Urea-polyethylene complexes

An answer to the question posed above can be found in the following study of free radicals trapped in a urea-polyethylene complex (UPEC). Figure 7.7 in Section 7.4.1 showed ESR spectra observed at 320 K which demonstrated the difference between the spectra of the alkyl radicals trapped in UPEC and those in solution grown crystals. The conclusion was that the radical sites in the UPEC were more mobile than in the crystals. The difference between the decay behavior of the alkyl radicals trapped in the complex and in the solution grown crystals is shown in Fig. 7.16. It can be said that the free radicals in the complex have a very long life time at 318 K and the decay rate in the complex at 411 K is of the same order as that in... 

Reaction (C) The reaction is important in a chain mechanism during the auto oxidization process. Direct observation of the reaction was successfully carried out by Hori et al. [32] for a urea-polyethylene complex (UPEC). Figure 7.30 shows the variations of ESR spectra for peroxy radicals in the UPEC due to heat treatment in vacuum at 361 K. Spectrum (a) was obtained from peroxy radicals and spectrum (d) was that obtained from alkyl radicals as shown in Section 7.4.1. Figure 7.31 shows the variation of the concentrations of various radicals with the time of heat treatment at 361 K. It was found that 20% of the radicals decayed, but ca. 80% of the decaying peroxy radicals were converted into alkyl radicals. A few authors reported that peroxy radicals trapped in polytetraflu-oroethylene converted into alkyl fluoro radicals by heat treatment in a similar manner. Some authors claimed that the back reaction of Eq. (7.16) occurred in that case ... 

Experiments on two types of materials will be presented, namely (1) a powder of solution-grown polyethylene, Sholex 6050, and (2) a urea-polyethylene inclusion complex, in which polyethylene is of the same origin. The inclusion complexes of urea and low-molecular-weight hydrocarbon molecules have been studied extensively for more than thirty years, but the urea-polyethylene complex (UPEC) was successfully prepared rather recently. We prepared the complex according to the method described by Monobe et al. 

In Sects. 5 and 6, a few investigations of urea-polyethylene complexes (UPEC) were discussed. The UPEC is an interesting material because a single polyethylene chain is located in an hexagonal canal of urea molecules and it must be expected that the polyethylene chain can behave differently from the bulk systems like solution-grown crystals or materials recrystallized from the melt. The inclusion complex system composed of short hydrocarbon molecules and urea molecules was studied more than 30 years ago. The crystalline structures of urea-hydrocarbon complexes are known The urea-polyethylene complex system was prepared rather recently by Monobe et al. , replacing the hydrocarbon molecules in the urea-hydrocarbon complex by... 

Urea-polyethylene complexes have been prepared by two methods [6,7] (a) a solid urea-polyethylene solution in xylene, with a small amount of aniline, and (2) a solid urea with a seed polyethylene solution of xylene. Polyethylene molecules perpendicular to the lamellae exist as extended... 

Motional Character of the Polyethylene Chain in the Urea-Polyethylene Inclusion Complex... 

PTHF chains have an a -trans conformation that is similar to polyethylene in the urea adduct or to -alkane complexes with urea. However, the stoichiometries of the adduct are uncertain and the diffraction patterns of the urea adducts even with /i-paraffin showed a high degree of disorder for the guest molecules. Some authors claim, particularly for the n-alkane adducts, that there may be some kind of conformational defects whereby some of the torsion angles could be gauche. Therefore, Chenite and Brisse reinvestigated the structures of PTHF-urea systems in more detail [15]. 

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