Elsevier

Progress in Organic Coatings

Volume 101, December 2016, Pages 440-454
Progress in Organic Coatings

Conductive polypyrrole and acrylate nanocomposite coatings: Mechanistic study on simultaneous photopolymerization

https://doi.org/10.1016/j.porgcoat.2016.07.023Get rights and content

Highlights

  • Conductive nanocomposite coating of polypyrrole and acrylate polymer was prepared.

  • A method involving simultaneous photopolymerizations was implemented.

  • The conductive and binding properties of the nanocomposite coating were optimized.

  • The morphology and dispersion of PPY/Ag particles in acrylate polymer were studied.

Abstract

Facile preparation of conductive polypyrrole coatings with optimized binding and conductive properties for various electronic applications such as printing circuit boards and conductive inks using simultaneous photopolymerization method was investigated in this article. This method involves independent polymerization processes of pyrrole and acrylate monomers occurring simultaneously to produce conductive coating. While AgNO3 was used as oxidizing agent for photopolymerization of pyrrole, Irgacure 907 was used as the photoinitiator for the acrylate monomer, 1,6-hexanediol diacrylate. Based on Real-time Fourier Transform Infrared (RT-FTIR) spectroscopy characterization on polymerization rates and mechanisms, it was deduced that the selection of proper initial concentrations of monomers and their corresponding photoinitiators was essential to obtain optimized conductive and binding properties of the final coating. In addition, microscopic techniques such as TEM and SEM revealed that the size, morphology and dispersion of polypyrrole were also affected by the initial concentrations. Finally, conductivity measurements obtained via four-point probe method and conductive AFM were correlated with the spectroscopic and microscopic characterization to determine the optimal conditions for the preparation of conductive polypyrrole/acrylate coatings via this method.

Graphical abstract

Conductive nanocomposite coating of polypyrrole and acrylate with optimized conductive and binding properties was synthesized in a single step simultaneous photopolymerization.

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Introduction

Intrinsically electronic conductive polymers have attracted great attention for their potential applications in printed circuit boards, flexible electronics, organic thin film transistors (OTFTs), sensors and various electronic devices due to their lower weight than metals, ease of preparation, and a wide range of controllable properties [1], [2], [3], [4], [5]. The past few decades have seen significant development in this field in terms of fundamental scientific research and many industrial applications. Several attempts have been made to fabricate micro-patterns, structures of conductive polymers, and their coatings on various substrates via chemical oxidation and electropolymerization processes [6], [7], [8], [9]. While some of the advantages that electropolymerization method offers are controllability on thickness of coatings and conductivity; the advantages of chemical oxidation method are feasibility of large-scale production and variability of precursors [10]. However, solubility of conductive polymers has been a major issue in both these methods in order to obtain thin films with improved adherence to various substrates.

The field of photo-induced chemistry offers a wide variety of tools that are useful in obtaining desired chemical transformations with an application of light for a broad range of applications. Among the photo-induced chemical reactions, photodoping and photopolymerization are notable techniques for the deposition of metals into polymers [11], [12]. Photodoping involves the incorporation of metals such as silver into various materials such as glass and polymer upon illumination of light. During the process, the charged metallic species migrate into the materials under the influence of intrinsic electric fields caused by the exposure of light. Photopolymerization is a process in which a liquid transforms into solid by the irradiation of generally UV light or electron beam. This phenomenon causes significant changes to physical properties of material such as viscosity, solubility, adhesion, color, and electrical conductivity. This technique for material synthesis has been found highly useful in obtaining microfabrication and coatings directly onto the substrate where there are limitations such as temperature and solubility. Complex nano- and micro-sized patterns and structures have been fabricated using photopolymerization onto printed circuit boards, miniature wiring boards, and biomaterial scaffolds [13], [14].

Recent studies on photopolymerization of pyrrole in the presence of electron acceptors, for example silver salts, excited by UV light have shown interesting results for the incorporation of metal nanoparticles into polypyrrole [15], [16], [17]. Previously, some reports on multiphoton-sensitized polymerization of pyrrole, self-sensitized photopolymerization of pyrrole, and photopolymerization of pyrrole using ruthenium, cobalt, ferrocene and copper complexes as electron acceptors were published [18], [19], [20], [21]. The photopolymerization method has been successfully used to manufacture composites of polypyrrole/metal nanoparticles for applications such as gas sensors and humidity sensors [22], [23]. However, so far very few reports have been published on the use of photopolymerization because of lower yields and inferior conductivity of polypyrrole obtained in this method as compared to chemical and electrochemical methods. One of the major advantages of photopolymerization is that it can be readily applied with the ease of design and control to make polymer coatings directly onto conducting and non-conducting substrates.

In this paper, the preparation of conductive polymer coatings of polypyrrole and acrylate via simultaneous photopolymerization was reported. This method facilitates the fabrication of conductive polymer coatings for various applications such as printing conductive inks and semiconductor devices. In this method, the reaction mixture contains pyrrole with its corresponding photo-activated oxidant and acrylate monomer with its corresponding photo-initiator. This reaction mixture will be converted into a hybrid coating through photopolymerization processes of both pyrrole and acrylate occurring simultaneously. While polypyrrole serves as conductive path link, acrylate polymer acts as binder to bind polypyrrole particles as well as strongly adhere to the substrate. Due to the fact that it is a single step process, it minimizes total preparation time and eliminates the processibility of conductive polymer. In this work, Real-time Fourier Transform Infrared (RT-FTIR) spectroscopy was used to investigate polymerization conversion of pyrrole to polypyrrole in the presence of UV activator, AgNO3. Subsequently simultaneous photopolymerization, which is the combination of polypyrrole formation and acrylate polymer formation occurring together in a reaction mixture, was also studied using RT-FTIR. Finally, coatings were made on aluminum substrate and characterized using four-point probe method and conductive AFM in order to measure conductivity and surface roughness of coatings. Further, coatings prepared on epoxy substrate were analyzed using transmission electron microscope (TEM) and scanning electron microscope (SEM).

Section snippets

Materials

Pyrrole was purchased from Sigma Aldrich, distilled under vacuum, and stored in refrigerator around 0 °C prior to use. AgNO3 and methanol were also purchased from Sigma Aldrich. 1,6-hexanedioldiacrylate (SR238) and Irgacure 907 were obtained from Sartomer. EPO-TEK® 377 purchased from Epoxy Technology, Inc., (USA) of the dimensions of 1 × 1 inch was used to prepare epoxy substrates for the coating applications.

Investigation of polymerization conversions using real-time FTIR

Real-time FTIR (RT-FTIR) technique was used to determine the polymerization conversion

Photopolymerization of pyrrole

Real-time Fourier Transform Infrared (RT-FTIR) spectroscopy is one of the most valuable techniques in measuring the polymerization rates qualitatively by monitoring the changes in the IR absorption characteristics of the reactive groups such as acrylates, methacrylates, epoxies, double bonds and thiol groups. In addition, it can further be used to calculate the degree of conversion at any time during the polymerization process. It has often been effectively used to investigate the kinetics of

Conclusion

An attempt was made to prepare conductive polypyrrole coatings with optimized binding properties onto various substrates such as aluminum, glass, and epoxy via simultaneous photopolymerization of pyrrole and UV curable acrylate monomer. In order to match the polymerization processes of pyrrole and acrylate in the final coatings, their individual photopolymerization processes were investigated using RT-FTIR. It was found that the increase in AgNO3 concentration in the formulations with

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements

The authors would like to thank the U.S Army Research Laboratory under grant numbers W911 NF-04-2-0029, W911NF-09-20014, W911NF-10-2-0082, and W911NF-11-2-0027 for supporting this research.

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