Free-Volume Related Structural-Changes in the Epoxy Polymer Tgddm

Positron lifetime measurements have been carried out in an amine-cured epoxy polymer TGDDM/DDS/BFE as a function of annealing temperature. The measured spectra are best fitted to three component analysis. From the measured long lifetime component the average size of the free volume holes has been calculated following Nakanishi et al.'s treatment. Variation of lifetime parameters with temperature indicates the preferential trapping of positronium atoms in the regions of increased free volume hole sizes. On the other hand, positrons are being trapped at micro-voids in the ordered regions. The slow increase of free volume upto 170-degrees-C may be due to the thermal expansion and we measure 170-degrees-C as the T(g) for this epoxy. Lifetime data are also used to calculate the trapping rates in the ordered and disordered regions of the epoxy based on Goldanskii's kinetic relations

Structural phase-transition in a superionic conductor na2(la,al)zr(po4)3 - positron-annihilation study

Positron lifetime measurements are made in a Nasicon analogue Na2(La, Al)Zr(PO4)3 with 2.8 wt% of ZrO2 in the nutrient under an isochronal annealing cycle from 25 to 275 °C in steps of 25 K. The analysis of the lifetime spectra is best fitted to two lifetime components. The second lifetime component is attributed to the positron trapping at structural vacancies, vacancyclusters, and tetrahedral voids present in the material. A diffuse phase transition observed at 125 °C confirms the fact that the thermal effct associated with the transition in a Nasicon is weak and is spread over a large temperature range. Positron trapping rate is calculated using the two state trapping model. The activation energy is also calculated using the same, and is found to be 0.11 eV

Positron lifetime study of the free-volume properties in the polymer polyacrylonitrile

Positron lifetime measurements have been made in the polymer polyacrylonitrile, in the iodine doped state as well as the undoped state. The measured lifetime spectra were well resolved into three components. The behaviour of the intermediate component and its intensity seems to indicate an initial positron trapping at the interfaces between the amorphous and the crystalline regions and a later trapping by the kinks of the crystalline region. From the behaviour of the average free-volume size and its density we interpret that the iodine atoms initially occupy the free-volume sites in the amorphous region and at larger concentrations they form complexes with the nitrile group of the polymer

Thermally induced microstructural changes in undoped and doped polyacrylonitrile- A positron-annihilation study

Positron-lifetime measurements have been made in undoped and polymers 0.8 wt %-iodine-doped polyacrylonitrile (PAN) under annealing from room temperature to 160-degrees-C. From the measured lifetimes, the free-volume size is calculated following the treatment of Nakanishi, Jean, and collaborators. The variation in free-volume size as function of temperature seems to suggest molecular-chain-folding processes in amorphous regions of the polymer, positron trapping at kink-type defects, and at the interface between crystalline and amorphous regions at lower temperatures. This study indicates that in the undoped state, the polymer has a glass transition temperature at 90-degrees-C which is shifted toward lower temperature in the doped state by 30-degrees-C. The results also suggest the degradation of the polymer in the two states after 100-degrees-C and the effect of annealing is only on the size but not on the density of free-volume sites

Positron Lifetime Study on Microstructural Changes in Iodine‐Doped Polymer PPO

Positron lifetime measurements are made in iodine doped (0.2 wt%) poly(2,6-Dimethyl-1,4-Phenylene Oxide) films under an isochronal annealing cycle in the temperature range 25 to 200-degrees-C. The measured lifetime spectra are best fitted to three component analysis. The annihilation parameters as a function of isochronal annealing temperature show some interesting microstructural changes. The average free volume size at each temperature is calculated from the ortho-positronium lifetime using an equation of Nakanishi based on a theoretical model of Tao. The variation of lifetime parameters as a function of isochronal annealing temperature seems to suggest that there is a structural relaxation process in the temperature range 25 to 70-degrees-C which may be due to different types of rotations and oscillations of the methyl group present in the polymer. The ortho-positronium yield increases in the range 55 to 160-degrees-C indicating the increase in the electron mobility. Comparison of lifetime parameters of undoped and doped states of the polymer indicates that the iodine dopant diffuses mostly into the disordered micro regions of the polymer

Dobbler broadened and ionic conductivity studies on NASICON analogue

Doppler broadened lineshape and ionic conductivity measurements have been made in the NASICON analogue Na2(La, Al)Zr(PO4)3 synthesized with four different concentrations of ZrO2 (2, 2.2, 2.8 and 5.2 by wt.%) in the nutrient at room temperature. The lineshape parameter S is calculated for each concentration and it shows maximum value and the a.c. ionic conductivity sigma-shows minimum value for the material with 2.8 (wt.%) concentration of ZrO2 in the nutrient. An attempt has been made in correlating the changes in S parameter with sigma. The results show that the defect density is maximum for the material with 2.8 (wt.%) concentration of ZrO2 for which the conduction paths of Na+ ion are restricted

Radiation Induced Changes in the Polymer PPO. A Positron Annihilation Study

Positron lifetime measurements are made on gamma-irradiated poly(2,6-dimethyl-1,4-phenylene oxide) films as a function of the absorbed radiation dose in the radiation dosage range (0 to 24) x 10(-5) Gy. All the measured lifetime spectra are well fitted to the three component analysis. Variations in the lifetimes and intensities of the first two components indicate that there is radiation induced molecular motions associated with the formation of free radicals. The average free volume size is calculated using the value of the o-Ps lifetime at each irradiation step. The average free volume size is calculated using the 0.162 nm3 over the absorbed radiation dose, while the number density of free volume sites is unaffected due to irradiation in the present dosage range

Positron-annihilation studies on nasicon analogs containing cation vacancies

Positron annihilation studies were carried out on the Nasicon analogue Na2(La, AI)Zr(PO4)3 compound for three different concentrations (2-2, 2"8 and 5.2 by wt.%) of ZrO 2 in the nutrient. Angular correlation study of annihilated photons reveals that the defect concentration is maximum for 2.8 (wt.%) of ZrO 2. Further, positron lifetime studies indicate that the positrons are trapped at cation vacancies. Application of a twostate trapping model to this system made it possible to evaluate the lifetime of positrons in the Bloch state and of positrons trapped at cation vacancies