Amorphous activation process

For semi-crystalline polymers with melting points of more than 100 °C above the glass transition temperature and for amorphous polymers far above the glass transition temperature Tg (at around T = Tg + 190°C), the shift factors obtained from time-temperature superposition can be plotted in the form of an Arrhenius plot for thermally activated processes ... 

The growth kinetics describes the nucleation processes on the atomic scale. Thermally activated processes as adsorption, desorption, and diffusion at the surface and in the volume, nucleation, and crystallization/ recrystallization determine the film structure and can be controlled by the substrate temperature and the growth rate. Using a diagram ln(J ) over 1/ T, R being the deposition rate and T the growth temperature, three different growth modes (epitaxial, polycrystalline, and amorphous) can be... 

The discussion above has been in terms of results of activation temperatures between 215 and 300°. As may be seen from Table VIII, the fraction of multiply exchanged 2-hexene resulting from process (8) changes little until after an activation temperature of 300°. It then declines and becomes very small by 400° on both crystalline and amorphous catalysts. Process (5), formation of exchanged 1-hexene, declines steadily with activation temperature and becomes very small at 400°, particularly on amorphous catalysts. 

A model Phillips catalyst for ethylene polymerization has been prepared by spin coating of a Cr(III) precursor (Cr(acac)3) on a flat silicon wafer (100) covered by amorphous silica. The spin coating parameters were chosen in order to obtain a homogeneous film. The model catalyst was submitted to an activation process. The surface concentration of Cr decreased from about 0.8 to 0.4 Cr atom/nm as the temperature increased from 150 to 550°C. Direct information concerning the surface molecular species and the environment of Cr was provided by ToF-SIMS and XPS. At 350°C, the catalyst precursor was decomposed Cr species were in the form oxide and surface-anchored chromates. Upon final activation at 650°C for 6 h, Cr species were below the XPS detection limit however the model catalyst was active for ethylene polymerization at 160°C and 2 bar pressure. 

A form of Fick s law describes the diffusion of gases through the amorphous polymer matrix. The diffusion coefficient has been observed to follow an Arrhenius relationship, characteristic of an activated process. 

A further experiment was carried out to study the possible role of tars in the oxidation process which transforms the imine into the oxime by 0-insertion. Indeed, no decisive data are available to exclude any influence of the presence of tars on the catalyst surface on the reaction of oxidation of the imine to oxime. On the other hand, the many evidences seem to indicate a possible correlation between the oxidation power exhibited by the simple amorphous silica samples and the presence of organic residues irreversibly adsorbed. In particular, an important indication is the extrapolation of the rate of formation of the oxime at t=0 h. The value obtained is about zero suggesting that the pure silica can not catalyze the oxime formation. In order to confirm this hypothesis, other catalytic tests were carried out under standard conditions and the first hour of reaction was studied in more detail. The results, reported in figure 6, showed that the rate of formation of the oxime at very beginning of the test with the time-onstream is really null. This datum demonstrates that the simple silica can not generate the oxime and that the oxidizing power is related to the presence of the tars and that the activation process which takes place in the first 10 h of the reaction is due to the increase of the tars. 

An unsolved issue of the ac relaxation is to know if it is really a thermally activated process. The Arrhenius plots are foimd to be non-linear [167, 169], but this can be attributed to the overlapping of different thermally activated processes. Arguments based on the concept of activation entropy led to the suggestion that the ac relaxation is substantially cooperative [169]. This was attributed to the participation of the amorphous region in this process, which will increase the complexity of the motions involved. 

Delayed elasticity is a property that is characteristic of a disorderly molecular arrangement in amorphous material or of disordered regions in crystalline material [190]. It is caused by thermally activated processes and may therefore involve entropy-elastic forces. Two observations suggest that the time-dependent elasticity... 

Activated carbon in its broadest sense includes a wide range of processed amorphous carbon-based materials. It is not truly an amorphous material but has a microcrystalline stracture. Activated carbons have a highly developed porosity and an extended interparticulate surface area Their preparation involves two main steps the carbonization of the carbonaceous raw material at temperatures below 800 C in an inert atmosphere and the activation of the carbonized product Thus, all carbonaceous materials can be converted into activated carbon, although the properties of the final product will be different depending on the nature of the raw material used, the nature of the activating agent and the conditions of the carbonization and activation processes. 

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