Radiation resistance polymers

HEXAFLUOROBENZENE The development of commercial routes to hexafluoroben2ene [392-56-3] included an intensive study of its derivatives. Particularly noteworthy was the development of high temperature lubricants, heat-transfer fluids, and radiation-resistant polymers (248). 

Many challenging industrial and military applications utilize polychlorotriduoroethylene [9002-83-9] (PCTFE) where, ia addition to thermal and chemical resistance, other unique properties are requited ia a thermoplastic polymer. Such has been the destiny of the polymer siace PCTFE was initially synthesized and disclosed ia 1937 (1). The synthesis and characterization of this high molecular weight thermoplastic were researched and utilized duting the Manhattan Project (2). The unique comhination of chemical iaertness, radiation resistance, low vapor permeabiUty, electrical iasulation properties, and thermal stabiUty of this polymer filled an urgent need for a thermoplastic material for use ia the gaseous UF diffusion process for the separation of uranium isotopes (see Diffusion separation methods). 

The radiation sensitivity of polymers and monomers is characterized by a G value the number of radicals formed per 100 e.v. (16 aJ) absorbed. Radiation sensitive groups include -COOH, C-halogen, -S02-, -NH2 and -C=C, Radiation resistant groups are aromatic rings. It appears that the presence of aromatic moieties also offers some degree of radiation protection to the polymer chain as a whole. 

Clough, R.L., Radiation resistant polymers, in Encyclopedia of Polymer Science and Engineering, Vol. 13, Kroschwitz, J.I., Ed., Wiley, New York, 1986, 669. 

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. 

Recently there has been increasing interest in studies of the effects of high energy radiation on polymers. Some of this interest has arisen because of the use of polymers as resists in the microchip industry, and some through the search for radiation resistant polymers for the aerospace and other high technology industries. 

Radiation-sensitive polymers are used to define pattern images for the fabrication of microelectronic devices and circuits. These polymers, called resists, respond to radiation by either chain scission (positive resists) or by crosslinking (negative resists). In positive resists, the exposed areas dissolve selectively by chemical developers in negative resists, the exposed areas are insoluble and remain after development. 

Sulfur dioxide was the major volatile product and was used as a probe to correlate the radiation resistance with polymer structure. The use of biphenol in the polymer reduced G(SO ) by 60% compared with bisphenol A based systems (Bis-A PSF). Surprisingly, the isopro-pylidene group was shown to be remarkably radiation resistant. The ultimate tensile strain decreased with dose for all polysulfones investigated and the rate of decrease correlated well with the order of radiation resistance determined from volatile product measurements. The fracture toughness (K ) of Bis-A PSF also decreased with irradiation dose, but the biphenol based system maintained its original ductility. 

The mechanical properties exhibited by a polymer after irradiation are a complex function of molecular weight and molecular weight distribution and the number and type of new structures formed. Thus, it is difficult to draw structure/radiation resistance conclusions from the change in the mechanical properties alone. However, the changes in mechanical properties are direct indications of the ultimate usefulness of the polymer in a radiation environment. 

In this paper, we examine the relationship between radiation resistance and polymer structure using volatile product and mechanical property measurements. 

The dependence of the volatile product yield with structure can be a very sensitive probe of radiation resistance and the protective effect of aromatic rings. G(H ) was observed to decrease from 5.6 to 0.038 for cyclohexane (3) and benzene (A) after gamma irradiation at ambient temperature. Since all polymers under investigation contained the sulfone moiety, G(SO ) (Table III) is an ideal probe for radiation resistance for this series. 

This research demonstrates the utility of a well-defined set of polymers with carefully controlled structure for relating structure to radiation resistance. The presence of the isopropylidene group in the polymer apparently had little effect on the radiation resistance of the polymer, as determined from volatile product yields, contrary to initial expectations. G(CH ) was extremely small, indicating that isopropylidene bond scission is of a low probability. This was further confirmed from G(SO ) measurements. 

The increase in the modulus for Bis A PSF and Hq/Bp PSF with irradiation indicated that crosslinking predominated for both polymers and that the crosslink structures were probably basically similar. Hq/Bp(50) PSF was considerably more radiation resistant than Bis-A PSF, as shown by the rate of decrease in the elongation at failure. For both polymers, there was an initial rapid decrease in the elongation at failure followed by a slower decrease. This effect was also demonstrated by the variation in the fracture toughness (KI(.) with irradiation for Bis-A PSF. This work with cobalt-60 gamma radiation complements earlier studies of these materials using high dose rate electron beam irradiation (6). 

Polymer encapsulation technology (PET) was designed to stabilize radioactive materials and wastes. Polymer encapsulation uses nonvolatile polymers with excellent heat resistance, low water solubility, chemical stability, and excellent radiation resistance. Once materials have been mixed with the encapsulant, the mixture expands and hardens. This process prevents radioactivity from escaping and confines radioactive particles to the polymer structure. 

A novel approach to increase the sensitivity and the spatial resolution is to use a polymer that can be cut just in half by ionizing radiation. Because the selective scission of the polymer skeleton does not leave a longer fragment that is difficult to be dissolved, the sensitivity and the resolution are expected to be improved [14,15] by using such a polymer as a radiation resist (see Fig. 1). 

Although the irradiation of 200 kGy decomposes about 80% of polystyrene in toluene by the dissociative electron attachment, the yield of the decomposition is only 20% for solid toluene. Because of its low efficiency of scission, the coupled polystyrene may not be a polymer suitable as a radiation resist. However, the present study has shown that a polymer that can be decomposed into two equivalent skeletons by ionizing radiation is possible to be... 

We report here plasma etch rate data for a series of vinyl resist polymers with a wide range of side chain substituents. The results of this study are valuable because they provide, when combined with other radiation chemical test data. Improved design criteria for making improved high performance radiation resists. Structural fomulae and chemical nomenclature plus acronyms for the vinyl polymer systems studied are compiled below ... 

Areas of application of radiation degradation of polymers can be usefully classified into (1) modification of polymers, e.g. molar mass or structure, (2) radiation-sensitive polymers, and (3) radiation-resistant polymers. Within the constraints of the required material properties of the polymers, either maximization or minimization of the response to radiation is usually the objective. 

The presence of aromatic groups in polymers greatly reduces their radiation sensitivity. Aromatic polysulfones are commercially important engineering plastics with high temperature resistance and also show good radiation resistance (16). Development of polymers with improved radiation resistance should be possible by copolymerization of other aromatic structures into the chain. 

The GPC technique was used to determine gel formation. Figure 13 shows the soluble fraction of aromatic polysulfone I measured with this technique after irradiation at 30 and 150°C. The relative radiation resistance of different polymers can be obtained by comparison of the gel doses (the highest dose for complete solubility of the polymer) provided that the initial molar masses of the of the polymers are known, or from G(S) and G (X) values these can be derived from the dose dependence of the soluble fractions beyond the gel dose, using a Charlesby-Pinner, or Saito-type plot with allowance for the molar mass distribution. 

The ESR measurements illustrated above require only small doses of radiation and are non-destructive of polymer. They may provide the best method for preliminary evaluation of the relative radiation resistance of different polymers. We are currently investigating whether this idea is quantitatively valid for a variety of polymers. 

This study is investigating the possibility of obtaining a silicone polymer having good radiation resistance, with retention of elastomeric properties. The main area of interest is the resistance to radiation of blends and block copolymers in which an aromatic component can form a separate microphase... 

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