The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study

Positron lifetime measurements, performed in the temperature range 80–300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ₃ and τ₄, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm³ (PE) and between 0.152 and 0.372 nm³ (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of T_g = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at T_g of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 10²⁷ holes/m³ is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513–1528, 1998     

Positron annihilation in amine-cured epoxy polymers—pressure dependence

Positron annihilation spectroscopy has been used to study free volume in an arnine-cured epoxy as a function of external pressure at temperatures above and below the glass transition temperature. The observed ortho-positronium lifetime τ₃ and formation probability I₃ decreased with increasing pressure. The decrease in τ₃ is interpreted in terms of a corresponding decrease in average free-volume hole size over the range from 0.135 to 0.045 nm³. The fractional free-volume and the free-volume compressibility in the epoxy are calculated as functions of pressure at 100°C.     

Ortho‐positronium lifetime distribution analyzed with MELT and LT and free volume in poly(ε‐caprolactone) during glass transition,melting, and crystallization

Positron annihilation lifetime spectroscopy (PALS) was used to study the free volume behavior in the temperature range between 100 and 370 K in semicrystalline poly(ε‐caprolactone) (PCL). For the analysis of the spectra we used the well‐known routine MELT as well as the new program LT8.0, which allows both discrete and log‐normal distributed annihilation rates. From experiments, confirmed by the analysis of simulated spectra, we found that MELT returns too large values for the o‐Ps lifetime τ₃ associated with too small intensities I₃. This is due to the underestimation of the width of o‐Ps lifetime distribution in MELT (the spectra analyzed contained 3 million counts). The same effects were observed in the parameters obtained from the discrete term analysis. LT, however, returns, when allowing the o‐Ps lifetime to be distributed, rather accurate values for τ₃, I₃, and the width (standard deviation σ₃) of the o‐Ps lifetime distribution. The effect of the glass transition, melting, and crystallization on the annihilation parameters was observed. These results were compared with differential scanning calorimetry (DSC) and pressure–volume–temperature (PVT) experiments. From this comparison, the number density of holes and the fractional free (hole) volume have been estimated. At a “knee” temperature T_k ≈ 1.5 T_g, a leveling off of the o‐Ps lifetime τ₃ and a distinct decrease in the width, σ₃, of its distribution was observed; the latter effect was detected for the first time. Fast motional processes and/or the disappearance of the dynamic heterogeneity of the glass and the transition to a homogeneous liquid are discussed as possible reasons for these effects. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3077–3088, 2003     

Positron annihilation lifetime studies of free volume in a polymer electrolyte PEO complexed with NH4I

Positron lifetime spectroscopy has been applied to study the temperature dependence of free-volume properties in a solvent-free polymer–salt complex polyethylene oxide (PEO) doped with ammonium iodide (NH₄I, with NH ≈ 0.076) in the temperature range of 298–353 K. The observed lifetime spectra were resolved into three components and the longest lifetime, τ₃, was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by the free volume. The lifetime component, τ₃, and its intensity, I₃, both showed a significant variation with temperature, which followed a different course in the heating and cooling cycle. Changes in the temperature coefficient of τ₃ and I₃ were observed at T ≈ 328 K, the melting point of the sample. This behaviour is correlated to the temperature variation of the electrical conductivity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 969–976, 1998     

Experimental free volume aspects of the polymer rheology as obtained by positron annihilation lifetime spectroscopy

The ortho‐positronium (o‐Ps) annihilation parameters, i.e. the mean o‐Ps lifetime, τ₃, and the o‐Ps relative intensity, I₃, in cis‐1,4‐polybutadiene (cis‐1,4‐PBD) and polyisobutylene (PIB) over a wide temperature range including the glass‐liquid transition have been measured by means of positron annihilation lifetime Spectroscopy (PALS). From them the free volume microstructural characteristics, i.e. the mean free volume hole size, V_h, and the free volume hole fraction, f_h, have been determined via a semiempirical quantum‐mechanical model of o‐Ps in a spherical hole or a phenomenological model of volumetric and free volume hole properties, respectively. Consequently, the literature rheological data for both the above‐mentioned polymers have been related to the free volume hole fractions via the WLF‐Doolittle type equation. It has been found that i) in the case of PIB this equation holds over 130K above the glass transition temperature T_g and ii) in the case of cis‐PBD the WLF‐Doolitle equation is valid in the temperature range over 60K above 1.3T_g, but below 1.3T_g down to T_g the modified WLF‐Doolittle‐Macedo‐Litovitz equation with the additional activation‐energy term describes the shift factor data better.     

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.     

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

We 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, T_50G, is connected with the mean o-Ps lifetime, τ₃ (T_50G) = (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, V_h (T_50G) = (102 ± 17) ų, 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 T_50G for both typical fragile PVME and strong PIB polymer and emphasizes the essential role of free volume in the spin probe dynamics.     

Contributions of side groups to the water diffusion in cured epoxy resins (2) study on physical aging

Novolac epoxy resins cured with novolac resin, novolac acetate resin, novolac butyrate resin, and novolac phenylacetate resin named as EP, EPA, EPB, and EPP, respectively, were prepared. Their physical aging behavior at a T_g‐30 °C (30 °C below glass‐transition temperature) was examined by positron annihilation lifetime spectroscopy and differential scanning calorimetry. The ortho‐positronium annihilation lifetime τ₃ variation extent of EP is less apparent than that of the other three esterified samples during physical aging. The time dependence of ops intensity I₃ agreed with the Kohlrausch‐Williams‐Watts (KWW) equation. The relaxation time (τ₀) and nonexponential parameter were calculated. The free volume and enthalpy relaxation rate characterized by the reciprocal of τ₀ and ?ΔH/?logt, respectively, exhibit the same order—EPP > EPB > EPA > EP. These results suggest that the extend and rate of relaxation are not only related to the frozen free volume produced by quenching but also significantly influenced by segmental mobility of the network that attributed to the side‐group flexibility and their interaction with networks. This work also supports the fact that side‐group flexibility and the free‐volume fraction and distribution act in concert to control the water‐diffusion behavior in epoxy networks. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1135–1142, 2003     

Transition and relaxation processes of polyethylene,polypropylene, and polystyrene studied by positron annihilation

Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ₃, and its intensity, I₃, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, T_g. For PE, above T_g (140 K), the value of τ₃ increased, but the temperature coefficient of I₃ was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (T_g = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1601–1609, 1997     

Positron annihilation lifetime study of PTFE/silica composites

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of PTFE/silica composites. The positron lifetimes (τ_n) and intensities (I_n) of PTFE and the composites (30-62% silica) were measured at room temperature as a function of specimen thickness. Four lifetime components were found in PTFE and the composites. The longer lifetime components, τ₃ = 1.4 ns and τ₄ = 4.4 - 4.1 ns, were interpreted to be due to the presence of two different sized free volume cavity distributions within the PTFE/silica composites. A strong silica concentration dependence was found in the bulk intensities (I_3b and I_4b). The I_3b value increased from 13.0% in PTFE to 28.2% in the 62% composite, while the I_4b value decreased from 17.5% in PTFE to 4.5% in the 62% composite. The smaller-void size, free volume fraction (τ₃I_3b) values increased linearly between 0 and 100% silica concentration, while the larger void size, free volume fraction (τ₄I_4b) values decreased nonlinearly with silica concentration. Since silica has a long lifetime component (τ₃ = 1.6 ns), this behavior is ascribed to silica particles occupying the large free volume cavities (370 ų) in the PTFE/silica composites. © 1996 John Wiley & Sons, Inc.     

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.     

Characterization of poly(silylenemethylene)s by positron annihilation lifetime spectroscopy. I.

Amorphous and crystalline poly(silylenemethylene)s with the repeating PhRSiCH₂ (R : Me or Ph) units were characterized by positron annihilation lifetime spectroscopy (PALS) to gain insights into the molecular motions of these polymers. The temperature dependence of the ortho-positronium lifetime (τ₃) and intensity (I₃) was examined from 50 to 470 K for each sample. The glass transition temperature of each polymer was easily distinguished by a change in the slope of τ₃ spectrum. Both polymers exhibited a steep drop of I₃ at 130–140 K being probably assignable to the transition arising from the motions of phenyl groups, which was almost undetectable by means of differential scanning calorimetry or dynamic mechanical analysis. Several other transitions of these polymers detected by PALS are also discussed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 755–761, 1998     

STUDIES ON MICELLE BEHAVIORS OF STYRENE-BUTADIENE-STYRENE (SBS) TRIBLOCK COPOLYMERS IN METHYL ETHYL KETONE (MEK) BY POSITRON ANNIHILATION TECHNIQUE

Positron annihilation technique was used to study the micelle behaviors of two SBS triblock copolymers in MEK solvent at different temperatures. Annihilation lifetime τ_3 of ortho-positronium (o-Ps) exhibited an obvious transition from shorter lifetime to longer lifetime with temperature. It was attributed to the change of micelle behavior of SBS copolymer molecules in the solution. Experimental results of sedimentation velocity of ultracentrifuge were also reported.     

Frontiers of positron and positronium chemistry in condensed media

Our work proves that positron annihilation spectroscopy is an excellent tool to follow the structural changes in chemical species. We present four examples to support the above statement. The sizes of defects in electrodeposited chromium layers were studied and estimated on the basis of positron lifetime spectra decomposed into two components. Vacancies, di-vacancies and vacancy-clusters could be identified in the electrodeposites. The changes of the size distribution of the free volume units in poly(methylmetacrylate) on the dependence of molecular weight and dispersity were described by the correlation between the lifetime ofortho-Ps and the free volume units in polymers. It was found that the free volume is affected by both the molecular weight and dispersity. The effect of dispersity was explained by the sample preparation technique, namely by the application of high pressure. The ortho-para conversion ofortho-Ps was used to follow the partial spin-crossover in [Fe(1-ethyl-1H-tetrazole)₆](BF₄)₂. The spin-crossover temperature was identified to be 105 K. A conformal structural transformation was found in [Zn(1-propyl-1H-tetrazole)₆](BF₄)₂ between 170 and 90 K by positronium lifetime measurement and the role of (BF₄)²⁻ anion, in this transformation, was proved by¹⁹F NMR spectroscopy.     

Study of free volume parameters of polystyrenes of different molecular architecture by positron annihilation lifetime measurement

Differences of size, content and size distribution of free volumes in linear branched and three-armed polystyrenes, synthesized by radical and anionic processes, were observed by positron annihilation lifetime measurements. The temperature dependence of an average free volume radius was quite similar among polystyrenes of different architectures and molecular weight distributions. The free volume radius increased with temperature, from 0.27 nm (T:60 K) to 0.29 nm (T:260 K) and 0.30 nm (T _g:363 K), then to 0.35 nm (423 K), showing turning at and transition temperature. The free volume content decreased from 60 K to 220 K to 300 K showing peculiar minimum at 220 K to 300 K depending on the molecular shape, increased above 320 K, upto 340 to 360 K. The free volume contents decreased with an increase of molecular weight and by an addition of oligomer or plasticiser, suggesting differences in relaxation time or molecular motion between the edge and middle portions of molecular chain and filling effect of smaller molecules in free volumes, respectively. The apparent free volume fraction showed clear variations atT andT _g. Size distribution of free volumes suggested more complicated behavior of free volume upon the molecular relaxations and filling effect.     

A study on free volume of acrylonitrile‐styrene copolymer and methyl methacrylate‐styrene copolymer by using positron annihilation

The o‐Ps lifetime τ₃ and the intensity I₃ of ST‐AN copolymers and ST‐MMA copolymers have been determined by using the positron annihilation technique. The average free volume hole radius R is estimated according to Tao's and Eldrup's model. The result shows that the average free volume hole size mainly attributes to lateral group volume and polarity of macromolecular chain as well as polymerizing temperature, and the o‐Ps intensity I₃ to the effect of the lateral group volume and the polarity. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 465–472, 1999     

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.     

Micro-structure of gamma-ray irradiated polyethylenes studied by positron annihilation

Effects of gamma-ray irradiation on high density (HDPE) and low density (LDPE) polyethylenes were measured by positron annihilation at temperatures between 100 and 420 K. The effect of the irradiation on the intensity I₃ and the lifetime τ₃ of the longest lived component was significant below the glass transition temperature T_g, while they were little affected above T_g. For LDPE a marked minimum was observed in the I₃ vs T curve. The lower edge of the minimum, corresponding to glass II and glass I transitions, was found to shift to lower temperatures by the irradiation. In HDPE both I₃ and τ₃ decreased on irradiation mainly due to radiation induced free radicals. The gel fraction of HDPE was small when irradiated below T_m, while it rose sharply on irradiation near T_m. Positron lifetime parameters of this highly crosslinked HDPE showed a distinct difference compared to HDPE irradiated below T_m. Usefulness of the positron annihilation method is discussed in conjunction with studying micro-structure of polymers.     

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.     

Investigation of transitions and relaxation processes in polystyrene by using positron annihilation

The glass transition and relaxation processes in polystyrene resins with the number average molecular weight ranging from 7.0·10² to 9.8·10⁴ were studied with the positron annihilation technique. The pick-off annihilation lifetime of ortho-positronium (₃) and its intensity (I ₃) were measured in the temperature range from 20 to 430 K. The glass transition temperature (T _g) was determined as an onset temperature coefficient of ₃.T _g shows the molecular weight dependence in these samples. BelowT _g, local motions were detected by measurements ofI ₃. The local motions could be observed above 100 K in this experiment.I ₃ show the minimum at around 250 K and it does not show molecular weight dependence.