Characterising poly (vinyl chloride)/Aliquat 336 polymer inclusion membranes: Evidence of phase separation and its role in metal extraction
Introduction
The development of polymer inclusion membranes (PIMs) has increased rapidly over the last two decades as a potential alternative to the conventional solvent–solvent extraction process for metal ion recovery [1], [2]. PIM film consists of a polymer, an extractant and if necessary a plasticizer. Extractant is an essential component which functions as a guest host specific molecule that provides selective membrane permeability for target species [3]. PIMs consisting of poly-vinyl chloride (PVC) and Aliquat 336 were first applied by James et al. [4] for the construction of ion selective electrodes more than four decades ago. Since then, PVC/Aliquat 336 PIMs have been one of the most studied PIM systems for the extraction of metallic ions from the aqueous phase. Previous studies have shown successful extraction of metal ions and small organic molecules using PVC based PIMs containing Aliquat 336 [5], [6], [7], [8], [9], [10]. However, it is not yet clear whether the PVC/Aliquat 336 PIM is a solid homogenous solution or a two phase heterogeneous mixture.
The mechanism of facilitated transport in PIMs is still open to speculation given the lack of understanding about the nature of their homogeneity. For a solid solution, the metal ion with the aid of an extractant is transported through a polymer matrix. For a heterogeneous solid, the metal ion with the aid of extractant is transported through continuous channels within a polymer matrix. In some instances, a combination of these two extremes may occur.
In recent years, several studies have been conducted to investigate the homogeneity of PIMs. Through scanning electron microscopy analysis, Xu et al. [11] speculated that at above 30 wt.% Aliquat 336, the interior structure of PVC/Aliquat 336 PIMs contained micro channels filled with Aliquat 336. They also showed that there exists a critical Aliquat 336 content in PIMs of 30–40 wt.% for the transport of Cd (II) to occur. Although the critical Aliquat 336 content has been confirmed by several other studies [11], [12], their speculation about the existence of micro channel in PIMs has not been substantiated. In fact, it has been contradicted by a recent study by St John et al. [13] who employed high resolution synchrotron-based fourier-transform infrared spectroscopy and proton-induced X-ray emission microspectrometry (μ-PIXE) to study the homogeneity of PVC/Aliquat 336 PIMs. μ-PIXE results reported by them showed that PVC based PIMs containing 10–40 wt.% Aliquat 336 are homogenous at the micro-scale which is comparable to the scale investigated by Xu et al. [11].
In this paper, PVC based PIMs were prepared with different Aliquat 336 concentrations. This work aims to determine whether the PIMs produced are a solid solution or mixture by application of differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques to characterise the thermal transitions. This approach clarifies the miscibility of PVC and Aliquat 336. Extraction of Cd (II) and Zn (II) were also investigated in order to observe any correlation between solid structure, thermal analysis and membrane function.
Section snippets
Reagents
All reagents were obtained from Sigma Aldrich, Australia. High molecular weight poly (vinyl chloride) (PVC) and Aliquat 336 (tricaprylmethylammonium chloride) were used as the base polymer and extractant respectively. The weight-average molecular weight of this PVC is 80,000 g/mol. Aliquat 336 is a mixture of tri-alkyl methyl ammonium chloride salts produced from the methylation of Alamine 336, with the substituent alkyl chain length containing between 6 and 12 carbon atoms. HPLC grade
Thermal analysis
DSC analysis of the PVC as supplied showed a glass transition temperature (Tg) of 85 °C (Fig. 1) which is consistent with literature values [15], [16]. In contrast, DSC analysis of the PVC cast from THF solution exhibited a Tg of 63 °C (Fig. 1). PIMs containing 10–40 wt.% Aliquat 336 exhibited a Tg in the range of 55–63 °C (Fig. 2). PIMs containing 50–70 wt.% Aliquat 336 exhibited a Tg that was too subtle for designation using the described experimental procedure. DSC analysis of the supplied neat
Conclusion
In the present study, the miscibility of the PVC/Aliquat 336 PIMs at various Aliquat 336 concentrations was investigated by measuring the Tg. The DMA has provided a distinct result for Tg of PVC/Aliquat 336 PIMs and a Tm of Aliquat 336. Results reported here indicate that PVC/Aliquat 336 PIMs were phase separated with two distinctive phases observed by DMA, that is an α transition at 71 ± 8 °C and β transition at −18 ± 1 °C where the later was not detected by DSC. The DMA results also suggest that
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