Mechanical and relaxation properties of -irradiated PVA doped with ferrous sulphate

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1 Polymer Degradation and Stability 66 (1999) 173±177 Mechanical and relaxation properties of -irradiated PVA doped with ferrous sulphate F. Sharaf *, S.A. Mansour, A.M.Y. El-Lawindy Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt Received 2 February 1998; received in revised form 1 June 1998; accepted 4 June 1998 Abstract Films of pure poly(vinyl alcohol) (PVA) and samples doped with di erent concentrations of heptahydrated ferrous sulphate were prepared using a casting method. Mechanical and relaxation properties were measured before and after irradiation. The modulus of elasticity, yield strength, tensile strength ductility and relaxation time were produced from true stress±true strain and stress relaxation measurements. The measured parameters were found to be dependent on both FeSO 4 concentration and on -doses. A cyclic e ect occurs. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Special attention has been paid, in recent years, to conductive±insulator systems due to their increasing applications in industry [1±3]. At the same time, one of the most important and widely studied areas of applied polymer science is that concerned with understanding and controlling the mechanical behavior of semi-crystalline polymers [4±7]. Irradiation of a polymer causes structural and chemical changes that lead to changes in its physical properties. Cross-linking and degradation may occur depending on the process that predominates under the ionizing irradiation conditions. In hydrocarbon polymers, hydrogen atoms are abstracted from the molecular chains and free radicals are formed. These radicals may subsequently recombine to form branched or crosslinked structures [8]. The direct scission of carbon±carbon bonds is also possible [9]. The present work is devoted to studying the e ect of adding di erent concentrations of FeSO 4 on the mechanical and relaxation properties of PVA before and after -irradiation to di erent doses. The optical absorption, refractive indices and Mossbauer e ect were previously measured for such samples [10]. The samples containing higher concentrations of * Corresponding author. Fax: address: fsharaf@ccis.suez.eun.eg (F. Sharaf) FeSO 4 were found to be more stable against irradiation. Also, a cyclic e ect was pronounced for the measured optical properties. 2. Experimental technique Poly(vinyl alcohol) (PVA), molecular weight , supplied by Merck of Germany, was used as a starting material. Pure and di erent concentration (5, 10, 15, 20, 25, 30% by weight of polymer) FeSO 4 doped PVA lms were produced using a casting method, as previously reported [11]. Films of 0.1 mm thickness were obtained. The irradiated samples were obtained by exposing unirradiated samples to di erent gamma exposure doses of 2, 4, 6, 8 and 10 Mrad, using Co-60 cell-200 of Atomic Energy of Canada. The samples were prepared as strips of approximate dimensions cm. A digital force gauge (Hunter Spring- ACCU Force II, 0.01 N resolution) was used for the mechanical measurements. All measurements were performed at room temperature (20 C). The length resolution was 0.02 cm. The modulus of elasticity (P.E. 12%), yield strength (P.E. 10%), tensile strength (P.E. 10%), and ductility, (PE 2%) for all samples, before and after irradiation, were computed from the true stress±true strain measurements. The stress relaxation was carried out under 10% extension ratio. The stress was automatically recorded every 2 min /99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S (98)

2 174 F. Sharaf et al. / Polymer Degradation and Stability 66 (1999) 173± Results and discussion Fig. 1 shows the true stress±strain behavior of PVA doped with di erent FeSO 4 concentrations (5, 10, 15, 20, 25 and 30%). In previous work [6], it was found that the addition of FeSO 4 causes an increase in the intramolecular forces forming a charge transfer complex which prevents the sliding of macromolecules over each other. This may explain the increase of the modulus of elasticity, yield strength, tensile strength and ductility as FeSO 4 is increased, as shown in Table 1. For pure PVA the true stress±strain behavior indicates a typical glassy state, which is accompanied by the formation of necking. A remarkable observation, after the addition of FeSO 4, was that the true stress±strain dependence is rapidly varying indicating a change from a rubber-like to a semi-crystalline state. The state of such crystallinity manifests itself by the occurrence of three distinct lines in the behavior of true stress±strain dependence [7]. The rst portion corresponds to elastic strain obeying Hooke's law. As the slope of this portion is monotonically increasing with increasing FeSO 4 concentration, a developed amorphous state may be identi- ed. The second portion corresponds to the beginning of the neck formation. The length of this portion decreases with increasing FeSO 4 concentration. Accordingly, at this stage of deformation, orientation of crystallites occurs in an initially isotropic specimen. Also the neck formation is shifted to higher strain values and it disappears gradually with increasing FeSO 4 concentration. These observations mean that the partially crystalline PVA is becoming more crystalline by increasing the FeSO 4 concentration. The slope of the third portion is higher than that of the rst, which means less deformation at high strains. Accordingly, the addition of FeSO 4 to PVA polymer changes it from a soft and tough state to a hard and tough one due to the increase of intramolecular interactions. The e ect of di erent -irradiation doses on the true stress±strain behavior of pure PVA is shown in Fig. 2. At small doses, one observes less deformation due to cross-linking which tends to increase the amorphous state. In another words, the -irradiation at doses up to 4 Mrad changes the sample from soft and tough to hard and tough state due to the increase of cross-linking at low doses. At higher dose, the sample is becoming soft and weak. The e ect of -irradiation on the true stress±strain behavior of di erent concentrations of FeSO 4 doped PVA samples was measured. Fig. 3 illustrates these measurements for 5% FeSO 4 and 30% FeSO 4 concentrations, respectively. Fig. 4 represents the calculated mechanical parameters for pure and di erently doped PVA samples at di erent -irradiation doses. These curves show the drastic changes in the mechanical properties due to the e ect of doping and -irradiation. At higher concentration of FeSO 4 the samples are more hard and ductile. Also, cross-linking predominates over degradation up to higher exposure dose, up to 8 Mrad in the case of 30% FeSO 4. This re ects the higher stability of such samples against ionizing radiation when they contain higher FeSO 4 concentration. Measurements of the relaxation time may indicate the degree of interaction between the Fe atoms and the macromolecules of polymeric chains. Long relaxation times indicate higher degrees of interaction while short Fig. 1. The true stress±true strain behavior of PVA doped with different concentrations of FeSO 4. Table 1 The calculated modulus of elasticity in kn/m 2, yield strength in kn/ m 2, tensile strength in kn/m 2 and ductility FeSO 4 (conc. %) Modulus of elasticity Yield strength Tensile strength Ductility (%) Fig. 2. The e ect of -irradiation on the true stress±true strain behavior of pure PVA samples.

3 F. Sharaf et al. / Polymer Degradation and Stability 66 (1999) 173± Fig. 5. The stress relaxation measurements for di erent concentrations of FeSO 4 doped PVA samples. Fig. 3. The e ect of -irradiation on the true stress±true strain behavior of di erent concentrations (5 and 30%) of FeSO 4 doped PVA samples. Fig. 6. The stress relaxation measurements for pure PVA samples exposed to di erent -irradiation doses. Fig. 4. The calculated mechanical parameters for pure and di erent concentrations of FeSO 4 doped PVA samples.

4 176 F. Sharaf et al. / Polymer Degradation and Stability 66 (1999) 173±177 relaxation time indicates smaller degrees of interaction. Stress relaxation measurements were performed and are shown in Fig. 5 for di erent concentrations of FeSO 4 and in Fig. 6 for pure PVA samples at di erent irradiation doses. Also the e ect of -irradiation on samples doped with di erent concentrations of FeSO 4 were measured. Fig. 7 illustrates such measurements for 5% FeSO 4 and 30% FeSO 4, respectively. The stress relaxation curves were tted to the following equation t ˆ 0 e t= where (t) and 0 are the true stress at time=t and 0, respectively, and is the relaxation time. The calculated relaxation times are presented in Fig. 8. The stability of the polymer against -irradiation at higher FeSO 4 concentration is obvious. These results are in agreement with previous [10] optical and Mossbauer measurements. 4. Conclusions Fig. 7. The stress relaxation measurements for di erent concentrations (5 and 30%) of FeSO 4 doped PVA samples exposed to di erent -irradiation doses. We conclude that the addition of heptahydrated ferrous sulphate to PVA causes an increase in the interand intramolecular interactions between the ferrous atoms and chains of the polymer matrix. Consequently the samples are changed from soft and tough to hard and ductile. As a result of -irradiation, the mechanical properties, modulus of elasticity, yield strength, tensile Fig. 8. The calculated relaxation times for all samples before and after irradiation.

5 F. Sharaf et al. / Polymer Degradation and Stability 66 (1999) 173± strength and ductility, show a cyclic behavior. At higher concentrations of FeSO 4 the samples show a noticeable stability against -irradiation where cross-linking predominates over degradation. Acknowledgements The authors are deeply grateful for the kind help and the fruitful discussions of Professor Dr M. El- Ocker and of Professor Dr A.M. Sanad, Faculty of Science, Al-Azhar University. Thanks are due to Mr A.M. Abdel-Salam for his great help during the data collection. References [1] Bueche FJ. Appl Phys 1973;44:532. [2] Chung KT, Sabo A, Pica AP. J Appl Phys 1982;53:6967. [3] Mosad MM. Mater Sci Lett 1990;9:32. [4] Schultz J. Polymer material science. Prentice-Hall, [5] Osaki K, Inoue T, Ahn KH. J Non-Newtonian Fluid Mech 1994;54:109. [6] Govaert LE, Reijs T. Polymer 1995;36(18):3589. [7] Liu Y, Truss RW. J Polym Phys 1994;32:2037. [8] Tager A. Physical chemistry of polymers. Moscow: Mir, [9] Kuleznev VN, Shershnev VA. The chemistry and physics of polymers. Moscow: Mir, [10] Sharaf F, El-Lawindy AMY, Mansour SA. Egyptian J Solids 1997;20(2):235±53. [11] Sharaf F, El-Eraki MHI, El-Gohary AR, Ahmed FMA. Polym Degrad Stab 1995;47:3.