Structural effect of ferrocenecarboxymethylated polymers on their electrical behavior under the exposure to methanol and acetone vapors

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Abstract

Functionalized ferrocenecarboxymethylated polymers, i.e. poly(vinylbenzyl ferrocenecarboxymethylate) (PVBFCC), poly(vinylbenzyl ferrocenecarboxymethylate-co-ethoxyethylmethacrylate) with the mole ratio between vinylbenzyl ferrocenecarboxymethylate and ethoxyethylmethacrylate of 75:25 (Co-PVBFCC 75/25) and 50:50 (Co-PVBFCC 50/50), and ferrocenecarboxymethylated polysulfone (BPSFCC) were investigated for their electrical behavior under the vapors of methanol and acetone and nitrogen gas. Electrical conductivity responses of the four ferrocenecarboxymethylated polymers in the presence of lithium perchlorate were measured when they were exposed to nitrogen gas, and methanol and acetone vapors. Main factors that affect the electrical conductivity and sensitivity of these electroactive ferrocene polymers originate from the polymer structure and the type of the passing gas or vapor. Ferrocenecarboxymethylated polymers with higher degrees of substitution of ferrocene units possess good electrical conductivity under the atmosphere of mixed N2/methanol vapor, while ferrocenecarboxymethylated polymers with more flexible chain and/or larger free volumes give higher electrical conductivity under the atmosphere of mixed N2/acetone vapor. The gas or vapor molecule with higher polarity and smaller size enhances the electrical conductivity of the ferrocene polymers. Our results clearly indicate that the synthesized ferrocenecarboxymethylated polymers have potential to be used as methanol or acetone sensor materials.

Introduction

Gas sensors play an increasingly important role in monitoring the environment in providing information on quality control of air pollution. A gas sensor can be made from various materials such as metal oxides [1] and conductive polymers (CPs) [2], [3]. However, CPs have several advantages over other materials since they have lighter weight, lower price and can be used to detect a lower level of gas concentration at room temperature. The most studied CPs are polypyrrole, polyaniline and polythiophene [4]. The applications of conductive polypyrrole and polyaniline as gas sensors are mainly associated with the detection of volatile organic compounds, ammonia, chlorine, carbon dioxide [5], [6], [7], or initial fire smoke [8]. Previously, many researchers have utilized systems of polymer blends or polymer composites in obtaining the desired properties [9], [10], [11], [12], [13]. The common characteristic that is associated with the electrical conductivity of these CPs is the conjugation of double bonds along their main chains for electron movement [14], [15]. However, it is possible to use non-conjugated conductive polymer materials as gas sensor as well. When the nonconductive polymers are transformed to CPs, both electrical conductivity and their desired physical properties can be obtained. Ferrocene polymer is a non-conjugated electroactive or conductive polymer. The two main possible architectures of ferrocene containing polymers are the main-chain polymers and the side-chain polymers [16]. Manners and co-workers prepared a ferrocene polymer by polymerization of ferrocenophanes with silicon bridges [17]. Dhanalakshmi and Sundarajan [18] synthesized conjugated polymers with ferrocene units as pendants or as end-groups by using the classical metathesis polymerization catalyst, W(CO)6, under photo-irradiation. Cazacu et al. [19] prepared the new polymeric structures containing ferrocene units along the chains, namely poly(silyl ester)s. Since these polymers contain the silyl ester groups along the chain, they are hydrolytically degradable. There are some reports on the application of ferrocene polymers as chemical sensors such as carbon monoxide sensors [20], and glucose biosensor [21], [22]. Casalbore-Miceli et al. [23] studied the interactions between humidity and ferrocene-functionalized polythiophene for humidity sensor application. The ferrous ion of the ferrocene unit can be oxidized to ferric ion when it is connected to voltage source and the electron from the nearby ferrocene unit can jump to the oxidized ferrocene unit which produces the electrical conductivity in the ferrocene polymers. Matsuura and Matsukawa [24], [25] performed tight-binding crystal orbital calculations in order to examine the electronic properties of various ferrocene polymers and they attempted to determine the effect of the bridging moieties, i.e. unsaturated-hydrocarbon bridges and heteroatom bridges, on the electrical conduction along the polymer chain. The principle of electrical conductivity of ferrocene polymer is the electron hopping from an iron atom in the ferrocene unit to the next one. It is expected that we can use this type of electroactive polymers in gas sensor applications to detect various gases possessing different influences on the electron hopping ability.

In our work, we investigated the effect of structures of ferrocenecarboxymethylated polymers on their electrical conductivity responses towards methanol and acetone vapors. Ferrocene unit was attached as a pendant group to the chloromethylated polystyrene and bromomethylated polysulfone main chain. The influence of polymer backbone flexibility on electrical behavior was also examined by incorporating various amounts of flexible co-monomer, 2-ethoxyethylmethacrylate, into the chloromethylated polystyrene backbone.

Section snippets

Materials

Vinylbenzyl chloride (VBC) (Fluka, mixture of ∼70% meta and ∼30% para isomer, ≥95.0%) and 2-ethoxyethylmethacrylate (EEM) (Aldrich, 99%), were extracted with 0.5% (w/v) sodium hydroxide solution to remove the inhibitors, i.e. t-butyl catechol, and nitromethane. Then they were washed with water and dried overnight over anhydrous magnesium sulfate. Benzoyl peroxide (BPO) (Panreac), chloroform, toluene, methanol, polysulfone (Aldrich, average Mn ∼26,000), bromomethyl methyl ether (BMME),

Synthesis and characterization of ferrocenecarboxymethylated polymers

All of the polymer supports were obtained from the free radical polymerization of the corresponding monomers, followed by the esterification of these halomethylated polymers with sodium ferrocenecarboxylate salt via the phase transfer catalysis. This provided the four different types of ferrocenecarboxymethylated polymers with well-defined structures, as supported by FT-IR and 1H NMR data. The esterification reaction is shown in Scheme 1 and all of the structures of the synthesized

Conclusions

The factors that affect the electrical conductivity and sensitivity of ferrocenecarboxymethylated polymers are both the polymer structures and the type of exposing gas or vapors. Ferrocenecarboxymethylated polymers with higher amounts of ferrocene moieties have good electrical conductivity under the atmosphere of mixed N2/methanol vapor, while ferrocenecarboxymethylated polymers with more flexible structures and/or having larger free volume have higher electrical conductivity under the

Acknowledgements

We would like to acknowledge the funding and supports from the Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University; the Functional and Sensing Materials Research Unit, National Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials; the Conductive and Electroactive Polymers Research Unit of Chulalongkorn University; the Royal Thai Government; and the Thailand Research Fund (TRF-BRG). The authors also would

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