Crosslinking, radiation chemical

The radiation chemical yields are expressed in terms of G-values. G(scission), G(s), equals the number of main chain scissions produced per 100 eV of energy absorbed and G (cross-linking), G(x), the number of crosslinks formed per 100 eV absorbed. The G-value is a structure dependent constant similar to quantum efficiency in photochemistry. 

The recombination between two alkyl radicals is believed to be the main source of radiation-induced crosslinks in polyethylene [21], so that it is important to study the mechanism for formation and reaction of the alkyl radical. We have applied the ESE method to elucidate the paramagnetic relaxation mechanism and the spatial distribution of the alkyl radical, in order to get further insight into the radiation-chemical reactions of polyethylene resulting in the formation of crosslinks. 

In general, one may cmiclude that both chemical and radiation-chemical approaches provide effective reductimi of metal irais directly in IPEC matrices. This leads to the formatimi of stabilized NPs. However, it was found that the chemical method is applicable mfly for a preparatiOTi of NPs in thermally crosslinked IPECs. Apparently, a poor regularity of the chemical processes causes noncrosslinked matrices to disintegrate. In cmitrast, the irradiation technique allows one to derive nanocomposites from both crosslinked and nmicrosslinked IPEC Aims due to specific control over the radiatimi-induced processes. 

Above a certain degree of crosslinking, strain at break and impact strength decrease again, and the plastic becomes hard and brittle. Radiation-chemical degradation also causes a decrease in mechanical properties with increasing radiation... 

Radiation-induced changes in plastics can be reduced or increased by certain measures. Stabilization is desired for plastics used, e.g., in nuclear reactor construction, or intended to be sterilized by radiation. Here, irradiation doses can reach levels of several kGy at which plastic properties would already begin to change. Sensibilization, on the other hand, is desired for radiation-chemical processes to reduce the doses required for crosslinking or for other reactions, i.e., to lower the cost of irradiation. Stabilization and sensibilization thus can affect various parallel reactions either uniformly or selectively [710]. 

In the creation of network structures by chemical bonding (covalent bonding), there is a method of (1) crosslinking at the same time as polymerization or (2) crosslinking by chemical reaction after linear polymer chains have been synthesized. The latter method can be further divided into the addition polymerization in the presence of divinyl conqjounds (radical polymerization, anionic polymerization, ionic polymerization, etc.) or the formation of crosslinked structures by polycondensation of multifunctional compoimds. In the addition reaction, free radical polymerization is generally utilized. In this free radical polymerization method, initiators are usually used, but light, radiation, and plasmas can also be used. 

Table 5.4 Radiation chemical yields C(S) and C(X) for main-chain cleavage and crosslinking, ...

Figure 5.8 Irradiation of poly(ethylene oxide) in vacuo with Co-y-rays. Radiation chemical yields of crosslinking and main-chain scission as a function of temperature. Adapted with permission from Ref [65] 1970, Wiley-VCH.

The scission and crosslinking reactions have the greatest effect on the material properties of polymers and measurement of the radiation chemical yields of these two processes is very important for quantifying degradation of polymers by radiation(4). The main methods used to determine the yields of scission and crosslinking are listed in Table 2. 

The G values quoted in the following tables are defined as the radiation chemical yields of individual atomic or molecular events for 100 eV of energy absorbed by the system. G(R) represents the yield of free radieals, G(X) the yield of crosslinks, G(S) the yield of main chain scission, and G(products) the yield of produet moleeules per 100 eV of absorbed radiation. 

This explains the predominance of radiation-chemical reactions as a means of initiating crosslinking in thermoplastic materials. In principle the effect can be induced by electromagnetic waves such as X-rays or gamma rays or by corpuscular radiation (beta rays) such as accelerated electron beams, for example. The differences between... 

Attractive blends for PEMs with high proton conductivity have been made from sulfonated PES, PSU, polyetherketone (PEK), PEEK or poly(2,6-dimethyl 1,4-phenylene ether) (PPE) blended with polybenzimidazole (PBI) or polyetherimide (PEI). To preserve the desired PEM performance, the blends are often crosslinked by radiation, chemical reaction of ionic interactions. For long-term PEM applications it is important that membranes resistance to mechanical, chemical and thermal degradation is maximized. Accelerated aging tests should follow several membrane functionalities, for example conductivity, membrane integrity and permeability. The tests should also identify a possible cross-correlation of effects, namely stress on thermal and/or chemical degradation. 

There is not enough space here to give a detailed classification, but only to delineate the major families from which resins for industrial coatings may be selected. Resins may be divided into two groups according to their modes of film formation which may or may not involve a chemical reaction. In the first, the components must react together to form a crosslinked structure which may require heat, radiation or catalysis to effect the reaction. The bulk of resins used in industrial finishes are of this type. They are commonly referred to as chemically convertible or, simply, convertible. 

Both radiation and chemical crosslinking can produce amorphous, mechanically stable networks. Radiation crosslinking has a practical advantage in that polymer elec-... 

Another way of modifying unsaturated PHAs in the bulk is by crosslinking of the material. This has been accomplished by either chemical reaction with sulfur or peroxides [109, 110], or by radiation curing [91, 111]. In all cases, crosslinking altered the ultimate material properties drastically, yielding a true rubbery material. The advantages of applying rubbers from crosslinked PHAs over the use of current rubbers will be elaborated in Sect. 4.5. 

However, the chemical changes observed in low molecular weight compounds can be quite misleading as models for polymers. Difficulties include the high concentration of end groups, e.g. COOH in N-acetyl amino acids, which can dominate the radiation chemistry of the models. Low molecular weight compounds are usually crystalline in the solid state and reactions such as crosslinking may be inhibited or severely retarded. 

---