ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, Серия Б, 2012, том 54, № 1, с. 132-143
КОМПОЗИТЫ
УДК 541(135+64+183)
EFFECT OF SURFACE MODIFICATION OF BENTONITE NANOCLAY WITH POLYMERS ON ITS STABILITY IN AN ELECTROLYTE SOLUTION1
© 2012 г. A. Khoshniyat"b , A. Hashemi4, A. Sharif a, J. Aalaie", and C. Duobisc
a Polymer Science and Technology Division, Research Institute of Petroleum Industry (RIPI),Tehran, IRAN b Composite group, Iran Polymer and petrochemical Institute, Tehran, IRAN c Department of Chemical Engineering, Ecole Polytechnique, Montreal, Quebec, Canada e-mail: s.a.hashemi@ippi.ac.ir (S. A. Hashemi) and sharifa@ripi.ir (A. Sharif)
Received April 26, 2011 Revised Manuscript Received August 10, 2011
Abstract—Recently surface modification of clay minerals has become increasingly important for improving the practical applications of clays and clay minerals. In this research work surface modifications of bentonite nanoclay particles were carried out by chemical grafting of different copolymers including poly(1-vinyl pyr-rolidone-co-styrene) (PVP), poly(methyl methacrylate-co-methacrylic acid) (PMMA) and poly (acryla-mide-co-diallyl dimethyl ammonium chloride) (PDADMAC). The efficiency of the grafting reactions was investigated using FTIR, TGA and XRD methods. It was shown that PVP as well as PDADMAC copolymers had been grafted successfully on the exterior of the nanoclay tactoids surfaces, while most of the PMMA molecules had entered the galleries and grafted on the inner surfaces of the nanoclay. Turbidimetry, zeta poten-tiometry and dynamic light scattering (DLS), were employed to analyze the dispersion stability of the unmodified and modified nanoclays in an electrolyte solution with high salinity. The turbidimetry results showed that stabilities of unmodified and modified nanoclay particles in the electrolyte solutions decreased as the time passed from their preparation, and after 24 hr they settled completely in the solutions. However, rates of the settlement of unmodified and modified particles differed from each other, that of the modified ones being lower. This difference was attributed to the grafted copolymers on the modified particles surfaces. Also, the results of the zeta potentiometry and DLS were in harmony with the turbidimetry observations.
INTRODUCTION
Electrolyte-nanoclay suspensions are an important part of several industries like paints, enhanced oil recovery and drilling, paper and water treatment [1, 2]. A major challenge for practical applications of this kind of electrolyte is their strong tendency to agglomeration, rapid sedimentation and, consequently, limited mobility of their colloids in the aquatic environment [3]. Stability of clays is one of the most important characteristics of these colloidal systems and its degree can be determined by the balance of attractive and repulsive forces acting between the colloidal particles. Colloidal interactions are usually discussed in the framework of DLVO (Derjaguin—Landau—Verwey— Overbeek) theory [1, 4—7]. Two approaches have been commonly used in improving electrolyte-colloid stability: electrostatic repulsion and steric stabilization [3, 6]. Electrostatic repulsion is achieved by changing the surface charge while steric stabilization is typically attained by the adsorption of long-chain organic mole-cules (e.g., polymers) on the clay surfaces. Combined electro-steric stabilization is also possible with the use of ionic polymeric molecules (e.g., a poly-electrolyte) [7—12]. Steric stabilization of colloidal particles is imparted by macromolecules that are at-
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tached (i.e., by grafting or physical adsorption) to the surfaces of colloids/particles, preventing them from collecting or flocculating [3, 13—17]. One of the main advantages of steric stabilization over electrostatic stabilization or double layer repulsion is its relative insen-sitivity to the presence of electrolytes. Low amounts of electrolyte may induce flocculation of the particles, unless they are protected by the presence of polymers attached to their surfaces. Other advantages include the effectiveness of steric stabilization in both aqueous and non-aqueous media, and the possibility to achieve stabilization at both low and high volume fractions of the dispersed phase. The latter is favorable for suspensions in many practical applications that involve a relatively high volume or weight fraction of solids [17].
Surface modification of nanoclays by polymers has been found to be one of the most effective methods to change their surface characteristics. There are two main approaches for the surface modification of nan-oclays with polymers: physical adsorption and chemical grafting of functional polymers to the surfaces of the nanoclays. The physical adsorption of polyethylene oxide, polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol and poly acrylamide on clay mineral surfaces have been studied by various research groups [8, 18—21]. The advantage of the physical attachment method is that the main structure of the
Table 1. Characteristics of the used polymeric materials
Copolymer Copolymer code Copolymer type Material status Brookfield viscosity, Cps Mn (Mw), g/mol Density, g/cm3
Poly(1-vinyl pyrrolidone-co-styrene) Poly(methyl methacrylate-co-methacrylic acid) Poly acrylamide-co-diallyl dimethyl amoniom chloride PVP PMMA PDADMAC Random Block Block 38% wt as emulsion in water white powder 10% wt in water 200-800 900-25000 15000 (34000) 1.04 1.02
nanoclay is not altered. However, the main potential disadvantage is the weak bonding forces between the adsorbed molecules and the nanoclay. Chemical grafting of functional polymers to the surface of nanoclays has been proposed to help solve this problem. Recent studies regarding the grafting of amphiphilic block co-polymer brushes on nanoparticle surfaces have shown the possibility of changing the interfacial energy at the solid—liquid interface over a very broad range between hydrophilic and ultrahydrophobic behaviors [3, 10, 15, 18, 19]. As examples, Minko et al. [15] introduced a method of fabricating stimuli-responsive core—shell nano-particles through grafting of poly (styrene-^-2-vinylpyridine-^-ethylene oxide) and poly (styrene-^-4-vinylpyridine) block copolymers on silica nanoparticles. Also, a cationic macromonomer, 2-methacryloylethyl-hexadecyldimethylammonium bromide MA16, and a cationic macroinitiator, cationic acrylic/styrene oligomer end-capped with a nitroxide, were used by Di-aconu et al. [22] to modify pristine Na-montmorillo-nite, to enhance compatibility between the clay platelets and the host acrylic polymer matrix in waterborne nanocomposites. They showed that both cationic species were successfully exchanged in the montmorillo-nite. Their organically modified clays were finally used for the synthesis of acrylic/clay waterborne nanocom-posites. Zhang et al. [3] developed a method for the preparation of a stable suspension of reactive nanos-cale zero-valent iron (nZVI) by grafting of polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A) on the nZVI surface.
Nevertheless, although a considerable amount of research has been undertaken regarding the modification of nanoclay particles with the aim of increasing their stability in various media, the behavior of the modified nanoclays in electrolyte media with high salinity has not been properly discussed yet and further investigations are needed to clarify remaining questions in this area. Furthermore, to the best of our knowledge, no information is available on the kinetics of the sedimentation of the un-modified and modified nanoclays in salt-concentrated electrolytes.
Therefore, the goal of this work was to modify nan-oclay surfaces by the copolymer grafting approach in order to prepare stable clay suspensions in an electrolyte with high salinity. The modified nanoclays were characterized by thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier transform infra-
red spectroscopy (FTIR). In addition, the stability of the suspensions prepared by dispersing the modified clays in the electrolyte medium was determined by spectrophotometry, zeta potentiometry and dynamic light scattering (DLS) methods.
EXPERIMENTAL
Materials
Calcium bentonite nanoclay (Nanomer) was obtained from Aldrich. The concentration of the nano-clay hydroxyl groups, which are necessary for its further modifications, was determined by pH-metry as 8wt%. Poly (1-vinyl pyrrolidone-co-styrene) (PVP) poly(methyl methacrylate-co-methacrylic acid) (PMMA) and poly acrylamide-co-diallyl dimethyl ammonium chloride (PDADMAC) were purchased from Sigma-Aldrich and used for the modification of the Nanomer. Table 1 shows the characteristics of these three copolymers. Also, y-aminopropyltriethoxysilane, APTES, (Mw = 221.37 g/mol) and bromoacetylbromide, (Mw = = 201.86 g/mol), as coupling agents, as well as tet-rahydrofuran (THF), dichloromethane and ethanol as solvents, were obtained from Merck and used as-received.
Modification of the Nanoclay Surface
A three-stage procedure based on Liu et al. [23, 24] was used in this research work in order to modify the surface of the Nanomer. Three grams of the Nanomer particles, dried overnight in a vacuum oven at 100°C, were dispersed in 100 ml dried toluene and homogenized with an agitation speed of10000 rpm for 15 min. Then, 5 ml of APTES was added to the prepared mixture followed by refluxing for 7 h at 40°C. The resultant modified nano-clay with APTES, APTES-Nano-mer, was filtered, washed thoroughly with ethanol and dried overnight in a vacuum at 50°C (step 1). In the second step, the APTES-Nanomer surface was brominated at room temperature by interacting for 15 h with 2 ml bromoacetylbromide, dissolved in 100 ml THF. The brominated APTES-Nanomer, Br-APTES-Na-nomer, was then filtered, washed thoroughly with eth-anol and dried overnight in a vacuum at 50°C (step 2). In the final
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