Quantitative approach for the prediction of preferential sorption in the case of pervaporation of a physico-chemically similar binary mixture

Selective transport by pervaporation of physico-chemically similar and polar components through a polar membrane is complicated due to competitive sorption and diffusion phenomena. In the present study the mechanism of preferential sorption has been predicted for such a system, methanol-ethylene glycol-cellophane. The mechanism has been explained in terms of intercrystalline swelling of the polymer matrix in presence of increasing methanol concentration in the feed. The reduction in preferential sorption at increasing wt% of methanol in the feed may be due to the increasing accessibility of the membrane towards ethylene glycol. This phenomenon has been quantitatively explained by considering a non-linear dependence on concentration of the binary liquid-polymer interaction parameter. Theoretical sorption data have been derived from the Flory-Huggins thermodynamics by using the swelling equilibrium condition. The coupling and plasticization phenomena in sorption are explained in terms of liquid-polymer interaction parameters. The theoretical results show good agreement with previously published experimental data.     

Mass and momentum transfer in polymers. III. Effects of liquid penetrants on tensile stress relaxation of amorphous polymers

Sorption, diffusion, swelling, and tensile stress relaxation measurements were made at room temperature (23°C) for the systems poly(n-butyl methacrylate) (PBMA) with liquid methanol and ethanol, and poly(methyl acrylate) (PMA) with liquid water. Stress relaxation curves for the fully swollen polymers could be superimposed approximately with those for the dry polymers by appropriate shifting along the long axes. For PMA–water the measured curve for stress relaxation with concurrent sorption could be predicted accurately by using a moving boundary theory with data measurements of stress relaxation of the unswollen and swollen polymer combined with sorption data. The modified moving boundary theory is generalized to include the effects of dimension changes through swelling and the larger effects of plasticization associated with sorption of liquids. This improved theory accurately predicts measured curves of stress relaxation with concurrent sorption for the PBMA–alcohol systems from individual stress relaxation, sorption, diffusion and swelling data. The general approach should be applicable to other amorphous polymer–liquid swelling agent systems. The anisotropic nature of swelling of polymer films and its effect on calculated diffusion coefficients are discussed briefly.     

CO2-induced plasticization phenomena in glassy polymers

A typical effect of plasticization of glassy polymers in gas permeation is a minimum in the relationship between the permeability and the feed pressure. The pressure corresponding to the minimum is called the plasticization pressure. Plasticization phenomena significantly effect the membrane performance in, for example, CO₂/CH₄ separation processes. The polymer swells upon sorption of CO₂ accelerating the permeation of CH₄. As a consequence, the polymer membrane loses its selectivity. Fundamental understanding of the phenomenon is necessary to develop new concepts to prevent it.In this paper, CO₂-induced plasticization phenomena in 11 different glassy polymers are investigated by single gas permeation and sorption experiments. The main objective was to search for relationships between the plasticization pressure and the chemical structure or the physical properties of the polymer. No relationships were found with respect to the glass-transition temperature or fractional free volume. Furthermore, it was thought that polar groups of the polymer increase the tendency of a polymer to be plasticized because they may have dipolar interactions with the polarizable carbon dioxide molecules. But, no dependence of the plasticization pressure on the carbonyl or sulfone density of the polymers considered was observed. Instead, it was found that the polymers studied plasticized at the same critical CO₂ concentration of 36±7 cm³ (STP)/cm³ polymer. Depending on the polymer, different pressures (the plasticization pressures) are required to reach the critical concentration.     

Solution polymerization of polybenzimidazole

Polybenzimidazoles (PBI) are an important class of heterocyclic polymers that exhibit high thermal and oxidative stabilities. The two dominant polymerization methods used for the synthesis of PBI are the melt/solid polymerization route and solution polymerization using polyphosphoric acid as the solvent. Both methods have been widely used to produce high‐molecular weight PBI, but also highlight the obvious absence of a practical organic solution‐based method of polymerization. This current work explores the synthesis of high‐molecular weight meta‐PBI in N,N‐dimethyl acetamide (DMAc). Initially, model compound studies examined the reactivity of small molecules with various chemical functionalities that could be used to produce 2‐phenyl‐benzimidazole in high yield with minimal side reactions. ¹H NMR and FTIR studies indicated that benzimidazoles could be efficiently synthesized in DMAc by reaction of an o‐diamine and the bisulfite adduct of an aromatic aldehyde. Polymerizations were conducted at various polymer concentrations (2‐26 wt % polymer) using difunctional monomers to optimize reaction conditions in DMAc which resulted in the preparation of high‐molecular weight m‐PBI (inherent viscosities up to 1.3 dL g^?1). TGA and DSC confirmed that m‐PBI produced via this route has comparable properties to that of commercial m‐PBI. This method is advantageous in that it not only allows for high‐polymer concentrations of m‐PBI to be synthesized directly and efficiently, but can be applied to the synthesis of many PBI derivatives. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1795–1802     

Synthesis and properties of a new fluorine‐containing polybenzimidazole for high‐temperature fuel‐cell applications

An amorphous, organosoluble, fluorine‐containing polybenzimidazole (PBI) was synthesized from 3,3′‐diaminobenzidine and 2,2‐bis(4‐carboxyphenyl)hexafluoropropane. The polymer was soluble in N‐methylpyrrolidinone and dimethylacetamide and had an inherent viscosity of 2.5 dL/g measured in dimethylacetamide at a concentration of 0.5 g/dL. The 5% weight loss temperature of the polymer was 520 °C. Proton‐conducting PBI membranes were prepared via solution casting and doped with different amounts of phosphoric acid. In the methanol permeability measurement, the PBI membranes showed much better methanol barrier ability than a Nafion membrane. The proton conductivity of the acid‐doped PBI membranes increased with increasing temperatures and concentrations of phosphoric acid in the polymer. The PBI membranes showed higher proton conductivity than a Nafion 117 membrane at high temperatures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4508–4513, 2006     

Diffusion and sorption of methanol and water in Nafion using time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy

Direct methanol fuel cells (DMFCs) are promising portable power sources. However, their performance diminishes significantly because of high methanol crossover (flux) in the polymer electrolyte membrane (e.g., Nafion 117) at the desired stoichiometric methanol feed concentration. In this study, the diffusion and sorption of methanol and water in Nafion 117 were measured using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. This technique is unique because of its ability to measure multicomponent diffusion and sorption within a polymer on a molecular level in real time as function of concentration. Both the effective mutual diffusion coefficients and concentrations of methanol and water in Nafion 117 were determined with time-resolved FTIR-ATR spectroscopy as a function of methanol solution concentration. The methanol flux, calculated from FTIR-ATR, matched that determined from a conventional technique (permeation cell) and increased by almost 3 orders of magnitude over the methanol solution concentration range studied (0.1-16 M). Furthermore, the data obtained in this study reveal that the main contribution to the increase in methanol flux is due to methanol sorption in the membrane.     

傅立叶变换红外光谱技术在超临界CO2作用聚合物体系中的应用

超临界CO₂（scCO₂）作为一种物理化学性质优良、具有高扩散速率及优良溶解性能的溶剂,在科学研究及工业生产中广受青睐。将scCO₂应用于聚合物体系中,CO₂ 与聚合物间特殊的相互作用有利于CO₂分子在聚合物中的吸附与扩散。同时通过CO₂的吸附及其对聚合物的溶胀和塑化作用,聚合物所处微观化学环境以及整体结构性质会发生一定的变化。由于傅立叶变换红外光谱（FTIR）技术能够有效地考察化学环境变化对分子结构造成的影响,这一表征技术在超临界CO₂作用体系中广为应用。本文主要选取了近年来利用FTIR技术考察scCO₂作用于聚合物体系的一些实例,从CO₂-聚合物相互作用机理,scCO₂对聚合物或生物大分子的加工过程的影响两方面,阐述了利用红外光谱技术在scCO₂作用体系中的应用以及前景。     

Water vapor sorption and diffusion in glassy polymers

By establishing relationships between polymer structure and gas permeation behavior, significant advances have been made in designing materials for membrane separation of gas mixtures. However, the situation is not so well understood when water vapor is one of the components since water molecules may interact with the polymer (plasticization) or each other (clustering) in ways that complicate the structure-property relationships. In addition, accurate measurement of water sorption, diffusion, and permeation is more complicated than for gases because of the unique hydrogen bonding capability of water, e.g., its tendency to strongly adsorb on high-energy surfaces and high heat of vaporization. A progress report on a broad program to understand water sorption and diffusion in glassy polymers that may be of interest for membrane applications is outlined; specific strategies include studies of structurally related polymers and miscible blends of hydrophobic/hydrophilic polymer pairs.     

Suppression of CO2-plasticization by semiinterpenetrating polymer network formation

CO₂-induced plasticization may significantly spoil the membrane performance in high-pressure CO₂/CH₄ separations. The polymer matrix swells upon sorption of CO₂, which accelerates the permeation of CH₄. The polymer membrane looses its selectivity. To make membranes attractive for, for example, natural gas upgrading, plasticization should be minimized. In this article we study a polymer membrane stabilization by a semiinterpenetrating polymer network (s-ipn) formation. For this purpose, the polyimide Matrimid 5218 is blended with the oligomer Thermid FA-700 and subsequently heat treated at 265°C. Homogeneous films are prepared with different Matrimid/Thermid ratios and different curing times. The stability of the modified membrane is tested with permeation experiments with pure CO₂ as well as CO₂/CH₄ gas mixtures. The original membrane shows a minimum in its permeability vs. pressure curves, but the modified membranes do not indicating suppressed plasticization. Membrane performances for CO₂/CH₄ gas mixtures showed that the plasticizing effect indeed accelerates the permeation of methane. The modified membrane clearly shows suppression of the undesired methane acceleration. It was also found that just blending Matrimid and Thermid was not sufficient to suppress plasticization. The subsequent heat treatment that results in the s-ipn was necessary to obtain a stabilized permeability. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1547–1556, 1998     

Hydration and interfacial water in Nafion membrane probed by transmission infrared spectroscopy

Infrared spectroscopy was applied to probe water inside pores and channels of Nafion membrane exchanged with either proton (H+) or sodium ions (Na+). Transmission measurements were performed on freestanding Nafion 112 (approximately 50 microm thickness) in a cell that enabled adjustment of the relative humidity. Experiments that employed Na+-exchanged Nafion focused on relative humidity environments at or below about 32% generated through the use of humectants. Under these conditions, narrow features in the O-H stretching spectral region near 3650-3720 cm(-1), previously attributed to interfacial water, were detected and matched to bands in vibrational sum frequency (VSF) spectra of water/air, water/organic, and salt-solution/air interfaces. The features correspond to the stretching mode of the "free" OH group of water oriented with one hydrogen atom toward other water molecules and interacting through hydrogen bonding and the other straddling the interface extending into fluorocarbon-rich regions (approximately 3668 cm(-1)) or air-filled segments (approximately 3700 cm(-1)) in the membrane. For membrane exchanged with H+, -SO3- groups were easily shifted to -SO3H as water was removed upon exposure to a few Torr of vacuum at 95 degrees C. In contrast, residual water was retained by membrane exchanged with Na+ after exposure to these conditions for up to 72 h. The permeation of methanol and acetone into Na+-exchanged Nafion 112 was also examined. The C-H and O-H stretching modes of methanol were perturbed in a manner that suggests the polymer disrupts hydrogen bonding interactions within the solvent, similar to the effect it exerts on pure water. For acetone, the C-H stretching modes were not shifted appreciably compared to those of the bulk liquid. However, the carbonyl band was affected, indicating the likely importance of dipolar interactions between solvent molecules and polar groups on the polymer. Control experiments performed with poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) membrane did not show evidence for water or methanol permeation, which demonstrates the critical role played by the ion-filled channels and pores in facilitating solvent transport within Nafion membrane.     

Polymeric Membrane Nanofiltration and Its Application to Separations in the Chemical Industries

The application of membrane technology, particularly water-based nanofiltration, as a separation process in the chemical industries has increased tremendously in recent years. However, the use of membranes capable of molecular separation in non-aqueous systems (e.g. nanofiltration) is a relatively new and growing application of membrane technology. The main challenge in applying polymeric nanofiltration membranes to non-aqueous systems is that the polymers developed for water-based applications are not suitable. Polyimide is a particularly interesting polymer as it has excellent chemical resistance, and membranes produced from it provide desirable separation properties – i.e. economically viable flux and good separation of nanoscale molecules. Various research works have shown that commercial polyimide organic solvent nanofiltration (OSN) membranes, trademark STARMEM™, 1 are robust and suitable for performing molecular separations. This work will discuss in detail the use of STARMEM™ in a pharmaceutical application. The EIC-OSN process was developed for separating the enantiomers of chiral compounds in pharmaceutical applications. High optical purity (94.9%) of (S)-phenylethanol from rac-phenylethanol was achieved through the use of STARMEM™122. Process simulation of the ideal eutomer-distomer system predicted that the highest theoretical resolvability from this process would be 99.2%. Other application areas of OSN are varied, including purification and fractionation in the natural products industry, homogeneous catalyst recovery, monomer separation from oligomers, etc. Currently, OSN is used in a small number of processes including a very large petrochemical application, but it has the potential to be applied to a wide range of separations across the full spectrum of the chemical industries.     

Fourier transform infrared study on the sorption of water to various kinds of polymer thin films

The state of sorbed water and the sorbing processes of water to various polymer thin films were studied with Fourier transform infrared (FTIR) spectroscopy. To prepare the polymer films, we used poly(ethylene glycol)s of different molecular weights and various kinds of vinyl polymers, such as poly(2‐methoxyethyl acrylate). The O? H stretching band of water sorbed in the films increased gradually on contact with water vapor at 50% relative humidity and leveled off. When O? H stretching bands of water sorbed to polymer films were compared, the peak positions and profiles of water sorbed to the polymeric materials with the same hydrogen‐bonding site were similar. A hybrid density‐functional method supported the assignment of the peaks. Furthermore, the diffusion coefficient (D) of water vapor in the polymer films was estimated by time‐resolved measurements of the sorbed water at the very initial stage (0–830 s). It was clearly shown that the D values of water vapor in the polymer materials with a strong hydrogen‐bonding site were smaller than those in hydrophobic polymers. The usefulness of the FTIR technique to investigate water sorption to polymer materials was definitely demonstrated. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2175–2182, 2001     

Atomistic packing models for experimentally investigated swelling states induced by CO2 in glassy polysulfone and poly(ether sulfone)

Atomistic packing models have been created, which help to better understand the experimentally observed swelling behavior of glassy polysulfone and poly (ether sulfone), under CO₂ gas pressures up to 50 bar at 308 K. The experimental characterization includes the measurement of the time‐dependent volume dilation of the polymer samples after a pressure step and the determination of the corresponding gas concentrations by gravimetric gas‐sorption measurements. The models obtained by force‐field‐based molecular mechanics and molecular dynamics methods allow a detailed atomistic analysis of representative swelling states of polymer/gas systems, with respect to the dilation of the matrix. Also, changes of free volume distribution and backbone mobility are accessible. The behavior of gas molecules in unswollen and swollen polymer matrices is characterized in terms of sorption, diffusion, and plasticization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1874–1897, 2006     

Physico-chemical analysis of semi-crystalline PEEK in aliphatic and aromatic solvents

Polyether ether ketone (PEEK) is a semi-crystalline thermoplastic polymer having excellent mechanical and thermal properties. Exposure of this polymer to aliphatic and aromatic solvents can lead to degradation or swelling of the polymeric material. The present work described the plasticization and stability analysis of semi-crystalline PEEK under different aromatic and aliphatic solvent environment. A variety of solvents (acetone, benzene, benzyl alcohol, chloroform, methanol, and toluene), based on their Hildebrand’s Solubility Parameter, were chosen for investigation. The physico-chemical characteristics of virgin and treated polymeric samples were investigated using Gas Chromatography–Mass Spectrometry (GC–MS), Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Fourier Transform Infrared Spectroscopy (FTIR) techniques. The results indicated that the solvent exposure did not significantly affect the thermal behavior and chemical structure of the polymer. However, it seems that certain components of the polymer were leached into the solvent phase as revealed by the GC–MS analysis. The present study identified PEEK as a potentially suitable polymer for the applications where high resistance to aliphatic and aromatic solvents is needed.     

Micro ATR‐FTIR study of the hydration state of poly(N‐tert‐butylacrylamide‐co‐acrylamide) copolymers during phase transition in aqueous media and in the mixture of water–methanol

Coil‐globule transition of poly(N‐tert‐butylacrylamide‐co‐acrylamide) P(NTBAM‐co‐AM) copolymers is investigated in the aqueous solution and in the mixture of water–methanol by micro ATR‐FTIR spectroscopy technique. In this study the microstructure and its changes in the hydration states of the distinct groups of these copolymers are investigated by micro ATR/FTIR technique. The results showed that by heating the solution above the LCST hydrogen bonding between C?O and water was decreased but the hydrogen bonding between polymeric chains increased, which prove the aggregation of polymer chain during phase separation. The chemical shifts of IR bands are also studied in the mixture of water–methanol. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 356–363, 2010     

Adsorption and conformation of carboxymethyl cellulose at solid-liquid interfaces using spectroscopic, AFM and allied techniques

Carboxymethyl cellulose (CMC) is a polysaccharide which is widely used in many industrial sectors including food, textiles, paper, adhesives, paints, pharmaceutics, cosmetics and mineral processing. It is a natural organic polymer that is non-toxic and biodegradable. These properties make it ideal for industrial applications. However, a general lack of understanding of the interaction mechanism between the polysaccharides and solid surfaces has hindered the application of this polymer. In this work, adsorption of CMC at the solid-liquid interface is investigated using adsorption and electrophoretic mobility measurements, FTIR, fluorescence spectroscopy, AFM and molecular modeling. CMC adsorption on talc was found to be affected significantly by changes in solution conditions such as pH and ionic strength, which indicates the important role of electrostatic force in adsorption. The pH effect on adsorption was further proven by AFM imaging. Electrokinetic studies showed that the adsorption of CMC on talc changed its isoelectric point. Further, molecular modeling suggests a helical structure of CMC in solution while it is found to adsorb flat on the solid surface to allow its OH groups to be in contact with the surface. Fluorescence spectroscopy studies conducted to investigate the role of hydrophobic bonding using pyrene probe showed no evidence of the formation of hydrophobic domains at talc-aqueous interface. Urea, a hydrogen bond breaker, markedly reduced the adsorption of CMC on talc, supports hydrogen bonding as an important factor. In FTIR study, the changes to the infrared bands, associated with the CO stretch coupled to the CC stretch and OH deformation, were significant and this further supports the strong hydrogen bonding of CMC to the solid surface. In addition, Langmuir modeling of the adsorption isotherm suggests hydrogen bonding to be a dominant force for polysaccharide adsorption since the adsorption free energy of this polymer was close to that for hydrogen bond formation. All of the above results suggest that the main driving forces for CMC adsorption on talc are a combination of electrostatic interaction and hydrogen bonding rather than hydrophobic force.     

Sorption and diffusion of methanol and water in PVDF‐g‐PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)‐graft‐poly(styrene sulfonic acid) (PVDF‐g‐PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in‐solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF‐g‐PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF‐g‐PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG‐NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF‐g‐PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3277–3284, 2000     

Role of Surface Molecular Architecture and Energetics of Hydrogen Bonding Sites in Adsorption of Polymers and Surfactants

Hydrogen bonding is generally thought to be an ubiquitous adsorption mechanism, which often foils selective adsorption schemes. Through investigation of hydrogen bonding energy and its dependence on surface molecular architecture, it may be possible to develop new methodologies to control the adsorption of surfactants and polymeric flocculants, depressants, and dispersants used in particulate processing industries. A model system using St?ber silica spheres and polyethylene oxide, a polymer known for its ability to form hydrogen bonds, was examined. The effect of two different surface treatments of the silica particles, calcination and rehydroxylation, upon the adsorption of two polymer molecular weights was studied. The adsorption behavior was then linked to the respective surface structures via characterization of the surfaces using FTIR, NMR, and Raman techniques. In this paper role of hydrogen bonding sites and surface architecture on adsorption is discussed. Copyright 2000 Academic Press.     

The impact of water and hydrocarbon concentration on the sensitivity of a polymer-based quartz crystal microbalance sensor for organic compounds

Long-term environmental monitoring of organic compounds in natural waters requires sensors that respond reproducibly and linearly over a wide concentration range, and do not degrade with time. Although polymer coated piezoelectric based sensors have been widely used to detect hydrocarbons in aqueous solution, very little information exists regarding their stability and suitability over extended periods in water. In this investigation, the influence of water aging on the response of various polymer membranes [polybutadiene (PB), polyisobutylene (PIB), polystyrene (PS), polystyrene-co-butadiene (PSB)] was studied using the quartz crystal microbalance (QCM). QCM measurements revealed a modest increase in sensitivity towards toluene for PB and PIB membranes at concentrations above 90 ppm after aging in water for 4 days. In contrast, the sensitivity of PS and PSB coated QCM sensors depended significantly on the toluene concentration and increased considerably at concentrations above 90 ppm after aging in water for 4 days. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR–FTIR) showed that there is a change in the sorption mechanism at higher toluene levels for PS and PSB. Positron annihilation lifetime spectroscopy (PALS) studies were performed to investigate the free volume properties of all polymers and to monitor any changes in the free volume size and distribution due to water and toluene exposure. The PALS did not detect any considerable variation in the free volume properties of the polymer films as a function of solution composition and soaking time, implying that viscoelastic and/or interfacial processes (i.e. surface area changes) are probably responsible for variations in the QCM sensitivity at high hydrocarbon concentrations. The results suggest that polymer membrane conditioning in water is an issue that needs to be considered when performing QCM measurements in the aqueous phase. In addition, the study shows that the hydrocarbon response is concentration dependant for polymers with a high glass transition temperature, and this feature is often neglected when comparing sensor sensitivity in the literature.     

Novel method for the preparation of ionically crosslinked sulfonated poly(arylene ether sulfone)/polybenzimidazole composite membranes via in situ polymerization

A series of ionically crosslinked composite membranes were prepared from sulfonated poly(arylene ether sulfone) (SPAES) and polybenzimidazole (PBI) via in situ polymerization method. The structure of the pristine polymer and the composite membranes were characterized by FT-IR. The performance of the composite membranes was characterized. The study showed that the introduction of PBI led to the reduction of methanol swelling ratio and the increase of mechanical properties due to the acid–base interaction between the sulfonic acid groups and benzimidazole groups. Moreover, the oxidative stability and thermal stability of the composite membranes were improved greatly. With the increase of PBI content, the methanol permeability coefficient of the composite membranes gradually decreased from 1.59 × 10⁻⁶ cm²/s to 1.28 × 10⁻⁸ cm²/s at 30 °C. Despite the fact that the proton conductivity decreased to some extent as a result of the addition of PBI, the composite membrane with PBI content of 5 wt.% still showed a proton conductivity of 0.201 S/cm at 80 °C which could actually meet the requirement of proton exchange fuel cell application. Furthermore, the composite membranes with PBI content of 2.5–7.5 wt.% showed better selectivity than Nafion117 taking into consideration the methanol swelling ratio and proton conductivity comprehensively.