Free Volume Holes Research Articles

Effect of adding feldspar on free volume properties of crosslinked polyester studied by positron annihilation lifetime spectroscopy

AbstractPositron annihilation lifetime spectroscopy (PAL) was applied to study the feldspar effect on the free volume properties of crosslinked polyester based on neopentyl glycol, succinic acid, phthalic anhydride, and maleic anhydride. The measurements have been carried on the polyester resin samples cured with three crosslinking agents namely styrene (SS) or styrene/methyl methacrylate (SM) or styrene/acrylonitrile (SA) comonomers mixtures in the ratios of 2 : 1 and loaded with different concentrations of feldspar in the range from 0 to 80 wt %. The free volume parameters (size and fractions of holes) depend on type of the crosslinking agent as well as the feldspar content added to the polyester. The results are supported by a significant variation in the nanoscale free volume hole size distributions. Moreover, the correlation between positron annihilation parameters and electrical parameters was discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Correlation Study between Free-Volume Holes and Molecular Separations of Composite Membranes for Reverse Osmosis Processes by Means of Variable-Energy Positron Annihilation Techniques

The nanoscopic structure of three kinds of commercially available composite membranes (LF10, NTR729HF, and NTR7250) for water purification has been studied by energy-variable positron annihilation. The membranes consist of a functional polymer film and porous polysulfone substrate supported on a nonwoven polyester fabric. Positron annihilation γ-ray and positron lifetime techniques were employed to probe the layer structure of the composite membranes and subnanometer-sized free-volume holes in the functional films, respectively. It was found that the functional film on the porous substrate in LF10 contains two different layers on the substrate, while that in NTR729HF and NTR7250 consists of a single top layer. Comparison of the rejections of different uncharged solutes, determined by a pressure-driven experiment, with the hole size of the functional film, obtained from the positron lifetime data, revealed that the holes probed by positrons can well explain the hindering effect of molecular transport. More...

Free volume holes diffusion to describe physical aging in poly(mehtyl methacrylate)/silica nanocomposites

The spontaneous thermodynamically driven densification, the so-called physical aging, of glassy poly(mehtyl methacrylate) (PMMA) and its nanocomposites with silica has been described by means of the free volume holes diffusion model. This mechanism is able to account for the partial decoupling between physical aging and segmental dynamics of PMMA in nancomposites. The former has been found to be accelerated in PMMA/silica nanocomposites in comparison to "bulk" PMMA, whereas no difference between the segmental dynamics of bulk PMMA and that of the same polymer in nanocomposites has been observed. Thus, the rate of physical aging also depends on the amount of interface polymer/nanoparticles, where free volume holes disappear after diffusing through the polymer matrix. The free volume holes diffusion model is able to nicely capture the phenomenology of the physical aging process with a structure dependent diffusion coefficient.

The Spin Probe Dynamics and the Free Volume in a Series of Amorphous Polymer Glass‐Formers

AbstractWe report the results of a combined study of the local structure and the reorientation dynamics in a series of five amorphous polymers of different fragility: cis‐trans‐1,4‐poly(butadiene) (c‐t‐1,4‐PBD), cis‐1,4‐poly(isoprene) (cis‐1,4‐PIP), poly(isobutylene) (PIB), poly(vinyl methylether)(PVME) and poly (propylene glycol) (PPG) by using two different probe methods. The reorientation dynamics of the molecular spin probe 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) from electron spin resonance (ESR) is related to the annihilation behaviour of the atomic ortho‐positronium (o‐Ps) one as obtained by positron annihilation lifetime spectroscopy (PALS). It was found that a slow to fast transition in the spin probe rotation mobility at the operationally defined spectral temperature parameter, T50G, is connected with the mean o‐Ps lifetime, τ3 (T50G) = (2.04 ± 0.26) ns. Consequently, using the free‐volume concept of the o‐Ps annihilation in terms of a quantum‐mechanical model of o‐Ps lifetime this transition can be connected with the occurrence of the mean free volume hole, Vh (T50G) = (102 ± 17) Å3, nearly independent of the chemical composition and the basic structural relaxation parameters of the amorphous polymers investigated. Finally, the free volume hole distribution aspect of the slow to fast transition indicates the presence of a sufficient free volume fluctuation at T50G for both typical fragile PVME and strong PIB polymer and emphasizes the essential role of free volume in the spin probe dynamics.

Testing of the structure of macromolecular polymer films containing solid active pharmaceutical ingredient (API) particles

The aim of the present study was to investigate the structure of free films of Eudragit ® L 30D-55 containing different concentrations (0%, 1% or 5%) of diclofenac sodium by positron annihilation spectroscopy. The data revealed that the size of the free-volume holes and the lifetimes of ortho-positronium atoms decreased with increase of the API concentration. Films containing 5% of the API exhibited a different behavior during storage (17 °C, 65% relative humidity (RH)) in consequence of the uptake of water from the air.

Correlation between Free-Volume Properties and Pervaporative Flux of Polyurethane−Zeolite Composites on Organic Solvent Mixtures

Micron-sized zeolite particles were incorporated into a polyurethane (PU) matrix to prepare ethylbenzene-selective membranes. The resulting composite membranes were used in the pervaporation (PV) of ethylbenzene/styrene (EB/ST) mixtures. The sorption, diffusion, and PV permeation behaviors as a result of zeolite addition were elucidated. Zeolite is less chemically compatible with organic solvents than PU and the PU-zeolite composites, which exhibited suppressed solvent solubilities compared with pristine PU. However, these membranes favor EB transport by diffusion selectivity. The diffusivity and permeation flux increases in parallel with the enlarged radius of the free-volume hole size (R(4) increasing from 3.46 to 3.64 Å using positron annihilation lifetime spectroscopy analysis) by increasing the zeolite content from 0 to 23%. The enlarged free volume at a zeolite loading of 23% promoted pure solvent diffusivities by 10% higher than that of the unfilled film. During the PV operation on the EB/ST mixture, a significant diffusion-coupling was observed, and the permeant diffusion coefficients from the binary mixture exceeded the pure solvent diffusivity. The permeation flux was greatly improved (up to 0.72 kg/m(2)·h) by zeolite addition without any detrimental effect on the separation efficiency.

Free volume of poly(perfluorosulfonic acid)/SiO2 composite proton exchange membranes by 129Xe NMR

Poly(perfluorosulfonic acid)/silica (PFSA/SiO2) composites were investigated by 129Xe NMR spectroscopy and relaxometry. 129Xe chemical shift extrapolated to zero pressure was used for calculation of average free volume hole size. This quantity reaches a maximum at 2wt.% SiO2 that could be correlated to the performance of composites proton exchange membrane. 129Xe longitudinal magnetization relaxation revealed a bimodal distribution of the free volume that was explained by the presence of xenon atoms in the backbone and pendant–chain domains. Thus, the free volume is heterogeneous and depends on the content of SiO2. Implications of the free volume changes for the hydrogen crossover through PFSA/SiO2 membranes are also discussed.

Positron Annihilation Study on Nickel and Iron Nano-Particles in Natural Rubber Composites

Problem statement: The effect of Ni and Fe nano-particles as a filler on the free volume properties Of Natural Rubber (NR) was studied using Positron Annihilation Lifetime Spectroscopy (PALS). Approach: The PAL measurements revealed that the free volume properties are strongly affected by the amount and type of filler. Results: Particularly speaking, the free volume fraction dramatically decreased by increasing the filler content. Besides, the addition of nano-prticles created new positron trapping sites at filler-rubber interfaces. Furthermore, correlations were made between the free volume hole sizes (Vh) and each of the mechanical and electrical properties successively. A negative correlation was observed between Vh and hardness in the Ni-rubber composites while a positive counterpart was found in the case of Fe-rubber composites. Finally, the polarity of the fillers, being higher than that of the rubber itself, leads to an increase in electrical parameters and an inhibition of o-Ps formation. Conclusion: These results indicate that the investigated composites are considered to be insulating materials as their conductivity values are in the order of an insulator range.

Permeability of Polymer/Clay Nanocomposites

ABSTRACTBecause of their large interfacial area, the presence of nanoplatelets in the polymeric matrix decelerates the process of diffusion of gases through the material. The particles are impermeable barriers to the diffusing molecules, forcing them to follow complicated paths, increasing, thus, the diffusion length. The barrier properties of the nanocomposites depend on the properties of the polymeric matrix, the volume fraction of the nanoplatelets, their aspect ratio, their orientation, and their interactions with the matrix. The mobility of the molecules is hindered by the crystallinity but it is facilitated by the free volume within the material. The size and shape of the free volume holes in the polymer affect, thus, the rate of diffusion. Interactions between the nanoparticles and the matrix may lower the barrier properties because they may increase the free volume in the material. The estimation of free volume in the nanocomposite is important for the proper choice of components and the manufacturing of nanocomposite coatings with optimum barrier properties. Detailed information about the diffusion mechanisms at atomic and molecular levels can be obtained using the approach of free volume.

Microstructural studies of poly (perfluorosulfonic acid) membrane doped with silver nanoparticles using positron annihilation spectroscopy

The Nafion -117 (H+ form) was converted to Ag+ form by ion exchange. The silver nanoparticles were formed in the membrane by reduction of Ag+ (of Ag+ form membrane) using an ionic reducing agent, NaBH4, and a neutral reducing agent, formamide. The free volume hole sizes in the nanoparticle doped and undoped Nafion were measured by positron annihilation lifetime spectroscopy (PALS). The surface modifications in the membrane by incorporation of nanoparticles were investigated using slow positron beam. The changes in the bulk free volume of Nafion with presence of nanoparticles were found to be nominal. In contrast, the surface microstructure appears to have undergone significant change.

Physical aging of polystyrene/gold nanocomposites and its relation to the calorimetric Tg depression

The aim of this work is to study the effect of gold nanoparticles on the segmental dynamics, glass transition (Tg) and physical aging of polystyrene (PS). To do so, PS/gold nanocomposite samples containing 5 and 15 wt% of 60 nm spherical gold nanoparticles, surface-treated with thiolated-PS, were prepared. The segmental dynamics of PS, as measured by means of broadband dielectric spectroscopy (BDS), was found to be unchanged in the presence of gold nanoparticles. Conversely, the calorimetric Tg of PS was shown to decrease with increasing the amount of gold nanoparticles in the samples. Furthermore, by measuring the amount of recovered enthalpy of PS—by means of DSC—after annealing at temperatures below Tg for various aging times, the physical aging was shown to speed up with increasing the nanoparticles weight fraction, i.e. the amount of PS/gold interface in the hybrid material. Thus, the main conclusion of our work is that PS molecular mobility and the out-of-equilibrium dynamics are markedly decoupled in these nanocomposites. The significant effect of the amount of PS/gold interface on both the physical aging rate of PS and the depression of the calorimetric Tg in the presence of nanoparticles is quantitatively accounted for by a model based on the diffusion of free volume holes towards polymer interfaces, with a diffusion coefficient depending only on the molecular mobility.

Low energy positron annihilation study of composite reverse osmosis membranes

Positron annihilation with a slow positron beam was applied to the characterization of composite reverse osmosis membranes. The results, obtained at different positron incident energies, indicated that the membranes are asymmetric with respect to the pore structure, consisting of a thin top layer with little porosity and an underlying thick porous layer. A relationship between the longest positron lifetime near the membrane surface and the salt rejection rate was discussed in terms of the free-volume hole size for the thin top layer.

Phase Transitions in Polymers Containing Long Self-Assembled CH<sub>2</sub> Sequences in the Side Chain: A Positron Lifetime Study

Positron lifetime annihilation spectroscopy (PALS) was used to study the temperature- dependence of sub-nanometer size local free volumes in two atactic poly(n-octadecyl methacrylate)s (PODMA) with different molecular weight. These materials exhibit short range layered structure with a self-assembly and crystallization of side chains. From the ortho-positronium (o-Ps) lifetime the size of free volume holes and its distribution are calculated. At a temperature Tm=311 ± 5 K, the mean hole volume <vh> shows an abrupt and strong increase from 0.15 nm3 to 0.2 nm3, which comes from the melting of side-chains. The reverse effect, attributed to side chain crystallization, was observed during cooling the samples at slightly lower temperatures. The lifetime result of PODMA is compared with semifluorinated polyesters in which the side chain has an oxydecylperfluorodecyl structure (-O-(CH2)10-(CF2)9-CF3). Long chain polymers without side-chains such as polyethylene and short side-chain poly(n-alkyl methacrylate)s: poly(methyl methacrylate) (PMMA) and poly(n-hexyl methacrylate) (PHMA) are also compared with these polymers.

Preliminary Positron Lifetime Results on Free Volumes in Cyclodextrins

Positron annihilation lifetime spectroscopy was used to study the free-volume parameters in various pure -, - and -cyclodextrins samples and, in the case of β-cyclodextrin, with inclusion of S-carvone and thymoquinone. The results clearly indicate the presence of long lifetime components related to Ps-formation. The data show that the addition of S-carvone to β-cyclodextrin results in a decrease of o-Ps lifetime that we ascribe to a reduction of free volume holes from 81.8 to 63.7 Å3. The long lifetime component disappears when thymoquinone is added to -cyclodextrin, indicating this substance acts as an o-Ps quencher. For all samples studied, a decrease in the long lifetime component values was observed with increasing source in situ time, a result that might be attributed to the irradiation of the sample by the 22Na positron source.

Slow Positron Studies in Polymers

The diffusion trapping model has been applied to slow positron annihilation in He+ irradiated polystyrene and polystyrene – polystyrene bilayers. The S-parameter and the positron lifetime have been calculated as a function of the incident positron energy. The effect of the fluence upon the nature of the S-parameter curve has been discussed. It has been found that a change in fluence affects positronium formation. The transition rate for surface to positronium formation has been found to be dependent upon the fluence and the atomic number of the irradiated ion. The lifetime results show that, at low energy, the o-Ps annihilates mainly at the polymeric surface. The free volume hole concentration is found to decrease at low energy, and becomes constant at higher energies.

Free volume distribution at the Teflon AF®/silicon interfaces probed by a slow positron beam

We performed positron annihilation lifetime spectroscopy experiments at Teflon AF®/silicon interfaces as function of the positron implantation energy to determine the free volume hole size distribution in the interfacial region and to investigate the width of the interphase. While no interphase was detected in very short chained solvent-free, thermally evaporated Teflon AF®, an interphase of some tens of nm in extension was observed for high molecular weight spin-coated Teflon AF® films. Influences of the native oxide layer on the data evaluation could be ruled out.

Physical aging in PMMA/silica nanocomposites: Enthalpy and dielectric relaxation

We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided.

Free volume in imidazolium triflimide ([C3MIM][NTf2]) ionic liquid from positron lifetime: Amorphous, crystalline, and liquid states

Positron annihilation lifetime spectroscopy (PALS) is used to study the ionic liquid 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide [C(3)MIM][NTf(2)] in the temperature range between 150 and 320 K. The positron decay spectra are analyzed using the routine LifeTime-9.0 and the size distribution of local free volumes (subnanometer-size holes) is calculated. This distribution is in good agreement with Fürth's classical hole theory of liquids when taking into account Fürth's hole coalescence hypothesis. During cooling, the liquid sample remains in a supercooled, amorphous state and shows the glass transition in the ortho-positronium (o-Ps) lifetime at 187 K. The mean hole volume varies between 70 Å(3) at 150 K and 250 Å(3) at 265-300 K. From a comparison with the macroscopic volume, the hole density is estimated to be constant at 0.20×10(21) g(-1) corresponding to 0.30 nm(-3) at 265 K. The hole free volume fraction varies from 0.023 at 185 K to 0.073 at T(m)+12 K=265 K and can be estimated to be 0.17 at 430 K. It is shown that the viscosity follows perfectly the Cohen-Turnbull free volume theory when using the free volume determined here. The heating run clearly shows crystallization at 200 K by an abrupt decrease in the mean <τ(3)> and standard deviation σ(3) of the o-Ps lifetime distribution and an increase in the o-Ps intensity I(3). The parameters of the second lifetime component <τ(2)> and σ(2) behave parallel to the o-Ps parameters, which also shows the positron's (e(+)) response to structural changes. During melting at 253 K, all lifetime parameters recover to the initial values of the liquid. An abrupt decrease in I(3) is attributed to the solvation of e(-) and e(+) particles. Different possible interpretations of the o-Ps lifetime in the crystalline state are briefly discussed.

Enthalpy Recovery of PMMA/Silica Nanocomposites

In this work, we have studied the effect of silica particles on the physical aging of nanocomposites based on poly(methyl methacrylate) (PMMA). To do that, we have followed the enthalpy relaxation by means of differential scanning calorimetry (DSC). In agreement with previous results carried out by means of broadband dielectric spectroscopy (BDS), we observe an acceleration of the physical aging process of PMMA nanocomposites in comparison to bulk PMMA. The fitting of the enthalpy relaxation results to the well-known Tool−Narayanaswamy−Moynihan (TNM) model gives rise to equal structural parameters for all investigated samples, including bulk PMMA. This implies that the molecular mechanism for physical aging in PMMA is not affected by the presence of silica particles. The only parameter changing is the pre-exponential factor setting the time scale of physical aging. The values obtained are correlated to the area/volume ratio of silica particles in the polymer and thereby to the silica interparticle distance in the nanocomposites. This latter observation is an indication that the physical aging process is driven by the diffusion of free volume holes toward polymer interfaces, as already proposed in the past.

Investigations of (PVA)0.7(NaBr)0.3(H2SO4)xM solid acid polymer electrolyte using positron annihilation lifetime spectroscopy

The free-volumes of solid polymer electrolytes (SPE) were characterized using positron annihilation lifetime (PAL) spectroscopy, FTIR, and scanning electron microscope (SEM) techniques. The SPE based on poly(vinyl alcohol) (PVA) and sodium bromide (NaBr) complexed with sulfuric acid (SA) H2SO4 at different weight percent ratios were prepared using solution cast technique. The PAL results indicate that a higher SA content (more than 0.87 mol/L) in (PVA)0.7(NaBr)0.3 matrix increase the free-volume hole size from 58 A3 to 87 A3. The increase in the SPE free-volume with higher SA content was associated with a decrease in the SPE crystallinity. It is postulated that the incorporated SA interrupt polymeric chain packing and retard crystallization during electrolyte films formation. The FTIR spectral studies indicate that the SA content higher than 0.87 mol/L induces chemical modifications within the PVA, which results in chain scission. The PAL study shows that the chain scissions within the polymer matrix affect the free volume hole density (I3) and hence the microstructure. I3 was found to be decreased from ≃11 to ≃6 %, resulting in lower fractional free-volume holes in the SPE films. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2038–2044, 2010