Mg(OH)2 Complex Nanostructures with Superhydrophobicity and

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Mg(OH)2 Complex Nanostructures with Superhydrophobicity and Flame Retardant Effects Huaqiang Cao,* He Zheng, Jiefu Yin, Yuexiang Lu, Shuisheng Wu, Xiaoming Wu, and Baojun Li Department of Chemistry, Tsinghua UniVersity, Beijing 100084, China ReceiVed: May 23, 2010; ReVised Manuscript ReceiVed: September 6, 2010

Complex Mg(OH)2 nanostructures are synthesized via a biomolecule-assisted hydrothermal route. The assynthesized Mg(OH)2 nanostructures are dispersed into an acrylonitrile-butadiene-styrene (ABS) copolymer by mechanical kneading, which shows an excellent flame-retardant behavior. Also we demonstrated for the first time that the complex Mg(OH)2 nanostructures can find application in self-cleaning for its superhydrophobicity with water contact angle over 150° and sliding angle of 1°. 1. Introduction Magnesium hydroxide [Mg(OH)2, MH] is an important inorganic material, which has many industrial applications, including synthesizing halogen-free flame retardants for polymers,1,2 preparing foodstuff starch esters,3 treating wastewater,4 and desulfurizing waste gases.5 Recently, it has become of great interest to synthesize MH nanostructures with various sizes, shapes, and dimensions. Many methods have been developed to synthesize nanostructures with different morphologies, for example, one-dimensional (1D) nanorods synthesized via a solvothermal method,6 1D needlelike nanocrystals synthesized via reverse precipitation,7 twodimensional (2D) platelike nanocrystals synthesized via a hydrothermal route,8 spherelike nanostructures synthesized via a solution-based chemical process,9 and three-dimensional (3D) flowerlike nanostructures synthesized via a hydrothermal reaction,10 which present different physicochemical properties. Acrylonitrile-butadiene-styrene (ABS), being a noncharring polymer upon combustion, is an important commercial engineering thermoplastic material due to its excellent mechanical properties, chemical resistance properties, ease of processing, and recycling ability.11 However, it still has some shortcomings, such as inflammability. It is difficult to obtain a halogen-free retardant system for ABS.12 Thus, it is a great challenge for scientists to improve the properties of ABS.13 It is necessary to solve this problem by modifying polymers by adding flameretardant compounds to decrease their flammability.2,11,14,15 However, many environmental laws have been enacted to prohibit the application of halogen-containing flame retardants in polymer materials around the world. MH crystal is an important inorganic halogen-free and smoke-suppressing flame retardant, due to its better thermal stability, smoke suppression property, and flame retardancy, compared with other inorganic flame retardants, even in comparison with aluminum trihydroxide.16 It undergoes decomposition at 340-490 °C, which is one of the most important merits. Furthermore, MH is an environmentally friendly additive which has been extensively used in the halogen-free flame-retarded (HFFR) polymeric materials. It was demonstrated that fiber- and lamellalike MH, being a flame retardant, presented excellent functions in polymers.16 However, many research studies indicated that the MH crystals * To whom correspondence should be addressed. E-mail: hqcao@ mail.tsinghua.edu.cn.

should be improved, such as decreasing the usage amount and increasing the flame-retardant efficiency.17,18 It was reported that the addition level of MH reached up to 60 wt % in order to achieve acceptable combustion resistance,19,20 which in turn led to the decrease of the mechanical properties of polymers. It is possible to solve these problems by using micro/nanostructured MH as a flame retardant in ABS. It is known that the focus of nanotechnology research has, in recent years, been steadily moving away from the preparation of high-quality nanomaterials and the understanding of their physicochemical properties to practical applications.21 Herein, we report a synthesis of a flowerlike MH complex nanostructure used as an additive in an ABS matrix. In the present paper, novel MH nanoflowers with flame retardant effect were designed and successfully fabricated by a simple amino-acid-assisted hydrothermal approach. We also demonstrated for the first time that the MH nanoflowers may find applications in self-cleaning for its superhydrophobicity. 2. Experimental Section Synthesis. Growth of flowerlike MH nanostructures was performed through a self-assembly manner using a hydrothermal synthesis route. MgCl2 · 6H2O (analytical pure, AR) and glycine [CH2(NH2)COOH, AR] were used without further purification. In a typical synthesis, 4 mmol of MgCl2 · 6H2O was added into 20 mL of deionized water forming solution A, while 8 mmol of glycine was dissolved into19.14 mL of deionized water, with 0.86 mL of 10 M NaOH solution added, forming solution B. Solution B was added into the stirred solution A within 30 min at room temperature. The resulting mixture was transferred to and sealed in a Teflon-lined autoclave, heated to 240 °C, and maintained at this temperature for 48 h. After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water one time, and then absolute alcohol three times, followed by drying at 60 °C for 7 h. Characterization. The phase analysis was performed with an X-ray diffractometer (XRD) (Bruker D8 advance) operating at 40 kV × 40 mA. Morphology was studied with scanning electron microscopy (SEM, KYKY 2000), field emission scanning electron microscopy (FE-SEM) (JSM-7401F), and transmission electron microscopy (TEM) (JEM-2010F). The high-resolution TEM (HRTEM) images and selected area electron diffraction (SAED) analysis were taken with a JEOL

10.1021/jp107216z  2010 American Chemical Society Published on Web 09/20/2010

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Figure 1. SEM images of products (a) MH-1, (b) MH-2, (c) MH-3, (d) MH-4, (e) MH-5, (f) MH-6, (g) MH-7, (h) MH-8, (i) MH-9, and (j) MH-10.

JEM-2010 electron microscope, operating at 200 kV. Fourier transform infrared (FT-IR) analysis of the sample coated in KBr taken from a compressed pellet was performed by using NICOLET 560 Fourier transform infrared spectrophotometer. Wetting Behavior Test. Water contact angle (CA) and sliding angle measurements were carried out on water droplet (drop volume 9 µL) and water droplet on an optical contact angle meter (Data physics Inc., OCA 20) at ambient temperature. The as-synthesized MH (5 mg) was dispersed in 5 mL of ethanol with ultrasonic treating for 10 min, followed by drying at 80 °C for 30 min. The glass was modified by the MH ethanol solution, followed by treating with 1H,1H,2H,2H-perfluorodecyltriethoxysilane methanol solution ([V(1H,1H,2H,2H-perfluorodecyltriethoxysilane):V(methanol) ) 2:98]), air drying for 1 h, and then drying at 120 °C for 1 h. The CA measurement was carried out after the preparation of samples of four days’ exposure to an atmosphere at room temperature (ca. 32 °C) as well as relative humidity of 21%. Fire Test. ABS-based nanocomposites (filled with different content of MH nanoflowers) were prepared in a two roll mixing mill (SK-160B, Shanghai Rubber Machinery Works) at 190 °C for 30 min with a speed of 18 rpm, then processed in a Platen Vulcanizing Press (QLB-350 × 350 × 2, Shanghai First Rubber Machinery Works) at 150 °C under 20 MPa for 10 min. The filler contents of were 1 and 5 wt % in terms of the as-synthesized MH nanoflowers. The Cone calorimeter tests were carried out on 100 mm ×100 mm ×3 mm compressed-molded samples placed horizontally under a heat flux of 35 kW m-2 according to ISO 5660. Mechanical Properties Test. ABS and ABS-based nanocomposites (filled with different content of MH nanoflowers) were measured on a 125 mm ×7 mm ×3 mm (length × width × height) compressed-molded sample by an electronic universal testing machine (Z004, Zwick/Roll, Zwick GmbH & Co.) with an acting load velocity of 20 mm min-1 at room temperature. Thermogravimetric Analysis (TGA). TGA experiments were performed using a TGA Q5000 V3.5 Build 252 thermal analyzer in air under air flow rate of 10 mL min-1. The samples were heated in platinum pans from room temperature up to about 900 °C at a heating rate of 10 °C min-1. 3. Results and Discussion To investigate the reaction evolution of MH nanostructures in our system, a series of contrast experiments were performed. The first study was the effect of the reaction time. Figure 1 shows SEM images of the final MH products synthesized in identical concentrations (Mg2+ concentration ) 4 mmol/40 mL, glycine concentration ) 8 mmol/40 mL) and the identical reaction temperature of 240 °C, but for different reaction times, that is, 30 min, 8 h, 24 h, and 48 h, with these being denoted as MH-1, MH-2, MH-3, and MH-4, correspondingly. Accord-

Figure 2. XRD patterns of (a) MH-1, (b) MH-2, (c) MH-4, (d) MH5, (e) MH-7, (f) MH-9, and (g) MH-10.

ing to SEM observation, the morphology of the product appeared as a platelike structure when the reaction time was 30 min (Figure 1a). When the reaction time was prolonged to 8, 24, or 48 h, flowerlike structures were obtained (Figure 1b-d). We also investigated the effect of the reaction temperature on the morphology of the products. For this series of experiments, we carried out the experiment in identical concentrations (Mg2+ concentration ) 4 mmol/40 mL, glycine concentration ) 8 mmol/40 mL) and the identical reaction time (8 h), but for different reaction temperatures, that is, 210 and 240 °C, these being denoted as MH-5 (Figure 1e) and MH-2 (Figure 1b), respectively, and similar flowerlike superstructures were obtained. The effect of concentration of Mg2+ and glycine on the morphology of the products was also investigated. After changing the concentration of Mg2+ from 4 mmol/40 mL (denoted as MH-2, Figure 1b) to 20 mmol/40 mL (denoted as MH-6, Figure 1f) with other reaction conditions remaining constant (the identical reaction time of 8 h, the identical reaction temperature of 240 °C, and identical ratio of reactions (Mg2+/ glycine) of 1:2, respectively), similar flower-like superstructures were obtained. The effect of the ratio of reaction agents (Mg2+/glycine) changing from 1:1 (Mg2+ concentration ) 8 mmol/40 mL, denoted as MH-7, Figure 1g) to 1:2 (Mg2+ concentration ) 8 mmol/40 mL, denoted as MH-8, Figure 1h), 1:4 (Mg2+ concentration ) 4 mmol/40 mL, denoted as MH-9, Figure 1i), and 1:8 (Mg2+ concentration ) 4 mmol/40 mL, denoted as MH10, Figure 1j), with the identical reaction temperature of 240 °C and the identical reaction time of 12 h, was that platelike structures (Figure 1g,h) and flowerlike superstructures (Figure 1i,j) were obtained, respectively. Figure 2 shows the XRD patterns for the as-synthesized samples. In the 2θ range 10-70°, various diffraction peaks were observed, which were assigned to (001), (011), (012), (110), (111), and (103) planes of the hexagonal phase of MH (JCPDS 83-0114). No impurity peaks were found in Figure 2, indicating that the MH crystals obtained via our method consist of a pure phase. The Fourier-transform infrared (FT-IR) spectrum recorded from the MH nanoflowers shows absorption bands at 3700,

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Figure 3. FT-IR spectrum of MH-4 nanostructure.

3460, 1640, 1410, 582, and 428 cm-1 (Figure 3). The sharp and strong peak appearing at 3700 cm-1 can be attributed to the O-H stretching vibrations in the Mg(OH)2 crystal structures.22-26 It is known that infrared technique relying on shifts and the width of the -OH stretching bands can be used to detect hydrogen bonds.27 Usually, hydrogen bonding results in decreased frequency and a broadening of the absorption band.28 A broad band between 3100 and 3600 cm-1, centering at 3460 cm-1, indicates that it belongs to the O-H stretching vibrations of adsorbed water molecules and the surface hydroxyls disturbed by the hydrogen bonds.29 The absorption band at 1640 and 1410 cm-1 can be attributed to bending vibrations of Mg-OH and -OH bonds in the crystal structure, respectively.24 The bands at 582 and 428 cm-1 are assigned to deformation vibrations of Mg-O-Mg.23,26 From the FT-IR data, we can further identify the as-synthesized sample to be MH. The morphology and size of a typical MH nanostructure were further characterized by low magnification SEM and high magnification FE-SEM, as shown in Figure 4. Interesting, lowmagnification SEM images (Figure 4a,b) show that the products consist of flowerlike complex nanostructures ca. 11.4 µm in diameter of a single superstructure (Figure 4b) assembled by nanoplates, as a second order structure, 60 nm in thickness (Figure 4d). The structure and morphology of MH were further studied by TEM (Figure 4e) and electron diffraction (ED) (Figure 4f). According to the TEM observation, the MH nanoflowers are composed of platelike structures. The ED indicates that the MH nanoflowers belong to crystalline hexagonal MH. The phase identification of the sample has been corroborated by using XRD technique. The possible growth mechanism of the complex flowerlike MH nanostructures is presented in Figure 5. It is composed of three stages: nucleation, growth, and assembly, termed as the N-G-A mechanism.30 The first stage is the nucleation process, i.e., the initial reaction between Mg2+ and glycine [CH2(NH2)COOH] ions [CH2(NH3+)COO-], which is too fast to generate the nuclei. The CH2(NH3+)COO- plays an important role in controlling nucleation and growth of MH nuclei. Glycine ions may act as a bidentate ligand to form relatively stable Mg2+ complex 1 {Mg(OH)2[CH2-(NH2)-COOH]2}.31 It can be explained that the sp3d2 hybrid orbital of Mg2+ is empty and ready to accept an electron pair. Mg2+ ion is a well-defined electron pair accepting a hard Lewis acid.16 It is believed to generate complex 1(in the solution as a precursor prior to the hydrothermal process), abiding by R.G. Pearson’s hard-soft acid base theory (HSAB theory).32 After a hydrothermal treatment process, MH nuclei are generated. The second stage is the MH nanoplate formation. The nanoplates can be regarded as 2D nanoparticles. The formation of nanoplates is determined by not only the surface area but also the surface energy.31 It is known that MH

Cao et al. is a lamellar structure with a plane composed of oxygen and magnesium ions and each Mg2+ ion is O-6-fold coordinated.23 Layered (2D) MH (Figure 5b) is typified by its anisotropic character. The atoms within the layers (ab plane) are covalently bonded, while the layers are stacked together by hydrogen bonding along the c axis via OH-, which leads to the forming of nanoplates through the oriented attachment mechanism.33 The third stage is the self-assembly process of nanoplates via hydrogen bonding (aggregation), leading to the formation of hierarchical architecture: flowerlike superstructures. All selfassembling systems are driven by some principle of energy minimization.34 Hydrogen bonds are weaker energy (20 kJ mol-1) compared with covalent bonds (about 500 kJ mol-1); however, they are favorable for self-assembling superstructures without chemical reactions. Furthermore, compared with the thermal energy (2.4 kJ mol-1), the hydrogen bonds are strong enough to hold the superstructures together.18 The hydrogen bonds between the plates of MH hold the flowerlike superstructures together, which has been demonstrated by the FT-IR data (Figure 3) and SEM and TEM observations (Figure 1 and 4). Surface wettability of the as-synthesized MH nanostructures was studied by measurement of the water contact angle (CA) using a water droplet of 9 µL. The CA value of the glass surface was 91.2° ( 1.8° after its initial coating with a methanol solution of 1H,1H,2H,2H-perfluorodecyltriethoxysilane [(C2H5O)3Si(CH2)2C8F17] [V(1H,1H,2H,2H-perfluorodecyltriethoxysilane):V(methanol) ) 2:98] (Figure 6a showing the CA of 92.9°). The CA value changed to 146.5° ( 4.9° after the glass surface was coated with as-synthesized MH nanostructures ethanol solution four times, then treated with a methanol solution of 1H,1H,2H,2H-perfluorodecyltriethoxysilane [V(1H,1H,2H,2Hperfluorodecyltriethoxysilane):V(methanol) ) 2: 98] (Figure 6b showing the CA of 146.8°). However, after the glass being treated with as-synthesized MH nanostructures ethanol solution eight times, then treated with a methanol solution of 1H,1H,2H,2Hperfluorodecyltriethoxysilane [V(1H,1H,2H,2H-perfluorodecyltriethoxysilane): V(methanol) ) 2:98], the CA value changed to 151.1 ( 1.5° (Figure 6c showing the CA of 150.6°), as well as corresponding sliding angle of 1° (Figure 6d-h and Supporting Information, Movie S1). These results imply the real possibility of introducing large-scale industrial fabrication of superhydrophobic surfaces with novel self-cleaning and anticorrosion properties.35 It has been demonstrated that the wetting phenomenon is determined by both the chemical composition and the textured topography of the surface.36 Usually, the solid surfaces with CA over 150° are defined as superhydrophobicity.37 Lotus leaf presents the lotus effect, i.e., superhydrophobicity, allowing water droplets to freely roll off the leaf into nearly perfect spheres, while allowing oil to selectively spread. The lotus effect can be attributed to the surface structure of lotus leaf, which is composed of unique micro- and nanosized second-order structures. This leads to a lotus leaf natural low-energy surface. This nature phenomenon of a plant has inspired materials scientists to biomimetically synthesize complex nanostructures for generating superhydrophobicity which can be applied in an environment protection field.31,38-41 This work indicates that the surface wettability of a solid can be changed via combing the generation of physical roughness (such as complex MH nanostructures with micro- and nanosized second-order structures) with chemical surface treatment. The cone calorimeter is one of the most effective methods for evaluating the flammability of materials.12,42 The evolution of heat release rate (HRR), in particular its maximum peak

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Figure 4. ( a) and (b) SEM images, (c) and (d) FE-SEM images, (e) TEM image, and (f) corresponding electron diffraction (ED) pattern of MH nanostructures.

Figure 5. (a) Formation of MH flowerlike superstructures, including three stages: nucleation, oriented growth, and self-assembly, termed as N-G-A mechanism. (b) Model fragment of a structural layer of MH.

(PHRR), is regarded to be an important parameter to evaluate the fire safety, and so is the time to ignition (TTI, tign), the period required for the entire surface of the sample to burn with a sustained luminous flame, which reflects the degree of difficulty in igniting a material.43 HRR, TTI, and other parameters were recorded simultaneously. The cone calorimeter experimental results for ABS (sample-ABS) and corresponding nanocomposites, i.e., ABS filled with 1 wt % MH (sample-AM-1) and 5 wt % MH (sample-AM-2), as prepared by simple melt blending are shown in Figure 7 and Table 1. The behavior of the pristine ABS, i.e., unfilled ABS, is typical of noncharring thermoplastics. The peak HRR reaches ca. 663

( 17 kW m-2, and the combustion is complete after ∼400 s. The thermal behavior of sample-AM-1 is similar to that of ABS. However, the thermal behavior of another sample, sample-AM2, is quite different. Although the MH content is only increased from 1 wt % to 5 wt % in the sample, the maximum values of HRR are changed from 652 ( 6 for sample-AM-1 to 430 ( 19 kW m-2 for sample-AM-2 and the corresponding combustion time is changed from ∼400 to ∼580 s, which indicates that the combustion time of sample-AM-2 is more extended. It is interesting that the time to ignition (TTI, tign) is also affected by the content of filled MH. The tign of ABS (sampleABS) is only 30 ( 2 s, and the tign of sample-AM-1 is 29 (

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Figure 6. Surface wetting behavior of the glass and the membranes modified by the as-synthesized MH nanostructures. Water contact angle (CA) measurements of (a) glass coated with 1H,1H,2H,2H-perfluorodecyltriethoxysilane, (b) glass treated with the Mg(OH)2 nanostructures methanol solution four time-cycle, and then 1H,1H,2H,2H-perfluorodecyltriethoxysilane molecules. (c) Glass treated with the MH nanostructures methanol solution eight time-cycle, and then 1H,1H,2H,2H-perfluorodecyltriethoxysilane molecules. (d-h) The sliding angle measurement of the membrane treated with the MH nanostructures methanol solution eight time-cycle, and then 1H,1H,2H,2H-perfluorodecyltriethoxysilane molecules.

Figure 7. Heat release rate curves for ABS resin and its nanocomposites with different Mg(OH)2 content.

TABLE 1: Cone Calorimetry Data for ABS and ABS/MH Nanocomposites at 35 kW/m2 terma

ABS 1 wt % Mg(OH)2 5 wt % Mg(OH)2 (sample-ABS) (sample-AM-1) (sample-AM-2)

30 ( 2 tign [s] tPHRR [s] 169 PHRR [kW/m2] 663 ( 17 THR [MJ/m2] 107.2 ( 5.4 ASEA [m2/kg] 1322 ( 21 AMLR [g/s] 0.078 ( 0.006

29 ( 1 190 652 ( 6 113.0 ( 3.1 1342 ( 35 0.083 ( 0.002

67 ( 5 245 430 ( 19 96.8 ( 1.5 1190 ( 5 0.054 ( 0.004

a tign ) time to ignition; PHRR ) peak heat release rate; tPHRR ) time to reach the PHRR; THR ) total heat release; ASEA ) average specific extinction area; AMLR ) average mass loss rate.

1 s, while sample-AM-2 reaches 67 ( 5 s, being almost double that of tign(ABS) and tign(S-MH-1). Another important parameter is the time taken to reach the PHRR (tPHRR). The tPHRR of sampleABS is ∼169 s, and that of sample-AM-1 is ∼190 s, while that of sample-AM-2 is ∼245 s, reaching a 44.9% increase compared with sample-ABS. The increase of tPHRR is very important because it provides crucial time for saving lives and property. The above results validate 5 wt % MH loading (sample-AM-2) endowing excellent fire retardancy on ABS. For ABS/MH nanocomposites, when the MH contents is 1 wt % (sample-AM-1), PHRR displays a 1.7% reduction and the tign and average mass loss rate (AMLR) exhibit little change compared to pure ABS copolymer. However, when 5 wt % MH loading was added into ABS copolymer (sample-AM-2), compared with ABS resin, the peak heat release rate (PHRR) and average specific extinction area (AMLR) present ∼35% and ∼31% reduction, respectively, while the total heat release (THR) presents ∼10% reduction. These results indicate that sampleAM-2 nanocomposite does not burn out. Both AMLR and THR

data of sample-AM-2 are reduced significantly, suggesting the network structures of MH formed at 5 wt % MH loading. The improved fire resistance displayed by the nanocomposite sampleAM-2 could be explained by the chemical structures of the nanofillers and the combustion products, i.e., by the stabilizing π-π electronic interactions between the unsaturated structures of the carbonaceous amorphous char. It is believed that the redox reaction, taking place during the burning of MH flame-retarded ABS, may play a part in the smoke suppression of MH.34,44,45 MgO generated via the decomposition of Mg(OH)2 owns the catalytic activity which can catalyze the redox reaction C + O f CO2 as well as CO + O f CO2, via promoting C and CO decomposed from polymer ABS to CO2. This behavior leads to the decrease of the concentration of toxic fumes. The excellent flame retardant effects of the as-synthesized MH can be attributed to the complex micro/nanosuperstructures. The complex MH superstructure is composed of nanoplates as a second structure with large surface area, which favors the catalytic reaction. By the way, the MgO, being an excellent refractory material, can cover the surface of burning ABS and form barrier, which functions as thermal insulation, isolating oxide and preventing molten drops.43 The TGA and differential TG (DTG) curves for sample-ABS and sample-AM-2 are shown in Figure 8. The detailed TGA and DTG data for pure ABS and its nanocomposite are summarized in Table 2. The ABS undergoes two-step thermal decomposition (Figure 8a), where end-chain and random scission occurs. There is a remarkable mass loss in the 300-500 °C temperature range, attributed to the main chain pyrolysis.46,47 The first step is its main decomposition process, beginning at ∼378 °C with a maximum rate of weight loss at ∼427 °C, leading to about ∼89% weight loss and the formation of ∼11 wt % charred residue. It is attributed to the main chain pyrolysis.46 The second decomposition step is observed between 500 and 650 °C, starting at 529 °C, accompanied by the maximum rate of weight loss at 546 °C. It can be attributed to the degradation of the carbonaceous residue formed during the first step. ABS polymer gives no char residue upon combustion. However, sample-AM-2 undergoes its first decomposition step appearing in the temperature range 250-450 °C (Figure 8b). The first step is its main decomposition process, beginning at ∼369 °C with a maximum rate of weight loss at ∼421 °C, which can be attributed to the decomposition of MH. This is partly caused by the degradation of Mg(OH)2 at lower temperature. It is known that the thermal decomposition temperature

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Figure 8. TGA and DTG curves: (a) sample-ABS resin and (b) its nanocomposite sample-AM-2.

TABLE 2: TGA Results for the Thermal Degradation of the Pure ABS Resin (Sample-ABS) and Its Nanocomposite (Sample-AM-2) first step sample

ABS loading (wt %)

Mg(OH)2 loading (wt %)

char at 900 °C (wt %)

Tbegin (°C)

sample-ABS sample-AM-2

100 95

0 5

0.34 4

300 250

a

second step

Tend (°C)

weight loss (wt %)

Tbegin (°C)

Tend (°C)

relativea loss (wt %)

500 450

88.89 84.13

500 450

650 550

97.4 75.9

Relative loss ) (weight loss in the second step)/residue at 500 °C × 100%.

TABLE 3: Mechanical Properties of ABS and ABS/MH Nanocomposites

sample

flexural strength (Mpa)

breaking load (N)

fracture thickness (mm)

fracture width (mm)

elastic modulus (Gpa)

sample-ABS sample-AM-1 sample-AM-2

59.670 60.251 57.524

22.783 23.005 21.964

3 3 3

7 7 7

2.68 2.63 2.69

of pure Mg(OH)2 crystal begins at around 340 °C, which is due to the transition of phase Mg(OH)2 f MgO.48 Although the onset of weight loss of sample-AM-2 occurs at a lower temperature than that of pure ABS (sample-ABS), sample-AM-2 gives a larger residue from the first step of degradation. The endothermic decomposition of MH, accompanied by the release of water, provided an effective heat sink mechanism in comparison with ABS resin. Also, MH acted as an inorganic filler, and the MgO layer functions as a barrier. The second decomposition step is observed between 450 and 550 °C, starting at 458 °C, accompanied by the maximum rate of weight loss at 486 °C. In the second step, the relative loss of sample-AM-2 is 75.9%, far less than that of pure ABS, 97.4%. Thus, the formation of the intercalated structure will also influence the second degradation step of nanocomposite sample-AM-2 in the char layer, and enhance the stability of the ABS matrix. Table 3 represents the mechanical properties of ABS nanocomposites at various as-synthesized MH nanostructure loading. The flexural strength of ABS nanocomposites, compared with that of sample-ABS, is slightly increased at 1 wt % MH loading (sample-AM-1), while it is slightly decreased at 5 wt % MH loading (sample-AM-2). Also, the trend of the elastic modulus was similar to that of flexural strength for sample-ABS, and its nanocomposites, i.e., sample-AM-1 and sample-AM-2. On the basis of the results from flexural strength and elastic modulus,

it is not difficult to draw a conclusion that the fire retardant MH nanostructures filled within ABS matrix do not decrease the mechanical properties of ABS. Conclusions In summary, Mg(OH)2 complex nanostructures were synthesized by a hydrothermal route, which had all the characteristics needed to be a successful flame retardant filler in ABS without destroying the mechanical properties of ABS nanocomposites. This kind of flame retardant is expected to be used in the market in the future. It has been first demonstrated that the flowerlike Mg(OH)2 nanostructures own superhydropobicity, which can find practical applications in environmental protection. This work demonstrates that the biomolecule-assisted synthesis route indeed provides a helpful hand to nanotechnology. Obviously, the superhydrophobic MH nanostructures may find applications in protecting cables in winter by avoiding ice formation on the surface of the cable. Acknowledgment. The authors gratefully thank to the financial support from the National Natural Science Foundation of China (No. 20921001 and 20535020), the Innovation Method Fund of China (No. 20081885189), and the National High Technology Research Development Program of China (No. 2009AA03Z321). Supporting Information Available: Movie of the sliding angle measurement of as-synthesized Mg(OH)2 complex nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Braun, U.; Schartel, B. Macromol. Chem. Phys. 2004, 205, 2185. (2) Lv, J.; Qiu, L.; Qu, B. Nanotechnology 2004, 15, 1576.

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