Elsevier

Synthetic Metals

Volume 209, November 2015, Pages 304-312
Synthetic Metals

Photopolymerization of pyrrole/methacrylate mixtures using α-cleavage type photoinitiators in combination with iodonium salt

https://doi.org/10.1016/j.synthmet.2015.08.004Get rights and content

Highlights

  • Pyrrole (Py) was photopolymerized in mixtures with a methacrylate monomer.

  • α-Cleavage type photoinitiators were used in combination with an iodonium salt.

  • Irradiation with 365-nm light enabled the polymerization of 2-mm thick layers.

  • Phase separation did not occur in mixtures pyrrole/methacrylate.

  • The electrical conductivity increased markedly in the mixture containing 50 wt% Py.

Abstract

Hybrid systems formulated with pyrrole and methacrylate monomers were photopolymerized with both UV (λ = 365 nm) and visible (λ = 470 nm) light. It is known that conducting polymers produced by electrochemical or chemical synthesis are intractable solids or powders which display poor mechanical properties and low processability. An advantage of the photopolymerization process is that it allows processability and mechanical properties of final polymers to be optimized by incorporating flexibilizers and different additives into photopolymerizable formulations. Mixtures pyrrole/methacrylate were photoactivated with the iodonium salt p-(octyloxyphenyl)phenyliodonium hexafluoroantimonate (Ph2ISbF6), in combination with 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-methoxy-2-phenylacetophenone (BZME) or the pair camphorquinone (CQ)/ethyl-4-dimethylamino benzoate (EDMAB). The Ph2ISbF6 in combination with DMPA or BZME was an efficient photoinitiator system under irradiation at 365 nm. Conversely, in mixtures photoactivated with Ph2ISbF6/CQ/EDMAB the polymerization of both pyrrole and methacrylate was comparatively slow. Microscopy studies revealed the absence of phase separation indicating that the mixtures pyrrole/methacrylate resulted in the formation of an interpenetrating polymer network. The electrical conductivity of the hybrid polymers increased markedly with the amount of polypyrrole as a result of the formation of a conducting polymer network in the insulating BisEMA matrix.

Introduction

Organic conducting polymers such as polypyrrole have attracted increasing attention over the last two decades for applications of technological interest including the manufacture of rechargeable batteries [1], corrosion prevention coatings [2], printed circuit boards [3] conducting inks [4] and polypyrrole-coated silver nanoparticles with antibacterial activity [5]. Most of the π-conjugated electronically conducting polymers, including polypyrrole, are synthesized by either electrochemical [6], [7], [8] or chemical processes [9], [10], [11], [12]. Electrochemical oxidation of pyrrole forms a film of conducting polymer at the electrode surface while chemical synthesis proceeds via the oxidation of pyrrole with an oxidant such as ferric chloride [9]. The polymer produced by these methods is generally an intractable solid or powder which displays poor mechanical properties and low processability. Several attempts have been made to improve the mechanical properties of polypyrrole by forming blends or composites with other polymers. Electrical semi-conducting composites have been prepared by the polymerization of pyrrole inside porous polymethylmethacrylate [13], poly(vinyl alcohol) [14] and poly(vinylchloride) matrixes [15]. Migahed et al. [16] synthesized conducting composites by polymerization of pyrrole in ethylene-vinylalcohol copolymer. Recently, Takano et al. [17] prepared conductive films based on pyrrole-cellulose acetate.

Despite the extensive results published on chemical and electrochemical polymerization of pyrrole, reports on the photochemical polymerization are relatively scarce [18], [19], [20], [21], [22], [23]. An advantage of the photopolymerization process is that it allows mechanical properties of polymer films to be optimized by incorporating different additives and flexibilizers into photopolymerizable formulations. The objective of this study was to examine the photoinduced polymerization of pyrrole in combination with a methacrylate monomer. Polymeric materials based on mixtures pyrrole/methacrylate have the potential to combine the electronic conductivity of polypyrrole with the simplified processing procedures and attractive mechanical characteristics of methacrylate polymers. To the authors' knowledge, there have been no reports on the simultaneous photopolymerization of pyrrole and methacrylates. Methacrylate monomers are readily photopolymerized by a free radical mechanism while pyrrole polymerizes by a cationic mechanism. The selected pyrrole/methacrylate system was photoactivated with an iodonium salt in combination with 2,2-dimethoxy-2-phenylacetophenone, 2-methoxy-2-phenylacetophenone and the pair camphorquinone/ ethyl-4-dimethylamino benzoate. The extent of reaction of the individual monomers was followed by UV–vis spectroscopy and Fourier transform infrared in the mid region (MIR). Conversion values at the surface of thick specimens (∼2 mm) were evaluated by attenuated total reflectance (ATR). Electrical conductivity of photocured mixtures was assessed.

Section snippets

Materials

Pyrrole (Py) (Sigma–Aldrich, Buenos Aires, Argentina) was distilled twice under reduced pressure and stored in a refrigerator at about 5 °C before use. The methacrylate monomer 2,2-bis[4-(2-methacryloxyethoxy)phenyl]propane (BisEMA) was from Esstech, Essington, PA. The iodonium salt was p-(octyloxyphenyl)phenyliodonium hexafluoroantimonate (Ph2ISbF6) (OMAN 071, Gelest Inc., Philadelphia, USA). The free radical photoinitiators 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-methoxy-2-phenylaceto

Results and discussion

This section is divided into three parts concerned first with studies of photopolymerization of mixtures pyrrole/methacrylate by UV–vis, followed by measurements of conversion of the individual monomers by FTIR, and finally electrical conductivity characterizations.

Conclusions

Mixtures pyrrole/methacrylate photoactivated with an iodonium salt in combination with DMPA or BZME were efficiently polymerized under irradiation at 365-nm. Conversely, in mixtures photoactivated with iodonium salt in combination with the CQ/EDMAB pair irradiated at 470 nm the polymerization of both pyrrole and methacrylate was very slow.

Microscopy studies revealed the absence of phase separation indicating that the combination of pyrrole/methacrylate resulted in the formation of an

Acknowledgements

The financial support provided by the ANPCyT through PICT 1008 (2010) and CONICET is gratefully acknowledged.

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