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Effects of Surfactant Loadings on the Dispersion of Clays in Maleated Polypropylene Zhongfu Zhao, Tao Tang,* Yongxin Qin, and Baotong Huang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China Received April 3, 2003. In Final Form: July 11, 2003 A series of organically modified clays (OMCs) with a surfactant loading range from 0.625 to 2.5 times the cation exchange capacity (CEC) were melt-mixed with maleated polypropylene (PPMA). Wide-angle X-ray diffraction and transmission electron microscopy results of these nanocomposites show that dispersion of clays becomes unfavorable in the PPMA matrix during melt intercalation as the surfactant loading increases in the process of modifying clays, though larger interlayer distances are obtained in their corresponding OMCs. It is even important that clays uniformly disperse at the nanoscale level in the PPMA matrix when the surfactant loadings are below the CEC, which implies that incomplete exchange of inorganic cations in the process of modifying clay benefits the dispersion of clays in the PPMA matrix.
Introduction Polymer melt intercalation is a promising approach to fabricate polymer/clay nanocomposites because it can be performed by using conventional polymer-processing techniques and it is environmentally benign.1 For most polymers, melt intercalation can be used to synthesize their nanocomposites containing a low weight percentage of clays, but the clays must be premodified with organic surfactants.2,3 Generally, it is believed that inorganic cations in the interlayer space should be completely exchanged with organic surfactants in order to intercalate polymer into clays, and clays were modified with excess surfactants in many reports of preparing various polymer/ clay nanocomposites.4-7 However, Vaia et al.8,9 believed that complete separation of the clay layers depends on the establishment of very favorable polymer-clay interactions. As more surfactants are used, the higher density of surfactants in the interlayer space would weaken the interaction between the polymer and the clays. Obviously, organic surfactant loading in organically modified clays (OMCs) has a great effect on formation of polymer/clay nanocomposites. In most polymer/clay nanocomposites, a low weight percentage (1-10%) of clays is usually required, which means an even lower weight percentage of surfactants in these nanocomposites. To exactly investigate the effect of surfactant loadings on the dispersion of the clay layers, nanocomposites containing a high weight percentage of clays are needed. Polypropylene is one of the most widely used polyolefin polymers. Since it does not include any polar groups in its backbone, polypropylene should be modified with maleic anhydride or other functional groups to prepare polypropylene/clay nanocomposites. It has been * Corresponding author. Tel: +86-431-5262004. Fax: +86-4315685653. E-mail:
[email protected]. (1) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (2) Alexandre, M.; Beyer, G.; Henrist, C.; Cloots, R.; Rulmont, A.; Je´roˆme, R.; Dubois, P. Chem. Mater. 2001, 13, 3830. (3) Alexandre, M.; Dubois, P. Mater. Sci. Eng. 2000, 28, 1. (4) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (5) Wang, Z.; Pinnavaia, T. J. Chem. Mater. 1998, 10, 3769. (6) Xu, R.; Manias, E.; Synder, A. J.; Runt, J. Macromolecules 2001, 34, 337. (7) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 891. (8) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 7990. (9) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000.
reported that maleated polypropylene (PPMA)/OMC ) 2:1 (w/w) nanocomposites were successfully prepared.10-12 In this letter, PPMA was used as a model polymer and the effect of cationic surfactant loadings on the dispersion of clays in PPMA/clay nanocomposites was systematically investigated. Materials and Experiments The materials used in this paper are clays (Na+montmorillonites) with cation exchange capacities (CECs) of 119 mequiv/ 100 g from Kunimine Co., octadecylamine from Wako Pure Chemical Industries Co., Osaka, Japan, and PPMA from Sanyo Chemical Industries. The acid value of PPMA is 52 mg KOH g-1, and the softening temperature is 145 °C. OMCs were synthesized by cation exchange reaction. Octadecylammonium chloride (octadecylamine protonated with equal molar concentrated HCl in 1000 mL of hot deionized water (about 80 °C)) was poured into the hot dispersion of clays (16 g). The mixtures were stirred vigorously for 30 min, giving white precipitates. Then, they were filtered with nylon cloth, washed with hot deionized water (about 80 °C), and dried in the air. Under the same conditions, the loading levels of surfactants with respect to CEC were varied systematically as 62.5%, 100%, 125%, 156%, 200%, and 250% and OMC62.5, OMC100, OMC125, OMC156, OMC200, and OMC250 were synthesized, respectively. To obtain the adsorbed level of surfactants, the OMCs were characterized by thermogravimetric analysis (TGA) according to weight loss from 200 to 500 °C,13 and they were 32.0%, 65.4%, 80.5%, 123.3%, 145.7%, and 174.3% with respect to CEC, respectively. Obviously the surfactants are incorporated into the layered silicates and the adsorption level of the surfactants increases monotonically with the surfactant loading level. In a Brabender mixer, various PPMA/clay ) 2:1 (w/w) nanocomposites were made from OMC62.5, OMC100, OMC125, OMC156, OMC200, and OMC250, respectively, by directly melt-mixing OMC and PPMA at a temperature of 180 °C for 15 min, and the mixer rotate speed was 100 rpm. These nanocomposites were correspondingly abbreviated as PCN62.5, PCN100, PCN125, PCN156, PCN200, and PCN250. Wide-angle X-ray diffraction (WAXD) was carried out with a Rigaku model Dmax 2500 with Cu KR radiation, and the (10) Kato, M.; Usuki, A.; Okada, A. J. Appl. Polym. Sci. 1997, 66, 1781. (11) Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. J. Appl. Polym. Sci. 1998, 67, 87. (12) Kawasumi, M.; Usuki, A.; Hasegawa, N.; Kato, M.; Okada, A. Macromolecules 1997, 30, 6333. (13) Xie, W.; Gao, Z. M.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Chem. Mater. 2001, 13, 2979.
10.1021/la034575w CCC: $25.00 © 2003 American Chemical Society Published on Web 07/30/2003
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Figure 1. WAXD profiles of various PPMA/clay nanocomposites.
Figure 2. Comparison of the interlayer distances of clay in the cases of various OMCs and in the cases of PPMA/clay nanocomposites. interlayer distances (d001) were estimated from the (001) peaks in the WAXD pattern with the Bragg formula. The morphology of the composites was studied by transmission electron microscopy (TEM) on ultrathin sections of the composites using a JEOL2010 transmission electron microscope.
Results and Discussion OMCs were characterized with WAXD (see the Supporting Information), and their d001 values were 1.78, 2.03, 2.36, 2.90, 3.71, and 3.87 nm for OMC62.5, OMC100, OMC125, OMC156, OMC200, and OMC250, respectively. In the experimental range, the interlayer distances gradually become larger with the increase of the surfactant loadings in the process of modifying clays, which should be attributed to the intercalation of surfactants into the interlayer space of the clay. WAXD was carried out on various PPMA/clay nanocomposites, and Figure 1 displays their WAXD patterns. Obviously, the (001) peaks of clays move to higher angles in the sequence PCN62.5, PCN100, PCN125, PCN156, PCN200, and PCN250, and their d001 values were calculated to be 6.90, 6.13, 4.75, 4.05, 4.10, and 3.98 nm, respectively. It is apparent that the interlayer distances of clays in these nanocomposites increase with the decrease of the surfactant loading in their corresponding OMCs. Besides, there are broader peaks (including one stronger peak and one weak one) for PCN156 and PCN125, which means silicates have at least two kinds of dispersion states in their matrix. Clearly, there are transition states of silicate dispersion as the interlayer distances of clays in these nanocomposites increase, which will be further explained with TEM results later. The d001 values of clays before and after melt-mixing are compared in Figure 2. The interlayer distances for these nanocomposites do not increase as those of the corresponding OMCs do. The interlayer distances decrease
Figure 3. TEM results of various nanocomposites: (a) PCN62.5; (b) PCN100; (c) PCN125; (d) PCN156; (e) PCN200; (f) PCN250.
in the former case but increase in the latter case as the surfactant loadings increase. Besides, the change of the interlayer distance before and after melt-mixing becomes smaller as surfactant loadings increase. The morphology evolution of these composites was investigated by TEM (Figure 3). Silicate plates are shown as dark lines in the images, and they have a broad distribution of diameters. Figure 3a shows TEM photographs of PCN62.5, in which the individual silicate plates are well dispersed in the polymer matrix and exfoliated morphology is achieved. However, the dispersion of silicates becomes poorer and poorer in these nanocomposites as the surfactant loading increases, which agrees well with the results of WAXD experiments. Images b
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and c in Figure 3 show TEM photographs of PCN100 and PCN125, in which individual silicate plates are dispersed more closely and the distance between adjacent clay plates decreases. Images d-f in Figure 3 display the TEM results of PCN156, PCN200, and PCN250, in which one can observe a tightly stacked layer structure. It has unequivocally been demonstrated that clays disperse at the nanoscale level in PCN62.5 in this work, implying that incomplete exchange of inorganic cations in the interlayer space benefits the dispersion of clays in the PPMA matrix. From the above trend, it could also be inferred that dispersion of clays in the PPMA matrix becomes unfavorable as the surfactant loading in their corresponding OMCs increases. As the surfactant loadings increase, the initial interlayer distances of the OMCs become much larger. However, according to the thermodynamic treatment for the intercalation process Vaia et al. had elaborated,8 the increased conformational entropy of the tethered surfactant chains to compensate the penalty of polymer confinement during the melt-mixing process would decrease in this case, which makes the intercalation process entropically unfavorable. Besides, effective dispersion of clays also depends on the establishment of very favorable polymer-clay interactions to overcome the penalty of polymer confinement.8 As the surfactant loading level increases, the density of surfactants on the clay surfaces increases, which weakens the favorable interaction between polymer and clays. On the other hand, it is necessary to premodify clays with surfactants for effective dispersion
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of clay layers in the PPMA matrix. Maybe optimal exchange of inorganic cations exists below CEC, which will be reported in detail later. Conclusions In summary, the dispersion of clays in the PPMA matrix during melt intercalation becomes thermodynamically unfavorable as the surfactant loading level increases in the process of modifying clays, although the initial interlayer distances become larger in the corresponding OMCs. It could be concluded that incomplete exchange of inorganic cations in the interlayer space benefits the dispersion of clays in the PPMA matrix. Importantly, our work has proved that this conclusion applies well to other polymers, such as polyamide and polystyrene, which is being researched in detail. Acknowledgment. The present work was supported by the Ministry of Science and Technology of China (Project No. G1999064801), the National Natural Science Foundation of China (Project No. 20174039), and the Project for Outstanding Youth Researcher from Jilin Province. Supporting Information Available: WAXD profiles of various OMCs as a function of the surfactant loading. This material is available free of charge via the Internet at http://pubs.acs.org. LA034575W
