Fractional Free Volume Research Articles

Molecular dynamics simulation of epoxy resin properties at different C=N contents

In response to the green development strategies of countries all over the world, research on degradable epoxy resins has attracted widespread attention. The introduction of reversible covalent bonds in the conventional cross-linked structure of epoxy resins is one of the methods to achieve degradation of epoxy resins, and most researchers use molecular dynamics simulations in their preliminary studies to investigate the feasibility of the introduction of reversible covalent bonding schemes. The purpose of this paper is to investigate the feasibility of introducing C=N into the cross-linked structure of epoxy resins. Four formulation schemes of vanillin-based monoepoxides with the curing agent 4,4′-methylenebis(cyclohexylamine) were designed, and the molecular dynamics simulation method was used to cross-link them. The changes in the cross-linking degree, structural parameters before and after cross-linking, free volume fraction, and C=N content before and after cross-linking were investigated. The effects of different C=N contents on the thermal properties such as glass transition temperature and thermal expansion coefficient, as well as the mechanical properties such as the elastic modulus and shear modulus of this epoxy resin, were investigated. The bond-breaking characteristics of C=N, C–N, and C–O were compared by density of states and differential charge density simulations. Then the degradation mechanism of epoxy resin after the introduction of C=N was illustrated. The results show that as the specific gravity of the curing agent molecule increases, the cross-linking degree tends to increase. The cross-linked model has reduced volume, increased density, decreased energy, and a more stable structure. After crosslinking, the gaps between the segments in the system become smaller, and the fraction of free volume decreases as the proportion of crosslinking agent molecules increases. The C=N content in epoxy resin shows an increasing trend first and then decreases with the increase in the proportion of the curing agent. The glass transition temperature of the material increases with the increase in C=N content, while the coefficient of thermal expansion decreases with the increase in C=N content. The elastic modulus and shear modulus of the material show an increasing trend with the increase in C=N content, with a relatively gradual change in magnitude. Compared with C–N and C–O bonds, the C=N bond is weaker in strength, has a greater polarity, and is more prone to cleavage and degradation.

Nitrile-Containing Spiro-Twisted Benzoxazines for All-Solid-State Polymer Electrolytes in Lithium-Ion Batteries

In this study, we developed new cross-linkable benzoxazine molecules comprising a spiro-twisted core and benzyl cyanide or benzonitrile groups to prepare semi-interpenetrating polymer network (s-IPN) electrolytes. The features of spiro-twisted core and cross-linking network derived from the cured benzoxazines play a crucial role in creating a high free volume and inhibiting the crystallinity. In addition, the incorporation of the nitrile group onto the crosslinked spiro-twisted molecular chains by covalent bonding would certainly enhance the miscibility of SPE membranes with lithium salts while maintaining good mechanical properties. On the other hand, the -CN groups would have more space for rotation and vibration to improve lithium migration owing to the presence of a high fractional free volume of spiro-twisted matrix. Along blending with poly (ethylene oxide) (PEO) electrolytes, a new type of CN-containing semi-interpenetrating polymer networks for solid polymer electrolytes (SPEs) has been achieved.

Revisiting experimental techniques and theoretical models for estimating the solubility parameter of rubbery and glassy polymer membranes

Estimation and correlation of the Hildebrand solubility parameter (δ) of polymers and small molecules is a common practice in membrane material science and is accomplished by experimental and numerical routes. In this paper, we revisit, update, and compare both routes to enhance the accuracy in the determination of δ. Best practices for the experimental determination of polymer solubility parameters are provided, and the viability of Dynamic Light Scattering (DLS) was demonstrated as an alternative to conventional time- and material-consuming techniques, such as Ubbelohde viscometry and swelling measurements. Glassy and rubbery polymers, including high fractional free volume (FFV) microporous polymers such as PIM-1 and poly(1-trimethylsilyl-1-propyne) (PTMSP), are among the samples included in this study with great relevance to membrane science. In an attempt to enhance the accuracy of numerical estimate of polymer solubility parameters via the group contribution method, we provide updated group contribution parameters, along with their uncertainty, according to the technique recently reported by Smith et al. These updated group contribution parameters result in a mean absolute relative error of 9.0% in predicting the solubility parameter on a test set of 40 polymers, which is on par with the average 10% error reported previously. We also show, using machine learning techniques, that augmenting the group contribution model with extra parameters or non-linear relationships does not improve its accuracy. Results of the updated group contribution technique and dynamic light scattering measurements were compared to experimental viscometry on four test polymers, and the difference between the three techniques is compared.

Molecular simulation of CO2 capturing by dual functionalized phosphonium-based amino acid ionic liquids

Various solvents have been proposed considering the importance of CO2 gas removal from the environment. Over the past few decades, ionic liquids (ILs) have elicited much attention due to their unique properties. In this regard, capturing the CO2 gas by phosphonium-based amino acid ILs (AAILs) was studied by molecular dynamics (MD) simulation. The effect of cation functionalization was studied. The anion was glycinate, [GLY]-, and the cation was tetrabutylphosphonium, [P4444]+, functionalized by different functional groups including acidic (−COOH), alcoholic (−OH), amine (−ΝΗ2), and ether (−OCH3). Density, absorption energy, radial distribution function (RDF), mean square displacement (MSD), concentration profile, free volume, and fractional free volume were under investigation. According to density values calculated from MD simulation, functionalized AAIL with a carboxylic acid group ([P4444COOH][Gly]) is the most concentrated system with the maximum interaction energy between AAIL and the trapped CO2 gas equal to − 1.917 kcal.mol−1. In addition, the lowest volume variation was observed through this physical absorption. RDF analysis reveals that anion interaction with CO2 gas is the most considerable one in the case of [P4444COOH][Gly] AAIL. The CO2 uptake is 2:1 for functionalized AAILs that shows cation functionalization improves CO2 gas absorption capacity; each mole of [P4444COOH][Gly] AAIL is capable of surrounding 2.11 mol of CO2. Finally, the gas diffusion coefficient is the lowest one in [P4444COOH][Gly] AAIL as the solvent-free volume is observed as a continuous channel to trap CO2 gas with the lowest gas diffusivity parameter.

Molecular simulation on the mechanical and thermal properties of modified insulating paper cellulose doped with trapezoidal polyphenylsesquisiloxane at different temperature

AbstractWith the further development of the transmission and distribution network, the requirements for the insulating system of power equipment are further deepened. As one of the important factors, insulating paper is of great value to improve its performance. The pure cellulose model and trapezoidal polyphenylsesquisiloxane (T‐PPSQ) composite model are established to analyze the mechanical and thermal properties of insulating paper cellulose. The effects of T‐PPSQ doping at different temperature on properties of cellulose are investigated by comparing the radial distribution function, mechanical parameters, glass transition temperature, mean square displacement (MSD), and free volume fraction (FVF) through molecular dynamics method. The results show that T‐PPSQ can improve the ability of cellulose to resist shear stress at different temperatures, showing good mechanical properties. When the temperature is 423 K, the mechanical performance parameters elastic modulus, bulk modulus and shear modulus can be increased by 47.2%, 18.2%, and 38.56% at most. The glass transition temperature of the insulating paper cellulose with T‐PPSQ increases by 64 K, the MSD and the FVF all decrease at different temperature which demonstrated it has improved thermal stability. The research in this paper is of great significance for improving the life of the insulating system of power equipment.

Mapping positron annihilation lifetime spectroscopy data of a polymer to classical molecular dynamics simulations without shifting the glass transition temperature.

Positron annihilation lifetime spectroscopy (PALS) enables the nondestructive measurement of nanoscale cavities in materials. In this study, a strategy was proposed for mapping PALS measurement data of isotactic polypropylene to classical molecular dynamics (CMD) simulations. The discrepancy between simulated and experimental glass transition temperatures was resolved by shortening the polymer chains, rather than adjusting for the temperature, using the Williams-Landel-Ferry (WLF) equation. The effective probe radii of ortho-positronium (o-Ps), determined by comparing PALS data with CMD simulations, were ∼0.8nm, which was consistent with the o-Ps size given by the solution of the Schrödinger equation. The free-volume fraction corresponding to the effective probe radius was 12.3% at the glass transition temperature, close to the value estimated using Simha-Boyer theory. The cavity number density was proportional to the effective probe radius and decreased with temperature. The o-Ps effective probe radius was proportional to both the critical probe radius and the -1/3 power of the monomer number density, and increased with increasing temperature. These findings suggest that combining PALS measurements with CMD simulations may provide insight into cavities in polymeric materials without relying on the WLF equation.

Molecular dynamics study on the mechanism of poly(amic acid) chemical structure on thermal imidization process and polyimide architecture

In this work, the thermal imidization of 3,3′, 4,4′-biphenyltetracarboxylic dianhydride/p-phenylenediamine (BPDA/PDA), pyromellitic dianhydride/4,4′-oxydianiline (PMDA/ODA), 4,4′-oxydiphthalic anhydride/4,4′-oxydianiline (ODPA/ODA), and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride/2,2′-bis(trifluoromethyl)benzidine (6FDA/TFMB) polyimides (PIs) were investigated based on molecular dynamics (MD) simulation. It was found that two kinds of imide bonds were formed, i.e., inter-molecular and intra-molecular imide bonds, which results in differences in the final architecture of the target PI systems. The BPDA/PDA and PMDA/ODA PIs present crosslinking network structure, while the ODPA/ODA PI shows a hybrid structure composed of crosslinking and linear fragments, and the 6FDA/ODA PI displays linear and branched structures. Compared with the previous view of linear PI molecular chains, these results agree well and better account for the thermoset of PMDA/ODA and BPDA/PDA, thermoplasticity of ODPA/ODA, and solubility of 6FDA/TFMB. The mechanism causing the above differences was studied from radial distribution function (RDF), coordination number, fractional free volume (FFV), and density field analysis. By comparing the experimental and simulated values of glass transition temperature and modulus of PI models, the rationality of inter-molecular imide bonds is proved. In addition, through the PAA/DMF compound models, it was found that the increase of solvent content hindered the formation of inter-molecular imide bonds. This study provides new insights into the thermal imidization process and the final architecture of PIs derived from poly(amic acid).

Salt transport of the carboxylated aromatic polyamide: Efforts to elucidate the structure-function relation of polyamide TFC desalination membranes

Five carboxylated polyamides with the number average molecular weight in the range of 15000–17000 Da, are synthesized via low-temperature solution polycondensation to study the effect of carboxyl group (-COOH) content on the carboxylated polyamide salt transport property. The glass transition temperature (Tg) of the carboxylated polyamide increases with the –COOH content due to the formation of more hydrogen bonds, which also leads to a decrease in free volume fraction (FFV). Interestingly, we observe that polymer having the –COOH content of 80%–100% possesses similar water uptake and KW, which provides a special perspective that how the –COOH group influences salt transport by excluding the effect of water content in polymer membranes. Water sorption can disrupt the hydrogen bonding within the –COOH groups, so setting out –COOH groups to bind water, leading to lower free water content and enhanced water/salt sorption selectivity. As the content of –COOH increases, the more negatively charged surface and stronger hydrogen bond interactions result in significant decrease in salt permeability (PS) and salt diffusivity (DS). These results implicate that the introduction of more –COOH groups to the polyamide layer can enhance the water/salt selectivity of TFC membrane without by changing the water sorption.

Selective separation of water and methanol from hydrogen and carbon dioxide at elevated temperature through polyimide and polyimidazole based membranes

Methanol (CH3OH) produced by carbon dioxide (CO2) hydrogenation represents a compelling strategy to generate a viable hydrogen (H2) carrier for energy storage and transportation, while mitigating carbon emissions from industry. The CO2 hydrogenation reaction suffers from a lower thermodynamic equilibrium compared to the conventional syngas route to methanol, and as such there is significant recycling of reagents. A membrane reactor (MR) configuration is ideally suited to overcome this equilibrium limitation if the products (CH3OH and H2O) can be selectively removed. However, the presence of H2 require non-porous polymeric membranes to achieve this selectivity, which represents a challenge given the elevated temperatures required for catalyst functionality. Here, three thermally resilient glassy polymeric membranes (two polyimides and polybenzimidazole) were investigated for their permselectivity towards CH3OH and H2O at temperatures from 100 to 200 °C under pressurized conditions. The permeability for both vapours were demonstrated to be solubility dependent for the three membranes over the temperature and activity ranges studied. This enabled all three membranes to demonstrate selectivity for CH3OH and H2O, compared to CO2 and H2, under single and mixed gas conditions. The greatest permeability and selectivity were for H2O in each polymer, due to this vapor's high condensability and small kinetic diameter, with permeability decreasing with temperature. Correspondingly, selectivity against H2 and CO2 decreased with temperature, as these lighter gas' permeability increased with temperature, but selectivity for the vapours was retained. As such, the high fractional free volume polyimide membrane represents the most suitable candidate for a CO2-hydrogenation MR.

Assessment of bidirectional relationships between brain imaging-derived phenotypes and stroke: a Mendelian randomization study

BackgroundStroke is a major cause of mortality and long-term disability worldwide. Whether the associations between brain imaging-derived phenotypes (IDPs) and stroke are causal is uncertain.MethodsWe performed two-sample bidirectional Mendelian randomization (MR) analyses to explore the causal associations between IDPs and stroke. Summary data of 587 brain IDPs (up to 33,224 individuals) from the UK Biobank and five stroke types (sample size range from 301,663 to 446,696, case number range from 5,386 to 40,585) from the MEGASTROKE consortium were used.ResultsForward MR indicated 14 IDPs belong to projection fibers or association fibers were associated with stroke. For example, higher genetically determined mean diffusivity (MD) in the right external capsule was causally associated with an increased risk of small vessel stroke (IVW OR = 2.76, 95% CI 2.07 to 3.68, P = 5.87 × 10−12). Reverse MR indicated that genetically determined higher risk of any ischemic stroke was associated with increased isotropic or free water volume fraction (ISOVF) in body of corpus callosum (IVW β = 0.23, 95% CI 0.14 to 0.33, P = 3.22 × 10−7). This IDP is a commissural fiber and it is not included in the IDPs identified by forward MR.ConclusionsWe identified 14 IDPs with statistically significant evidence of causal effects on stroke or stroke subtypes. We also identified potential causal effects of stroke on one IDP of commissural fiber. These findings might guide further work toward identifying preventative strategies at the brain imaging levels.

Molecular dynamics coupled with experimental study of the effects of polyethylene glycol on the structure and antifouling properties of polyethersulfone membranes

AbstractPolyethylene glycol (PEG), as a hydrophilic molecule, exerts a significant influence on the morphology and performance of membranes. However, the mechanism by which PEG influences polyethersulfone (PES) membranes remains unclear. In this work, the addition of varying concentrations (4, 12, 24 wt%) and molecular weights (200, 400, and 600 Da) of PEG on the PES membrane was investigated by molecular dynamics (MD) simulation and experimental study. Compared to the concentration of PEG (4, 12, and 24 wt%), the molecular weight of PEG (PEG200, PEG400, and PEG600) had little effect on the membrane pore size. The radial distribution function and fractional free volume results of MD simulation both proved that. Additionally, both the experimental adsorption capacity (QHA) and the calculated adsorption energy (Eads) revealed that the adsorption of humic acid (HA) on the PES membrane with 4 wt% PEG was higher than that on the pure PES membrane, which meant that PEG was not beneficial to the anti‐fouling property but improved the water flux of PES membranes. The results of this study not only provide theoretical predictions and a scientific basis for the formulation of PES membranes but also promotes an approach for exploring the mechanism by which additives affect polymer membranes.

Tetramine-Based Hyperbranched Polyimide Membranes with Rigid Crosslinker for Improved Gas Permeability and Stability.

Triamine-based HBPI membranes are known for high gas separation selectivity and physical stability, but their permeabilities are still very low. In this study, we utilized a tetramine monomer called TPDA (N,N,N',N'-tetrakis(4-aminophenyl)-1,4-benzenediamine) as a crosslinking center and incorporated an additional diamine comonomer called DAM (2,4,6-trimethyl-1,3-diaminobenzene) to enhance gas separation performance, especially gas permeability. The findings demonstrated that the resultant 6FDA-DAM/TPDA membranes based on tetramine TPDA exhibited a greater amount of free volume compared to the triamine-based HBPI membranes, resulting in significantly higher gas permeabilities. Furthermore, the higher concentration of DAM component led to the generation of more fractional free volumes (FFV). Consequently, the gas permeabilities of the 6FDA-DAM/TPDA membranes increased with an increase in DAM content, with a minimal compromise on selectivity. The enhanced gas permeabilities of the 6FDA-DAM/TPDA membranes enabled them to minimize the footprint required for membrane installations in real-world applications. Moreover, the 6FDA-DAM/TPDA membranes exhibited remarkable durability against physical aging and plasticization, thanks to the incorporation of a hyperbranched network structure.

Chemical topology molecular engineering of CO2-philic membranes toward highly efficient carbon capture

Membrane gas separation processes represent a crucial platform for achieving carbon neutral climate goals. CO2-philic membranes offer excellent solubility selectivity due to their high affinity, thereby allowing the separation of gas fractions with similar molecular sizes or smaller than CO2 molecules. This level of separation is difficult to achieve with conventional diffusion-dominated separation membranes. However, the gas permeability of PEO membranes is often hampered by dense chain packing and crystallization behavior. To overcome this challenge, we employed a molecular engineering approach to develop a unique PEO membrane with a hyperbranched chemical topology connection. The packing density of the polymer chains was effectively regulated by employing cross-linker molecules with distinct spatial configurations, while the free volume fraction was successfully reinforced. This unique molecular engineering strategy led to the creation of a PEO membrane exhibiting superior performance that surpassed the limit of the 2019 upper-bound line. This work demonstrated a molecular design strategy for the precise control of structural attributes in crosslinked PEO networks, offering an alternative solution for developing CO2 capture membranes and enabling further opportunities for rational membrane material design.

Efficient extraction of S-heterocyclic aromatics from fuel oil with oxalic acid-based deep eutectic solvent

The presence of thiophenes in fuel oil not only affects the quality of oil products, but also causes serious environmental problems. The efficient extraction of S-heterocyclic aromatics from fuel oil by π-electron donor-based (i.e., oxalic acid) deep eutectic solvents (DESs) is proposed for the first time. The distribution coefficient and separation factor of n-heptane + 2-methylthiophene/3-methylthiophene/thiophene in 100 DESs were predicted by COSMO-RS model, and the optimal DESs (i.e., Tetraethylammonium chloride-oxalic acid / Tetraethylammonium bromide-oxalic acid) was selected as the potential extractants. The selectivity and extraction capacity of the two DESs were measured and compared by liquid–liquid equilibrium experiments. The thermodynamic consistency of the experimental data was tested by Othmer-Tobias and Hand equations, which show that R2 is>0.95, and the experimental results have high reliability. The interaction mechanism between the extractant and the target systems was analyzed by molecular dynamics simulation and quantum chemical calculation. The results of the mean square displacement and fraction free volume show that thiophene has higher displacement and diffusion in DES2 (i.e., tetraethylammonium bromide and oxalic acid) compared to 2-methylthiophene and 3-methylthiophene molecules. Moreover, the interaction analysis show that electrostatic energy is the main effect between molecules. Finally, the weak interaction between different molecular pairs was identified by the molecular surface electrostatic potential and the independent gradient model. This work provides insight into the design and screening of novel DESs for the extraction and separation of S-heterocyclic aromatics.

Fluorinated-cardo-based Co-polyimide membranes with enhanced selectivity for CO2 separation

The application of fluorinated polyimides in the field of gas separation membranes is becoming increasingly widespread due to their strong thermal and chemical stability. However, overcoming the separation trade-off between gas permeability and selectivity of polyimides remains a key hurdle, requiring the design of new polyimides with tailorable free volume and microstructures. Herein, we synthesized a series of polyimides containing rigid cardo groups with C-F bonds (i.e. FFDA) to increase the packing efficiency of polymer chains and thus enhance the gas selectivity of membranes. Moreover, DAM segments were copolymerized with FFDA to finely regulate the fractional free volume and produce 6FDA-FFDA/DAM copolyimide to boost gas separation performance. The CO2 permeability and CO2/CH4 selectivity of fluorinated-cardo-based copolyimide 6FDA-FFDA/DAM (1:1) are 203.4 Barrer and 43.4, respectively, performing beyond the 2008 Robeson upper bound for CO2/CH4 separation. The design principles in this work lay the groundwork for fabricating desirable polymeric membranes with tailored microstructures and properties for selective CO2 separation.

Isomer and substituent engineering of TADF emitters toward tunable room-temperature phosphorescence

Metal-free organic phosphorescent materials are crucial for the advancement of optoelectronics and bioelectronics. Nonetheless, achieving rational control over the performance of room-temperature phosphorescence (RTP) presents a significant challenge. In this study, two sets of isomeric thermally activated delayed fluorescence (TADF) emitters with a donor–acceptor (D-A) configuration are designed and synthesized. Their RTP efficiency and lifetime are regulated by tuning their energy levels and molecular packing modes through isomer and substituent engineering. All of these materials exhibited afterglow emission, with the exception of 4-CzAIPh. A comprehensive investigation was conducted on the packing modes in the crystalline state, which revealed that the three RTP materials possess strong π-π interactions and numerous intermolecular forces, providing stability to the long-lived triplet excitons of RTP. Especially, 4-CzAIAd demonstrated the fastest intersystem crossing rate, the smallest fractional free volume and relatively larger spin–orbit coupling between its singlet and triplet states, as well as between its triplet state and ground state, which explains its longest phosphorescence lifetime of 301 ms.

Inner-selective polyethersulfone-polydimethylsiloxane (PES-PDMS) thin film composite hollow fiber membrane for CO2/N2 separation at high pressures

Carbon dioxide (CO2) is the dominant greenhouse gas (GHG) and thus CO2 capture from flue gas of power plants, the main anthropogenic source of emission, is a key approach to mitigating the climate change. Thin-film composite (TFC) membranes have received extensive research attention recently as a promising post-combustion CO2 capture (CO2/N2) technology owing to its energy-efficiency, design versatility and upscaling potential. However, studies with the most industrially relevant membrane configuration, the hollow fibers, are rather limited for this application. Herein, we developed an inner-selective polyethersulfone-polydimethylsiloxane (PES-PDMS) TFC hollow fiber membrane by inner coating PDMS in the lumen of PES hollow fibers and then pressurizing the hollow fibers for CO2/N2 separation. The optimal PES-PDMS TFC membrane can achieve a CO2 permeance of 2150 GPU and a CO2/N2 selectivity of 20 when the operating pressure was between 20–22 bar, which holds good potential for post-combustion CO2 capture. The high separation performance arises from a combination of the stretched PES and PDMS layers. The micro-deformation of both layers under high pressures at the lumen side could possibly reduce the selective layer thickness. It may also induce polymer chain orientation, increase the fractional free volume, and potentially create free volume with smaller sizes, thus enhancing the CO₂ permeability and CO₂/N₂ selectivity from the PDMS and PES layers.

Enhancing CO2/(light gases) separation performance of Pebax-based mixed-matrix membranes by [BMIM][AC] ionic liquid

Incorporating the proper additive with an affinity towards target gases is considered a helpful strategy for simultaneously improving membrane permeability and selectivity. In this research, the superior features of polyetherblockamide polymer (Pebax1657), 1–Butyl3–methylimidazolium acetate ([BMIM][AC]) ionic liquid (IL), and aluminum oxide (Al2O3) nanoparticles were blended to produce efficient ternary mixed-matrix membranes for CO2 removal from light gases. The fabricated membranes structures have been evaluated by FTIR3ATR, FESEM, XRD, and DSC analyses. The permeation rates of unmixed CH4, N2, and CO2 gases through the resulting membranes were obtained at 25 °C and a feed pressure range of 4–10 bar. The gas permeability results showed that embedding Al2O3 nanoparticles as the loading up to 6 wt% increased the CO2/(light gases) separation performance. Besides, adding the IL into the polymer matrix boosted the membranes gas separation efficiencies caused by the increased CO2 solubility and their fractional free volume. The results also revealed that the resulting membrane containing 15 wt% of [BMIM][AC] IL and 6 wt% of Al2O3 nanoparticles, which has the best gas separation efficiency, is the optimum ternary mixed-matrix membrane. The comparison between the optimized membrane and the unloaded one efficiencies demonstrated that CO2/N2 and CO2/CH4 selectivities, along with the CO2 permeability, rose from 63.40, 19.93, and 115.39 Barrer for the unloaded membrane to 81.18, 25.14, and 166.42 Barrer for the optimum membrane (around 28 %, 26 %, and 44 % increments), at 10 bar and 25 °C, respectively.

6FDA-DABA polyimide, polyphenylene, and polyimide-polyphenylene multiblock synthesis, characterization, and membrane gas separation performance and modeled transport

A 6FDA-DABA (6FD) polyimide, polyphenylene (PP), and multiblock copolymer (6FD-PP) materials were synthesized using sequential polycondensation and Diels-Alder reactions. PP and 6FD solubility parameters were 19.4 (MPa)1/2 and 22.4 (MPa)1/2, as predicted using Group Contribution Theory. Due to their solubility parameter differences, PP and 6FD solutions in tetrahydrofuran became immiscible when mixed at all concentrations. However, 6FD-PP multiblock copolymer solutions were clear, and their solution-cast films were robust and creasable. Homopolymer and block copolymers estimated fractional free volume (FFV) ranged from 0.174 (6FD) to 0.196 (PP). Their densities and FFV largely obeyed the rule of mixtures, suggesting a "blend-like" morphology. The glass transition temperature (Tg) of 6FD and PP were 285 °C and 400 °C, which became two distinct Tgs at 340 °C and 420 °C for 6FD-PP-[14k:13k]. Larger PP block lengths appeared to suppress the first-order polyimide transition entirely. SAXS and AFM morphological analysis revealed two distinct domains with an average separation distance ranging from 9.0 to 47.7 nm, dependent on PP block length. Membrane gas separation performance was compared to Robeson's upper bound. 6FD-PP multiblock copolymer gas transport properties were found to be highly dependent upon copolymer composition and FFV. Increasing PP content within a copolymer significantly improved gas permeability at the expense of He/CH4 and CO2/CH4 selectivity. Membrane CO2 permeability ranged from 4.36 Barrers using 6FD to 155 Barrers with PP. Their CO2/CH4 selectivity varied between 80.6 (6FD) to 12.7 (PP). 6FD-PP-[14k:13k] membrane's O2 permeability was between these polymers. The membrane had a 30% improvement in its ideal O2/N2 selectivity. The N2/CH4 selectivity ranged from 3 to 1.70, suggesting that 6FD-PP multiblock systems could be efficiently tailored from N2 to CH4 selectivity. Five gas permeability models (rule-of-mixture, Maxwell, EBM, laminate, and blend) were used to study 6FD-PP block copolymer transport behavior. Among them, a blend model had the most reasonable gas permeability prediction. 6FD-PP multiblock membranes improved several ideal gas pairs leading to more permeable and selective materials. The synthetic approach using block copolymerization creates an opportunity for gas separation improvement and tuneability of specific gas pairs.

Effect of fluorine hybridization on dielectric response of epoxy polymer

Dielectric response of polymer dielectrics determines their applications in electronic devices. Although fluoropolymers were widely accepted in communication technology, electronic packing and electrical insulating for their extraordinary dielectric properties, the underlying effect of fluorine hybridization on the dielectric response of polymers is still unclear. Here, surface fluorination was conducted to construct fluorine hybrid epoxy resin (EP), and the dielectric response of EP before and after fluorination was studied. It was found that at low temperature, the electronic polarization was dominated on the dielectric response. The fluorine hybridization reduced the electronic polarization, which led to a lower permittivity of the fluorinated EP. However, at a higher temperature, the fluorine hybridization increased the thermal expansion coefficient and enlarged the free volume fraction, which allowed flexible reorientations of dipoles and led to an enhanced dipolar polarization, and thus, it proved an enhanced permittivity for fluorinated EP. Therefore, the effect of fluorine hybridization on dielectric response of EP polymers was temperature-dependent, with the low-temperature-dominated electronic polarization and high-temperature-dominated dipolar polarization, respectively. This work revealed the influence of fluorine hybridization on dielectric response of epoxy polymers and could promote the application of organic dielectrics in communication and electronic devices.