Regular ArticleSynthesis and characterization of polyaniline-silica composites: Raspberry vs core-shell structures. Where do we stand?
Graphical abstract
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 (Dh ≈ 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.
Section snippets
Chemicals
The following chemicals were purchased and used as received: tetraethoxysilane 98% (TEOS) from Acros, ethanol 99.7% from VWR, ammonia 35% (NH3) and ammonium persulfate (APS) from Acros Organics, Aniline 99.5% (Ani) from VWR, hydrochloric acid 1.0 M (HCl) from Fisher and Ludox® TM-40 colloidal silica from Aldrich.
Characterization
Scanning Electron Micrographs (SEM) were obtained using a HITACHI S4800 FEG HR Scanning Electron Microscope or a FEI Quanta FEG 200 apparatus without previous metallization of the
Synthesis and characterization of silica particles
Monodisperse silica particles were synthesized through the sol-gel method proposed by Stöber et al. [31] and Matsoukas and Gulari [32]. Two different concentrations of ammonia were used: 0.3 and 1.0 mol L−1. Fig. 1 shows SEM micrographs of the silica particles obtained using these two ammonia concentrations. As expected, the silica nanoparticles were all spherical and uniform in size. By counting 100 spheres through SEM observations mean diameter DSEM was estimated to be 285 ± 5 nm for silica
Conclusions
The initial objective of this work was to prepare SiO2-PANI core-shell particles. For this, we followed a reported procedure which used medium acidic pH below the pKa of aniline molecules and in a region where silica nanoparticles were slightly negatively charged to favor electrostatic interactions between anilinium cations and the silica surface [22]. The course of the polymerization was followed by pH and temperature changes. Using large particles, the temperature profiles resembled those
Acknowledgements
N.R. thanks the Ministère de l’Enseignement Supérieur et de la Recherche for a PhD scholarship. Mr Didier Cot from the Institut Européen des Membranes of the University of Montpellier is gratefully acknowledged for the SEM images reported here. The authors also thank Mr Maxime Guerineau from the laboratoire Interdisciplinaire Carnot de Bourgogne Dijon for his help with BET measurements. We are also thankful to Marie-Laure Léonard from ESIREM Dijon, France for obtaining the TGA results reported
References (45)
- et al.
Conducting polyaniline blends and composites
Prog. Polym. Sci.
(1998) - et al.
Conducting polymers prepared by oxidative polymerization: polyaniline
Prog. Polym. Sci.
(1998) - et al.
Zeta-potential measurements on conducting polymer-inorganic oxide nanocomposites particles
J. Colloid Interface Sci.
(1995) - et al.
In situ polymerized polyaniline films 4. Film formation in dispersion polymerization of aniline
J. Colloid Interface Sci.
(2002) - et al.
Fabrication of semiconducting polyaniline/nano-silica nanocomposite particles and their enhanced electrorheological and dielectric characteristics
Colloids Surfaces A – Physicochem. Eng. Aspects
(2011) - et al.
Enhanced electrorheological activity of polyaniline coated mesoporous silica with high aspect ratio
J. Colloid Interface Sci.
(2016) - et al.
Conducting polymer colloïdal silica composites
Polymer
(1991) Hartree-Slater subshell photoionization cross-sections at 1254 and 1487 eV
J. Electron Spectrosc. Relat. Phenom.
(1976)- et al.
Preparation of conductive PDDA/(PEDOT:PSS) multilayer thin film: influence of polyelectrolyte solution composition
J. Colloid Interface Sci.
(2014) - et al.
Controlled growth of monodisperse silica spheres in micron size range
J. Colloid Interface Sci.
(1968)
Dynamics pf growth of silica particles from ammonia-catalyzed hydrolysis of tetra-ethyl-orthosilicate
J. Colloid Interface Sci.
Adsorption and stability of colloidal silica
Colloids Surfaces A – Physicochem. Eng. Aspects
Preparation of uniform silica/polypyrrole core/shell microspheres and polypyrrole hollow microspheres by the template of modified silica particles using different modified agents
J. Colloid Interface Sci.
Novel strategy to prepare polyaniline-modified SiO2/TiO2 composite particles
Synth. Met.
Applications of advanced hybrid organic-inorganic nanomaterials: from laboratory to market
Chem. Soc. Rev.
Polymer/silica nanocomposites: preparation, characterization, properties, and applications
Chem. Rev.
“Synthetic metals”: a novel role for organic polymers (Nobel lecture)
Angew. Chem.-Int. Ed.
Particle-size distributions of polyaniline silica colloidal composites
Langmuir
Surface characterization of conducting polymer-silica nanocomposites by X-Ray photoelectron spectroscopy
Langmuir
Polyaniline dispersions.6. Stabilization by colloidal silica particles
Macromolecules
The carbonization of colloidal polyaniline nanoparticles to nitrogen-containing carbon analogues
Polym. Int.
In situ polymerized polyaniline films. 2. Dispersion polymerization of aniline in the presence of colloidal silica
Langmuir
Cited by (26)
Concentration-dependent construction of raspberry-like SiO<inf>2</inf> shell on spherical FeNi towards improved electromagnetic performance from kHz to GHz
2023, Journal of Materials Research and TechnologyPreparation of SiO<inf>2</inf>@EVA core–shell particles via post-emulsification method with polydisperse SiO<inf>2</inf> particle sizes and the critical thickness-to-diameter ratio in PA6/SiO<inf>2</inf>@EVA system
2023, Applied Surface ScienceCitation Excerpt :However, perfect core shell particles with accurate rubber shell thickness cannot be obtained in the previous works. Several SiO2 cores might be encapsulated in one rubber shell and raspberry-like or dumbbell-like structures may also be formed [20–22]. Although seed emulsion polymerization has been proved to be an effective way to accurately control the rubber shell thickness, but it can only be used with some specific monomers [9,10].
Mono and binary mixture removal of eriochrome black T and Cr(VI) from water by SiO<inf>2</inf>/polyaniline composite
2023, Materials Chemistry and PhysicsCitation Excerpt :The synthesis of SiO2/polyaniline composite is based on an electrostatic attraction between positively charged anilinium monomers and silica NPs. Roosz et al. [19] reported that the diameters of SiO2 NPs impact the final composite structure. A non-uniform PAni shell was obtained using the Large SiO2 NPs.
Use of hydrogen bonded layer-by-layer assemblies for particle manipulation
2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Tannic acid (TA) having a Mw of 1 701 g/mol was provided from Fischer Sci. Colloidal silica nanoparticles were purchased from AkzoNobel (BINDZIL, 30/220, 30%, d≈ 12 nm) or synthesized in our laboratory (r = 165 nm) by the method described in detail previously [49]. Borosilicate glass spheres (BSG) (r = 5 µm) were provided from SPI Supplies.
Conductive polymer-based composite photocatalysts for environment and energy applications
2022, Conjugated Polymers for Next-Generation Applications, Volume 1: Synthesis, Properties and Optoelectrochemical DevicesMesoporous silica decorated with L-cysteine as active hybrid materials for electrochemical sensing of heavy metals
2021, Journal of Environmental Chemical Engineering