Chemistry of Radiation Degradation of Polymers - American Chemical

Chapter 24

Chemistry of Radiation Degradation of Polymers James H. O'Donnell

Downloaded by STANFORD UNIV GREEN LIBR on May 6, 2013 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch024

Polymer Materials and Radiation Group, Department of Chemistry, University of Queensland, Brisbane, Queensland, Australia 4072 High energy radiation produces ionization and excitation in polymer molecules. These energy-rich species undergo dissociation, abstraction and addition reactions in a sequence of reactions leading to chemical change. Scission and crosslinking of the polymer molecules, formation of small molecules and modification of the chemical structure of the polymer are responsible for the changes in material properties. The identification and measurement of these changes utilizing solution characterization methods, ESR spectroscopy and pulse radiolysis of radical and ionic intermediates, GC/MS analysis of volatile products and NMR and FTIR spectroscopy of chemical structure will be considered. The yields of the different chemical changes assist in understanding the mechanism of radiation degradation. Relationships between the chemical structure of the polymer and the radiation resistance are of fundamental importance tor practical applications. Knowledge of the chemistry of radiation degradation of polymers is becoming of increasing importance on account of the utilization of polymeric materials in a variety of radiation environments, the use of radiation for sterilization of medical equipment, and the development of radiation processes for modifying the properties of polymeric materials. There have been investigations of the effects of radiation on polymeric materials since the construction of the first nuclear power plants in the 1950's, and this application has provided a major incentive for research in the field. There is now renewed interest in the radiation degradation of polymers with the need to establish the safe service lifetimes of polymer materials as the ages of these nuclear facilities increase. Moreover, there are new applications of polymer materials in radiation environments which require greater knowledge of the anticipated rate of deterioration in their performance. Particle accelerators for high energy nuclear physics require polymeric materials to be used in radiation environments at extremely low temperatures, e.g. in super-conducting magnets. Satellites, space stations and space vehicles require light weight polymeric materials resistant to radiation under conditions of nigh vacuum and alternating high and low temperatures.

0097-6156/91/0475-0402S06.00/0 © 1991 American Chemical Society

In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Chemistry of Radiation Degradation of Polymers 403

Types and Characteristics of Radiation Radiation, in the present context, means radiation with sufficient energy to produce ionization (2,2). In practice, this is most likely to be 7-rays from radioactive Co (the radioactive isotope of cobaltoroduced by neutron irradiation of ^ C o in a nuclear reactor) or from 'Cs (the radioactive isotope of caesium recovered from thefissionproducts of uranium in nuclear reactor fuel), or electrons produced as high energy beams in accelerators. There are heavier particles, including protons, alpha particles, and ions of higher elements in the periodic table produced by radioactive isotopes and, increasingly, by accelerators. To summarize: Downloaded by STANFORD UNIV GREEN LIBR on May 6, 2013 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch024

1 3

Photon

Particle

X-rays 7-rays

electrons protons α-particles heavy ions

Absorption of radiation Photons transfer energy to substrate molecules by three processes. Very high energy photons (>4 MeV) produce an electron and a positron (pair production) which subsequently undergo annihilation; this is not normally a significant process for irradiation of polymers. Most energy is deposited in the substrate by Compton scattering, whereby the ejection of a valence electron is accompanied by deflection of the incident photon by the electron cloud around the atom. At lower energies, the incident photon is completely absorbed by the substrate atom to produce ionization. Thus, the radiation chemistry of photon radiation (7 and X rays) is largely due to electrons. Tne energy absorption in a polymer from irradiation by electrons occurs through the two processes of ionization and excitation. Both ions and excited species may be involved in subsequent chemical reactions or they may be neutralized or deactivated. The relative importance of chemical change resulting from ionic, radical and excited state intermediates is an important aspect of research into the radiation chemistry of polymers. Measurement of Dose Absorbed Any quantitative estimates of chemical changes produced by radiation require measurement of the energy absorbed in the substrate. Therefore, dosimetry is a vital part of radiation chemistry. A variety of techniques have been used to measure absorbed dose; in practice the choice depends on convenience. Some common dosimeters are: Calorimetry Faraday cup Photographic film Coloured ions in glass Ceric ion Polyethylene (H2)

Ionization chamber Solid-state electronics Dyes in plastic Ferrous ion (Fricke) Alanine (ESR)

Colour development in plastic film is very convenient in industrial facilities and especially for electron accelerators. Ferrous and ceric ion dosimeters are chemical systems which are widely used for 7-irradiation.


RADIATION EFFECTS ON POLYMERS

404 Radiation Units

Thexlassical unit for the intensity of a radiation source was the curie (Ci) = 3.7 χ 10 disintegrations per second; the modern SI unit is the becquerel (Bq) = 1 disintegration/s. The absorbed dose was measured in electron volts (e V) per g (convenient for electron accelerators), and then in rad (=0.01J/kg), and the SI unit is the gray (Gy) = 1 joule/kg. Thus, the amount of chemicalchange can be related to the energy absorbed using mol per kg. The G value equal tathe number of events per 100 eV (equivalent to 16 atto joules; 1 aJ= 10' J) of energy absorbed has been customarily used to measure radiation chemical yield, but /imol/J is now recommended (1 μπιοΙ/J = 10 G). Αυ


18

Penetration of Radiation The depth profile of the absorbed energy is extremely important for electrons and X-rays in the mm range and for positive ions, when the penetration depth decreases rapidly into the μπι range with increasing mass. A typical dose versus depth profile for electrons is shown in Figure 1. Tne surface effect, i.e. the variation in dose by about 60% between the surface and the maximum, and the decrease in dose beyond the maximum at about 50% penetration cause substantial inhomogeneity in dose through a thick sample (relative to the penetration range) and a serious over-estimation of the dose for thin samples unless the dosimeter has the same thickness as the sample. Aspects of Radiation Chemistry Direct and Indirect Action. High-energy radiation is absorbed in a polymer by interaction with the valence electrons of the atoms. It is non-specific spatially and with respect to different atoms and groups, except for their electron densities, in contrast to photochemistry, where the photons are absorbed by specific chromophores, so that there is high specificity of energy absorption within molecules. An important example is irradiation of polymers in solution. High-energy radiation is absorbed by both the polymer ana the solvent producing ions, radicals and excited states. The chemical changes in the polymer will resultfromreactions of these species formed in the polymer (direct action) and in the solvent (indirect action). By contrast, the solvent is normally chosen to be non-absorbing in photochemistry. Energy Transfer. Although the absorption of energy is spatially and molecularly random, chemical change may be quite non-random. One important process enabling this spatial selectivity is energy transfer, which may be intra- or intermolecular. Energy transfer along polymer molecules and energy trapping is currently an importantfieldof research in radiation chemistry of polymers. As a result of energy transfer, molecular components present in only small amounts may be the main sites of chemical change. Hydrogen Transfer. Polymers contain hydrogen atoms as a major component of their molecular structures and C-H scission is an important radiation chemical reaction followed by abstraction reactions by the liberated Η atoms. The intermediate radical and ionic species may undergo Η atom transfer, a process driven by the excess energy in the system and by tne formation of radicals of lower energy. Η transfer can proceed in a sequence of steps, resulting in movement of radical and ionic sites over long distances, and this process may play an important role in crosslinking.


24. O'DONNELL

405 Chemistry of Radiation Degradation of Polymers


Primary and Secondary Products. Some molecular structures are particularly reactive with intermediate species in radiation chemistry. A very important example is the reaction of C=C double bonds with radicals (scavenging). The C=C bonds may be present in the original polymer, or formed during irradiation, e.g. by elimination of H2. Thus, an unsaturated molecule containing C=C bonds may be a primary product of irradiation, and these C=C bonds may react with radical intermediates to form secondary products. The yields of botfi the primary and secondary products will be dose dependent as shown in Figure 2. Temperature Rise from Energy Absorption. Absorption of radiation in a polymer is thefirststage in a sequence of processes which leads to chemical change. However, only a portion of the energy is utilized in the chemistry and most is converted into molecular excitation or heat. Polymers have low coefficients of thermal transfer so that the absorption of energy can lead to significant temperaturerises.This will be particularly important for electron irradiation at high dose rates. Not only do the rates of chemical reactions increase with temperature (the normal Arrhenius behaviour), but if the glass transition (Tg) or melting (Tm) temperatures are exceeded, dramatic changes in material properties can occur. Small molecular products are always formed on irradiation of polymers. They are frequently trapped in glassy or crystalline polymers, but cause foaming above Tg or Tm. Post-Irradiation Effects. Some chemical change occurs during irradiation of polymers and some occurs subsequent to irradiation. There are several causes of these post-irradiation effects. Radicals are trapped in irradiated polymers and react over a period of time, the rate depending on the reactivity of the * radicals, the mobility of the matrix, and the diffusion of oxygen into the sample. Oxidative reactions are normally degradative, i.e. lead to scission, and cause deterioration in mechanical properties. Post-irradiation reactions of radicals can be observed by electron spin resonance (ESR) spectroscopy if the polymer is irradiated below Tg or Tm. Irradiation in air leads to the formation of peroxides and these compounds have characteristic rate versus temperature relationships for decomposition, usually with significant rates in the range 50-150 °C. Therefore, polymers irradiated in air are likely to deteriorate slowly on standing at ambient temperature and more rapidly if heated. Small molecule products formed during irradiation will have slow rates of diffusion in glassy and crytalline polymer, except for H2. Consequently, hydrocarbons, carbon monoxide, carbon dioxide and monomers will be trapped in the polymer, causing internal stresses which may be very large. This internal stress results in the development of crazing and 0 1 cracks in the polymer with time. This phenomenon is readily demonstrated in glassy polymers such as poly(methyl methacrylate). Radiation Environment. There are two important characteristics of the environment in which a polymer is subjected to irradiation: (1) chemical, and (2) thermal. The chemical environment is normally vacuum or gaseous. The gas may be nitrogen, helium, air, oxygen, chlorine, sulfur dioxide, etc. Chemically "inert" gases may not be inert to radiation, as energy transfer can occur to other components and result in excitation and chemical reaction. Oxygen readily forms peroxy species by addition to radicals and has a major enhancement effect on the radiation degradation of most polymers. The use of reactive gas environments has great potential for radiation chemical modification of polymers, especially for surface treatments, but has not been investigated widely.



406


Penetration /

mm

Figured. Depth profile for the dose absorbed in a substrate of density lg/cnr for 1 MeV electrons.

Dose

Figure 2. The dose dependence of primary (—) and secondary (—) products in radiolysis.


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Chemistry of Radiation Degradation of Polymers 407

The Sequence of Events in Radiation Chemistry The effects of radiation on polymers result from a sequence of events which can be divided into: (1) physical - energy absorption and transfer, (2) physicochemical - ionization and excitation, (3) chemical - radical-molecule, ionmolecule, radical-radical and ion-ion reactions, (4) morphological structural changes, (5) material properties.


Effects of Radiation on Polymeric Materials The properties of polymeric materials are affected by radiation as a result of the chemical changes in the polymer molecules. These changes include molecular weight and structure. Secondary effects are on crystallinity, and on molecular architecture in the amorphous phase. "Degradation" of polymers is frequently taken to mean reduction in molecular weight, or deterioration in some desirable property, but these are unnecessarily narrow definitions, as an increase in modulus due to crosslinking may be beneficial in one application, but the accompanying decrease infracturestrain may be a disadvantage in another application. Therefore, it is appropriate to consider degradation as any change in the molecular, morphological or material properties of a polymer resulting from irradiation. Radiation-Induced Changes in Polymers The molecular changes produced in polymers by radiation may be classified into: 1. scission and crosslinking of the polymer molecules, leading to decrease or increase in molecular weight and the possible development of an insoluble or gel (g) fraction of the polymer above the gel dose, Dg, wnich increases with dose up to a limit, which depends on the ratio of scission to crosslinking. 2. evolution of small molecular products, such as H2, CO, CO2, CH4, depending on the composition of the polymer. Identification and measurement of these volatile products provides valuable information about the mechanism of the radiation chemical reactions. However, they can contaminate the environment of the polymer - a serious problem in space applications of polymeric materials. Also, for irradiations at temperatures below Tg of amorphous polymers, the normally volatile products are likely to be trapped in the polymer, and the internal stresses will cause post-irradiation deterioration of the polymer through crazing and cracking. 3. changes in the molecular composition and structure of the polymer molecules, including loss and formation of unsaturation. An example is coloration in poly(vinyl chloride), PVC, from transparent through yellow and orange to black after low radiation doses due to the formation of conjugated C=C unsaturation. Measurement Techniques Molecular Weight. Scission and crosslinking decrease or increase, respectively, the molecular weights of the polymer molecules. Therefore, measurements of the changes in molecular weight averages or distribution with dose can quantify these processes. Viscometric measurements in solution to give [η] indicate whether crosslinking or scission predominates. If only scission occurs and the mol. wt. distribution is initially tne most probable, then [η] values will give Mv,



408


provided that Κ and a for the Mark-Houwink equation are known. Mn may be derived from Mv if the breadth of the MWD is known, which then enables the scission yield, G(S), to be calculated. However, [η] values can be very misleading if the initial mol. wt. distribution is differentfromthe most probable. An initially broad distribution will narrow to the most-probable distribution with irradiation if there is only scission, and G(S) will be over-estimatedfrom[η] values. Crosslinking reduces the ratio of hvdrodynamic volume to mol. wt. compared to linear molecules and [η] values will then lead to underestimation of the mol. wt. Estimated corrections have been published. Gel permeation chromatography, GPC, is used very widely to determine average molecular weights (and the mol. wt. distribution) on account of the experimental simplicity of the technique. Calibrations using monodisperse polystyrene standards must be corrected to the polvmer under investigation, either by Q factors or the universal calibration, although characterized, polydisperse samples can be used. GPC suffersfromthe same hydrodynamic volume problem as viscometry when crosslinking occurs, but this problem is frequently ignored. Measurements of Mn by osmometry or of Mw by light scattering provide absolute average molecular weights which enable reliable values or G(S) and G(X) to be derived for the polymer. However, these methods require careful experimental technique, and are limited by the solubility of the polymers in suitable solvents, especially for compatibility with the membrane in osmometry. The techniques are also limited by tne mol. wt. range of the instruments. Ultracentrifugation has been largely neglected as a technique for measuring the mol. wt. averages and distributions of irradiated polymers, perhaps because it is a technique used mainly by biochemists. Sedimentation velocity measurements can give the mol. wt. distribution and Mw. Sedimentation equilibrium enables Mw and Mz to be calculated. Greater use of this technique is warranted. Soluble Fractions. Polymers in which crosslinking predominates over scission, as determined by G(S)

Chemistry of Radiation Degradation of Polymers - American Chemical

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