Free Volume Elements Research Articles

Boosted Intracavity Aperture in Macrocyclic Amines Enabling Finely Regulated Microporous Membranes.

Finely tuning the pore structure of traditional nanofiltration (NF) membranes is challenging but highly effective for achieving efficient separations. Herein, we propose a concept of using macrocyclic amines (1,4,7-triazacyclononane, 3A; 1,4,7,10-tetraazacyclododecane, 4A1; and 1,4,8,11-tetraazacyclotetradecane, 4A2) with different intra-annular apertures to finely modulate the pore structure of microporous membranes via interfacial polymerization (IP). The boost in the intracavity size of the building blocks results in heightened steric hindrance of these amine monomers, leading to a controlled increase in membrane pore size, as demonstrated by both film characterizations and multiscale simulations. In conjunction with the increased intracavity size, the water permeability follows an augmented trend of 3A-TMC, 4A1-TMC, and 4A2-TMC (TMC: trimesoyl chloride) while exhibiting increased molecular weight cut-offs due to larger free-volume elements and stronger pore interconnectivity. Our proposed macrocyclic amine design strategy provides a guideline for finely regulated microporous membranes with high potential in NF-related applications.

Surface Microstructure Study on Corona Discharge-Treated Polyethylene Using Positron Annihilation Spectroscopy.

The microstructure and chemical properties of the corona discharge process could provide an effective method for predicting the performance of high-voltage cable insulation materials. In this work, the depth profile of the microstructure and chemical characteristics of corona discharge-treated PE were extensively investigated using Doppler broadening of position annihilation spectroscopy accompanied with positron annihilation lifetime spectroscopy, attenuated total reflectance Fourier transform infrared spectra, Raman spectra and contact angle measurement. By increasing corona discharge duration, the oxygen-containing polar groups, including hydroxyl, carbonyl and ester groups, strongly contribute to the deterioration of hydrophobicity and the enhancement of hydrophilicity. And the mean free volume size, with a broadening distribution, decreases slightly. The line shape S parameter decreases because of the decrease in free volume elements and the appearance of oxygen-containing groups. Also, the thickness of the degradation layer, determined from the S parameter with positron injection depth, increases and diffuses into the PE matrix. A linear S-W plot within the degradation layer of different corona treatment duration samples indicates the defect type does not change. The S parameter decreases and the W parameter increases with an increasing corona duration. Using a slow positron beam, the nondestructive probe can be used to profile the microstructure and chemical environment across the corona discharge damage depth, which is beneficial for investigating the surface and interfacial insulation materials.

Supramolecular structure of epoxy oligomers

AbstractBased on the generalized results of experimental methods for studying the viscosity of oligomers—DSC, electron microscopy, NMR spin echo, x‐ray diffraction analysis, edge wetting angles, laser microinterference, atomic force microscopy, dynamic light scattering, plastic flow—an analysis of the features of the supramolecular organization of melts and solutions of epoxy oligomers was carried out. In terms of thermofluctuation approach, a qualitative assessment of the structural elements forming a macroscopic polymer body is given, and direct morphological evidence of their structure, thermodynamic stability, size evolution with temperature changes and elastic‐strain effects is presented. A mathematical apparatus has been developed to describe the destruction of domains in solutions and melts of oligomers. Analytical equations have been proposed for calculating the time of thermofluctuation relaxation of domains. Based on the analysis of the supramolecular structure of dian epoxy oligomers by transmission electron microscopy and translational mobility of oligomer macromolecules in the high temperature region (Tg + 150°C), it has been suggested that a structure such as “flickering clusters” of free volume elements is formed in the oligomer melts.Highlights Phase diagrams of aromatic and aliphatic epoxy oligomers have been constructed. The influence of molecular weight on the activation energy of diffusion of viscous flow is estimated. It has been shown that the supramolecular structure of epoxy oligomers in the melt contains “flickering clusters.” The lifetime of density fluctuations of epoxy oligomers domains in melts has been calculated.

Understanding the influence of sodium chloride concentration on ion diffusion in charged polymers

Previous observations made on sulfonated polysulfones equilibrated with solutions of increasing salinity, where salt diffusion coefficients increase concurrently with reductions in water content driven by osmotic de-swelling, are currently unexplained. To further explain the molecular underpinnings of these observations, we characterized the influence of external salt solution concentration on the average salt and ion diffusion properties of two sulfonated polymers: a model cross-linked material (XLAMPS) and a series of sulfonated polysulfones (HQ:BP – XX). Analysis of the average salt and co- and counter-ion diffusion coefficients in these polymers suggest that the increase in charge screening (of electrostatic interactions between anionic sulfonate groups and co-ions) that occurs with increasing salinity affects the salt diffusion coefficients of sulfonated polysulfone in a manner that is different from what would be expected based on changes in water content alone and from what was observed for XLAMPS. Free volume theory describes the influence of these electrostatic effects on salt diffusion using a parameter related to the characteristic free volume element size necessary for diffusion. The results and analysis suggest that charge screening reduces the characteristic free volume element size necessary for diffusion in sulfonated polysulfone, which provides an explanation for the observed increase in average salt diffusion coefficients with increased salinity.

Permeability of porous membrane polymers modified by supercritical carbon dioxide

An approach to predict the gas permeability of membrane polymers after supercritical CO2 treatment is proposed. The approach is based on the connection of the temperatures of secondary relaxation transitions with the effective sizes of mobile free volume elements in the polymers. The correlation between permeability of nitrogen and effective sizes of mobile holes for a set of polymers is established. The effect of supercritical CO2 on the nitrogen permeability of polycarbonate, polysulfone, polyvinylbutyral is analyzed by FTIR spectroscopy of low-molecular weight conformationally-inhomogeneous compounds introduced in the polymers. Membranes were exposed at 40 MPa and 333 K for 4 h through static treatment and dynamic treatment separately. For polyvinylbutyral, the nitrogen permeability did not change after supercritical CO2 modification while for polysulphone, the effective volume of mobile holes increased, but the nitrogen permeability decreased. For polycarbonate after supercritical CO2, the effective volume of mobile holes and the nitrogen permeability increased.

Ultrafast Interfacial Self-Assembly toward Bioderived Polyester COF Membranes with Microstructure Optimization.

The precise manipulation of the microstructure (pore size, free volume distribution, and connectivity of the free-volume elements), thickness, and mechanical characteristics of membranes holds paramount significance in facilitating the effective utilization of self-standing membranes. In this contribution, the synthesis of two innovative ester-linked covalent-organic framework (COF) membranes is first reported, which are generated through the selection of plant-derived ellagic acid and quercetin phenolic monomers in conjunction with terephthaloyl chloride as a building block. The optimization of the microstructure of these two COF membranes is systematically achieved through the application of three different interfacial electric field systems: electric neutrality, positive electricity, and negative electricity. It is observed that the positively charged system facilitates a record increase in the rate of membrane formation, resulting in a denser membrane with a uniform pore size and enhanced flexibility. In addition, a correlation is identified wherein an increase in the alkyl chain length of the surfactants leads to a more uniform pore size and a decrease in the molecular weight cutoff of the COF membrane. The resulting COF membrane exhibits an unprecedented combination of high water permeance, superior sieving capability, robust mechanical strength, chemical robustness for promising membrane-based separation science and technology.

Pendant Group Functionalization of Cyclic Olefin for High Temperature and High-Density Energy Storage.

High-temperature flexible polymer dielectrics are critical for high-power-density energy storage and conversion under harsh operating conditions. These types of dielectrics will need to simultaneously possess a high bandgap, dielectric constant and glass transition temperature - a substantial challenge when designing novel dielectric polymers. In this work, by varying halogen substituents of an aromatic pendant hanging off a bicyclic mainchain polymer, a class of high-temperature olefins with adjustable thermal stability are obtained, all with uncompromised large bandgaps. Halogens substitution of the pendant groups at para or ortho position of polyoxanorborneneimides (PONB) imparts it with tunable high glass transition temperature from ∼220 to 245°C, while with also moderate dielectric constant of ∼ 2.8-3.0 and high breakdown strength of ∼625-800MV/m. A high energy density of 7.1 J/cc at 200°C is achieved with p-POClNB, representing the highest reported energy density among all-organic homo-polymer dielectrics. Molecular dynamic simulations and ultrafast infrared spectroscopy were used to probe the free volume element distribution and chain relaxations of the polymers to provide insights to the dielectric thermal properties. An increase in free volume element is observed with the change in the pendant group from fluorine to bromine at the para position; however, a decrease in free volume element is observed as we change the pendant group from fluorine to chlorine at the ortho position because of the steric hindrance. Overall, the dielectric constant and band gap remain stable while the glass transition temperature changes more obviously. Consequently, by proper designing the pendant groups, the thermal stability of PONB can be improved for harsh condition electrification. This article is protected by copyright. All rights reserved.

Polyamide reverse osmosis membrane compaction and relaxation: Mechanisms and implications for desalination performance

This work investigates the compaction and relaxation behavior of composite reverse osmosis (RO) polyamide (PA) selective layers, utilizing non-equilibrium molecular dynamic (NEMD) simulations and well-controlled permeation experiments. Composite PA-RO membranes are prepared by interfacial polymerization of para and meta diamine monomer blends to achieve different PA film crosslinking degrees (CD). Wet-testing results suggest that “tighter” (higher CD) composite RO membranes undergo 65 % less compaction and recover 17 % more of their initial permeability (termed “relaxation”) when the pressure is relieved compared to lower CD PA layers. NEMD simulations provide a visual and quantitative characterization of PA layer's free volume changes in operando. NEMD simulations also corroborate experimental findings and elucidate the viscoelastic properties of the PA layer that govern compaction and relaxation behavior. The mechanisms of compaction under water permeation derived from NEMD simulations further support the notion of viscous flow of water through interconnected free volume elements (i.e., “pores”) within crosslinked PA films.

Scalable Preparation of Ultraselective and Highly Permeable Fully Aromatic Polyamide Nanofiltration Membranes for Antibiotic Desalination.

Membranes are important in the pharmaceutical industry for the separation of antibiotics and salts. However, its widespread adoption has been hindered by limited control of the membrane microstructure (pore architecture and free-volume elements), separation threshold, scalability, and operational stability. In this study, 4,4',4'',4'''-methanetetrayltetrakis(benzene-1,2-diamine) (MTLB) as prepared as a molecular building block for fabricating thin-film composite membranes (TFCMs) via interfacial polymerization. The relatively large molecular size and rigid molecular structure of MTLB, along with its non-coplanar and distorted conformation, produced thin and defect-free selective layers (~27 nm) with ideal microporosities for antibiotic desalination. These structural advantages yielded an unprecedented high performance with a water permeance of 45.2 L m-2 h-1 bar-1 and efficient antibiotic desalination (NaCl/adriamycin selectivity of 422). We demonstrated the feasibility of the industrial scaling of the membrane into a spiral-wound module (with an effective area of 2.0 m2). This module exhibited long-term stability and performance that surpassed those of state-of-the-art membranes used for antibiotic desalination. This study provides a scientific reference for the development of high-performance TFCMs for water purification and desalination in the pharmaceutical industry.

Copolyimide molecular brushes with poly(methacrylic acid) side chains as additive to polyphthalamide membrane: Organization, physical, and transport properties

AbstractMacromolecules of complex architecture find application as modifiers of commercial polymeric membrane materials. In this work, copolyimide molecular brushes (coPI) composed of polyimide backbone and poly(methacrylic acid) side chains were used to modify poly(m‐phenylene iso‐phthalamide) (PPA). Structure, physical, mechanical, and transport properties of dense nonporous PPA/coPI membranes containing up to 10 wt% coPI were studied. The effect of included coPI on the membrane structure was estimated using atomic force microscopy and mechanical tests. The coPI modifier contributes to the additional formation of free volume elements evenly distributed throughout the membrane. Transport properties of PPA/coPI membranes were investigated via sorption tests and pervaporation separation of methanol (MeOH) and methyl tert‐butyl ether (MTBE) mixtures. The inclusion of coPI modifier in the PPA membrane leads to an increase in the total flux of the membrane. The highest separation factor was found for the PPA/coPI membrane containing 10 wt% coPI; transport properties of the best membrane were compared with the literature data on separation of the azeotropic MeOH–MTBE mixture.

Evolution of free volume elements in amorphous polymers undergoing uniaxial deformation: a molecular dynamics simulations study

Amorphous polymers are considered promising materials for separation applications due to their excellent transport properties and low fabrication costs.

Study of the free volume distribution in the blend of polyvinyl chloride and polyvinyl butyral using FTIR spectroscopy

The local molecular dynamics and the free volume distribution in the blend of polyvinyl chloride and polyvinyl butyral in the ratio 1 : 1 were studied by FTIR spectroscopy. The freezing temperatures of the conformational equilibria of five conformationally inhomogeneous compounds in the blend under study were determined, and the effective sizes of the mobile free volume elements in the blend were determined.

Structure, Dynamics, and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations

Structure, Dynamics, and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations

Simultaneously enhanced gas separation and anti-aging performance of intrinsic microporous polyimide by dibromo substitution

Physical aging is a huge challenge hindering the practical gas separation application of high-performance intrinsic microporous polyimides (PIM-PIs). Herein, through precise molecular design, bromine was introduced into the PIM-PI chain to both improve its separation and anti-aging properties. To clarify the anti-aging effect of bromo groups in PIM-PIs, two fluorene-based diamine isomer monomers, 9,9-bis(3-bromo-4-aminophenyl)-fluorene (BBAPF), 2,7-dibromo-9,9-bis(4-aminophenyl)-fluorene (DBBAPF), and their corresponding PIM-PIs (BAPI, BBPI, DBPI) were synthesized. Compared with pristine BAPI membrane, the permeability of BBPI is improved about 50%, which can be attributed to the bromo groups near the amino group suppressing chain densification packing and providing more free volume element (FVE). In addition, the substituted bromo group increased the chain rigidity of the polymer and hindered its rotation, resulting in a lower release of fractional free volume (FFV) during aging process. Particularly, the permeability of the BBPI was maintained at approximately ∼70% with slightly increased selectivity after 250 days, while the pristine BAPI only retained about 50% permeability of N2 (CH4). Our findings proposed an effective strategy for designing microporous polymers with improved permeability and anti-aging properties for practical applications in gas separation membranes.

A new dip-coating approach for plasticization-resistant polyimide hollow fiber membranes: In situ thermal imidization and cross-linking of polyamic acid

Polymeric hollow fiber membranes (HFMs) are in high demand for gas separation due to their superior processability and cost-effectiveness, despite exhibiting high vulnerability to plasticization. In this study, we present a new dip-coating technique for fabricating defect-free and plasticization-resistant polyimide HFMs on a cross-linked polyimide/polysilsesquioxane hollow fiber support. This is achieved through an in situ process involving thermal imidization and cross-linking of a polyamic acid (PAA) precursor solution. The use of a thermally stable cross-linked nanoporous hollow fiber support ensures the coating of various glassy PAA solutions prepared by different dianhydrides and diamine monomers without disturbing its internal pore structure. The resulting polyimide hollow fiber membranes enhanced CO2/CH4 separation performance compared to as-synthesized polyimide solution-coated counterpart. It is because in situ thermal imidization and cross-linking of PAA solutions induced the formation of more imine linkages, possibly generating larger free volume elements due to the disturbance of intersegmental chain packing while maintaining sufficient chain rigidity. Additionally, the membrane’s practical applicability was confirmed as the hollow fibers demonstrated resistance to plasticization when subjected to an equimolar CO2/CH4 mixed feed of approximately 50 bar. Our simple but versatile dip-coating approach allows for the formation of highly plasticization-resistant polyimide HFMs.

Synthetically tunable polymers, free volume element size distributions, and dielectric breakdown field strengths

Four polyetherimide (PEI) polymers were synthesized with different end groups, while leaving the molecular weights, glass transition temperature, thermal properties, and dielectric constant essentially unchanged. The Restricted Orientation Anisotropy Method (ROAM), an ultrafast infrared laser technique, was used to measure the films’ free volume elements (FVEs) radius probability distribution (RPD) curves. This technique exploits the observation that a vibrational probe’s molecular reorientation dynamics inside a polymer film’s FVEs are sterically restricted by the surfaces of the FVEs. The measured RPD curves displayed significant changes in shape and center positions for the PEI polymers with different end cap. The results demonstrate that structural changes to the end groups of a polymer can significantly modify its microscopic morphology, which is consistent with other experiments that observed changes in macroscopic observables. The thin film samples’ breakdown fields (EBD) were measured, and a correlation between polymer films having a higher probability of large FVEs and lower EBDs was observed. This work demonstrates that the nanoscopic chain packing structure that controls the nature of the FVEs and breakdown field properties of PEI are synthetically tunable, without the need to change the main chain chemical structure. The results provide some experimental evidence for the theoretical model of electron-acceleration inside FVEs as the microscopic origin of polymer film dielectric breakdown. The results also provide a framework for the study of other polymer dielectrics, and suggest that design of high breakdown field polymer films may involve minimizing FVE sizes, particularly the large FVE tails of the size distribution.

Microstructure optimization of bioderived polyester nanofilms for antibiotic desalination via nanofiltration

The successful implementation of thin-film composite membranes (TFCM) for challenging solute-solute separations in the pharmaceutical industry requires a fine control over the microstructure (size, distribution, and connectivity of the free-volume elements) and thickness of the selective layer. For example, desalinating antibiotic streams requires highly interconnected free-volume elements of the right size to block antibiotics but allow the passage of salt ions and water. Here, we introduce stevioside, a plant-derived contorted glycoside, as a promising aqueous phase monomer for optimizing the microstructure of TFCM made via interfacial polymerization. The low diffusion rate and moderate reactivity of stevioside, together with its nonplanar and distorted conformation, produced thin selective layers with an ideal microporosity for antibiotic desalination. For example, an optimized 18-nm membrane exhibited an unprecedented combination of high water permeance (81.2 liter m-2 hour-1 bar-1), antibiotic desalination efficiency (NaCl/tetracycline separation factor of 11.4), antifouling performance, and chlorine resistance.

Structural engineering on 6FDA-Durene based polyimide membranes for highly selective gas separation

In this work, four diamine monomers, selected based-on steric hindrance and affinity with gas to be separated, were separately introduced into 6FDA-Durene to prepare co-polyimides. The influences of incorporating diamine monomer geometry on free volume elements (FFV, FAV, cavity size distribution, surface area and cavity connectivity) and gas transport properties (solubility and diffusion) of synthesized co-polyimide membranes were systematically investigated. The introduction of second amino monomer can increase gas selectivity, however leaded to lower gas permeability. Gas solubility and solubility selectivity of membrane remained almost stable with introduction of different amino monomer. While the gas diffusion changed remarkably, and the diffusion selectivity increased by about 170%. The FDDA membrane with crown ether structure displayed the highest gas selectivity (increased by 122%, 121%, 158%, 115% and 115% for H2/N2, H2/CH4, O2/N2, CO2/N2 and CO2/CH4, respectively, compared to 6FDA-Durene membrane), whose selectivity was relatively high among the PI-based membranes. This can be explained that the crown ether group granted the membrane with largest FAV ratio of specific gas pair, as well as the smallest pore size that would cause stronger size sieving effect. These results indicated that gas separation property of polyimide membranes can be effectively adjusted via tuning monomer structure, which is easily synthesized or has been commercialized, to realize the industrial application of membrane-based gas separation.

Intrinsically microporous polyimides from p-phenylenediamine with fused cyclopentyl substituents for membrane-based gas separation

In this study, a novel diamine, 1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene-9,10-diamine (DMNDA), was synthesized in six-steps using dicyclopentadiene and p-benzoquinone as the starting materials. Next, four polyimides of intrinsic microporosity (PIM-PIs) were prepared by the high-temperature polycondensation of DMNDA with four typical dianhydrides in m-cresol. The fractional free volumes (FFV) of these polymers were in the range of 0.193–0.216, decreasing in order of SBIDA > 6FDA > BTA > CpODA. Furthermore, the introduction of bulky fused cyclopentyl substituents resulted in free volume elements of ∼ 0.5 nm, evidenced by visible shoulders in the WAXD patterns. Consequently, the gas permeability and perm-selectivity of 6FDA-DMNDA were greater than those of the benchmark polymer, 6FDA-Durene. SBIDA-DMNDA and BTA-6FDA-DMNDA exhibited high H2 permeability, but modest H2/CH4 and H2/N2 selectivity, approaching the 2008 Robeson upper bounds. Furthermore, SBIDA-DMNDA showed a CO2 permeability of 1439 barrer and a CO2/CH4 selectivity of ∼ 14 at 10 bar, approaching the 2018 upper bound for CO2/CH4 mixed-gas separation.

Towards guided metallic ions migration in network glass-formers: The Positronics approach in application to lithium tetraborate glass

The Positronics approach grounded on positron annihilation lifetime spectroscopy treated in terms of x3-x2-CDA (coupling decomposition algorithm) is used to recognize nanostructurization-driven volumetric effects in lithium tetraborate glass g-Li2B4O7 under transition to crystalline state, doping by Ag and/or Gd activators and 2-h above-Tg annealing in air environment. Lithium vacancies and vacancy complexes are shown to be principal free-volume elements responsible for positron trapping and decaying of bound positron-electron (positronium, Ps) states in these glasses. Expected devitrification (glass-to-crystal) transition corresponds to direct (Ps-to-positron) trapping conversion in volume-contracted network due to transformation of Ps-hosting sites in positron traps.Plausible migration scenarios for metallic activators in g-Li2B4O7 include occupation of vacancy-type positron traps and Ps-decay holes by Ag+ and/or Gd3+ ions, totally reducing positron-trapping and Ps-decaying contributions. Effect of thermal annealing in metal-doped lithium tetraborate glass is not related to simple de-occupation of vacancy-type defects due to escape of Ag + or Gd3+ ions. Annealing-stimulated transformations are guided by relaxation towards defect-free glass structure, being disturbed by competitive defect formation and destruction processes accelerated by interaction with oxygen. Since non-elementary nature of trapping-modification processes in doped glasses subjected to annealing, the formalism of x3-x2-CDA cannot be reasonably applicable.