Polymer cross-linking radiation chemistry

It is important to recognize that polypropylene, which is the major constituent of TPO, is a typical degrading-type polymer in the radiation chemistry of polymers, i.e., once a free radical is formed on a polymer chain, the free radical unzips the chain rather than cross-links. CASING effect was first found with polyethylene [24], which is a typical cross-linking-type polymer. The same CASING effect, however, could not be anticipated with the treatment of the degrading-type polymers because the degradation of substrate polymer enhances the extent of weak boundary layer. 

Feller, R. L., "Cross-Linking of Methacrylate Polymers by Ultraviolet Radiation", Preprints of papers presented at the New York Meeting, Division of Paint, Plastics and Printing Ink Chemistry, American Chemical Society, Sept., 1957, 17, No. 2, 465-470. 

Controlling the Crosslinking Density. The selective, controlled manipulation of polymers using radiation chemistry can be achieved by preferential irradiation shielding. By using a shield made of selected materials, of a certain shape, the overall properties of the irradiated polymer may be controlled and tailored. Thus, the cross-linking density can be locally controlled in a direction perpendicular to the direction of irradiation by (34) ... 

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 history of radiation chemistry of polymers started in the early 1950s (Chapiro 1962 Charlesby and Alexander 1955). Poly(Ai-vinyl pyrrolidone) (PVP) was and still is an often applied polymer, also as hydrogel, in medicine and pharmacy. In 1955 Charlesby and Alexander first reported on cross-linking of PVP (Charlesby and Alexander 1955). Since then various other water soluble polymers have been radiochemically cross-linked, even for creating new biomaterials (Hoffman 1981). Hydrogels can be synthesized by radiation techniques in different ways ... 

The radiation chemistry of resist polymers is governed by two main processes, namely, backbone scission and cross-linking. Whether the polymer will act as a positive or negative resist resin under lithographic imaging depends on whether backbone scissioning or the cross-linking process is dominant. ... 

The two main classes of NMR experiment applied to the study of the radiation chemistry of polymers are concerned with measurement of either the chemical structure of the polymer after irradiation, or the changes in the physical properties brought about by cross-linking or scission. In Sections 3-5 of this review the use of NMR to identify changes in chemical structure is discussed. The changes in the NMR line shape and relaxation times induced by radiation, and the use of these methods to determine radiochemical yields, are discussed in Section 2. 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

The chemistry of typical free-radical polymerizations involves an initiation, propagation, chain transfer, and termination step leading to the formation of a cross-linked polymer system (36). The initiation step (radical formation step) utilizes chemistries that when subjected to thermal or ultraviolet radiation form radicals that react with activated monomers, such as a methacrylate. A wide variety of thermal, ultraviolet, visible, and redox initiators are commercially available. Typical thermal initiators include the class of azo compounds, such as azobisisobutylonitrile (AIBN), and peroxide initiators, such as the per-oxydicarbonates and the hindered peroctoates. Polymerization conditions vary... 

Free-Radical Intermediates. From an early time the participation of free radicals in the radiation chemistry of polymers has been well understood. Charlesby (125) in 1952 invoked a free-radical mechanism of cross-linking of polyethylene, although for other materials the participation of ionic species has also been suggested (126-137). The primary radicals observed at 77 K, at which temperature a significant proportion of the radicals are assumed to be trapped and prevented from further reaction, are the secondary alkyl radicals I (49,138-148). 

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