Behavioural factors change

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

Near the glass temperature and at larger temperatures, this conversion between temperature and time is not valid anymore because not the relaxation processes, but the sliding of chain molecules past each other determines the elastic behaviour. Although this process is thermally activated as well, the dependences are more complicated because raising the temperature also increases the free volume and thus has an additional effect on the movement of the molecules. Therefore, the conversion factor changes to... 

In this discussion, entropy factors have been ignored and in certain cases where the difference between lattice energy and hydration energy is small it is the entropy changes which determine whether a substance will or will not dissolve. Each case must be considered individually and the relevant data obtained (see Chapter 3), when irregular behaviour will often be found to have a logical explanation. 

From the foregoing sections it will be realised that the aging and weathering behaviour of a plastics material will be dependent on many factors. The following agencies may cause a change in the properties of a polymer ... 

In practice there are a number of other factors to be taken into account. For example, the above analysis assumes that this plastic is Newtonian, ie that it has a constant viscosity, r). In reality the plastic melt is non-Newtonian so that the viscosity will change with the different shear rates in each of the three runner sections analysed. In addition, the melt flow into the mould will not be isothermal - the plastic melt immediately in contact with the mould will solidify. This will continuously reduce the effective runner cross-section for the melt coming along behind. The effects of non-Newtonian and non-isothermal behaviour are dealt with in Chapter 5. 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

The shortcomings of such a code are mainly attributable to the limited number of environments considered because specific behaviour is generally related to specific metal/electrolyte behaviour which can result in substantial potential changes or even polarity reversals. A polarity reversal implies a large potential at one or both metal electrodes and may be attributed to two main factors ... 

An investigation has been made of the factors which control / and D conduction and it has been found that the difference is only one of degree and not of kind . Thus, if the varnish films are exposed to solutions of decreasing water activity, then the resistance falls with increasing concentration of electrolyte, but a point is eventually reached when the type of conduction changes and the films exhibit /-type behaviour. It appears that D films can be converted into / films, the controlling factor being the uptake of water. 

The above explanations, (i)—(iv), of S—T behaviour suggest that changes at the reaction interface may include the following factors which... 

The concept of a characteristic reaction temperature must, therefore, be accepted with considerable reservation and as being of doubtful value since the reactivity of a crystalline material cannot readily be related to other properties of the solid. Such behaviour may at best point towards the possible occurrence of common controlling factors in the reaction, perhaps related to the onset of mobility, e.g. melting of one component or eutectic formation, onset of surface migration or commencement of bulk migration in a barrier phase. These possibilities should be investigated in detail before a mechanism can be formulated for any particular chemical change. 

It was found that both the catalytic rates and the selectivity to the various products can be altered significantly (rate changes up to 250% were observed) and reversibly under NEMCA conditions. Depending on the product, electrophobic or electrophilic behaviour is observed as shown in Fig. 8.57. In addition to the selectivity modification due to the different effect on the rate of formation of each product, acetaldehyde, which is not produced under open circuit conditions is formed at negative overpotentials (Fig. 8.58). Enhancement factor A values up to 10 were observed in this complex system.59... 

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