Formation of novel polymeric films derived from 4-hydroxybenzoic acid

https://doi.org/10.1016/j.matchemphys.2011.03.053Get rights and content

Abstract

This work reports electrochemical and morphological studies of formation of poly(4-hydroxybenzoic acid), prepared in different pH, on the graphite surface. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), electrochemical quartz crystal microbalance (EQCM) and atomic force microscopy (AFM) have been carried out to study the modified electrodes formed. The electrodes modified with poly(4-HBA), prepared in pH 0.5 and 7.0, presented oxidation/reduction peaks, but no peak was detected to modified electrodes prepared in pH 12.0. Voltammetric studies showed decrease in oxidation/reduction currents and slower electron transport across the polymer for all pH values, however, the electron transport was higher when the polymer was prepared in acid medium. EIS analysis indicated that the charge transfer resistance for poly(4-HBA) electropolymerized at pH 12.0 was about 2 and 1.4 times higher when compared to pH 0.5 and 7.0, respectively. Studies through EQCM showed higher amount of polymer mass deposited in acid medium. Images by AFM indicated that the topography is affected by pH value, whereas films prepared in acidic pH conditions presented higher roughness.

Highlights

► Graphite electrodes modified with poly(4-hydroxybenzoic acid) prepared in pH 0.5, 7.0 and 12.0. ► Electron transport was higher in poly(4-hydroxybenzoic acid) prepared in acid medium. ► Poly(4-HBA) prepared in pH 12.0 presents higher charge transfer resistance. ► Polymers prepared in pH 0.5 and 7.0 present globular morphology and in pH 0.5 higher roughness.

Introduction

Polymers are an important class of materials with electrical properties that allow a wide range of electronic, optoelectronic and biotechnological applications such as in rechargeable batteries [1], molecular electronics [2], electronic displays [3], solar cells [4], ion exchange membrane in fuel cells [5], diodes [6], capacitors [7], field-effect-transistors [8], printed circuit boards [9], chemical sensors [10], drug release systems [11], biosensors [12] and others.

Conducting polymers present unusual properties such as electrical conductivity, low ionization potential, low-energy optical transitions and high electron affinity due to the charge mobility along the backbone of the polymer chain promoted by the delocalization of the electrons based on overlap of the orbital of successive carbon atoms with π bonding [13]. On the other hand, non-conducting polymers present high resistivity, permselectivity and self-limited growth. These polymers show properties that make them materials with interesting properties such as increase of the electrocatalytic properties for the development of analytical devices and support matrix for immobilization of biomolecules [14], [15].

The production of polymers may be by chemical or electrochemical process. The electrochemical synthesis of polymers has become a very attractive route since it is fast and easily handled method. It also presents high reproducibility and stability, thickness control and the reactions can be carried out at room temperature and ambient pressure [16], [17], [18], [19]. Insulating or conducting polymers can be preformed by electrochemical doping/dedoping, and the conformation of electroactive polymers can be controlled by electrochemical processes, producing dramatic effects on the morphology of the polymer films [20].

In addition, polymeric coatings can be applied to a wide range of electrode materials, including graphite, glassy carbon, Au and Pt. Electrodes covered with polymers, besides increasing the flexibility of choosing of the material, allowing to obtain higher surface coverage and increasing the amount of electroactive material attached to the surface [21].

Electropolymerization studies in solutions with different pH values are important in order to produce different polymeric films on the electrode surface. For example, the electrooxidation of phenols and derivatives in basic medium occurs at potentials more cathodic than in acid medium, due to increase in the electronic cloud by the presence of an oxygen atom in phenoxide form, increasing the resonance effect on the aromatic ring [14], [16], [22], [23], [24]. The presence of oxygen atoms bonded directly to an aromatic ring in the monomer facilitates the polymerization process [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25] and the polymer obtained presents mechanical resistance and high stability.

To the best of our knowledge, this is the first report on the electrochemical polymerization of 4-hydroxybenzoic acid (4-HBA) in graphite electrode, at different pH conditions. Cyclic voltammetry, electrochemical impedance spectroscopy, electrochemical quartz crystal microbalance and atomic force microscopy have been combined to study this electropolymerization.

Section snippets

Reagents

The monomer 4-hydroxybenzoic acid (Acros Organics, 99%) (2.50 mmol L−1) was prepared in HClO4 (Merck, 98%) solution (0.5 mol L−1). The adjustment of pH was carried out using NaOH solution. All solutions were prepared freshly, before each electropolymerization, and deaerated with N2 for ca. 40 min. All chemicals were of analytical grade and used without further purification, using highly purified water (18.2  cm) from Master System (GEHAKA).

Formation and electrochemical/morphological characterization of poly(4-HBA)

A conventional three-electrode electrochemical cell, with a

Electrochemical behavior of 4-hydroxybenzoic acid monomer in different pH values

Fig. 1 presents the effect of the pH on the electrochemical behavior of 4-HBA on graphite electrode.

The inset of Fig. 1 shows that a protonated/depronotated process in 4-HBA can occur due to variation of the pH conditions. In highly acidic medium, 4-HBA has the carboxyl group protonated (structure I). With the increase of pH values occurs the deprotonation of the carboxyl group originating the structure II. In highly basic medium, above of the pK2, –OH and –COOH are deprotonated (structure III).

Conclusions

This paper presents, for the first time in literature, the electropolymerization of 4-hydroxybenzoic acid in different pH values. The electrochemical oxidation of 4-hydroxybenzoic acid, investigated by means of cyclic voltammetry and electrochemical quartz crystal microbalance, produced polymers onto graphite surface.

The electrodes modified with poly(4-HBA), prepared in pH 0.5 and 7.0, presented oxidation/reduction peaks, but no peak was detected in the modified electrode prepared in pH 12.0.

Acknowledgements

The authors are grateful for the financial support fromConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). Also, we would like to thank teacher Abílio Borghi for the review of the English manuscript.

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