WO1992018582A1

WO1992018582A1 - Tritiated light emitting polymer compositions

Info

Publication number: WO1992018582A1
Application number: PCT/GB1991/000586
Authority: WO
Inventors: Colin David Bell; John Hewer Coles Howse
Original assignee: Amersham International Plc
Priority date: 1990-04-11
Filing date: 1991-04-12
Publication date: 1992-10-29
Also published as: GB2242908A; GB2242908B; CA2106879A1; GB9008268D0; EP0579600A1

Abstract

Transparent light-emitting compositions comprise a mixture of a polymer, e.g. a poly (vinyl aromatic) hydrocarbon labelled with tritium and a first and preferably second organic compound. The first organic compound, which is at least partly bonded to the polymer, may be a known scintillant such as 2,5-diphenyloxazole. The second compound may be a known spectral shifter such as 2,2'-p-phenylene-bis-(5-phenyloxazole).

Description

TRITIATED LIGHT EMITTING POLYMER COMPOSITIONS

This invention concerns light-emitting polymer compositions based on tritium and methods of making them. Tritium has a half life of 12.43 years, and a maximum beta-emission energy of 18.6 keV. When tritium gas or tritium labelled compounds or tritium labelled polymers are mixed with inorganic scintillators such as zinc sulphide, the mixture emits a greenish light and can be used as a light source. This has been known for many years (see US Patent 3033797 and 3342743). The subject has been reviewed in TRITIUM AND ITS COMPOUNDS 2nd Edition (E.A. Evans Published by Butterworths 1974, page 18-20). A disadvantage of such products is that they are necessarily opaque, owing to the presence of zinc sulphide, and so emit only light generated at the surface. A transparent product would emit light generated below the surface.

Radioactive light sources are discussed by G. Foldiak (Industrial Applications of Radioisotopes , Elsevier, 1986, p.386-7), who teaches use of tritium as a gas in thin-walled glass vessels. US Patent 4,889,660 teaches the production of tritiated light emitting polymer sources by mixing a tritiated polymer with a phosphor which may be partly soluble in the polymer. The specific activity is at least 265 Ci/gram of polymer, a high value perhaps intended to counteract the low efficiency by which energy from radioactive decay is transferred to the phosphor. In the Examples, deuterium i s used; it would be difficult or impossible to replace deuterium by tritium at this high specific activity viz 265 Ci/gram polymer because of the radiation damage that would result. Stable light sources of clear scintillating plastic containing carbon-14 (half life 5730 years, beta emission energy 156 keV maximum) have been described by Facey (J. Scientific Instruments, 43(1966)658-659). The low molar specific activities achievable with carbon-14 (maximum theoretical 62.4 millicuries per milliatom of carbon) severely limits the use of this radionuclide in any form for light sources where the highest light intensities are required. In addition to technical limitations, the cost of carbon-14 is several orders of magnitude higher than for tritium and is commercially unattractive for uses in light sources. Tritium is available in megacurie quantities at 100 per cent isotopic purity with a specific activity 29 curies per milliatom of hydrogen. This makes tritium technically and commercially attractive for use in the invention herein described.

For the detection and measurement of radiation, it has been known for many years to use both liquid scintil lators and plastic scintillators. These comprise solutions of organic luminescent substances such as para-terphenyl , in liquids or in solid polymers. Plastic scintillators were reviewed by E. Schram in "Organic Scintillation Detectors" published by Elsevier 1963, pages 67-74. But the scintillators did not contain any internal source of radiation. This invention provides in one aspect a light-emitting composition comprising an intimate mixture of a polymer labelled with tritium and a first organic compound which emits light when subjected to radiation generated by tritium, wherein the first organic compound is at least partly bonded to the polymer. When the f-irst organic compound is at least partly bonded to the polymer, it is found that energy from radioactive decay is more efficiently transferred to the organic compound. Thus a brighter phosphorescence is achieved per unit of radioactivity. Whether the organic compound is bonded to the polymer or not can be determined by Mass Spectrometric measurement on pyrolysed samples or by a simple test: the composition is dissolved in a suitable organic solvent and the solution subjected to chromatography. If complete separation results, then the organic compound was not bonded to the polymer. If no separation or only incomplete separation results, then the organic compound was at least partly bonded to the polymer. The nature of the bond is immaterial. Bonding occurs for example when the polymer is formed from its constituent monomer or monomers in the presence of the organic compound.

The composition includes a first organic compound (including organometal 1 ic compounds) which emits light when subjected to beta- or other radiation generated by radioactive decay of tritium. A number of such compounds have been known for many years and are widely used in liquid scintillation counting for radioactivity measurements (see for examples in Radioisotope Laboratory Techniques by R.A. Faires and G.G.T.Boswell , 4th Edn. Butterworths , 1981 page 161- 165). These include paraterphenyl , 2 ,5-diphenyloxazole (PPO), tetraphenylbutadiene , phenylbiphenyloxadiazole (PBD), 2-(4-tert.butylphenyl)-5-(4-biphenylyl)-1,3,4- oxadiazole(butyl-PBD) , and 2 ,2 ' -p-phenylene-bis- (5- phenyloxazole) (POPOP) . As is known, mixtures of these compounds may be used and may be more effective than the individual compounds.

The composition may also contain a second compound which changes the wave length of the emitted light. Again, such spectral shifters are known, and include diphenyl stilbene, POPOP, and 7-H-benzimidazo (1,2-b)benz(d,e)isoquinoline-7-one(BBQ). BBQ for example changes the wave length of the emitted light to 480 to 500 Angstroms. A combination of POPOP or PPO and BBQ or butyl-PBD has been found particularly effective in our work.

Polymers labelled with tritium are well known, and are most conveniently prepared by labelling a monomer or co-monomer with tritium prior to polymerisation. The polymer should preferably be clear for maximum efficiency at the wave length of the emitted light, and should preferably be resistant to damage by self-irradiation (E.A. Evans, Tritium and Its Compounds, 2nd Edition, Butterworths, London pages 720-721). On these grounds, polymers of vinyl-aromatic hydrocarbons, such as styrene and vinyltoluene, are preferred. A G (scission) value of 0.04 is quoted for irradiation of polymethylstyrene and a G (cross- linking) value of 0.02 which is much less than for other known polymers, see Polymer Photophysics and Photochemistry by J. Guillet Pub. Cambridge University Press, 1985 page 353 et seq. Additional stability is however conferred on the composition by the presence of the first, and optionally second, organic compounds, by virtue of their conversion of beta radiation energy into light, reducing the proportion of energy available for self-irradiation of the polymer.

The extent of tritium labelling is a compromise between several factors. By incorporating 2 atoms of tritium per monomer molecule, it is possible to achieve activities of 600 Ci/g. Such monomers may be diluted with non-radioactive monomer to achieve the overall specific radioactivity required. Activities below about 1 Ci/g are rather unlikely to be useful as illuminating devices but may have a use as light sources for calibration. As the tritium labelled polymer is an expensive material relative to the other components, it will be preferred; to use the minimum required to achieve the desired light output. All polymers including polystyrene suffer from radiation damage, and at high levels of activity this may lead to darkening with less of light output, and eventually to embrittlement and degradation. Labelling to an activity of from 1 to 100 Ci/g, particularly 5 to 25 Ci/g, of composition may be appropriate in many cases, with activities towards the lower end of that range where a service life of more than five years is required.

Tritiated vinyl aromatic monomers may be made by the catalytic partial reduction by tritium of substituted acetylenes. For the purpose of this invention, reduction is carried out with tritium- hydrogen mixtures up to 100 per cent isotopic abundance of tritium as required, preferably in the presence of a platinum or palladium catalyst or other suitable hydrogenation catalyst. After the reduction it is preferable to remove by filtration or by distillation any catalyst from the tritiated monomer. It is also preferable to dilute the tritiated monomer with non- radioactive monomer(s) which have been purified either by distillation or by passage through a column of neutral alumina.

Preferably the tritium-labelled monomer and/or unlabelled polymerisation monomer is deuterated. By using fully deuterated styrene and tritiated styrene, it is possible to double the light output in comparison with non-deuterated monomers.

The concentration of the first organic compound, and the second organic compound if used, should be enough to efficiently convert the beta- radiation into visible light, but not so great as to inhibit polymerisation of the monomer mix or to substantially harm the properties of the polymer. While optimum concentration may vary depending on the nature of the polymer, the extent of tritium labelling, and the nature of the organic compound(s), suitable concentrations are likely to lie in the range 10-250 ⁵ grams of organic compound(s) per litre of non-volatile monomer mix. The concentration of scintillators are optimised for the light output required but too high a concentration will result in self-absorption of the light and thus reduce the efficiency - see Design ¹⁰ Principles of Fluoresence Radiation Converters by

G. eil in Nuclear Instruments and Methods 87, 111-123 (1970).

A cross-linking agent may be included in the monomer mix and may be beneficial in increasing light ¹^ output, as discussed below. For example, with a vinyl aromatic system, up to 50 g/1 of divinylbenzene may be useful.

In another aspect, the invention provides a method of making a light-emitting polymer composition, ⁰ which method comprises providing a reaction mix comprising at least one polymerisable organic monomer, preferably a vinyl-aromatic hydrocarbon, labelled with tritium, and a first organic compound which emits light when subjected to radiation generated by tritium, and ⁵ subjecting the reaction mix to polymerisation conditions. The first organic compound, and a second organic compound if used, are preferably present in solution in the monomer. Preferably, polymerisation of the monomer or monomers is effected in the substantial ° absence of oxygen. Polymerisation may be effected in the presence of Ziegler-Natta catalysts, or by the use of free-radical polymerisation initiators, or simply by heating the reaction mix. The use of Ziegler-Natta catalysts or other suitable catalysts may promote formation of an isotactic polymer which may improve transfer of energy from radioactive decay to the organic compound. Examples of suitable initiators include α-azo-butyronitrile (AZBN); 2,2 ' -azo-bis (2- methylpropionitri le) ; and benzoyl peroxide. During the polymerisation it is believed that, under the influence of the initiator, some covalent bonds are formed between the first and second organic compounds and the polymerising monomers, so that "antennae" are produced which increase the efficiency of light output by ion fluorescence. When the polymerisation reaction is exothermic, careful temperature control of the reaction mix may be needed to avoid thermal decomposition of the organic scintillators. The reaction mix may be shaped prior to polymerisation to generate plastic sheets of desired thickness, rods, filaments, microbeads, capillary tubing, or other desired shapes. After polymerisation, the solid products can also be cut and shaped as desired.

Upon polymerisation of the monomer mix, the light emitted by the composition increases, to an extent that is dependent on various factors. Use of purer reagents; increasing the hardness of rigidity of the product (and for this reason a cross-linking agent may be beneficial); cooling the product, stretching the product or otherwise inducing crystallisation; all these steps may increase light output from a given composition.

The light emitting compositions of this invention are useful wherever a continuous or . intermittent independent light source is required and power lines or batteries cannot conveniently be provided. Some examples are:

Production of electricity by combination with photocells

Light sources for signs, gun-sights, markers on instruments.

Large light sources on airfields and other situations where remote lighting may be required (see also G. Foldiak in Industrial Application of Radioisotopes, Pub. Elsevier 1986, p.386 et seq. and A Novel Light-Collection System for Segmented Scintillation-Counter Calorimeters, V.Eckardt, R. Kalbach, A. Manz, K.P. Pretzl, N. Schmitz and D. Vranic, Nuclear Instruments and Methods, 155, 389-398 (1978).

EXAMPLE 1

Formulation for yellow/green light

Toluylacetylene (37μl) was reduced with tritium (20 Ci) in the presence of palladium or platinum (8mg). The product was removed by distillation under vacuo and the resulting tritiated vinyltoluene was added to a solution of PPO and BBQ in vinyltoluene. Polymerisation of this and the following examples in suitable moulds was achieved by heating in a temperature controlled oven, increasing the temperature gradually from 25°C to 130°C over 6 days and maintaining the mixture at 130°C for a further 24 hours. The polymer mixture was then allowed to cool to room temperature over 12 hours yielding a polymer with a yellow/green emission. Note that all polymerisations were carried out under an atmosphere of inert gas (dry nitrogen). An increase in signal (light output) was observed as the radioactive monomer mixture polymerised and typical light intensities of six microlamberts have been measured. EXAMPLE 2

Formulation for blue light 2-(4-tert-butylphenyl )-5-(4-biphenylyl)- 1, 3, 4 - oxadiazole (butyl-PBD) 25 - 100 g/1 POPOP 1 - 20 g/1

*vinyltoluene 1 - 4 g/1 vinyltoluene balance to 1 litre.

*Formed by reduction of toluylacetylene with tritium, up to an activity of approximately 50 Ci/g as in Example 1.

EXAMPLE 3

Formulation for red light Butyl PBD 35g/l ** peri di - naphthylene (perylene) 5g/l **

5, 6, 11, 12-tetraphenylnaphthacene (rubrene) 2g/l ** dimethyl POPOP 1g/l **

** (see IB Berlman and Y Ogdan, Nucl Inst.

+ Meth, 178 (1980) 411-413) * vinyltoluene 1-4g/l vinyltoluene balance to 1 litre

The main advantages of this invention over previous plastics as light sources are: greater efficiency of light output using tritium incorporated into the polymers; by using the preferred monomers described, tritium released on storage is negligible (no detectable levels) providing improved radiological safety over the examples of the prior art; improved stability of the polymer light sources against deterioration by self-irradiation; actual isotopic abundance of tritium in the final light emitting polymer composition can be less than 1 per cent if required. EXAMPLE 4 3

(a) Reduction of Phenyl Acetylene to [ H] Styrene

In a small hydrogeπation flask (ca. 5ml) was placed phenylacetylene (196 μl), styrene (or styrene- dg, i.e. styrene with 8 deuterium atoms instead of hydrogen atoms, or pentafluorostyrene) containing stabliliser (1 ml) and 10% palladium-on-charcoal catalyst. The mixture was stirred rapidly in the presence of tritium gas (ca. 40 ml, 100 Ci) over 4 hours when uptake of tritium gas has ceased. The [ H) styrene was distilled from the catalyst in vacuo to give a clear solution of [ H] styrene monomer (1.2 ml).

(b) Formation of Light Emitting Polymer

To the above [ H] styrene monomer (43.6 Ci) was added styrene (or styrene-d₈ or pentafluorostyrene) monomer (1.5 ml with stabiliser removed by passage over an alumina N column before use) in which was dissolved divinylbenzene (cross linker monomer) (60 μl), butyl-

PBD (300 rag), and AZBN (3 mg). The mixture (2 x 1.45 ml) was placed in two glass tubes and sealed under nitrogen. The tubes were heated at 70°C for 18 hours. After cooling the polymer was removed from the sealed tubes and pressed between glass plates in a heated press at 120°C to give a flat sheet and assembled between blue photocells.

The following table shows the number of scintillation counts recorded in unit time under various circumstances. Monomer Scintillation Counts in

Monomer Solution Polymer after 70°C/18 hrs

401238

1683467

358952

These results show an increase in the scintillation counts on polymerising the monomer solution. This increase was seen when the monomer was styrene or pentaf luorostyrene, but was very much more marked when the monomer was styrene-d_fi.

Claims

1. A light-emitting composition comprising a mixture of a polymer labelled with tritium and a first organic compound which emits light when subjected to radiation generated by tritium, wherein the first organic compound is at least partly bonded to the polymer.

2. A composition as claimed in Claim 1, which is clear or translucent at a wavelength of the emitted light.

3. A composition as claimed in Claim 1 or Claim 2, wherein there is also present at least one second compound which changes the wavelength of the emitted light.

4. A composition as claimed in any one of Claims 1 to 3, wherein the polymer is a poly (vinyl aromatic) hydrocarbon.

5. A composition as claimed in any one of Claims 1 to 4, formed by subjecting to polymerisation conditions a reaction mixture comprising at least one polymerisable monomer labelled with tritium and the first, and if present the second compound.

6. A composition as claimed in Claim 5, wherein the tritium-labelled monomer and/or an unlabelled polymerisation monomer is deuterated.

7. A composition as claimed in Claim 5 or Claim 6, wherein the reaction mix comprised a vinyl aromatic hydrocarbon monomer labelled with tritium in which the first, and if present the second compound is present in solution.

8. A composition as claimed in any one of Claims 1 to 7, which is labelled to an activity of 1-100 Ci/g.

9. A method of making a light-emitting polymer composition, which method comprises providing a reaction mix comprising at least one polymerisable monomer labelled with tritium and a first organic compound which emits light when subjected to radiation generated by tritium, and subjecting the reaction mix to polymerisation conditions.

10. A method as claimed in Claim 9, wherein polymerisation is effected in the presence of free- radical polymerisation initiator and in the substantial absence of oxygen.

11. A method as claimed in Claim 9 or Claim 10, wherein the reaction mix is formed into a desired shape prior to polymerisation.