Free Volume Behavior Research Articles

Free-volume analysis of the structural and dielectric properties of PMMA/TeO2 composites via positron annihilation lifetime spectroscopy

PMMA (Poly methyl methacrylate) composites have promising potential for applications in optics and dielectric energy storage. To investigate the effects of TeO2 contents in PMMA in this respect, we studied their composites on light absorption in the UVvisible range, structural analysis via X-ray diffraction, dielectric characteristics, and free volume behavior. We added up to approximately 7 % of the weight concentration of the TeO2 micropowder to the PMMA matrix for effective dispersion because of the limitations of ultrasonic agitation. The XRD data demonstrate a widening of the peaks with a filler content of up to 4.8 % and agglomeration at 7 %, indicating TeO2 particle contact. An increase in absorbance was observed in the UVvisible spectrum of the PMMA/TeO2 composites with increasing TeO2 content up to 4.8 %. The dielectric constants exhibit a positive correlation with higher quantities of TeO2, and the polarization mechanism is interpreted by interface polarization and free volume. The free volume measured by positron annihilation lifetime spectroscopy (PALS) increases to 4.8 % but decreases to 7 %. The effect of the free volume within the matrix on the polarization distribution of TeO2 influenced the dielectric constant and the loss factor, which increased to 4.8 % and then decreased to 7 % due to agglomeration.

Combinatory effects of nanocarbon- and radiation-induced networks on free-volume heterogeneity, nonisothermal crystallization, and electrorecovery

Electro shape-memory polymers (eSMPs) are highly regarded for their ability to undergo preordained transformations in response to electrical cues, a feature intrinsically linked to the prevailing electrical conduction pathways and macromolecular configurations. Alterations through the introduction of nanoparticles or the implementation of crosslinking techniques can produce synergistic or antagonistic shifts in the free volume and nonisothermal crystallization behavior. To date, such effects have remained largely unexplored. In this study, we developed gamma radiation-processed eSMPs that exhibit superior electrorecovery capabilities (approximately 100% recovery at 7.5 V) intrinsically connected to the specific radiation dose administered. Furthermore, we examined the changes in the free volume, extensional viscosity, rheology, and nonisothermal crystallization of these electroconductive composites. Our findings highlight the necessity of a critical radiation dose for the induction of electroshape recovery, and evidence suggests that the radiation dose substantially reduces the crystallization temperature under nonisothermal conditions while simultaneously broadening the crystallization process, especially at high cooling rates. Analyses of extensional stress further underscored the critical role of radiation dose. Positron annihilation lifetime spectroscopy (PALS) studies revealed that both the NCB and radiation dose contribute to an overall reduction in the free-volume size.

Free-Volume analysis of PAMAM dendrimers by means of positron annihilation lifetime spectroscopy

Polymeric structures known as dendrimers branch around a nucleus and contain functional groups that can affix surface groups to their ends. The inherent structural free volume of these materials makes them attractive for various important applications in the fields of biotechnology and nanotechnology. The purpose of this research was to examine the free volume behavior of ester- and acid-terminated poly(amidoamine) (PAMAM) dendrimers cored with tris(2-aminoethyl)amine (TREN) for the first three generations (0.5, 1.5, and 2.5 generations). Positron annihilation life spectroscopy (PALS) was used to analyze and discuss the size and fractions of hole-free volume generated by thermal effects in the temperature range of 10 °C to 80 °C. In addition, these PALS results may be related to the entanglement and deformability of the generation-dependent architecture and the influence of the surface groups.

Interfacial-interaction induced modifications in segmental dynamics and free volume of the poly(ethyleneimine) canopy in nanoparticle-organic hybrid materials: an investigation by temperature dependent broadband dielectric and positron annihilation spectroscopy.

Broadband dielectric spectroscopy, positron annihilation lifetime spectroscopy and differential scanning calorimetry are used to investigate the molecular dynamics, free volume and thermal behaviour of a poly(ethyleneimine), PEI, canopy in liquid-like nanoparticle-organic hybrid materials (NOHMs) consisting of alumina nanoparticles and nanorods as inorganic nanocores. It is confirmed that the highly branched PEI canopy in liquid-like NOHMs possesses an ordered structure having less entanglements of the side chains as compared to neat PEI. The size of the free volumes associated with the PEI canopy for nanoparticle- and nanorod-based NOHMs is larger and smaller, respectively, as compared to neat PEI. The time scales characterizing the segmental dynamics and side chain relaxations of the canopy in the nanorod-based NOHM are slowed down drastically as compared to the nanoparticle-based NOHM and neat PEI. These results confirm that the shape of the inorganic cores plays a deterministic role toward the structural arrangement (free volume) and segmental dynamics of the canopy polymer in liquid-like NOHMs.

Controlling Fractional Free Volume, Transport, and Co-Transport of Alcohols and Carboxylate Salts in PEGDA Membranes.

Understanding multi-component transport through polymer membranes is critical for separation applications such as water purification, energy devices, etc. Specifically for CO2 reduction cells, where the CO2 reduction products (alcohols and carboxylate salts), crossover of these species is undesirable and improving the design of ion exchange membranes to prevent this behavior is needed. Previously, it was observed that acetate transport increased in copermeation with alcohols for cation exchange membranes consisting of poly(ethylene glycol) diacrylate (PEGDA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and that the inclusion of poly(ethylene glycol) methacrylate (PEGMA) (n = 5, n represents the number of ethylene oxide repeat units) could suppress this behavior. Here, we further investigate the role of PEGMA in modulating fractional free volume and transport behavior of alcohols and carboxylates. PEGDA-PEGMA membranes of varied membranes are fabricated with both varied pre -polymerization water content at constant PEGMA (n = 9) content and varied PEGMA content at two pre -polymerization water contents (20 and 60 wt.% water). Permeability to sodium acetate also decreases in these charge-neutral PEGDA-PEGMA membranes compared to PEGMA-free films. Therefore, incorporation of comonomers such as PEGMA with long side chains may provide a useful membrane chemistry structural motif for preventing undesirable carboxylate crossover in polymer membranes.

Reversible linear-compression behavior of free volume in a metallic glass

Free volume is one of the key structural concepts widely used to describe various glass phenomena. However, free volume is challenging to be rigorously defined in glass structures and directly determined by experiments. Here, we employed metallic glass as a simple glass model system and studied the structural evolution of an as-spun (hyperquenched) and another thermally annealed metallic glass with different amounts of free volume by in situ high-pressure synchrotron x-ray diffraction. Therefore, the effect of free volume on the compression behavior of metallic glasses can be clarified by comparing the two samples at identical experimental conditions. We find that a higher amount of free volume causes both lower density and bulk modulus as expected at ambient conditions. Surprisingly, during compression, the decrease of free volume has a constant rate up to $\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ and appears to be mostly reversible upon decompression without obvious permanent densification after pressure release. These results suggest an elastic process in metallic glasses under pressure. Implications of these results for the structural models based on the free-volume concept are discussed. The typically assumed glass structural model with isolated domains of ``liquidlike'' (free-volume rich) region embedded in a solidlike (free-volume poor) matrix seems unlikely. Instead, a more ``homogeneous'' model over a wide range of length scales such as a nested fractal network of liquid- and solidlike regions seems more consistent with the experimental observation.

Penetration of Hydrogen into Polymer Electrolyte Membrane for Fuel Cells by Quantum and Molecular Dynamics Simulations.

The advent of the Hydrogen Society created great interest around hydrogen-based energy a decade ago, with several types of vehicles based on hydrogen fuel cells already being produced in the automotive sector. For highly efficient fuel cell systems, the control of hydrogen inside a polymer-based electrolyte membrane is crucial. In this study, we investigated the molecular behavior of hydrogen inside a polymer-based proton-exchange membrane, using quantum and molecular dynamics simulations. In particular, this study focused on the structural difference of the pendent-like side chain polymer, resulting in the penetration ratio of hydrogen into the membrane deriving from the penetration depth of the membrane’s thickness while keeping the simulation time constant. The results reveal that the penetration ratio of the polymer with a shorter side chain was higher than that with the longer side chain. This was justified via two perspectives; electrostatic and van der Waals molecular interactions, and the structural difference of the polymers resulting in the free volume and different behavior of the side chain. In conclusion, we found that a longer side chain is more trembling and acts as an obstruction, dominating the penetration of hydrogen inside the polymer membrane.

Functionalization of fluorinated ionic liquids: A combined experimental-theoretical study

We present new experimental and modelling data concerning imidazolium based-FILs synthesized with a hydroxyl group in the end of the cationic hydrogenated side chain and compared them with the analogous non-functionalized FILs in order to verify their suitability in the biomedical field. The thermophysical and thermodynamic properties of the neat compounds and the self-aggregation behaviour of FILs in aqueous solutions were measured and compared with theoretical results from the soft-SAFT equation of state, in good agreement with each other. Results showed that the presence of the hydroxyl group increases the density and viscosity of pure compounds and aqueous mixtures, whereas the thermal stability, melting, free volume, ionicity and self-aggregation behaviour decrease. These properties are improved with respect to the conventional perfluorosurfactants for the desired application, due to the full miscibility in water and the promising improved biocompatibility.

Systematic estimation and interpretation of fractional free volume in 1-alkyl-3-methylimidazolium-based ionic liquids

With the aim of providing a plausible explanation for fractional free volume (FFV) behavior in ionic liquids (ILs) such as 1-alkyl-3-methylimidazolium-based cations paired with common anions like [BF4]−, [PF6]−, and bis(trifluoromethane)sulfonamide ([NTf2]−), we comprehensively estimated the van der Waals volume of the ions and subsequently determined the FFV behavior present within these liquids with the aid of quantum chemical calculations. Unlike the values seen in previous studies, our results showed an increase in the FFV character of the IL under investigation when either the cation's alkyl chain was lengthened and/or the size of the anion was expanded. The sizes of cations and anions present in the IL were shown to affect the FFV values in different ways: whereas the effect of the anion's size could be accounted for by looking at changes in the Coulomb interaction between cations and anions in the polar (charge localized) domains, the effect exerted by the alkyl chain was found to be related to the alkyl chain's dependence on the FFV obtained for alkanes due to the introduction of non-polar (alkyl group aggregated) domains to ILs with increasing alkyl chain lengths. An empirical equation was proposed in order to calculate FFV values for alkyl-imidazolium-based ILs.

Slip avalanche in nanoscratching of metallic glasses

Slip avalanches, similar to discrete earthquake events, of Zr-, Co-, and Ce-based metallic glasses during nanoscratching were investigated. Differing from the conventional continuum approach, mean-field theory, which is an inherently-discrete model, was applied to analytically compute intermittent slip avalanches. Mean-field theory was first connected with the potential energy barrier and concentration of free volume in order to study the stick-slip behavior. The results suggest that the motion behavior of free volume affects the critical slip avalanche size.

Foams Made of Engineered Recombinant Spider Silk Proteins as 3D Scaffolds for Cell Growth

Materials for tissue engineering have to be biocompatible and have to support cell adhesion, proliferation and differentiation. Additionally, in case of soft tissue engineering the mechanical properties have to accommodate that of the tissue with mechanical integrity until the artificial scaffold is replaced by natural extracellular matrix. In case of artificial 3D scaffolds, it is of critical importance to be able to tune the mechanical properties, the inner free volume (i.e., pore size) and degradation behavior of the employed biomaterial. Here, the potential of recombinant spider silk proteins was evaluated concerning their processing into and application as 3D scaffolds for soft tissue engineering. Highly porous foams made of the recombinant spider silk protein eADF4(C16) and a variant containing an RGD motif were fabricated by salt leaching yielding mechanically robust scaffolds. In contrast to other salt-leached silk scaffolds, the swelling behavior of these scaffolds was low, and the mechanical properties in the range of soft tissues. The pore size and porosity of the foams could be adjusted by the salt crystal size. Fibroblasts adhered and proliferated well in foams made of the spider silk RGD variant but not in the foams of the nonmodified one.

Free volume and damping in a miscible high performance polymer blend: Positron annihilation lifetime and dynamic mechanical thermal analysis studies

ABSTRACTHigh performance polymer blend of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) was examined for their free volume behavior at different compositions of PEI using positron annihilation lifetime spectroscopy (PALS). The damping property of the blend was studied using tan‐δ obtained from dynamic mechanical thermal analysis (DMTA). The dependence of tan‐δ on temperature revealed that the blend is miscible in all compositions, in agreement with earlier studies. The tan‐δ peak height is found to increase with increase in free volume fraction for the entire blend composition signifying that the free volume plays an imperative role in understanding the damping property. Using DMTA, master curves were obtained at a reference temperature T0 by applying the time‐temperature‐superposition (TTS) using Williams‐Landel‐Ferry (WLF) relationship. From the shift factor aT, the WLF coefficients and were evaluated, using which the free volume fraction was found. Both PALS and DMTA methods were found to give similar results for the dependence of free volume for various PEI contents studied in this blend. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42961.

Free volume study on the miscibility of PEEK/PEI blend using positron annihilation and dynamic mechanical thermal analysis

High performance polymer blend of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) was examined for their free volume behaviour using positron annihilation lifetime spectroscopy and dynamic mechanical thermal analysis methods. The fractional free volume obtained from PALS shows a negative deviation from linear additivity rule implying good miscibility between PEEK and PEI. The dynamic modulus and loss tangent were obtained for the blends at three different frequencies 1, 10 and 100 Hz at temperatures close to and above their glass transition temperature. Applying Time-Temperature-Superposition (TTS) principle to the DMTA results, master curves were obtained at a reference temperature To and the WLF coefficients c01 and c02 were evaluated. Both the methods give similar results for the dependence of fractional free volume on PEI content in this blend. The results reveal that free volume plays an important role in determining the visco-elastic properties in miscible polymer blends.

The effect of polymeric excipients on the physical properties and performance of amorphous dispersions: Part I, free volume and glass transition.

To investigate the structural effect of polymeric excipients on the behavior of free volume of drug-polymer dispersions in relation to glass transition. Two drugs (indomethacin and ketoconazole) were selected to prepare amorphous dispersions with PVP, PVPVA, HPC, and HPMCAS through spray drying. The physical attributes of the dispersions were characterized using SEM and PXRD. The free volume (hole-size) of the dispersions along with drugs and polymers was measured using positron annihilation lifetime spectroscopy (PALS). Their glass transition temperatures (Tgs) were determined using DSC and DMA. FTIR spectra were recorded to identify hydrogen bonding in the dispersions. The chain structural difference-flexible (PVP and PVPVA) vs. inflexible (HPC and HPMCAS)-significantly impacts the free volume and Tgs of the dispersions as well as their deviation from ideality. Relative to Tg, free volume seems to be a better measure of hydrogen bonding interaction for the dispersions of PVP, HPC, and HPMCAS. The free volume of polymers and their dispersions in general appears to be related to their conformations in solution. Both the backbone chain rigidity of polymers as well as drug-polymer interaction can impact the free volume and glass transition behaviors of the dispersions.

Positron annihilation study of free volume in electron irradiated Zr50Cu40Al10 bulk glassy alloy: Effects of thermal relaxation before irradiation

Change in free volume reflects various properties such as hardness and ductility of glassy alloys. The electron- and ion- irradiations affect the free volume of ZrCuAl bulk glassy alloys that is associated with the mechanical properties. In this study, as-quenched Zr50Cu40Al10 bulk glassy alloys and structural relaxed one by annealing for 5 hours at 673K below glass transition temperature (Tg) were irradiated by 8 MeV electrons with a maximum fluence of about 2×1018 e−/cm2 at room temperature. The behaviour of free volume in these samples was investigated by positron annihilation techniques. X-ray diffraction measurements showed that no crystallization occurred after the irradiation. We observed the positron lifetime increased by the irradiation for both as-quenched and structural relaxed samples. The increases in positron lifetime at 2×1018 e−/cm2 were 9 psec for as-quenched and 12 psec for relaxed sample, respectively. In addition, the increase in positron lifetime with irradiation fluence was clearly different for the two types of sample. These facts imply that the thermal relaxation before irradiation of Zr50Cu40Al10 bulk glassy alloy influences the radiation effects, especially the free volume change.

Conformational analysis of random copolymers and influence of their compositions on physical properties

Considering four aromatic copolymers as an example, it was shown that the Kuhn segment of those random copolymers calculated by Monte Carlo method had a nonlinear dependence on the copolymer’s composition. It was also shown that the behavior of glass transition temperature, free volume, and dielectric permittivity versus conformational rigidity was similar to the behavior of aromatic homopolymers.

Montmorillonite nanoclay filler effects on electrical conductivity, thermal and mechanical properties of epoxy‐based nanocomposites

AbstractEpoxy‐based nanocomposites with 2, 5, and 7 wt% of montmorillonite (MMT) nanoclay were prepared using high shear melt mixing technique. The microstructural features of the nanocomposites were investigated by transmission electron microscopy (TEM). The thermal and mechanical properties were measured using differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), and dynamic mechanical analyzer (DMA). Further, the effect of voltage, temperature, seawater aging on the electrical conductivity (σDC) of the nanocomposites was also measured. To understand the free volume behavior upon filler loading, and to observe the connectivity between microstructure and other properties, positron annihilation lifetime spectroscopy was used. The TEM results revealed that MMT nanoparticles were uniformly dispersed in the epoxy matrix. Experimental results showed that the inclusion of 2 wt% MMT nanofiller increased the Tg, electrical conductivity, thermal stability, modulus, free volume of the epoxy nanocomposite significantly. This is well explained from the results of Tg (DSC and DMA), thermal stability, TGA residue, free volume analysis, and electrical conductivity. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

Subnanometre size free volumes in amorphous Verapamil hydrochloride: A positron lifetime and PVT study through Tg in comparison with dielectric relaxation spectroscopy

Positron annihilation lifetime spectroscopy (PALS), a method well established for the study of polymers, is employed to characterize the temperature dependence of the free volume through T g in the amorphous pharmaceutical Verapamil hydrochloride. From the PALS spectra analyzed with the routine LifeTime9.0 the size (volume) distribution of local free volumes (subnanometre-size holes), its mean, 〈 v h 〉 , and mean dispersion, σ h, were calculated. A comparison with the macroscopic volume from PVT-experiments delivered the hole density and the hole free volume fraction and in that way a complete characterization of the free volume microstructure. These data are used in correlation with structural ( α-) relaxation data from broad-band dielectric spectroscopy in terms of the Cohen–Turnbull free volume model. An extension of this model, distinctions in the free volume behaviour of the glassy and supercooled-liquid state and different ways of extrapolating the equilibrium part of the free volume into the temperature range of the glass are discussed. The potential of the PALS method for the study of pharmaceuticals is briefly reviewed and some recently developed applications (analysis of density fluctuations) are illuminated.

Behavior of free volume in ZrCuAl bulk metallic glass after irradiation

Free volumes in Zr50Cu40Al10 bulk metallic glasses irradiated by 100 MeV and 200 MeV Xe ions and 2 MeV electrons at room temperature have been investigated by using positron annihilation techniques, the micro Vickers hardness and X-ray diffraction measurements. No crystallization took place due to both ion and electron irradiations. The coincidence Doppler broadening spectra of annihilation gamma rays also remained unchanged by ion and electron irradiation. Meanwhile, an increase and a decrease of positron lifetimes after electron and ion irradiation, respectively, were observed. This suggests the structural modification of free volumes. The response of positron lifetime to ion and electron irradiations may be connected with the tendency of hardness.

Free volume and crystallinity of poly(ethylene naphthalate) treated in pressurized carbon dioxide

The free volume and crystallization behaviour of poly(ethylene naphthalate) (PEN) treated in pressurized carbon dioxide was studied, using Positron Annihilation Lifetime Spectroscopy (PALS) and Differential Scanning Calorimetry (DSC). PALS probes the values of free volume cavity sizes in materials, thus making it possible to investigate the effect of pressurized carbon dioxide treatment on free volume hole sizes in PEN. The crystallinity and melting behaviour of PEN was analyzed with temperature modulated DSC. We found that the carbon dioxide pressure during treatment of PEN was the prime parameter affecting the value of free volume after the treatment. The free volume sizes were observed to be insensitive to the other two parameters, temperature and time. Increasing the time of the treatment however, increased the crystallinity of PEN. Interestingly, this was not coupled to a decrease in ortho-Positronium intensity as was expected, indicating that positronium may form in the crystalline fraction of the polymer as well as in the amorphous fraction.