Polar Influence of the Organic Modifiers on the Structure of Montmorillonite in Epoxy Nanocomposites

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ScienceDirect J. Mater. Sci. Technol., 2013, 29(11), 1040e1046

Polar Influence of the Organic Modifiers on the Structure of Montmorillonite in Epoxy Nanocomposites Fu-Chun Liu1)*, En-Hou Han1), Wei Ke1), Nan Tang2), Junbiao Wan2), Guilai Yin2), Jingwei Deng2), Kangwen Zhao2) 1) State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2) Jiangxi Electric Power Research Institute, Nanchang 330096, China [Manuscript received September 19, 2012, in revised form February 26, 2013, Available online 30 August 2013]

The epoxy nanocomposites with similar amines (CH3(CH2)17NH2 and CH3(CH2)17N(CH3)3Cl) treated montmorillonite clays have been investigated by wide-angle X-ray scattering, transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). Different nanocomposite structures, intercalation and exfoliation were formed by the reaction of octadecyltrimethylammonium chloride-exchanged and octadecylamine-exchanged clays with epoxy resin and phenalkamine as the curing agent, respectively. Results showed that the exfoliated nanocomposite can be obtained when octadecylamine with the lower polarity was used as a modifier. However, the intercalated nanocomposite can be obtained when octadecyltrimethylammonium chloride with higher polarity was used as a modifier. KEY WORDS: Polymer; Clay; Exfoliation; Intercalation

1. Introduction It is well known that organic coatings are an important method to protect metals from corrosion. For increasing the barrier properties of the coating, various pigments, such as anticorrosion pigments are incorporated. Recently it is reported that TiO2, SiO2, ZnO, Fe2O3 nanopigments[1e4] play a significant role in corrosion resistance. Besides nanometer oxide, layered silicate is promising in polymer modification. Polymer-layered silicate nanocomposites exhibit excellent mechanical[5], barrier[6], thermal[7e9], electrical properties[10,11] after alkylammoniumexchanged smectite clays as the reinforcement phase in selected polymer matrices. Epoxy is one of the most important types of polymer used in coatings, adhesives, fiber-reinforced materials. For obtaining desired epoxy nanomaterials, highly exfoliated clay is necessary. Hence, various materials and methods were investigated, such as different types of clays[12], different types of epoxies[13e15], curing agents[16,17], modifiers and processing techniques[18e23]. Corresponding author. Ph.D.; Tel.: þ86 24 23915895; Fax: þ86 24 23894149; E-mail address: [email protected] (F.-C. Liu). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.08.014 *

Suitable modifiers for clay modification are amines[24,25], alkylammonium[26], ammonium salt[27,28], phosphorus compounds[29e31], organic acid[32e34], polyhedral oligomeric silsesquioxane[35], polymer[36], and other cationic surfactants[37,38]. Recently, silane as a surface modifier has attracted significant interest of scientists and engineers. The silane-modified clay improves elastic modulus, tensile strength[39,40] and fracture toughness[41], anticorrosive properties[42], thermal and scratch resistance performances[43,44] of the epoxy nanocomposites. Above-mentioned modifiers can improve the nanocomposite properties, because molecular structures of modifiers play an important role in nanocomposite materials. Ryznarova et al.[45] investigated four different clay surface modifiers of alkylammonium chloride on the structure of epoxyeclay nanocomposites. It is found that protonated and functionalized forms of clay surface modifiers are able to catalyze intragallery polymerization of epoxy, which results in gradual increase in the dspacing during curing and higher degree of dispersion leading to improved mechanical properties[45]. Thelakkadan et al.[46] analyzed intercalated and exfoliated nanocomposites with two types of quaternary ammonium-modified montmorillonite clays. The storage modulus and the glass transition temperature, and AC dielectric breakdown strength of the exfoliated nanocomposite are higher than those of the intercalated nanocomposite. Moreover, accumulated space charge for exfoliated nanocomposite is lower than that for intercalated nanocomposite.

F.-C. Liu et al.: J. Mater. Sci. Technol., 2013, 29(11), 1040e1046

Dai et al.[47] studied the effect of intercalating agent on the physical properties of the epoxy/clay nanocomposite materials. The results revealed that the quaternary alkylphosphonium saltmodified clay showed better dispersion capability than quaternary alkylammonium salt-modified clay. The better dispersion of the quaternary alkylphosphonium salt-modified clay in epoxy resin led to more effectively enhanced physical property. Wang et al.[48] synthesized epoxyeclay nanocomposites by epoxy resin and clay cured with different amine curing agents. The clay treated with 2,4,6-tris [(dimethylamino) methyl] phenol curing agent, can exfoliate at 35  C after 60 h, but, for the other clay treated with p,p-diamino-diphenyl-methane, exfoliation of the clay layers does not occur. They thought that the relative curing speed between the interlayer and extralayer was the most important factor determining clay exfoliation. Within nearly 20 years of the extensive study of epoxy/clay nanocomposites, mechanisms of surface modifier were rarely reported. However, the physical and chemical properties of the surface modifiers play an important role in clay structure. The influence of similar amines as surface modifiers on the structure of epoxy nanocomposites has been investigated. Different exfoliation and intercalation structures were obtained in the same epoxy matrix, and their formation mechanisms were discussed in this work. 2. Experimental 2.1. Materials Montmorillonite was obtained from Zhangjiakou Chemical plant, Hebei province, China. Its cation exchange capacity (CEC) value, as determined by ICP (inductively coupled plasma) spectrometry, was 114.55 mequiv/100 g. E-51, epoxy resin, the diglycidyl ether of bisphenol A (DGEBA) with a molecular weight of 380 g/mol, was obtained from Shanghai Resin Co., Ltd. Its molecular formula is

Phenalkamine NX-2003 from Cardolite Company was used as a curing agent for the epoxy resin. Octadecylamine and octadecyltrimethylammonium chloride (70%) were purchased from Chemical Reagents and Glass Apparatus Branch, Shenyang Pharmaceutical Co., Ltd, China. 2.2. Method of the montmorillonite modification 25 g of montmorillonite (denoted as MT) was added to a 5 l flat-bottom flask loaded with 2 l deionized water, and mixed at room temperature for 24 h using a magnetic stirrer equipped with a stirring rod to make solution A; solution B was prepared from a certain dosage (according to Eq. (1)) of CH3(CH2)17NH2 or CH3(CH2)17N(CH3)3Cl and 250 ml deionized water, but stirred for only 30 min, then was adjusted to pH ¼ 3e4 with HCl solution. Solution A and solution B were mixed at 80  C for 24 h. Afterward, the sediments were cleaned many times by filter centrifuging so that there was no chloride detected with 0.1 mol/l

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AgNO3 solution. And then sediments were dried at 60  C in a vacuum oven. The clays were ground with a mortar and pestle, and sized by 325 meshes. The particles were collected and the powder was put in bottles, which was stored in a desiccator. The products made in this way were denoted as 1MT clay (modified by CH3(CH2)17NH2) and 2MT clay (modified by CH3(CH2)17N(CH3)3Cl). The dosage of the modifier was calculated according to Eq. (1). CEC  25 gðfor montmorilloniteÞ  1:2 ¼ ðX =Mw of modifierÞ  1  1000

(1)

where X denotes the dosage of the modifier, Mw denotes the molecular weight of the modifier, CEC denotes the amount of cation exchange, the constant of 1.2 (>1) denotes the superfluous amount of the modifier added to ensure sufficient intercalation of the montmorillonite. 2.3. Preparation of montmorillonite/epoxy nanocomposite coatings The mixture of DGEBA and montmorillonite was prepared by dispersing montmorillonite powder in DGEBA by high speed stirring at 70e80  C for 1 h. One part of the mixture was retained for X-ray diffraction (XRD). The other part was stirred with NX-2003 hardener at a ratio of 100:55, and then free films were made on plastic base with a thread applicator. The DGEBA/montmorillonite/ NX-2003 mixture was heated at 120  2  C. The montmorillonite content was 10% by the weight of the dried epoxy coating. The oven temperature was calibrated by using a thermometer. 2.4. Characterization of nanocomposites The wide-angle X-ray diffraction (XRD) spectra of the samples were recorded using a Rigaku D/MAX 2400 X-ray diffractometer

with CuKa radiation (l ¼ 0.1540562 nm) at a scanning rate of 2 / min and a diffraction beam graphite monochromator for monochromatization. The tube voltage is 50 kV, and the tube current was 182 mA. The samples for transmission electron microscopy (TEM) study were prepared as described in Section 2.3, then microtomed into 60e90 nm thick slices with a Reichert-Jung Ultracut-E ultramicrotome and examined in a JEOL-200FX electron microscope with an accelerating voltage of 100 kV. Thermal gravimetric analysis (TGA) was performed using a Pyris Diamond TG/DTA (PerkinElmer Inc.) at a heating rate of 10  C/min from 50 to 800  C to investigate the thermal stability of the modified and unmodified montmorillonite. Differential scanning calorimetry (DSC) was employed to investigate the effect of the clay on curing course of the epoxy. Samples of pure epoxy, 1MT/epoxy, 2MT/epoxy with the hardener were studied using a Perkin-Elmer DSC model 7 at a heating rate of 5  C/min to determine the polymerization temperature under N2 protection.

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The modified and unmodified montmorillonite powder was pressed to form a wafer by a HY-12 sheeting machine (Tianjin City Optical Instrument Factory) for measuring contact angles. Montmorillonite/epoxy coatings were prepared with a thickness of 50  5 mm. An OCA15 model optical contact angle instrument (German Dataphysics Company) was employed to determine the contact angles of the powder and the films, and the final value was the average of five measurements. Some parameters were obtained by the measurements or calculation by the software attached to the instrument. qw and qe represent the water contact angle and the ethylene glycol contact angle, respectively; sds and sdl refer to the London dispersive component of the surface free energy of a solid and a liquid related to long-range forces, respectively; and sps , spl refer to the polar component of the surface free energy of a solid and a liquid, respectively, ss and sl refer to the total free surface energy of a solid and a liquid, respectively. 3. Results and Discussion 3.1. Structure of epoxy/clay nanocomposites Montmorillonite is a layered clay mineral, which belongs chemically to the family known as the 2:1 phyllosilicates. The layer thickness is about 1 nm with the lateral dimensions of the layers changing from 20 nm to tens of microns depending on the source of the clay[45]. A stacked array of montmorillonite sheets separated by a regular spacing is known as a gallery. In the galleries many metal cations, such as Naþ, Kþ, Ca2þ, Mg2þ, can be adsorbed because of the relatively weak forces between the layers, and these cations are easy to exchange with the cations in water. It is difficult to intercalate the epoxy resin into the gallery directly; therefore an intercalation agent is used to build a bridge between the epoxy resin and montmorillonite. The intercalation agent expands the interlayer spacing and the polarity of the montmorillonite is transformed from hydrophilicity into hydrophobicity, thus improving the affinity and compatibility between it and the resin. After organic treatment of montmorillonite, organic cations replace the Naþ resulting in change of the interlayer spacing. Fig. 1 shows the XRD patterns of Naþmontmorillonite, montmorillonite modified with octadecylamine (1MT), and the mixture of 1MT and epoxy resin, from which the change in the interlamellar spacing is evident. The XRD pattern of the pristine Naþ-montmorillonite has a silicate (001) reflection at 2q ¼ 7.02 , which corresponds to a layer d-spacing of 1.26 nm. When the montmorillonite was modified with octadecylamine, the interlayer spacing changed from 1.26 nm

Fig. 1 Wide-angle XRD patterns of: (a) MT; (b) 1MT; (c) 1MT swollen in the epoxy resin.

Fig. 2 Wide-angle XRD patterns of: (a) MT; (b) 2MT; (c) 2MT swollen in the epoxy resin.

(2q ¼ 7.02 ) to 3.04 nm (2q ¼ 2.9 ). When epoxy resin was mixed with the modified montmorillonite, the peak occurred at 2q ¼ 2.38 , indicating that the epoxy molecules had intercalated the interlayer (d ¼ 3.71 nm), which is close to the value of 3.75 nm reported for montmorillonite solvated by epoxy[49]. The interlamellar spacing between the layers of the clay, d001, was calculated according to the Bragg equation. Fig. 2 shows XRD patterns of MT, octadecyltrimethylammonium chloride treated montmorillonite (2MT), and the mixture of 2MT and epoxy resin. Organically modified montmorillonite powders showed a strong X-ray diffraction peak with a characteristic interlamellar spacing of about 2.15 nm (2q ¼ 4.11 ) (Fig. 2(b)). However, when the montmorillonite was expanded with DGEBA, the interlamellar spacing increased to about 3.43 nm (2q ¼ 2.57 ) (Fig. 2(c)), indicating an intercalation of the clay by the epoxy. Fig. 3 shows the XRD patterns of the 1MT and 2MT clay/ epoxy nanocomposites cured at 120  C. For 1MT clay/epoxy nanocomposite, the diffraction peak at 3.71 nm (2q ¼ 2.38 ) had disappeared completely. The interlamellar spacing had increased to >5.88 nm, which was calculated according to the Bragg equation considering limitation of X-ray diffractometer (2q ¼ 1.5 ), and the clay exfoliated completely. The wide peak at 2q ¼ 6 e7 possibly resulted from constraints of the polymer chains, which occurred because of long distance order and short distance disorder. For 2MT clay/epoxy nanocomposite, the interlamellar spacing had increased at the beginning of 3.43 nm,

Fig. 3 Wide-angle XRD patterns of 1MT and 2MT after reaction with the epoxy resin and NX-2003 curing agent.

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Table 1 Basal spacing (d001, nm) of alkylammonium-exchanged montmorillonite Gallery cation

Initial cation orientationa

Airdried

Epoxysolvated

Calculated value

CH3(CH2)17NH3þ CH3(CH2)17N(CH3)3þ

0.7 0.4

3.04 2.15

3.71 3.43

3.72 3.72

a

Orientation of the alkylammonium ion under air-dried conditions.

Fig. 4 TEM micrograph of 1MT nanocomposites.

which indicates that an intercalation structure had formed during curing process. The interlamellar spacing of the cured clay/epoxy mixture was 5.32 nm (2q ¼ 1.66 ). Fig. 4 shows the TEM micrograph of 10% 1MT nanocomposite. The nano-clay platelets are quite well-dispersed in the matrix, despite of high magnification, with an exfoliated structure dominated morphology. The basal spacing in the clay is up to 100 nm, which is consistent with the XRD data in Fig. 3. Fig. 5 shows the TEM micrograph of 10% 2MT nanocomposite. The nano-clay platelets are adequately well-dispersed in the matrix, with an intercalated structure dominated morphology. The basal spacing in the clay is about 50 nm, which is consistent with the XRD data in Fig. 3. Lan et al.[50] investigated the influence of various chain lengths of the alkylammonium ion on epoxide intercalation, and found that the basal spacing for a series of epoxy-solvated CH3(CH2)n1NH3þ-montmorillonite with n increased in proportion to the chain length of the onium ion. They proposed Eq. (2) for calculating the basal spacing (nm): d001 ¼ 0:127ðn  1Þ þ dA þ dM

(2)

where (n1) is the number of eCH2e groups in the onium ion chain; dA is the basal spacing of Naþ-montmorillonite. 0.127 nm is the contribution due to eCH2e chain segments when the chain adopts an all-trans configuration. dM is the Van der Waals radius of the eCH3 end group. Table 1 shows the expansion of the basal spacing for epoxysolvated CH3(CH2)17NH3þ-montmorillonite and CH3(CH2)17

N(CH3)3þ-montmorillonite. The basal spacing calculated by Eq. (2) is consistent with that from XRD for epoxy-solvated CH3(CH2)17NH3þ-montmorillonite. The basal spacing of epoxysolvated CH3(CH2)17N(CH3)3þ-montmorillonite is less than calculated value. Three methyl groups on CH3(CH2)17N(CH3)3þ could become a steric hindrance and restrain the diffusion of the modifier molecule into the intragalleries. Figs. 6 and 7 show the exfoliation process of 1MT clay and the intercalation process of 2MT clay, respectively. More octadecylamine can enter intergallery of the clay than octadecyltrimethylammonium. After epoxy intercalation, the basal spacing became larger. Finally, epoxy resin cured with the hardener in intergallery prompted formation of the intercalated nanocomposite for CH3(CH2)17N(CH3)3þ-montmorillonite and exfoliated nanocomposite for CH3(CH2)17NH3þ-montmorillonite. The orientation of the epoxy-intercalated gallery is approximately perpendicular to the onium ion. Assuming that the onium ion in the intergallery adopts all-trans configurations, the angle between the onium ion and a vertical line, b, is calculated to be 23.5 for CH3(CH2)17N(CH3)3þ, and 7.3 for CH3(CH2)17NH3þ, from the data of Table 1. In terms of the basal spacing of MT, 1.26 nm, and the Van der Waals dimension of an epoxy molecule, 1.46 nm  0.44 nm  0.35 nm[50], epoxy molecules can enter clay intergalleries easier and faster with an increase in the basal spacing. 3.2. TGA of unmodified and modified montmorillonite Fig. 8 shows the TGA as a function of temperature for MT, 1MT, 2MT. Fig. 8 clearly demonstrates the effect of octadecylamine and octadecyltrimethylammonium chloride modification of the clay surface on the decomposition temperature (Td) of the clays. The unmodified MT clay was thermally stable up to 582  C, whereas the organically altered 1MT clay and 2MT started to decompose at 198 and 214  C, 384 and 368  C below MT, respectively. The drop in Td was concomitant with a 32% and 24% weight loss, which equals to the quantity of CH3(CH2)17NH2 and CH3(CH2)17N(CH3)3Cl modifier within clay gallery approximately, respectively. 3.3. DSC analysis

Fig. 5 TEM micrograph of 2MT nanocomposites.

Table 2 lists the onset reaction temperature, peak temperature of pure epoxy, epoxy with 1MT and 2MT cured by NX-2003. Despite the onset reaction temperature of epoxy containing 1MT was slightly lower than those of pure epoxy and epoxy containing 2MT, the peak temperature of epoxy containing 1MT was almost the same as that of the epoxy containing 2MT, which can been seen in Fig. 9. Therefore, for the peak temperature and dynamic DSC curves, there is a slight difference, that is, clay intercalated with CH3(CH2)17NH2, and CH3(CH2)17N(CH3)3Cl

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F.-C. Liu et al.: J. Mater. Sci. Technol., 2013, 29(11), 1040e1046

Fig. 6 Schemes for the exfoliation process of 1MT clay.

has indistinctive catalytic effect on polymerization. This result is different from the viewpoint of Lu et al.[51]. In their paper, clay was modified by CH3(CH2)17NH2, CH3(CH2)17N(CH3)3Cl, and then was added into epoxy resin and P, P0 -diamino-diphenyl-

methane (EP/DDM), respectively. They found that clay modified by CH3(CH2)17NH2 may evidently decrease the polymerization temperature in EP/DDM. It is likely that the reaction activation energy at room temperature is lower than that at middle

Fig. 7 Schemes for the intercalation process of 2MT clay.

F.-C. Liu et al.: J. Mater. Sci. Technol., 2013, 29(11), 1040e1046

Fig. 8 Thermogravimetric decomposition curves for three montmorillonites: MT, 1MT and 2MT. Table 2 Onset and peak temperature ( C) of pure epoxy, epoxy with 1MT and 2MT cured by NX-2003 Samples

Onset

Peak

Pure epoxy cured by NX-2003 Epoxy with 1MT cured by NX-2003 Epoxy with 2MT cured by NX-2003

48.1 46.7 50.4

85.4 85.7 87.1

temperature, and the reaction at room temperature is not as fierce as that at middle temperature. 3.4. Polarity of the modified clay and the epoxy/clay nanocomposites Fig. 10 shows the relationship between the contact angle and the clay content for two kinds of nanocomposite coatings. The water contact angle decreased with increasing clay content, which is possibly related to the surface roughness because with increasing clay content the surface roughness increases. The water contact angle of CH3(CH2)17N(CH3)3Cl modified clay coating decreased more rapidly than that of CH3(CH2)17NH2 modified clay coating because the former was more hydrophilic. According to these results, it can be inferred that it is possible to modulate the contact angle of the coating in order to meet different needs by changing the hydrophobic character and the clay content. Table 3 shows contact angles of pure, 5% 1MT and 5% 2MT cured epoxy mixture and the surface tension of the polar and

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Fig. 10 Water contact angles of 1MT and 2MT nanocomposite coatings.

Table 3 Contact angles of pure, 5% 1MT and 5% 2MT cured epoxy mixture and the surface tension of the polar and disperse components of them

qw (deg.) qe (deg.) sds (mN/m) sps (mN/m) ss (mN/m) Pure 5% 1MT 5% 2MT

79.3 79.9 73.8

45.6 39.4 41.9

29.99 37.90 26.67

6.07 3.86 15.38

36.06 41.76 42.05

disperse components of them. It can be seen that the surface free energies of the epoxy films with 5% 1MT and 5% 2MT are greater than that of the pure epoxy film. Owing to different modifying agents used, the surface free energy of the 5% 1MT epoxy film is slightly lower than that of 5% 2MT epoxy film. In view of the polar component of the surface free energy, sps of the 5% 2MT epoxy film is significantly larger than that of the 5% 1MT epoxy film. sps is related to Debye-inductive polarization, Keesom-orientational polarization forces, and hydrogen bonding[47,48]. It indicates that the polarity of the 5% 2MT epoxy film is higher than that of the 5% 1MT epoxy film, and this result is consistent with the difference in contact angle with water. In order to explain the polarity of clay/epoxy films, the contact angles of MT, 1MT modified montmorillonite and 2MT modified montmorillonite were measured at zero time. Table 4 shows water contact angles and ethylene glycol contact angles, the polar and disperse components of surface tension of the unmodified and modified montmorillonite. The surface energy of CH3(CH2)17NH2 modified montmorillonite and its polar component are lower than that of CH3(CH2)17N(CH3)3Cl modified montmorillonite and its polar component, which is consistent with the results of the corresponding nanocomposite films. Lowering the polarity of montmorillonite is beneficial to the formation of an exfoliated structure of epoxy/clay nanocomposites. On the other hand, a film made from montmorillonite with a higher polarity has a lower hydrophobicity, and the film made from montmorillonite with a smaller polarity has a higher hydrophobicity. Table 4 Water contact angles, ethylene glycol contact angles, surface tension parameters of the unmodified and the modified montmorillonite

Fig. 9 Dynamic DSC of pure epoxy (a), epoxy with 1MT (b) and 2MT (c) cured by NX-2003.

MT 1MT 2MT

qw

qe

sds

sps

ss

(deg.)

(deg.)

(mN/m)

(mN/m)

(mN/m)

17.14 122.3 119.9

17.86 63.42 63.5

31.21 48.81 58.23

37.44 4.53 20.76

68.65 53.34 78.99

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4. Conclusion Surface treatment by amine and ammonium salt can change the dispersive component, the polar component of the surface free energy and the total surface energy of the clay. Exfoliated clay/epoxy nanocomposites are useful, and the results of surface free energy and XRD measurements have shown that lower polar octadecylamine modified clay is easier to exfoliate than the pristine clay, while higher polar octadecyltrimethylammonium chloride modified clay is easier to intercalate than the pristine clay. It is easier to attain a coating with a larger contact angle with water by using a nanocomposite coating prepared by clay treated with a lower polar surface modifier. Acknowledgments We gratefully acknowledge the support from the National Key Technology R&D Program (Grant No. 2012BAB15B00) and State Grid Practical Project e Investigation on the key technologies on development and application of anticorrosive material of power transmission and transformation equipment (Grant No. 521820130014) for supporting these studies. REFERENCES [1] H. Shi, F. Liu, E. Han, Y. Wei, J. Mater. Sci. Technol. 23 (2007) 551e558. [2] H. Shi, F. Liu, L. Yang, E. Han, Prog. Org. Coat. 62 (2008) 359e 368. [3] Z. Wang, F. Liu, E. Han, W. Ke, S. Luo, Chin. Sci. Bull. 54 (2009) 3464e3472. [4] F. Liu, L. Yang, E. Han, J. Coat. Technol. Res. 7 (2010) 301e313. [5] P.C. LeBaron, Z. Wang, T.J. Pinnavaia, Appl. Clay Sci. 15 (1999) 11e29. [6] T.L. Wang, W.S. Hwang, M.H. Yeh, J. Appl. Polym. Sci. 104 (2007) 4135e4143. [7] A.J. Gu, G.Z. Liang, Polym. Degrad. Stab. 80 (2003) 383e391. [8] G. Das, N. Karak, Polym. Degrad. Stab. 94 (2009) 1948e1954. [9] G. Das, N. Karak, Prog. Org. Coat. 69 (2010) 495e503. [10] S. Raetzke, Y. Ohki, T. Imai, T. Tanaka, J. Kindersberger, IEEE Trans. Dielectr. Electr. Insul. 16 (2009) 1473e1480. [11] T. Tanaka, Y. Ohki, M. Ochi, M. Harada, T. Imai, IEEE Trans. Dielectr. Electr. Insul. 15 (2008) 81e89. [12] X. Kornmann, H. Lindberg, L.A. Berglund, Polymer 42 (2001) 1303e1310. [13] O. Becker, R. Varley, G. Simon, Polymer 43 (2002) 4365e4373. [14] D. Ratna, O. Becker, R. Krishnamurthy, G.P. Simon, R.J. Varley, Polymer 44 (2003) 7449e7457. [15] M. Harada, A. Ueda, H. Miyazaki, M. Ochi, J. Appl. Polym. Sci. 113 (2009) 2256e2263. [16] T.P. Mohan, M.R. Kumar, R. Velmurugan, Polym. Int. 54 (2005) 1653e1659. [17] T.P. Mohan, M.R. Kumar, R. Velmurugan, J. Mater. Sci. 41 (2006) 2929e2937. [18] A. Yasmin, J.L. Abot, I.M. Daniel, Scripta Mater. 49 (2003) 81e 86. [19] T.K. Oh, M. Hassan, C. Beatty, H. El-Shall, J. Appl. Polym. Sci. 100 (2006) 3465e3473. [20] M. Hernandez, B. Sixou, J. Duchet, H. Sautereau, Polymer 48 (2007) 4075e4086.

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