Temperature dependence of the free volume in polytetrafluoroethylene studied by positron annihilation spectroscopy

The temperature dependence of the free volume holes in pure polytetrafluoroethylene (PTFE) and doped with 25% glass have been studied in the temperature range (30–250 °C) using positron annihilation lifetime spectroscopy. The data clearly revealed the glass transition temperatures for pure and doped PTFE are 130 and 110 °C, respectively. As the temperature increases, the free volume distribution becomes positioned at larger free volume hole size. A good correlation between the electrical conductivity and the o-Ps parameters was achieved.     

Study of the size and numerical concentration of the free volume of carbon filled HDPE composites by the positron annihilation method

In this paper, the size and numerical concentration of free volume of high density polyethylene/carbon black (HDPE/CB) composite were investigated by positron annihilation lifetime spectroscopy (PALS). The PALS were measured in two series of samples, one with various CB contents in the composites and the other with changing the temperature of HDPE/CB composite containing 25 phr CB. It was found that the important parameters of PALS show their fluctuation around the percolation threshold. The conductivity of HDPE/CB is controlled by CB contribution, and that can be reflected in o-Ps lifetime. The temperature dependence of positron lifetimes reveals that the existence of glass transition temperatures and the size of free volume holes increases when temperature increases above glass transition. The results observed from the second set of samples suggest that positive temperature coefficient is in some way related with free volume expansion. The experiment facts implied that the conductivity of HDPE/CB was related with not only the size of free volume holes but also the number of free volume holes. The Doppler-broadening of HDPE/CB was also investigated.     

Free volumes in liquid-crystalline main-chain polymer probed by positron annihilation

Free volumes in a copolymer consisting of p-hydroxybenzoic acid (HBA) and poly(ethylene terephthalate) (PET) were probed by positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 30 and 230°C. Above the glass transition temperature (ca. 60°C), the lifetime of ortho-positronium (o-Ps) annihilated in the free volumes and its annihilation probability were found to increase with increasing specimen temperature. These facts were attributed to the increase both in the size of the free volumes and in the concentration of such regions due to rearrangements of molecules. From the observed lifetime of o-Ps, it was found that the size of the free volumes increases from 0.05 nm³ to 0.1 nm³ in the temperature range between 30 and 230°C. At solid-mesophase transition temperature (150–220°C), the size of the free volumes monotonously increases, while the increase in the concentration of such regions saturates at 174°C. The clear relationship between the data obtained by the positron annihilation and those obtained by differential scanning calorimetry was confirmed. ©1995 John Wiley & Sons, Inc.     

Delayed formation and localisation of positronium in polymers at low temperatures

A theoretical model of the positron annihilation lifetime spectrum including the mechanisms of slow positronium (Ps) localisation and delayed Ps formation from a positron and a trapped electron was developed. The model was applied to two series of spectra for low-density polyethylene and high-density polyethylene (HDPE) collected at constant temperature (much below the glass temperature) as a function of measurement time. The Ps internal relaxation time and time of localisation of Ps in a free volume centre were determined. The results show that after long irradiation of the polymer a dominant fraction of positrons (unbound in Ps) annihilate from the trapped states. On the basis of parameters determined from the HDPE lifetime spectra, two S(t) curves (for sample in darkness and in light) were calculated. The predicted shapes of S(t) well agree with literature data obtained with the age–momentum correlation (AMOC) experiment. According to the new model the shapes of the para-Ps and the ortho-Ps (p-Ps) components are non-exponential. In spite of this, the multi-exponential decomposition of a polymer spectrum enables to determine correctly the value of the o-Ps lifetime, however the other parameters determined from the spectrum have no simple physical meaning.     

Influence of vinylidenefluoride on the physical ageing of poly(chlorotrifluoroethylene)

The influence of vinylidenefluoride (3% composition) on the physical ageing of poly(chlorotrifluoroethylene) at 70 °C has been investigated using the positron annihilation lifetime technique. Both the ortho-positronium (o-Ps) pick-off lifetime and its intensity show a systematic variation with ageing time. The o-Ps intensity exhibits non-exponential character which can be fitted with two additive exponential decay curves and the free volume is found to exhibit the Doolittle type of free volume relaxation. The relaxation times were evaluated from the structural relaxation function and the non-exponentiality parameter was estimated using the Kohlrausch-Williams-Watts (KWW) function, which indicates the deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seems to label the different kinetic units/irregularity in the chain backbone of PCTFE structure participating in the relaxation process. Physical ageing seems to yield close packing of polymer chains.     

Thermal variation of free-volumes size distribution in polypropylenes. Probed by positron annihilation lifetime technique

Positron annihilation lifetime measurement was applied to the study of free-volume properties in three kinds of polypropylene as a function of temperature in the range of 25–180°C at thermal equilibrium. Positron lifetime data for polypropylenes were analyzed with a Laplace inversion technique in order to obtain continuous positron annihilation lifetime (PAL) distributions. At each temperature, four distinct PAL distributions were recognized. The distribution of the longest lived component was associated with a pick-off annihilation of ortho-positronium (o-Ps) trapped in free-volume of amorphous region, which grew bigger as the temperature increased. The hole radius distributions of free-volumes were estimated from the results of o-Ps lifetime distributions. A detailed analysis showed a mean radius of free volumes was 0.34 nm at room temperature and that was 0.42 nm near the melting point for each specimen. The distributions of hole radii of free volumes were found to be broader after thermal treatments. The relaxation of free volumes was attributed to the thermal equilibrium and the evacuation of included molecules in free volumes. © 1995 John Wiley & Sons, Inc.     

A positron annihilation lifetime study of isotactic polypropylene

Positron lifetime studies were performed on well-characterized annealed and quenched samples of isotactic polypropylene. The positron experiments were conducted from ?20 to 110°C as a function of both heating and cooling. Of the three decaying exponential components resolved from the lifetime spectra, only the long-lifetime ortho-positronium (o-Ps) pickoff component was affected by the changes in temperature. The behavior of both the lifetime and intensity of the o-Ps component was interpreted with the aid of x-ray diffraction, densitometry, and optical microscopy examinations and results from previously reported investigations of the thermal transition behavior of polypropylene. The present experiments demonstrate that o-Ps lifetimes were similar for both the annealed and quenched samples, independent of thermal cycling, while the o-Ps component intensity was significantly larger for the quenched material during heating, with both sample types exhibiting a significant hysteresis upon cooling. These results suggest that the mean free-volume cavity size is independent of prior thermal treatment, while the density of free-volume sites is a sensitive function of structure and prior thermal history. The variations of lifetime and of intensity with temperature have provided insight into polypropylene's glass transition phenomena.     

Comments on the relation: positronium lifetime – free volume size parameters of the Tao–Eldrup model

The author is discussing the parameters appearing in the Tao–Eldrup (TE) model describing the ortho-positronium (o-Ps) lifetime dependence on the size of free volume in which Ps is trapped. Parameter values are not universal, applicable to all media. The Ps penetration to the bulk should depend on Ps work function; the o-Ps decay rate is strongly influenced by the contact density factor.     

Positron age–momentum correlation studies of free volumes in polymers

Positron age–momentum correlation (AMOC) spectroscopy, which can sensitively probe momentum distributions of positrons and positronium (Ps), was conducted for studying the pick-off process of the triplet bound state ortho-Ps (o-Ps) with electrons at the walls of the free volumes in polymers. Influences of different chemical elements forming free volume were investigated. It was found that the momentum distribution of o-Ps pick-off annihilation sensitively depends on the electronic state in the free volumes. The feasibility of the chemical analysis relevant to the free volume in polymers is discussed.     

Studies of crystallization of SiO2-glass by positron annihilation

The crystallization of silica glass has been studied by positron lifetime spectroscopy and accompanying investigations using X-ray diffraction and nuclear magnetic resonance (NMR). To this end isothermal and isochronal heat treatments were performed in the range from 700 °C up to 1600 °C. The lifetime spectra are analyzed by two lifetime components. The decrease of the short lifetime (200 ps) is attributed to the increasing volume fraction of the crystalline phase. The long lifetime (1000 ps) is related with the pick-off annihilation of theo-Ps states in cavities.With starting formation of crystallization nuclei the long lifetime increases which is explained by expanding cavities at the interface between crystalline phase and amorphous matrix.     

Free volumes in solids under high pressure

A review of experimental work on ortho-positronium (o-Ps) lifetime in solids under the pressures up to 1 GPa is presented. Among the effects observed at high pressure one can mention: the disappearance of the energy level for Ps at the reduction of free volume size; pressure induced phase transitions; variation of Ps formation intensity with time; increase of o-Ps lifetime after intercalation of high pressure gas to the paraffin samples.     

Positron annihilation study on free volume of amino acid modified,starch-grafted acrylamide copolymer

Free volume measurements using positron annihilation lifetime spectroscopy was performed for uncrosslinked and crosslinked starch-grafted polyacrylamide, and their modified amino acid samples including some of their iron(III) complexes. The measurements were performed at room temperature. The analysis of lifetime spectra yielded mostly three lifetime components. It was observed that the values of the short lifetime component τ₁ are slightly higher than the lifetime associated with the self-decay of para-positronium atoms in polymers. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume has also been calculated from the lifetime data. The avrage value of this parameter of the crosslinked polymer were found to be higher than those of the uncrosslinked polymer.     

A positron annihilation lifetime study of isothermal structural relaxation in bisphenol-A polycarbonate

Positron annihilation lifetime spectroscopy has been used to study the isothermal relaxation response of compression molded bisphenol-A polycarbonate at temperatures of 263, 273, and 303 K. The temperature dependence of both the lifetime and intensity of the ortho-Positronium (o-Ps) pickoff component is discussed in terms of ductile-to-brittle transition behavior and free volume theory. An additive exponential model and the Williams–Watt model were used to analyze the relaxation as a function of temperature and provided results consistent with the anticipated molecular mobility of polycarbonate at sub-T_g temperatures.     

Positron annihilation in hypercrosslinked polystyrenes

Positron annihilation in hypercrosslinked polystyrene, called CPS(0.3)150E, has been studied as a function of temperature in the range from 35 to 370 K. The positron lifetime spectra were resolved into five components using the PATFIT and MELT programs. The annihilation rate constant of the longest-lived component was found to increase linearly with increasing temperature. Remarkably, the distribution of o-Ps lifetimes resolved by MELT was shown to broaden with decreasing temperature. Moreover, an effect of the oxygen pressure on the positronium characteristics was also investigated in another hypercrosslinked polystyrene, CPS(0.3)100E. The oxygen pressure varied from vacuum to 6 atm. The annihilation rate constants of the long-lived components were observed to increase linearly as a function of the oxygen pressure, which demonstrates a dependence of the kinetic of o-Ps quenching on the pore size.     

Positron annihilation lifetime and differential scanning calorimetric study of immiscible NBR/PE blends

The temperature dependence of the mean size of nanoscale free‐volume holes, 〈V_h〉, in polymer blend system consisting of polar and nonpolar polymers has been investigated. The positron lifetime spectra were measured for a series of polymer blends between polyethylene (PE) and nitrile butadiene rubber (NBR) as a function of temperature from 100 to 300 K. The glass transition temperatures (T_g) for blends were determined from the ortho‐positronium (o‐Ps) lifetime τ₃ and the mean size of free‐volume holes 〈V_h〉 versus temperature as a function of wt % of NBR. The T_gs estimated from the PALS data agree very well with those estimated from DSC in view of different time scales involved in the two measurements. Both DSC and PALS results for the blends showed two clear T_gs of a two‐phase system. Furthermore, from the variation of thermal expansivity of the nanoscale free‐volume holes, the thermal expansion coefficients of glass and amorphous phases were estimated. Variations of the o‐Ps formation probability I₃ versus temperature for pure PE and blends with low wt % of NBR were interpreted on the basis of the spur reaction model of Ps formation with reference to the effects of localized electrons and trapping centers produced by positron irradiation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 227–238, 2009     

Column packings for high-performance liquid chromatography and positron annihilation lifetime spectroscopy

Positron annihilation measurements as a function of temperature and the length of bonded alkyl groups have been carried out on silica gel samples. Silica gel samples were bare and bonded with alkyl group from C1 up to C18. The diameters of pores were deduced from the lifetime of trapped ortho-positronium (o-Ps), and it was found that o-Ps lifetime provides reasonable information on the pore sizes for both bare and alkyl bonded silica gels.     

Study of molecular motions in alkyl cyanobiphenyls using positron lifetime spectroscopy

Positron lifetime measurements have been carried out in two liquid-crystal-forming homologues of alkyl cyanobiphenyls, 7CB and 10CB. In each of these two compounds measurements were performed during the heating cycle of samples prepared either by quenching or by slow cooling from the respective liquid crystalline phase. In both compounds, the behavior of quenched and slow-cooled samples is found to be different. Unlike the slow-cooled sample, the material in the quenched sample seems to have transformed into a glassy solid. Theo-Ps pick-off lifetime in the quenched sample exhibits a strong temperature dependence. In each case, its value exhibits six broad peaks at various characteristic temperatures which have been ascribed to various motions associated with the molecules of these compounds. The quenched samples of 7CB and 10CB exhibit glass transitions at 268 and 266 K, respectively. The present work demonstrates an interesting application of positron lifetime spectroscopy (PLS).     

Free volume and microstructural investigation of poly(ethylene terephthalate)-g-acrylic acid (PET-g-AA) copolymer films

Acrylic acid (AA) was grafted onto PET films by preirradiation method using a ⁶⁰Co γ-radiation source. Microstructural investigation of the PET-g-AA copolymers has been carried out by wide angle X-ray diffraction (WAXD), dynamic mechanical analysis (DMA) and positron annihilation lifetime spectroscopy (PALS) techniques. The WAXD results showed only marginal variation in the crystallinity of the graft copolymers. Dynamic mechanical analysis showed an increase in the temperature of the α-relaxation and the tan δ value of subroom temperature secondary relaxations. From the PALS results, a complex variation of the free volume parameters, i.e. the o-positronium lifetime (τ₃), its intensity (I₃), fractional free volume (f_v) and the intermediate lifetime component was observed for the graft copolymers. Multiple phenomena of the effect of secondary relaxations, additive behavior of the individual polymer free volume parameters and effects from interfacial defects have been invoked to understand the fluctuational nature of the free volume properties in the graft copolymers.     

Free volume evolution in polyoxymethylene studied by positron annihilation spectroscopy

The free volume hole and its distribution in polyoxymethylene have been studied over the temperature range (5–90 °C) by positron annihilation lifetime spectroscopy. At a certain temperature (20 °C) the variation of ortho-positronium lifetime shows a distinct increase in its slope. The hole volume shows a small linear increase with temperature below glass transition temperatue and a steeper increase above it. A linear relation between ln(σT^0.5) and 1/V_h was observed and the critical hole volume was estimated.