ZnAl-LDH nanocomposites prepared by melt and solution intercalation

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Materials Chemistry and Physics 109 (2008) 206–211

Structural characterization and related properties of EVA/ZnAl-LDH nanocomposites prepared by melt and solution intercalation Ming Zhang, Peng Ding, Longchao Du, Baojun Qu ∗ State Key Laboratory of Fire Science and Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Received 16 April 2007; received in revised form 5 November 2007; accepted 13 November 2007

Abstract The crystal morphological structures, thermal and mechanical properties of ethylene-vinyl acetate copolymer/layered double hydroxide (EVA/LDH) nanocomposties prepared by melt and solution intercalation methods have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and mechanical measurements. The XRD data show that the exfoliated EVA/LDH nanocomposites can be obtained by controlling the LDH loading of about 10% for the melt intercalation and 5% for the solution intercalation. The TEM images verify that the exfoliated nanocomposites are of the nanoscale dispersion of LDH layers in the EVA matrix. The thickness and size of single LDH platelet are 1 nm and 100–150 nm. The TGA data give evidence that the thermal degradation temperatures of the EVA/LDH nanocomposite samples are 13–35 ◦ C higher than that of pure EVA resin when 50% weight loss was selected as a point of comparison. The data from the mechanical test show that the tensile strength values of the EVA/ZnAl-LDH nanocomposite samples are 2.5–4.8 MPa higher than that of pure EVA resin, and their tensile modulus increases continuously with the increase of LDH loadings, while the elongation at break is almost the same as that of the pure EVA resin. © 2007 Elsevier B.V. All rights reserved. Keywords: Layered double hydroxide; Ethylene-vinyl acetate copolymer; Exfoliated structures; Thermal property; Mechanical property; Melt intercalation; Solution intercalation

1. Introduction In recent years, polymer/layered double hydroxide (LDH) nanocomposites have been attracted considerable interest due to their novel mechanical, thermal, optical, and physic-chemical properties, which are rarely present in the pure polymer or micro-scale compostites [1–3]. LDHs are host–guest materials consisting of positively charged metal hydroxide sheets with intercalated anions and water molecules [4]. Several methods have been developed in our laboratory for preparing polymer/LDH nanocomposites by various intercalation methods, such as melt intercalation, solution intercalation, in situ polymerization and so on [5–9]. For example, the solution intercalation of maleric anhydride grafted polyethylene (PE-g-MA) into MgAlLDH has been reported, in which the MgAl-LDH was exfoliated in PE-g-MA matrix when the 5 wt% contents of LDH was con-

∗

Corresponding author. Tel.: +86 551 3607245; fax: +86 551 3607245. E-mail address: [email protected] (B. Qu).

0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.013

trolled [5]. The exfoliated LLDPE/LDH nanocomposites with 20 wt% modified MgAl-DS can be obtained by melt intercalation [6]. Ethylene-vinyl acetate copolymer (EVA) is widely applied for cable, wrapper, adhesive, drug, and so on [10–13], due to unique properties such as good adhesion, flexibility, toughness and biocompatibility. With increasing the content of vinyl acetate (VA), EVA of plastic properties becomes rubber [14–16]. However, some properties of EVA, such as thermal stability, tensile strength, and so on, need to be improved for further applications. Polymer/LDH nanocomposites can obtain superior thermal and mechanical properties than pure polymer itself [17–19]. Moreover, EVA with the VA polar group has good compatibility with inorganic clay. As far as we are aware, however, there are few studies on the EVA/LDH nanocomposites. In the present work, the morphological structures, thermal and mechanical properties of the EVA/LDH nanocomposites by melt and solution intercalation methods have been studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and mechanical measure-

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ments. The main purpose of this work is to enhance the thermal stability and mechanical properties of EVA materials for the applications of special wire and cable. 2. Experimental 2.1. Materials EVA copolymer containing 18 wt% vinyl acetate was purchased from SAMSUNG General Chemicals (Korea). Al(NO3 )3 ·9H2 O and Zn(NO3 )2 ·6H2 O (analytical pure) were supplied by Shanghai Zhenxing Chemicals No. 1 Plant. Sodium dodecyl sulfate (SDS), NaOH (analytical pure), and xylene were obtained from China Medicine (Group) Shanghai Chemical Reagent Corporation (China). All these commercial chemicals were used as received without further purification.

2.2. Preparation of samples ZnAl-LDH was synthesized by a spontaneous self-assembly method according to the literature [19] The pH value of 30 mL aqueous solution containing Zn(NO3 )2 (0.03 mol), Al(NO3 )3 (0.01 mol), and C12 H25 SO4 Na (3.0 g) was adjusted to about 10 with 1 mol L−1 NaOH aqueous solution. The obtained slurry was aged for 3 days at 80 ◦ C, then filtered, and washed by distilled water. ZnAl-LDH powder was obtained by drying at 60 ◦ C in an oven. EVA/ZnAl-LDH nanocomposites samples, respectively, were obtained by melt intercalation of mixing the EVA resin with the desired amount of ZnAlLDH powders in a SXX-300 mixer with a rotor speed of 64 rpm at 150 ◦ C and by solution intercalation of desired amount LDH in 50 mL xylene solutions of EVA for 24 h with stirring under a N2 atmosphere. Then the mixture obtained by solution intercalation was poured into 50 mL ethanol. The precipitate was filtered, washed by ethanol, and dried for 12 h under vacuum at 120 ◦ C. The samples prepared by melt intercalation or solution intercalation are labeled as EVA-x or sEVA-x, respectively, where s means the solution intercalation and x is the contents of LDH.

2.3. Characterization The X-ray diffraction (XRD) was performed using a Rigaku D/Max-rA rotating anode X-ray diffractometer equipped with a Cu K␣ tube and Ni filter (λ = 0.1542 nm). The transmission electron microscopy (TEM) images were obtained on a Hitachi H-800 transmission electron microanalyzer with an accelerated voltage of 200 kV and a camera length of 0.8 m. The sample was ultramicrotomed with a diamond knife on an LKB Pyramitome to give about 100 nm thick slices. The slices were transferred from water to a 200-mesh Cu grid. The thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50H thermoanalyzer. In each case an 18 mg sample was examined under a nitrogen flow rate of 6 × 10−5 m3 min−1 at a scan rate of 10 ◦ C min−1 from room temperature to 700 ◦ C. The mechanical properties were measured with an Instron Universal tester (model 1185) at 25 ◦ C with a crosshead speed of 25 mm min−1 . The dumb-bell shaped specimens were prepared according to ASTM D412-87. Five samples were analyzed to determine the average values in order to obtain reproducible results. The actual deviations of error analysis for the mechanical data, such as tensile strength, elongation at break, and the tensile modulus at 100% elongation, were within ±5%.

3. Results and discussion 3.1. Crystal morphological structures of EVA/ZnAl-LDH nanocomposites Fig. 1 gives the wide angle XRD patterns of sEVA-2, sEVA-5, EVA-2, and EVA-10 samples with different loading of ZnAlLDH. Apparently these nanocomposite samples prepared by solution and melt intercalation methods show similar diffraction peaks of ZnAl-LDH layers marked by the asterisks (*) in

Fig. 1. Wide angle XRD patterns of various samples: (A) sEVA-2, (B) sEVA-5, (C) EVA-2, (D) EVA-10.

Fig. 1, which indicates that the ZnAl-LDH layered framework is preserved in the EVA/LDH nanocomposites. Fig. 2A and B show the low angle XRD patterns in the range of 2θ ≤ 10◦ for various EVA/ZnAl-LDH samples prepared by solution and melt intercalation methods, respectively. The basal spacing of ZnAlLDH as a control sample is determined as being 2.85 nm from the (0 0 3) diffraction peak at 2θ = ∼3.10◦ (Fig. 2A(a)). It can be seen from the EVA/ZnAl-LDH samples that the (0 0 3) diffraction peaks disappear in sEVA-2, sEVA-5, EVA-2, EVA-5, and EVA-10 samples, which indicates that the ZnAl-LDH layers have been exfoliated in these samples. When the ZnAl-LDH loadings further increase to 10 wt% for solution intercalation and 20 wt% for melt intercalation, the sEVA-10 and EVA-20 samples show intercalation or partly exfoliated structures because of the relatively weak (0 0 3) diffraction peaks appeared again at the same positions as the ZnAl-LDH control samples. These results show that the melt intercalation favors the exfoliation of ZnAlLDH layers compared with the solution intercalation. This can be due to the strong shear force interaction during the melt mixing to promote the EVA chains intercalated into the ZnAl-LDH platelets, which makes the ZnAl-LDH to be exfoliated easily to be dispersed in the EVA matrix. When the EVA/ZnAl-LDH nanocomposites were cooled from the melt state, the exfoliated structural dispersion was kept in the EVA matrix as reported on the PE/MgAl-LDH nanocomposites prepared by melt intercalation [20]. Fig. 3 shows the typical TEM images of exfoliated EVA/ZnAl-LDH nanocomposite samples with 5 wt% ZnAlLDH prepared by solution and melt intercalation methods. It can be seen that the exfoliated ZnAl-LDH sheets are dispersed disorderly in the EVA matrix. The positions pointed by the arrows in Fig. 3A and B show some ZnAl-LDH layers nearly vertical to the cutting section of the TEM specimen. However, the homogenous dispersion state in the sample prepared by solution intercalation (Fig. 3B) seems better than that of the sample prepared by melt intercalation (Fig. 3A). The ZnAl-LDH sheets

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evidence of nanoscaled dispersion of ZnAl-LDH layers in the EVA matrix. The similar observation has been reported in our previous work [21]. 3.2. Thermal properties of EVA/ZnAl-LDH nanocomposites Fig. 4 illustrates the thermal degradation behaviors of pure EVA and various EVA/ZnAl-LDH samples prepared by melt intercalation and solution intercalation, respectively. It can be seen that all nanocomposite samples show enhanced thermal stabilities compared with pure EVA. The thermal degradation process of pure EVA resin occurs in two steps. The first degradation step at the range of 335–400 ◦ C can be assigned to the evolution of acetic acid, whereas the second degradation step at the range of 425–550 ◦ C is due to the degradation of ethylenebased chains in EVA [22]. There is about 25 wt% weight loss at the range of 200–400 ◦ C for the EVA/ZnAl-LDH samples, which can be attributed to the decomposition of ZnAl-LDH and the thermo-oxidation degradation of EVA [23]. It can be seen from Table 1 that the thermal degradation temperatures of the EVA-2, EVA-5 and EVA-10 prepared by melt intercalation are 448, 448, and 461 ◦ C when 50% weight loss is used as a point of comparison. At the same time, the thermal degradation temperatures of the sEVA-2, sEVA-5 and sEVA-10 samples prepared by solution intercalation are 451, 464, and 470 ◦ C, respectively, when 50% weight loss is used as a point of comparison. These data indicate that the thermal degradation temperatures increase with increasing the LDH loadings. This is probably because the more loading ZnAl-LDH layers favor the obstruct effect and formation of char in the polymer, which increases the thermal stability of the nanocomposites. It can be found that the thermal degradation temperatures of exfoliated samples via solution intercalation are higher 3–16 ◦ C than those of the corresponding exfoliated samples via melt intercalation. This is probably because the more LDH prepared by solution intercalation can homogenously disperse in the EVA matrix than that prepared by melt intercalation, as shown in Fig. 3A and B.

Fig. 2. Low angle XRD patterns in the range of 2θ < 10◦ for various samples prepared by (A) solution intercalation: (a) ZnAl-LDH, (b) sEVA-10, (c) sEVA5, (d) sEVA-2; (B) melt intercalation: (a) ZnAl-LDH, (b) EVA-20, (c) EVA-10, (d) EVA-5, (e) EVA-2.

in the EVA-5 sample show somewhat aggregating, as seen in Fig. 3A. Fig. 3C shows the TEM image of the single platelet of ZnAl-LDH in the sEVA-5 sample. There is no change of the ZnAl-LDH structure in the exfoliated processes. The thickness and the lateral size of singlet exfoliated ZnAl-LDH layer are about 1 nm and 100–150 nm, respectively. This gives a positive

3.3. Mechanical properties of EVA/ZnAl-LDH nanocomposites The data of tensile strength (TS), elongation at break (EB), and tensile modulus at 100% elongation (M100 ) with the ZnAl-

Table 1 Compositions, preparation method, morphological structures, and corresponding thermal degradation temperature of various samples Sample code EVA

Loading of ZnAl-LDH (wt%) 0

Morphological structuresa

Thermal degradation temperatureb (◦ C)

–

435

EVA-2 EVA-5 EVA-10 EVA-20

2 5 10 20

Exfoliated Exfoliated Exfoliated Partly exfoliated

448 448 461 –

sEVA-2 sEVA-5 sEVA-10

2 5 10

Exfoliated Exfoliated Intercalation

451 464 470

a b

Analyzed by XRD patterns. Analyzed by TGA and 50 wt% weight loss as a comparison point of temperature.

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Fig. 3. TEM images of the nanocomposite samples: (A) EVA-5; (B) sEVA-5; (C) single LDH platelet of sEVA-5.

LDH loadings in various EVA/ZnAl-LDH samples prepared by melt intercalation and solution intercalation are listed in Table 2. The data demonstrate that the samples with the 2–5 wt% ZnAlLDH loadings via melt intercalation show enhanced tensile strength (about 21.5 MPa) compared with pure EVA (18.7 MPa). But when the ZnAl-LDH loadings further increase to 20 wt%, the tensile strength values drop down very quickly to 13.7 MPa. This could be caused by the excessive ZnAl-LDH loadings, which leads ZnAl-LDH layers to be aggregated in the sample.

It can be seen that the EVA/ZnAl-LDH nanocomposite samples with the 2–10 wt% ZnAl-LDH loadings via solution intercalation have apparently enhanced tensile strength (21.2–23.5 MPa) compared with the pure EVA no matter whether the ZnAl-LDH layers in these samples are exfoliated or not. The elongation at break of samples via melt intercalation or solution intercalation basically keeps constant within 10 wt% loading of ZnAl-LDH and is approximately close to the 1450% value of pure EVA. It can been found that the tensile modulus M100 values

Table 2 Mechanical properties of pure EVA and various EVA/ZnAl LDH samples with different contents of ZnAl-LDH Sample code

Elongation at break (%)

Tensile strength (MPa)

Tensile moduls at 100% elongation (MPa)

EVA

1453 ± 29

18.7 ± 0.4

42.8 ± 1.3

EVA-2 EVA-5 EVA-10 EVA-20

1450 1470 1400 1183

sEVA-2 sEVA-5 sEVA-10

1475 ± 30 1455 ± 29 1365 ± 27

± ± ± ±

29 29 28 24

21.4 21.5 18.5 13.7

± ± ± ±

0.4 0.4 0.4 0.3

23.5 ± 0.5 21.2 ± 0.4 21.2 ± 0.4

45.6 56.9 66.7 72.1

± ± ± ±

1.4 1.7 2.0 2.2

47.5 ± 1.4 54.3 ± 1.6 68.1 ± 2.0

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of ZnAl-LDH, their loadings and preparation methods, while the elongation at break basically keeps constant within 10 wt% loading of ZnAl-LDH no matter whether the preparation method is solution and melt intercalation or not. References

Fig. 4. TGA profiles of pure EVA and various nanocomposite samples prepared by (A) melt intercalation; (B) solution intercalation.

(45.6–72.1 MPa) of samples via melt intercalation or solution intercalation increase with increasing the LDH loadings, much higher than 42.8 MPa of pure EVA. The enhanced mechanical properties are mainly due to the compatible interaction of the ZnAl-LDH layers with EVA resin. 4. Conclusion The EVA/ZnAl-LDH nanocomposites have been prepared by melt and solution intercalation of EVA chains into organic modified ZnAl-LDH interlayers. The exfoliated EVA/LDH nanocomposites can be obtained by controlling the ZnAlLDH loading. The melt intercalation benefits the exfoliation of ZnAl-LDH, whereas the exfoliated structures of EVA/LDH nanocomposites via solution intercalation are more homogeneous dispersion than that via melt intercalation. The thermal degradation temperatures of nanocomposite samples are 13–35 ◦ C higher than that of pure EVA. The samples prepared by solution intercalation have higher degradation temperatures than those of the samples with the same ZnAl-LDH loadings via melt intercalation. The data from the mechanical test show that the enhanced tensile strength and the tensile modulus of the nanocomposite samples mainly depends on the dispersion state

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