Papers by Keyword: Free Volume

Abstract: Ortho-positronium pick off annihilation lifetime gives the information of the microscopic free volume of polymers. Polyimide polymers such as Kapton are applied in many fields, but it was impossible to apply the positron lifetime method for free volume investigation because of no positronium formation. Here, we apply the idea of the free positron annihilation probability that is sum of the probability of annihilation by the positron and electrons on the molecular chain where the positron localized and that for the annihilation with the electrons on the neighboring molecular chains. The second term is probably affected by the free volume change. We have successfully shown the temperature dependence and the electron beam irradiation effect on the free volume change by observing the free positron annihilation lifetime for Kapton.

Abstract: The in situ positron annihilation spectroscopy measurement has been developed and applied to investigate the structural change in free volume on the tensile deformation of linear low-density polyethylene. The pick-off lifetime of ortho-positronium (Ps) decreases by applying the strain and an aging variation cannot be observed. On the contrary the fraction of Ps formation gradually decreases day by day and becomes constant after several days. Further, after release of strain, it returns to the original value. The reason is considered to be that the molecular chains become rigid gradually during deformation and they lose the flexibility.

Abstract: Positron annihilation lifetime spectroscopy (PALS) is used to study the free-volume in β-cyclodextrin (CD) with the encapsulation of two different essential oils: garlic (Allium sativum) and marjoram (Origanum majorana). The data reveal the presence of a long lifetime component related to the o-Ps. It is observed that the addition of garlic (or marjoram) oil to β-CD results in a decrease of the o-Ps lifetime which was ascribed to a reduction of the free volume holes from 83.6 to 67.7 ų (79.1 to 72.6 ų). Different mechanisms for the encapsulation of garlic and marjoram oils in β-CD are suggested by the PALS parameters.

Abstract: Positron lifetime spectroscopy was employed for investigation of the size distribution of free volume holes in Nafion N-1110 membrane. Size distribution of free volume holes was determined in the as-received and dried membrane and in the membrane immersed in water at various temperatures. It was found that absorbed water causes reduction of the mean size of free volume holes and increase of their dispersion and concentration. This indicates that free volumes are split by swollen ionic clusters absorbing water. The kinetics of water uptake and the amount of absorbed solvent was determined by in-situ measurement of the weight increase.

Abstract: The relaxation behaviors of PS below Tg and its variation trend along with aging time were studied by DSC, fluorescence spectrum and XRD. On the basis of DSC results, the kinetics of the volume and enthalpy relaxation process were analyzed, and the influence of the aging time on the relaxation behaviors were discussed. The results of fluorescence spectrum and XRD indicated the presence of the rearrangement of polymer chains in the physical aging of polystyrene.

Abstract: The electrochemical performance of polyaniline (PANI)/stainless steel electrodes prepared in H₂SO₄ and p-toluene sulfonic acid (p-TSA) using potentiodynamic method were investigated in corresponding acids and three kinds of sodium salts, respectively, by cyclic voltammetry, galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy (EIS). It is concluded that the counter anion of acid employed in the electropolymerization plays a key role in the free volume of PANI. The kinetic parameters of PANI are strongly influenced by the free volume both in acid and in salt solutions. In addition, the utilizing of salt solution is beneficial to the kinetic analysis of the energy-storage process of PANI, although the capacitance of PANI in salt solution decays on cycling due to the lack of protons.

Abstract: PolymethylMethacrylate (PMMA) composites with different weight content of organic rectorite (OREC) are prepared via melt blending. The results of X-ray diffraction (XRD) show that the layer distance of OREC is much larger than that of the pristine rectorite. SEM images reveal that the OREC filler has been well dispersed in matrix. With the addition of OREC, Bending modulus increased obviously. Ortho-positronium (o-Ps) lifetimes show that the free volume sizes of PMMA/OREC composites are larger than those of the PMMA matrix. The relationship between the atomic-scale free volume size and mechanical properties is preliminarily discussed in this work.

Abstract: 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.

Abstract: The structure of isotactic polypropylene (PP) and its blends with ethylene-propylene-diene terpolymer EPDM (0-100%) containing the unvulcanized and the vulcanized rubber phase were studied using positron annihilation lifetime (PAL) spectroscopy technique and thermostimulated luminescence (TSL). Cross-linking in the polymer blends is one of the effective ways to create novel materials with effective industrial properties. Meanwhile, composition of such systems is very complicated since it contains microphases of the dispersed components, cross-linked or not, elements of the cross-linking agents, which have some distribution between the blend components. This distribution, heterogeneity and also variation of elementary free volumes due to cross-linking are the points of interest in the study of polymer blend structures.

Abstract: Polymerisation and polymer modification are often carried out by means of radiation methods, since irradiation can modify the structure of materials and improve their performance. On the other hand, combined action of ionising radiation and oxygen may induce a degradation of the polymer. Physical changes of different polyesters subjected to gamma irradiation up to 1 MGy are discussed and evidenced by various techniques, mainly viscometry, differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Viscometry monitors variations in the molecular weight. Calorimetric measurements allow one to detect crystallinity changes. PALS evidences possible irradiation effects on the free volume; furthermore, it supplies information on the chemical modifications induced by oxygen. The results can be explained in terms of the main mechanisms induced by gamma irradiation (chain scission, crosslinking and oxydative degradation), although their importance depends on the dose.