Electrorheology of polyindole

doi:10.1016/j.polymer.2021.123448

Polymer

Volume 217, 5 March 2021, 123448

https://doi.org/10.1016/j.polymer.2021.123448 Get rights and content

Highlights

•
Polynidole particles were prepared using acetic acid and ammonium persulfate at different oxidant-to-indole mole ratios.
•
The sample prepared at the mole ratio [APS]/[Indole] = 1has the shortest response time to applied electric field.
•
A dependence of created chain-like structures on conductivity and dielectric properties was investigated.

Abstract

Polyindole particles were prepared by the oxidative polymerization, where ammonium peroxydisulfate was used as an oxidant at several mole ratios to indole. Prepared spherical micrometre-sized polyindole particles were further used as a dispersed phase in silicone-oil electrorheological (ER) fluids and their rheological behaviour was investigated in the presence and in the absence of an electric field. While different oxidant-to-indole mole ratios did not affect the size and morphology of the particles, their electric and dielectric properties were significantly changed. The highest ER effect was observed for the ER fluid based on the particles with low oxidant-to-indole mole ratio (1:1). Such system exhibited yield stress around 100 Pa at 10 wt% concentration of solid phase. Chain-like structures created upon an application of an electric field were further investigated using an optical microscopy, where significant disparities were found according to electric and dielectric properties of the particles. These findings clearly show that varying mole ratio between the indole and oxidant provides systems with tuneable ER effect.

Graphical abstract

Introduction

Conducting polymers, such as polyaniline [1] or polypyrrole [2], represent a class of materials for the applications especially in energy conversion devices. Electrical properties are of a prime interest but the electrochemical activity, responsivity or chemical performance come to the forefront. Ring-substituted derivatives have subsequently been prepared and also tested in this direction. In the case of pyrrole, the studies have further been extended to the oxidative polymerization of related heterocyclic compounds, viz., imidazole, benzimidazole and indole. If the conductivity of the oxidation products was determined, it has always been considerably lower compared with polypyrrole [3,4]. The polymeric character has not been unambiguously proved and oxidation products are rather oligomers only [5]. Despite these facts they have found notable and non-conventional uses.

Conducting polymers or their oligomers are commonly used among organic particles as a dispersed phase in ER fluids [6,7]. Among their highest advantage belongs their tuneable conductivity through doping processes and easy and inexpensive synthesis. Polyindole (PIn) is a representative of conducting polymers constituting of a benzenoid ring connected to a pyrrole one. Several papers dealing with ER performance of silicone oil suspensions based on PIn particles [8,9] and their composites with other polymers [[10], [11], [12]] or clays [8,13] have recently been published. Polyindole particles were found as a suitable material for ER fluids due to their tuneable conductivity, pronounced ER effect and high colloidal stability leading to long-term stability, which is important from application point of view. In all cases iron(III) chloride was used as an oxidant. Nevertheless, morphology, shape and size of PIn particles can be further controlled with other chemicals and surfactants present during the synthesis [14]. Polyindole particles used in electrorheology prepared in various ways have not been so far investigated. From above-mentioned materials, the oxidation products of indole were selected for this study. So far the preparation of polyindole has been reported to produce materials for the application in heterogeneous catalysis [15] or as electrocatalysts [16] and components for supercapacitor electrodes [17] or antibacterial materials [18]. Iron(III) chloride was used as a typical oxidant of indole in organic medium [3,4,10,16,19,20] but other oxidants as well as aqueous media have also been recently exploited [5,15,17,18,21]. It was proposed that the indole molecules are linked through α and β positions of pyrrole moiety [4,5,[17], [18], [19],[22], [23], [24]] (Fig. 1) or with the help of the positions on an adjacent benzenoid ring [16,20].

In the present contribution, PIn prepared under various oxidant-to-monomer mole ratios have been considered for the application in electrorheology. The PIn-based compositions have already been investigated in this direction [9,10,25] but, according to our best knowledge, the behaviour of PIn prepared in this specific way, have not yet been addressed as dispersed phase in ER materials. Such application requires the particles polarizable in electric field but, at the same time, their limited conductivity is essential [[26], [27], [28]]. When suspended in insulating liquid, the suspension viscosity can be controlled by the application of electric field as a result of chain-ordering by induced dipole–dipole interactions [29]. The optimized conductivity of particles is required to reduce the currents drifting through the suspensions and, consequently, improving the ER efficiency.

The present study is therefore focused on the preparation of PIn particles by the oxidative polymerization where ammonium peroxydisulfate (APS) was used as an oxidant at various oxidant-to-monomer mole ratios. Such synthesis leads to fabrication of PIn globules with various conductivity. After dispersion of the particles in silicone oil, the resulting ER fluids were investigated from the dielectric and rheological point of view in detail and they were observed to display the tuneable electro-responsive characteristic in the presence of external electric field.

Section snippets

Preparation of polyindoles

Indole 50 mmol (Aldrich) was dissolved in 150 g of 80% acetic acid (Lach-Ner, Czech Republic) and 100 g of demineralized water was added. Ammonium peroxydisulfate, p. a., 50–125 mmol (Lach-Ner, Czech Republic) was dissolved in a mixture of 100 g of 80% acetic acid and 100 g of demineralized water. The solution of APS was poured into the solution of indole at room temperature. The oxidant-to-indole mole ratio was chosen as [APS]/[Indole] = 1, 1.5, 1.75, 2, 2.25, and 2.5. The reaction immediately

FTIR spectra

In FTIR spectra of samples prepared with various mole ratios [APS]/[Indole] (Fig. 2) we observed an enhanced absorption in the region above 2000 cm⁻¹ characteristic for the presence of polarons in the conducting polymers. The band with maximum at 3400 cm⁻¹ belongs to the N–H stretching vibrations, and the peak at 1620 cm⁻¹ to the corresponding N–H deformation vibrations. They are both well distinguished in the spectrum of indole monomer. This supports idea that there are still N–H bonds on the

Conclusions

Polyindole particles were successfully prepared by the oxidation of indole with ammonium peroxydisulfate at various mole ratios. While the various mole ratios of oxidant and monomer did not affect the morphology and size of the particles, their electric and dielectric properties differed significantly, due to their various protonation. Particles were after deprotonation further used as a dispersed phase in electrorheological fluids. The highest shear stress at low shear rates (≈100 Pa) was

CRediT authorship contribution statement

Tomáš Plachý: Methodology, Investigation, Writing-original draft. Jan Žitka: Conceptualization; Methodology. Miroslav Mrlík: Writing-original draft. Funding acquisition; Supervision. Pavel Bažant, Markéta Kadlečková: Methodology. Miroslava Trchová:: Methodology. Jaroslav Stejskal: Writing–review & editing.

Declaration of competing interest

The authors declare that they have no competing interests.

Acknowledgment

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic, DKRVO (RP/CPS/2020/003).

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Electrorheology of polyindole

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of polyindoles

FTIR spectra

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgment

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