Extra Free Volume Research Articles

Optimization of the Thermally Conductive Low-k Polymer Dielectrics Based on Multisource Free-Volume Effects.

Polymers/polymer matrix composites possessing low dielectric constants (low-k polymer dielectrics) contribute to the advance of electronics, for instance, microprocessor chips, mobile phone antennas, and data communication terminals. However, the intrinsic long-chain structural characteristic results in poor thermal conductivities, which draw heat accumulation and undermine the outstanding low-k performance of polymers. Herein, multisource free-volume effects that combine two novel kinds of extra free volume with the known in-cage free volume of polyhedral oligomeric silsesquioxanes (POSSs) are discussed to reduce the capacity for dielectric constant reduction. The multisource free-volume effects of POSSs are associated with the thermal conductive network formed by the hexagonal boron nitride (BN) in the polymer matrix. The results show a decent balance between low-k performance (dielectric constant is 2.08 at 1 MHz and 1.98 at 10 GHz) and thermal conductivity (0.555 W m-1 K-1, 4.91 times the matrix). The results provide a new idea to maximize the free-volume effects of POSSs to optimize dielectric properties together with other desired performances for the dielectrics.

Sliding behaviour of carbon nanothread within a bundle embedded in polymer matrix

Interfacial shear strength determines the load transfer efficiency between nanofillers and polymer matrix, and thus the overall mechanical performance of polymer nanocomposites. Through atomistic simulations, this work systematically assessed the sliding behaviours of one-dimensional carbon nanothread (NTH) from a bundle configuration embedded in the poly (methyl methacrylate) (PMMA) matrix. It is found that the functionalized NTH exhibits remarkably enhanced interfacial shear strengths due to the strong mechanical interlocking effect, about an order of magnitude higher than the pristine sample. However, excess volume is generated within a functionalized bundle structure, leading to a weaker non-bonded interaction for a functionalized NTH within the bundle than that only embedded in the polymer matrix. With covalent cross-linking, the surrounding NTHs can be simultaneously dragged out from the PMMA matrix while pulling out the central NTH. Such phenomenon attributes to the filler-filler interactions and filler-matrix interactions. Further studies reveal that no extra free volume is generated within the bundle structure at high temperatures, which retains the effective load transfer efficiency. In comparison, free volume tends to be generated at the interface between NTHs and PMMA matrix at a higher temperature. These findings could be beneficial for fibre design and high-performance polymer nanocomposites with 1D nanomaterials.

High-pressure hydrogen decompression in sulfur crosslinked elastomers

High-pressure storage and cyclic (de)pressurization of hydrogen gas is known to result in degradation and failure of gas canisters, hoses, linings, and O-rings as the relatively small hydrogen molecule can readily permeate most materials. Hence, identifying material compositions that are less susceptible to hydrogen-induced damage is of significant importance to the hydrogen energy infrastructure. Here, we use classical atomistic molecular dynamics simulations to study hydrogen exposed ethylene-propylene-diene monomer (EPDM) rubber, an elastomer typically used in O-rings. We make chemical modifications to the model by adjusting the crosslink density and report on gas solubility, diffusivity, and molecular restructuring in response to rapid decompression. Our simulations indicate that increases in crosslink density can reduce volumetric expansion during decompression and result in smaller free volume pore sizes. However, these favorable properties for sealing materials come with a tradeoff. At pressure, crosslinks introduce extra free volume, providing potential sites for gas localization, the precursor to cavitation-induced failure.

Effect of Intermittent Ultrasonic Vibration-Assisted Compression on the Mechanical Properties of Zr-Based Amorphous Alloys

Bulk amorphous alloys have some good mechanical properties due to their special atomic arrangement and are now popular in the field of materials. Zr-based amorphous alloys have good mechanical properties, but they are different from lattice slip materials with high ductility. When these materials are compressed and deformed, it generates a concentrated elastic force in the shear zone that causes instantaneous amorphous fracture. The extremely poor plasticity of Zr-based amorphous materials highlight their shortcomings and make them difficult to use in engineering applications. In this paper, it is found that the plasticity of Zr-based amorphous alloys is enhanced to a certain extent by intermittent ultrasonic vibration-assisted compression (IUVC). The ultrasonic vibration stress of IUVC can increase the extra free volume of Zr-based amorphous alloys and increase their degree of “rejuvenation”, which is manifested as an increase in plasticity. To explore how IUVC affects the plasticity of Zr-based amorphous alloys, we design experiments to analyse the effects of different intermittent times, pre-pressures and ultrasonic amplitudes on the plasticity of amorphous alloys.

Enhanced desulfurization performance of polyethylene glycol membrane by incorporating metal organic framework MOF-505

• MOF-505/PEG mixed matrix membranes were prepared. • The filled MOF-505 provided extra free volume for the membranes. • The active metal sites of MOF-505 have selective adsorption for thiophene. • Both permeability and selectivity of the MOF-505/PEG MMMs were increased. The metal–organic framework MOF-505 particles were synthesized and incorporated into the polyethylene glycol (PEG) matrix to prepare mixed matrix membranes. A series of methods were used to characterize the structure and morphology of MOF-505 particles and membranes. The prepared MOF-505 had a well-developed pore structure, a large number of metal sites for selective adsorption of thiophene and good compatibility with polymer matrix. Accordingly, the desulfurization performance of the membrane was improved effectively. The effects of the content of MOF-505, operating temperature and feed sulfur content on membrane performance were investigated by pervaporation experiments. The membrane with 3 wt% MOF-505 showed the optimal desulfurization performance with a permeation flux of 2.66 kg/(m 2 ·h) and an enrichment sulfur factor of 8.15, which were increased by 158% and 25% versus pristine membrane, respectively.

Pebax‐based mixed matrix membranes loaded with graphene oxide/core shell ZIF‐8@ZIF‐67 nanocomposites improved CO2 permeability and selectivity

AbstractIn order to facilitate CO2 transport in Pebax‐based membranes, graphene oxide (GO)/core shell ZIF‐8@ZIF‐67 nanocomposites were loaded in Pebax copolymer to improve CO2 permeability and selectivity. The 0.5 wt% GO doped core shell ZIF‐8@ZIF‐67, which gave highest CO2 adsorption capacity of 1.12 mmol/g, was used as nanocomposite. The incorporated GO/core shell ZIF enhanced CO2 adsorption via unsaturated metal sites (Zn‐O or Co‐O), because O atoms in GO substituted for N atoms coordinated with Zn and Co single atoms in core shell ZIF‐8@ZIF‐67. Positron annihilation lifetime spectroscopy indicated that GO‐templated core shell ZIF nanocomposites generated extra free volume and provided low‐resistance channels to facilitate CO2 transport. Fourier transform infrared spectroscopy analysis revealed that hydrogen bonds were generated between Pebax polymer chains and GO‐templated core shell ZIF which improved swelling resistance and reduced interface defects. Therefore, Pebax‐based MMMs loaded with 5 wt% GO/core shell ZIF‐8@ZIF‐67 exhibited optimum CO2 permeability (173.2 barrers) and ideal selectivity of CO2/N2 (61.9) and CO2/H2 (11.6), which were 99.7%, 66.4%, and 20.8% higher than Pebax membranes and surpass Robeson 2008 upper bound. The tensile strength increased by 17.6% to 28.8 MPa and elongation at break increased by 7.61%–554.6% when pure Pebax membranes were incorporated with 2.5 wt% GO/core shell ZIF‐8@ZIF‐67.

Investigation of the microporous organization of 1,2-disubstituted polyacetylenes using low-temperature argon sorption and small-angle X-ray scattering

The extra free volume of glassy, highly permeable 1,2-disubstituted polyacetylenes is associated with the microporous organization of these polymers. In this work, the microporosity of poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(4-methyl-2-pentyne) (PMP) is investigated by two independent methods—convenient low-temperature Ar sorption and less commonly used small-angle X-ray scattering (SAXS). The Ar sorption data were analyzed in connection with the specific surface area, pore volume, and micropore size distribution. It is shown that the higher specific surface area, total pore volume, and micropore volume of PTMSP than those of PMP are associated with higher gas permeability coefficients. It was found that the Brunauer–Emmett–Teller-specific surface values obtained for PTMSP and PMP from argon sorption isotherms correlate with the specific surface values obtained from the SAXS method. The correlation of the specific surface values obtained from fundamentally different methods makes it possible to evaluate the small-angle scattering method as adequate for assessing the microporosity of polymeric materials. The microporosity of poly(1-trimethylsilyl-1-propyne) (PTMSP) and poly(4-methyl-2-pentyne) (PMP) is investigated by two independent methods—convenient low-temperature Ar sorption and less commonly used small-angle X-ray scattering (SAXS). The Brunauer–Emmett–Teller-specific surface values obtained for PTMSP and PMP from Ar sorption isotherms correlate with the specific surface values obtained from the SAXS method. The correlation of the specific surface values obtained from fundamentally different methods makes it possible to evaluate the small-angle scattering method as adequate for assessing the microporosity of polymeric materials.

Effects of severe-strain-induced defects on the mechanical response of two kinds of high-angle grain boundaries

ABSTRACT Grain boundaries of metallic materials subjected to severe plastic deformation exhibit significantly enhanced diffusivity and excess energy compared with their relaxed poly- or bi-crystalline counterparts even when the macroscopic degrees of freedom are the same in both types of grain boundaries. Boundaries of excess energy are/can be relaxed by annealing. As a first step in accounting for this experimentally observed high-energy state of general high-angle grain boundaries subjected to severe plastic deformation, a concept of localised basic shear units and the presence of localised extra free volume in these units situated in different locations in the grain boundaries, which was originally proposed to explain steady-state structural superplastic flow, is made use of. Using MD simulation, the mechanical response of these modified grain boundaries is compared with that of their relaxed state. The results are also compared with a case of a homogeneous distribution of extra free volume within the grain boundary. The localised shear units containing extra free volume introduced in the grain boundaries are found to alter their physical and mechanical features strongly, which, in turn, drastically affect, consistent with experimental results, the mechanical response of the heavily deformed material.

Synthesis and fabrication of adsorptive carbon nanoparticles (ACNs)/PDMS mixed matrix membranes for efficient CO2/CH4 and C3H8/CH4 separation

Polydimethylsiloxane (PDMS) mixed matrix membranes (MMM)s containing microporous adsorptive carbon nanoparticles (ACNs) are developed for CO2/CH4 and C3H8/CH4 separation. The ACNs are produced by the pyrolysis of a phenolic resin precursor at 700 °C and transformed into fine particles through ball-milling. For functionalization of the particles and widening of the pores, the prepared ACNs undergo ozone treatment to increase the sorption capacity and to reduce diffusional limitations. Successful accomplishment of modification is evidenced by pore size distribution, X-ray diffraction, and Fourier transform infrared spectra analyses. MMMs containing different amounts of both native ACNs (ACN-N) and oxidized ACNs (ACN-O) are evaluated in terms of permeability, solubility and diffusivity coefficients for target gases. Based on the results, higher pressure and lower temperature promote gas separation due to capillary condensation of penetrants in the micropores of the particles present in the MMMs. Activation energy of permeation, diffusion, and enthalpy of sorption are also determined for MMMs and compared to the pure PDMS membrane. The real selectivity in PDMS/ACN-O (10%) membrane for CO2/CH4 and C3H8/CH4 are 57.3% and 53.1% higher than those in pure PDMS membrane. This enhancement is attributed to the selective adsorption and surface diffusion of the more adsorbable component in ACNs as well as partial diffusion hindrance of less adsorbing component (CH4) in the presence of more adsorbable component (e.g., CO2, C3H8) onto the surface of ACN pores. Also, CO2 and C3H8 permeability improved by 61.6% and 47.7% respectively, compared to the pure PDMS membrane and is attributed to the formation of extra free volume in PDMS matrix as a result of chain packing disruption in the presence of ACNs. According to the findings, the fabricated membranes are promising candidates for natural gas sweetening and hydrocarbon recovery.

Correlation of grain boundary extra free volume with vacancy and solute segregation at grain boundaries: a case study for Al

Grain boundary extra free volume (GB EFV) can be considered as fundamental microstructural parameter for polycrystalline or nano-crystalline materials. Here, we present a systematic first principles study on a group of representative symmetric tilt grain boundaries of Al with various EFVs subjected to vacancy formation and Mg segregation. All grain boundaries were constructed using the coincident site lattice (CSL) and the structural unit (SU) models. It was found that the SU model is superior to the CSL in describing FCC-Al GBs, the same as we previously revealed for BCC-Fe. The predicted relation between GB misorientation angle and EFV, and the predicted EFV criteria for a stable GB, both agree with available experimental observations. Vacancy formation and Mg segregation show stronger preference to those GBs with high EFV values, due to the resultant high levels of atomic disorder. These findings not only provide a new, atomistic perspective on the significance of EFV, but also suggest a viable means of predicting GB properties based on direct experimental characterisation of GB EFVs.

Atomic structure of Fe90Sc10 glassy nanoparticles and nanoglasses

Glassy nanoparticles of a Fe90Sc10 alloy have been studied using small- and wide-angle X-ray scattering. The results indicate that the surface shells have an electron density that is lower than the electron density in the cores of the glassy nanoparticles. The lower electron density seems to result primarily from a lower atomic packing density of the surface shells rather than from compositional variations due to surface segregation. During the consolidation of the glassy nanoparticles, the short-range order and the extra free volume content present in the shells may be transferred into the interfaces of the resulting bulk Fe90Sc10 nanoglasses.

Theory and calculations of colloidal depletion interaction

Colloidal suspension is composed of particles with sizes between 1 nm and 1 m, suspended in liquid phase. The interaction between the particles consists of a hard core repulsive interaction and other kinds of repulsive and attractive interacions. Hard interaction forbids the particles from occupying the same places, resulting in a depletion effect. When big colloid particles are immersed in a colloid of small particles, each big particle has a depletion layer where the small particles cannot enter due to the hard interaction. The depletion layers of two big particles overlap when they are close enough so that extra free volume of the small particles increases and therefore the entropy of the small particles increase, thus an effective interaction between big particles is induced. This effective interaction is the so-called depletion interaction. In this review the concepts and an intuitive explanation of depletion interaction of colloidal suspensions are presented. The numerical calculation methods, including the acceptance ratio method, Wang-Landau-type method, and density functional theory method, are briefly reviewed. Several useful analytic approximations are presented. Stating from the depletion interaction between two flat plates, the Derjaguin approximation is introduced through the Asakura- Oosawa model. With this approximation, the approximate formulas of depletion interaction between two hard spheres, between a hard sphere and a hard wall, and between a hard sphere and curved hard walls in a small hard sphere colloid are derived. The depletion interaction between two hard spheres in a thin rod colloid and a thin disk colloid are also derived in the Derjaguin approximation.

Polyimide mixed matrix membranes for CO2 separations using carbon–silica nanocomposite fillers

Mixed matrix membranes (MMMs) have a potential to improve the separation performance of polymeric membranes while maintaining their advantages of easy processing and lower costs. In this work, series of MMMs were developed via solution casting by adding porous carbon–silica nanocomposite (CSM) fillers to a readily available Matrimid® membrane. CSMs were prepared by a hard template synthesis technique to get a tuneable porosity and surface chemistry which is controlled by the optimization of the filler porosity using carbon deposition, the pyrolysis conditions, and the maximization of polarity via oxygen functional groups. SEM images of the synthesized MMMs confirmed the good adhesion and dispersion of the fillers within the polymer matrix. The separation results demonstrate that the overall separation efficiency is increased by the addition of a carbon phase, providing an increased affinity for the CO2 gas molecules next to the creation of extra porosity and free volume. It was showed that significantly improved CO2 mixed gas selectivity and permeability for CO2:N2 and CO2:CH4 gas mixtures at 9bar and 308K was achieved. For gas mixtures with a 50:50 (CO2:N2) feed composition, a 2-fold and 6-fold increase of the mixed gas selectivity (up to 42.5) and permeability (up to 27 Barrer) compared to unfilled PI was achieved, respectively. The performance of the membranes was compared to the existing literature data.

DFT Study of Gold Surfaces–Ligand Interactions: Alkanethiols versus Halides

The variation in the surface reactivity of gold nanorods (GNR) is explored by density functional theory (DFT) simulations of the different facets. The physical and chemical properties of Au310 and Au520 surfaces, recently detected on the GNR tips, were compared with conventional Au100 and Au111 surfaces. Due to the tight gold packing, the sloped Au111 surfaces have the highest binding energy with rigid ligands, whereas the Au310 have the lowest. The observed correspondence between the ligand binding energies and the surface moduli of elasticity demonstrates that the overall monolayer structure, stability and reactivity is a balance between the ligand induced surface strain and the interligand strain. The GNR tips are able to distribute the ligand induced strain with minimal surface reorganization due to a high modulus of elasticity combined with extra free volume within the ligand shell. A simulation of the halide-surface interaction, consisting of multiple bond breaking and forming points, showed that the halides will induce the same surface strain but halides with reduced ligand-to-ligand interaction can move freely around the gold surface. More importantly, the charge transfer between halide to gold depends mainly on this mobility and is a factor in the role of halides as shape directing agents in gold nanoparticle synthesis. These simulations reveal some of the key factors that must be considered to effectively functionalize GNRs for specific applications.

Elevated pervaporation performance of polysiloxane membrane using channels and active sites of metal organic framework CuBTC

Hybrid membranes with polydimethyl siloxane (PDMS) as the polymer matrix and metal organic framework (MOF) CuBTC ([Cu3(BTC)2]) as the filler were fabricated and applied in pervaporative separation of thiophene from model gasoline. CuBTC submicroparticles with size of 0.2–0.3μm were synthesized through a facile method at room temperature. The morphology, crystal structure and pore parameters of CuBTC, together with the morphology, functional groups, thermal and mechanical property, free volume characteristics, sorption and separation performance of PDMS–CuBTC hybrid membranes were studied. Primary functions of CuBTC particles are summarized as follows: (i) the channels of CuBTC provide expressways with low resistance for the transport of permeates; (ii) the submicroparticles interfere with the packing of PDMS chains, generating extra free volume of the membrane; (iii) the active metal sites of CuBTC facilitate the transport of thiophene; and (iv) the channels of CuBTC sieve thiophene from the bulk of model gasoline. As a result, the normalized flux and selectivity of PDMS membranes are elevated simultaneously. The optimal separation performance is achieved when the weight ratio of normal CuBTC to PDMS reaches 8%, with a permeation flux of 194.2kgμm/(m2h) and an enrichment factor of 5.2 (increase by 100% and 75% compared with PDMS control membrane, respectively).

Loading rate effect on the creep behavior of metallic glassy films and its correlation with the shear transformation zone

Abstract The room temperature creep behaviors of metallic glassy films with a series of loading rates were systematically investigated by a spherical-tip nanoindentation. The results show that the role of loading rate on the creep deformation is material-dependent and can be intrinsically related to the shear transformation zone (STZ). Smaller STZs facilitate the more dependency of creep on the loading rate and, interestingly, a rate independency was observed in Ni–Nb, whose STZ volume is particularly larger than those in other samples. The deformation mechanism of the creep flow is discussed in the light of STZs combined with extra free volume, providing detailed explanation for the creep diversity on the loading rate in different metallic glass systems.

Epoxy Resin/Carbon Black Composites Below the Percolation Threshold

A set of epoxy resin composites filled with 0.25-2.0 wt.% of commercially available ENSACO carbon black (CB) of high and low surface area (CBH and CBL respectively) has been produced. The results of broadband dielectric spectroscopy of manufactured CB/epoxy below the percolation threshold in broad temperature (200 K to 450 K) and frequency (20 Hz to 1 MHz) ranges are reported. The dielectric properties of composites below the percolation threshold are mostly determined by alpha relaxation in pure polymer matrix. The glass transition temperature for CB/epoxy decreases in comparison with neat epoxy resin due to the extra free volume at the polymer-filler interface. At room temperature, the dielectric permittivity is higher for epoxy loaded with CBH additives. In contrast, at high temperature, the electrical conductivity was found to be higher for composites with CBL embedded. The established influence of the CB surface area on the broadband dielectric characteristics can be exploited for the production of effective low-cost antistatic paints and coatings working at different temperatures.

Liquid permeation through PTMSP: One polymer for two different membrane applications

The extra high free volume glassy polymer poly[1-(trimethylsilyl)-1-propyne] (PTMSP) is a perspective membrane material for two inherently different applications: (i) organic solvent nanofiltration (OSN), when a high solvent flux through the membrane is expected, and (ii) high pressure membrane contactors for CO2 capture that requires the absence of liquid permeation (no liquid leakage) through the membrane. The transport of wetting/non-wetting liquids through PTMSP was studied. Filtration experiments of binary water–ethanol mixtures revealed that no liquid permeation through PTMSP took place at trans-membrane pressures up to 30bar if the alcohol content is 40wt% or lower. The further increase of the ethanol concentration (50wt% and higher) provides the hydrodynamic flux through PTMSP; its value increases with increasing ethanol content. No single transport model available in the literature can satisfactorily describe the water–ethanol permeation through PTMSP. In this work, a new semi-empirical model was developed and it was successfully applied for describing the experimental data on liquids permeation through PTMSP dense membranes at pressure differences up to 150bar. The model takes into account the fractional free volume of polymer, swelling degree of the polymer in solvent environment, membrane compaction under elevated pressure difference across the membrane and physical–chemical macroscopic parameters of the liquid and polymer such as surface tension, viscosity and molar volume. The sorption and swelling degree of PTMSP were measured for the water–ethanol mixtures under study. The sorption and swelling data and liquid permeation results for binary ethanol–water mixtures revealed that, in spite of remarkable content of the liquid in the free volume elements of PTMSP (i.e. sorption is up to 0.42g/g and swelling degree is up to 0.12), there is no liquid flux through PTMSP at pressure differences of 10–30bar. Thus, the threshold values of the PTMSP swelling degree was estimated on the level of 0.12–0.19. It was shown that PTMSP assures zero liquid flux (no liquid leakage) for the PTMSP/liquid system the corresponding parameters of which are below the threshold region (important for high pressure membrane contactor applications). When sorption and swelling for a PTMSP/liquid system are above the threshold levels, the PTMSP membrane exhibits a certain hydrodynamic flux for the corresponding liquid (important for OSN applications).

Broadband dielectric/electric properties of epoxy thin films filled with multiwalled carbon nanotubes

Many attempts have been made to fully explore flexibility, resistance to corrosion, and processing advantage of epoxy resin filled with carbon nanotubes (CNTs) as conductive filler, although sometimes with a certain degradation of polymers’ intrinsic properties. It is important to move the percolation threshold into the region of smaller CNTs’ concentration. The results of a broadband dielectric investigation of multiwalled CNT (MWCNT)/epoxy resin composites in wide temperature range from room temperature to 450 K were analyzed for percolation. Far below the percolation threshold (0.25 wt. % MWCNT) the dielectric properties of the composite are mostly determined by alpha relaxation in pure polymer matrix and the freezing temperature decreases due to the extra free volume at the polymer–filler interface. Close to the percolation threshold, the composite shows the negative temperature coefficient effect in the temperature region, where the pure polymer matrix becomes conductive. The activation energy of DC conductivity increases with the MWCNT concentration far below the percolation threshold and decreases close to it (1.5 wt. % MWCNT). The dielectric analysis of the MWCNT/epoxy resin reveals a significant influence of the polymer matrix on the temperature dependence of composite dielectric properties.

Effect of sand blasting on structural, thermal, and mechanical properties of Zr58.3Cu18.8Al14.6Ni8.3 bulk metallic glass

Abstract In the present work sand-blasting-induced structural, thermal, and mechanical effects were studied in Zr58.3Cu18.8Al14.6Ni8.3 bulk metallic glass. It was observed that sand blasting favors disordering of the atomic configuration, formation of free volume and evolution of multiple intersecting shear bands. As a result, considerable compressive plasticity was achieved in sand-blasted samples. It was also observed that in order to superimpose the effect of pre-straining and extra free volume for improving compressive plasticity, there exists a limit to both parameters.