Molecular modeling of poly(benzoxazole-co-imide) membranes: A structure characterization and performance investigation
Graphical abstract
Free-volume morphologies and sorption isotherms of various gases of the PBO, PBO-PI, and PI membrane models.
Introduction
Thermally rearranged poly-benzoxazole (TR-PBO) membranes show promising performance in the gas separation process [1], [2], [3], [4], [5]. The development of PBOs can be traced back to the 1960s, when PBOs received considerable attention and were shown to be naturally very rigid and to possess stable thermal properties [6], [7], [8]. Soon after, PBO was adopted for gas separation in membrane technology due to its excellent properties in severe conditions [1], [2], [3]. The TR-PBO membrane contained high free volume with a unique shape resembling a series of bottlenecks connecting adjacent cavities, leading to the membrane's excellent permeability and selectivity [4], [5]. For further analysis, positron annihilation lifetime spectroscopy (PALS) was used to characterize the TR-PBO membrane. In PALS analysis, a bimodal profile of free volume size was discovered in TR-PBO membranes, allowing for the improvement of gas permeability with good selectivity [9]. With regard to gas transport behaviors, the effective free volume in the TR-PBO membrane was shown to be released after the thermally rearranged reaction, resulting in increasing gas diffusivity and solubility while maintaining comparable selectivity [10], [11], [12]. Due to the high free volume of TR-PBO, researchers sought to fabricate a membrane using TR-PBO and highly selective materials to overcome the upper bound of gas separation [13], [14], [15], [16], [17], [18]. The TR-poly(benzoxazole-co-imide) ;membranes composed of TR-PBO and polyimide were fabricated using different compositions [13], [14]. Thermal rearrangement process enhances the backbone rigidity and free volume of the co-polymer membranes to improve gas separation performance. These TR-poly(benzoxazole-co-imide) membranes exhibit good permeability, and their gas selectivity is improved with an increase in polyimide content, which is proportionally more concentrated around the Robeson Upper Bound. Pyrrolone segments were added to the TR-PBO membranes to enhance the selectivity, and these modified membranes exhibited a selection factor twice as high as that of the pure TR-PBO membrane [15]. Previous efforts also attempted to control the conditions of thermal treatments to adjust the formation of the thermally rearranged benzoxazole segment in the membrane to fine-tune the relationship between permeation and separation factors [15], [16]. TR-poly(benzoxazole-co-amide) membranes have been studied and they showed good hydrogen separation performance [17]. Moreover, cross–linked TR-poly(benzoxazole-co-imide) membranes have been investigated to improve gas separation performances [18].
Recently, theoretical study at the microscopic scale has been regarded as a promising method for developing and designing novel materials [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In a microstructure analysis, the torsional angle was adapted to investigate the packing efficiency of polymeric membranes [19]. Simulated wide-angle X-ray diffraction has been used to accurately describe the bulk membrane free space [20]. In addition, the molecular dynamics (MD) technique has enhanced several methods for analyzing the cavity or free-volume size distribution, including the concepts of an energetic sizing algorithm [21], geometrics [22], and image analysis [23]. With regard to gas diffusion and sorption behaviors, the MD and Monte Carlo (MC) simulations have been applied and shown to be highly consistent with experimental results. The MC method has been applied to study the effects of material structure, fabrication parameters, and operational conditions on the gas sorption behaviors [24], [25], [26]. The MD simulation has been used to investigate the areas of thermal motion mechanisms, mobility, and self-diffusivity of small molecules in the membrane matrix [27], [28], [29]. Through the solution–diffusion model, the simulated gas permeability can be estimated based on the solubility and diffusivity, and such estimations have compared favorably with experimental results [19], [20], [30], [31].
The TR-PBO membrane has shown promising performance in applications for gas separation. Combining the highly permeable TR-PBO membrane and highly selective membrane materials is a key criterion in the development of useful membranes in membrane technology. The molecular simulation technique has been shown to be both feasible and valuable in investigations of membrane structure and performance at an atomic scale. Therefore, the aim of the present work was to evaluate the performance of a poly(benzoxazole-co-imide) membrane composed of PBO chains with high permeability and segments and polyimide chains with good selectivity using a molecular simulation technique. Three types of membrane models were constructed in this study: poly-benzoxazole (PBO), poly(benzoxazole-co-imide) (PBO-PI), and polyimide (PI) membranes. The physical properties of these membranes for model construction were based on previous studies [13]. Details of the model construction are illustrated in Section 2.
Section snippets
Theoretical method
In this work, three types of membrane models, PBO, PBO-PI, and PI, were constructed to analyze how the rigid benzoxazole segments affect membrane structure and gas transport behaviors. The MD technique was used to construct the membrane model and to analyze the membrane structure and gas diffusion. The MC method was applied to simulate the gas sorption behaviors in the membrane. All molecular models were constructed using the Materials Studio software from Accelrys Inc. The model construction
Model validation
This work aims to analyze the structural properties and transport behaviors of TR-PBO membranes using molecular models. The TR-PBO membrane reveals a typically looser structure and larger free volume due to its intrinsically rigid chains. Thus, we first constructed four types of TR-PBO membrane models to validate the accuracy and feasibility of this simulation work. Four types of TR-PBO models, aPBO, tPBO, sPBO, and cPBO, were constructed for molecular modeling. These TR-PBO membranes were
Conclusions
The MD and MC techniques were successfully used to characterize the membrane structure and gas transport behaviors of the PBO, PBO-PI, and PI models. In the micro-structure analyses, the torsional angle and WAXD analyses demonstrated that the membrane contained more benzoxazole segments, contributing to the highly stiffened chains. Therefore, the membrane with the benzoxazole structure had a lower packing efficiency, as reflected by the broader torsional angle profile and larger d-spacing
Acknowledgments
The authors would like to express their appreciation for financial support from the National Research Council of Taiwan under Grant numbers NSC-99–2221-E-002–240-MY3 and NSC-101–2811-E-002–063. The computational resources from the National Center for High-Performance Computing (Taiwan, ROC) is highly appreciated. This research was also supported by Korea CCS R&D Center (KCRC), funded by the Ministry of Education, Science and Technology in Korea, all of which we gratefully acknowledge.
References (34)
- et al.
Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)
J. Membr. Sci.
(2012) - et al.
Gas sorption and characterization of thermally rearranged polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)
J. Membr. Sci.
(2012) - et al.
Sorption and transport of small gas molecules in thermally rearranged (TR) polybenzoxazole membranes based on 2,2-bis (3-amino-4-hydroxyphenyl)-hexafluoropropane (bisAPAF) and 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)
J. Membr. Sci.
(2013) - et al.
Highly permeable and selective poly(benzoxazole-co-imide) membranes for gas separation
J. Membr. Sci.
(2010) - et al.
Effect of the chemical structure of various diamines on the gas separation of thermally rearranged poly(benzoxazole-co-imide)(TR-PBO-co-I) membranes
J. Membr. Sci.
(2013) - et al.
Thermally rearranged (TR) poly(benzoxazole-co-pyrrolone) membranes tuned for high gas permeability and selectivity
J. Membr. Sci.
(2010) - et al.
Thermally rearranged (TR) poly(benzoxazole-co-amide) membranes for hydrogen separation derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB), 4,4′-oxydianiline (ODA) and isophthaloyl chloride (IPCl)
J. Membr. Sci.
(2013) - et al.
Cavity size, sorption and transport characteristics of thermally rearranged (TR) polymers
Polymer
(2011) - et al.
Grand Canonical Monte Carlo simulations for energy gases on PIM-1 polymer and silicalite-1
Chem. Eng. Sci.
(2012) - et al.
Investigation of CO2-induced plasticization in fluorinated polyimide membranes via molecular simulation
J. Membr. Sci.
(2012)
Free volume and alcohol transport properties of PDMS membranes: insights of nano-structure and interfacial affinity from molecular modeling
J. Membr. Sci.
Analysis of dual-mode model parameters for gas sorption in glassy polymers
J. Membr. Sci.
Synthesis and properties of fluorine-containing aromatic polybenzoxazoles from bis(o-aminophenols) and aromatic diacid chlorides by the silylation method
Macromolecules
Rigid-rod polymers. 1. Synthesis and thermal properties of para-aromatic polymers with 2,6-benzobisoxazole units in the main chain
Macromolecules
Rigid-rod polymers. 2. Synthesis and thermal properties of para-aromatic polymers with 2,6-benzobisthiazole units in the main chain
Macromolecules
Polymers with cavities tuned for fast selective transport of small molecules and ions
Science
Tuning microcavities in thermally rearranged polymer membranes for CO2 capture
Phys. Chem. Chem. Phys.
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