Assembly of nanocomposite PEBAX membranes: A complementary study of affinity and clusterization phenomena
Highlights
► Pebax and KET used as nanocomposite membranes. ► The reciprocal affinity indicates good miscibility with KET up to 50 wt% and discrepancies at 70 wt%. ► Analysis show segregation phenomena. ► KET migrates toward polar moieties of Pebax. ► Water prefers to interact with KET.
Introduction
The design of new functional polymeric membranes that exhibits perm-selective properties is an important target for membrane technology [1], [2], [3], [4]. One of the most common strategies to change and improve the performance of raw materials foresees the incorporation of suitable structural and chemical elements into the polymer matrixes in order to control structure–property relationships [5], [6]. A more flexible method for achieving this goal is to blend different compounds, selected on the basis of their intrinsic chemical and morphological characteristics [7], even if miscibility and compatibility are critical issues to be considered [8]. In any case, the possibility of manipulating the membrane affinity through blending strategies appears to be a promising tool for enhancing the process efficiency, especially when the sorption selectivity is the predominant parameter in the overall transport [7]. In this respect, thermoplastic elastomers (TPE) block co-poly(ether/amide) is a class of attractive solubility-selective membrane materials, which finds important applications in vapour and gas separation, packaging, coatings, biomedical devices and so on [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The performances of block co-poly(ether/amide) 80PTMO/PA12 (PEBAX@2533, PEBAX) membranes are well known because they can be changed from highly breathable [19] to a barrier film by blending the polymer with specific organic modifiers [4], [5]. At the molecular level the affinity of PEBAX blended with N-ethyl-o/p-toluene sulphonamide (KET) membranes to specific polar species, including water molecules, was demonstrated to be the result of ‘domino processes’ involving hydrogen bonding [20]. However, a deeper understanding of how the local changes in the membrane structure, owing to the presence of the filler and the modification of filler–polymer and filler–penetrate–polymer interactions, can affect the performance of the membrane is needed: therefore it is necessary to turn the observation length from macro- to nano-scale. This is because it could facilitate the selection of new advanced materials on the basis of structural models. The focus of our research was to explain why water molecules prefer to interact with membranes containing 50 wt% in the first stage of the dissolution process. The subject of this combined study was the assessment of the determining factors which control miscibility and compatibility of the systems at molecular level, i.e., the interactions among chemical modifiers, polymeric matrix and guest molecules, in relation to the modifier/membrane/penetrant interactions and to solubility processes. Insightful indications about the effective distribution and arrangement of KET molecules, on multiple scales, from macro down to nano, are yielded by integrating experimental findings from calorimetry and infrared spectroscopy measurements with molecular dynamics (MD) simulations. Both approaches have also been used to establish the role of molecular rearrangement and clusterization phenomena in packed membranes that appears to be more evident on the nano-scale when a large amount of modifier is used. The type and frequency of intermolecular interactions between modifier–polymer, modifier–modifier and, then, modifier–polymer–water have been also examined and discussed, while analysing length scale more in depth.
Section snippets
Materials and membrane preparation
PEBAX@2533 was used to prepare flat nanocomposite isotropic membranes from alcoholic mixtures (2-propanol/n-butanol 50/50 v/v) of the polymer and N-ethyl-o/p-toluene sulphonamide (KET), ranging in concentration from 30 wt% to 70 wt%. The solutions were stirred at 70 °C in order to guarantee complete dissolution of the polymer. The clear dopes were put into Petri dishes inside a pre-equilibrated climatic chamber (T=25 °C, RH=50%) until formation of isotropic membranes. Annealing at 70 °C for a
Material affinity to water
Generally, the dissolution of the penetrating species into these elastomeric membranes is mainly due to its participation in favourable interactions with the polymeric material [4], [13]. Specifically, the supra-molecular chemistry of the membrane surface plays a discriminating role in interactive processes, which involve a polymer matrix and penetrating molecule. By controlling the chemical nature and the distribution of hydrophilic and/or hydrophobic domains on the surface, it is possible to
Conclusions
In this study affinity, miscibility and clusterization phenomena have been investigated for 80PTMO/PA12 (PEBAX) membranes modified by KET. The focus was to explain why water molecules prefer to interact with membranes containing 50 wt% in the first stage of the dissolution process. Experimental and theoretical assessment have been sinergically integrated in order to estimate the distribution and arrangements of modifiers into the matrix and to relate this to the macroscopic affinity behaviour.
Acknowledgements
This study is a part of the research programme financed by the EC FP6 Project “Computer-aided molecular design of multifunctional materials with controlled permeability properties” NMP3-CT-2005-013644 and by EU N° 218331 “Setting up research-intensive clusters across the EU on characterisation of polymer nanostructures” NaPolyNet-FP7-NMP-2007-CSA-1 Coordination and support action.
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