Polystyrene radiation effects

D. Li, H. Xia, J. Peng, M. Zhai, G. Wei, J. Li, and J. Qiao, Radiation preparation of nano-powdered styrene-butadiene rubber (SBR) and its toughening effect for polystyrene and high-impact polystyrene, Radiat. Phys. Chem., 76(11-12) 1732-1735, November-December 2007. 

Recent progress in the radiation effects of ion beams on polymers are reviewed briefly. Our recent work on the radiation effects of ion beams on polystyrene thin films on silicon wafers and time resolved emission studies on polymers are described. 

The present paper reviews the application of ion beams with energy above 10 keV to polymers and describes our recent work on the radiation effects of ion beams on solid polystyrene films studied by solubility change and time-resolved spectroscopy. 

The present chapter describes mainly the radiation effects of various ion beams on spin-coated polystyrene and PMMA films studied mainly by product analysis and by nanosecond ion beam pulse radiolysis. 

Radiation effects of ion beams on polymers such as polystyrene have been studied using very quantitative, homogeneous, and energetically accurate irradiation data obtained by time-resolved and product analysis [30]. Recently main chain scission, ablative decomposition, and positive-negative inversion of PMMA induced by various ion beams have been investigated. The dependence of the beam energy and atomic number of incident ion beams on radiation effects has been considered. 

A number of investigations on radiation effects of polymers at ambient temperature have been carried out and summarized in some recent publications [38-40]. However, there are very few experimental results on cryogenic temperature irradiation of conventional polymers. Figure 5 shows the summary of early experiments on irradiation of polymers at 77 K up to y-ray dose of 2 x 106 Gy [41,42]. As is evident from Fig. 5, the candidate polymers that can be used at cryogenic temperatures are only aromatic based epoxy resins, poly-imides, and polystyrene. This means that the choice of polymers for superconducting magnets to be operated in a radiation environment is rather limited. Therefore, the intention of this chapter is to give information on the radiation tolerance of recently developed polymers. 

Radiation-induced Degradation.—There have been several reports on radiation effects in polymers,288 including single crystals,287 fluoropolymers,288 polyamides,289 polysiloxanes,270 polyethylene and its copolymers,271 polypropylene,272 polyolefins,273 polystyrene and its copolymers,274 poly(vinyl chloride) and related polymers,275 rubbers,278 polysulphones and other sulphur-containing polymers,277 polycarbonate,278 nylon,279 poly(vinylpyridines),280 and wool.281... 

Dong, W., Chen, G., Zhang, W., Radiation effects on the immiscible polymer blend of nylonlOlO and high-impact strength polystyrene (II) Mechanical properties and morphology. Radiation Physics and Chemistry 2001,60, 629-635. 

Ghiszewski, W., Zagorski, Z. P. (2008). Radiation effects in polypiopylene/polystyrene blends as the model of aromatic protection effects. NuMeonika, 55(7), 21-24. 

Radiation effects are remarkably sensitive to molecular structure. G(S) and G(X) values vary from 0.1 to 10 (as can be seen in Table 5.4). The G(X) of polyethylene is about one order of magnitude larger than that of polystyrene, which demonstrates that aromatic rings exert a protective effect. 

The theory of radiation-induced grafting has received extensive treatment. The direct effect of ionizing radiation in material is to produce active radical sites. A material s sensitivity to radiation ionization is reflected in its G value, which represents the number of radicals in a specific type (e.g., peroxy or allyl) produced in the material per 100 eV of energy absorbed. For example, the G value of poly(vinyl chloride) is 10-15, of PE is 6-8, and of polystyrene is 1.5-3. Regarding monomers, the G value of methyl methacrylate is 11.5, of acrylonitrile is 5.6, and of styrene is >0.69. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

A substantial intramolecular protective effect by phenyl groups in polymers is shown by the low G values for Hz and crosslinking in polystyrene (substituent phenyl) and in polyarylene sulfones (backbone phenyl), as well as many other aromatic polymers. The relative radiation resistance of different aromatic groups in polymers has not been extensively studied, but appears to be similar, except that biphenyl provides increased protection. Studies on various poly(amino acid)s indicate that the phenol group is particularly radiation resistant. 

The Acid Effect. The possible mechanistic role of hydrogen atoms in the current radiation grafting work becomes even more significant when acid is used as an additive to enhance the copolymerisation. At the concentrations utilised, acid should not affect essentially the physical properties of the system such as precipitation of the polystyrene grafted chains or the swelling of the polyethylene. Instead the acid effect may be attributed to the radiation chemical properties of the system. Thus Baxendale and Mellows (15) showed that the addition of acid to methanol increased G(H2) considerably. The precursors of this additional hydrogen were considered to be H atoms from thermalised electron capture reactions, typified in Equation 5. 

Creep rates of three glassy polymers are much greater during electron irradiation than before or after. Radiation heating is eliminated as a possible cause. Essentially the same concentration of unpaired electrons and ratio of cross-linking to scission were found in polystyrene samples in the presence or absence of stress. The effects of radiation intensity, stress, and temperature on creep during irradiation are examined. The accelerated creep under stress is directly related to a radiation-induced expansion in the absence of stress. This radiation expansion is decreased by increase in temperature or plasticizer content and decrease in sample thickness. It is concluded that gas accumulation within the sample during irradiation causes both the expansion under no stress and the acceleration of creep under stress. 

The calculated temperature rise for the other two polymers used is less than for polystyrene. Their normal creep rates show a dependence on temperature similar to polystyrene in this temperature range, so that again the effect of the temperature rise on creep rate is negligible relative to the change caused by the radiation. 

Effect of Temperature. The effect of sample temperature on the creep rate of polystyrene in the absence and presence of radiation is shown in Figure 9. The actual creep rate 10 to 12 minutes after application of stress is plotted as a function of the reciprocal of the absolute temperature. 

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