Synthesis and characterization of a benzoyl modified Pebax materials for gas separation applications

Highlights

• Benzoyl group has been grafted to Pebax®2533, obtaining a new product (BP2533).

• Crystallinity of amide blocks in BP2533 disappears at high substitution degree.

• Crystallinity of Polyether blocks in BP2533 increase with the substitution degree.

• Gas permeability slightly increases in BP”533 with respect to pristine Pebax.

Abstract

Pebax copolymers produced by Arkema are widely employed for different applications, including active molecular carriers and membranes for gas separation. In the present work, a new modification approach for Pebax®2533 is presented, along with the characterization of the newly obtained materials.

Pebax was modified by grafting, through a nucleophilic acyl substitution, a benzoyl group on Polyamide12 block. The yield of the reaction was confirmed by FTIR and NMR analysis, while thermal DSC and TGA characterizations were then carried out on the polymeric products characterized by different degrees of substitution to understand their properties. Finally, self-standing films were obtained by casting and gas permeation tests were conducted at 35 °C using CO₂, N₂, CH₄, O₂ and He, in order to understand the potentiality of the new material as membrane for gas separation.

DSC showed that in the modified Pebax, named “Benzoyl-P2533” (BP2533), the crystalline phase of the Nylon block was canceled, as expected, but at the same time the degree of crystallinity of the block of Polytetramethyleneoxide increased from 19%, measured for the unmodified Pebax, to a max of 35% for the fully substituted material. For this reason, gas permeability showed small but consistent increment, in the order of 10–11% for most of the gas tested, with the only exception being helium, where the increment resulted to be around 48%. As a consequence, the overall selectivity of CO₂ against helium dropped with respect to pristine Pebax. For all the other gases, on the other hand the selectivity with respect to CO₂ remained substantially constant, resulting in slight but neat improvement of the ability of the new material to separate this gas.

Introduction

Pebax block copolymers, produced by Arkema, have found widespread use in different applications, from sport equipment to industrial and medical devices, thanks to the many existing grades, the tunable mechanical properties and the ease of processing [[1], [2], [3], [4]]. In the last two decades, then, these materials become also particularly studied, in view of possible application as membranes for CO₂ capture [5]. Membranes for gas separation are known indeed to have high potential in enhancing separation performances with low installation costs and low environmental impact [6]. For this reason they have been widely studied for CO₂ separation applications [[7], [8], [9]], both in view of the increasing attention given to global warming issues [10,11], and of the still existing economic interest in natural gas treatments [[12], [13], [14]].

Unfortunately, many polymeric membranes still suffer from performance limitations which hinder their deployment in real world applications. The well-known Robeson's Upper bounds, as an example, is the graphical representation of the trade-off existing between selectivity and permeability which somewhat prevents many membrane materials, to achieve of both high separation performance and high productivity [15]. Over the years, the effort of scientific community brought to a continuous increase of performances, but more work needs to be done to obtain further improvements and make membranes for gas separation more and more competitive with respect to other technologies [7,14,[16], [17], [18], [19], [20]].

Different approaches are usually considered to improve the permselective performance of a membrane. It is common, for example, to employ different types of permselective fillers to enhance the gas transport properties of a standard polymeric membrane, developing the so called “Mixed Matrix Membranes” (MMM) [18,[21], [22], [23]], or to produce membranes containing mobile carriers like ionic liquid or amino acids into the polymer backbone to obtain, among the others, “Ionic Liquid Membranes” (ILM) [[24], [25], [26], [27]] or “Facilitated Transport Membranes” (FTM) [2,23,[28], [29], [30], [31], [32]], which both showed high potential in developing innovative membranes with high gas separation capacity. Based on these findings, the research today is mostly focused on the addition of the right fillers or carriers, in the proper amount and combination, in order to increase the overall performances of the final membrane.

Whichever is the approach considered (MMM, ILM, FTM), however, the hosting polymer chosen for the addition is of high importance in order to obtain membranes able to overcome the Robeson's upper bound. Pebax block copolymers have been often considered to that aim; they are, indeed, highly versatile materials and possess a reasonably good ability to separate CO₂ from other gases such as nitrogen or methane [2,5,33].

Polyether block, in particular, is known to have high affinity to carbon dioxide, while the nylon moiety gives high mechanical strength and somewhat prevent the crystallization of polyether phase. High crystallinity of linear polyethers indeed strongly reduce the gas permeability, making unattractive their direct use as membrane materials. Crosslinking [34,35], blending [36], and copolymerization [[37], [38], [39]] as in Pebax, are among the strategies used to create polyether based membranes for CO₂ capture.

Among the different types of Pebax available, then, the most popular for this application are surely Pebax®1657, which has been the base of many researches on membranes in the last years [33,[40], [41], [42], [43], [44], [45], [46], [47]] and Pebax®2533, endowed with a higher content of polyether blocks, which presents lower CO₂/N₂ selectivity compared to Pebax®1657 (about 25–34 against 43–80), but higher overall permeability for CO₂ (about 130–351 Barrer against 45–80 Barrer¹) [[48], [49], [50], [51], [52]].

Interestingly, beside their starting properties, Pebax materials present different reactive groups which can be exploited to carry on reactions that may strongly affect different characteristics of the material, such as solubility, compatibility with other chemicals/fillers, transport properties and so on. Most of the current studies, however, focus on the addition of fillers or carriers, while the possible effect related to matrix modification is subjected to limited investigation. Few works are available attempting Pebax modification in literature, like the one carried out by Yuan et al. [53] who modified Pebax®2533 by crosslinking it with Nafion to create pervaporation membranes, or Jomekian et al. [40] who investigated the possible interaction of Pebax®1657 with 3-Di-n-butyl-2-mthylimidazolium chloride (DnBMCl) ionic liquid (IL) and ZIF8.

In this context, the present work focuses on a new chemical modification of a common type of Pebax, which is reported together with the analysis of its effect on material structure and transport properties. The final goal is indeed not only related to the production of a polymer suitable to be used directly as a membrane material, but more in general to develop a new procedure to introduce different functional groups in the polymer chains to tailor their properties and also increase their affinity with different types of filler/additives for the production of new types of gas separation membranes.

The chemical modification carried out in this work is based on the substitution of the hydrogen in the amide group of the Polyamide block with a bulky aromatic group, the benzoyl, theoretically leading to a further boost in permeability, due to the reduction of Nylon crystallinity and the subsequent increment of polymer chains mobility. In addition, Meshkat et al. [47], have recently described the use of the aromatic benzoic acid into a similar dense polymer, Pebax®1657, which brought to interesting permselective improvements, such as the rising of CO₂ permeability and the increase of both CO₂/N₂ and CO₂/CH₄ selectivity. Thus, even if not in mobile carboxylic acid form, but grafted into polymer's backbone, the addition of the aromatic benzoyl group seemed promising in term of permselective improvements.

In the present work, a thorough analysis of the reaction process and of the materials properties has been performed together with a wide characterization in term of gas permeability to have a clear view of the impact of the modification on the copolymer structure and properties. FTIR, NMR, TGA and DSC analysis have been considered and the permeability of five different gases, namely CO₂, N₂, CH₄, O₂, and He, has been tested at 35 °C and 1 bar of differential pressure.