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LDH nanocomposites via nanoplatelet-like LDHs modified with N-Lauroyl-glutamate
Composites Science and Technology 81 (2013) 37–41
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
A new method to prepare exfoliated UV-cured polymer/LDH nanocomposites via nanoplatelet-like LDHs modified with N-Lauroyl-glutamate BingZhi Guo a,b, Yun Zhao a,b, QiangTao Huang b, QingZe Jiao a,b,⇑ a b
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100083, PR China Beijing Institute of Technology, Zhuhai, Zhuhai 519085, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 18 February 2013 Accepted 22 February 2013 Available online 4 March 2013 Keywords: A. Nanocomposites A. Polymers B. Thermal properties Layered double hydroxides
a b s t r a c t A novel approach to prepare the exfoliated polymer/layered double hydroxide (LDH) nanocomposites is reported. The key features of this method are synthesis of nanoplatelet-like organic LDH modified with N-Lauroyl-glutamate (LDH-LG) and blending LDH-LG with polyester acrylate followed by UV curing. The LDH-LG was prepared by adjusting the proportion of the oil phase in the O/W type microemulsion. From X-ray diffraction analysis and high resolution transmission electron microscope observation, the nanoscale organic LDH particles were homogeneously dispersed in the polymer matrix. The addition of LDH-LG into polyester acrylate improved the wear resistance of the nanocomposite film significantly. The coefficients of static friction, dynamic friction and gloss of nanocomposite with 2 wt% of LDH-LG decreased to 0.114°, 0.062° and 87.5° compared with 0.856°, 0.758° and 94.0° of the pure polymer, respectively. The thermal properties of the polymer/LDH nanocomposites were also discussed. Ó 2013 Published by Elsevier Ltd.
1. Introduction Polymer/inorganic nanocomposites, where in the inorganic components are dispersed in nanometer scale and thus can maximize their functions, have attracted much of current interests [1,2]. Moreover, on account of the unique properties such as enhanced mechanical and thermal properties, reduced flammability and high chemical stability, polymer/layered crystal nanocomposites are recognized as one of the most important organic/inorganic hybrid materials [3,4]. Many studies have been focused on exfoliated polymer/montmorillonite nanocomposites. However there are few reports of exfoliated polymer/LDH nanocomposites due to delamination difficulties of LDHs. The strong electrostatic interaction between highly charged hydroxide layers and the intercalated anions hinders the exfoliation of LDH layers [5,6]. LDHs, important layered inorganic materials, are generally xþ n represented by the formula ½MII1 x MIII x ðOHÞ2 ðA Þx=n mH2 O, whereII III in M , M , and A represent divalent (or monovalent) cation, trivalent cation and the interlayer anion, respectively. The interlayer anion (An) may vary over a wide range and has an exchangeable capacity [7]. For preparing exfoliated polymer/LDH nanocomposites the modification of LDH with an organic surfactant containing both alkyl anion and reactive group is frequently applied. Hsueh and Chen ⇑ Corresponding author at: School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100083, PR China. Tel.: +86 13801049336; fax: +86 07563622606. E-mail address: [email protected] (Q. Jiao). 0266-3538/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.compscitech.2013.02.022
[8] reported the preparation of epoxy/LDH nanocomposites by thermal polymerization using the LDH intercalated by the synthesized amino laurate. Yuan and coworkers [9] prepared exfoliated UV-cured polymer/LDH nanocomposites through intercalation of a photoinitiator into LDH interlayer. They also reported fabrication of exfoliated UV-cured polymer/LDH nanocomposites via preexfoliated organic LDH [10]. In order to synthesize a pre-exfoliated hybrid LDH, the LDH was intercalated by sodium dodecyl sulfate and a silane coupling agent was then grafted onto the interlayer of LDH to supply epoxy group. Finally trimethylolpropane thioglycolic acetate as a trithiol terminal A3 monomer was induced to the epoxidized LDH, obtaining a thiol-endcapped LDH (LDH-SH) hybrid with the pre-exfoliated microstructure [10]. In this study, we propose a novel approach to prepare the exfoliated polymer/LDH nanocomposites. Firstly, LDH nanoplatelets modified with N-Lauroyl-glutamate (LDH-LG) was prepared by a microemulsion method. Then, the LDH-LG was blended with polyester acrylate. Finally, a completely exfoliated polymer/LDH nanocomposite was obtained by coating and UV curing of the blend. The microstructure, thermal and mechanical properties of UV-cured nanocomposites were examined in details. 2. Experimental 2.1. Materials N-Lauroyl-glutamate sodium (LG-Na), YIFN Guangzhou Chemical Co., Ltd. (Guangzhou, China). Mg(NO3)26H2O, Al(NO3)39H2O,
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B. Guo et al. / Composites Science and Technology 81 (2013) 37–41
sodium carbonate, sodium hydroxide and acetone were all purchased from Tianjin Damao Chemical Reagent (Tianjin, China). N-octane, Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Polyester acrylate, Jiangmen HengGuang New Material Co., Ltd. (Jiangmen, China). Tripropylene glycol diacrylate (TPGDA) and trimethylol propane triacrylate (TMPTA), were all from Taiwan Eternal Chemical Co., Ltd. (Zhuhai, China). Photoinitiator agent is Benzophenone (BP), which was obtained from Runtec Chemical Co., Ltd. (Changzhou, China). All chemicals were used as received without further purification. 2.2. Preparation of nanoplatelet-like organic LDH (LDH-LG) The LDH-LG is achieved in a microemulsion composed of octane/LG-Na/water [11]: A mixture of 50.0 g octane and 4.5 g LG-Na was added into 58.5 mL of de-ionized water, and simultaneously maintained pH value at 10.0 by drop-wise addition of 2 mol L1 NaOH solution. An aqueous solution containing magnesium nitrate (Mg(NO3)2) and aluminum nitrate (Al(NO3)3) with Mg2+/Al3+ molar ratio of 2:1 (3.84 g(0.015 mol)Mg(NO3)26H2O + 2.81 g(0.0075 mol)Al(NO3)39H2O + 60 mL H2O) was added drop-wise into the microemulsion. To maintain the pH value of 10.0 of the system, an aqueous solution of 2 mol L1 of NaOH was added simultaneously. The obtained slurry was stirred and kept at 85 °C for 24 h. The precipitates were filtered and washed with de-ionized water repeatedly to eliminate the unreacted LG-Na, further washed three times with acetone. Most of the wet product was added to acetone to form the LDH-LG acetone suspension. A part of the wet product was dried under vacuum overnight and ground into white powders. The magnesium/aluminum layered double hydroxides (MgAlLDH) with molar ratio of Mg2+/Al3+ = 2 were synthesized by the nucleation crystallization separation method [12]. 2.3. Preparation of UV-cured polymer/LDH nanocomposites The typical preparation procedure of UV-cured polymer/LDH nanocomposites was as follows: 20.0 g polyester acrylate, 10.0 g TMPTA and 10.0 g TPGDA were mixed homogeneously, and the LDH-LG was added into the resin with the respective concentration of 0.5, 1 and 2 wt%. A desired amount of LDH-LG acetone suspension was first dispersed in the resin. After stirring vigorously at the speed of 3000 rad/min for 4 h and sonicating for 1 h to achieve the complete dispersion, acetone was removed under vacuum.
Then, 2.0 g of photoinitiator (BP) was added, and stirred again at room temperature for 2 h. The final resin was applied to glass substrates to get films, and then exposed on UV curing machine for a certain time (1kw, XH-101-200, Shenzhen, China) with conveyer speed of 2.0 m min1. 2.4. Characterization Fourier transform infrared (FTIR) spectra were performed on a Bruker VERTEX-70 spectrometer with a disc of KBr. The X-ray diffraction patterns were recorded on a Shimadzu XRD-6000 X-ray powder diffractometer (Cu Ka radiation, k = 0.15406 nm), the scan ranges of specimens were collected from 2h = 0.5–2° and 2–70°. The morphology of LDH-LG in polymer matrix was obtained using JEOL JEM-2100 transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The samples were prepared with an ultramicrotome equipped with a diamond knife to give 60-nm thick slices. The thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50H thermoanalyzer. Shimadzu DSC-60 differential scanning calorimeter (DSC) was used to test the thermal performance of polymer/LDH nanocomposites. All the samples were examined under nitrogen atmosphere at a heating rate of 10 °C/ min. Film wear resistance was tested by 10001 wear instrument. The coefficient of film was tested using a MXD-01 coefficient of friction instrument. The 60° gloss of film was measured on a GZ-1 intelligent gloss instrument. 3. Results and discussion 3.1. Characterization of the LDH-LG The structure of LDH-LG synthesized using the microemulsion method was identified by XRD pattern shown in Fig. 1. It can be seen that (0 0 3) characteristic diffraction peak of LDH materials is not observed in the range of 2–70° and 0.5–2°, which is usually at 2h = 11.7°. The nanoplatelet-like LDHs with a limited number of layers (1–5) would exhibit a weak and significantly broadened (0 0 3) reflection based on Ref. [13]. The resulting LDH-LG with no (0 0 3) reflections shows that the LDH-LG is possibly having a monolayer thickness. Also we are sure that the LDH-LG is not exfoliated nanosheets, which are obtained via anion exchange and delamination of LDH nanoplatelets, thereby showing XRD patterns centered at 2h of about 25° [14–17]. In addition to the (0 0 3) peak, there is a broad reflection at 2h of around 20°, which is absent in
Fig. 1. XRD patterns of LDH-LG: (a) wide-angle X-ray diffraction and (b) small-angle X-ray diffraction.
B. Guo et al. / Composites Science and Technology 81 (2013) 37–41
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Fig. 4. HR-TEM micrographs of UV-cured polymer/LDH nanocomposite at 2 wt% LDH-LG loading. Fig. 2. FTIR spectrum of MgAl-LDH and LDH-LG.
Fig. 5. TG curves of polymer/LDH nanocomposites with various LDH-LG contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, and (d) 2 wt%. Fig. 3. TG profiles of MgAl-LDH and LDH-LG.
carbonate-intercalated LDHs. This peak is attributed to the scattering of the X-rays by the carbon chain of LG [18]. Fig. 2 shows the FTIR spectra of MgAl-LDH and LDH-LG. In the spectrum of MgAl-LDH, the broad and strong absorption band at 3500 cm1 can be attributed to the stretching vibration of O–H groups. A strong absorption band at 1360 cm1 is due to the presence of carbonate. The spectrum of LDH-LG displays the characteristic absorption bands of –CH2 or –CH3 in the aliphatic chains of LG at 2850, 2920 cm1. Strong absorption peaks at 1580 cm1 is assigned to the deformation vibration of RCONH– in LG, 1645 and 1408 cm1 peaks are caused by stretching vibrations of C–O in LG molecule carboxyl, demonstrating the successful modification of LDH by LG. Fig. 3 reveals the TGA curves of MgAl-LDH and LDH-LG at a heating rate of 10 °C/min. Two distinguishable weight loss of the MgAl-LDH occurred in the range of 100–230 °C and 300–580 °C. In the first one, a weight loss of 14.5% is attributed to the evaporation of the physically adsorbed and intercalated water [19]. The second step of weight loss about 27.5% can be ascribed to the dehydroxylation of the hydroxide layers and elimination of intercalated carbonate anion [20]. The TGA data of LDH-LG exhibits similar tendency, and the temperature of weight loss is almost at the same range as that of MgAl-LDH. In the first step, the weight loss of 14.5% from room temperature to 230 °C can be attributed to the loss of water. The main weigh loss stage occurred at the temperature of 300–580 °C is due to the thermal decomposition of LG chain
Fig. 6. DSC heating curves of polymer/LDH nanocomposites with various LDH-LG contents:(a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, and (d) 2 wt%.
on the LDH layer and dehydroxylation of the LDH sheet. The remaining weight percent of the inorganic compositions of the MgAl-LDH and LDH-LG are 54.3% and 37.8%, respectively. The total weight loss for the same region of LDH-LG is 16.5% higher than that for MgAl-LDH. These results are in agreement with the FTIR and XRD measurements.
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Table 1 Properties of pure polymer and polymer/LDH nanocomposites. Sample
Wear number (times)
Width of the scratches (cm)
Coefficient of static friction
Coefficient of dynamic friction
Gloss (°)
Pure polymer 0.5 wt% LDH-LG 1 wt% LDH-LG 2 wt% LDH-LG
2000 2500 2500 2500
0.65 0.48 0.33 0.23
0.856 0.369 0.195 0.114
0.758 0.243 0.077 0.062
94.0 94.3 89.5 87.5
3.2. Morphology and dispersion of LDH-LG in nanocomposites TEM instrument can be directly employed to visualize the exact intercalation or exfoliation degree of filler in the polymer matrix. Fig. 4 shows the HR-TEM micrograph of polymer/LDH nanocomposite with 2 wt% of LDH-LG. The dark lines present LDH platelets. The HR-TEM observation indicates the formation of completely nanoplatelet-like LDHs. Therefore the preparation of exfoliated polymer/LDH nanocomposites has been successful. 3.3. Thermal properties of the UV-cured nanocomposites In order to better understand the effect of LDH-LG on the thermal stability of polymer matrix, TGA analysis was operated to investigate the thermal degradation of pure polymer and polymer/LDH nanocomposites. Typical thermogravimetric profiles of weight loss for neat polymer and polymer/LDH nanocomposites at a heating rate of 10 °C/min are illustrated in Fig. 5. The onset temperatures of degradation for pure polymer, and polymer/LDH nanocomposites with 0.5, 1, and 2 wt% of LDH-LG loading are determined to be 190 °C, 173 °C, 179 °C and 180 °C, respectively. These results indicate that the incorporation of organicallymodified LDH did not always improve the thermal stability of the nanocomposites as those reported in the literatures [8–10]. From these experimental data, it can be seen that the presence of LDH-LG in UV-curable polyester acrylate induced the worse thermal stability. Similar phenomenon was also reported for synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites [21]. The decrease of thermal degradation temperature might be caused by alkaline LDH sheets in UV-curable polyester acrylate matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the UV-curable polyester acrylate resulting in their worse degradation stability [22]. Another possible reason is that the nano-LDH-LG layers would play a role of hindering the growth of three-dimensional network structure of the cured film in the curing process to a certain extent. Therefore smaller molecular weight of cured membrane leads to the reduction of thermal stability. Fig. 6 shows the DSC traces of UV-cured pure polymer and UVcured polymer/LDH nanocomposites in the range of 30–450 °C during the melting process. As can be seen from Fig. 6, a shoulder peak at 400 °C weakens gradually with increasing the amount of LDH-LG. It has completely disappeared when the content of LDHLG is up to 2 wt%. The interaction between the molecular chain of the polymer matrix and the LDH layer partially eliminates the stress generated during the crystallization process, thereby hinders the formation of different crystalline isomer. Another possible reason is that the nano-scale LDH-LG layers plays a role of impeding the movement of the polymer chains, which slows the growth rate of crystallization of the polymer and reduce the crystals formed with different molecular weight. 3.4. Properties of UV-cured polymer/LDH nanocomposite film The wear resistance, coefficient of static friction, dynamic friction and gloss of nanocomposite films are listed in Table 1.
As shown in Table 1, the wear resistance of the film is improved significantly by introduction of LDH-LG. The coefficients of static friction, dynamic friction and gloss of nanocomposite with 2 wt% of LDH-LG decrease to 0.114°, 0.062° and 87.5° compared with 0.856°, 0.758° and 94.0° of the pure polymer, respectively. The LDH-LG layers which are dispersed in the polymer matrix uniformly, play a role of physical crosslinking point to the polymer network, improve the strength of polymer film, and also reduce the flexibility. Therefore, the wear resistance of the polymer film is significantly improved with the increase of the LDH-LG content. The uniformly dispersed LDH-LG layers have better compatibility with the matrix. As a hard phase, LDH-LG layers make the matrix to be strengthened. Meanwhile, nano-scale layers can make the film surface smooth, which contributes to a lower coefficient of friction. Nano-scale layers can also effectively prevent the adhesion transfer of the matrix [23], which is conducive to the formation of fine particulate wear debris, and inhibit the generation of large particles of debris. Fine particulate wear debris generated in the friction surface play the role of lubrication, thereby the friction coefficient of the system is reduced.
4. Conclusion A novel approach to synthesize the exfoliated polymer/LDH hybrid nanocomposites was proposed via nanoplatelet-like organic LDHs. Three polymer/LDH nanocomposites were prepared by loading different amounts of LDH-LG into an UV-curable polyester acrylate matrix. The HR-TEM observation confirmed the formation of exfoliated microstructure of the UV-cured nanocomposites. The film performance test indicated that the addition of LDH-LG into polyester acrylate could improve the wear resistance of the film significantly and cut down the friction coefficient of the film. The thermal stability was decreased compared with the pure polymer. Besides, the organic LDH nanoplatelets can be used to other polymers for preparing exfoliated polymer/LDH nanocomposites.
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B. Guo et al. / Composites Science and Technology 81 (2013) 37–41 [10] Yuan Y, Zhang Y, Zhang Y, Shi WF. A novel approach for preparing exfoliated UV-cured polymer/LDH nanocomposites via pre-exfoliated organic LDH. Appl Clay Sci 2011;53:608–14. [11] Xi H, He J, Evans DG, Duan X. Delamination of layered double hydroxides in microemulsion. Chin J Inorg Chem 2004;20:1217–22. [12] Duan X, Jiao QZ, Li L. A new method of synthesis monodispersed ultrafine layered materials. 2001. [CN 1288078]. [13] Hu G, Wang N, O’Hare D, Davis J. Synthesis of magnesium aluminium layered double hydroxides in reverse microemulsions. J Mater Chem 2007;17:2257–66. [14] Li L, Ma RZ, Ebina Y, Iyi N, Sasaki T. Positively charged nanosheets derived via total delamination of layered double hydroxides. Chem Mater 2005;17:4386–91. [15] Liu ZP, Ma RZ, Osada M, Iyi N, Ebina Y, Takada K, et al. Synthesis, anion exchange, and delamination of Co–Al layered double hydroxide: assembly of the exfoliated nanosheet/polyanion composite films and magneto-optical studies. J Am Chem Soc 2006;128:4872–80. [16] Shao MF, Xu XY, Han JB, Zhao JW, Shi WY, Kong XG, et al. Magnetic-fieldassisted assembly of layered double hydroxide/metal porphyrin ultrathin films and their application for glucose sensors. Langmuir 2011;27:8233–40.
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[17] Zhao ZP, Qi YL, Wei M, Zhang FZ, Xu SL. Layer-by-layer assembly and morphological characterizations of DNA/layered double hydroxide thin films. Mater Lett 2012;78:62–5. [18] Gunawan P, Xu R. Synthesis of unusual coral-like layered double hydroxide microspheres in a nonaqueous polar solvent/surfactant system. J Mater Chem 2008;18:2112–20. [19] Zhao Y, Li F, Zhang R, Evans DG, Duan X. Preparation of layered doublehydroxide nanomaterials with a uniform crystallite size using a new method involving separate nucleation and aging steps. Chem Mater 2002;14:4286–91. [20] Lin JJ, Juang TY. Intercalation of layered double hydroxides by poly(oxyalkylene)-amidocarboxylates: tailoring layered basal spacing. Polymer 2004;45:7887–93. [21] Lim ST, Hyun YH, Choi HJ, Jhon MS. Synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites. Chem Mater 2002;14:1839–44. [22] Cam D, Marucci M. Influence of residual monomers and metals on poly (Llactide) thermal stability. Polymer 1997;38:1879–84. [23] Li YJ, Hu XL, Shen BX, Yang XH, Chen DM, Dong YM. The properties of modified nano-SiO2/acrylics composite coating. J Xiamen Univ (Nat Sci) 2008;47:696–700.
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
A new method to prepare exfoliated UV-cured polymer/LDH nanocomposites via nanoplatelet-like LDHs modified with N-Lauroyl-glutamate BingZhi Guo a,b, Yun Zhao a,b, QiangTao Huang b, QingZe Jiao a,b,⇑ a b
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100083, PR China Beijing Institute of Technology, Zhuhai, Zhuhai 519085, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 18 February 2013 Accepted 22 February 2013 Available online 4 March 2013 Keywords: A. Nanocomposites A. Polymers B. Thermal properties Layered double hydroxides
a b s t r a c t A novel approach to prepare the exfoliated polymer/layered double hydroxide (LDH) nanocomposites is reported. The key features of this method are synthesis of nanoplatelet-like organic LDH modified with N-Lauroyl-glutamate (LDH-LG) and blending LDH-LG with polyester acrylate followed by UV curing. The LDH-LG was prepared by adjusting the proportion of the oil phase in the O/W type microemulsion. From X-ray diffraction analysis and high resolution transmission electron microscope observation, the nanoscale organic LDH particles were homogeneously dispersed in the polymer matrix. The addition of LDH-LG into polyester acrylate improved the wear resistance of the nanocomposite film significantly. The coefficients of static friction, dynamic friction and gloss of nanocomposite with 2 wt% of LDH-LG decreased to 0.114°, 0.062° and 87.5° compared with 0.856°, 0.758° and 94.0° of the pure polymer, respectively. The thermal properties of the polymer/LDH nanocomposites were also discussed. Ó 2013 Published by Elsevier Ltd.
1. Introduction Polymer/inorganic nanocomposites, where in the inorganic components are dispersed in nanometer scale and thus can maximize their functions, have attracted much of current interests [1,2]. Moreover, on account of the unique properties such as enhanced mechanical and thermal properties, reduced flammability and high chemical stability, polymer/layered crystal nanocomposites are recognized as one of the most important organic/inorganic hybrid materials [3,4]. Many studies have been focused on exfoliated polymer/montmorillonite nanocomposites. However there are few reports of exfoliated polymer/LDH nanocomposites due to delamination difficulties of LDHs. The strong electrostatic interaction between highly charged hydroxide layers and the intercalated anions hinders the exfoliation of LDH layers [5,6]. LDHs, important layered inorganic materials, are generally xþ n represented by the formula ½MII1 x MIII x ðOHÞ2 ðA Þx=n mH2 O, whereII III in M , M , and A represent divalent (or monovalent) cation, trivalent cation and the interlayer anion, respectively. The interlayer anion (An) may vary over a wide range and has an exchangeable capacity [7]. For preparing exfoliated polymer/LDH nanocomposites the modification of LDH with an organic surfactant containing both alkyl anion and reactive group is frequently applied. Hsueh and Chen ⇑ Corresponding author at: School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100083, PR China. Tel.: +86 13801049336; fax: +86 07563622606. E-mail address: [email protected] (Q. Jiao). 0266-3538/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.compscitech.2013.02.022
[8] reported the preparation of epoxy/LDH nanocomposites by thermal polymerization using the LDH intercalated by the synthesized amino laurate. Yuan and coworkers [9] prepared exfoliated UV-cured polymer/LDH nanocomposites through intercalation of a photoinitiator into LDH interlayer. They also reported fabrication of exfoliated UV-cured polymer/LDH nanocomposites via preexfoliated organic LDH [10]. In order to synthesize a pre-exfoliated hybrid LDH, the LDH was intercalated by sodium dodecyl sulfate and a silane coupling agent was then grafted onto the interlayer of LDH to supply epoxy group. Finally trimethylolpropane thioglycolic acetate as a trithiol terminal A3 monomer was induced to the epoxidized LDH, obtaining a thiol-endcapped LDH (LDH-SH) hybrid with the pre-exfoliated microstructure [10]. In this study, we propose a novel approach to prepare the exfoliated polymer/LDH nanocomposites. Firstly, LDH nanoplatelets modified with N-Lauroyl-glutamate (LDH-LG) was prepared by a microemulsion method. Then, the LDH-LG was blended with polyester acrylate. Finally, a completely exfoliated polymer/LDH nanocomposite was obtained by coating and UV curing of the blend. The microstructure, thermal and mechanical properties of UV-cured nanocomposites were examined in details. 2. Experimental 2.1. Materials N-Lauroyl-glutamate sodium (LG-Na), YIFN Guangzhou Chemical Co., Ltd. (Guangzhou, China). Mg(NO3)26H2O, Al(NO3)39H2O,
38
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sodium carbonate, sodium hydroxide and acetone were all purchased from Tianjin Damao Chemical Reagent (Tianjin, China). N-octane, Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Polyester acrylate, Jiangmen HengGuang New Material Co., Ltd. (Jiangmen, China). Tripropylene glycol diacrylate (TPGDA) and trimethylol propane triacrylate (TMPTA), were all from Taiwan Eternal Chemical Co., Ltd. (Zhuhai, China). Photoinitiator agent is Benzophenone (BP), which was obtained from Runtec Chemical Co., Ltd. (Changzhou, China). All chemicals were used as received without further purification. 2.2. Preparation of nanoplatelet-like organic LDH (LDH-LG) The LDH-LG is achieved in a microemulsion composed of octane/LG-Na/water [11]: A mixture of 50.0 g octane and 4.5 g LG-Na was added into 58.5 mL of de-ionized water, and simultaneously maintained pH value at 10.0 by drop-wise addition of 2 mol L1 NaOH solution. An aqueous solution containing magnesium nitrate (Mg(NO3)2) and aluminum nitrate (Al(NO3)3) with Mg2+/Al3+ molar ratio of 2:1 (3.84 g(0.015 mol)Mg(NO3)26H2O + 2.81 g(0.0075 mol)Al(NO3)39H2O + 60 mL H2O) was added drop-wise into the microemulsion. To maintain the pH value of 10.0 of the system, an aqueous solution of 2 mol L1 of NaOH was added simultaneously. The obtained slurry was stirred and kept at 85 °C for 24 h. The precipitates were filtered and washed with de-ionized water repeatedly to eliminate the unreacted LG-Na, further washed three times with acetone. Most of the wet product was added to acetone to form the LDH-LG acetone suspension. A part of the wet product was dried under vacuum overnight and ground into white powders. The magnesium/aluminum layered double hydroxides (MgAlLDH) with molar ratio of Mg2+/Al3+ = 2 were synthesized by the nucleation crystallization separation method [12]. 2.3. Preparation of UV-cured polymer/LDH nanocomposites The typical preparation procedure of UV-cured polymer/LDH nanocomposites was as follows: 20.0 g polyester acrylate, 10.0 g TMPTA and 10.0 g TPGDA were mixed homogeneously, and the LDH-LG was added into the resin with the respective concentration of 0.5, 1 and 2 wt%. A desired amount of LDH-LG acetone suspension was first dispersed in the resin. After stirring vigorously at the speed of 3000 rad/min for 4 h and sonicating for 1 h to achieve the complete dispersion, acetone was removed under vacuum.
Then, 2.0 g of photoinitiator (BP) was added, and stirred again at room temperature for 2 h. The final resin was applied to glass substrates to get films, and then exposed on UV curing machine for a certain time (1kw, XH-101-200, Shenzhen, China) with conveyer speed of 2.0 m min1. 2.4. Characterization Fourier transform infrared (FTIR) spectra were performed on a Bruker VERTEX-70 spectrometer with a disc of KBr. The X-ray diffraction patterns were recorded on a Shimadzu XRD-6000 X-ray powder diffractometer (Cu Ka radiation, k = 0.15406 nm), the scan ranges of specimens were collected from 2h = 0.5–2° and 2–70°. The morphology of LDH-LG in polymer matrix was obtained using JEOL JEM-2100 transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. The samples were prepared with an ultramicrotome equipped with a diamond knife to give 60-nm thick slices. The thermogravimetric analysis (TGA) was performed on a Shimadzu TGA-50H thermoanalyzer. Shimadzu DSC-60 differential scanning calorimeter (DSC) was used to test the thermal performance of polymer/LDH nanocomposites. All the samples were examined under nitrogen atmosphere at a heating rate of 10 °C/ min. Film wear resistance was tested by 10001 wear instrument. The coefficient of film was tested using a MXD-01 coefficient of friction instrument. The 60° gloss of film was measured on a GZ-1 intelligent gloss instrument. 3. Results and discussion 3.1. Characterization of the LDH-LG The structure of LDH-LG synthesized using the microemulsion method was identified by XRD pattern shown in Fig. 1. It can be seen that (0 0 3) characteristic diffraction peak of LDH materials is not observed in the range of 2–70° and 0.5–2°, which is usually at 2h = 11.7°. The nanoplatelet-like LDHs with a limited number of layers (1–5) would exhibit a weak and significantly broadened (0 0 3) reflection based on Ref. [13]. The resulting LDH-LG with no (0 0 3) reflections shows that the LDH-LG is possibly having a monolayer thickness. Also we are sure that the LDH-LG is not exfoliated nanosheets, which are obtained via anion exchange and delamination of LDH nanoplatelets, thereby showing XRD patterns centered at 2h of about 25° [14–17]. In addition to the (0 0 3) peak, there is a broad reflection at 2h of around 20°, which is absent in
Fig. 1. XRD patterns of LDH-LG: (a) wide-angle X-ray diffraction and (b) small-angle X-ray diffraction.
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Fig. 4. HR-TEM micrographs of UV-cured polymer/LDH nanocomposite at 2 wt% LDH-LG loading. Fig. 2. FTIR spectrum of MgAl-LDH and LDH-LG.
Fig. 5. TG curves of polymer/LDH nanocomposites with various LDH-LG contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, and (d) 2 wt%. Fig. 3. TG profiles of MgAl-LDH and LDH-LG.
carbonate-intercalated LDHs. This peak is attributed to the scattering of the X-rays by the carbon chain of LG [18]. Fig. 2 shows the FTIR spectra of MgAl-LDH and LDH-LG. In the spectrum of MgAl-LDH, the broad and strong absorption band at 3500 cm1 can be attributed to the stretching vibration of O–H groups. A strong absorption band at 1360 cm1 is due to the presence of carbonate. The spectrum of LDH-LG displays the characteristic absorption bands of –CH2 or –CH3 in the aliphatic chains of LG at 2850, 2920 cm1. Strong absorption peaks at 1580 cm1 is assigned to the deformation vibration of RCONH– in LG, 1645 and 1408 cm1 peaks are caused by stretching vibrations of C–O in LG molecule carboxyl, demonstrating the successful modification of LDH by LG. Fig. 3 reveals the TGA curves of MgAl-LDH and LDH-LG at a heating rate of 10 °C/min. Two distinguishable weight loss of the MgAl-LDH occurred in the range of 100–230 °C and 300–580 °C. In the first one, a weight loss of 14.5% is attributed to the evaporation of the physically adsorbed and intercalated water [19]. The second step of weight loss about 27.5% can be ascribed to the dehydroxylation of the hydroxide layers and elimination of intercalated carbonate anion [20]. The TGA data of LDH-LG exhibits similar tendency, and the temperature of weight loss is almost at the same range as that of MgAl-LDH. In the first step, the weight loss of 14.5% from room temperature to 230 °C can be attributed to the loss of water. The main weigh loss stage occurred at the temperature of 300–580 °C is due to the thermal decomposition of LG chain
Fig. 6. DSC heating curves of polymer/LDH nanocomposites with various LDH-LG contents:(a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, and (d) 2 wt%.
on the LDH layer and dehydroxylation of the LDH sheet. The remaining weight percent of the inorganic compositions of the MgAl-LDH and LDH-LG are 54.3% and 37.8%, respectively. The total weight loss for the same region of LDH-LG is 16.5% higher than that for MgAl-LDH. These results are in agreement with the FTIR and XRD measurements.
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Table 1 Properties of pure polymer and polymer/LDH nanocomposites. Sample
Wear number (times)
Width of the scratches (cm)
Coefficient of static friction
Coefficient of dynamic friction
Gloss (°)
Pure polymer 0.5 wt% LDH-LG 1 wt% LDH-LG 2 wt% LDH-LG
2000 2500 2500 2500
0.65 0.48 0.33 0.23
0.856 0.369 0.195 0.114
0.758 0.243 0.077 0.062
94.0 94.3 89.5 87.5
3.2. Morphology and dispersion of LDH-LG in nanocomposites TEM instrument can be directly employed to visualize the exact intercalation or exfoliation degree of filler in the polymer matrix. Fig. 4 shows the HR-TEM micrograph of polymer/LDH nanocomposite with 2 wt% of LDH-LG. The dark lines present LDH platelets. The HR-TEM observation indicates the formation of completely nanoplatelet-like LDHs. Therefore the preparation of exfoliated polymer/LDH nanocomposites has been successful. 3.3. Thermal properties of the UV-cured nanocomposites In order to better understand the effect of LDH-LG on the thermal stability of polymer matrix, TGA analysis was operated to investigate the thermal degradation of pure polymer and polymer/LDH nanocomposites. Typical thermogravimetric profiles of weight loss for neat polymer and polymer/LDH nanocomposites at a heating rate of 10 °C/min are illustrated in Fig. 5. The onset temperatures of degradation for pure polymer, and polymer/LDH nanocomposites with 0.5, 1, and 2 wt% of LDH-LG loading are determined to be 190 °C, 173 °C, 179 °C and 180 °C, respectively. These results indicate that the incorporation of organicallymodified LDH did not always improve the thermal stability of the nanocomposites as those reported in the literatures [8–10]. From these experimental data, it can be seen that the presence of LDH-LG in UV-curable polyester acrylate induced the worse thermal stability. Similar phenomenon was also reported for synthetic biodegradable aliphatic polyester/montmorillonite nanocomposites [21]. The decrease of thermal degradation temperature might be caused by alkaline LDH sheets in UV-curable polyester acrylate matrix that can catalyze the depolymerization and/or inter- and intra-molecular transesterification reactions of the UV-curable polyester acrylate resulting in their worse degradation stability [22]. Another possible reason is that the nano-LDH-LG layers would play a role of hindering the growth of three-dimensional network structure of the cured film in the curing process to a certain extent. Therefore smaller molecular weight of cured membrane leads to the reduction of thermal stability. Fig. 6 shows the DSC traces of UV-cured pure polymer and UVcured polymer/LDH nanocomposites in the range of 30–450 °C during the melting process. As can be seen from Fig. 6, a shoulder peak at 400 °C weakens gradually with increasing the amount of LDH-LG. It has completely disappeared when the content of LDHLG is up to 2 wt%. The interaction between the molecular chain of the polymer matrix and the LDH layer partially eliminates the stress generated during the crystallization process, thereby hinders the formation of different crystalline isomer. Another possible reason is that the nano-scale LDH-LG layers plays a role of impeding the movement of the polymer chains, which slows the growth rate of crystallization of the polymer and reduce the crystals formed with different molecular weight. 3.4. Properties of UV-cured polymer/LDH nanocomposite film The wear resistance, coefficient of static friction, dynamic friction and gloss of nanocomposite films are listed in Table 1.
As shown in Table 1, the wear resistance of the film is improved significantly by introduction of LDH-LG. The coefficients of static friction, dynamic friction and gloss of nanocomposite with 2 wt% of LDH-LG decrease to 0.114°, 0.062° and 87.5° compared with 0.856°, 0.758° and 94.0° of the pure polymer, respectively. The LDH-LG layers which are dispersed in the polymer matrix uniformly, play a role of physical crosslinking point to the polymer network, improve the strength of polymer film, and also reduce the flexibility. Therefore, the wear resistance of the polymer film is significantly improved with the increase of the LDH-LG content. The uniformly dispersed LDH-LG layers have better compatibility with the matrix. As a hard phase, LDH-LG layers make the matrix to be strengthened. Meanwhile, nano-scale layers can make the film surface smooth, which contributes to a lower coefficient of friction. Nano-scale layers can also effectively prevent the adhesion transfer of the matrix [23], which is conducive to the formation of fine particulate wear debris, and inhibit the generation of large particles of debris. Fine particulate wear debris generated in the friction surface play the role of lubrication, thereby the friction coefficient of the system is reduced.
4. Conclusion A novel approach to synthesize the exfoliated polymer/LDH hybrid nanocomposites was proposed via nanoplatelet-like organic LDHs. Three polymer/LDH nanocomposites were prepared by loading different amounts of LDH-LG into an UV-curable polyester acrylate matrix. The HR-TEM observation confirmed the formation of exfoliated microstructure of the UV-cured nanocomposites. The film performance test indicated that the addition of LDH-LG into polyester acrylate could improve the wear resistance of the film significantly and cut down the friction coefficient of the film. The thermal stability was decreased compared with the pure polymer. Besides, the organic LDH nanoplatelets can be used to other polymers for preparing exfoliated polymer/LDH nanocomposites.
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