Irradiation polymers

The radiation chemistry has been mainly discussed in terms of degradation reactions (as above) involving the loss of gaseous products and the irreversible change of the stoichiometry [203]. However, more recent results showed that polymers irradiated with radiation deposit-... 

Host irradiated polymers show a continuing change in properties for a long period after irradiation. These post-irradiation effects may be attributed to (1) trapped radicals which react slowly with the polymer molecules and with oxygen which diffuses into the polymer (2) peroxides formed by irradiation in the presence of air or trapped within polymers irradiated in vacuum or an inert atmosphere) and slowly decompose with formation of reactive radicals, usually leading to scission, (3) trapped gases in glassy and crystalline polymers which cause localized stress concentrations. 

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. 

Figure 23. Morphology of rat aortic smooth muscle cells 48 h after seeding on high-density (A, C, E) and low-density (B, D, F) polyethylene. A, B pristine polymers, C, D plasma-irradiated polymers, E, F polymers irradiated with plasma and subsequently grafted with glycine. Cell membrane stained with Texas Red C2-maleimide, the nuclei counterstained withHoechst 33342. Olympus IX 50 microscope, digital camera DP 70. Bar=200 pm.

When considering industrial use of polymer irradiation, the following four questions arise ... 

The cost of polymer irradiation depends on the type of polymer, formulation, and shape of the fabricated polymer. Upgrading polymers by electron beam radiation costs about 1 to 4 cents per pound. Some of this cost can be equalized through material saving in view of property improvement. Surface treatment might be as low as 0.1 to 0.2 cents per pound. 

This book is not intended to give a complete survey of polymer irradiation. For this, we have to refer to such excellent texts as Atomic Radiation of Polymers by Charlesby and Radiation Chemistry of Polymeric Systems by Chapiro. Since these books were issued in 1961 and 1962, more experimental research has been done and more theories have been developed by these authors, who are also the authors of the first two chapters of this volume. New trends have been discovered, and more light has been shed on polymer irradiation by the authors of the following 18 chapters. Thanks to their work and efforts, polymer irradiation is making inroads into the plastic and related industries. 

Radioprotection. The processes of crosslinking and degradation observed in polymers irradiated in the pure state can also be observed in polymers irradiated in solution. The presence of a solvent can intervene in the reaction in several ways thus it allows increased polymer mobility, and some of the radiolytic products of the solvent may react with the polymer or with the polymer radicals, etc. The polymer-water system is of particular interest in that it provides a simple model for some radiobiological systems and can be analyzed far more readily. 

An irradiation-induced expansion could conceivably be caused by the ions, formed as precursors of the radicals, or by thermalized electrons trapped within the polymer. Irradiation induces electrical conductivity in polymers, and this conductivity decays after irradiation is ceased (4, 5). The decay process is accelerated by increased temperature or plasticity of the specimen, presumably by facilitating leakage of the trapped electrons or ions to ground. One might speculate that the sample expands upon irradiation because of the local mutual electrical repulsions of like charges which are trapped in the polymer matrix, and that both increased temperature and plasticizer content diminish this expansion because of charge leakage out of the specimen. It is difficult to prove or disprove this hypothesis. 

Very primary events in the chemical effect of radiations on matter are excitation and ionization of molecules, which result in the formation of neutral free radicals and radical ions. These reactive species play vital roles in the radiation-induced chemical reactions. As they are paramagnetic with an unpaired electron, electron spin resonance (ESR) spectroscopy has been a useful method for elucidating the mechanism of radiation-induced reactions in solid matter where radical species can be trapped temporarily. Since the early days of the chemical application of ESR, this method has been applied very often to the identification and quantification of free radicals in polymers irradiated by radiation [1]. This is probably because, from the view-point of fundamental research, a variety of free radicals are readily trapped in solid polymers and, from the view-point of applied research, these free radicals have close correlation with radiation-induced crosslinking and degradation of polymers. 

Figure 10 CD spectra of orthocopolymer 25 (r = 1) in methylene chloride and tetrahy-drofuran (1 1) solution (T,V), meta-terpolymers 24 (r = 0.02) in rc-hexane solution (A,A), and orthoterpolymer 25 (r = 0.04) in rc-hexane solution ( , ) irradiated with CPL. The filled up-triangle, down-triangle, and square are CD spectra of the polymers irradiated with r-CPL, and the open up-triangle, down-triangle, and square are CD spectra of the polymers irradiated with /-CPL. (Reproduced from Ref. 80 2000, American Chemical Society.)...

Table IV. G(H2> Values for Polymers Irradiated in Film at 35 C under Vacuum to a 2-3 Mrad Dose...

Monomer Irradiation Energy Co-polymer Irradiation Input Co-polymer efficiency... 

FtKIire I , Three types of molecular rcnncnlarion or phwochiumii liquid-cryslalline polymers irradiated with linearly polarised light. 

Method of crosslinking Copolymerization with crosslinking monomers Crosslinking of water-soluble polymer Irradiation of radioactive ray Self-crosslinking Introduction of crystal structure... 

Scheme 6 Industrial applications of monomer and polymer irradiation.

It is known that aromatic compounds containing heteroatoms of nitrogen may have light stabilizing effect at polymers irradiation. Some derivates of carbazole, according to literature and patent data, may be CC and antioxidants during oxidative destruction of polymers. 

Photochemical Properties. A simple experiment was performed to compare the photochemical activity of the MP with the TM polymers. Solutions of TM1 and MP1 in quartz cuvettes with the same absorptivity were irradiated with 60 mj cm-2. The UV-Vis spectra before and after irradiation are shown in Fig. 8. A comparison of the absorption bands after 100 pulses shows that about 50% of TM1 and only 20% of MP1 are decomposed. This confirms clearly that the triazene-containing polymers decompose photochemically much more easily than the polymers without this group. It is important to point out that TM1 contains the same structural unit as MP1 (Scheme 3), but with the additional triazene unit in the repetition unit of the polymer. Irradiation of low concentrations of the polymer in solution can be interpreted as pure photochemical decomposition with nearly no thermal influences. 

Fig. 84 Comparison of fluorescence and optical microscopic images of poly(butyl methacrylate) PBMA (a) and (c), and poly(ethyl methacrylate) PEMA (b) and (d) target polymer sin-face following 355-nm laser-induced molecular transfer of pyrene contained in triazene polymer. Irradiation dose 5 pulses, 200 mj cm-2. The bar denotes 100 jltm in each case. REPRINTED WITH PERMISSION OF [Ref. 360], COPYRIGHT (1998) Elsevier Science...

In this article, the fundamental steps in the photodegradation of polymers are described and ESR studies of polymers irradiated with ultraviolet light are summarized. 

ESR spectra of this polymer irradiated with ultraviolet light have been reported by Kelleher et al. (106) Symons et al. (107) and Tsuji et al. (108). This polymer in the solid state gave an ESR spectrum as shown in Fig. 21, even before irradiation. Irradiation at room temperature caused the increase of this spectral intensity. This apparent eight-line spectrum with a separation of about 5.5 gauss was identified as due to substituted phenoxy radicals (107). 

Type 3 random polymers irradiating from a fullerene spheroid (starburst type or cross-link type) ... 

In view of the specific phase structure of the polymer irradiated, it is of interest to analyze the optical properties (extinction) of Ag nanoparticles embedded in the amorphous carbon matrix (C matrix). For this system, the extinction cross-section spectra versus particle size dependence (Figure 8.6)... 

Lion that the polymer irradiated is completely carbonized, which was used in the simulation (Figure 8.6), does not become a reality when the process lasts for a long time. Below, the variation of the extinction spectra with amount of carbon in the PMMA layer is analyzed in terms of a model that considers the optical properties of silver MNPs covered by the amorphous carbon sheath. 

---