The enhancement of mechanical properties of P84 hollow fiber membranes by thermally annealing below and above T_g

Highlights

• P84 HFMs were annealed below/above T_g to investigate mechanical properties.

• Mechanical strength of P84 HFMs began to increase when annealed below T_g.

• Mechanical strength of P84 HFMs increased greatly when annealed above T_g.

• P84 HFMs can be cross-linked by thermally annealing below/above T_g.

Abstract

In this paper, BTDA-TDI/MDI (P84) co-polyimide hollow fiber membranes (HFMs) were prepared by dry-jet wet spinning technique. The P84 HFMs were thermally annealed below and above P84 T_g (321.5 °C) to investigate their mechanical and gas separation properties. Thermally annealing at different temperature below and above T_g influenced the tensile strength, Young's modulus and the elongation at break significantly. Compared with the pristine P84 HFMs, the tensile strength and Young's modulus of the thermally annealed P84 HFMs decreased slightly when the thermally annealing temperature was 180, 200 and 250 °C. However, the tensile strength of the thermally annealed P84 HFMs began to increase when the thermally annealing temperature approached to its T_g (such as 300 °C), which was induced by the increased molecular weight (Mw) and CTCs (charge transfer complexes) formation. Compared with the thermally annealed P84 HFMs below T_g, the HFMs' tensile strength, Young's modulus and the elongation at break increased greatly with thermally annealing above T_g, which was due to the obvious cross-linking reaction of P84. To further understand the effect of thermally annealing below and above T_g, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) with attenuated total reflection (ATR) mode, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), thermogravimetric-mass spectroscopy (TG-MS) measurements were explored.

Introduction

Over the last decades, membrane gas separation processes have attracted more and more attention. There are some advantages of membrane technology such as higher energy efficiency, economic efficiency and environmental friendliness, compared with traditional processes which relied on cryogenic distillation, absorption and adsorption [[1], [2], [3]]. Glassy polymers show very attractive separation characteristics (O₂/N₂, CO₂/CH₄ and H₂/N₂) and thermal stability in comparison with rubbery polymers [4]. Up to now, cellulose acetate, polysulfone, poly (phenylene oxide) and polyimides are usually used as membrane materials to separate mixed gas [[5], [6], [7]]. The aromatic P84 co-polyimide which consists of rigid chains has shown promising properties for gas separation [[8], [9], [10], [11]]. Except for good gas separation performance, the membranes should have sufficient anti-plasticization property and mechanical strength because they are usually applied in rugged environments [12].

Hollow fiber configuration can provide higher surface area to volume ratio than other membrane configurations such as plate and spiral wound modules, so they are usually used in industrial gas separation. Some previous studies about hollow fiber gas membranes focused on the effect of spinning parameters and membrane materials on the gas separation performance [[13], [14], [15], [16]]. However, quite few studies have been reported on mechanical properties of the hollow fiber membranes (HFMs).

Based on this situation, some efforts have been made. Qin et al. [17] investigated the effects of dope extrusion velocity of HFMs on their permeability/separation properties and mechanical properties, finding that Young's modulus and the tensile strength of the fibers enhanced with the increasing dope extrusion velocity. Kung et al. [18] blended the polyimide Matrimid and polybenzimidazole (PBI) to fabricate HFMs and the fibers' tensile strength decreased with the increasing PBI contention. Zuo et al. [19] designed the dual-layer PVDF/Ultem HFMs with good mechanical properties. In Favvas's study [20], the multi-wall carbon nanotubes (MWCNTs) were added into P84 carbon HFMs, leading to the significantly increase in the tensile and flexural modulus of elasticity of the HFMs. To reinforce the membrane structure, Wang and Teoh [21] fabricated MMMs (mixed matrix membranes) by incorporating polytetrafluoroethylene (PTFE) particles or clay, the membranes exhibited good mechanical strength. However, for the membrane mechanical properties, it could be found that most of the researches focused on fabricating procedure, such as the polymer solution composition and membrane preparation conditions, and the study about post treatment was seldom reported.

Thermally annealing is a post treatment method of membranes, which is often used to improve the anti-plasticization property of the membrane to CO₂ and olefins. Bos et al. [22] had proved that heat treatment of the Matrimid membrane could suppress the CO₂ plasticization. Krol et al. [23] prepared Matrimid 5218 asymmetric HFMs, the suppression of propylene plasticization was attributed to the formation of CTCs after thermally annealing. Chung et al. [24] had applied thermal approaches to enhance anti-plasticization characteristics of the asymmetric 6FDA-2,6DAT HFMs. Xu [25] enhanced CO₂/CH₄ separation performance and plasticization resistance by thermal crosslinking of phenolphthalein-based cardo poly (arylene ether ketone) membrane. Above all, for much relevant works, the thermally annealing temperature was below the T_g of the membrane materials, and the researches mainly focused on the anti-plasticization properties but less on membrane mechanical properties.

Carbon molecular sieve (CMS) membranes [[26], [27], [28], [29], [30], [31]] are a new class of porous membranes which can surpass the Robeson's upper bound. There is growing interest in using CMS membranes for the separation of organic vapor mixtures and gas because of the high chemical stability. CMS membranes are obtained from the pyrolysis of polymeric membranes at a temperature above T_d (the decomposition temperature) which is much higher than the T_g of the membrane materials. Until now, a plurality of relevant published works about thermal treatment of gas membrane focused on the annealing (below T_g) and pyrolysis procedure. To our best knowledge, the mechanical properties of gas membrane by thermally annealing below and above T_g were seldom concerned, which played an important role in industrial application.

In this study, BTDA-TDI/MDI co-polyimide (P84), a commercial polymer synthesized by 1-1′-methylenebis [4-isocyanatobenzene] and 2,4-diisocyanato-1-methylbenzene with 5-5′-carbonylbis [1,3-isobenzofurandione], was used to fabricate HFMs for gas separation by dry-jet wet spinning technique. In order to improve the mechanical and solvent-resistant properties of the prepared HFMs, the pristine P84 HFMs were thermally annealed below and above T_g. The possible cross-linking mechanism was proposed. The tensile strength, Young's modulus, elongation at break and gas separation properties (CH₄, N₂, O₂, CO₂ and H₂) of the pristine and thermally annealed HFMs were investigated. The influence of thermally annealing temperature on the mechanical properties and gas separation properties was discussed.