ReviewUltrasound assisted in situ emulsion polymerization for polymer nanocomposite: A review
Introduction
The first attempt to use of the ultrasound in enhancement of chemical reaction rates was reported by Richards, A.L. Loomis in 1927 [1]. Since then the use of ultrasound in the synthesis has attracted a lot of interest, in various fields of chemistry, materials science and chemical engineering [2], [3], [4]. Cavitation as a phenomenon was first identified and reported by Thornycroft and Barnaby in 1895 [5]. When more powerful ultrasound at a lower frequency is applied to a system, it is possible to produce the chemical changes as a result of acoustically generated cavitation. The generation of radicals takes place due to the extreme environment created by acoustic cavitation [6], [7]. In addition, the physical effect of the medium on the wave is referred to low power or high frequency ultrasound. The physical effect in the fields of nanomaterials like thermal heating, mass transfer, emulsification, and surface cleaning are induced by cavitation [8].
Application of sonochemistry in materials science and nanotechnology has been covered and reviewed by a several authors [2], [8], [9], [10], [11], [12]. Early reviews on nanostructured materials prepared using ultrasound radiation and applications of ultrasound to materials chemistry were carried out by Suslick and Price [2], [9]. Then the review on “Ultrasound assisted chemical processes” was carried out by Ashokkumar and Grieser [10]. This review highlighted the area of synthesis of the nanomaterials using ultrasound assisted technique. They have reviewed the use of ultrasound in homogeneous (radical and pyrolytic) and heterogeneous (synthesis of metal and semiconductor particles, complex molecules such as protein and polymers) reaction systems. Further, they have described the effect of ultrasound on leather processing, drug delivery, intraparticle collisions, precipitation stripping of rare earth oxalates and water treatment. The review on sonochemistry for the fabrication of nanomaterials has been carried out by Gedanken [8]. He has reviewed the work related to materials science and nanotechnology, in which it has been concluded that the sonochemical method of synthesis is superior over all other existing techniques. The topics reviewed were preparation of amorphous products, insertion of nanomaterials into mesoporous materials, deposition of nanoparticles on ceramic and polymeric surfaces and the formation of proteinaceous micro- and nanospheres using cavitation technique. The recent reviews [11], [12] reported the sonochemical fabrication of polymer nanocomposites and sonochemical synthesis of nanostructured materials such as carbon nanotubes (CNT), organophillic clay, and metal nanoparticles.
In the area of polymer science, due to the intense conditions generated by acoustic cavitation, ultrasound acts as an initiator by breaking chemical bonds of molecules and thus enhances the rate of polymerization. Cao et al. [13] have studied the radical generation mechanism in ultrasonically irradiated emulsion copolymerization of styrene in the presence of a cationic surfactant (methacryloxyethyl dodecydimethyl ammonium bromide, C12N+) without adding any chemical initiators. Under ultrasonic irradiation condition, C12N+ undergoes bond scission between the two alkyl and ionic group thus producing much more original radicals to initiate the emulsion polymerization. Teo et al. [14] have reported the miniemulsion polymerization of methacrylate monomers in the presence of ultrasound without the use of external initiator and at 30 °C (room temperature synthesis using cavitation). They have observed that these cavitation based polymerization reactions follow pseudo-first-order kinetics and further it has been reported that a radical enters the monomer droplet containing a growing radical will lead to pseudo-instantaneous termination. The mechanism for the preparation of polymer latex particles involved in sonochemical process is very similar to that of a conventional miniemulsion polymerization process. Ultrasonically initiated emulsion polymerization of methyl methacrylate [14], [15], [16], [17], [18], [19], [20], styrene [17], [18], [21], [22], [23], [24], n-butyl methacrylate [25], n-butyl acrylate [19], [22], [26] were also studied. The rate of polymerization of methacrylate monomers [14] is strongly dependent on the temperature, vapor pressure and hydrophobicity of monomer and ultrasonic intensity, which were the important findings reported by Teo et al. [14].
For the preparation of polymer nanocomposites either organic molecules or inorganic particles can be encapsulated into inorganic materials or organic polymer latex, which could be accomplished by applying cavitation during the polymerization process. There have been several research articles which describe the preparation of polymer nanocomposite by conventional in situ emulsion polymerization, but attempt was not successful in making complete encapsulation [27], [28], [29], [30], [31], [32], [33]. Recently, numbers of attempts were made by the researchers in the preparation of polymer nanocomposites latexes using ultrasonically initiated in situ emulsion polymerization [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. This ultrasound assisted in situ emulsion polymerization technique provides an effective way of synthesizing finely dispersed inorganic particles in polymer latex along with many advantages such as higher polymerization rate, narrow molecular weight and particle size distribution, higher monomer conversion and no or small amount of external initiator requirement compared to convention in situ emulsion polymerization [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. Fillers, such as titanium dioxide, calcium carbonate, clays, several oxides and metal particles were encapsulated by means of ultrasound assisted in situ emulsion polymerization [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. Addition of untreated inorganic particles during emulsion polymerization makes system unstable and leads to agglomeration of particles. Hence, functionalization of inorganic fillers plays an important role during the encapsulation process. Surface modification of an inorganic particle with an organic molecule is one of the ways to reduce its surface energy and to increase its compatibility with the organic polymer matrix. The surface modification has been extensively carried out using different organic modifiers and surfactants, e.g. silane coupling agents, titanate coupling agents, or stearic acid, myristic acid, etc. [47], [48], [49], [50], [51].
Section snippets
Ultrasound assisted emulsion polymerization
It is well known that for emulsion polymerization, hydrophobic monomer, emulsifier and hydrophilic initiator are the main constituents could be used in the water phase (continuous phase). Polymers such as acrylonitrile butadiene styrene, polystyrene, poly-methyl methacrylate, etc. can be easily prepared via emulsion polymerization processes. These conventional emulsion polymerization processes have different disadvantages, e.g. high polydispersity of particles, and instability of colloidal
Polymer nanocomposites synthesis techniques and role of ultrasound in polymer nanocomposites preparation
Preparation of polymer nanocomposite from nano-inorganic additives and polymers has been accomplished by the means of surface encapsulation or surface grafting. Several methods have been implemented to synthesize polymer nanocomposites. Some of them are, suspension polymerization [58], [59] emulsion polymerization [60], intercalative polymerization [61], solution casting method [62], hybrid latex polymerization [63]. Improper dispersion of inorganic nanoparticles and weak bonding between
Types of polymer nanocomposites synthesized by ultrasound assisted in situ emulsion polymerization
Encapsulated polymer nanocomposites are intended to combine the best features of the organic and the inorganic compound. These hybrid particles can be defined as colloidal particles that include both organic and inorganic compounds. These polymer nanocomposites can be prepared by (a) combining organic and inorganic components, (b) reacting organic and inorganic precursors, or (c) polymerizing in situ organic and/or inorganic precursors in the presence of their corresponding compound. An
Conclusions
The current development of polymer nanocomposites prepared by ultrasound assisted in situ emulsion polymerization with inorganic particles such as inorganic oxides, metal, magnetic particle, CNT and clay have been reviewed. In this ultrasonic irradiation not only initiate polymerization of monomers such as MMA, nBA, styrene, etc. at room temperature while inorganic nanoparticles/clay layers were dispersed at the nanoscale, but can also shorten the polymerization time and save the energy. Thus,
References (158)
Using sonochemistry for the fabrication of nanomaterials
Ultrason. Sonochem.
(2004)- et al.
Radical generation process studies of the cationic surfactants in ultrasonically irradiated emulsion polymerization
Ultrason. Sonochem.
(2008) - et al.
Ultrasound initiated miniemulsion polymerization of methacrylate monomers
Ultrason. Sonochem.
(2008) - et al.
Ultrasound assisted polymerization of MMA and styrene in near critical CO2
J. Supercrit. Fluids
(2005) - et al.
Emulsion polymerization synthesis of cationic polymer latex in an ultrasonic field
J. Colloid Interface Sci.
(2002) - et al.
Ultrasonic initiation of polystyrene latex synthesis
Ultrason. Sonochem.
(2000) - et al.
Preparation of BA/ST/AM nano particles by ultrasonic emulsifier-free emulsion polymerization
Ultrason. Sonochem.
(2006) - et al.
Effect of ultrasonic frequency on polymerization of styrene under sonication
Ultrason. Sonochem.
(2001) - et al.
Polymerization rate and mechanism of ultrasonically initiated emulsion polymerization of n-butyl acrylate
Ultrason. Sonochem.
(2002) - et al.
Polymerizations in the presence of seeds: 3. Emulsion polymerization of vinyl acetate in the presence of quartz powder
Polymer
(1988)