Polyethylene main-chain scission

Figure 2. Adiabatic potential curves in the main chain scission of a model compound of polyethylene ethane (9) Alg (O) Alu (U) A2g (Q) A2u (A) Eg (A) Eu (-------------------------) singlet (---) triplet (4)...

In addition, Yoshida and Ranby (44) suggested that the broad line quartet could also be caused by radicals XII formed by main chain scissions, analogous with the interpretation of spectra for ultraviolet-irradiated polyethylene. 

It was shown in the pulse radiolysis of the aqueous solution of polyethylene oxide), for example, the peroxy radicals produced by the reaction of 02 combined and formed highly unstable oxyl radicals [73], The LSI decay-curve after the pulse observed with an 02-saturated solution showed two modes. The faster one obeyed a second order kinetics, suggesting that Eq. (17) was the rate determining step in the series of consecutive reactions. This reaction was followed by H-abstraction of OH radical, leading to the main-chain scission. 

Simultaneously with Charlesby s findings, work along similar lines was carried out in G. E. s Research laboratories in Schenectady (22) and also in Research Institutes in the Soviet Union, although the latter only became known several years later (23). The results of this research demonstrated that in addition to polyethylene, many other polymers could be cross-linked by radiation. These include silicones, rubber, poly (vinyl chloride), polyacrylates and, to a lesser extent, polystyrene. In contrast, polymers such as polymethacrylates, polyisobutylene, polytetrafluoroethylene and cellulose underwent "degradation" by main-chain scission. These early findings were confirmed and extended to other compounds by numerous studies. 

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

The most pronoimced decrease in the melting temperature is observed with isotactic polypropylene, indicating that the crystalline perfection of polypropylene is more highly affected by ionizing irradiation than that of polyethylene. Obviously, the process includes not only the main chain scissions but also the change in tacticity of polypropylene macromolecules. 

As a result of this extensive experimental program, it has been found that in rotational molding, polyethylene and polypropylene polymers show different degradation behavior. While in polypropylene the thermo-oxidative degradation causes mainly chain scission, in polyethylene crosslinking dominates. The use of increased amounts of antioxidant in the... 

The overall chemical effects in polyethylene are H2 formation, cross-linking, main chain scission, formation of main chain unsaturation (truns-vinylene and diene) and disappearance of vinyl end groups . At room temperature a yield of hydrogen of around 3.7 (lOOeV)" is found. There is a considerable temperature effect in crystalline samples at 130°C this is 6. 3(100 eV) and at 133 °C, in the melt, 6.2 (lOOeV)" ... 

In addition to main chain scission or cross-linking, gas formation is also observed as a result of irradiation. The gas in the hydrocarbon-based polymers mostly consists of hydrogen. The amount of gas produced depends on the nature of the polymer and also on dose, temperature, type of radiation, etc. In the case of polyethylene the G-value of gas production is high, G 0.32 pmol comparable to the gas yields observed in the radiolysis of hquid n-alkanes G 0.5-0.6 pmol In the radiolysis of polystyrene and polymethylstyrene the yield of gaseous products is only G 0.01 pmol 1 , that value is typical of aromatic compounds. The benzene rings attached to the main chain exert a protective effect against both the C-H and the C-C decompositions in the chain. 

High energy irradiation (electron beam or gamma) of polymeric materials results in a multitude of chemical reactions. The two main reactions are crosslinking, as observed in polyethylene and chain scission, for example polyisobutylene. In polypropylene with its structure in between polyethylene and polyisobutylene, both reactions are observed. The relative importance of chain scission (degradation) over crosslinking depends on the physical state, the irradiation parameters, presence of... 

Despite the increased probability of main-chain scission in polypropylene compared with polyethylene, there is no evidence of the formation of stable scission radicals on irradiation at low temperatures. Rather extensive ESR evidence has been accumulated that the stable free-radical intermediates are those listed below ... 

Three assumptions have been made to estimate the molecular length required to achieve main chain scission (a) the simultaneous and concerted motion of the monomer units (b) the equal interaction energy between the corresponding monomer units in adjacent polymer chains (c) the flow activation energy for the corresponding monomer is an approximation for E. In the case of polyethylene, E is 1.01 kcal/mole and the bond energy is about 82 kcal/mole. 

A related study has been the elucidation of the crosslink structures induced within polyethylene by high energy radiation. The secondary carbon radicals thus produced by C—H bond scission may diffuse by hydrogen atom abstraction. They have been shown to combine in pairs to form H type junctions, and to create Y type junctions by reactions with the vinyl end groups of the chains and with primary carbon radicals produced by main chain scission. In each case the shifts characteristic of the new structure were identified [32], The shifts of the H junctions are distinct, being 41.1, 31.9 and 28.7 ppm respectively at the (CH) junction and the first and second linked carbons, as is shown in Scheme 1, but the shifts of the Y junctions coincide with those at the roots of long branches, and their formation is recognised only when a careful comparison has been made of the areas of these shifts before and after irradiation. 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

Polyethylene displays good heat resistance in the absence of oxygen in vacuum or in an inert gas atmosphere, up to the temperature of 290°C. Higher temperature brings about the molecular-chain scission followed by a drop in the molecular-weight average. At temperatures in excess of 360°C the formation of volatile decomposition products can be observed. The main components are as follows ethane, propane, -butane, n-pentane, propylene, butenes and pentenes [7]. 

Oxidation during processing of polypropylene is principally accompanied by chain scission made evident by a reduction in melt viscosity. The oxidation during processing of polyethylene on the other hand is accompanied mainly by crosslinking. The following two tests are, therefore, used ... 

Figure 10.12 Chain scission in polyethylene oxide) matrix, (a) Carbonyl region of photoaged PEO samples irradiated at lOOmWcnr2 and 75°C for various times (0, 1,2 and 4 minutes). The main band at 1725cm-1 is attributed to formate functions, whereas the shoulder at 1 750cm"1 is assigned to esters, (b) Evolution of the storage modulus (G ), the loss modulus (G"), and the tangent of the phase angle tan(8) versus time (temperature 90°C). The increase in tan(8) is evidence of chain scission, (c) Endotherms of the fusion of PEOs samples recorded at...

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