Structure dependence of gamma ray irradiation effects on polyurethane and epoxy resin studied by PAL technique

Positron annihilation lifetime (PAL) technique has been employed tostudy the microstructural changes of polyurethane (PU), EUXIT 101and epoxy risen (EP), EUXIT 60 by Gamma-ray irradiation with thedose range (95.76 - 957.6) kGy. The size of the free volume hole andtheir fraction in PU and EP were determined from ortho-positroniumlifetime component and its intensity in the measured lifetime spectra.The results show that the irradiation causes significant changes in thefree volume hole size (Vh) and the fractional free volume (Fh), andthereby the microstructure of PU and EP. The results indicate thatthe γ-dose increases the crystallinity in the amorphous regions of PUand increase the cross-linking of EP.


Introduction
The studies of microstructure like free-volume properties at a molecular and atomic scale can provide a basic understanding of the mechanical and physical properties of polymers [1].
In PAL technique, one employs the positron emitted from a radioactive source as a probe to monitor the lifetime of the positron and positronium (Ps) (a bound atom consists of an electron and a positron) annihilation in the material under study [2].
The positron and positronium atoms entering the polymer can be trapped in open spaces like holes, voids or cavities.
The corresponding annihilation 0.511MeV photons come from these open spaces. Positrons produced by the radioactive isotope 22 Na are commonly used. The Ps atom formed can be in a singlet state parapositronium (p-Ps) or triplet state ortho-Positronium (o-Ps), with mean lifetime in free spaces of 0.125 ns and 140 ns, respectively [3].
In condensed medium, the mean lifetime of o-Ps is reduced to a few nanoseconds due to its interaction with surrounding molecules of the medium [4]. So the results for o-Ps lifetime and its probability are related to freevolume hole size (V h ), fraction freevolume (F h ) and free-volume distribution. The relation between the mean radius of free-volume hole size and the o-Ps lifetime τ o-Ps was found according to the semi-empirical formula [5]: where: R is the radius of the hole size and given by R= R 0 +∆R , R 0 is the radius of the spherical well potential, ∆R is the electron layer thickness. The free-volume ─ hole size (V h ) is: The fractional free-volume (the ratio of free-volume to the total volume) was found according to fitted empirical formula as [6]: whereA is a constant, its value between 1 and 2 for polymers and I o-Ps is the intensity of τ o-Ps .
PAL is widely used for investigation different aspects of polymer properties [7].
Positron and positronium lifetime in polymer materials may be affected by different factors such as change in degree of crystallinity, blending of polymer, temperature and irradiation of polymer [8][9][10][11]. The present paper is prepared to study the effect of Gamma irradiation on the microstructure of Polyurethane (PU), EUXIT 101 and Epoxy Resin (EP) EUXIT 60 (Swiss Chem.), and to find a correlation between the irradiation dose and the changes in V h and F h values.

Experimental details and data analysis
The investigated PU and EP were irradiated at room temperature to energetic Gamma ray, using 60 Co, which emits mono-energetic 1.17 and 1.33MeV, and has a half-life of 5.3 years. The dose rate was 0.57kGy/h. The samples were irradiated within the dose range (95.76 -957.6) kGy. The positron lifetime measurements were performed using a fast-slow coincidence system Fig. 1, with time resolution of 420±20 ps (FWHM). The positron source activity was 1.54µCi. The positron lifetime spectra were analyzed using PFPOSFIT program [12]. The free volume hole size, and the fractional free volume were calculated using Eqs.(1, 2 and 3) respectively.

Res
Th lifet Th frac    The fitting line in Fig. 3 represents the third-order of polynomial fit. F h = a 0 + a 1 X + a 2 X 2 + a 3 X 3 where X is the gamma-dose obtained for PU, and (a 0 , a 1 , a 2 and a 3 ) are coefficients. Increasing crystallinity in the amorphous sites reduces the freevolume hole size, and their number density (the regions where Ps atoms form and annihilate). The reduction of V h and F h is due to a combined effect of free radicals (induced as a result of γ-irradiation) and increasing crystallinity.
The positron and positronium lifetime spectra for un-irradiated and γ-irradiated EP samples were measured and analyzed using three lifetime components. The results are displayed in Table 2. The free volume hole size V h size and the fractional free volume hole size F h , were plotted as a function of γ-dose in Figs. 4 and 5 respectively. The values of V h and F h for un-irradiated samples are 0.113 nm 3 and 1.711%, respectively. The initial γ-dose in induces a percentage reduction of V h and F h values, indicating that γ-dose results in increasing of cross-linking of EP, and at the same time increasing, the number density of intermolecular spaces available for Ps formation, suggesting the predominant of crystallinity of EP chains due to γirradiation.  As the γ-dose increases up to a total dose of 861.84kGy, V h and F h values decrease gradually with increasing γdose to reach their minimum values. The total γ-dose induces percent reductions of 20.88% in V h and 31.12% in F h values. The reduction in V h and F h values is due to an increase of cross-linking and crystallinity in the amorphous regions of EP, as a resultant effect of γ-ray and the reaction of the reactive free radicals. The increasing of EP cross-linking with γ-dose led to reduce both of, the intermolecular spaces, and their number density.

Fig. 5: Fh for EP as a function of γ-dose.
A fitting curve was obtained also for the investigated EP as shown in Fig. 5 the free-volume hole fraction as a function of γ-dose. The fitting curve in Fig. 5 also represents the third-order polynomial fit.
These results indicate that, the increases of crystallinity in the amorphous regions of EP, resulting in reduction of degradation effect rate, thereby increasing of V h and F h indicates that the intermolecular spaces in EP structure becomes larger due to the effects of γ-dose without altering their number density, which refers as an increase of τ o-Ps value with constant Ps formation probability, I o-Ps . These results reinforced and agree with the idea of, "high γ-irradiation makes radial distribution of EP bonds between molecules in the polymeric chain. Thus, the sample will become more brittle" [13][14][15].
Ps formation and annihilation is strongly affected by many factors such as three dimensional polymer structure, chemical structure, crystallinity and free radicals. Where γ-irradiation, produce free radicals in EP, as a results of interaction of γ-ray with polymer chain in different chemical reaction, such as recrystallization, cross-linking and degradation.
These results are supported by the idea of depending the free-volume on the amount of hardness, since the decrease of free-volume brings the decrease of possible volume for plastic, deformation results in the increase of hardness [10,16]. Increasing the hardness of the material is due to reduction of free-volume because of increase cross-linking that takes place in the amorphous regions in EP.

Conclusions
Comparing the results of the γirradiation effects on PU and EP microstructure the following remarks are concluded: 1. The value of V h for un irradiated samples equal to (0.121and 0.113) nm 3 for PU and EP, respectively. Which means' that, EP is more hardness than PU, because of, the high elasticity of PU. Thus, The increases of crystallinity in the amorphous regions of EP, resulting in reduction of V h , than PU free-volume hole size. 2. Corresponding to V h , also τ o-Ps , values show that for un irradiated samples equal to (2.241and 2.172) ns for PU and EP, respectively. The increment of PU value, due increasing Ps formation hole size. 3. The final γ-irradiation value of τ o-Ps , have the same value for PU and EP, thereby, increasing crosslinking with increasing γ-dose for PU is greater than EP sample. And so PU is more radiation effects than EP sample.
The present results give evidence, that PAL technique can be sensitively employed as a microprobe to monitor the chemical microstructure changes of the polymer under study. Present work has studied the effects of γirradiation, thereby determining the specific dose to be used to enhance or degrade a physical property of the studied polymer.