Assembly of nanocomposite PEBAX membranes: A complementary study of affinity and clusterization phenomena

Abstract

The behaviour of a block co-poly(ether/amide) 80PTMO/PA12 (PEBAX) membrane blended with an amphiphilic filler, N-ethyl-o/p-toluene sulphonamide (KET), was studied both experimentally and theoretically in order to identify the affinity, the miscibility and the effective distribution and arrangement of KET molecules and the mechanism controlling water interactions on different length scales, from macro to nano. Both methods have also been used to establish the role of molecular rearrangement and clusterization phenomena of modifiers in packed membranes, and to relate this to the macroscopic affinity behaviour. The compatibility between KET and PEBAX and the presence of nanoclusters of KET inside the matrix have been studied at molecular level through modelling, calorimetry and infrared spectroscopy analyses. A good miscibility has been observed up to 50 wt% of modifier content but with discrepancies when KET is at 70 wt%, confirming a saturation of hydrophilic sites of the filler, resulting in a reduced affinity of the membranes to polar penetrant. An integrated experimental–theoretical approach has provided a more detailed investigation of three-component complex systems, in which the macroscopic properties and morphologies are determined by interactions in the nano-scale range, thus resulting in a reciprocal validation and useful correlations between MD and experimental analysis.

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.