1. Introduction
Significant progress has been made in the development of new polymers for the fabrication of gas separation membranes. Polymers used for gas separation membranes can be either rubbers, generally characterized by a relatively low selectivity but high permeability and thus enabling a great productivity, or glassy polymers, characterized by low permeability but high gas-pair selectivity and offering a good separation efficiency. This trade-off between permeability and selectivity, first introduced [
1], and updated by Robeson [
2] and then by Pinnau et al. [
3] is typical for polymeric membranes, and was theoretically explained by Freeman et al. [
4]. An exceptional novel class of so-called polymers of intrinsic microporosity (PIMs), introduced by Budd, McKeown and co-workers [
5], which combine an exceptionally high permeability with relatively high selectivities, is responsible for the large upward shift of the upper bound in recent years. PIMs owe their exceptional behaviour to extremely rigid [
6] and highly contorted [
7,
8] polymer chains, which do not allow efficient packing and is responsible for a high free volume in these materials.
While most high performing PIMs require complex and expensive synthetic procedures, the key challenge to improve the competitiveness of membrane separations over other gas separation techniques is the fabrication of inexpensive polymer membranes, having a good trade-off between permeability and selectivity. Commercial polymers used for gas separation membranes generally have a high selectivity, but low permeability. Matrimid
®5218, a commercial amorphous glassy polyimide, is one of those polymers, and improvements of its overall performance would necessarily require an increase in its permeability. One option to improve the permeability of a polymer is by blending with another polymer to combine synergistically the best properties of the two individual materials [
9,
10]. This is not an easy task, because due to only a very small gain in entropy, most polymers are not miscible at the molecular level. Early studies on the miscibility of Matrimid
®5218 were reported by Grobelny et al. [
11], and since then different polymers were blended with Matrimid, in order to increase its separation performance [
12,
13,
14]. The blend of Matrimid
®5218 with polyethersulfone (PES) yielded mechanically stable flat film [
15] and hollow fiber [
16] membranes for efficient CO
2/N
2 separation; its blend with polysulfone (PSf) enhanced the stability of Matrimid in CO
2/CH
4 binary mixtures, due to the mitigation of CO
2 plasticization [
17]. While the CO
2 and CH
4 permeability of polysulfone/Matrimid
®5218 blend membranes increase with the polyimide content, there is an optimum in the CO
2/CH
4 mixed gas selectivity (≃30) for the blend with 20% of Matrimid [
18].
With their unique properties, starting with PIM-1 [
19] that defined the 2008 Robeson upper bound [
2], numerous new PIMs with a wide range of different chemical structures [
6,
20,
21,
22,
23,
24,
25,
26,
27,
28] and increasingly efficient gas separation performance, have moved the Robeson upper bounds further for several gas pairs [
3]. Despite their high permeability, the sophisticated synthesis and high costs hinder PIMs from being the basis for large scale membrane production and industrial applications at present [
29]. In addition, for demanding separations, their modest selectivity needs improvement. The idea underpinning the present work is to blend PIMs with a highly selective commercial polymer such as Matrimid
®5218, in order to tailor their permeability and selectivity. In the last few years, the archetypal PIM-1 was blended with several other commercial polymers, especially with highly selective polymers such as polysulfone [
30] and polyimide [
31] in order to increase its selectivity. Blends of PIM-1 with polyphenylenesulfone (PPSU) and sulfonated polyphenylenesulfone (sPPSU) exhibited similar permeability of the polysulfone but enhanced selectivity compared to the neat PIM-1, with an additional anti-plasticization effect under mixture conditions [
30]. Similarly, the blending of carboxylated PIM-1 (cPIM-1) with the highly selective co-polyimide P84 demonstrated increased selectivity with a simultaneous reduction of the permeability, as the amount of P84 was increased in the blend [
31]. The first blend of PIM-1 with Matrimid was made by Yong et al. and they also studied the miscibility of Matrimid and Torlon with cPIM-1 [
32,
33,
34]. Addition of a small quantity of Matrimid in PIM-1 improved the O
2/N
2 separation performance, while a small amount of PIM-1 in an excess of Matrimid enhanced the CO
2/CH
4 gas separation performance. Moreover, they used the PIM-1/Matrimid blend to fabricate hollow fibers, demonstrating the greater versatility of the blend for obtaining an ultrathin dense layer, potentially suitable for industrial use [
35]. More recently, a novel blend membrane of PIM-1 and a Tröger’s Base (TB) polymer showed lower pure gas permeability but higher ideal selectivity than that of the pristine PIM-1 membrane [
36].
In the present paper, we study the properties of a blend membrane based on the highly permeable PIM-EA(H
2)-TB [
37,
38,
39,
40] and the highly selective Matrimid
®5218 (
Figure 1). PIM-EA(H
2)-TB is a member of the new class of PIMs consisting of Tröger’s Base and ethanoanthracene (EA), forming a particularly rigid polymer backbone. The equivalent polymer with methyl substituents on the two bridgehead positions of the ethanoanthracene unit, PIM-EA-TB, was shown to have a marked size-sieving behaviour that favours the diffusion of gases with smaller kinetic diameters, and surpasses the 2008 Robeson upper bound for the O
2/N
2, H
2/CH
4 and H
2/CO
2 gas pairs [
8,
41]. The PIM-EA(H
2)-TB is highly selective, as the analogous PIM-EA-TB, and was recently used to increase the hydrogen permeability of polybenzimidazole by blending [
40].
The aim of this work to enhance the permeability of Matrimid by the addition of PIM-EA(H2)-TB, and to find the desired combination of the high permeability of the PIM and the high selectivity of the polyimide. Detailed analysis of the gas transport parameters under single and mixed gas permeation conditions provides deep insight into the role of gas diffusivity and solubility in the overall transport properties of the novel Matrimid®5218/PIM-EA(H2)-TB blend. In particular, a novel mixed gas permeation setup with the unique possibility to determine the mixed gas diffusion coefficients will provide unprecedented information on the coupling effect between CO2 and CH4 or CO2 and N2 during permeation of the respective mixtures in the neat polymers and the blend.