Positron Annihilation Lifetime Spectroscopy Data Research Articles

Micropore Characterization of Mesocellular Foam and Hybrid Organic Functional Mesocellular Foam Materials

The micro- and mesoporous structures of microemulsion templated mesocellular foam (MCF) silica materials aged at different temperatures were investigated by using nitrogen sorption αs-comparison plots and positron annihilation lifetime spectroscopy (PALS). The impact of surface amino functionalization with (3-aminopropyl)triethoxysilane (APTES) on these structures was also studied. The nitrogen sorption αs-comparison plots show that the micropore volume of the MCF silica can be controlled by varying the synthesis aging temperature such that the higher the aging temperature, the smaller the micropore volume. The corresponding PALS data showed that this effect of temperature on micropore volume is a direct consequence of a temperature-dependent micropore population, with more micropores in the MCF material aged at lower temperature. After functionalization of the silica MCF materials with APTES, the BET surface area was seen to have reduced substantially, while maintaining the characteristic mesopore sizes....

Temperature dependence of free volume in atactic polypropylene

Positron annihilation lifetime spectroscopy data are often used to get information on the typical sizes of sub nanometric holes forming the free volume in polymers. To this purpose, the cavities are modelled as spheres or, more generally, using geometries which assume an isotropic expansion with the temperature. However, this guess could be unrealistic owing to the irregular shape of holes and the constrained movements of the macromolecules. In this work positron and dilatometric data are used to estimate the free volume fraction in atactic polypropylene. Comparison with the prediction of the Simha–Somcynsky theory supplies information on the thermal dependence of the volume of holes, whose behaviour can be interpreted in terms of anisotropic expansion.

Diffusion of water in nanoporous NF polyamide membrane

Diffusion of water sorbed in a nanofiltration (NF) polyamide membrane as studied by quasielastic neutron scattering (QENS) is reported here. The trimesoyl chloride–piperazine based NF membrane was synthesized by interfacial polymerization technique and was characterized by positron annihilation lifetime spectroscopy (PALS) and SEM techniques. PALS data shows that the membrane has an average pore size ∼4.6 Å. QENS data from water sorbed NF membrane show that the diffusion in the sorbed water occurs through jump diffusion with the jump lengths distributed randomly. Translational diffusion coefficient obtained for water sorbed in the NF membrane is found to be smaller than that of bulk water.

Cooperative rearranging region size and free volume in As–Se glasses

Glasses of the As–Se system have been used as model objects of the covalentdisordered inorganic polymers to investigate the correlation between the cooperativerearranging region (CRR) size determined at the glass transition temperature andthe free volume fraction in the glassy state. The CRR size has been determinedusing temperature modulated differential scanning calorimetry data according toDonth’s approach, while the free volume fraction in the investigated materialshas been estimated using positron annihilation lifetime spectroscopy data. Theobtained results testify that the appearance of open-volume defects greater than80 Å3 leads to a significant decrease in the CRR size.

Positronium as a probe in natural polymers: decomposition in starch

Ortho-positronium (o-Ps) is used as a probe in positron annihilation lifetime spectroscopy (PALS) experiments, to characterise the behaviour of free volumes in natural starch samples, as a function of temperature (T). Up to about 540 K, the o-Ps intensity, I(3), remains constant at 26.2% while its lifetime, tau(3), is found to increase linearly. Both parameters undergo a decrease above this T, due to the onset of decomposition, which results in a shrinking of the sample pellets. The results indicate that the glass transition temperature should be above 501 K. Data from thermal gravimetry analysis (TGA) measurements are well described by supposing a first order process for the survival probability (p) of the starch lattice, with an activation energy, E(act) = (1.52 +/- 0.05) eV, and a frequency factor, ln(k(0), s(-1)) = 25.3 +/- 0.4. In the decomposition region, the PALS data show the unexpected correlation (tau(3n))(3) = I(3n), linking the normalised values of tau(3), tau(3n), and of I(3), I(3n). This is explained by considering that the changes in I(3) with T arise from those in the surviving volume fraction of the lattice, p, whereas the changes in tau(3) reflect the shrinking of the radius of the free volumes, the latter decreasing in proportion to p(1/3). Quantitative approaches on these bases lead to satisfactory fitting of all PALS data, yielding an activation energy, E(act) = (1.53 +/- 0.03) eV, and frequency factor, ln(k(0), s(-1)) = 25.4 +/- 0.2, in excellent agreement with the values derived from TGA.

Temperature dependence of the free volume from positron lifetime experiments and its relation to structural dynamics: Phenylphthalein-dimethylether

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the free volume in the temperature range between 103 K and 393 K in phenylphthalein-dimethylether (PDE), a low-molecular-weight glass former. Using the routine LIFETIME9.0, the ortho-positronium (o-Ps) lifetime distribution was analyzed, and from this, the volume distribution gn(vh) of subnanometer-size holes was calculated. From a comparison of PALS and specific volume data, the number density and the volume fraction of holes were estimated. These free-volume data, as a function of temperature, were used to test the validity of the Cohen-Turnbull (CT) free-volume theory. It was found that the structural relaxation from dielectric spectroscopy can be described by the CT theory after introducing a corrected free volume (Vf-DeltaV), where DeltaV=0.014 cm3/g. The extended free-volume theory of Cohen and Grest can be fitted to the dielectric-relaxation and free-volume data, but the parameters of both fits are not consistent. PDE shows some peculiar features. The "knee" in the o -Ps lifetime expansion and crossover in temperature dependence of the frequency of the primary dielectric relaxation process occur at different temperatures. In addition, the change in the Vogel-Fulcher-Tammann parameters at TB/Tg=1.1 has no observable effect on the mean free volume vh (or Vf). The size of the smallest representative freely fluctuating subsystem, VSV estimated from the standard deviation sigmah of gn(vh), decreases from 4.1 nm3 to 2.6 nm3 when the temperature increases from T/Tg=1.0 to 1.15. Correspondingly, the length of dynamic heterogeneity, xi=VVS1/3, decreases from 1.6 nm to 1.4 nm. It is concluded that at T/Tg approximately 1.10=TB/Tg the system transforms from a heterogeneous to a homogeneous (true) liquid.

Advanced Algorithms for PALS Analysis of Self-Assembled Amphiphiles

Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self-assembly materials plays an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. We explore positron annihilation lifetime spectroscopy (PALS) as an alternative and/or complementary technique for this purpose, using the phytantriol/water system. While PALS is a well established technique for characterising many materials, the coexistence of aqueous and hydrophobic regions in a soft self-assembled amphiphile material poses a challenge to the analysis and interpretation of the results. In order to alleviate these difficulties we developed a computer program for general-purpose PALS data analysis called PAScual. Amongst the most salient features of this new code are the possibility to perform bounded fits and the option of using advanced algorithms to provide a more robust and unbiased fit: on the one hand, it incorporates a global nonlinear optimisation routine based on the Simulated Annealing algorithm and, on the other hand it gives information on the reliability of the results by means of a Markov Chain Monte-Carlo Bayesian Inference method. In this work we present the newly developed PALS data analysis techniques as well as the results for the phytantriol/water system, comparing them with additional data obtained from complementary techniques.

Temperature Dependence of the Free Volume in Amorphous Teflon AF1600 and AF2400: A Pressure−Volume−Temperature and Positron Lifetime Study

The microstructure of the free volume and its temperature dependence were studied for amorphous Teflon AFs using pressure−volume−temperature experiments (PVT) and positron annihilation lifetime spectroscopy (PALS). Two copolymers of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole units with a composition of 35:65 (AF 1600, Tg = 160 °C) and 13:87 mol parts (AF 2400, Tg = 240 °C) were investigated after drying the as-received materials. The results are compared with those for CYTOP (Tg = 105 °C) and the perfluoroelastomer PFE (Tg = −2 °C) as well as for conventional (non-fluorinated) polymers. The PVT data were fitted by the Tait equation and the Simha−Somcynsky (S−S) equation of state (eos). From the latter one the hole fraction h and the specific hole free volume, Vf = hV, were determined. From the PALS data published recently by some of the authors (Rudel et al. Macromolecules 2008, 41, 788) the free volume hole size distribution characterized by its mean, ⟨vh⟩, and standard dev...

The free volume in two untreated, pressure-densified, and CO2 gas exposed polymers from positron lifetime and pressure-volume-temperature experiments

AbstractChanges in the microstructure of the free volume and its temperature dependence in ethylene-norbornene copolymer and bisphenol-A polycarbonate due to densification under pressure and swelling with CO2 gas have been examined using positron annihilation lifetime spectroscopy (PALS) and pressurevolume- temperature (PVT) experiments. Employing the Simha-Somcynsky equation of state the specific hole free and occupied volumes were estimated. From the PALS spectra analyzed with the new routine LT9.0 the size distribution of subnanometre holes and its mean and mean dispersion were calculated. Above Tg, the dispersion mirrors the thermal fluctuations in the free volume. From comparison of PALS and PVT data the specific number of holes was estimated. It was found that the occupied volume has a constant and identical compressibility in the glassy and rubbery state. It shows no memory for the history of the glass and mirrors only the pressure and temperature during the measurement. The change in the total volume due to the pre-treatments of the polymers occurs exclusively in the hole free volume Vf and the relative change in Vf is one order of magnitude larger than in the total volume. PALS data show that the mean hole size and its dispersion in the glassy state is decreased due to densification and increased due to swelling. PVT data show that the volume changes are frozen in the polymer glasses and that, when heating the samples, the volume begins to recover at temperatures ca. 50 K in gas swollen and 20 K in densified polymers below Tg. The PALS data show a corresponding behaviour.

Testing ETE model, temperature dependences of PALS data

AbstractThree porous glasses produced by sol‐gel method were investigated using Positron annihilation lifetime spectroscopy (PALS) method. The theoretical curves calculated from the pick‐off annihilation model for cylindrical and spherical free volumes were compared to the results of PALS measurements done in the temperature range from 100 to 500 K. The o‐Ps mean lifetime values in the pores found in the experiment change with temperature in the glasses containing pores of R < 1.5 nm in the range up to 40 ns, while the model predicts these changes less then 10 ns. The meaningful discrepancy between experimental data and model predictions was found at low temperatures. The discrepancy diminishes with increasing pore sizes. In the glass containing the largest pores, R ∼ 2 nm, experimental values of o‐Ps lifetime agree well with model predictions. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Charged defects in chalcogenide vitreous semiconductors studied with combined Raman scattering and PALS methods

A combination of Raman scattering and positron annihilation lifetime spectroscopy (PALS) techniques to study charged defects in chalcogenide vitreous semiconductors (ChVSs) was applied for the first time in this study. In the case of Ge 15.8 As 21 S 63.2 glass, it is found that the main radiation-induced switching of heteropolar Ge–S bonds into heteropolar As–S ones, previously detected by IR fast Fourier transform spectroscopy, can also be identified by Raman spectroscopy in the depolarized configuration. Results obtained by Raman scattering are in good agreement with PALS data for the investigated glass composition.

Features of positronium migration in a solid containing nanopores, by an example of sorption and annihilation of positrons in vaterite

The features of migration of positrons in the bulk of vaterite (metastable modification of CaCO3) were studied using positron annihilation lifetime spectroscopy (PALS). Comparison of the lifetime of ortho-positronium determined from the PALS data, which corresponds to the time of its residence in the vaterite nanopores, with the absorption results (BET adsorption isotherm) allowed evaluation of the diffusion coefficient of positronium in the perfect sections of the vaterite crystal lattice, equal to 0.4 × 10−1 cm2 s−1.

Free volume in poly( n-alkyl methacrylate)s from positron lifetime and PVT experiments and its relation to the structural relaxation

Free volume data from positron annihilation lifetime spectroscopy (PALS) experiments are combined with a Simha–Somcynsky (S–S) equation of state analysis of pressure–volume–temperature (PVT) data to model free volume contributions to structural mobility in a series of poly( n-alkyl methacrylate)s. From the PALS data the glass transition temperature, T g, decreases (from 382 to 224 ± 5 K) and a given mean free volume is observed at lower temperatures as the side-chain length increases (going from methyl- to hexyl-). This is evidence of an internal plasticization whereby the side-chains reduce effective packing of molecules. By comparing PALS and PVT data, the hole number per mass unit, N h′, is calculated using different methods; this varies between 0.54 and 0.86 × 10 21 g −1. It is found that the extrapolated free volume becomes zero at a temperature T 0′ that is smaller than the Vogel temperature T 0 of the α-relaxation. The α-relaxation frequencies can be fitted by the free volume theory of Cohen and Turnbull, but only when the free volume V f is replaced by ( V f − Δ V) where Δ V( = E f( T 0 − T 0′), E f is the thermal expansivity of V f) varies between 0.060 and 0.027 ± 0.003 cm 3/g, decreasing with side-chain length, apart from poly( n-hexyl methacrylate) where Δ V increases to 0.043 ± 0.003 cm 3/g. One possible interpretation of this is that the α-relaxation only occurs when, due to statistical reasons, a group of m or more unoccupied S–S cells are located adjacent to one another. m is found to vary between 8 and 2 for poly(methyl methacrylate) and poly( n-butyl methacrylate), respectively. We found that no specific feature in the free volume expansion was consistently in coincidence with the dynamic crossover.

Positron/positronium annihilation as a probe for chemical environments of free volume holes in fluoropolymers

The sensitivity of the Doppler-broadening of positron annihilation radiation (DBAR) for the chemical environment of free volume holes was studied for fluorinated polymers. The free volume was characterized by positron annihilation lifetime spectroscopy (PALS). For this study, we prepared three series of copolymers of methacrylates with various semi-fluorinated methacrylates with increasing fractional number of fluorine atoms fF. Moreover some commercial fluoroelastomers and semicrystalline fluoropolymers were investigated. After the subtraction of a narrow para-positronium DBAR component, calculated on a simplified way using the PALS data, from the conventional Doppler broadened annihilation energy line, the remaining broad distribution shows no response to the size of free volume holes or the positronium formation probability but can be related to the nature and density of the chemical species in the vicinity of the holes. Element specific information of the annihilation site can be expressed in terms of an integral parameter, e.g. the normalised curve shape parameter Sbn of the broad DBAR component. We observed that Sbn does not show the expected exponential decrease but goes down linearly with fF in the range fF<0.45 and levels-off for fF≥0.5. This was explained assuming positronium localization and annihilation in two types of holes: holes, which have walls free of fluorine atoms and holes, which contain fluorine in their walls. A steeper, exponential-like decrease of Sbn with fF seem to occur for the fluoroelastomeric and semicrystalline fluoropolymers which contain not less than one F atom per two backbone carbons. In this, the case ortho-positronium localized at a hole will always find at least one fluorine atom in which neighbourhood it can annihilate.

On the correlation between void-species structure of vitreous arsenic selenide studied with X-ray diffraction and positron annihilation techniques

The structure of nanoscale void-species has been studied in glassy arsenic selenide g-As 2Se 3 by combining the first sharp diffraction peak in X-ray diffraction (FSDP-related XRD), treated within Elliott’s void-based model, and positron annihilation lifetime spectroscopy (PALS) within two-state positron trapping model. By suggesting that the same nanovoids are responsible for both FSDP and PALS data, the analytical correlation relationship between the FSDP position, Q 1, and nanovoid diameter, D, has been presented in the form of Q 1 = 2.3 · π/ D.

On the Thermal Expansion of Nanohole Free Volume in Perfluoropolyethers

To determine the free volume in polymers, positron annihilation lifetime spectroscopy data are transformed into nanohole volumes by modeling the cavities as spheres or, more generally, using geometries assuming an isotropic thermal expansion. However, this guess could be unrealistic owing to the irregular shape of nanoholes and constrained movements of the macromolecules. In this work, it is shown that a comparison of hole-lattice theory with positron and dilatometric data for a homologous series of perfluoropolyethers supplies information on the anisotropic expansion of nanoholes; the relation between volume and typical unconstrained size of the cavities can be expressed by a power law with noninteger exponents.

Microstructure of free volume in SMA copolymers II. Local free volume from PALS

The structure of the free volume and its temperature dependence between, at maximum 133 and 503K of copolymers of styrene with maleic-anhydride, SMA (0–35mol% MA), is studied by pressure–volume–temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this second part of the work, PALS data are reported from which the temperature dependence of the mean size and size distribution of local free volumes (subnanometer size holes) is analysed. The mean hole volume, νh, varies in PS between 80Å3 (133K) and 212Å3 (503K) and shows a systematic decrease with increasing MA content for a given temperature above Tg. The specific number of holes, Nh′, estimated from a comparison of PVT and PALS results, increases slightly with MA content from Nh′=(0.60±0.02)×1021g−1 to Nh′=(0.70±0.02)×1021g−1 which corresponds to Nh(Tg)=Nh′/Vg=0.62–0.82nm−3 (Vg is the material's specific volume at Tg) and 1/Nh(Tg)=1.6–1.2nm3 for the volume which contains one hole. The analysed size distributions of the holes above Tg follow the compressibility of the free volume as it is predicted by the theory of thermal volume fluctuation. We also comment on the connection between the hole size as measured by PALS and the size of a cluster of randomly distributed unoccupied cells as defined by the Simha–Somcynsky theory.

The Structure of the Free Volume in Poly(styrene‐co‐acrylonitrile) from Positron Lifetime and Pressure Volume Temperature (PVT) Experiments

AbstractSummary: The structure of the free volume and its temperature dependence between 25 and 190 °C of copolymers of styrene with acrylonitrile, SAN (0 to 50 mol‐% AN), is studied by pressure volume temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this second part of the work, PALS data are reported from which the temperature dependence of the mean size and size distribution of local free volumes (subnanometer size holes) is analysed. The mean hole volume, vh, varies in PS between 105 Å3 (27 °C) and 220 Å3 (190 °C), and shows a systematic decrease with increasing AN content, at Tg from 121 Å3 for PS to 102 (±2) Å3 for SAN50. The specific number of holes, $N_{\rm h} '$, estimated from a comparison of PVT and PALS results, is independent on the temperature and the composition of SAN, $N_{\rm h} '$ = (0.56 ± 0.02) × 1021 g−1 which corresponds to Nh(Tg) = $N_{\rm h} '$/V(Tg) = 0.59 nm−3 and 1/Nh(Tg) = 1.7 nm3 for the volume which contains one hole. The analysed size distributions of the holes above Tg follow the compressibility of the free volume as it is predicted by the theory of thermal volume fluctuation. Moreover, we showed that the bulk elasticity modulus follows mastercurves as a function of vh above and below Tg.Free volume hole radius distribution n(rh) in PS for three different temperatures.magnified imageFree volume hole radius distribution n(rh) in PS for three different temperatures.

Secondary precipitation in Al–Zn–Mg–(Ag) alloys

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after a preliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heat treatments giving a substantial benefit on the mechanical properties. In the present work, positron annihilation lifetime spectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondary ageing of Al–4wt.%Zn–3wt.%Mg– xAg, where x=0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterised by the substantially different evolutions that are observed. The results shed light on the interplay between the formation of coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates, which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherent precipitation that is obtained if the alloys are first aged to peak hardness. In the presence of Ag, on the contrary, the effects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted well before reaching peak hardness.

Ru-doped phosphazene membrane materials used for catalytic hydrogenation

Semicrystalline polydimethylphosphazene [−NP(CH 3) 2 − ] n (PDMP) and the catalyst Ru-doped PDMP, in both the virgin and hydrogenated forms, have been analysed using differential scanning calorimetry spectroscopy (DSC), 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction (XRD) and positron annihilation lifetime spectroscopy (PALS). These techniques in combination indicate that the catalyst Ru coordinates the polymer and alters the structural conformation of the polymeric chain. The PALS data show that this coordination induces structural ordering that results in a decrease in the size of the inter- and intra-chain spacing. This ordering is also evidenced by a decrease in the intensity of the 31P upfield resonance, an increase in T g and the appearance of a pre-melting endotherm.