Polyaniline/thermoplastic polyurethane blends: Preparation and evaluation of electrical conductivity
Introduction
Intrinsically Conducting Polymer (ICPs) are promising materials for various applications. Polyaniline (PAni) is a conducting polymer which has been extensively studied due to its environmental stability and simple methods of synthesis. Unfortunately, the poor processability of PAni and its inadequate mechanical properties limit its commercial applications. In order to overcome these problems, numerous methods have been studied [1], [2].
Techniques based on the dispersion of conducting polymer particles in a matrix comprised of common insulation polymers have been receiving increasing attention and are very attractive due to the possibility of combining the good processability and mechanical performance of the conventional polymer with the electrical and optical properties of PAni. However, polyaniline is usually immiscible when blended with other polymers, and a phase separation process restricts the formation of more integrated materials. If the compatibility of these blends were enhanced then the electrical properties would potentially increase. Less phase separated blends lead to materials with enhanced mechanical and electrical properties. Methods to improve compatibility in conducting polymer blends include the use of compatibilizers such as block copolymers or ionic polymers [3], [4], [5]. Another way to increase compatibility is to insert counter ions, such a functionalized protonic acids, between the polyaniline (PAni) chains making it possible to form blends with a conventional polymer. An example of such a functionalized acid is dodecylbenzenesulfonic acid (DBSA), which has long alkyl chains that increase the solubility of PAni.DBSA in toluene, xylene, etc., and acts as a surfactant, inducing compatibility with polymer matrices with a similar structure [6]. These blends are normally obtained by mixing soluble or fusible polyaniline with other polymers in solutions or melts [7], [8], [9], [10]. Blending of PAni with conventional polymers through solution processing has been reported to be efficient when both polymers are soluble in a common solvent.
The “in situ” emulsion polymerization of aniline (Ani) in the presence of a host insulation polymer is an interesting route through which more compatible conducting blends can be obtained. In this method, the oxidative polymerization of aniline is carried out by introducing an aqueous solution of an oxidant (ammonium persulfate) into an organic emulsion of the host polymer, aniline and a protonation agent. The blends are obtained by subsequent co-precipitation of the PAni and the host polymer.
The use of a thermoplastic elastomer with conducting polymers is very attractive due to the combination of mechanical properties and a processability which does not require vulcanization. Thermoplastic polyurethane (TPU) is one of the most versatile products in the group of engineering thermoplastics with elastomeric properties. TPU is characterized by a two-phase morphology in which a soft phase containing either polyesters or polyethers is reinforced by condensation with a hard domain consisting of an aromatic diisocyanate extended with a short-chain diol. The excellent physical properties, chemical resistance and abrasion resistance of TPU, as well as its ease of processing, make it of interest to blend TPU with other polymers, such as polyaniline. Several conducting polymer blends have been prepared by “in situ” emulsion polymerization of aniline in the presence of a thermoplastic elastomer [11], [12], [13], [14], [15], [16], but none of these preparations employed thermoplastic polyurethane as the host matrix.
This study investigated the influence of the blend preparation method on the electrical conductivity and morphology of the blend. To this aim, we prepared PAni.DBSA/TPU blends through two methods, solution casting and “in situ” emulsion polymerization.
Section snippets
Materials
Aniline (Ani) (analytical grade, Merck) was distilled twice under vacuum and stored under nitrogen with cooling. Ammonium peroxydisulfate (APS) (analytical grade, Merck) and dodecylbenzenesulfonic acid (DBSA) (technical grade, Pró-Quimica do Brasil) were used without further purification. The thermoplastic polyurethane (TPU – Desmopan 385 – ester based) was kindly supplied by Bayer.
The dodecylbenzenesulfonic acid aniline salt (DBSAn) was prepared by mixing DBSA and aniline, with a
Characteristics of Pani.DBSA samples used in the blends
The emulsion polymerization of dodecylbenzenesulfonic acid aniline salt (DBSAn) in an organic medium, such as chloroform, toluene, and other solvents [17], opened new possibilities for the preparation of conducting blends via a one-step “in situ” method. Considering that THF is the common solvent for PAni.DBSA and TPU, it was used in this study as an organic medium in the emulsion polymerization of DBSAn with the aim of obtaining conducting PAni.DBSA/TPU blends through “in situ” or solution
Conclusions
Conducting PAni.DBSA/TPU free-standing films can be obtained by solution casting or “in situ” methods. TPU was immiscible with PAni.DBSA, resulting in a gross-phase separation in the blends with a low electrical conductivity prepared through the casting process. The different methods used to prepare the PAni.DBSA/TPU blends significantly affected the conductivity values and percolation threshold. The “in situ” blend preparation method was able to produce PAni.DBSA/TPU blends with enhanced
Acknowledgement
The authors would like to thank CAPES for the financial support.
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