Free Volume Distribution Research Articles

Comparison of the structural and physical properties of nitrocellulose plasticized by N-butyl-N-(2-nitroxy-ethyl) nitramine and nitroglycerin: Computational simulation and experimental studies.

We performed experimental studies on the effects of N-butyl-N-(2-nitroxy-ethyl) nitramine (Bu-NENA) and nitroglycerin (NG) on the plasticization of nitrocellulose (NC). The tensile strength and elongation at break of NC/Bu-NENA measured at -40 °C show increments of about 23% and 33% respectively, compared with those of NC/NG, suggesting that Bu-NENA has a higher plasticizing efficiency than NG. Scanning electron microscope (SEM) observation reveals that NC shows better miscibility with Bu-NENA than NG, which was further evidenced by the mesoscopic morphologies derived from mesodynamics simulations. Furthermore, molecular dynamic simulations were conducted to explore the mechanical strengthening mechanism of plasticizers. The simulation results such as free volume (Vfree), binding energy (Ebind) and radial distribution function (RDF) were obtained. NC/Bu-NENA shows a 28% improvement in Vfree compared to NC/NG, suggesting that Bu-NENA molecules demonstrate higher mobility in NC matrix. The larger Ebind of NC/Bu-NENA than NC/NG indicates the stronger interaction between NC and Bu-NENA. Furthermore, g(r) values correlated to the atom pair interaction determined from RDF curves reveal that NC/Bu-NENA shows higher intensities of both hydrogen bond and van der waal force than those of NC/NG blend. These computational and experimental studies reveal great potential in design and fabricating low vulnerable insensitive propellants.

Polyamides with phosphaphenanthrene skeleton and substituted triphenylamine for gas separation membranes

Molecular dynamics (MD) and Monte Carlo (MC) simulations were used to study the structural and gas transport properties of a group of three highly permeable glassy polyamides containing phosphaphenanthrene skeleton and substituted triphenylamine side groups. The polymer chains were relaxed using a combination of canonical and isothermal-isobaric ensemble simulations to obtain the final equilibrium density. The grand canonical Monte Carlo method coupled with a canonical MD simulation was used to study the solubility of small gases in these polymers. The accuracy of the methods used in this work was tested and validated by comparing the physical and gas transport properties of gases with the available experimental data. The presence of large bulky side groups along the rigid backbone of the polymer, hinders its efficient packing and results in a superior free volume distribution. The computed diffusivities and solubilities of gases are significantly higher for the polyamides containing both phosphaphenanthrene and trityl-substituted triphenylamine side groups. The thermal motion of the gases, which results in frequent jumps from one free volume region to another, is enhanced with the availability of larger cavities. These molecular details can provide critical information for the experimental design of highly selective and efficient gas separation membranes.

Free volume distribution and structural inhomogeneity in Ni50V50 amorphous alloy

The structure of Ni50V50 amorphous alloy was investigated by using neutron, X-ray diffraction and reverse Monte Carlo (RMC) modelling. The RMC configuration was decomposed into polyhedral holes whose faces are all triangles consisting of chemical bonds. Free volumes in the respective polyhedral holes were evaluated with reference to the packing efficiency of Ni-V metastable BCC phase. The tetrahedral holes, which account for about 70% of the whole space, are regarded as densely packed units because the average packing efficiency of them is approximately equal to that of the corresponding crystal phase. On the other hand, various types of polyhedral holes, which have a certain free volume, are observed and some of them are connected with each other. The densely packed coordination polyhedra consisting only of tetrahedral holes tend to be clustered.

Free volume characteristics on water permeation and salt rejection of polyamide reverse osmosis membranes investigated by a pulsed slow positron beam

Polyamide thin film composite reverse osmosis membranes on porous polysulfone support were prepared using m-phenylenediamine, trimesoyl chloride and dimethyl sulfoxide (DMSO) with various contents as precursors via an interfacial polymerization. The membrane with heterogeneous multilayer structure was depth-profiled by positron annihilation lifetime spectroscopy based on a pulsed slow positron beam and the thickness of the polyamide active layer can be estimated through fitting the results of depth dependence of o-Ps intensity. The free volume characteristics in the polyamide selective layer of the membranes were evaluated and were correlated with the permeation properties. With increasing DMSO content, mean free volume size (VFV) was enlarged, which was responsible for the enhanced water and sodium chloride permeation. However, the salt rejection was significantly decreased with a slight increase in the VFV around a critical value of 88.0 A3, which was the average size of hydrated sodium and chloride ions. It was well interpreted for that the broadened free volume distribution enables an amount of larger free volumes to be involved in the salt permeation, although little change in VFV was found.

Fluctuations of conformational mobility of macromolecules around the glass transition.

The heterogeneity of local dynamics in disordered systems is behind some key features of glass transition. In order to improve our understanding of the molecular dynamics in disordered systems in the vicinity of the glass transition, different parameters have been proposed to quantitatively describe dynamical heterogeneity. In the case of polymers, free volume models relate the macromolecular mobility to the free or accessible volume. The relationship between dynamic heterogeneity and fluctuations of accessible volume seems straightforward. In the present work, the heterogeneity of local dynamics in polymeric systems is analyzed by computer simulation with the bond fluctuation model. The value of the accessible volume around each polymer chain is evaluated from a snapshot or static structure at each system state, resulting in a distribution of accessible volume that reflects system heterogeneity. The relationship between the average value and the standard deviation of free volume distributions at different temperatures fits a master curve for different systems, regardless of the specific inter- and intramolecular interaction potentials that define each material. The dynamic slowdown around the glass transition is accompanied by a clear evolution of the mean value and shape of the accessible free volume distribution. The relative fluctuation of the dynamically accessible volume has been used as a parameter to quantitatively describe heterogeneity. The fluctuation varies with temperature with remarkable differences between the liquid and glassy states of the systems studied, presenting a peak at the glass transition temperature, which can be interpreted as a reflection of the distribution of local glass transition temperatures.

Supported PEBA-zeolite 13X nano-composite membranes for gas separation: Preparation, characterization and molecular dynamics simulation

Poly ether block amide (PEBA) incorporated with zeolite 13X nanoporous was deposited onto a PSf/PE layer. The fabricated Mixed Matrix Composite Membranes (MMCMs) were weighed up by XRD, SEM, FTIR and AFM analysis. The influence of PEBA concentration, number of coated layers, zeolite loading, PDMS cover layer and a set of conditions for operating a particular process on the gas separation performance of the fabricated membranes were specified through pure gases permeability examination. The best CO2 permeability and CO2/N2 and CO2/CH4 selectivity for the MMCM loaded with 1 wt% zeolite 13X showed an increase in 200%, 1030% and 3633% compared to the neat membrane. CO2/N2 and CO2/CH4 selectivity increased 153% and 18.24%, after coating a PDMS cover layer on the PEBA-zeolite (1%)/PSf/PE, respectively. Molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) have been performed to reckon the influence of PEBA and zeolite 13X contents on gas separation and thermodynamic properties. Four different Neat and MMMs were constructed in simulation part regarding the incorporation of 0, 0.5, 1 and 2 wt% of zeolite nano-fillers. Fractional free volume (FFV), Density, X-ray diffraction (XRD) and radial distribution function (RDF) were performed to understand the nanostructure properties of the membranes. Having used three different gases (N2, CO2 and CH4), the permeability and selectivity of constructed membranes have been studied.

Thermal annealing on free volumes, crystallinity and proton conductivity of Nafion membranes

Free volumes, backbone mobility, crystallinity and proton conductivity of Nafion membranes upon thermal annealing were intensively studied. The development of size and distribution of free volumes in Nafion membranes as a function of environmental humidity was found to be closely associated with the adsorbed water molecules and the Nafion backbone mobility. Results suggested a gradual decrement in Nafion backbone mobility as a function of annealing temperature, due to the thermal induced recrystallization. This was further supported by a linear decrement in the o-Ps intensity as a function of membrane crystallinity, because o-Ps atoms mostly annihilated in the amorphous phase in Nafion. Particularly, a significant increment in crystallinity was found in Nafion membranes annealed at temperatures higher than 140 °C. The present results showed that annealing at ∼140 °C of Nafion membranes is favourable for their proton conductivity because of the low crystallinity.

Water and Salt Transport Properties of Triptycene-Containing Sulfonated Polysulfone Materials for Desalination Membrane Applications.

A series of triptycene-containing sulfonated polysulfone (TRP-BP) materials was prepared via condensation polymerization, and the desalination membrane-relevant fundamental water and salt transport properties (i.e., sorption, diffusion, and permeability coefficients) of the polymers were characterized. Incorporating triptycene into sulfonated polysulfone increased the water content of the material compared to sulfonated polysulfone materials that do not contain triptycene. No significant difference in salt sorption was observed between TRP-BP membranes and other sulfonated polysulfone membranes, suggesting that the presence of triptycene in the polymer did not dramatically affect thermodynamic interactions between salt and the polymer. Both water and salt diffusion coefficients in the TRP-BP membranes were suppressed relative to other sulfonated polysulfone materials with comparable water content, and these phenomena may result from the influence of triptycene on polymer chain packing and/or free-volume distribution, which could increase the tortuosity of the transport pathways in the polymers. Enhanced water/salt diffusivity selectivity was observed for some of the TRP-BP membranes relative to those materials that did not contain triptycene, and correspondingly, incorporation of triptycene into sulfonated polysulfone resulted in an increase, particularly for acid counterion form TRP-BP materials, in water/salt permeability selectivity, which is favorable for desalination membrane applications.

Tracking free volume changes in bisphenol-a based polycarbonate sheets after treatment with liquid acetone

Free volume properties, size and distribution, in the bisphenol-a based polycarbonate (PC) sheets were investigated as a function of the treatment duration by liquid acetone using the positron annihilation lifetime (PAL) spectroscopy. Along with increasing the PC treatment duration, the free volume sizes decreased and the holes-distribution broadened, which in turns led to the polymer crystallization induction. From the PAL and WAXD data, it has been confirmed that the crystallization capability of the amorphous PC tremendously enhanced, with a rapid increase of the PC crystallinity along with increasing the treatment duration by liquid acetone. These findings were further confirmed by the surface chemistry and topography characterization techniques which revealed the formation of crystalline structure after liquid acetone treatment. The solvent-induced crystallization of PC sheets promoted restrictions to the infrared induced molecular vibrations the surface functional groups which was reflected as drop in peaks intensities and shift of the carbonyl peak of the Fourier transform infrared (FTIR) spectrum.

Positronium probes free volume to identify para- and meta-aramid fibers and correlation with mechanical strength

We adopt here a novel approach to identify the para- and meta-aramid fibers using the available free volume in them. Positron Annihilation Lifetime Spectroscopy (PALS) has been employed to characterize the free volume in these fibers. The free volume content in the para-variety (Kevlar) is less and the free volume distribution is narrow as compared to its meta-counterpart (Nomex). The results are further validated by characterizing the free volume content in Nomex IIIA fibers, which is a blend composed mainly of para- and meta-aramid fibers. The free volume results are in good correlation with the mechanical properties of these fibers obtained by Universal Testing Machine (UTM), structural studies by X-ray diffraction (XRD) and storage modulus using Dynamic Mechanical Thermal Analysis (DMTA). The thermal decomposition temperature (Td) of the fibers is characterized by thermo gravimetric analysis (TGA). We also show here the first results of an appreciable glass transition temperature (Tg) not only for the Nomex fibers but also for its blend Nomex IIIA using the combined results of Differential Scanning Calorimetry (DSC) and DMTA.

Enhancement of plasticity in Zr-Cu-Ni-Al-Ti bulk metallic glass by heterogeneous microstructure

Icosahedral phase was found as a primary precipitated phase in the Zr70−xCu12.5Ni10Al7.5Tix (x=0–8) BMGs with two or three steps of crystallization process. Remarkable plasticity was realized in a wide range of alloy composition. The enhanced plasticity is attributed to the presence of icosahedral medium-range order clusters and the local heterogeneous distribution of free volume.

Molecular dynamics study of the interaction between nanoscale interstitial dislocation loops and grain boundaries in BCC iron

Atomic simulations are used to investigate the interaction between nanoscale interstitial dislocation loops and grain boundaries (GBs), the subsequent evolution of the GBs' structures, and the resulting impact on mechanical properties, in BCC iron. The interaction between loops and GBs — Σ3{111} and Σ3{112} — is affected by the angle (θ) between the Burgers vector and the normal to the GB plane, as well as by the distribution of free volume (FV) and stress. Loops can be totally absorbed by Σ3{111} boundaries, while the interaction with Σ3{112} boundaries is found to change the Burgers vector and habit plane after absorption, but to otherwise leave the loop intact, resulting in selective absorption. When θ=90o, no absorption occurs in Σ3{112}. The stress accumulation induced by the absorption affects the local mechanical properties of GBs. In nanocrystalline iron sample, a similar phenomenon is also observed, resulting in rearrangement of GBs and grain growth.

Correlating Free-Volume Hole Distribution to the Glass Transition Temperature of Epoxy Polymers.

A new algorithm is developed to quantify the free-volume hole distribution and its evolution in coarse-grained molecular dynamics simulations of polymeric networks. This is achieved by analyzing the geometry of the network rather than a voxelized image of the structure to accurately and efficiently find and quantify free-volume hole distributions within large scale simulations of polymer networks. The free-volume holes are quantified by fitting the largest ellipsoids and spheres in the free-volumes between polymer chains. The free-volume hole distributions calculated from this algorithm are shown to be in excellent agreement with those measured from positron annihilation lifetime spectroscopy (PALS) experiments at different temperature and pressures. Based on the results predicted using this algorithm, an evolution model is proposed for the thermal behavior of an individual free-volume hole. This model is calibrated such that the average radius of free-volumes holes mimics the one predicted from the simulations. The model is then employed to predict the glass-transition temperature of epoxy polymers with different degrees of cross-linking and lengths of prepolymers. Comparison between the predicted glass-transition temperatures and those measured from simulations or experiments implies that this model is capable of successfully predicting the glass-transition temperature of the material using only a PDF of the initial free-volume holes radii of each microstructure. This provides an effective approach for the optimized design of polymeric systems on the basis of the glass-transition temperature, degree of cross-linking, and average length of prepolymers.

A fractional transient model for the viscoplastic response of polymers based on a micro-mechanism of free volume distribution

In the present work, the nonlinear viscoelastic/viscoplastic response of polymeric materials is described by introducing essential modifications on a model developed in previous works. A constitutive equation of viscoelasticity, based on the transient network theory, is introduced in a more generalized form, which takes into account volume changes during deformation. This time-dependent equation accounts for the nonlinearity and viscoplasticity at small elastic and finite plastic strain regime. The present description was proved to be more flexible, given that it contains a relaxation function that has been derived by considering instead of first order kinetics a fractional derivative that controls the rate of molecular chain detachment from their junctions. Therefore, the new equation has a more global character, appropriate for cases where heavy tails are expected. On the basis of the distributed nature of free volume, a new functional form of the rate of plastic deformation is developed, which is combined with a proper kinematic formulation and leads to the separation of the total strain into the elastic and plastic part. A three-dimensional constitutive equation is then derived for an isotropic, compressible medium. This analysis was proved to be capable of capturing the main aspects of inelastic response as well as the instability stage taking place at the tertiary creep, related to the creep failure.

Mechanical properties of spinodal decomposed metallic glass composites

Controlling and manipulating heterogeneity through spinodal decomposition is one of few effective ways in making better metallic glass matrix composites. Here using a finite element modeling we investigate how the geometry, orientation, size, and statistical and spatial distribution of varying free volume heterogeneities in spinodal decomposition affect mechanical properties of the composites. Among the plethora of factors, orientation and statistical distribution of free volumes in the spinodal microstructures are identified as two key ones critically influencing the mechanical responses. The size effect is also discovered and found to be governed by the minimum size in shear banding initiation and propagation.

Effect of Chain Dynamics, Crystallinity, and Free Volume on the Barrier Properties of Poly(ethylene terephthalate) Biaxially Oriented Films

The primary focus of this work was to evaluate the relationship between free volume distribution, chain motion, crystallinity, and the gas properties of PET upon strain-induced crystallization (SIC) at different stretch ratios. The formation of a three-phase structure containing rigid amorphous phase, mobile amorphous phase, and crystalline phase upon SIC was confirmed by differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Dynamic mechanical analysis and PALS indicated that there was a significant reduction in the fractional free volume upon orientation at an extension ratio of 3 × 3. Sub-Tg relaxation studies indicated that the activation energy of mechanical relaxation decreased with increasing the stretch ratio. Gas transport studies revealed that the reduction in permeability coefficient was mainly due to reduction in diffusivity. Permeation studies using gas molecules with different sizes revealed that strain-induced crystallization affects the free volume distri...

Structure Manipulation in Triptycene-Based Polyimides through Main Chain Geometry Variation and Its Effect on Gas Transport Properties

Two new triptycene-based polyimides, 6FDA-1,4-trip_ortho and 6FDA-2,6-trip_para, were synthesized to investigate the effect of varying polymer backbone geometry on chain packing and gas transport properties. Changing the imide linkage geometry from para to ortho reduced gas permeabilities by ∼48% due to more efficient chain packing of the asymmetric ortho structure, which is demonstrated by decreased d-spacing and fractional free volume. Varying the triptycene orientation from the 1,4- to 2,6-connection also caused a decrease in permeability (e.g., 29% decrease for PCO2). This is likely the result of reduced chain mobility, as evidenced by increased Tg, and a shift in free volume distribution toward smaller cavities, as supported by smaller d-spacing. Physical aging studies show that the equilibrium specific volume of these isomeric polymers is similar, as evidenced by nearly identical gas transport properties exhibited by all aged samples.

Transport Diffusion of Light Gases in Polyethylene Using Atomistic Simulations.

We explore the temperature dependence of the self-, corrected-, and transport-diffusivities of CO2, CH4, and N2 in a polyethylene (PE) polymer membrane through equilibrium molecular dynamics simulations. We also investigate the morphology of the polymer membrane based on the intermolecular radial distribution function, free volume, and pore size distribution analysis. The results indicate the existence of 1.5-3 Å diameter pores in the PE membrane, and with the increase in the temperature, the polymer swells linearly with changing slope at 450 K in the absence of gas and exponentially in the presence of gas. The gas adsorption isotherms extracted via a two-step methodology, considering the dynamics and structural transitions in the polymer matrix upon gas adsorption, were fitted using a "two-mode sorption" model. Our results suggest that CO2 adsorbs strongly, whereas N2 shows weak adsorption in PE. The results demonstrate that CO2 is more soluble, whereas N2 is least soluble. Further, it is found that an increase in the temperature negatively impacts the solubility of CO2 and CH4 but positively for N2; this reverse solubility behavior is due to increased availability of pores accessible to N2, which are kinetically closed at the lowest temperatures. The reported self-diffusivities of the gases from our simulations are on the order of 10-6 cm2/s, consistent with the experimental evidence, whereas transport-diffusivities are 2 orders of magnitude higher than self-diffusivities. Furthermore, the temperature dependence of the self-diffusivity follows Arrhenius behavior, whereas the transport-diffusivity follows non-Arrhenius behavior having different activation energies in low and high temperature regions. Also, it is seen that loading has little effect on the self- and corrected-diffusion coefficients of all gases in the PE membrane.

Permeation and separation of SO2, H2S and CO2 through thermally rearranged (TR) polymeric membranes

Abstract The permeabilities of sulfur dioxide and hydrogen sulfide are reported here for two copolymer based TR membranes, as a function of feed partial pressure and temperature. The two TR membranes are formed from the precursor co-polyimide 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA), 3,3′-dihydroxyl-4,4′-diaminobiphenyl (HAB) and 2,4,6-trimethyl-m-phylenediamine (DAM) (6FDA-HAB-DAM), where one of the membranes undergoes crosslinking with 3,5-diaminobenzoic acid (DABA) during TR. It was observed that SO2 permeability decreased with increasing partial pressure and temperature in both TR membranes. This is indicative that SO2 permeability is dominated by SO2 solubility within the TR membranes, and is attributed to the high condensability of SO2. Both polymeric membranes displayed selectivity for SO2 against CO2, with separation performance similar to other classes of polymeric membranes. H2S permeability was observed to decrease with increasing partial pressure and temperature for the cross-linked TR (XTR) membrane. The non-cross-linked TR membrane displayed constant H2S permeability with pressure and permeability increased with increasing temperature. Hence, permeability within the XTR membrane was H2S solubility dominated, while in the non-crosslinked TR membrane permeability was H2S diffusivity dominated. This was attributed to the difference in free volume and cavity size distribution in the TR membrane as a result of crosslinking. It was observed that both membranes displayed H2S/CO2 selectivity of less than one, indicative that they favor CO2 over H2S.

Free volume gradient effect on mechanical properties of metallic glasses

Gradient of free volume distribution in metallic glasses is omnipresent either naturally or artificially in synthesis and application. But how the spatial inhomogeneity affects mechanical properties remains poorly understood. Here we probe this issue using finite element modeling. We find that the strength and toughness improves with increasing gradient; and the larger the gradient, the more effective. But the plasticity is marginally enhanced or even decreases. Too high the gradient value leads to brittle fracture caused by abrupt release of the large stress concentrated at the gradient region. The effects are represented in their relations with the gradient energy coefficient.