عنوان البحث(Papers / Research Title)
Fractional Free-Volume dependence of Oxygen in Beta-Irradiated Polystyrene Probed by Positron life time
الناشر \ المحرر \ الكاتب (Author / Editor / Publisher)
احسان ضياء جواد البيرماني
Citation Information
احسان,ضياء,جواد,البيرماني ,Fractional Free-Volume dependence of Oxygen in Beta-Irradiated Polystyrene Probed by Positron life time , Time 5/26/2011 8:42:36 AM : كلية التربية للعلوم الصرفة
وصف الابستركت (Abstract)
Positron annihilation spectroscopy (PAS) is a useful tool to investigate
الوصف الكامل (Full Abstract)
Fractional Free-Volume dependence of Oxygen in Beta-Irradiated Polystyrene Probed by Positron life time
Positron annihilation spectroscopy (PAS) is a useful tool to investigate the structure of the influence of radiation damage to the structure of polymeric materials. In this technique, the lifetime of positron and positronium are considered as non-destructive probe to study the characteristics parameters of these materials. Because of the relatively small size of the Ps probe (1.06Ao) compared to other probes, PAS is particularly sensitive to small hole and a free volume of angstroms in size and at a time of molecular motion from 10-10s and longer [1].
Positron annihilation lifetimes are usually determined by detecting the prompt g-ray (1.28MeV) from the nuclear decay that accompanies the emission of a positron from 22Na radioisotope and the annihilation g-rays (0.511MeV). The application of this technique is to study the microscopic properties of the matter, e.g., the effect of irradiation. The PAL spectra of most polymers have a long-lived component, which is attributed to pick-offannihilation of ortho-positronium (o-Ps) formed in amorphous region, the lifetime of the long-lived component (t3) is a measure of the size of interstitial space among polymer chain, which as referred to as “free space hole”.
The intensities of this component (I3) is considered to contain information about the concentration of the free space hole. In conventional positron annihilation experiment, the positrons are injected into a solid with a mean energy of 200 keV [2]. They slow down to thermal energies within (1-10 ps) [2] by ionization and excitation reaction in the solid. During this time they penetrate a distance of (10 –1000)mm depending on the density of the solid (the penetration depth is roughly inversely proportional to the density). To understand the mechanism of positrons and positronium interactions with materials and also of positronium formation one has to deal with the models so far suggested for that: such as the “Ore gap”model, the “Spur” model, the “Free volume”model… etc.
In this work the free volume model is mainly to be used. Oxygen during irradiation combine with active sites on the polymer chain to form peroxide links to reduce the net scission rate [3]. In PE and PS, cross-linking is the most important result of ionizing radiation, and the free radicals are the intermediate processes. If oxygen admitted to an irradiated polymer, the radicals are converted to peroxy radicals. The peroxy radicals are unstable so they form carbonyl (C=O) and hydroxyl (OH) groups, where, the oxygen destroyed the primary free radicals and peroxy intermediate is formed [4].
PE and PS, show oxygen effects that are dependent on sample thickness, dose rate and also the chemical structure, which is an important factor in oxidative scission [4]. In both polymers (PE and PS) irradiation with oxygen gives a much large scission to cross-linking ratio, although cross-linking predominates in vacuum irradiation in PE and PS. [3].
Al-Bayati [5] employed PALS to investigate g-irradiation effect on polystyrene (PS) and polymethylmethacrylate (PMMA) in air at room temperature with dose up to 1340 kGy. It was found that the initial irradiation dose induced percentage increases in free volume and free volume fraction.
These low doses induce degradation in the polymer chain except for PS, in which an opposite effects, the cross-linking, are dominant. Suzuki et.al., [6] studied the radiation effect on positronium formation in low temperature PE. In the case of non irradiated PE samples, they found at low temperature below the glass-transition temperature, the intensity of the long-lived component of positronium, I3, increased due to an increase in the concentration of trapped electron. However, the increase in I3obtained in a few MGy g-irradiated samples became very small due to the effect of induced radicals. Also, they have observed that the trapped electrons were affected by the intensity of the positron source used for a PALS experiment. Several other works of polymers have been studied by using PALS technique to study the irradiation effects such as:
u.v-irradiation of acrylonitrile-butadiene-styrene (ABS) and polycarbnate (PC) [7], b-irradiated of (PP) and (ABS) [5], g-irradiated of Teflon [8], g-irradiated of epoxy [5,9] and g-irradiated tissue [10]. As explained in the literature review, it is noticed that the effects of the low b-dose rang (0.3-12.3)Gy have not been studied for polymers by means of PALS teqnique and most works published used high b-doses (Mrad), in spite of the fact that b-rays is ionizing particles and have mass, therefore, the low doses effects are also important. In the present study the polymer; namely PS are studied in the presence of air and in vacuum to explain the oxygen effect during b-irradiation on the o-Ps lifetime. To the best of our knowledge b-irradiation effect under vacuum for the investigated samples in this work are not studied before.
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