Crystallization shrinkage

It is important to point out that gas adsorption might induce crystal shrinkage increasing the flux trough defects [35]. Therefore, defect-free and well-intergrown membranes are desirable in gas separation applications. 

The degree of crystallization increases with increasing cavity temperature prevention or reduction of post-crystallization (shrinkage), causes changes in toughness properties. High cavity temperatures provide a more uniform structure in the surface layers. 

At HOY speeds, the rate of increase in orientation levels off but the rate of crystallization increases dramatically. Air drag and inertial contributions to the threadline stress become large. Under these conditions, crystallization occurs very rapidly over a small filament length and a phenomenon called neck-draw occurs (68,75,76). The molecular stmcture is stable, fiber tensde strength is adequate for many uses, thermal shrinkage is low, and dye rates are higher than traditional slow speed spun, drawn, and heat-set products (77). 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

Amorphous nylons are transparent. Heat-deflection temperatures are lower than those of filled crystalline nylon resins, and melt flow is stiffer hence, they are more difficult to process. Mold shrinkage is lower and they absorb less water. Warpage is reduced and dimensional stabiUty less of a problem than with crystalline products. Chemical and hydrolytic stabiUty are excellent. Amorphous nylons can be made by using monomer combinations that result in highly asymmetric stmctures which crystalline with difficulty or by adding crystallization inhibitors to crystalline resins such as nylon-6 (61). 

Wood While fairly inert chemically, wood is readily dehydrated by concentrated solutions and hence shrinks badly when subjected to the action of such solutions. It is also slowly hydrolyzed by acids and alkalies, especially when hot. In tank construction, if sufficient shrinkage once takes place to allow crystals to form between the staves, it becomes very difficult to make the tank tight again. 

---