Synthesis and characterization of polyaniline-silica composites: Raspberry vs core-shell structures. Where do we stand?

Abstract

The synthesis of polyaniline-silica composites has been reinvestigated in view of the opposing results found in the literature. Firstly, we synthesized silica particles with tunable size using the Stöber process. These silica particles have been fully characterized before being used as solid support for the polymerization of aniline. This polymerization was performed according to a published procedure where the pH of the reaction mixture was below the pKa of aniline but at a value where the silica particles surface was still slightly negatively charged. The objective of this procedure was to favor electrostatic interactions between anilinium cations and the silica surface to lead to the formation of silica-polyaniline core-shell particles. Several sets of nanocomposites were prepared under different experimental conditions (oxidant/aniline ratio, silica concentration, temperature, silica particles diameters). The study evidenced that under all the conditions used the formation of core-shell nanoparticles is impossible. However, using different particle sizes, noticeable morphological differences were observed. The use of large silica particles led to the formation of non-uniform polyaniline-silica composites whereas the use of smaller particles always led to raspberry-like morphology as reported by other groups in highly acidic media. The difference in morphology led to different electrical properties with electrical conductivities measured at room temperature ranging from 1 .6× 10⁻³ to 2.5 × 10⁻⁵ S cm⁻¹.

Introduction

Organic-inorganic hybrid materials have received much attention in recent years [1]. Indeed, the resulting materials may present unique chemical and physical properties coming from the synergistic effect of both components. More particularly, the preparation, characterization and applications of polymer-silica composites are the most commonly reported in the literature [2]. Silica nanoparticles (NPs) exhibit interesting properties, such as good thermal and chemical stabilities, have tunable size and can be easily chemically modified. Intrinsically conducting polymers such as polypyrrole, polyaniline (PANI) or polythiophene can exist in different oxidation states and respond to external stimuli by changing one of their characteristics (color, conductivity, permeability) [3]. In particular, PANI is a non-toxic, thermally stable and low cost polymer with relatively high conductivity that has been used as antistatic coating, electrode materials, corrosion inhibitor and sensors [4], [5]. However, PANI is insoluble in most common solvents which renders its processing difficult. One of the objectives is to reduce product cost without losing the desired properties of the components. Moreover, for some applications, like ink-jet printing, stable aqueous dispersions are needed. Armes and collaborators were the first to report the synthesis and characterization of colloidal polyaniline-silica composites by chemical oxidative polymerization of aniline in the presence of colloidal silica (D ≈ 20 nm). The submicronic aggregates formed (D = 250 ± 50 nm) showed raspberry morphology in which the silica particles were glued together by the PANI [6], [7], [8], [9], [10]. At the meantime, it was shown that polymerization of aniline at surfaces (silica gel, polymer particles) occurred before precipitation polymerization in the bulk. Based on the concept of heterogeneous catalysis this has been attributed to higher reactivity of adsorbed molecules compared to that occurring in the bulk. The adsorption of oligomers produces the initiation centers which start the growth of PANI chains [11], [12], [13], [14]. Moreover, there have been some evidences that the presence of insulating silica does not affect significantly the electrical properties. Indeed, the incorporation of inorganic materials such as silica into PANI has even been shown to enhance conductivities [15], [16], [17]. PANI-silica nanocomposites have also been used as electrorheological fluids showing higher yield stress, higher polarizability and faster response to electric field compared to pure PANI [18], [19], [20].

Core-shell structures based on a silica core and a PANI shell were firstly reported by Armes and collaborators in 1991 [21]. In this study, PANI-silica composites were prepared by in-situ polymerization of aniline in the presence of micrometer silica particles using a low oxidant and monomer concentration to slow the reaction down and to promote polymerization on the surface rather than in the bulk. In this case, SEM micrographs have shown that the silica particles were not uniformly coated with polyaniline. In 2006, Jang and collaborators described the formation of SiO₂-PANI core-shell nanoparticles less than 30 nm in diameter with a thin PANI layer (2 nm) obtained by in-situ polymerization [22]. Later on, the same authors obtained core-shell structures using self-stabilized dispersion polymerization carried out in an aqueous/organic liquid system at −30 °C. Particles with diameter ranging from 18 to 130 nm were obtained depending on the silica NPs used. The polymer chains were grown parallel to the silica resulting in an enhancement of electrical conductivities [17]. Note that other examples of core-shell SiO₂-PANI particles can be found in the literature but in these cases, a chemical grafting of the monomer on the silica surface was performed prior to polymerization [23]. Recently, two groups have reported the synthesis of monodisperse mesoporous carbon nanospheres (MCNs) by combining polymerization of aniline with co-assembly of colloidal silica nanoparticles. Starting from commercial SiO₂ nanoparticles (7–42 nm) they performed the polymerization of aniline and obtained polyaniline-silica nanospheres where SiO₂ NPs were uniformly embedded in the PANI matrix. After carbonization and removal of the SiO₂ template, MCNs with the desired mesopores were obtained [24], [25].

Taking this literature into account, it seems that polymerization of aniline in the presence of small silica NPs has led to discrepancy results. Armes, Müllen and Asefa’s works look much alike [21], [24], [25]. In their work, raspberry like structures were obtained while Jang and collaborators described the formation of core-shell SiO₂-PANI particles [22]. This latter result was rather intriguing to us [22]. Indeed, the formation of the core-shell structure was ascribed to the fact that anilinium monomer adsorbed onto negatively charged silica surface. The synthesis was performed at pH = 3 and the authors claimed that the zeta potential of Ludox TM-40 silica at this pH was −50.3 mV. This value is much higher than those classically observed with this kind of particles (ξ = −5 mV) [24], [26]. In the present paper, we performed an in depth study of the chemical polymerization of aniline in the presence of silica particles using a procedure close to the one described by Jang and collaborators [22]. In most studies, commercial silica sols have been used. However, these sols have been stabilized by adding counter-cation like NH₄⁺, Na⁺, Li⁺ at pH where they are negatively charged. The amount and nature of the stabilizers used are sometimes difficult to determine. Therefore, we decided to prepare our own silica particles of different sizes (D_h ≈ 90 and 300 nm) by Stöber process and to characterize them carefully. Then, we performed the polymerization of aniline in the presence of silica particles under different experimental conditions (temperature, concentration of aniline, concentration of silica, particles size). The resulting PANI-silica composites were finally characterized by FT-IR, SEM observations and electrical conductivities measurements.