Polymerization of Anilinium/DBSA in the Presence of Clay Particles: Catalysis and Encapsulation.
This paper describes polymerization of anilinium-DBSA complex to PANI-DBSA (DBSA doped polyaniline) in an aqueous dispersion in the presence of mica or talc clays. It is found that the clay presence significantly accelerates the polymerization kinetics by metal ions, such as [Fe.sup.+2] ions, present in the clays. Polymerization occurs preferentially on the clay particles' surface, causing their encapsulation with PANI-DBSA shells. Further coating with PANI-DESA takes place on the already coated particles with thin PANI-DBSA shells by an autoacceleration effect of anilinium-DBSA polymerization by PANI. It is suggested that the catalytic/autoacceleration effect dominating in the coating process of clay particles with PANI-DBSA can be extended to other particle/coating systems, which are already under investigation.
Electrically conducting plastics comprise a matrix and a dispersed conductive filler at a content exceeding a threshold concentration (percolation concentration). Typical conductive fillers are carbon black, carbon fibers, metal particles and fibers, or intrinsically conductive polymers (ICP) such as polypyrrole, polyaniline (PANI) and polythiophene. It has been a challenge in polymer/conductive filler systems to minimize the conductive component concentration because of processing performance and cost considerations. In this regard conductive fibrous, rather than particulate, particles are sometimes preferred, as well as immiscible multi-component systems, where the conductive particles preferentially locate in one of the phases, or at the interface (1). In a different approach, polystyrene suspension particles were coated with thin ([sim] 1 [micro]m thick) metal shells using an electroless deposition procedure, which requires a catalyst to induce the redox reaction. Thus, prior to deposition the catalyst wa s adsorbed on the polystyrene particles' surface and subsequently metal deposition took place, selectively upon the catalyzed surfaces to produce the thin metal shells coating the PS particles (2). Compression molded samples of these metal coated PS particles were found to be highly conductive at extremely low metal concentrations, less than 1v/v percent (random distribution of the metal particles requires [greater than]15v/v percent metal). Metal-coated particulate and fibrous fillers, and PANI coated carbon black particles are commercially available (3, 4). The concept of polymerizing a monomer in the presence of particles of catalyzed surfaces can be widely employed to encapsulate inorganic or organic particles with conducting or insulating polymers.
Micas are members of a class of silicates known as phyllo- (or sheet) silicates, a term that reflects their crystal structure. Mica has a perfect basal cleavage, which enables its splitting into thin sheets. This is an expression of the layered crystal structure of the mica lattice. A common form of mica is muscovite. Mica-type silicates are flake-shaped aggregates consisting of stacked units without regular features. Mica is characterized by hydrophilic behavior and swelling capacity, which is essential for an efficient intercalation.
Several researchers have invented methods to introduce conjugated polymeric chains within nanometric void spaces of inorganic host materials by in situ polymerization (5-7). Closs and co-workers studied the interlayer adsorption of aniline into [Cu.sup.+[2.sub.-]] and [Fe.sup.+[3.sub.-]] montmorillonites from aqueous solutions. IR spectroscopy has indicated that aniline adsorbed in [Cu.sup.+[2.sub.-]] montmorillonite is oxidized to radical species, which polymerize, whereas the [Cu.sup.+2] ions, rapidly re-oxidize, acting as initiators (8, 9).
Mehrotra and Giannelis fabricated highly oriented multilayered films of PANI by intercalative polymerization of aniline in a synthetic mica-type silicate. Gallery [Cu.sup.+2] ions, introduced by intercalative ion exchange process, serve as oxidation centers for the polymerization of aniline .
Stejskal et al. polymerized aniline in the presence of ultrafine colloidal silica particles in an aqueous medium, to form colloidally stable PANI-silica particles. These dispersions yield microstructured composites comprising sub-micrometer PANI particles dispersed in a matrix of insulating silica particles .
The objective of the present work has been to encapsulate mica particles with PANI shells, i.e., to produce conductive PANI/mica particles. To achieve this goal a method was developed by which aniline preferentially polymerizes upon the mica particles to form the encapsulating PANI shells. This paper summarizes the preliminary results obtained and suggests the coating mechanism involved.
Mica R120 was supplied by Micronized Products, USA. This muscovite mica is a product ground in water to give high aspect ratio plate-like particles having a size distribution of 45-150 [micro]m . Talc R00058 was received from Tosaf, Israel.
Anilinium-Dodecyl Benzene Sulfonic Acid (DBSA) complex was polymerized in an aqueous dispersion using a polymerization process, developed in our laboratories [11, 12].
Anilinium-DBSA complex was prepared in the presence of mica, by mixing aniline (6 g), DBSA (21.6 g) and as received mica (25 g) in water (600 g) for 3 h. The dispersion was cooled ([sim]0[degrees]C), and a solution of ammonium peroxydisulfate (15 g dissolved in 50 g water) was added dropwise. The polymerization process was carried out at [sim]0[degrees]C for 5 h. A color change from white (anilinium-DBSA complex in water) through blue to dark green was observed. At the final stage of polymerization a green PANI-DBSA/mica dispersion was obtained.
Prior to the polymerization process (described in Procedure 1), the mica powder was dispersed in 600 mL deionized water. The dispersion obtained was stirred overnight.
Prior to the polymerization process (described in Procedure 1), the mica powder was dispersed in 600 mL deionized water and treated in an ultrasonic bath for 24 h at room temperature.
Polymerization of Anilinium-DBSA in the Presence of [Fe.sup.+2] Ions
[FeCO.sub.3] salt (2.2 g) was added (no mica powder present) to the anilinium-DBSA dispersion and subsequentiy polymerization took place according to Procedure 1.
Polymerization of Anilinium-DBSA in the Presence of [Al.sup.+3] Ions
[Al.sub.2][([SO.sub.4]).sub.3]16[H.sub.2]O salt (2.2 g) was added (no mica powder present) to the anilinium-DBSA dispersion and subsequently polymerization took place according to Procedure 1.
Precipitation of PANI-DBSA/Mica From an Aqueous Dispersion
Methanol was added to the PANI-DBSA/mica aqueous dispersion to precipitate the doped PANI-DBSA/mica particles (methanol: dispersion volume ratio was [sim]1:1). The precipitate was filtered, washed with methanol and dried in a vacuum oven (60[degrees]C, overnight). The color of the powder obtained was dark green.
The volume conductivity of room temperature compression molded strips ([sim]50 atm) was measured by the "two electrode technique" (DIN 53596) using a Keithley Electrometer 614 and high 240A voltage supply. Samples were coated with a silver paint, to completely eliminate the sample-electrode contact resistance.
Scanning Electron Microscopy (SEM) studies of powders were performed using a Jeol-JSM 5400 machine, at an accelerating voltage of 10 kV. Samples were gold sputtered prior to observation.
RESULTS AND DISCUSSION
Aniline and DBSA at a stoichiometric ratio react to form a fine dispersion of solid needle-like anilinium-DBSA complex, which upon the addition of an oxidizer (ammonium peroxydisulfate) polymerizes to form a PANI-DBSA stable dispersion in an aqueous medium. In this polymerization method DBSA, a functionalized protonic acid, acts as a surfactant and a dopant and also provides the acidic character to the polymerization medium.
Table 1 depicts the dispersion's color change during the polymerization process. The polymerization process of anilinium-DBSA in an aqueous medium is characterized by a typical color change from white (anilinium-DBSA complex in water) through blue (after an induction period of 90-120 mm) and turquoise to dark green. UV-VIS studies have indicated that the average oxidation state of the formed PANI chains vary during polymerization from doped pernigraniline to doped emeraldine and correlate with the dispersion's color . Thus, monitoring the color changes during the polymerization of anilinium-DBSA offers a satisfactory quantitative and simpler way to follow the polymerization progression. The polymerization of anilinium-DBSA is characterized by a relatively long induction period compared to anilinium-HC1 polymerization, which is characterized by 10-20 min induction period [13, 14]. Haba et al.  have described that in the initial stage of PANT polymerization, the average oxidation state is of pernigrani line (characterized by a blue color), in accordance with results of MacDiarmid et al. . When the dispersion's color turns to turquoise (superposition of blue and green), some of the PANI chains are still in the pernigraniline oxidation state, and some have already proceeded to the emeraldine oxidation state. As the polymerization process progresses, the pernigraniline oxidation state is being converted into the emeraldine oxidation state (characterized by the emerald green color). It should be noted that mica presence within the polymerization medium significantly accelerates the anilinium-DBSA polymerization process by reducing the typical long induction period (reduction of [sim]1 order of magnitude) and also by reduction of the total polymerization time. The PANI-DBSA/mica ratio used in this procedure is 25/75 respectively. By some pretreatment procedures of the mica prior to the anilinium-DBSA polymerization, a further reduction in the induction time period and in the total polymerization time is achi eved. An ultrasonic bath treatment (Procedure 3) significantly affects the PANI-DBSA polymerization process. A color change to pale blue is observed already during the addition step of the ammonium peroxydisulfate (APS), contrary to an induction time of [sim]2 h in ordinary anilinium-DBSA polymerization. The final color change to dark green is observed already 30 min after the APS addition and thus a much faster polymerization process of anilinium-DBSA is realized. The hydrophilic nature and swelling capacity of mica, its extensive stirring in water, and the ultrasonic treatment step cause some disintegration of the mica aggregates and thus an increase of the total surface area exposed to the reactants.
It is thus suggested that in the early stages of polymerization, the PANI particles formed deposit onto the mica particles' surface, which has a catalytic effect on the polymerization reaction. Most silicates, including mica, have a negative surface charge. Since the PANI chains are polycations, the resulting attractive electrostatic interactions may play a role in the polymerization coating process by PANI of the mica particles. Further oxidation of anilinium-DBSA is autoaccelerated by the already existing PANI coating , i.e. the presence of PANI promotes the polymerization of aniline. Because of this autoacceleration effect, polymerization continues to take place preferentially upon the surface of the mica particles. Thus, a location preferred polymerization process on the surface of the mica particles, rather than random deposition of PANI-DBSA particles throughout the whole reactor's volume, is observed.
The electrical conductivity of compression molded samples prepared by the different polymerization procedures is summarized in Table 2. The PANI-DBSA materials polymerized in the presence of mica have lower conductivities ([sim]1 order of magnitude lower) compared to the neat PANI-DBSA (Table 2). The differences of the electrical conductivity values among the PANI-DBSA/mica materials are relatively small and the conductivity of the ultrasonically pretreated PANI-DBSA/ mica material (Procedure 3) is the closer to the reference PANI-DBSA.
Figure 1 shows typical SEM micrographs of the PANI-DBSA/mica materials obtained by the different polymerization procedures. Micrographs of reference neat mica before polymerization (Fig. la) and reference neat PANI-DBSA powder (Fig. 1b) are also shown for comparison. The original mica aggregates (Fig. la) are flake-like and consist of stacked units without regular features. Relatively sharp edges and corners and a wide particle size distribution characterize most of the original mica particles. However after polymerization, these particles change their original shape and are covered with a layer of polymer (PANI-DBSA) (Fig. 1c, d). The sharp edges of the mica particles have disappeared and a smooth surface morphology, typical of the neat PANI-DBSA (Fig. 1b), is observed. By dispersing and stirring the mica powder in water for 24 h prior to polymerization (Procedure 2), a slight increase in the surface area of mica results (Fig. 1e, f), owing to some breakage by the intensive mixing. A significant surface are a increase due to breakage and delamination is achieved by the ultrasonic bath treatment (Figs. 1g, h.).
The muscovite mica used in this study contains metallic ions, such as [Al.sup.+3], [Fe.sup.+2]. Polymerization experiments of anilinium-DBSA in the presence of [Fe.sup.+2] ions have shown a significant catalytic effect on the polymerization process, presumably through a redox-type reaction, similar to the acceleration phenomenon observed by the mica particles (Table 1). Other experiments have shown that the presence of [Al.sup.+3] ions within the polymerization medium did not significantly accelerate the polymerization process of anilinium-DBSA (Table 1). The electrical conductivity of PANI-DBSA polymerized in the presence of [Fe.sup.+2] ions is similar to that of neat PANI-DBSA, while the conductivity of PANI-DBSA polymerized in the presence of [Al.sup.+3] ions is slightly lower. These results prove that the catalytic effect of mica on the polymerization kinetics of anilinium-DBSA is actually caused by the of [Fe.sup.+2] ions present in mica. This catalytic effect causes polymerization to occur preferential ly on the mica particles' surface in the first polymerization stage. Thereafter, polymerization further proceeds onto the mica particles' surface to thicken the already existing thin PANI-DBSA shells by the autoacceleration effect (PANI affecting anilinium-DBSA polymerization). This combined catalytic/autoacceleration effect is the mechanism causing mica particles' encapsulation with PANI-DBSA shells without formation of separate PANI-DBSA particles. In fact, this approach was utilized in the past to coat PS particles with metal shells (2). It is thus suggested that the catalytic/autoacceleration mechanism can be generalized to include various coated particles (coatings and particles) through catalyzing a given particle surface prior to polymerization.
Polymerization of anilinium-DBSA in the presence of talc particles has also shown a similar behavior regarding the accelerated polymerization kinetics. Morphological studies have indeed confirmed that a layer of PANI-DBSA covers the talc particles surface without the formation of separate PANI-DBSA particles.
In summary, the preliminary study presented in this article shows that polymerization of anilinium-DBSA in an aqueous dispersion takes place preferentially on the mica and talc particles' surface owing to the presence of [Fe.sup.+2] ions in the crystal lattice of these clays, which serve as catalyzing centers. The thin PANI shells formed enhance further aniline polymerization upon these shells by an autoaccleration effect of aniline polymerization in the presence of PANI. Parameters of the encapsulation process and the catalytic/autoaccelration mechanism will be further studied in future experiments, which will also include implementation of the idea in other systems of particles (quartz, glass beads and fibers, carbon black, etc.) undergoing encapsulation by PANI-DBSA.
(*.) Corresponding author.
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|Author:||SEGAL, E.; AVIEL, O.; NARKIS, M.|
|Publication:||Polymer Engineering and Science|
|Date:||Aug 1, 2000|
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