Crystallization, cold under strain

Cyclodimer 3 proved to be somewhat difficult to manipulate, thus contributing to the complexity of its characterization. The bowed diacetylenic linkages revealed in the X-ray data impart surprising physical characteristics to the molecule. The energy-rich hydrocarbon was sufficiently strained that it decomposed explosively upon grinding (i. e. preparing a Nujol mull) or when heated above 80°C. At room temperature, crystals blackened within a few days and apparently auto-polymerized, even when stored under vacuum in the dark. Only dilute solutions of 3 in benzene or pyridine were fairly stable over time, especially when stored cold under an inert atmosphere. 

The manner in which a metal deforms after its yield strength has been exceeded by the applied stress depends on many factors. Parameters controlling the deformation process include the alloy s composition, its class (i.e., whether a, a -i- p, or P), its condition (i.e., whether quenched— e.g., P r, a + P-annealed, low-temperat ire aged, etc.), and the rate and temperature at which the deformation is carried out. Some observables or results of the deformation process include the anomalous stress-strain behavior alluded to above and discussed in Chapter 12,phase transformation under stress (i.e., transformation-assisted deformation), and texturization (i.e., the development of preferential crystal orientation or the formation of deformation cells or subbands in response to heavy cold work). 

The orientation process aligns the molecules and makes it easier to form crystalline domains. The crystalline structure is the preferred state to which PET will revert unless molecular motion is restricted. The typical level of crystallinity in cold mold blown bottles, like CSD bottles, is 20-25%. This crystallization is formed as the material is being stretched and is referred to as strain-induced crystallization (SIC). The crystallites formed under SIC conditions are numerous and remain small and therefore different from those formed under thermal crystallization conditions, which result in large spherulites. Spheru-lites will grow until they impinge on one another. At a certain size, the spherulites start to scatter incident light and will appear opaque white rather than the clear colorless amorphous PET. To preserve bottle clarity, the crystallites must remain small. 

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