Diffusion Behavior Of Water Molecules Research Articles

Influence of electric field on moisture-absorbing characteristics at the FRP/RPUF interface: experimental study and molecular dynamics simulation

The interface between fiber-reinforced polymer (FRP) and rigid polyurethane foam (RPUF) is a crucial component of composite cross-arm, not only operating within high electric field environments but also vulnerable to water-induced deterioration. In this paper, the moisture-absorbing characteristics and aging mechanism at the FRP/RPUF interface under the influence of an electric field were investigated through accelerated aging experiments, molecular dynamics (MD) simulations, reactive force filed (ReaxFF) simulations and Density Functional Theory (DFT) analysis. The results indicated that the moisture-absorbing characteristics of the FRP/RPUF system could be divided into two stages: Stage I, dominated by free diffusion, and Stage II, dominated by physical absorption. In Stage I, the electric field inhibited the diffusion behavior of water molecules by affecting the mean square displacement (MSD) of water molecules and the free volume of the FRP/RPUF system. During Stage II, the intrusion of water deepened the aging degree of the system, resulting in the emergence of a large number of free volumes and noticeable channels for water transport at the interface. The electric field enhanced the chemical reaction activity of epoxy resin and polyurethane by influencing their frontier molecular orbital energy, thereby promoting the occurrence of hydrolysis reactions. This intensified the physical moisture absorption process, ultimately promoting the Stage II process.

Water/organic liquid interface properties with amine, carboxyl, thiol, and methyl terminal groups as seen from MD simulations.

Molecular dynamics simulations were performed to study structural and dynamic properties of polar butanamine/water/butanamine, pentanoic acid/water/pentanoic acid, butanethiol/water/butanethiol, and nonpolar pentane/water/pentane systems. The mass density profiles along the interface normal to the organic liquid/water system, the difference in the local structure of H O molecules in bulk and in the vicinity of interface, as well as the diffusion behavior of water molecules at the interface with above-mentioned organic liquids have been investigated. Our MD simulation has shown that the diffusion of water molecules across the water/organic liquid interface is influenced by the hydrogen bonds between water molecules and the terminal groups of organic liquids. It was found that the loss of the hydrogen bonds in the nonpolar organic liquid leads to a decrease in the value of the normal component of the diffusion coefficient , while the tangential diffusion coefficients, both and , increase.

Molecular-level evaluation of thermal field confinement effects on moisture distribution at the interface of natural ester oil-paper insulation: An investigation of electric field correlation

Oil-paper insulation is commonly used in transformers and is subject to dynamic changes in moisture content and distribution due to the influence of multiple physical fields such as temperature and electric fields. Moisture at the interface of oil-paper insulation can distort the interfacial electric field and cause insulation failure, but it is difficult to measure and characterize. In this study, molecular dynamics was used to simulate the diffusion behavior of water molecules at the interface of natural ester oil-paper insulation at temperatures ranging from 298 to 373 K, and the effect of electric fields on water diffusion was also analyzed. The results showed that without an electric field, water molecules in natural ester insulating oil tend to cluster at 298 K. Water molecules gradually diffuse towards the interface of the natural ester oil-paper insulation, but most remain at the interface and are difficult to penetrate into the cellulose insulating paper. In addition, normal diffusion is gradually enhanced at 298–328 K, reaching a minimum at 343 K. The interaction energy between water molecules and natural ester insulating oil decreases with increasing temperature, while the interaction energy with cellulose insulating paper decreases and then increases, reaching a minimum at 343 K. The combined influence of temperature and electric fields leads to an increase in the number of water clusters at the oil-paper interface. The interaction energy between water molecules and both natural ester insulating oil and cellulose insulating paper is enhanced, inhibiting the diffusion of water into the insulating paper. It was also found that the ability of water to diffuse differs significantly when the electric field direction is normal to the interface compared to when it is tangential to the interface. This is due to the fact that the water molecules when subjected to electric field action turn to polarize, and their electric field direction diffusion coefficient decreases, but still much higher than the non-electric field action direction. Further, the study found that the natural ester oil-paper insulation interface polarization is directional, and water molecules polarized in the direction normal to the insulation interface require a higher electric field strength.

Characterization of Cross-Linking in Guar Gum Hydrogels via the Analysis of Thermal Decomposition Behavior and Water Uptake Kinetics

This study aimed to explore a test method for evaluating the effective cross-linking density of hydrogels. A guar gum–epichlorohydrin hydrogel (GEH) was prepared using guar gum (GG) as the raw material and epichlorohydrin (ECH) as the cross-linking agent. The thermal and mechanical properties, equilibrium swelling rate (ESR), water uptake (WU), and mass cross-linking degree of the hydrogels were assessed. Furthermore, the diffusion behavior of water molecules in the freeze-dried GEH was investigated. The experimental results showed the significance of the initial decomposition temperature (Ti) and final decomposition temperature (Tf) of the freeze-dried GEHs in determining the effective cross-linking density. The water uptake kinetics of the freeze-dried GEH was consistent with the linear fitting of the pseudo-second-order kinetic model and nonlinear fitting of the Fickian diffusion model, suggesting that chemisorption dominated the water absorption process in the GEH. Therefore, the effective cross-linking density of the hydrogels could be determined from the thermodynamic analysis and the diffusive behavior of water molecules in the gels. The thermal stability and water diffusion kinetics of the hydrogels were closely linked to the effective cross-linking density and pendant modification.

Understanding the effect of moisture on interfacial behaviors of geopolymer-aggregate interaction at molecular level

The interaction between geopolymer and aggregate largely determines the mechanical properties and durability of the geopolymer concrete. The effects of moisture on interfacial behavior of geopolymer-aggregate interaction are poorly understood, especially at molecular level. Herein, molecular dynamics (MD) simulation was employed to reveal the interactive behaviors of geopolymer-aggregate interfacial system with the participation of moisture. Full atomistic models adopted for MD simulations were constructed using the sodium aluminum silicate hydrate (N-ASH) gel model and the main chemical components of the aggregates, SiO2 and CaCO3. Then the wetting characteristics of aggregate surfaces, interfacial characteristics and mechanical behaviors of the geopolymer-aggregate interfacial systems containing interfacial moisture were elucidated and compared. It is found that the SiO2 surface is hydrophobic while the CaCO3 surface exhibits hydrophilic characteristics. Interfacial moisture participates in electrostatic interaction, H-bond interaction and coordination interaction in geopolymer-aggregate interface area. Appropriate interfacial water is beneficial to the interfacial interaction of geopolymer-aggregate system, but excessive water will increase the risk of interfacial failure. The interfacial moisture affects the diffusion behavior of water molecules and Na+ ions in geopolymer to the interfacial region, and the formation of H-bonds and coordination bonds at the interface. Mechanically, with the participation of interfacial moisture, the geopolymer-SiO2 interfacial system possesses stronger tensile strength, and a greater risk of shear failure than that of geopolymer-CaCO3. The above atomic-level findings may facilitate a better design and fabrication of geopolymer concrete in engineering.

The Effects of Urea and Caprolactam on the Diffusion Behavior of Water in Polyvinyl Alcohol (PVA): A Molecular Dynamics Simulation Study

The effects of compound plasticizer urea/caprolactam (UC) on the diffusion behavior of water in mixtures with poly(vinyl alcohol) (PVA) were studied using a Molecular Dynamics (MD) simulation method. Five simulation models of PVA composites with different plasticizer contents were constructed to investigate the variation of the intermolecular interaction as well as the diffusion behavior of water molecules. Results showed that the predominant interaction between the functional groups of UC and PVA consisted of hydrogen bonds. As the plasticizer content increased, diffusion coefficients of water in PVA systems increased due to the comparably weak diffusion resistance. It was also found that the rotation of the PVA chains and the small molecules became faster with increasing UC content, and the relaxation time became shorter.

Molecular Dynamics Study on the Diffusion Behavior of Water Molecules and the Dielectric Constant of Vegetable/Mineral Oil Blends.

Insulating oil plays a crucial role in internal insulation of oil-impregnated transformers. It has been demonstrated in a variety of experimental studies that mineral oil (MO) and vegetable oil (VO) can be blended in different ratios to improve insulation properties; however, the mechanisms underlying this phenomenon remain unclear. In this study, a molecular dynamics (MD) simulation approach was used to investigate diffusion of water molecules in VO/MO blends and dielectric constants of a mixture. The results show that the diffusion coefficient of water molecules is negatively correlated with the proportion of VO; thus, addition of VO helps to improve the insulation properties of a mixture. Due to introduction of strong polar functional groups, a decrease in the diffusion behavior of water molecules can be attributed to an increase in the interaction energy and formation of hydrogen bonds between water molecules and the mixed oil system. There is a direct correlation between the dielectric constant of a mixture and VO content; however, it is very sensitive to water content. The presence of strong polar water molecules or functional groups in a mixture leads to an increase in the dielectric constant, which results in a reduction in insulating properties. Accordingly, presence of polar groups plays an important role in determining the insulating properties of a mixture. To increase the insulation performance of a mixture, it is important to consider the diffusion-inhibiting and dielectric effects of the stronger polar groups in vegetable oil compared to those in mineral oil.

Diffusion behavior of water molecules in binary mixed vegetable oils under the synergistic effect of temperature and nano-TiO2

Temperature and water are two important factors affecting the insulating properties of power insulating oil. Meanwhile, nanoparticles have received attention from researchers as the latest improvement method for the electrical performance and insulation properties of insulating oil. Therefore, in this study, we constructed models of pure binary mixed vegetable insulating oils and mixed insulating oils modified with nano-TiO2. The diffusion behaviour of water in the binary mixed vegetable insulating oil under the synergistic effect of temperature and nanoparticles was investigated. Results indicate that the diffusion coefficient of water molecules in the nano-TiO2 modified vegetable insulating oil decreased up to 30.7 % compared to the unmodified vegetable insulating oil. With the introduction of nanoparticles, the free volume of the oil model is reduced by 18.23 % on average, and the space for water molecules to move is greatly reduced. The average interaction energy of oil and water molecules decreased by 9.53 %, and a large number of water molecules formed hydrogen bonds with O atoms on the nanoparticles. The nano-TiO2 particles have a negative electrostatic potential distributed on the surface and have a strong electrostatic attraction to the free H3O+ in the oil. Compared with pure oil, water molecules in modified oil need higher temperature and higher kinetic energy to get rid of hydrogen bond with nanoparticles. Therefore, the introduction of nanoparticles has largely reduced the influence of temperature on the diffusion of water molecules.

Simulation of the diffusion behavior of water molecules in palm oil and mineral oil at different temperatures

The presence of moisture accelerates the aging of oil-paper insulation systems and reduces the insulating performance. In this study, the molecular dynamics method was used to investigate the diffusion behavior of water molecules in palm oil and compared with the diffusion behavior of water molecules in mineral oil. The mineral oil-water mixture model and vegetable oil-water mixture model were established; then, molecular dynamics calculations were performed on the established models from 323 K to 363 K. The free volume, diffusion coefficient, number of hydrogen bonds and interaction energy were analyzed. The results showed that the diffusion of water molecules in the two oils was gradually increased with increasing temperature, but the diffusion capability of water molecules in mineral oil was about 1.5–2 times higher than that in palm oil. In addition, the effect of temperatures on water molecules diffusion was found to be greater in mineral oil. This occurs because hydrogen bonds can be formed between palm oil and water molecules, which enhances the binding energy between palm oil and water molecules; moreover, palm oil provides less free space for diffusion of water molecules compared with mineral oil. Thus limiting the diffusion of water molecules in palm oil. Furthermore, because palm oil has a strong adsorption capacity for water molecules, and the water solubility of palm oil is relatively large, therefore, it is beneficial to increase the breakdown voltage of palm oil.

Electrical Breakdown Mechanism of Transformer Oil with Water Impurity: Molecular Dynamics Simulations and First-Principles Calculations

Water impurity is the essential factor of reducing the insulation performance of transformer oil, which directly determines the operating safety and life of a transformer. Molecular dynamics simulations and first-principles electronic-structure calculations are employed to study the diffusion behavior of water molecules and the electrical breakdown mechanism of transformer oil containing water impurities. The molecular dynamics of an oil-water micro-system model demonstrates that the increase of aging acid concentration will exponentially expedite thermal diffusion of water molecules. Density of states (DOS) for a local region model of transformer oil containing water molecules indicates that water molecules can introduce unoccupied localized electron-states with energy levels close to the conduction band minimum of transformer oil, which makes water molecules capable of capturing electrons and transforming them into water ions during thermal diffusion. Subsequently, under a high electric field, water ions collide and impact on oil molecules to break the molecular chain of transformer oil, engendering carbonized components that introduce a conduction electronic-band in the band-gap of oil molecules as a manifestation of forming a conductive region in transformer oil. The conduction channel composed of carbonized components will be eventually formed, connecting two electrodes, with the carbonized components developing rapidly under the impact of water ions, based on which a large number of electron carriers will be produced similar to “avalanche” discharge, leading to an electrical breakdown of transformer oil insulation. The water impurity in oil, as the key factor for forming the carbonized conducting channel, initiates the electric breakdown process of transformer oil, which is dominated by thermal diffusion of water molecules. The increase of aging acid concentration will significantly promote the thermal diffusion of water impurities and the formation of an initial conducting channel, accounting for the degradation in dielectric strength of insulating oil containing water impurities after long-term operation of the transformer.

The Effect of Nano-Silica Modified With Silane Coupling Agents on the Diffusion Behavior of Water Molecules in Palm Oil Based on Molecular Simulation

To enhance the compatibility of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and palm oil, four different types of silane coupling agent (KH550, KH560, KH570 and KH792) were adopted to modify the surface of silica. It turns out that when using the KH792 to modify the surface of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , the interfacial bonding between silica and palm oil was maximum and their overlapping area was the largest, up to 12 Å. Thus, it can be concluded that the relatively optimal silane coupling agent is KH792. And when nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , whose surface was modified with KH792, was added to the palm oil, the interaction energy between palm oil and water molecules was enhanced and the diffusibility of water molecules in oil was decreased. In addition, the addition of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> to the oil reduced the diffusible space of water molecules provided by oil and further restricted the diffusion of water molecules in oil. Therefore, it can be seen that the addition of nano-SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> to palm oil may cause the restriction of the diffusion of water molecules in it.

Differences analysis of water molecular diffusion behaviors in vegetable oil and mineral oil under temperature field

The existence of water will greatly affecf the insulation performance of transformer insulation system. In order to research the difference of diffusion behavior of water molecules in mineral oil and vegetable oil, the diffusion behavior of water molecules in two oil media was studied by using molecular simulation method. The results show that the diffusion motion of water molecules in two oil media was gradually intensified with the increase of temperature. By comparing the diffusion behavior of water molecules in vegetable oil and mineral oil, it is found that the hydrogen bond formed between vegetable oil and water can limit the diffusion motion of water molecules in vegetable oil. Moreover, the interaction energy between water molecules and vegetable oil is greater than that with mineral oil. In addition, the FFV of water molecules in mineral oil is greater than that in vegetable oil, which indicates vegetable oil provides less free space for the diffusion of water molecules. This provides a basis for understanding the difference of diffusion behavior of water molecules in two oil media from a micro perspective.

Electrodialysis based direct air dehumidification: A molecular dynamics study on moisture diffusion and separation through graphene oxide membrane

To compromise shortages of traditional dehumidification methods, an electrodialysis based direct air dehumidification with higher efficiency and respondency was proposed. This technique could be accomplished via directional migration of charged water molecules and subsequent separation from air composition by the membrane. Notably, the water molecules diffusion and separation through the membrane are challenging and fundamental tasks to support the proposed novel dehumidification strategy. The purpose of this paper is to evaluate the dynamic diffusion behavior of water molecules across the double-layered nanoporous graphene oxide (2NPGO) membrane under the external electric field. Thence, molecular dynamics (MD) simulations of atmospheric gas, including water and nitrogen, were conducted. Once the electric field was applied, the self-diffusion coefficient of water increased from 2.421 × 10-9 m2·s−1 to 7.566 × 10-8 m2·s−1. In the presence of nitrogen, an electric field strength of 10-6 V/ Å would contribute to the increased permeation rate of 2.83 × 103 mol·m-2s−1 for water and a selectivity of 4.918 over nitrogen. Further study in hydrogen bond evolution and interaction energy variation versus electric field strength illustrated the mechanism of enhanced diffusion of water molecules.

Simulation effect of SiO2 nanoparticles on the water molecules diffusion inside insulating oil at different temperatures

The performance of transformer insulating oil will gradually decrease with the long-term operation of transformer. Previous researches have shown that temperature is one of the important factors affecting the insulation performance of transformer insulation oil, and the addition of nanoparticles can effectively improve the insulation properties of oil. In order to further reveal the influence of temperature and nanoparticles on the diffusion ability of water molecules in insulating oil, the diffusion behavior of water molecules in pure oil model (AW-1) and model (AW-2) containing nano-SiO2 particles at 303K, 323K, 343K, 363K and 383K temperatures are studied in this paper. The simulation results showed that nano-SiO2 particles can effectively reduce the influence of temperature on the diffusion behavior of water molecules in oil. In two models, the free volume fraction of moisture in oil will be improved with the increase of temperature, which provides more space for the movement of water molecules. However, in the AW-1 model, the growth rate of the free volume fraction is around 2.2 %, while that in the AW-2 model is around 1.5 %. Moreover, according to the analysis of the radial distribution function (RDF) between different atoms in the two models, the covalent bond and the hydrogen bond between the atoms weakens due to the increase of temperature. In addition, with the increase of temperature, the interaction energy between water molecules and oil gradually decreases in two models. In the AW-1 model, with the increase of temperature, the reduction rate of interaction energy is about 4.5 %, in the AW-2 model, the reduction rate of interaction energy is about 2.5 %, which indicates that the influence of temperature on the AW-2 model is smaller. Therefore, adding nanoparticles to the oil can effectively restrain the movement of water molecules in the oil, so as to improve the insulation performance of transformer insulation system.

Miscibility and diffusivity of water in organic acids: Molecular dynamics simulations

Molecular dynamics simulations were performed to study both the structural and dynamic properties of n-alkyl carboxylic acid/water/n-alkyl carboxylic acid system with different alkyl chain length, such as acetic (CH3COOH) (A/W/A system), butyric (CH3(CH2)2COOH) (B/W/B system) and hexanoic (CH3(CH2)4COOH) (H/W/H system) acids. The mass density profiles along the interface normal to the organic acid/water system, the difference in the local structure of H2O molecules in bulk and in the vicinity of interface for A/W/A, B/W/B and H/W/H systems, as well as the diffusion behavior of water molecules at the interface with abovementioned organic acids, in the wide range of the temperature changes, have been investigated. Because of the central importance of water to the biology and pharmacology a number of new calculated regimes of miscibility and diffusivity of water molecules in carboxylic acids help us to better analyze such organic systems.

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Diffusion behavior of particles in hydrogels is important both for the fundamental understanding of mass transport and for practical applications and has been investigated for a long time. There are three major theories describing the diffusion behavior of small particles in a polymer network: obstruction, hydrodynamic, and free volume theories. Although many researchers have examined these three theories, their applicability is still unclear due to ambiguity stemming from the heterogeneity of conventional hydrogels. Recently, we have developed a near-ideal hydrogel called Tetra-PEG gel that provides a unique possibility to correlate gel structure with properties. In this study, we measured the diffusion coefficient of water molecules (D) in Tetra-PEG gels by pulsed field gradient spin-echo 1H NMR. By comparing D and the correlation length of a polymer network (ξ) measured by small-angle neutron scattering, we observed an identity formula similar to the hydrodynamic theory (D/D0 = exp(−d/ξ), where D0 is t...

Non-affine deformation of free volume during strain dependent diffusion in polymer thin films

We employ molecular dynamics simulations to understand the influence of non-affine deformation and recovery of free volume on the diffusion behavior of water molecules in polymers, as a function of tensile strain. This study is analogous to strain dependent diffusion of water in polymer thin films which undergo folding in response to water, which is not completely explored. Results reveal that diffusion coefficient of water molecules increases upto 20% strain followed by gradual reduction at higher strain. Non-affine deformation analysis indicates the coupled behavior of relaxation of polymer chains and recovery of free volume. Relaxation process at lower strain coalesces the free volume regions, enhancing the diffusion coefficient. Meanwhile, larger strain dissociates the highly deformed free volume regions. Interestingly, the dissociated regions undergo negligible changes in orientation and shape resulting in reduced diffusion coefficient. The above mechanisms coupled with hydrogen bonds are primarily responsible for shape change in polymer films.

Diffusion Behavior of Water Molecules in Hydrogels with Controlled Network Structure

Hydrogels have attracted attention as biomaterials because of the similar property to biological soft tissues. One of the applications of hydrogels for biomaterials is drug release carrier. For the application, it is important to understand and to control the diffusion behavior of molecules in hydrogels. There are three major models describing diffusion behavior of small molecules in the polymer solution. First is the Ogston model, in which polymer network is considered to behave as an obstacle preventing the diffusion of particles. Second is the hydrodynamic model, in which the existence of polymer increases the effective viscosity and decreases diffusion speed of particles. Third is the free volume model, in which particles diffuse passing through free volume of solvent and polymer. Although the validity of these models has been investigated by many researchers, there is no clear experimental evidence to support any models. Thus, even now, many models are left without validation. The muddling of models was assisted by the heterogeneity in conventional hydrogels. To understand the diffusion behavior of small molecules in a hydrogel, the validation of these models using a homogeneous hydrogel is necessary. Recently, we have developed a homogeneous hydrogel, Tetra-PEG gel, by combining two kinds of tetra-armed pre-polymers. It has been revealed that Tetra-PEG gel has an extremely homogeneous network structure, and network structure parameters (polymer volume fraction : φ 0, molecular weight between crosslinks : M w, binding ratio : p) of Tetra-PEG gel can be controlled independently. In this study, we investigated the effect of network structures on the self-diffusion coefficient of water molecules (D) in Tetra-PEG gels and in pure water (D 0) by Pulsed field-gradient spin-echo Nuclear Magnetic Resonance (PGSE-NMR). The values of D/D 0 decreased as an exponential function with an increase in φ 0 (0 - 0.15), and was independent of M w (10k, 20k g/mol) (Figure 1). This result well agreed with three models, and the difference among three models was negligible. On the other hand, D/D 0 decreased with a decrease in p (0 - 1) (Figure 1). This result was not reproduced by any three models, suggesting these models were not valid to describe diffusion behavior of small molecules in a hydrogel. Here, we tested another model proposed by M. Tokita et al. In the model, the diffusion of probe molecules is mainly determined by the correlation length of the polymer network (blob size : ξ) and the hydrodynamic radius of the probe molecules (R h) as, D/D 0 = exp(-R h/ξ). In Figure 2, D/D 0 in Tetra-PEG gels were plotted against 1/ξ. The values of xwere estimated by our previous small angle neutron scattering experiment. As shown in the figure, all the data including p-tune results fall onto a guide showing D/D 0 = 0.97exp(-2.8/ξ). The experimental values of the front coefficient (0.97) and R h (2.8Å) were close to the theoretical value (1.0) and the hydrodynamic radius of a water molecule (3.0Å), respectively. These results suggest that Tokita’s model is the correct model to describe the diffusion behavior of small molecules in a hydrogel. Figure 1

Molecular and condition parameters dependent diffusion coefficient of water in poly(vinyl alcohol): a molecular dynamics simulation study

Diffusion behavior of water molecules in poly(vinyl alcohol) (PVA) was investigated by molecular dynamics (MD) simulation, and the diffusion coefficient of water molecules in PVA was calculated based on the Einstein’s relation. The effects of polymerization degree, temperature, density, and pressure on the diffusion coefficient were conducted to gain insights into their diffusion mechanisms under different processing conditions. The results showed that the simulation data of diffusion coefficient was consistent with experimental values. With the increase of polymerization degree of PVA, the diffusion coefficient began with a sharp decrease and then became stable, while the diffusion coefficient increased with increasing temperature. It was also found that PVA matrix with a smaller cell density had a larger free volume, and then, the diffusion coefficient was larger. The H2O sorption isotherm could be described by the Langmuir sorption isotherm, and the diffusion coefficient decreased with the increase of pressure.

Structural and water diffusion of poly(acryl amide)/poly(vinyl alcohol) blend films: Experiment and molecular dynamics simulations

To study the effects of composition ratios and temperature on the diffusion of water molecules in PVA/PAM blend films, five simulation models of PVA/PAM with ten water molecules at different composition ratios (4/0, 3/1, 2/2, 1/3, 0/4) were constructed and simulated by using a molecular dynamics (MD) simulation. The diffusion behavior of water molecules in blends were investigated from the aspects of the diffusion coefficient, free volume, pair correlation function (PCF) and trajectories of water molecules, respectively. And the hydrophilicity of blend composite was studied based on the contact angle and equilibrium water content (EWC) of the blend films. The simulation results show that the diffusion coefficient of water molecules and fractional free volume (FFV) of blend membranes increase with the addition of PAM, and a higher temperature can also improve the diffusion of water molecules. Additionally, the analysis of PCFs reveals the main reason why the diffusion coefficient of water in blend system increases with the addition of PAM. The measurement results of contact angle and EWC of blend films indicate that the hydrophilicity of blend films decreases with the addition of PAM, but the EWC of blends increases with the addition of PAM.