Fractional Free Volume Research Articles

Molecular design of free volume structure in thermally rearranged polybenzoxazole membranes for gas separation studied by positron annihilation

AbstractIn this work, 2,2‐bis(3‐amino‐4‐hydroxyphenyl) hexafluoropropane (APAF) was used as the fixed diamine monomer, and 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 4,4′‐oxydiphthalic anhydride (ODPA), 3,3′,4,4′‐biphenyltetracarboxylic diandhydride (BPDA) and pyromellitic dianhydride (PMDA) were selected as four different dianhydride monomers, respectively, to prepare the thermally rearranged polybenzoxazole (TR‐PBO) gas separation membranes with different structures via a thermal rearrangement reaction at 450°C. The microstructure of TR‐PBO membranes was investigated using X‐ray diffraction and positron annihilation spectroscopy. All the TR‐PBO membranes exhibit bimodal free volume structure, containing ultramicropores (<0.7 nm) and micropores (<2 nm) with size of 3.6–4.6 and 7.6–8.4 Å, respectively. As the steric hindrance of the bridging groups in the dianhydride monomers increases, the radius and number of pores in the membranes increases, leading to an increase in fractional free volume (FFV). The pure gas permeation test shows a positive correlation between gas permeability and FFV in the TR‐PBO membranes, indicating that an increase in FFV is favorable for the diffusion of gas molecules. Particularly, the ultramicropores play a decisive role on the gas selectivity. All the TR‐PBO membranes exhibit excellent gas permeaselectivity. Among them, the H2/CH4 separation performance of APAF‐6FDA and APAF‐ODPA membranes approaches the 2008 Robeson upper bound, which is due to the high number of both micropores and ultramicropores. Our results demonstrate that the free volume structure of polymer membranes can be designed by selecting monomers with different structures and utilizing thermal rearrangement reactions, allowing for the preparation of gas separation membranes with high permeability and selectivity.

Understanding the Interface Enhancement Mechanisms of CFRP with the Polydopamine-Polyetheramine Interphase at the Molecular Level.

In this work, polydopamine (PDA) and polyetheramine D230 were selected to construct the PDA-D230 interphase between the carbon fiber (CF) and epoxy matrix. Density functional theory (DFT) and molecular dynamics (MD) simulations were performed to explore the interface enhancement mechanisms of a carbon fiber reinforced polymer (CFRP) with the PDA-D230 interphase from the molecular level. The adsorption characteristics of a PDA molecule on the CF surface were investigated using the DFT method. The results show that stronger π-π stacking interactions are formed due to the structure and orientation preference of the PDA molecule. The interfacial structures and properties of CFRP with the PDA-D230 interphase are derived from MD simulations. The PDA-D230 interphase on the CF surface induces stronger interfacial interaction energy, leading to the better load transfer between the CF and epoxy matrix. The existence of the PDA-D230 interphase on the CF surface can decrease the mean-square displacement (MSD) value and the free volume fraction of CFRP, which restricts the movement of epoxy atoms and inhibits the translational and rotational motion of epoxy chains. Compared with the epoxy using pristine CFs as reinforcement, the interfacial shear stress (ISS) of CFRP with the PDA-D230 interphase is improved by 13.1%. Our results provide valuable insights into the interface characteristics of CFRP with the PDA-D230 interphase, which are of great significance for exploring the strengthening mechanisms for CFRPs with the PDA-D230 interphase.

Molecular Dynamics Simulation and the Regeneration and Diffusion Effects of Waste Engine Oil in Aged Asphalt Binder.

In recent years, the potential of waste engine oil (WEO) as a rejuvenator for aged asphalt binders has gained significant attention. Despite this interest, understanding WEO's regeneration mechanism within aged asphalt binders, particularly its diffusion behavior when mixed with both aged and virgin asphalt binders, remains limited. This study adopts a molecular dynamics approach to constructing models of virgin, aged, and rejuvenated asphalt binders with different WEO contents (3%, 6%, 9%, and 12%). Key properties such as the density, glass transition temperature, cohesive energy density, solubility parameter, viscosity, surface free energy, fractional free volume, and diffusion coefficient are simulated. Additionally, models of rejuvenated asphalt binder are combined with those of aged asphalt binder to investigate mutual diffusion, focusing on the impact of WEO on the relative concentration and binding energy. The findings reveal that WEO notably decreased the density, viscosity, and glass transition temperature of aged asphalt binders. It also improved the molecular binding within the asphalt binder, enhancing crack resistance. Specifically, a 9% WEO content can restore the diffusion coefficient to 93.17% of that found in virgin asphalt binder. Increasing the WEO content facilitates diffusion toward virgin asphalt binders, strengthens molecular attraction, and promotes the blending of virgin and aged asphalt binders.

Experimental measurement and molecular dynamics simulation analysis of thermal aging performance in composite solid propellants

As a complex mixed system, the aging mechanism of high-energy composite materials involves the aging of each component itself, interactions between components, and transformation of microstructural and thermodynamic properties. The study of a single component or performance parameter has great limitations for elaborating the cognitive complexity of aging processes, making it difficult to reveal the underlying principles of physicochemical property transformation. Thus, it can only be applied as an empirical summary. In view of this, this study comprehensively utilized numerical analysis and experimental detection to explore the evolution of micromolecular conformation and changes in macro aging performance from four aspects of composite systems, including binder aging, plasticizer migration, bond interface debonding, and mechanical property transformation. There was good consistency observed between molecular simulation and thermal aging experiments, indicating that there was basically no post curing process for HTPB propellants using TDI as the curing agent. In the early stage of aging, scission chain was the main process and, in the later stage of aging, oxidation crosslinking the main process. Oxidative crosslinking increased the mean square radius of gyration and glass transition temperature of the binder system, reduced the peak value of radial distribution function, and revealed the reason for the change of propellant hardness and CC and CO double bond content. However, chain degradation increased the number of polar groups, enhanced intermolecular polarity, reduced the solubility parameter, and increased the free volume fraction of the mixed system and the dioctyl sebacate diffusion coefficient. This resulted in an increase in the tangent value of the HTPB propellant loss angle from 0.43 to 0.44° and a decrease in the glass transition temperature from 203.3 to 202.0 K. The binding energy at the interfaces of AP/HTPB and Al/HTPB increased first and then decreased, which was consistent with SEM images of bonding interfaces. The joint effect of binding energy and mechanical properties of the binder system promoted the transformation of the stress-strain curve of aging propellant.

Migration of polydimethylsiloxane chain segments toward the surface of UV-cured waterborne poly(urethane-acrylate) films

Waterborne polyurethane (WPU) has poor water resistance and low surface hydrophobicity due to the incorporation of hydrophilic chain-extender. The water resistance and the surface hydrophobicity can be improved by incorporation of polydimethylsiloxane (PDMS), which is attributed to the migration of PDMS chain segments toward the surface of WPU film. However, whether the PDMS chain segments can migrate toward the surface of WPU film after UV-curing, resulting in higher hydrophobicity, needs further investigation? We prepared the different WPUA films by changing the molar ratio of aminopropyl-terminated polydimethylsiloxane (APT-PDMS) to hydroxyethyl acrylate (HEA), to deeply understand the migrating phenomenon of PDMS chain segments toward the surface of WPUA films. The migration cause of PDMS chain segments and the influence of chemical cross-linking on the migration were investigated by thermal annealing and solvent immersion of WPUA films. In addition, the migration activation energy and the relaxation time were studied by the migration rate of PDMS chain segments in the thermal annealing and the rheological behavior of WPUA film, aiming to elucidate the migration of PDMS chain segments depending on the chain motion and the fractional free volume. This investigation provides a simple and feasible strategy for improving the hydrophobicity of waterborne polyurethane.

Correlation between structure and dynamics of liquid GeO2 under compression

Correlation between structure and dynamics of GeO2 system under compression is investigated by molecular dynamics and Monte-Carlo methods. Results show that structural phase transition occurs at the range of oxygen packing factor from 0.55 to 0.60. Self-diffusion coefficient of Ge and O exhibits anomalous and depends on percentage of GeOx polyhedrons (x = 4,5,6) and packing factor/ free volume of model. The self-diffusion coefficient can be determined via percentage of GeOx units and free volume fraction. The data mining technique is applied to clarify the polyamorphism, structure and dynamics heterogeneities. The cause of anomalous diffusion has been elucidated.

Molecular Dynamics Simulations on Epoxy Resin Composite via Grafting Acryloyl-chloride to Inhibit Electron Transport and Improve Thermal-mechanical Properties

Electrical, thermal, and mechanical properties of cross-linked epoxy resin (EP) modified by the chemical grafting of acryloyl chloride (AC) were studied to explore the trapping mechanism of charge transport inhibition. The bound state traps deriving from grafted molecules were analyzed by first-principles calculations combined with electron transmission spectra to study the underlying mechanism of the electrical properties. In contrast to pure EP, the EP-graft-AC (EP-g-AC) represents significantly depressed conductivity due to the electron scattering from polar-groups of the grafted AC molecule. The substantial deep traps are generated in EP-g-AC molecules by the polar group of grafted AC and accordingly decrease charge mobility and raise the charge injection barrier, consequently suppressing space charge accumulation and charge carrier transport. EP-g-AC polymer acquires a significant amelioration in thermal and mechanical properties, as indicated by the higher cohesive energy density, glass transition temperature, and decomposition temperature in consistence with the lower thermal vibrations compared with pure EP polymer, except that the resulting higher fractional free volume is not preferable, which is attributed to the mixing incompatibility of the grafted AC molecules with EP molecular-chains.

Barrier Properties of Biodegradable Aliphatic–Aromatic Copolyesters (PBXT Series)

AbstractIn this study, a series of biodegradable poly(butylene alkylene carboxylate‐co‐terephthalate) copolymers with varying methylene numbers in the alkylene units (0, 2, 4, and 8) are synthesized. These copolymers, namely, poly(butylene oxalate‐co‐terephthalate) (PBOT), poly(butylene succinate‐co‐terephthalate) (PBST), poly(butylene adipate‐co‐terephthalate) (PBAT), and poly(butylene sebacate‐co‐terephthalate) (PBSeT), are prepared with nearly identical structural and molar compositions. The objective of this study is to comprehensively examine the impact of alkylene unit length on the barrier properties of the materials, delving into aspects such as crystallinity, free volume, molecular chain mobility, as well as adsorption and diffusion of gases within the materials. The findings indicate a decrease in crystallinity from 17.2% for PBOT to 10.2% for PBSeT as the alkylene chain length increases, while maintaining the same chemical composition. Concurrently, the fractional free volume increases from 0.9% to 2.6%, and the melt flow activation energy decreases from 106.6 to 52.1 kJ mol−1. Importantly, theoretical calculations are performed, demonstrating that the predominant site for gas adsorption and diffusion is within the material's free volume. These combined observations indicate a gradual decrease in barrier properties as the number of methylene groups increases.

Vinyl chloride capture by ionic liquids: Structural effects and molecular thermodynamic insights

Ionic liquids (ILs) have been proposed as potential environmentally friendly and efficient solvents for gas absorption and purification. In this study, we propose the utilization of ILs for vinyl chloride (VC) capture. The structural effects, including the cationic skeleton and alkyl chain length on the cation, as well as the anionic structures such as alkyl chain length for phosphate and sulfate anions and the fluorination effect, were systematically investigated at a microscopic level using COSMO-RS mode and quantum chemical calculations. ILs screening results reveal that noncyclic cationic skeletons with long alkyl chains on both cations and phosphate/sulfate-based anions, without fluorinated derivatives, are favorable for VC absorption due to their large fractional free volume and nonpolar nature. Both the anion and cation contribute to the absorption process; however, the anion effect plays a more significant role. The interaction between VC and the cation is mainly governed by van der Waals forces, while both hydrogen bonding interactions and van der Waals forces exist between VC and the anion.

Intrinsically Microporous Polyimides Derived from 2,2'-Dibromo-4,4',5,5'-bipohenyltetracarboxylic Dianhydride for Gas Separation Membranes.

This work aims to expand the structure-property relationships of bromo-containing polyimides and the influence of bromine atoms on the gas separation properties of such materials. A series of intrinsically microporous polyimides were synthesized from 2,2'-dibromo-4,4',5,5'-bipohenyltetracarboxylic dianhydride (Br-BPDA) and five bulky diamines, (7,7'-(mesitylmethylene)bis(8-methyldibenzo[b,e][1,4]dioxin-2-amine) (MMBMA), 7,7'-(Mesitylmethylene)bis(1,8-dimethyldibenzo[b,e][1,4] dioxin-2-amine) (MMBDA), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-2,8-diamine (TBDA1), 4,10-dimethyl-6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-3,9-diamine (TBDA2), and (9R,10R)-9,10-dihydro-9,10-[1,2]benzenoanthracene-2,6-diamine (DAT). The Br-BPDA-derived polyimides exhibited excellent solubility, high thermal stability, and good mechanical properties, with their tensile strength and modulus being 59.2-109.3 MPa and 1.8-2.2 GPa, respectively. The fractional free volumes (FFVs) and surface areas (SBET) of the Br-BPDA-derived polyimides were in the range of 0.169-0.216 and 211-342 m2 g-1, following the order of MMBDA > MMBMA > TBDA2 > DAT > TBDA1, wherein the Br-BPDA-MMBDA exhibited the highest SBET and FFV and thus highest CO2 permeability of 724.5 Barrer. Moreover, Br-BPDA-DAT displayed the best gas separation performance, with CO2, H2, O2, N2, and CH4 permeabilities of 349.8, 384.4, 69.8, 16.3, and 19.7 Barrer, and H2/N2 selectivity of 21.4. This can be ascribed to the ultra-micropores (<0.7 nm) caused by the high rigidity of Br-BPDA-DAT. In addition, all the bromo-containing polymers of intrinsic microporosity membranes exhibited excellent resistance to physical ageing.

Ultrasound-assisted preparation of crumb rubber modified asphalt: Characterization and molecular dynamics simulation

The traditional method of crumb rubber modified asphalt (CRMA) preparation can improve the stability but is not conducive to cleaner production. From the perspective of process optimization, ultrasound is introduced into the crumb rubber modified asphalt preparation process, and the high temperature, high pressure and high energy accompanied by the cavitation effect are used to generate free radical triggering chemical effects at the two-phase interface between crumb rubber and asphalt. In this paper, the influence of ultrasound on the compatibility and stability of crumb rubber modified asphalt system was evaluated by combining experiment and molecular dynamics simulation. An experimental platform for ultrasound-assisted preparation of crumb rubber modified asphalt was established. The effects of ultrasonic on the physical properties of asphalt and crumb rubber modified asphalt were studied through Brookfield viscosity test, thin film oven aging test and Fourier infrared spectroscopy. The base, aged, and ultrasonically treated asphalt samples were simulated using molecular dynamics, and various parameters such as Cohesive Energy Density (CED), Mean Square Displacement (MSD), and Free Volume Fraction (FFV) of asphalt and its components were calculated and analyzed. The results showed that under the ultrasonic-assisted preparation, the agglomeration phenomenon of rubber particles is weakened, the particle size is reduced, the penetration degree is reduced, and the compatibility between rubber and asphalt is improved. Under the action of ultrasonic, the MSD and diffusion coefficient of crumb rubber modified asphalt are much higher than other kinds of asphalt, the dipole moment is increased, and the molecular motion range is wider. This indicates that the diffusion ability of crumb rubber modified asphalt is significantly enhanced under the external force of ultrasonic simulation. The molecular bonds of crumb rubber modified asphalt changes greatly, the energy decreases obviously, and the system stability increases. In conclusion, ultrasonic action on crumb rubber modified asphalt preparation is a green and effective way to improve system stability and compatibility.

Reducing hydroxide transport resistance by introducing high fractional free volume into anion exchange membranes

The efficiency of ion transport in solid polyelectrolytes is significantly hindered by tight polymer chain entanglement compared to that in liquids. In this study, a series of hyperbranched poly(aryl piperidinium) anion exchange membranes (AEMs) with 1,3,5-triacetylbenzene as a branching point were synthesized. The hyperbranched structure results in weaker chain entanglement and a larger interchain correlation distance, leading to a higher fractional free volume, thereby greatly reducing ion transport resistance. Consequently, a remarkably higher OH− conductivity of the as-prepared hyperbranched AEM (296 mS cm−1) is achieved compared to that of non-hyperbranched AEMs (180 mS cm−1). Furthermore, the hyperbranched AEMs exhibit excellent fuel cell and water electrolysis performance, with a peak power density of 1.53 W cm−2 in AEMFCs and a current density of 1 A cm−2 at a voltage of 2 V in AEMWEs, respectively.

White Matter Microstructure Analysis in Subjective Memory Complaints and Cognitive Impairment: Insights from Diffusion Kurtosis Imaging and Free-Water DTI.

Dementia is characterized by a cognitive decline in memory and other domains that lead to functional impairments. As people age, subjective memory complaints (SMC) become common, where individuals perceive cognitive decline without objective deficits on assessments. SMC can be an early sign and may precede amnestic mild cognitive impairment (MCI), which frequently advances to Alzheimer's disease (AD). This study aims to investigate white matter microstructure in individuals with SMC, in cognitively impaired (CI) cohorts, and in cognitively normal individuals using diffusion kurtosis imaging (DKI) and free water imaging (FWI). The study also explores voxel-based correlations between DKI/FWI metrics and cognitive scores to understand the relationship between brain microstructure and cognitive function. Twelve healthy controls (HCs), ten individuals with SMC, and eleven CI individuals (MCI or AD) were enrolled in this study. All participants underwent MRI 3T scan and the BNI Screen (BNIS) for Higher Cerebral Functions. The mean kurtosis tensor and anisotropy of the kurtosis tensor showed significant differences across the three groups, indicating altered white matter microstructure in CI and SMC individuals. The free water volume fraction (f) also revealed group differences, suggesting changes in extracellular water content. Notably, these metrics effectively discriminated between the CI and HC/SMC groups. Additionally, correlations between imaging metrics and BNIS scores were found for CI and SMC groups. These imaging metrics hold promise in discriminating between individuals with CI and SMC. The observed differences indicate their potential as sensitive and specific biomarkers for early detection and differentiation of cognitive decline.

Green environmental protection and sustainable utilization of straw: Investigation for molecular dynamics of highland barley straw fiber to enhancing road performance of asphalt

This study explores the impact of highland barley straw (HBS) fiber, an environmentally friendly material, on the molecular dynamics and performance of asphalt. Despite the widespread use of fibers in enhancing asphalt performance, molecular-scale investigations are lacking. In this study, molecular dynamics software was employed to model matrix asphalt and three types of HBS fiber-modified asphalt with varying cellulose contents (8.62 wt%, 15.88 wt%, 22.06 wt%). Thermodynamic parameters mean squared displacements (MSD), radial distribution function (RDF), free volume fraction (FFV), and mechanical parameters were analyzed through simulation to understand the diffusion behavior of HBS cellulose molecules in asphalt and study the modification mechanism. The findings revealed that the 8.62 wt% cellulose-modified asphalt had the slowest diffusion rate, with the most favorable temperature for asphalt diffusion being 298.15 K. Integration of HBS cellulose molecules facilitated the self-aggregation process of asphaltene-asphaltene and improved the compatibility between asphaltene-resin, resulting in asphalt stabilization. However, the study found that moisture damage weakened the water resistance of HBS cellulose-modified asphalt binder. Specifically, modifying asphalt with 8.62 wt% cellulose increased the Young modulus by 28.37%, bulk modulus by 39.77%, and shear modulus by 26.45%, demonstrating the overall improvement achieved with the combined effect of 8.62 wt% HBS cellulose-modified asphalt. These findings have significant implications for repurposing HBS as a solid waste material and enhancing the low-temperature crack resistance of asphalt binders.

The Incorporation of Graphene Nanoplatelets in Tung Oil-Urea Formaldehyde Microcapsules: A Paradigm Shift in Physicochemical Enhancement.

Tung oil (TO) microcapsules (MCs) with a poly(urea-formaldehyde) (PUF) shell were synthesized via one-step in situ polymerization, with the addition of graphene nanoplatelets (GNPs) (1-5 wt. %). The synergistic effects of emulsifiers between gelatin (gel) and Tween 80 were observed, with gel chosen to formulate the MCs due to its enhanced droplet stability. SEM images then displayed an increased shell roughness of the TO-GNP MCs in comparison to the pure TO MCs due to the GNP species on the shell. At the same time, high-resolution transmission electron microscopy (TEM) images also confirmed the presence of GNPs on the outer layer of the MCs, with the stacked graphene layers composed of 5-7 layers with an interlayer distance of ~0.37 nm. Cross-sectional TEM imaging of the MCs also confirmed the successful encapsulation of the GNPs in the core of the MCs. Micromanipulation measurements displayed that the 5% GNPs increased the toughness by 71% compared to the pure TO MCs, due to the reduction in the fractional free volume of the core material. When the MCs were dispersed in an epoxy coating and applied on a metallic substrate, excellent healing capacities of up to 93% were observed for the 5% GNP samples, and 87% for the pure TO MC coatings. The coatings also exhibited excellent corrosion resistance for all samples up to 7 days, with the GNP samples offering a more strenuous path for the corrosive agents.

Meta-aramid film with both flexibility and strength: Combined effect of aromatic ether bond and micro-branched structure

Developing Meta-aramid materials that combine flexibility and strength will greatly expand their application fields. In previous work, it was demonstrated that aromatic ether bonds can contribute to improving flexibility. In addition, micro-branched chain structures can reduce intermolecular forces by increasing fractional free volume, while newly formed hydrogen bonding can maintain polymer strength. Therefore, two flexible polymers were obtained by incorporating 4,4′-diaminodiphenyl ether (ODA) and 4,4′-oxybis [3-(trifluoromethyl)benzenamine] (OTB) to the molecular chains, respectively, and flexible films were prepared by a combination of solution casting and dry-wet processing methods. By studying the properties of copolymer films with 10% ODA and 10% OTB added, it was found that they can maintain thermal stability and mechanical strength while improving the flexibility of the films. The AFM results visually showed that the DMT modulus of the modified films decreased at the micro level and DMA indicated that the introduction of ODA and OTB can shorten the molecular relaxation time, which reflected the improvement of molecular chain flexibility. Through molecular dynamics simulations, the differences were compared in molecular chain conformation, mean square displacement and hydrogen bonding after the introduction of ODA and OTB and the combined mechanism of aromatic ether bonds and CF3 side groups was confirmed in improving flexibility and maintaining mechanical strength.

Determination of optimum limestone filler content in asphalt mastic by dielectric spectroscopy

The rheological and dielectric characteristics of asphalt mastics with different ratios of limestone filler were analyzed and compared with each other for road pavement applications. The dielectric properties of asphalt mastics are studied as a function of frequency (0,1–106 Hz) at different temperatures ((-)40°C−40°C). The glass transition temperature and the relaxation behaviour were determined by Vogel–Fulcher-Tamman and Cole–Cole equations, respectively. The free volume fraction was calculated by the Dolittle equation. The filler ratio dependence of the dielectric properties was attributed to the variation of the void fraction. This result was related to the glass transition temperature and ductility. The sample with 6 wt% limestone filler showed the highest dielectric constant value among all samples for all temperatures due to the excellent adhesion between binder and filler. Consequently, it was determined to be the optimum value to design hot mix asphalt for use in territories with high-temperature differences.

Statistical thermodynamics to assess probability of water channel formation in membrane

In this work, the probability of water channel formation was calculated for several polymeric materials, and with that information, the desalination capacity of each material was assessed. Using a 3D lattice model for a flat sheet, the Gibbs free energy change (ΔG) that results from the formation of the first water channel from the top to the bottom surface of the sheet was calculated as a function of the hydrophilicity/-phobicity of the polymeric material (w), the total fraction of water in the 3D lattice (x), and the thickness of the lattice (n). With this result, an equation was further derived to evaluate (xb) the fraction of water which is related to the channel formation, and (xb/x), the ratio of xb to x. It was found that the values of xb are closely related to the fractional free volume (FFV) measured by positron annihilation lifetime spectroscopy (PALS) and the percent free volume of percolated open spaces where water could pass through obtained by molecular dynamics simulation (MDS).These equations were then used to assess the usefulness of each material for making reverse osmosis membranes. When xb is too large, too many channels form, and those channels join to form large pores through which salt can pass, rendering reverse osmosis unsuccessful. On the other hand, when xb is too small, not enough water channels are formed, so reverse osmosis cannot occur then either. It was found that xb was too small for polyvinylidene fluoride (PVDF) and polyethersulfone (PES); suitable for cellulose acetate (CA), aromatic polyamide (APA) and aromatic polyamidehydrazide (APAH); and too large for cellophane (CE). This paper determines which circumstances allow water to form channels in a membrane, and it provides a novel method to assess a material's suitability for reverse osmosis.

Molecular dynamics simulation for damping properties of PDMS composite enhanced by the last generation siloxane dendrimer alcohol with hydroxyl terminal groups

In this study, the damping properties of PDMS composite enhanced by last generation siloxane dendrimer alcohol with hydroxyl terminal groups G3-OH were evaluated by all atomic molecular dynamics simulation (AAMDS) method in terms of damping loss factors (DLFs) tanδ, compositional miscibility (|δi−δ0|), cohesive energy density (CED), binding energy (EB) and maximum free volume fraction (FFVmax). Based on the total atomic charge distribution, further, the potential damping mechanism in composite PDMS/G3-OH(100/47) was explored. The simulation results showed that the |δi−δ0| of composite PDMS/G3-OH(100/47) was 0.0939 (J/cm3)0.5, whose good compatibility between the two components laid a solid foundation for the excellent damping performance. As the argument, the tanδ, CED, EB and FFVmax of composite PDMS/G3-OH(100/47) were 0.8073, 1.668 (J/m3)×108, −19877.607 kcal/mol as well as 39.85 % respectively, which were better than 1.2439, 1.390 (J/m3)×108, −14539.327 kcal/mol and 40.71 % of pure PDMS matrix material PDMS/G3-OH(100/0). Furthermore, the multiple hydrogen bond mechanism can perfectly explain the structure-activity relationship between the internal structure and the excellent damping properties of composite PDMS/G3-OH(100/47). Specifically, a total of 19 α-type hydrogen bonds were formed in the composite PDMS/G3-OH(100/47). All in all, according to the evaluation of AAMDS method, the PDMS composite enhanced by 47-equivalent last generation siloxane dendrimer alcohol with hydroxyl terminal groups G3-OH had outstanding damping properties.

Preparation and characterisation of heat‐resistant fluorinated co‐polyimides and their gas transport characteristics

A series of fluorinated co-polyimide films based on the 6FDA-FTPPA/BAFL backbone were prepared from 4-(3-fluoro-4-(trifluoromethyl)phenyl)-2,6-bis(4-aminophenyl)pyridine (FTPPA),commercial 9,9-bis(4-aminophenyl)fluorene (BAFL), and 4,4’-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), with varying ratios of FTPPA:BAFL units (3:1, 2:1, 1:1, 1:2, and 1:3). Co-polyimides not only exhibit high thermal stability ( Tg &gt; 357°C), good optical properties (λcutoff &gt; 327 nm) and hydrophobicity (contact angle &gt;91.4°), but also favorable solubility properties in both high and low boiling organic solvents. The fractional free volume of the polymers was simulated using molecular mechanics and molecular dynamics and correlated with experimental gas separation data. The presence of bulky groups led to high FFVs (&gt;17.8%) and the formation of large microcavities with diameters ranging from 5.48 to 5.72 Å, which efficiently increased gas permeability. In pure gas permeation experiments, the permeability coefficients of CO2 and He for the 6FDA-FTPPA/BAFL (3:1) film were 41.48 and 79.38 bar, respectively, which were superior to those of commercial Matrimid and Kapton in terms of gas permeability. In especial, the 6FDA-FTPPA/BAFL (1:2) and 6FDA-FTPPA/BAFL (1:3) in co-polyimide membranes exhibited the most attractive separation performance for O2/N2 gas pairs, approaching the 1991 Roberson upper limit.