Fractional Free Volume Research Articles

Development of a universal atomistic cement model incorporating nanomaterials: From laboratory investigation to molecular simulation

Hydrated cement is considered one of the most complex binder systems. The development of models that suitably represent hydrated cement is still far from established due to the complexity of the hydration process, as well as the multiscale phase and different morphological properties. Therefore, this study attempts to develop a hydrated cement model using molecular-level simulation and laboratory testing. This model aimed to predict the mechanical properties of cement paste between 0.25 and 0.65 w/c ratios. In this regard, five cement paste mixtures with various w/c ratios were prepared and tested to evaluate their mechanical properties and micro-structures. Then, a novel modeling methodology using molecular dynamics (MD) simulation was carried out to develop an ingredient-based atomistic cement model. In this regard, the Dreiding force field (DFF) was used in all the simulation processes, starting from the cement clinker phases to the hydrated cement. The experimental results of cement paste mixtures revealed some useful data that was supportive of the simulations. Radial distribution function (RDF) and fractional free volume (FFV) were utilized to investigate the local atomic structure of the cement paste models. The calcium-silicate-hydrated (C–S–H) of the developed model was characterized through RDF and benchmarked to a well-known mineral analog, namely tobermorite. The developed model was computationally tested under different w/c ratios, and reliable results in terms of micro-structural matrix as well as mechanical properties were obtained. Then, the cement models incorporating different ratios of nano-silica were developed, and a good estimation of the mechanical properties was obtained. Because of the universal nature of the reported model in this research, it could be utilized to further predict the mechanical behavior and durability performance of the cement paste.

Construction and Investigation of Novel Cross-Linked Fluorine-Containing Sulfonated Polyimide Membranes for VFB Application.

Membrane with remarkable proton conductance and selectivity plays a key role in obtaining high vanadium flow battery (VFB) performance. In this work, the trade-off effect between proton conductance and vanadium ion blocking was overcome by the introduction of a cross-linking structure to prepare covalent cross-linked fluorine-containing sulfonated polyimide (CFSPI-PVA) membranes. Herein, the CFSPI-PVA-15 membrane possesses excellent comprehensive properties, including acceptable area resistance (0.21 Ω cm2), lower vanadium ion permeability (0.76 × 10-7 cm2 min-1), and remarkable proton selectivity (3.11 × 105 min cm-3) compared with the commercial Nafion 212 membrane. At the same time, the CFSPI-PVA-15 membrane exhibits higher coulomb efficiencies (97.26%-99.34%) and energy efficiencies (68.65%-88.11%) and a longer self-discharge duration (29.2 h) in contrast with the Nafion 212 membrane. Moreover, 500 cycles of the CFSPI-PVA-15 membrane at 160 mA cm-2 are also stably executed. The internal reasons for the improved chemical stability of the CFSPI-PVA-15 membrane are clarified from theoretical calculations with the mean square displacement value and fractional free volume. Therefore, the CFSPI-PVA-15 membrane exhibits great potential for application in VFB.

Investigating the Free Volumes as Nanospaces in Human Stratum Corneum Lipid Bilayers Using Positron Annihilation Lifetime Spectroscopy (PALS).

This work is the first one that provides not only evidence for the existence of free volumes in the human stratum corneum but also focuses on comparing these experimental data, obtained through the unique positron annihilation lifetime spectroscopy (PALS) method, with theoretical values published in earlier works. The mean free volume of 0.269 nm was slightly lower than the theoretical value of 0.4 nm. The lifetime τ3 (1.83 ns with a coefficient of variation CV of 3.21%) is dependent on the size of open sites in the skin. This information was used to calculate the free volume radius R (0.269 nm with CV 2.14%), free volume size Vf (0.081 nm3 with CV 4.69%), and the intensity I3 (9.01% with CV 10.94%) to estimate the relative fractional free volume fv (1.32 a.u. with CV 13.68%) in human skin ex vivo. The relation between the lifetime of o-Ps (τ3) and the radius of free volume (R) was formulated using the Tao-Eldrup model, which assumes spherical voids and applies to sites with radii smaller than 1 nm. The results indicate that PALS is a powerful tool for confirming the existence of free volumes and determining their size. The studies also focused on describing the probable locations of these nanospaces in SC lipid bilayers. According to the theory, these play an essential role in dynamic processes in biological systems, including the diffusion of low-molecular-weight hydrophobic and moderately hydrophilic molecules. The mechanism of their formation has been determined by the molecular dynamics of the lipid chains.

Revealing structure-property relationship of polyimide membrane for gas separation: Insight into monomer ratio via molecular simulation

The molecular design of polyimide (PI) offers a simple yet effective approach in improving gas separation efficiency by adjusting chain stiffness and fractional free volume (FFV). In this study, the configuration of 6FDA-DAM:DABA PI was tailored by varying the diamines ratio at 1:1, 2:3 and 3:2. Leveraging molecular dynamics simulation as a robust predictive tool, the impact of monomer ratio on the physical and gas transport characteristics of the resulting PIs was analyzed. The outcome demonstrated that the total dipole moment of polymer chains and the presence of bulky groups both affect a membrane's FFV. 6FDA-DAM:DABA (3:2) demonstrated the highest glass transition temperature (Tg) due to its highest DAM composition in the polymer backbone. Furthermore, diffusivity and solubility of the three PIs were also evaluated and compared with literature data. In overall, this study provided an alternative way of assessing the membrane properties beyond conventional experimental methods.

Investigation of branched sulfonated polyimide membranes containing self-synthesized trianhydride monomer for vanadium flow battery via a combined theoretical-experimental strategy

In this work, a new branched trianhydride monomer 1,3,5-tris(4-naphthyloxy-1,8-diacid) phthalic anhydride is established for improving the chemical/dimensional stability and proton conduction of the branched sulfonated polyimide (BSPI) membrane for application in vanadium flow battery (VFB). Compared with linear SPI-60 membrane containing conventional dianhydride monomer 1,4,5,8-naphthalenetetra-carboxylic dianhydride, BSPI-60 membrane exhibits remarkable resistance to vanadium ions, proton conduction and structural stability. Besides, the challenge of simultaneously improving vanadium ions resistance and proton conductance is tackled. The proton selectivity of BSPI-60 membrane achieves 1.11 × 105 S·min/cm3, which is 2.8 and 2.5 times higher than both SPI-60 (0.39 × 105 S·min/cm3) and commercial Nafion 212 (0.44 × 105 S·min/cm3) membranes, respectively. At the same time, BSPI-60 membrane exhibits higher coulomb efficiencies (97.2 %–99.3 %) and energy efficiencies (85.6 %–69.5 %) compared those of SPI-60 and Nafion 212 membranes at 100–300 mA/cm2. Remarkably, the 500 cycles of BSPI-60 membrane at 140 mA/cm2 are also stably executed. The internal reasons for enhanced chemical/dimensional stabilities and proton conduction of BSPI-60 membrane are clarified from theoretical calculations with mean square displacement value, fractional free volume and natural bond orbital charge via density functional theory and molecular dynamics simulation. Our study encompasses not only the synthesis of a novel branched trianhydride monomer but also the development of a BSPI membrane with a unique molecular structure specifically designed for VFB applications.

The influence of small molecule products on natural ester impregnated cellulose insulation paper under electric-thermal coupling field

Transformer oil-paper insulation materials are vulnerable to insulation failure due to the influence of voltage, high temperature, and other complex conditions. Additionally, during the operation of the transformer, the generation of harmful substances such as small molecule acids and water becomes a significant concern. In this study, we employed molecular simulation technology to investigate the effect of small molecule products on natural ester-impregnated cellulose insulation paper under electric-thermal coupling field. The results show that: under the action of electric-thermal coupling, the introduction of the electric field can weaken the molecular thermal motion to a certain extent, and the higher the temperature, the more pronounced the weakening effect of the electric field. When subjected to an electric-thermal coupling field, the mean square displacement of the cellulose molecular chains inside the models doped with water and formic acid increases compared to the oil-paper composite model, with an increase of 60.13 % and 98.25 %, respectively, at 700 K. The fraction of free volume in the model increases and the cohesive energy density decreases, the diffusion ability of molecules inside the model remains strong. The weakening effect of the electric field does not diminish the hydrolysis reaction between the small molecular products and the cellulose molecular chains. This leads to a decrease in model stability and ultimately to the breakdown of the insulation paper.

The improvement mechanism of styrene grafted nano-Al2O3 on the hydrophobicity of epoxy resin composites: Based on molecular dynamics and experimental research

In the environment with high humidity, water will penetrate into the epoxy resin, resulting in the deterioration of the electrical properties and the decreasing resistance for the heat and corrosion. This study aims to utilize styrene grafted Al2O3 to improve the wettability of the epoxy resin surface and reduce the infiltration of water molecules into the epoxy resin interior. Firstly, the effects of doping PS/Al2O3 on the internal structure of epoxy resin and the diffusion process of water molecules within it are analyzed through the method of molecular dynamics (MD). Subsequently, epoxy resin samples with mass fractions of 1%, 3%, and 5% are prepared, followed by testing for hydrophobicity, water absorption, and the quality of surface condensation under different temperature differentials. The results indicate that the PS/Al2O3 and epoxy resin exhibit a strong interaction, reducing the proportion of free volume fraction of epoxy resin, which lowers the diffusion rate of water molecules. To be specific, the nanoparticles capture water molecules by forming hydrogen bonds with them, limiting the displacement of water molecules and reducing the water absorption of the epoxy resin (maximum reduction of 87%). Moreover, the addition of PS/Al2O3 decreases the adhesion energy of the material surface to water molecules, increasing the static contact angle from 103.58 ° to 125.42 °. Additionally, the total weight of condensation droplets on the material surface is reduced (maximum reduction of 26.02%). These findings serve as a reference for modifying epoxy resin materials in hygrothermal environments.

BF3 Complexing Phosphate/Phosphonate Ionic Liquids: Synthesis, Characterization and Thermophysical Properties.

A new group of BF3 complexing phosphate/phosphonate ionic liquids (ILs) [Emim][X(BF3)2] (X=dimethyl phosphate, diethyl phosphate, methyl phosphonate, and ethyl phosphonate) were synthesized and characterized. Key thermophysical properties of the new complex ionic liquids, including density, viscosity, conductivity, surface tension, solid-liquid phase transition, and thermal stability were determined and compared with those of [Emim][X]. Some other important thermophysical properties such as isobaric thermal expansion coefficient, molecular volume, standard molar entropy, and lattice potential energy were obtained from measured density data, and the free volume was estimated by a linear equation presented in this article, while critical temperature, normal boiling temperature, and enthalpy of vaporization were estimated from measured surface tension and density data. Furthermore, Fragility study shows that [Emim][X(BF3)2] should be considered as fragile liquids, while [Emim][X] could be considered as extremely fragile liquids. The ionicity of [Emim][X(BF3)2] was predicted by Walden rule, and the result shows that these ILs fit well with Walden law. The key features of these complex ILs are their extremely low glass transition (-95.33~-98.46 °C) without melting, considerably low viscosities (33.876~58.117 mPa ⋅ s), and high values of free volume fraction (comparable to [Omim][BF4], [Emim][NTf2], and [Emim][TCB]).

Refractive index tuning and birefringence effect in crosslinked siloxane-acrylate copolymers for optical waveguide applications

Tunability of refractive index of polymeric waveguide material could be made by controlling free volume as well as polarizability of constituent materials. Two acrylate-co-polysiloxane series of different crosslinking network were synthesised by utilizing different crosslinker and monomer feeding ratio. Using Claussius-Mossotti/Lorentz-Lorentz (CM/LL) model, fractional free volumes were calculated showing a decrease in the experimental (nexp) compared to theoretical (ni) refractive indices. Curing agent with asymmetrical configuration induce a crosslink network of higher birefringence. They were respectively fabricated into multimode channel waveguide whose core-clad refractive index differences (Δn) show values in the range 0.0049–0.0408.

Exploring physical aging in PIM-1 using molecular dynamics

This comprehensive study explored the aging process of PIM-1, a ladder polymer of intrinsic microporosity (PIM), by applying molecular dynamics simulations for the first time. Through detailed analysis, our work illustrates the evolution of the polymer structure from a loosely packed, less dense state of the pristine polymer to a more tightly packed configuration due to physical aging. For this purpose, a novel molecular dynamics (MD) methodology was employed in the process toward equilibration of PIM-1. This structural transition was quantitatively captured by measuring key parameters such as density, fractional free volume (FFV), cohesive energy density (CED), d-spacing, surface area, and gas permeabilities. The simulations demonstrate a noticeable increase in density by approximately 7 % in aged PIM-1 compared to a fresh sample. This increase in density is accompanied by a corresponding decrease in FFV, suggesting a more compact molecular arrangement. The impact of these structural changes is evident in the gas transport properties. Permeabilities of all gases tested, He, H2, O2, N2, CO2 and CH4, decreased by 33 %–80 %. Moreover, the selectivity of gas pairs like CO2/CH4 and O2/N2 exhibited increasing trends due to aging, as previously reported in experimental work. Structural analysis performed on the fresh and aged structures indicated collapse of free volume over aging, by disappearance of pores larger than ∼6.5 Å. Furthermore, no intrachain rearrangement was observed during physical aging in the ladder PIM-1 structure; rather, the aging resulted in increased interchain packing efficiency. Our methodology can be employed to other PIM architectures, such as polyimides of intrinsic microporosity (PIM-PIs) as well as low-free volume glassy polymers.

Influence of graft-activated crumb rubber on the aging performance of modified bitumen: Experimental analysis and molecular dynamics simulation

This paper centers on the assessment of how graft-activated crumb rubber (GR) affects the aging properties of modified bitumen. Three bitumen samples, namely matrix bitumen (MB), crumb rubber modified bitumen (CB) and graft-activated crumb rubber modified bitumen (GB) were aged by rotating thin film oven test (RTFOT) and pressure aging vessel (PAV). The physical and rheological performance of the bitumen were assessed before and after aging, and the aging mechanism was analyzed using Fourier-transform infrared spectroscopy (FTIR) and molecular dynamics (MD) simulation. The results indicated that GR significantly improved bitumen performance in both high and low temperatures after being aged, enhancing its anti-aging properties. This positive effect was attributed to the physical swelling of GR in bitumen and chemical reactions between the amide group in GR and the anhydride group in bitumen. These reactions depleted the polar molecular content in bitumen, reducing its susceptibility to oxidation. A comparison of FTIR results between CB and GB demonstrated the absence of the anhydride group and reduced levels of the carbonyl index (CI) and sulfoxide index (SI) in GB, confirming the occurrence of chemical reactions. MD simulations further revealed that GB exhibited the highest fractional free volume (FFV) after being aged and displayed inferior agglomeration behavior towards light components compared to crumb rubber (CR). In conclusion, the combined experimental and analytical findings collectively affirmed that the primary factor contributing to the enhanced anti-aging properties of GB was the chemical reaction between GR and bitumen.

Mechanism Insights in Anticorrosion Performance of Waterborne Epoxy Coatings Reinforced by PEI-Functionalized Boron Nitride Nanosheets.

Functionalized hexagonal boron nitride nanosheets (BNNSs) have arisen as compelling anticorrosive additives, yet the precise mechanism of their corrosion resistance enhancement in coatings remains unclear. Here, polyethylenimine functionalized BNNSs (PEI-BNNSs) with approximately 6-11 layers were prepared through a "one-step" method. Then, the PEI-BNNSs/Waterborne epoxy (WEP) composite coatings were incorporated via the waterborne latex blending method for the anticorrosion of the Q235 substrate. The impedance modulus (|Z|f = 0.01 Hz) of 0.5 wt % PEI-BNNSs/WEP composite coating soaked in 3.5 wt % NaCl solution for 35 days increased by 4 orders of magnitude compared to pure WEP coating, exhibiting exceptional long-term resistance against corrosion. The positron annihilation lifetime spectroscopy and corrosion product analysis demonstrated that the reinforced anticorrosion capabilities are not solely ascribed to the "tortuous path effect" arising from BNNSs impermeability. These mechanisms also encompass the reduction in free volume fraction and radius of the free volume cavities within the composite coating brought about by the PEI molecules. Additionally, the increase in coating adhesion, promoted by PEI, plays an important role in augmenting the barrier properties against corrosive agents. This study provided a full comprehension of the role played by functionalized BNNSs in fortifying the anticorrosion attributes of WEP coatings.

Synthesis and preparation of thermoplastic silicone elastomer and molecular dynamics simulation of self healing and mechanical properties

Previous studies have shown that the self-healing properties of modified polydimethylsiloxane (PDMS) elastomer are primarily attributed to hydrogen bonding with modified silica. The addition of modified silica can improve the tensile strength of the PDMS matrix, as the modified silica and modified PDMS blend groups generate more hydrogen bonding interactions, resulting in better self-healing properties. However, as the proportion of silica in the system gradually increases, the rigid particles cause a slight decrease in the tensile property of the material. Consequently, it is crucial to strike a balance between the desired mechanical properties and the self-healing capabilities when investigating this subject matter.To investigate the self-healing behavior of PDMS elastomer, a molecular dynamics technique based on a microcracking model was employed. During the compounding process, isocyanate-modified nanosilica was added to the polysiloxane matrix.The ratio of soft and hard segments, which determines the number of hydrogen bonds, was optimized by varying the silica addition ratio from 1 % to 5 %. The performance of the samples was analyzed using several calculations, including Fraction of Free Volume (FFV), Mean Square Displacement function (MSD), and Relative Concentration (RC).The results revealed that the highest self-healing performance and fastest self-healing rate were achieved when the silica addition ratio was 3 %. All tested structures displayed efficient healing within a short time frame. The primary hydrogen bonding exchanges occurred between the urea and UPy groups within the system.Furthermore, the experimental results aligned with the findings from the molecular dynamics simulations, thereby validating this study.

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.