Polyoxymethylene irradiation

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... 

Gak et al. (100, 102) examined the nature of free radicals in polyoxymethylene. When formaldehyde homopolymer, PFA-OH was irradiated at —196° C, the initial rate of formation of the free radical — CH20- was found to be proportional to the square of the incident light intensity, which indicates the two-photon process of —CH20-radical formation. When irradiated PFA-OH was kept in the dark, some increase occurred in the concentration of -CH20- and there was a simultaneous fall in the concentration of -CH2 at temperatures above —196° C. The maximum rate was attained in the temperature range —130° C----123° C. This is caused by reaction (41). 

Table 52.28 gives the G(X) and the G(S) values for a list of different polymers which will not be discussed in detail. The polyoxymethylene, cellulose, and polyisobutylene are all readily degraded upon irradiation. 

Fig. 4.7. Decay curve of free radicals trapped in irradiated polyoxymethylene at 354 K after heating at room temperature for an extended time period , plot of raw data without any reduction O. plot after subtracting the amount of non-vanishing radicals obtained from Fig. 4-6 solid line indicates calculated decay curve with A = 5.0x 10 s" and B = 1.6x 10" s (Ref.

For polymers containing crystalline and amorphous parts, the rate of degradatitm decreases with increasing crystallinity when the degradation occurs mainly in the amorphous part as is the oxidation of polyethylene In cemtrast to this, oxidative degradation of polyoxymethylene and polypropylene undary-irradiation is not mudi... 

Plastics without oxygen in their molecular chains are always subject to strong oxidative degradation. For plastics with intramolecular oxygen (polyether, polyethylene oxide, and polyoxymethylene) the influence of external oxygen during irradiation is small. 

Dose dependence of molecular weight in polyoxymethylene copolymer during irradiation in a vaouum [737]... 

Stress-strain diagram of polyoxymethylene copolymer, irradiation in vacuum at 15 °C with 1 MeV electrons from a van de Graafgenerator [737]... 

Aromatic groups such as polyimides and polyetherimides are more resistant to irradiation compared to polymers that have groups such as C-Cl, for example, in PVC. Side chains such as methyl group in PP result in the production of CH4 on irradiation. The C-O-C linkage in polyoxymethylene is rather easily broken on... 

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