Polyethylene transitional phenomena

It has been also reported a study on the threading process of a a - cyclodextrin (a-CD) and polyethylene glycol (PEG), as a function of temperature and solvent composition. This reaction produces a polyrotaxane that eventually precipitates and forms a thick gel. Ceccato et al. [46] have proposed a molecular model for the interpretation of the temperature and solvent composition effect on the threading process. According of this model, the reaction can be depicted as a five - step phenomenon that mainly depend on the threading and sliding of a-CD and PEG. The transition... 

This phenomenon was observed with polymers some years ago [177—183]. The more recent investigations are due to Buben and Nikolskii [183]. These workers measured the emission from many amorphous and crystalline polymers and observed correlations between the temperature corresponding to the glow peak maxima and the structural transition temperatures of the materials. A detailed study of polyethylene thermoluminescence was made by Charlesby and Partridge [184]. The glow curve obtained after irradiation in vacuo possesses three peaks, a, j3 and y, whose luminescence intensities are proportional to the irradiation dose for doses below 5 x 104 rads. The maxima occurs, respectively, at —110, —65 and —27°C, when the total... 

With the increase of flow velocity, polymer melt extruded in the tube will become unstable due to the stick-slip transition near the tube wall, which makes the extrudate shows wave-like, bamboo-Uke, or spiral-like distortions. All these phenomena are known as melt-broken phenomena. In these cases, the shear rate suddenly rises, as illustrated in Fig. 7.17, thus this behavior is also called capillary-jet phenomenon. The string-Uke shark-skin phenomenon upon the extrusion of polyethylene melt can be attributed to the intermittent stick-slip transition near the tube wall of the exit (Wang 1999). 

It is interesting to note the annealing in the case of isotactic polypropylene. As shown in Fig. 10.9., heat treatment results in an up shift of the narrowing temperature in region II On the other hand, however, an effect of annealing appeared in the down shift of the transition (narrowing) temperature in the case of fractured polyethylene A similar phenomenon was also reported by Bullock et al. for polyethylene by the spin label method... 

To determine whether this phenomenon is associated solely with the increase in conductivity, we investigated a pigmented polyethylene system. This system does not become conductive at a critical volume concentration. The aim was to determine whether in these circumstances as well there would be a phase transition comparable with that found in the systems filled with carbon black. And indeed Irgalith yellow BAWP dispersed in a polyethylene matrix also displays a stagnation of density on the same scale as is found with carbon black in a variety of polymer matrices (Fig. 19.27). 

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