Crosslinking in polyethylene

Positive Ion. Many ion-molecule reactions have been suggested to explain unsaturation and crosslinking in polyethylene. The subject has recently been reviewed by the author (10). However, there has been practically no direct evidence of ion-molecule reactions. The author (10)... 

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. 

The direct detection of radiation induced crosslinks in polyethylene has been a major goal of radiation chemists for many years. It was recognized as early as 1967 that solution 13c nuclear magnetic resonance (NMR) spectroscopy could be used to detect structures produced in polymers from ionizing radiation. Fischer and Langbein(l) reported the first direct detection of radiation induced crosslinks (H-links) in polyoxymethylene using 13c NMR. Bennett et al.(2) used 13c NMR to detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the molten state. Bovey et al.(3) used this technique to identify both radiation induced H-links and long chain branches (Y-links) in n-alkanes... 

If the radicals of this radi< pair move away, the comlnnation of macroradicals may occur and crosslinks in polyethylene may be formed. Self-reaction within a parent radical pair remains, however, still more probable and reduces the yield of crosslinks. 

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). 

Radiation-induced crosslinking in polyethylene affects mainly the amorphous, interlamellar regions and leads to hardening and embrittlement [89]. 

Nomenclature for branch caihon resonances Determination of crosslinking in polyethylene Determination of thermal oxidation in polyethylene Determination of stcreoregulariiy of polymers... 

A similar example is the formation of nonstoichiometric interpolymeric complexes between mutually complementary polyelectrolytes — polycation and polyanion [69,70], They behave like true polymer networks and are capable of swelling the interpolymeric complexes between PAAc and polyethylene piperazine swells, for instance, 16-18 times [70], Also advantageous in this case is the possibility to carry out this type of crosslinking in open systems, such as soil. 

Stearamide is one of many electron donors which donate an electron to the cationic moiety in excited MAH or in propagating -MAH chains. This results in the inhibition of the homopolymerization of MAH and decreases the crosslinking of polyethylene and the degradation of polypropylene which accompany the peroxide-catalyzed reaction of MAH with these polyolefins (8,9). ... 

NMR resonances attributable to Y-links have been reported in polyethylene after irradiation to low doses. These crosslinks are suggested to be formed by reaction of a chain radical with a C=C double bond at the end of another molecule. 

Shen, F. W., and McKellop, H. A. (2002), Interaction of oxidation and crosslinking in gamma-irradiated ultrahigh molecular-weight polyethylene, J. Biomed. Mater. Res., 61(3), 430 139. 

The formation of oxygen anion radicals and molecules of ozone also should be counted with at the ionization initiation of oxidation [26]. Initiation reaction caused by oxygen anion radicals may play an important role within the polymer bulk while the effect of ozone forming in the surrounding air atmosphere will include only the formation of radicals on the polymer surface. The latent effect of ionization initiation on polymer oxidation which is very distinct may be documented on a relatively fast increase of concentration of carbonyl groups, observed over 1 year after irradiation crosslinking of polyethylene [27]. 

For example, with only 1% of the carbonyl group, the G value for crosslinking of polyethylene was doubled. Furthermore, as shown in Table TV there was a concurrent reduction in the... 

Polyethylene oxide has been shown to be an excellent mucoadhesive polymer. Low levels of polyethylene oxide are effective thickeners, although alcohol is usually added to water-based formulations to provide improved viscosity stability see Table II. Polyethylene oxide films demonstrate good lubricity when wet. This property has been utilized in the development of coatings for medical devices. Polyethylene oxide can be radiation crosslinked in solution to produce a hydrogel that can be used in wound care applications. 

Crosslinking of polyethylen and its copolymers EPM, EPDM, and EVA is mainly carried out by radicals generated from organic peroxides. Other crosslinking methods like electron-beam radiation and grafting of vinylsUanes assume a smaller role in this process. 

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