Effect of thermal history on the free volume properties of semi‐crystalline poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) membranes by positron annihilation lifetime spectroscopy

Free volume properties of a series of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) membranes, which were produced by various nonisothermal crystallization processes (rapid‐, step‐, and slow‐cooling processes), were investigated using positron annihilation lifetime (PAL) spectroscopy over a temperature range of 25–90 °C. From the annihilation lifetime parameters, the temperature dependence of free volume size, amount, size distribution, and fractional free volume and thermal expansion properties of free volume were discussed. A model which assumed that amorphous phase was subdivided into mobile and rigid amorphous fractions (MAF and RAF) in the semicrystalline polymer was considered to interpret the temperature dependence of those free volume properties. Morphological observation of the semicrystalline polymer by small‐angle X‐ray scattering (SAXS) indicated that the rapid‐cooled (cold‐crystallized) membranes showed a much thinner thickness of the repeating lamellar/amorphous layers and most likely higher amount of RAF, which restrained the chain motion, than the step‐ and slow‐cooled (melt‐crystallized) membranes. The difference of free volume properties among various PHBV membranes was created according to the crystalline structure of the polymer from different thermal history. The polymer crystallized with slower cooling rate induced higher crystallinity and resulted in less free volume amount and lower fractional free volume. In addition, the thermal expansion coefficients of free volume size were affected by the crystallization rate of PHBV polymer. Larger distribution of the free volume size of melt‐crystallized membranes was observed as a result of the bimodal distribution of the lamellar periodicity and less amount of RAF than that of the cold‐crystallized membranes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 855–865, 2009     

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.     

聚对苯二甲酸乙二酯的玻璃化转变和结晶化的正电子湮没研究

研究了聚对苯二甲酸乙二酯的玻璃化转变和结晶过程对正电子湮没寿命谱的影响.结果表明,结晶过程对正电子湮没的长寿命组份τ_3的寿命值不敏感,用这种方法测得的“T_g”不随结晶度而变化,而长寿命组份的相对强度则随结晶度的增加而减小.     

Effect of orientation on the free volume and oxygen transport of a polypropylene copolymer

The effect of uniaxial orientation on the free‐volume and oxygen‐transport properties of a propylene copolymer with 4.5 wt % ethylene was examined. The free‐volume hole size and hole density were measured with positron annihilation lifetime spectroscopy. Subsequently, the free‐volume characteristics were correlated with the oxygen‐transport properties. Orientation had only a small effect on the total amount of free volume: a small increase in the hole density was offset by a small decrease in the hole size. As a result, the oxygen solubility and amorphous‐phase density were unchanged by orientation. However, a pronounced decrease in the oxygen diffusivity when the draw ratio exceeded 6 indicated a change in the dynamic free volume. This was attributed to an increasing number of taut tie chains, which retarded oxygen diffusion. The reduced amorphous chain mobility was also manifest in the increased glass‐transition temperature, decreased bulk thermal expansivity, and decreased expansivity of free‐volume holes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1230–1243, 2005     

The effect of crystallinity on chain mobility and free volume in the amorphous regions of a miscible polycarbonate/polyester blend

Homopolymers and blends of polycarbonate/ThermX have been investigated by differential scanning calorimetry, dynamic mechanical analysis, density measurements, and positron annihilation lifetime spectroscopy. The study focuses on the chain mobility and free volume in the amorphous miscible regions of the blends and how this mobility and free volume are affected by increasing crystallinity. It is proposed that judicious annealing, which results in increasing crystallinity (while avoiding ageing or transesterification), induces a constraint on the amorphous region leading to an increase in T_g and coalescence of free volume sites. © 1994 John Wiley & Sons, Inc.     

Study of epoxy resin for cryogenic use by positron annihilation method

The molecular state of epoxy resins for cryogenic use has been studied in terms of positron lifetime measurement. The addition of plasticizer increased the free volume in the epoxy and hence the ductility of the epoxy at room temperature was increased. The fracture toughness at cryogenic temperatures, however, decreased. The increase of molecular weight between crosslinks decreased the free volume in the epoxy at room temperature and increased the fracture toughness slightly at cryogenic temperature. Based on the data the molecular state model desired for cryogenic application was proposed and the nano-sized silica dispersed epoxy was prepared by means of sol-gel method to follow the model. The cryogenic properties of the silica dispersed epoxy and the free volume evaluated by positron annihilation lifetime were also discussed.     

Effect of CO2 exposure on free volumes in polystyrene studied by positron annihilation spectroscopy

Free‐volume properties, size and distribution, in amorphous polystyrene exposed to CO₂ gases have been measured as a function of pressure to 800 psi (5.5 MPa), of time, and of temperature using positron annihilation lifetime spectroscopy. The free volume increases significantly and its distribution broadens as a function of pressure. The free volume relaxes as a function of time with a characteristic time of 15 h, and 5.7 h for 400, and 800 psi, respectively, after depressurizing under vacuum. A portion of free volume created by CO₂ exposure remains permanently in the polymer after CO₂ exposure. The glass transition temperature decreases significantly as a function of CO₂ pressure from the free‐volume data and is compared with the differential scanning calorimeter results. The observed free‐volume variations as a function of pressure, time, and temperature are discussed in terms of hole expansion, creation, free‐volume relaxation, plasticization, and hole filling in amorphous polymers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 388–405, 2008     

POSITRON LIFETIME STUDY OF THERMALLY INDUCED MICROSTRUCTURAL CHANGES IN NISTARI SILK FIBER

Positron lifetime measurements have been made in natural polymer-Nistari silk fibers as a function of isochronalannealing temperature in the range of 27℃ to 280℃. The variations in the positron results indicate the structural changesoccurring in the Nistari silk fibers and determine the glass transition temperature as 170℃. Activation energies weremeasured separately for the crystalline and amorphous regions indicating the versatility of the technique. These values areclose to the N--H bond dissociation energy, suggesting N--H bond dissociation as the most probable process occurringduring thermal treatment As an extension of the positron results, the molecular weight of the Nistari silk fibers wasdetermined to be 10.7×10~5 based on free volume, which lies within the range suggested for the silk fibers. There seems to bean indication that cross-linking changes the spiral structure of cotton fibers to network type. However, this needs to be validated by other techniques.     

Local free volume and structural relaxation studied with photoisomerization of azobenzene and persistent spectral hole burning in poly(alkyl methacrylate)s at low temperatures

The final extent of trans‐to‐cis photoisomerization of an azobenzene probe in various amorphous polymers has been used in previous studies for estimating local free volume and its fluctuation in polymer solids. However, there have been few studies on what kinds of molecular motion cause the fluctuation of local free volume at low temperatures. The onset of local structural relaxation (molecular motion) can be observed with temperature cycling experiments in persistent spectral hole burning (PSHB). Thus, in the present article, the fluctuation of local free volume observed in trans‐to‐cis photoisomerization of azobenzene is related to the local structural relaxation observed in PSHB for poly(alkyl methacrylate)s with various ester groups, i.e., methyl (PMMA), ethyl (PEMA), n‐propyl (PnPMA), isopropyl (PiPMA), and isobutyl (PiBMA) groups. In the final cis fraction, rapid decrease, from 20 to 4 K in PEMA, PnPMA, and PiPMA, and from 86 to 20 K in PiBMA, is observed. These temperature regions of the rapid decrease in final cis fraction in these polymers agree well with those where the hole width in PSHB temperature cycling experiments begins to increase for the same polymers. For example, PEMA begins its ester ethyl group rotation at 17 K, which was primarily observed with PSHB, causing the drastic decrease in final cis fraction of azobenzene from 20 to 4 K. The final cis fractions at 4 K for these poly(alkyl methacrylate)s reflect the intrinsic sizes of the local free volume, except in the case of PMMA, and are compared with the reported results of positron annihilation lifetime measurements. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3098–3105, 2000     

Detection of free volumes in polyaniline complexes with various acids by using positron lifetime spectroscopy

The positron lifetime spectroscopy (PLS), a non-destructive characterization method, utilizes positronium (Ps; an electron–positron bound state) as a probe and measures its lifetime in polymer free volumes. For the first time the free volumes have been estimated by PLS in polyaniline (PANI) complexes with various inorganic and organic acids. It was found that the o-Ps lifetime increases and the intensity decreases with increasing ionic radius of the counter-ions in PANI complexes. Obviously, larger counter anions result in enhanced mean size of the voids corresponding to the free volume in the bulk polymer.Electrical conductivity has been measured by conventional four-probe technique. The glass transition temperature and temperature of removal of the absorbed water have been determined by using differential scanning calorimetry. It was established fairly well correlation of the mentioned polymer parameters with the o-Ps lifetime and the free volume of PANI complexes, respectively. The greater free-volume results in a decrease of conductivity, glass transition temperature and temperature of removal of the absorbed water.     

嵌段共聚物溶液胶束温度行为的郑电子湮没研究

用正电子寿命技术研究了聚苯乙烯—二甲基硅氧烷／正庚烷二嵌段共聚物溶液胶束的温度行为．结果表明正电子湮没参数对溶液中出现的结构和微观环境的变化十分灵敏．在一定的温度范围o-Ps寿命和强度的陡然变化反映了溶液中共聚物分子聚集态随温度经历不同阶段的变化，超过临界胶束温度o-Ps寿命随温度增高而迅速地增大，这一行为反映了分子聚集体解离成自由共聚物分子的过程．     

聚对苯二甲酸乙二酯中正电子湮没寿命参数温度效应的研究

对聚对苯二甲酸乙二酯的非晶、低温热处理(54℃)、高温热处理(180℃)以及双轴拉伸等四种不同试样,测量了室温到180℃范围内不同温度在的正电子湮没寿命谱.根据最小二乘曲线拟合解谱结果,可知最长寿命成份的湮没参数τ和I的温度曲线分别灵敏地反映了聚对苯二甲酸乙二酯的玻璃转变过程和结晶过程.据此讨论了正电子湮没寿命参数与PET转变的关联以及PET玻璃转变的有关性质.     

Free volume studies in miscible polymer blend systems

This paper summarises the currently available literature concerned with measurement of free volume in miscible, amorphous polymer blends using positron annihilation lifetime spectroscopy (PALS) which probes excluded volume at the angstrom level. Previously reported data is compared with new data from a range of different blend systems. Miscible blends tend to show a negative deviation of free volume size (and to a lesser degree free volume fraction) on mixing due to the intimacy of packing of the blend component macromolecules. A largely immiscible system is also reported and shows a different behaviour (positive deviation of free volume size) and this is ascribed to additional free volume at the interface.     

Nuclear techniques characterization of short-range order in Zr-TM-Cu-Al-Ni (TM=Hf,Ti, Fe) bulk metallic glasses

In this paper an overview of recent and new perturbed angular correlation (PAC), Mössbauer (MS) and positron annihilation (PAS) experiments on Zr- and Hf-based bulk metallic glasses is presented. PAC results showed that all samples in their as-prepared amorphous state, irrespective of the preparation method used, are well described by a dense random packing of ions (DRP). Hf-based samples showed a greater electric field gradient (EFG) value than Zr-based ones for same composition. A general observation is that the relaxed amorphous and supercooled liquid has the same local arrangements than the initial amorphous state. The crystallization behaviour is not preparation-method dependant but composition and impurities have a major influence on it. Preliminary positron annihilation lifetime measurements on amorphous samples yielded one characteristic lifetime, with a value intermediate between those for the well-annealed and defected metallic constituents of the alloys. Mössbauer spectra of as-prepared amorphous alloys consist of a broad asymmetric doublet, originating from a distribution of electric field gradients characteristic for amorphous samples.     

Slow positron study on dielectric-coated mirror for free-electron-laser experiments

A positron lifetime study has been done on dielectric multilayer cavity mirrors for free-electron-laser experiments by the use of a variable-energy pulsed positron beam. A long-lived ortho-positronium component has been observed at low positron energy region, corresponding to the depth of the top amorphous SiO₂ layer. The intensity of the positronium component correlates with the degradation and restoration of the mirrors. We discuss the relation between the positronium intensity and degradation mechanism of the mirrors. The present studies revealed that the slow positron lifetime technique is highly sensitive to the properties of the mirrors and is useful for the evaluation of the mirrors.     

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.     

Vacancy spectroscopy of radiation cross-linked and degraded polytetrafluoroethylenes

Polytetrafluoroethylene samples having different morphology were prepared by radiation induced cross-linking and degradation, and positron annihilation lifetimes were measured to extract information about the vacancy structure from the orth-positronium (o-Ps) component. The o-Ps intensity I₃ was inversely correlated with the crystallinity confirming that this component is associated with the amorphous part. The o-Ps lifetime τ₃ was distinctly different depending on the phase of the polymer. Below the γ relaxation temperature, the vacancies produced by degradation could be seen from an increase in the τ₃ value, but at temperatures between the γ and β relaxations such vacancies became invisible due to the activated segment motion. And above the β relaxation temperature vacancies in the interface between crystalline and amorphous parts appear to affect τ₃. The o-Ps intensity did not show stable reproducible results. Accumulation of positive charge can be responsible for it. The results, not only unveiling the not yet well understood behavior of the o-Ps component, demonstrate the uniqueness of the vacancy spectroscopy using o-Ps as a probe.