Elsevier

Synthetic Metals

Volume 110, Issue 1, 15 March 2000, Pages 17-23
Synthetic Metals

Preparation and characterization of conductive polyaniline/silica hybrid composites prepared by sol–gel process

https://doi.org/10.1016/S0379-6779(99)00176-9Get rights and content

Abstract

A method had been developed to cast organic/inorganic hybrid composites by sol–gel process. This procedure was mainly controlled by the pH of mixing circumstances of organic/inorganic multicomponent solutions and was attributed to the solvent pH. Cast composite from m-cresol solution could not form a uniform hybrid composite film due to the indistinguishable growth and aggregation of silica at pH<7 in sol–gel processing. On the other hand, hybrid composite cast from N-methyl-2-pyrrolidone (NMP) solution could form a uniform, crack-free film because of its stable silica particle growth according to the mutual repulsion at pH>7. But bulk conductivity from m-cresol solution was higher than that from NMP solution, which resulted from the secondary-doping effect of the solvent. These conformational changes of the polymer chain and ability to form hybrid composite film were analyzed by means of Fourier-transform infrared (FTIR), wide-angle X-ray diffraction (WAXD), microdielectrometer, thermogravimetric analysis (TGA), and conductivity measurements.

Introduction

It is generally recognized that polyaniline is the foremost air-stable organic conducting polymer [1] and has the great potential for commercial applications because of its many promising properties [2].

However, because of its poor processability, it has not yet met its requirements for a wide range of commercial applications. Many researchers tried to find a solution for better processability such as mechanical blending [3], solution blending [4], electrochemical deposition [5], and chemical surface deposition [6]. The solution blending method, one of the promising methods, is solubilizing polyaniline in a variety of solvents (including non-polar solvents) with counterions used for doping polyaniline. Using this concept, a number of polymers which cannot be co-processed with polyaniline have the opportunity for solution blending and its composites have been made with a variety of commercial bulk polymers. These composites have complement properties of each polymer, polyaniline and the other bulk polymers, for better mechanical, physical and chemical properties. Most of all, composites incorporated by inorganic components instead of organic polymers especially have enhanced modulus, transparency, surface hardness and heat resistance [7].

During the last few years, the sol–gel process, with its associated mild conditions, offers a new approach to the synthesis of organic/inorganic composite materials with domain sizes down to the molecular size. Organic/inorganic composites can be prepared by dissolving preformed polymers into sol–gel precursor solutions followed by hydrolysis and condensation to form three-dimensional network phase of different morphological structures [7]. The polymer molecules are dispersed in the three-dimensional network of metal oxide such as silica. After evaporation of the solvent in the precursor solution, the gels are prepared, the pore size and the structures of which are determined by pH condition at the hydrolysis–condensation reaction.

So far, the conjugated conductive polymers such as polyaniline, polythiophene and polypyrrole have been examined to make hybrid composites, but these have been only specified in particle type. Therefore, we introduced the polyaniline/silica hybrid composite films by using sol–gel process at two different circumstances by varying the pH of the solvent for the future use in transparent electromagnetic shielding materials. The obtained hybrid films are characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD) and dielectrical analysis. The conductivity was also investigated.

Section snippets

Experimental

Aniline (Aldrich) was purified by distillation in vacuum before use and ammonium peroxydisulfate [(NH4)2S2O8] was used to polymerize the aniline. Camphorsulfonic acid (CSA), m-cresol, N-methyl-2-pyrrolidone (NMP), tetraethyl orthosilicate (TEOS, Aldrich), and oxalic acid were used as received.

Emeraldine base form of polyaniline was prepared based on the procedures reported by Wei et al. [8]. Finely ground emeraldine base powder was added to the solvent (NMP) to make 1 wt.% solution and

Conductivity study

The bulk conductivity of the polyaniline/silica sol–gel composite increases with increasing amount of polyaniline component, as shown in Fig. 1. The bulk conductivity rapidly increases with polyaniline content up to 20%, and then gradually increases with polyaniline contents in both composites. The camphorsulfonic acid-doped polyaniline, as cast from m-cresol, had a much higher conductivity than the one cast from NMP. That can be explained according to the so-called `secondary-doping-effect'

Conclusions

Organic/inorganic hybrid composite films have been made with polyaniline and silica by sol–gel process. Polyaniline/silica hybrid composite film was completed under NMP condition because NMP played an important role in the formation of stable silica network and hydrogen bonding interaction with polyaniline. On the other hand, m-cresol had no effect on the formation of stable silica network, so polyaniline did not hybridize with silica in the circumstance of m-cresol in contrast to that of NMP.

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