Polymers with Electron Beams

The mechanism of electron beam crosslinking is.mainly free radical in nature. It is believed to initially involve ejections of hydrogen radicals from materials like polyethylene as a result of bombardment with high energy electrons  

The hydrogen radical then abstracts another hydrogen from a neighboring polymer molecule and the two radical polymers combine or crosslink  

Wire coatings are often based on polyolefins or on poly(vinyl chloride). Many other electron-beam curable materials for surface coatings, however, are veiy similar in composition to the ultraviolet-light curable ones. No photoinitiators, however, are needed. 

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Overall a customer needs to know under what circumstances it is best to use either the electron-beam techniques of EDS and WDS or the X-ray technique of XRF for an analysis problem. If both are equally available, the choice usually resides in whether high spatial resolution is needed, as would be obtained only with electron-beam techniques. If liquids are to be analyzed, the only viable choice is XRF. If one s choice is to use electron-beam methods, the further decision between EDS and WDS is usually one of operator preference. That is, to commence study on a totally new sample most electron-beam operators will run an EDS spectrum first. If there are no serious peak overlap problems, then EDS may be sufficient. If there is peak overlap or if maximum sensitivity is desired, then WDS is usually preferred. Factored into all of this must be the beam sensitivity of the sample, since for WDS analysis the beam current required is lO-lOOx greater than for EDS. This is of special concern in the analysis of polymer materials. 

Fracture toughness may correlate with the 0 relaxation temperature for the polymer. After irradiation, the 0 relaxation temperature increases with a corresponding broadening and decrease in intensity which can be seen in Figure 1. This result is consistent with the results of Hinkley et. al. (13) who observed the same phenomenon for polyether sulfone irradiated with electron beam irradiation above Tg. 

Polymers of MMA, AAc, and MAA were grafted onto an ultrahigh molecular weight polyethylene (UHMWPE) fiber surface after pretreatment with electron beam irradiation [31]. Sundell et al. [32] pretreated a PE film with electron beams to facilitate the graft polymerization of vinyl benzylchloride onto the substrate. The inner surface of porous PE hollow fiber had also been modified by grafting of glycidyl methacrylate (GMA) polymer after electron beam irradiation [33]. 

Polymer Surface Analysis. The major technique used for the surface analysis of polymers has been X-ray photoelectron spectroscopy (XPS or ESCA). However, this technique is often not adequate to determine the molecular structure of polymers. This has prompted many workers to explore the potential of SIMS for this work (11-16). Significant problems encountered with ion beam bombardment in conjunction with electron beam charge neutralization have been drift in the polymer surface potential and thermal damage from the combined effects of the electron and ion beams. These problems do not exist when utilizing FAB in conjunction with photoelectron charge neutralization. 

In later reports, asymmetric PS- -P2VP were used for similar micelles formation and film deposition processes. After exposing the micelles with electron beam (e-beam) on selected areas, the chemical nature of the irradiated polymer was modified. Those nonirradiated areas could be removed selectively by washing with dimethyl formamide (DMF) or toluene, while the irradiated areas remained intact upon exposure to solvent. A schematic diagram of this process is shown in Figure 5.3. When the micelles remaining on the surface were exposed to... 

The direction of radical reactions that involve competitive, mixed-order kinetics could be changed as dose rate is increased. Bimolecular reactions would be favored in competition with unimolecular (or pseudo-unimolecular) reactions, because of the higher instantaneous radical concentrations. This effect could be encountered in lipids and in polymers. It is responsible for the greater retention of vitamin B] (thiamin) in pork when irradiated with electron beams than when irradiated with gamma rays (Figure 1) [6]. 

This study is divided into two parts the first involves electron beam exposures on COj isolated in rare gas solids to investigate the role of an inert solvent on radiation chemistry further experiments were conducted on diazoketones mixed into polymers to investigate the role of reactive solvents. The second part of the study deals with electron beam exposure of fluorinated acrylates and perfluorinated ethers. These reveal that highly fluorinated acrylates when mixed with non-fluorinated acrylic monomers may be used to produce radiation cured films with a broad range of contact angles. 

Zhao, L., Mitomo, H., Zhai, M. L., Yoshii, E, Nagasawa, N., Kume, T, Synthesis of antibacterial PVA/CM-chitosan blend hydrogels with electron beam irradiation. Carbohydrate Polymers 2003, 53(4), 439-446. 

O Development of Fuel Cell Polymer Electrolyte Membranes by Radiation-Induced Grafting with Electron-Beam Irradiation... 

Polymer Surface Oxidation Induced by Irradiation with Electron Beams Svorcik et al. [96] have irradiated PE foils, 15 pm thick (Mjj = 180,000, p = 0.945 g/cm ) in air and at room temperature, with a 14.89 MeV electron beam. The electron flux was 247 Gy/min and the samples were irradiated to fluences ranging from 57.6 to 576 kGy. 

Irradiated PVDF and poly(VDF-co-TrFE) copolymer possess ferroelectric properties that allow the use of such fluorinated polymer in the domain of captors, sensors, and detectors [47,194]. Another interesting property of crosslinked poly(VDF-co-HFP) copolymer is their insolubihty in organic solvent [195]. Cured fluorinated polymers can be processed as membranes for many electrochemical applications such as fuel cell and batteries [196]. For example, a poly(VDF-co-HFP) copolymer has been crossUnked with various systems such as polyols [197], by irradiation with electron beam or y-rays [197] or with aliphatic amines [198] in order to elaborate a solid polymer electrolyte for non aqueous lithium battery [197,198]. This electrolyte is particularly interesting for its ionic conductivity, its adhesion with an electro-conductive substrate and also remarkably enhanced heat resistance. 

A third disadvantage of studying polymers with electron microscopy is the radiation damage due to the electron beam. There are several primary and secondary irradiation effects, which can, on one side, damage the polymer, but on the other hand, contribute to a contrast enhancement, for example, if in polymer blends the thickness or the density in one part of the specimen is reduced by evaporation of volatile fractions of polymer chains, as in PVC/SAN blends (see Fig. 3.15) or if secondary cross-linking effects in semicrystalline polymers are stronger in the amorphous regions than in the crystalline ones, as in PEs [1,15,16]. 

Now-a-days electron beam radiation processing has wide applications particularly in the wire, cable, coating and the tire industries. This chapter deals with the processing of polymers by electron beam radiation and the properties as well as the applications of modified polymers. 

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