Irradiation polyethylene

Intentional hranching may improve the properties of the product polymer through grafting. A graft copolymer can he obtained by creating active sites on the polymer backbone. The addition of a different monomer then reacts at the active site and forms a branch. For example, polyethylene irradiated with gamma rays and then exposed to a reactive monomer, such as acrylonitrile, produces a polyethylene-polymer with acrylonitrile branches ... 

Figure 1. Ultraviolet absorption spectra of Marlex-5003 polyethylene irradiated to 421 Mrad at liquid nitrogen temperature in vacuo. Curve 1 spectrum taken at liquid nitrogen temperature (Int) after irradiation. Curve 2 spectrum taken at room temperature immediately after warming to room temperature (rt). Curve 4 spectrum taken at rt after standing for 46 hours at rt in vacuo. Curve 3 spectrum taken at Int after the 46 hours at rt...

Fig. 3. Dependence of the concentration of the alkyl radical in polyethylene irradiated by y-rays at 77 K in vacuum on the radiation dose (1 Mrad = 10 kGy). The insert shows the cw-ESR spectrum due to the alkyl radical...

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

The concentrations of structures produced in irradiated polyethylene are on the order of 1 per 10,000 carbon atoms for absorbed doses of approximately 2.0 Mrad. Although the approach of examining polyethylenes irradiated with absorbed doses less than the gel dose placed a premium on sensitivity, we were able to detect the first direct radiation induced long chain branches in high density polyethylene (4). 

Fig. 1. Change of ESR spectrum oflow-density polyethylene irradiated with ultraviolet light in nitrogen atmosphere with increasing temperature irradiation time, 2 hr, irradiation temp, at —196° C. G represents an amplifier gain setting. The separation between the two Mn++ peaks is 86.7 gauss. [Polymer J. 2, 606 (1971),...

Conjugated dienes, trienes and higher polymers were detected in polyethylene irradiated at room temperature [289] and the first two only for irradiations at 77°K [290]. The ultraviolet absorption bands are given in Table 6. The G value for diene formation is 0.25 at zero dose and 35° or 120°C. A value of 0.15 was obtained at the same temperatures for a 27 Mrad irradiation [291]. 

CHEMICAL YIELDS FOR CROSSLINKING (GCL) AND MAIN CHAIN SCISSION (Gcs) IN POLYETHYLENE IRRADIATED IN VACUO AT ROOM TEMPERATURE [287]... 

Figure 2. Ultraviolet absorbance as a function of wavelength. All spectra taken at liquid nitrogen temperature. Martex-6002 linear polyethylene irradiated in vacuum at liquid nitrogen temperature to 40 Mrads. Curve I, immediately after the irradiation curve 2, after annealing at 150°K. in vacuum for one hour curve 3, after annealing at room temperature in vacuum for three hours Curve 4, after exposure to air at room temperature for four days...

Another approach to the problem of the identification of transient species in irradiated polyethylene is through the use of infrared spectroscopy. Figure 4 illustrates some of the infrared absorption bands pertinent to this work. The spectra illustrated in Figure 4 were taken of polyethylene irradiated at 77 °K., dotted line, and then after heating to... 

Figure 5. Infrared spectra taken at liquid nitrogen temperature of Marlex-6009 polyethylene irradiated to 220 Mr ads at room temperature. Different spectra after different annealing times and temperatures...

An aliquot of the oil (1-10 g) was weighed into a clean quartz vial and sealed in a polyethylene irradiation vial. A solution of Cd(N03)2 in 0.1 M HNO3 containing 35.0 yg Cd/ml was used as a standard. Samples and standards were irradiated for 4-16 hours and allowed to decay for 4-8 hours after irradiation. Each oil sample was transferred to a 200-ml beaker, and the oil remaining in the irradiation vial was then washed out with benzene. 

The presence of H-crosslinks in polyethylene irradiated in solid state has been ruled out in previous NMR studies. A different behavior can take place in the molten state, where the mobility of radicals is higher. Under these conditions, NMR studies confirm the presence of some H-crosslinks (Perez and Vanderhart 1988). 

Ikada Y, K. Nakamura, S. Ogata, et al. 1999. Characterization of ultrahigh molecular weight polyethylene irradiated with gamma-rays and electron beams to high doses. / Poly Sci Part A Poly Chem 37 159-168. 

O Figure 30.4 illustrates a Cr, Au, and Mo pellet ready for irradiation next to a sample of ceramic material. They are irradiated in a polyethylene irradiation vial using a pneumatic transport system and 5 days after irradiation, the Cr, Au, Mo pellet is counted to measure the... 

Next to an open polyethylene irradiation vial is an assembly consisting of a 150 mg sample of ceramic material to the left and a smaller diameter neutron-spectrum monitor (in the form of a Cr, Au, Mo pellet) ready for irradiation... 

Dole and co-workers have reported yields of alkyl free radicals in polyethylene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore, Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay by a first-order process, and they attributed this to reactions between alkyl radicals within isolated spurs. The persistent free radicals on warming to room temperature are the allyl radicals II. The impact of long-term stability of radical species on the stability of polyethylene has been underlined by studies of Jahan and co-workers (150-157) of ultrahigh molecular weight polymer used in medical implants. 

Figure 5.9 Oxidative degradation of polyethylene irradiated in the presence of air at different dose rates (as indicated in the graphs), (a) Relative ultimate tensile strength and...

An alternative type of crosslink, known as a Y-link, has been periodically suggested, but only recently has evidence been obtained for formation of these crosslinks. Randall et al.(9) identified Y-links in the solution i C NMR spectra of polyethylene irradiated below the gel dose. They suggested that these crosslinks were formed by addition of a backbone radical to a terminal double bond on another molecule (mechanism II in Fig. 2). 

E. Suljovrujic. Post-irradiation effects in polyethylenes irradiated under various atmospheres. Radiation Physics and Chemistry 89,43-50, August (2013). 

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