Comparative studies on the corrosion protection effect of DBSA-doped polyaniline prepared from in situ emulsion polymerization in the presence of hydrophilic Na+-MMT and organophilic organo-MMT clay platelets
Introduction
Electronically conducting polymers have emerged as a new class of materials in the past decades and attracted extensive research activities due to they exhibit broad spectrum of potential commercial applications in electronic, optical, biological, and other civilian/defense industries. Among those conducting polymers, polyaniline (PANI) is a potential material for commercial applications due to its environmental stability, good processability and relatively low cost [1], [2]. It had been reported that PANI could be used to electronic devices and products such as light-emitting diodes (LED) [3], [4], EMI shielding [5], secondary batteries, electrochromic device [6] and electrorheological materials [7], etc. Recently, PANI had been explored as potential candidate of anticorrosion coating to replace the chromium-containing materials, which have adverse health and environmental concerns [8], [9], [10], [11], [12], [13]. Wessling claimed a full mechanism that the corrosion protection of PANI on steel is attributed to an increase in the corrosion potential and to the redox catalytic property of PANI in the formation of passive of layer of metal oxide [12]. Moreover, Wei et al. demonstrated the enhanced anticorrosion effect of PANI through a series of electrochemical measurements on cold-rolled steel (CRS) coupons in saline [13].
Layered materials, such as smectite clays (e.g., Na+-montmorillonite, Na+-MMT) evoked great research interests for the preparation of polymer–clay nanocomposite materials in the past decade, because of its small particle (<10 μm) and easy of intercalation [14]. The Na+-MMT, whose lamella is constructed from an octahedral alumina sheet sandwiched between two tetrahedral silica sheets, exhibits a net negative charge on the surface of layers. The cation such as Na+ or Ca2+ is absorbed on the surface to compensate the net negative charge [15]. The increase in interlayer spacing that occurs with swelling of the Na+-MMT clay in water is large and enables the particles to be penetrated by relatively large size molecules [16]. Currently, organophilically charged clay layers exchanged by cationic surfactants playing an important role for the intercalation [15], [17].
There are a number of reports on the preparation and properties for the nanocomposite materials of PANI with various layered materials through the conventionally oxidative polymerization reactions [18], [19], [20], [21], [22], [23], [24]. In the past years, we had demonstrated that the preparation and enhanced anticorrosion effect of polyaniline–clay nanocomposite materials in the form of coating through conventionally oxidative polymerization [25]. In that study, we found that the introduction of low organo-MMT content (e.g., 0.25 wt.%) into emeraldine base of PANI may increase the length of the diffusion pathways for oxygen and water as well as decrease the permeability of the coating, reflecting a significant enhancement in corrosion protection on metallic surface.
Recently, the research interests on developing polymer–clay nanocomposite materials have shifted to water-based system due to the concerns on environmental and health issues. For example, many research groups reported that preparation of PANI/Na+-MMT nanocomposite materials through in situ emulsion polymerization (i.e., water-based latex) [26], [27], [28]. PMMA–clay nanocomposite materials have also been prepared by suspension [29], [31] and emulsion polymerization [29], [30], [31], [32], [33]. Waterborne polyurethane–clay nanocomposite materials have also studied by Jeong and co-workers [34]. However, practical application of water-based polymer–clay nanocomposite materials (e.g., latex form) has seldom been mentioned. By screening considerable literatures involved with the preparation of polymer–clay nanocomposite materials, we do believe that the Na+-MMT clay may reveal a better dispersion in latex system than that of organo-MMT. We therefore envision that the corrosion protection effect of polymer/Na+-MMT clay nanocomposite materials prepared from emulsion polymerization should be effectively enhanced than that of polymer/organo-MMT clay nanocomposite materials. Therefore, in this paper, we present the first evaluation of corrosion protection effect of a series of polyaniline (PANI)/Na+-montmorillonite (MMT) clay nanocomposite materials by in situ emulsion polymerization through the standard electrochemical corrosion measurements. The organ-PCN materials were also prepared as control experiments. Effects of material composition on the gas permeability, optical properties and electrical conductivity of a series of PCN materials were also studied by gas permeability analyzer (GPA), ultraviolet–vis absorption spectra and four-point probe technique, respectively.
Section snippets
Chemicals and instrumentations
The Na+-montmorillonite with a cation exchange capacity (CEC) of 114 mequiv./100 g was purchased from Pai-Kong ceramic company, Taiwan. Hexadecyltrimethyl-ammonium chloride (Fluka, 98.0%) was used as intercalating agent. Aniline (Fluka, 98.0%), ammonium persulfate (APS) (Fluka, 98.0%) and dodecylbenzene sulfonic acid (DBSA) (Fluka, 90.0%) were functioned as oxidant and surfactant (acid dopant), respectively. Ammonia solution (Riedel-de Haen, 25%) and N-methyl-2-pyrrolidinone (NMP) (Mallinckrodt,
Results and discussion
Generally, base forms of polyaniline can be schematically represented by the following general formula:where the y value ranges from 1 for the fully oxidized polymer (pernigraniline) and to 0.5 for the half-oxidized polymer (emeraldine) and to 0 for the fully reduced polymer (leucoemeraldine). On the other hand, MMT is a clay mineral containing stacked silicate sheets measuring ∼1 nm in thickness and ∼220 nm in length [25]. It possesses a high aspect ratio and a plates morphology. The chemical
Conclusion
A series of nanocomposite materials that consisted of polyaniline and Na+-MMT clay platelets were successfully prepared through in situ emulsion polymerization reactions with APS and DBSA as initiator and surfactant. The as-synthesized nanocomposite materials were subsequently characterized by infrared spectroscopy, wide-angle powder X-ray diffraction pattern and transmission electron microscopy.
Corrosion protection effect of PCN materials at low Na+-MMT clay loading up to 3 wt.% compared to
Acknowledgement
The financial support of this research by the Center-of-Excellence Program on Membrane Technology, the Ministry of Education, Taiwan, R.O.C. and NSC 94-2113-M-033-008 is gratefully acknowledged.
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