 1-h mechanical stirring (200 rpm). Then the solution was mechanically stirred at 70 °C for 2−3 h in a water bath at the same speed (200 rpm) until the viscosity of the solution increased, which is essential to remove the bubbles and to prevent the sedimentation of the silica nanoparticles during the curing process. Generally, the curing process for cross-linking means the phase transition from liquid to solid at a critical extent of the reaction, which is called gelation (gel point).25 At the gel point, the steady shear viscosity of polymer is infinite, and the equilibrium modulus is zero.25 On the basis of this idea, the experiment at 70 °C for 2− 3 h in a water bath could make the viscosity of the Epon solution increase and help achieve the gel point. As the viscosity increased to certain extent, the solutions were poured into silicone molds (which can prevent the sedimentation of the nanoparticles at the curing temperature) and cured at 120 °C for 5 h, and then cooled down to room temperature naturally. The pure epoxy resin without adding silica nanoparticles was also prepared following the aforementioned curing procedures for comparison. In order to investigate the synergistic effects of the nitrogen/phosphorus on the flame-retardant properties of the epoxy nanocomposites, the H2SO4-doped f-silica/epoxy nanocomposites with f-silica loading of 5 wt % were also prepared following the aforementioned fabrication method and curing procedures with the same amount of H3PO4 for comparison.

2. EXPERIMENTAL SECTION 2.1. Materials. The Epon 862 (bisphenol F epoxy) and the curing agent, EpiCure W, were provided by Miller-Stephenson Chemical Company, Inc. The molecular structures of these chemicals are shown in Scheme 1. Aniline (C6H7N, 99.9%), Scheme 1. Molecular Structure of Epon 862 and the Used Curing Agent Epicure W

and ammonium persulfate (APS, (NH4)2S2O8, reagent grade, 98%) were purchased from Sigma Aldrich. Phosphoric acid (H3PO4, 85 wt %), sulfuric acid (H2SO4, 98 wt %), and methanol were obtained from Fisher Scientific. The silica nanoparticles with an average diameter of 60−70 nm (98+%, amorphous, specific surface area: 160−800 m2/g) were obtained from US Research Nanomaterials Inc. All the chemicals were used as received without any further treatment. B



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Figure 1. SEM images of (a) u-silica and (b) f-silica; (c) FT-IR spectra and (d) TGA curves of u-silica, pure H3PO4-doped PANI, and silica nanoparticles functionalized with H3PO4-doped PANI (f-silica).

2.4. Characterizations. The chemical structures of the usilica nanoparticles, f-silica nanoparticles, and pure PANI were analyzed by Fourier transform infrared spectroscopy (FT-IR, a Bruker Inc. Vector 22 coupled with an ATR accessory) in the range of 500−4000 cm−1 with a resolution of 4 cm−1. The thermal stability was studied in a thermogravimetric analysis (TGA, TA Instruments, Q-500) with a heating rate of 10 °C min−1 under an air and nitrogen flow rate of 60 mL min−1 from 25 to 800 °C, respectively. The viscosity of the liquid Epon nanosuspensions with silica nanoparticles was studied in a Rheometer (TA Instrumental, AR2000ex ETC system). The measurements were performed in a cone−plate geometry with a diameter of 40 mm and a truncation of 64 μm. The measurements were done using a steady state flow procedure and at 25 °C, respectively. The shear rate was carried out between 1 to 1000 s−1. The specimens placed between the cone and plate were allowed to equilibrate for approximately 2 min prior to each test. Dynamic mechanical analysis (DMA) was performed in the torsion rectangular mode by using an AR 2000ex (TA Instrumental Company) with a strain of 0.05%, a frequency of 1 Hz, and a heating rate of 2 °C min−1 in the range of 30− 200 °C. The sample dimensions are 12 × 3 × 40 mm3. The tensile tests were carried out following the American Society for Testing and Materials specifications (ASTM, 2002, standard D412-98a) in a unidirectional tensile test machine (ADMET

universal testing system, eXpert 2610). The parameters (displacement and force) were controlled by a digital controller (MTESTQuattro) with MTESTQuattroMaterials Testing Software. The dog-bone shaped samples were prepared according to the ASTM standard requirement in the silicone rubber molds. A crosshead speed of 1 mm min−1 was used, and the strain (mm/mm) was calculated by dividing the jogging displacement by the original gauge length. The fracture surface of the samples was observed on a JSM-6700F system with a JEOL field emission scanning electron microscope (SEM). The SEM specimens were prepared by sputter coating a thin gold layer approximately 3 nm thick. Microscale combustion calorimetry (MCC) was conducted on 3 ± 1 mg samples using a Govmark Microscale Combustion Calorimeter (Model: MCC-2) operated at a heating rate of 1 °C s−1 to 750 °C in the pyrolysis zone. The samples were tested according to ASTM guidelines (ASTM D7309−07). The combustion zone was set at 900 °C. The oxygen and nitrogen flow rates were set at 30 and 70 mL min−1, respectively. The heat release rate (HRR) in watts per gram of sample (W g−1) was calculated from the oxygen depletion measurements. The heat release capacity (HRC) in J g−1 K was obtained by dividing the sum of peak HRR by the heating rate in K s−1. The total heat release (THR) in kJ g−1 was obtained by integrating the HRR curve. The char yield was obtained by weighing the sample before and after the test. The reported values are the C



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temperature to 200 °C is due to the elimination of moisture and the doped H3PO4 in PANI.36 The second weight loss from 250 to 600 °C is due to the large-scale thermal degradation of the PANI chains.37 The silica loading in the f-silica estimated from the weight residue at 800 °C is about 73%. The initial calculated u-silica nanoparticle loading in the f-silica nanoparticles is 60 wt %. Therefore, the polymerization degree of the aniline to the PANI is around 67.5%. 3.2. Nanocomposites Formation Mechanism. In order to investigate the interaction between u-silica (or f-silica) and epoxy matrix to explore the nanocomposites formation mechanism, the DSC tests were performed as aforementioned in the Experimental Section. Figure 2 shows the DSC curves of

averages of three measurements on each compound with a deviation of ±5%. To investigate the nanocomposite formation mechanism, the u-silica and f-silica nanoparticle samples for the test were prepared without adding the curing agent in the following way: u-silica or f-silica (7.0 wt % loading) was mechanically (600 rpm) stirred with epoxy resin for one hour; then a small amount of epoxy resin, and epoxy resin suspensions with either u-silica or f-silica were used for differential scanning calorimetry (DSC, TA Instruments Q2000) tests. The DSC measurements were carried out to monitor the possible reaction between usilica (or f-silica) and epoxy resin. Briefly, the measurements were carried out under a nitrogen flow rate of approximately 20 mL/min at a heating rate of 10 °C/min from 30 to 70 °C, and an isothermal process at 70 °C was continued for 2 h. After that, the temperature was continuously increased to 120 °C at a heating rate of 10 °C/min, and an isothermal process was conducted at 120 °C for another 2 h.

3. RESULTS AND DISCUSSION 3.1. Functionalization of Silica Nanoparticles. The SEM images of the u-silica and f-silica nanoparticles are shown in Figure 1 (a,b). The u-silica nanoparticles are ball-like morphology, and the surface is relatively smooth, Figure 1a. The f-silica nanoparticles, Figure 1b, show particulate structure with a fairly uniform size distribution and is observed to be relatively loosely packed. The surface of the f-silica becomes rougher due to the polymerization occurred on the surface of usilica nanoparticles to form the core−shell structure.26 Figure 1(c) shows the FT-IR spectra of the u-silica, pure PANI and f-silica, respectively. In the FT-IR spectrum of the usilica, the absorption peak at 800 cm−1 is assigned to the bending vibrations of Si−O bonding.27 The absorption peak around 1067 cm−1 is due to the stretching vibration of the Si− O bond in the silanol groups (Si−OH).28 The presence of this peak indicates the incomplete condensation with the presence of a conspicuous amount of hydroxyl groups on the surface of the silica nanoparticles during the silica nanoparticle preparation process.29 These two peaks are also observed in the FT-IR spectrum of f-silica. In the FT-IR spectrum of pure H3PO4 doped PANI, the strong absorption peaks at 1557 and 1476 cm−1 correspond to the CC stretching vibration of the quinoid and benzenoid rings of the PANI polymer backbone, respectively.30 The band at 1287 cm−1 is related to the C−N stretching vibration of the benzenoid unit.31 The C−H in-plane bending vibration of the quinoid rings is observed to be located at 1238 cm−1.32 The band at around 790 cm−1 is due to the outof-plane bending of C−H in the substituted benzenoid ring.33 These characteristic peaks (1557, 1476, 1287, 1238, and 790 cm−1) confirm that the synthesized PANI is in EB salt form.34 These characteristic peaks also appear in the FT-IR spectrum of f-silica, however, a small shift (about 25 cm−1) is observed, indicating a strong interaction between the silica and PANI layers.35 This may arise from the existence of the hydrogen bonding between the silanol groups on the particle surface and the amine groups of the EB salt form PANI. Figure 1(d) shows the TGA curves of the u-silica, pure H3PO4-doped PANI, and f-silica conditioned in air. For the usilica nanoparticles, there is a slight weight loss (about 6 wt %) around 150 °C during the measured temperature range from 30 to 800 °C due to the loss of moisture. However, the H3PO4doped PANI is almost completely decomposed, and two-stage weight losses are observed. The first stage from room

Figure 2. DSC curves of (a) Epon monomers. (b) Epon suspension with 7.0 wt % loading of f-silica nanoparticles. (c) Epon nanosuspension with 7.0 wt % u-silica nanoparticles.

the epoxy resin monomers, epoxy resin nanosuspensions with 7 wt % loading of the u-silica, and f-silica nanoparticles. For the Epon monomers, Figure 2(a), two peaks are observed at 70 and 120 °C, and there is no exothermic peak obtained from 80 to 120 °C. In the Epon resin nanosuspensions with f-silica (Figure 2(b)) and u-silica (Figure 2(c)), the two peaks at 70 and 120 °C are also observed; meanwhile, the curing exothermic peak is obviously observed, illustrating that there is chemical reaction between the u-silica (or f-silica) and epoxy matrix. The reaction enthalpy for the Epon resin nanosuspensions filled with f-silica nanoparticles is about 6.431 J g−1. However, the reaction enthalpy for the Epon resin suspensions filled with u-silica nanoparticles is about 1.234 J g−1, indicating that the reaction between u-silica and Epon monomers is very trivial compared with that between f-silica and Epon monomers. The chemical reaction between the doped EB salt form PANI and epoxy has been reported in our previous papers,5,6 which arises from the condensation reaction from the amine groups and the epoxy groups. Ragosta et al.29 have explored the chemical reaction between the silica nanoparticles and epoxy matrix and believed that the presence of silanol groups (Si−OH) could react with the proton acceptor epoxy groups, which contributes to the enhanced tensile strength. 3.3. DMA Properties of Cured Epoxy and Nanocomposites. Figure 3 shows the DMA tests including storage modulus (G′), loss modulus (G″), and loss factor (tan δ) as a function of temperature from 30 to 200 °C for the cured pure epoxy and cured f-silica/epoxy nanocomposites with different fD



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epoxy (74 °C), owing to the increased stiffness and constriction between the epoxy polymer chains by the added f-silica nanoparticles, and the temperature shift increases with increasing f-silica loadings, which is also observed in the multi-walled carbon nanotubes (MWNTs)/polystyrene and polypropylene nanocomposites.39 In Figure 3(c), the peak of tan δ (indicating the glass transition temperature Tg, which is related to the degree of cross-linking in the materials6) for the cured f-silica/epoxy nanocomposites has also shifted to a higher temperature (2−10 °C) compared with that of the cured pure epoxy (87.5 °C), and the Tg increases with increasing the fsilica loading. The increased Tg is attributed to the fact that the added f-silica nanoparticles restrict the segmental movement of the polymer chains.6 Meanwhile, the Tg is described as a kinetic phenomenon caused by the continuous approach of free volume toward equilibrium during the heating procedure, which is caused by a rather sudden increase of free volume toward equilibrium near Tg.40 The addition of the f-silica nanoparticle can increase the free volume, and thus an increased Tg is obtained. The increased Tg is also observed in the silica epoxy hybrid nanocomposites prepared by the highshear mixing method in a sonication.41 Figure 4 shows the comparison of the DMA properties for 7.0 wt % f-silica/epoxy and u-silica/epoxy nanocomposites. From Figure 3, the G′, G″, and tan δ for the f-silica/epoxy nanocomposites (86, 87, and 97.5 °C for G′, G″, and tan δ, respectively) are observed to be higher than those of the usilica/epoxy nanocomposites (84, 82, and 93.5 °C for G′, G″, and tan δ), indicating that the functionalization could increase the cross-linking degree of epoxy.6 3.4. Tensile Mechanical Properties. Figure 5 shows the representative tensile strain−stress curves of the cured pure epoxy and epoxy resin nanocomposites filled with different loadings of the u-silica and f-silica nanoparticles, and the related tensile mechanical properties are listed in Table 1. For the 1.0 wt % silica/epoxy nanocomposites, the tensile strength of the fsilica/epoxy nanocomposites (around 108.1 MPa) is a little higher than that of the u-silica/epoxy nanocomposites (around 107.3 MPa). As the silica loading increases from 1.0 to 7.0 wt %, the tensile strength of the u-silica/epoxy nanocomposites changes negligibly (from 107.3 to 110.7 MPa). The tensile strength of the f-silica/epoxy nanocomposites also has small changes from 108.1 to 99.8 MPa as the f-silica loading increases from 1.0 to 7.0 wt % . Meanwhile, for all the u-silica/epoxy nanocomposites, the failure is attributed to the ductile failure mode, in which the yield appears in the strain−stress curve.6 Only f-silica/epoxy nanocomposites with silica loading of 1.0 wt % show a ductile failure, and the f-silica/epoxy nanocomposites with other loadings show a brittle failure, which is the same as that of the cured pure epoxy. For the elongation-at-break of the silica/epoxy nanocomposites, no obvious silica loading-dependent properties are observed. The Young’s modulus of the usilica/epoxy nanocomposites shows no particle loading-dependent properties. However, for the f-silica/epoxy nanocomposites, the Young’s modulus increases with f-silica loading increasing to 5.0 wt % and then decreases as the f-silica loading increases to 7.0 wt %. 3.5. Microstructure of Fracture Surface. Figure 6 shows the SEM microstructures of the tensile fracture surface of the cured pure epoxy and the epoxy nanocomposites filled with usilica and f-silica nanoparticles. For the cured pure epoxy, Figure 6(a,b), the fracture surface is very smooth together with the riverlike markings (commonly observed in the cured epoxy

Figure 3. (a) G′; (b) G″, and (c) tan δ vs temperature curves for the cured pure epoxy and its PNCs with different f-silica nanoparticle loadings.

silica loadings. In Figure 3(a), the temperature for the sharp decrease of the G′ (onset temperature) has been delayed by 6− 15 °C for the cured f-silica/epoxy nanocomposites compared to the onset temperature of the cured pure epoxy (71 °C); the delayed temperature increases with increasing f-silica loading. This large improved G′ is attributed to the reinforcement performance of f-silica, which restricts the mobility of epoxy polymer chains.38 In Figure 3(b), the peaks of G″ for the cured f-silica/epoxy nanocomposites have shifted to a higher temperature (about 4−13 °C) than that of the cured pure E



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Figure 5. Tensile stress−strain curves of the cured epoxy and epoxy nanocomposites filled with different loadings of u-silica and f-silica nanoparticles.

Table 1. Tensile Mechanical Properties of the Cured Epoxy and Silica/Epoxy Nanocomposites sample pure epoxy 1.0 wt % f-silica/epoxy 3.0 wt % f-silica/epoxy 5.0 wt % f-silica/epoxy 7.0 wt % f-silica/epoxy 1.0 wt % u-silica/epoxy 3.0 wt % u-silica/epoxy 5.0 wt % u-silica/epoxy 7.0 wt % u-silica/epoxy

tensile strength (MPa)

Young’s modulus (GPa)

elongation at break (%)

87.8 ± 3.7 108.1 ± 1.6

2.56 ± 0.5 2.87 ± 0.2

3.7 ± 0.1 5.66 ± 0.2

100.3 ± 1.5

3.19 ± 0.1

3.64 ± 0.7

99.8 ± 1.3

3.52 ± 0.3

3.21 ± 0.2

100.1 ± 0.5

3.33 ± 0.4

3.53 ± 0.1

107.3 ± 2.1

3.04 ± 0.1

4.77 ± 0.4

110.6 ± 3.7

3.33 ± 0.1

4.52 ± 0.4

110.7 ± 1.8

3.23 ± 0.1

5.31 ± 0.3

107.7 ± 0.9

3.25 ± 0.1

4.95 ± 0.2

strong silica fillers. The formation of the dimples is accompanied by the formation of new fracture surfaces with more fracture energy subsequently being dissipated, which is consistent with the epoxy/silica nanocomposites.45 For the silica/epoxy nanocomposites, the toughening mechanism (i.e., failure modes) is still open for discussion. Many mechanisms have been reported in the literature, such as crack deflection (where the crack front tilts and twists when it encounters the particles and hence passes around them),46 particle debonding and plastic void growth,46 and development of nanovoids.47 In the present work, the crack deflection may be the main reason for the failure mode of the silica/epoxy nanocomposites since the particle debonding and the nanovoids could not be observed, and a strong interfacial adhesion between silica nanoparticles and epoxy matrix is seen in the SEM images of the fracture surfaces, Figure 6(c−j). Figure 6(c,d,g,h) show the SEM images of the cured silica/epoxy nanocomposites filled with 1.0 and 7.0 wt % loading of u-silica nanoparticles, respectively, and Figure 6(e,f,i,j) depicts the SEM images of the cured silica/epoxy nanocomposites filled with 1.0 and 7.0 wt % of f-silica nanoparticles. From Figure 6(c,d), especially for the high loading u-silica/epoxy nanocomposites, Figure 6(g,h), the u-silica is observed to be agglomerated together. However, in Figure 6(e,f,i,j), even for the high loading f-silica/epoxy

Figure 4. (a) G′, (b) G″, and (c) tan δ vs temperature curves for PNCs filled with 7.0 wt % u-silica and f-silica nanoparticles, respectively.

tensile specimens),42 which is typical of a brittle failure; no obvious plastic deformation is occurred during the tensile tests.43 However, the fracture surface of the cured silica/epoxy nanocomposites becomes very rough and dramatically changes after addition of the silica nanoparticles, Figure 6(c−j). The riverlike markings are diminished and even disappear, and many dimples appear in the fracture surface of the silica/epoxy nanocomposites and the crack direction has been interrupted by the silica nanoparticles,44 which is beneficial for the load being effectively transferred from the weak epoxy matrix to the F



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weight loss (at around 350−430 °C) is attributed to the elimination of water molecules from oxypropylene groups (−CH2−CH(OH)−) and the breakdown of the epoxy network.49 The second weight loss (at about 450−600 °C) corresponds to the degradation of benzene rings of the cured epoxy PNCs due to the high C−C bonding energy.38,50 In the nitrogen (inert) atmosphere, Figure 6(B), the decomposition of epoxy and its nanocomposites is totally different from that in the air conditions, which only exhibits a single-stage degradation with the temperature from 30 to 800 °C.51 In Figure 6(A-a), for the u-silica/epoxy nanoxomposites, the first decomposition temperature (Td1, 5 wt % weight loss) is increased by 55 °C, and the second decomposition temperature (Td2, 60 wt % weight loss) is increased by 72 °C compared with the cured pure epoxy (311 and 440 °C for Td1 and Td2, respectively). For the H3PO4-doped f-silica/epoxy nanocomposites, Figure 6(A-b), the Td1 increases by 37 °C, and Td2 increases by 71 °C compared with those of the cured pure epoxy. For the H2SO4-doped f-silica/epoxy nanocomposites, the Td1 and Td2 are increased by 57 and 59 °C, respectively. These results indicate that the adding of u-silica and f-silica can increase the thermal stability of the epoxy matrix and the different dopant of f-silica can make the epoxy matrix have different thermal stability in the air atmosphere. In Figure 6(B), in the inert condition, the decomposition temperature (5 wt % weight loss) for the cured pure epoxy, silica/epoxy nanocomposites with 5 wt % loading of u-silica, and f-silica doped with H3PO4 and doped with H2SO4 are 363, 367, 363, and 374 °C, respectively. The weight residues at 800 °C of the silica/ epoxy nanocomposites with 5 wt % (normalized to u-silica: 3.8 wt %) loading of u-silica, and f-silica doped with H3PO4 and with H2SO4 are 14.72, 15.02, and 12.52%, respectively, which are improved compared with that of the cured pure epoxy (9.02%). This indicates that the adding of u-silica and f-silica can restrain the epoxy degradation.52 Figure 8 shows the heat release parameters obtained from MCC test for the cured pure epoxy and silica/epoxy nanocomposites. Generally, the HRR peak, THR, HRC, and char yield are important parameters to evaluate the flameretardant properties of the materials.53,54 In Figure 8, for the 1.0 wt % u-silica/epoxy nanocomposites, the HRR peak (about 652.2 W/g) is much higher than that of the cured pure epoxy (506 W/g), which means that the addition of u-silica did not provide the flame-retardant performance to the cured pure epoxy; inversely, it can support combustion at some extent. Actually, Kashiwagi et al.55 have found that the increased viscosity of the polymer matrix by the introduced nanoparticles can reduce the flammability of the matrix. Thus, in this work the increased HRR peak by adding u-silica nanoparticles to the epoxy matrix may be related to the fact that the decreased viscosity of the Epon monomers (Figure S1 in Supporting Information) can promote the combustion of the epoxy matrix. Meanwhile, the enlarged free volume for the epoxy arising from the existing nanoparticles could provide more opportunities for the decomposed volatiles to get burned to cause a higher HRR, similar to the aforementioned increased Tg. The heat-release parameters including HRR peak, THR, HRC, and char yield of the pure epoxy and epoxy nanocomposites filled with u-silica and f-silica are listed in Table 2. The HRR peaks for the epoxy nanocomposites filled with u-silica decrease with increasing usilica loadings, but they are still much higher than those of the epoxy nanocomposites filled with f-silica. The HRR peak, THR, and HRC values for the epoxy nanocomposites filled with f-

Figure 6. SEM microstructures of the fracture surface taken from tensile specimens of (a,b) cured pure epoxy; (c,d) 1.0 wt % u-silica/ epoxy PNCs; (e,f) 1.0 wt % f-silica/epoxy PNCs; (g,h) 7.0 wt % usilica/epoxy, and (i,j) 7.0 wt % f-silica/epoxy PNCs.

nanocomposites, the nanoparticle distribution is much better than the u-silica/epoxy nanocomposites. These results indicate that the functionalization improves the silica nanoparticle dispersion within the epoxy matrix. 3.6. Flame-Retardant Property Evaluation. Panels A and B of Figure 7 show respectively the TGA curves of pure epoxy and silica/epoxy nanocomposites with 5.0 wt % loading of u-silica, f-silica doped with H3PO4 and H2SO4 under air and nitrogen conditions. Under air conditions, Figure 6(A), the main degradation takes place in two stages in their decomposition profiles for the epoxy and silica/epoxy nanocomposites. The slight weight loss in the range of 250−320 °C in the nanocomposites is due to the homolytic scission of chemical bonds in the epoxy network.48 The first decomposed G



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Figure 7. TGA curves of the (a) cured pure epoxy, (b) epoxy filled with 5.0 wt % u-silica, (c) f-silica doped with H3PO4, and (d) f-silica doped with H2SO4.

doped PANI layer can significantly enhance the flame-retardant properties of the epoxy matrix. The char yield of the f-silica/ epoxy increases as the f-silica loading increases. Meanwhile, the HRR peak and HRC of epoxy nanocomposites filled with 5.0 wt % loading of f-silica doped with H2SO4 are higher than those of the epoxy nanocomposites filled with f-silica doped with H3PO4; however, the char yield of epoxy nanocomposites loaded with 5.0 wt % of f-silica doped with H2SO4 is lower than that of the epoxy nanocomposites filled with f-silica doped with H3PO4. These results indicate that the functionalization of silica by H3PO4-doped PANI is beneficial to the char formation during the combustion process; therefore, an improved the flame retardancy can be obtained as evidenced by the decreased HRR and increased residue. The flame-retardant compounds containing phosphorus and nitrogen are reported to mainly act as intumescent flame retardants resulting in a char layer in the condensed phase.56,57 Meanwhile, these nitrogen-containing compounds are usually gas sources and could produce incombustible gases without toxic smoke and fog when the materials are decomposing at high temperatures. These produced gases could dilute the concentration of oxygen near the flame and form a protective layer at the heating.58 Figure 9 shows the morphology after combustion under nitrogen conditions at 700 °C. The cured pure epoxy, Figure 9(a), shows the smooth and porous structure with many broken bubbles, indicating that the rapid volatilization has happened in the epoxy matrix because of the heat.59,60 The epoxy filled with f-silica doped with H2SO4, Figure 9(c), depicts a similar morphology with pure epoxy. However, the epoxy filled with f-silica doped with H3PO4 (Figure 9(b)) exhibits a more continuous and condensed-phase morphology, which further confirms that the phosphorus component can promote the char yield in the condensed phase. The microstructures of the char residues after the combustion in the nitrogen condition at 700 °C were investigated by SEM. Figure 10 shows the morphology of the char residues of the cured pure epoxy, epoxy filled with 5.0 wt % f-silica doped with H3PO4 and epoxy filled with 5.0 wt % fsilica doped with H2SO4. The char residues of the pure epoxy, Figure 10(a), exhibit a smooth and continual char layer, which means a rapid volatilization at the surface of the pure epoxy.59,60 For the epoxy filled with 5.0 wt % f-silica doped with H2SO4, Figure 10(c), the nanoparticles are observed to migrate and be

Figure 8. HRR vs temperature of (a) pure epoxy. Epoxy nanocomposites filled with the following at a loading of (b) 1.0, (c) 5.0, (d) 7.0 wt % of f-silica doped with H3PO4; (e) 1.0 wt % of u-silica; and (f) 5.0 wt % of f-silica doped with H2SO4.

Table 2. Heat-Release Parameters of the Measured Samples samples pure epoxy epoxy filled with u-silica 1.0 wt % 5.0 wt % 7.0 wt % epoxy filled with f-silica (H3PO4) 1.0 wt % (normalized to u-silica: 0.7 wt %) 5.0 wt % (normalized to u-silica: 3.8 wt %) 7.0 wt % (normalized to u-silica: 5.5 wt %) epoxy filled with f-silica (H2SO4) 5.0 wt % (normalized to u-silica: 3.8 wt %)

HRR peak (W/g)

THR (kJ/g)

HRC (J/g K)

char (%)

533.8

26.7

506

8.2

652.2 579.9 488.3

26.5 25.4 24.3

466 416 460

8.1 11.9 13.8

501.0

26.6

479

8.5

454.0

25.4

431

12.6

385.3

24.5

366

14.7

478.8

24.8

447

9.9

silica doped with H3PO4 are noticed to be much lower than those of the cured pure epoxy and decrease with increasing fsilica loadings, indicating that the introduction of the H3PO4H



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Figure 9. Micrographs of the (a) pure epoxy, and silica/epoxy nanocomposites filled with 5.0 wt % of (b) f-silica doped with H3PO4 and (c) f-silica doped with H2SO4 after combustion under nitrogen conditions from room temperature to 700 °C.

pushed to the surface of the matrix caused by the volatile products.61 However, for the epoxy filled with 5.0 wt % loading of f-silica nanoparticles doped with H3PO4, Figure 10(b), the nanoparticles are tightly cladded by the char layers, and no migration of the nanoparticles is observed. This indicates that the presence of the phosphorus component can insulate the heat transformation and promote the char formation,62 which is consistent with the results obtained from the MCC test.

4. CONCLUSION Epoxy resin nanocomposites reinforced with different asreceived silica and polyaniline-functionalized silica nanoparticle loading levels have been prepared. The DMA results show that the polymer chains have become stiffer after adding the functionalized silica nanoparticles, and the Tg is much higher than that of the cured pure epoxy and epoxy nanocomposites filled with the as-received silica nanoparticles. Enhanced tensile strength is obtained for the epoxy nanocomposites filled with both as-received silica and functionalized silica nanoparticles. The toughening fracture surface of the nanocomposites filled with both as-received and functionalized silica nanoparticles clearly translates the effective load transfer from the weaker epoxy matrix to the stronger silica nanoparticles. The functionalized silica/epoxy nanocomposites exhibit a better flame-retardant performance than the epoxy nanocomposites filled with the as-received silica nanoparticles. The DSC test is used to investigate the interactions between the epoxy polymer matrix and (1) the as-received silica which is from the silanol groups (Si−OH) on the surface of the as-received silica nanoparticles or (2) the functionalized silica nanoparticles and the amine groups in the polyaniline polymer backbone of the functionalized silica nanoparticles. These interactions are responsible for the observed enhanced mechanical properties. More decreased HRR peaks and more increased char yields are resepectively obtained by the TGA and MCC tests after adding

Figure 10. SEM micrographs of the char residue: (a) pure epoxy, (b) epoxy nanocomposites filled with 5.0 wt % f-silica doped with H3PO4, and (c) epoxy nanocomposites filled with 5.0 wt % f-silica doped with H2SO4.

f-silica nanoparticles doped with H3PO4 into the epoxy matrix compared with those of the nanocomposites with u-silica nanoparticles and f-silica nanoparticles doped with H2SO4, indicating the good flame-retardant performance from the H3PO4-doped f-silica nanoparticles in the epoxy matrix.

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ASSOCIATED CONTENT

S Supporting Information *

The shear rate vs viscosity of the Epon monomer and Epon suspensions with 7 wt % loading of u-silica. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

[email protected] (S.W.); [email protected] (Z.G.). Notes

The authors declare no competing financial interest. I



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Article

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ACKNOWLEDGMENTS This project is financially supported by the National Science Foundation Nanoscale Interdisciplinary Research Team and by Materials Processing and Manufacturing (CMMI 10-30755). H.G. acknowledges the support from China Scholarship Council (CSC) program.

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