Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Effect of polymer molecular weight on the physical properties and CO 2 /N 2 separation of pyrrolidinium-based poly(ionic liquid) membranes Liliana C. Tomé a,b, ⁎ , Diogo C. Guerreiro a , Raquel M. Teodoro a , Vítor D. Alves c , Isabel M. Marrucho a,b, ⁎ a Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal b Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal c LEAF – Linking Landscape, Environment, Agriculture and Food, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal ARTICLE INFO Keywords: PIL–IL composites Polymer molecular weight Membrane forming ability Thermal analysis Mechanical properties Gas permeation ABSTRACT Aiming at investigating the effect of the polymer molecular weight (M w ) on the physical and gas permeation properties of poly(ionic liquid)-ionic liquid (PIL–IL) composites, this work focuses on membranes based on variable M w pyrrolidinium-PILs having [C(CN) 3 ] – as counter-anion and different amounts (20, 40 and 60 wt%) of free [C 2 mim][C(CN) 3 ] IL. Although all the prepared composite materials have high thermal stability (T onset > 556 K) for post-combustion CO 2 separation, the evaluation of the film forming ability shows that it is not possible to obtain free standing PIL–IL membranes using the Low M w PIL (average < 100 kDa). The formed Medium M w (average 200 – 350 kDa) and High M w (average 400 – 500 kDa) PIL–IL membranes present similar mechanical properties in terms of Young´s modulus, tensile strength and elongation at break. The gas perme- abilities and diffusivities are dependent on the M w of the PIL used. The Medium M w PIL–IL membranes display higher CO 2 permeabilities (14.6 – 542 Barrer) than those (8.0–439 Barrer) observed for High M w PIL–IL com- posites. Despite the M w of the PIL used, the incorporation of high free IL contents increases both CO 2 perme- ability and CO 2 /N 2 permselectivity. Consequently, the finest CO 2 /N 2 separation performances, overcoming the 2008 upper bound in the Robeson plot, were obtained for the High and Medium M w PIL–60 IL composites, respectively, with CO 2 permeabilities of 439 and 542 Barrer and CO 2 /N 2 permselectivities of 64.4 and 54.0. 1. Introduction Among the broad range of diverse membrane materials investigated for gas separation over the past few years [1–3], polymeric ionic liquids or poly(ionic liquid)s (PILs), a subclass of polyelectrolytes that combine the chemical tunability of ionic liquids (ILs) with the common features of polymers [4], have emerged as new versatile task-specific materials for the development of high performance CO 2 separation membranes [5–7]. The potential of these functional ionic polymers has been exploited using different membrane arrangements, such as neat PIL membranes [8–11], PIL–IL composite membranes [12–14], PIL copo- lymer membranes [15–17], and PIL–IL–inorganic particle mixed matrix membranes [18–20]. The development of PIL–IL composite membranes, which combine the best properties of both ILs and PILs, allowed membranes with high CO 2 permeability and CO 2 /N 2 permselectivity, as well as good me- chanical properties. The proof-of concept was published by Bara et al. [21], who prepared PIL–IL composite membranes by polymerization of an IL monomer in the presence of 20 wt% of free (non-polymerizable) IL. The improved CO 2 separation performances obtained in the pre- sence of free IL [22,23] inspired other researchers to pursue this strategy. The majority of the PIL–IL composite membranes studied so far were prepared using PILs composed of imidazolium cation moieties in their polymeric backbone and fluorinated or cyano-functionalized as counter-anions [24–27]. Later on, and in order to understand the influence of the PIL poly- cation, our group investigated the gas permeation properties of PIL–IL membrane materials based on PILs having different cation functional- ities, such as imidazolium, pyridinium, pyrrolidinium, ammonium and cholinium [28]. The results showed that depending on the chemical structure of the polycationic PIL, the polymer chains interact and pack differently, thus affecting the gas transport. Nevertheless, polycation variations alone cannot promote the CO 2 permeability improvements needed for PIL–IL membranes to be considered competitive [28]. In light of this fact, and also considering that pyrrolidinium-based PILs can be prepared by anion metathesis reactions from a commercially https://doi.org/10.1016/j.memsci.2017.12.019 Received 20 October 2017; Received in revised form 30 November 2017; Accepted 9 December 2017 ⁎ Corresponding authors at: Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal. E-mail addresses: liliana.tome@itqb.unl.pt (L.C. Tomé), isabel.marrucho@tecnico.ulisboa.pt (I.M. Marrucho). Journal of Membrane Science 549 (2018) 267–274 Available online 10 December 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved. T