2D Lamellar Aluminophosphate Nanolayers for Enhancing Flame

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2D lamellar aluminophosphate nanolayers for enhancing flame retardancy and mechanical properties of polymers Saihua Jiang, Keqing Zhou, Yongqian Shi, Ningning Hong, Siuming Lo, Yuan Hu, and Zhou Gui Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 Nov 2013 Downloaded from http://pubs.acs.org on November 6, 2013

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Industrial & Engineering Chemistry Research

2D lamellar aluminophosphate nanolayers for enhancing flame retardancy and mechanical properties of polymers

Saihua Jianga, b, Keqing zhoua, Yongqian shia, b, Ningning Honga, Siuming Lob, Yuan Hua, b*, Zhou Guia*

a

State Key Laboratory of Fire Science, University of Science and Technology of China, Jinzhai Road 96,

Hefei, Anhui, 230027, P. R. China b

Department of Civil and Architectural Engineering, City University of Hong Kong and USTC-CityU

Joint Advanced Research Centre, Suzhou, P.R. China

∗

Corresponding author, Tel.: +86-551-3601664; fax: +86-551-3601664. [email protected] (Y. Hu);

Tel.: +86-551-3601288; fax: +86-551-3601669. [email protected] (Z. Gui).

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Abstract 2D lamellar aluminophosphate (LAP) is successfully exfoliated into nanolayers in polyurethane (PU) through the combination of organically modification and solution casting method. Morphological studies by XRD and TEM show that the exfoliated LAP nanolayers are dispersed well in the PU matrix. The introduction of small amounts of LAP nanolayers (≤5.0 wt%) results in obvious enhancements in the thermal properties and mechanical performance of PU/LAP nanocomposites. These enhancements are benefiting from the good dispersion and exfoliated morphology of stiff LAP nanolayers, and strong interfacial interaction between LAP nanolayers and the matrix. LOI and MCC results indicate that the LAP nanolayers incorporated also improve the flame retardancy of nanocomposites. Detailed flame-retardant mechanism is proposed. Physical barrier effect of LAP nanolayers and the graphitized char formation catalyzed by LAP play key roles in the flame retardancy enhancement.

Keywords: 2D aluminophosphate nanolayers; Polymer nanocomposite; Mechanical property; Flame retardancy

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1. Introduction Since first reported in 1982, aluminophosphate molecular sieves AlPO4-n (n denotes the structure type), have attracted the attentions from scientists and industry owning to their potential applications in the areas of catalysis, adsorption and host-guest assembly chemistry.1-4 Various aluminophosphates have been synthesized with three dimensional (3D) open-frameworks, 2D layer, and 1D chain structures over the past decades, among which the members of 2D lamellar aluminophosphate (LAP) show diverse stoichiometries, sheet structures, and sheet stacking sequences. Typically, LAPs consist of macroanionic sheets built up from alternation of Al-centered polyhedra (AlO4, AlO5, AlO6) and P-centered P(Ob)n(Ot)4-n tetrahedra (b, bridging; t, terminal; n=1-4). The protonated organic amine molecules or complex cations, acting as the structure-directing agents, reside in the interlayer region.5 Benefiting from the inherent lamellar structure, LAPs show many attractive characteristics including large surface area and aspect ratio, catalytic activity, high elastic modulus, thermal and chemical stability, and the potential to delaminate within polymers.6-9 However, currently, the study and application of AlPO4-n are still restricted to 3D bulk samples. To date, in addition to the use in gas separation membrane,6, 10 few studies have reported the new application of LAPs with respect to the bulk counterpart. Thus, broadening the application of LAPs is a promising avenue of research. As is well known, 2D nanolayers are potential fillers for polymeric nanocomposites owing to their outstanding physicochemical property and unique structure.11-12 The introduction of small amounts of 2D nanolayers could offer remarkable improvements in properties of polymers including thermal property, flame retardancy, mechanical performance, etc.13-18 For example, free-standing graphene or MoS2 (≤5 wt%) incorporated into polymers could retard the thermal degradation, decrease the heat release rate during polymer combustion, and enhance the tensile strength or storage modulus significantly. Physical 3



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barrier effect of layers which slows down the diffusion of gases and degradation products, and the interface interaction between layers and polymeric matrix, are main reasons for the improvements.13, 15-17 Similar to graphene and MoS2 nanosheets, LAP nanolayers are expected to improve the flame retardancy and mechanical properties of polymers. Moreover, AlPO4-n is a common solid acid catalyst, which could catalyze dehydrogenation of polymers and promote the charring of polymers.19-22 In our earlier work, we have found that LAPs could catalyze the formation of graphitized char during polymethyl methacrylate (PMMA) pyrolysis.22 The catalytic carbonization function is a favorable factor for the flame retardant system,14, 23-25 which further makes the LAP a promising candidate for flame-retardant applications in polymeric nanocomposites. In general, the properties of nanocomposites depend on their morphologies which can be classified as: immiscible, intercalated and exfoliated. The intercalation or exfoliation of 2D nanomaterials are crucial for the property enhancements of nanocomposites.13-18 However, in contrast to inorganic graphene analogues (IGAs) such as graphite oxide, MoS2 and hexagonal boron nitride (HBN), with a weak van der Walls force between the layers, the LAPs possess relatively strong interaction between the adjacent aluminophosphate nanolayers. 26-27 Thus, it is difficult for LAPs to be exfoliated or intercalated using simple mixing. Organic modification before adding into polymers is a good method for the dispersion and exfoliation of 2D nanomaterials in polymer matrixes.14, 28-29 This step is considered to be important for two reasons: (1) to make the material compatible with polymers and (2) to give room for the polymer chain to penetrate into the interlayer spacing of the layers. For example, Marand et al. have modified LAP using a cationic modified agent, cetyltrimethylammonium chloride (CTMAC). The as-synthesized CTMAC modified LAP (CTMAC-LAP) was then added into polyimide matrix.10 Due to the organic modification, CTMAC-LAP was successfully intercalated by polyimide chains. Nevertheless, strong

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ionic bonds and hydrogen bonds between CTMAC cations and the anionic LAP nanolayers still limit the penetration of polymer chains into the interlayer, and hence none exfoliated LAP have been observed in the nanocomposites.10 Recently, we have used a long-chain neutral modifier, dodecanamine (DDA), to modify the LAP, obtaining an organically modified LAP, DDA-LAP. The modification weakened the interaction between adjacent LAP nanolayers and increased the interlayer distances, making the penetration of PMMA chains into the LAP interlayer more easily, resulting in the partial exfoliation of LAP.22 However, there is still a long way to achieve full exfoliation of LAP in polymer nanocomposites. Solution blending and casting method is an effective way for the preparation of polymer nanocomposites.30-32 Due to the intrinsic van der Waals interactions, many 2D nanomaterials usually easily re-agglomerate such as graphene and MoS2,16, 30 which makes dispersion and exfoliation difficult. Therefore, it is required to disperse 2D nanomaterials in a compatible solvent before it is incorporated into the polymer matrix. DDA-LAP can be well dispersed in N, N-Dimethylformamide (DMF), which is beneficial for the dispersion of DDA-LAP in polymers by solution casting method. PU is a typical polar polymer, which can be dissolved in DMF. Neat PU is flammable and forms little residue during combustion. Herein, LAP was previously modified by DDA using a one-step method. The DDA-LAP obtained was then incorporated into PU matrix via solution casting method using DMF as THF solvent. Morphology studies show that the employed method provides the uniform dispersion of exfoliated LAP nanolayers in the PU matrix. The PU/DDA-LAP nanocomposites obtained exhibit significantly enhanced mechanical properties, glass transition temperature and flame retardancy. From structure and mechanism investigation, exfoliated morphology, good dispersion of DDA-LAP nanolayers and the strong interface interaction between the DDA-LAP nanolayers and PU matrix play key roles in the enhancements of thermal property and mechanical performance. Graphitized char formed by the

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catalysis of DDA-LAP as well as the physical barrier effect is responsible for the enhanced flame retardancy. Our work not only offers a convenient method for the exfoliation of LAPs, but also demonstrates that the 2D LAP nanolayers could improve the mechanical property and flame retardancy of polymers. 2. Experimental 2.1. Materials Tetramethylammonium hydroxide (TMAH) was supplied by Jiande Xinde Chemical Co., Ltd. DDA, DMF, ethanol, orthophosphoric acid (H3PO4), and aluminium chloride hexahydrate (AlCl3·6H2O), were of analytical grade and offered by Sinopharm Chemical Reagent Co., Ltd. Polyurethane 4190 (PU4190, polyol-polyester type, density of 1.12 g/cm3) was purchased from Zhejiang Tiantian Plastic Co. Ltd (Lishui, Zhejiang, China). 2.2. Preparation of DDA modified LAP (DDA-LAP) The DDA-LAP is synthesized as described in our previous paper.22 2.3. Preparation of PU/DDA-LAP nanocomposites The preparation of PU/DDA-LAP nanocomposites is illustrated in Scheme 1. First, PU (5g) was dissolved in DMF (100 mL) with refluxing and vigorous stirring under N2 atmosphere. Once the mixture became transparent, different amounts of DDA-LAP suspension (0.01 g/mL in DMF) was dripped into the above mixture. After refluxing for additional 48 h, the mixture was cast onto glass plates and dried at 80 °C for 72 h to form flat membranes. Finally, the membranes were further heated at 140 ºC for 2 days to remove residual DMF, affording PU/DDA-LAP nanocomposites. The composition of all samples is presented in Table 1.

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2.4. Characterization X-Ray Diffraction (XRD) was conducted on a Japan Rigaku D/Max-γA rotation anode X-ray diffractometer, employing a 2θ range from 1° to 10° at the scanning rate of 0.01°/s. The basal spacing of DDA-LAP nanolayers is calculated using the Bragg equation: 2dsinθ = nλ, where θ is half of the diffraction angle, d represents the distance between crystal planes, λ is the wavelength of X-ray and n is an integer. Transmission electron microscope (TEM, JEM-2100F, Japan Electron Optics Laboratory CO., LTD, Japan) was used to study the morphologies of PU/DDA-LAP nanocomposites. The nanocomposite ultrathin sections were firstly sliced by an Ultratome (Model MT-6000, Du Pont Company, USA), then transferred from water to carbon-coated copper grids and finally observed. Dynamic Mechanical Analysis (DMA) was conducted on a DMA Q800. The test frequency is 10 Hz, and the linear heating rate is 5 °C/min. The samples (20 × 4 × 3 mm3) were tested under a sinusoidal strain. Differential Scanning Calorimetry (DSC) was conducted on a DSC Q2000 instrument (TA Ltd, USA) at a heating rate of 10 °C/min under nitrogen atmosphere. Tg values were determined by the mid-point of the inflexion curve of the typical second heating. Thermal Gravimetric Analysis (TGA) was performed on a Q5000 thermo-analyzer instrument (TA Instruments, USA) from 30 to 700 °C under an air flow of 60 ml/min, employing a linear heating rate of 10 °C/min. Limiting Oxygen Index (LOI) values were measured by LOI Analyzer instrument, according to the ASTM standard D2863. Microscale combustion calorimeter (MCC) test was performed by using on a microscale combustion

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calorimeter (Govmak MCC-2). The test principle has been described in our earlier work.16 Scanning electron microscopy (SEM) of char residues was performed on a scanning electron Microscope (Hitachi X650). Laser Raman spectroscopy (LRS) measurements were recorded from 500 to 2000 cm-1 on a LabRAM-HR Confocal Raman Microprobe (Jobin Yvon Instruments, France) using a 514.5 nm argon ion laser. 2. Results and discussion The morphology of DDA-LAP was investigated by TEM (Figure 1a) and XRD (Figure 1b). From Figure 1a, the DDA-LAP exhibits a typical 2D layered structure and has a size of 0.3-1.0 µm. Figure 1b illustrates the XRD pattern for DDA-LAP in the range of 2θ= 1~10°. Three peaks in the low-angle region correspond to (001), (002) and (003) diffractions of the layered structure. The high intensity of the (001) peak suggests the highly ordered organization of DDA-LAP nanolayers. Large basal spacing (around 30.8 Å), calculated from the (001) diffraction peak at 2θ= 2.86° using Bragg equation, is due to the presence of long-chain DDA molecules in the interlayer. The intercalation of DDA not only enlarges the gallery between the nanolayers, but also improves the interfacial properties between the LAP nanolayers and PU matrix because the DDA is more compatible with PU chains than the pristine LAP. Small angle XRD patterns of PU nanocomposites with various DDA-LAP contents are also exhibited in Figure 1b. In the patterns of PU/1.0% DDA-LAP and PU/2.0% DDA-LAP, the characteristic diffraction peaks of DDA-LAP completely disappear, indicating that the long range ordering in the inorganic phase is completely destroyed and DDA-LAP nanolayers have been completely exfoliated in the PU matrix. In the case of PU/5.0% DDA-LAP, a very weak and broad (001) peak appears around 2θ=2.03°. Inorganic nanolayers at higher loading tend to agglomerate in polymer matrix due to the strong intermolecular

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interaction, which would retard the penetration of polymer chains into the interlayer 29. However, the left shift of (001) peak location, corresponding to the increase of basal spacing, suggests polymer chains have penetrated into the interlayer and further expanded the interlayer. Moreover, compared to pristine DDA-LAP, the (001) peak intensity is reduced sharply in PU/5.0% DDA-LAP, which means that the oriented distribution of DDA-LAP nanolayers has been destroyed by the polymer chains. Thus, most DDA-LAP is exfoliated in PU/5.0% DDA-LAP. To further investigate the morphology of DDA-LAP in PU matrix, the nanocomposites were characterized by TEM. Figure 2 illustrates the representative TEM images of PU/2.0% DDA-LAP (Figure 2a) and PU/5.0% DDA-LAP (Figure 2b) at different magnifications. For PU/2.0% DDA-LAP, at low magnification, the DDA-LAP are homogeneously distributed across the whole polymer; at high magnification, the LAP nanolayers are full exfoliated and individual LAP nanolayers are distributed well in the matrix. When higher level of DDA-LAP is introduced, the images for PU/5.0% DDA-LAP (Figure 2b) exhibit a well dispersed morphology at low magnification but partially exfoliated LAP nanolayers at high magnification. From Figure 2b, some intercalated LAP nanolayers with an increased basal spacing of around 4.4 nm can be easily found at high manificaiton, which is due to the increasing intermolecular interaction between LAP nanolayers at higher loading.29 These results are consisted with the XRD analysis. Both XRD and TEM results demonstrate that LAP nanolayers have been successfully exfoliated in the PU matrix. Furthermore, Figure 2c presents the optical photos of pure PU (A) and PU/DDA-LAP nanocomposites (B-D). It is evident that pure PU is highly transparent. Compared with pure PU, the transparency of nanocomposites decreases as DDA-LAP content rises, but all of them are still transparent. That further confirms the good dispersion of LAP nanolayers in the PU matrix. Both good dispersion and exfoliated morphology of LAP nanolayers contribute to the performance enhancements.

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Storage modulus (E’) of the samples vs temperature obtained from DMA tests are shown in Figure 3a and Table 1. With the incorporation of DDA-LAP (≤ 5.0 wt%), the storage moduli of PU/DDA-LAP nanocomposite are enhanced significantly compared to that of pure PU. For example, only 1.0 wt% DDA-LAP incorporated increases the storage modulus by 17.2%. The introduction of 5.0 wt% DDA-LAP leads to a 31.0% maximum increase in the storage modulus (3611 MPa) at -80 °C. It appears that three factors influence the mechanical properties of PU/DDA-LAP nanocomposites: (a) the mechanical strength of LAP nanolayers, which is the root for the mechanical property enhancements;6, 22 (b) the strong interaction between the LAP nanolayers and the PU matrix in nanocomposites such as hydrogen bonds and electrostatic interaction, which restrict segmental motions at the interface; 14-15, 22, 26-27 (c) the morphology and dispersion of LAP nanolayers in the PU matrix. For PU/DDA-LAP nanocomposites, good dispersion and exfoliated morphology of LAP nanolayers would result in the increase of interfacial area between nanolayers and polymer matrix. The increased interfacial area leads to increased interactions. Thus, continuous improvement in storage modulus of nanocomposite is obtained as DDA-LAP content rises. In addition, it is evident that the increment of storage modulus decreases with increasing DDA-LAP content, which is attributed to the increase of exfoliation difficulty for LAP nanolayers, as demonstrated by the XRD analysis. Given the characterizations above, good dispersion and exfoliated morphology of LAP nanolayers are the key factors in the further enhancement of mechanical properties. Polymer segmental mobility is one of the most important factors affecting viscoelastic behavior and gas diffusion in polymers and their composites. The effect of LAP nanolayers on the segmental motions expressed as Tg of PU in nanocomposites was evaluated. From Figure 3b, the peaks of the tan δ curves correspond to the Tg of polymers. The Tg of all PU/DDA-LAP nanocomposites are higher than that of pure PU. To further confirm this point, Tg values of all samples were also tested by DSC, as displayed in

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Figure 4. The Tg values calculated by DSC and DMA are listed in Table 1. The relationship between Tg and the DDA-LAP content in nanocomposites obtained from DSC measurement is in agreement with the DMA results. As DDA-LAP content rises, the Tg value of the nanocomposite increases. The increase of Tg is due to the great restriction of PU chain motions, which suggests the good interfacial interaction between the LAP nanolayers and the PU chains. Moreover, from Table 1, it is evident that the increment of Tg is decreased with the increase of DDA-LAP content. This tendency of Tg, similar to that of the storage modulus, can also be attributed to the difficulty increase of exfoliation and dispersion along with the increase of DDA-LAP content. The above results further confirm that good dispersion of exfoliated LAP nanolayers combining with the strong interaction between LAP nanolayers and polymer matrix, play key roles in the enhancements of mechanical performance and glass transition temperatures. Figure 5 shows the TGA curves of DDA-LAP, PU and the nanocomposites in air, and the corresponding data are given in Table 2. The temperatures at which 10% (T0.1), 50% (T0.5) mass loss occurs are used as the measure of initial degradation temperature and half degradation temperature, respectively. For DDA-LAP, the main loss starting from 250 °C is attributed to the loss of DDA molecules, with the aluminophosphate residue yield of 47.4 wt%.8, 22 From Figure 5 and Table 1, the nanocomposites are less thermally stable than PU when evaluated by T0.1. The increase of DDA-LAP leads to the decrease of T0.1, which is due to the early degradation of polymers catalyzed by LAP. LAP is a common solid acid catalyst, which catalyzes the char formation through promoting the dehydrogenation of polymers at relative low temperature, finally improving the thermal stability of residues at high temperature.19-22 This point can be confirmed by the better thermal stability of nanocomposites than that of PU at high temperature. For instance, the T0.5 increase gradually from 373.4 °C to 380.9 °C as DDA-LAP content rises. The temperature at the 90% mass loss rate (T0.9) are in order of PU (500.0 °C)

2D Lamellar Aluminophosphate Nanolayers for Enhancing Flame

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