Crystallization oriented melts

There are many modifications of lamellar and superlamellar structures in different polymers, for instance, the cross-hatched lamellae in a-iPP (see Fig. 2.2) or the so-called shish kebab structures in oriented PE and PP see Fig. 2.13. Such structures appear when highly oriented melts crystallize (in the form of a bundle of oriented, longer macromolecules, the shish, and smaller perpendicular lamellae of folded macromolecules, the kebabs ). A peculiar shape is formed by spiral lamellae in TPU (also see Fig. 4.95 in Section 4.5.3). 

If this sample contains also folded-chain crystals (reasons for their appearance during orientational crystallization were stated before), under isometric conditions they undergo melting at a higher temperature (at point 1 with respect to the oriented melt with transition to line A2) than under the conditions of free heating (point 1 with transition in the isotropic melt to line At). 

The most important conclusion of the foregoing investigations is evidence of the presence of two types of structures in samples crystallized from the oriented melt FCC that amount to 85-90% of the total mass of the sample and ECC with the melting temperatures exceeding by 5-6 K for polyethylene and 15-20 K for polypropylene the... 

This model of the structure of orientationally crystallized samples based on experimental data is in good agreement with the results of the foregoing thermodynamic analysis which resulted in relationships describing the formation of two structures, FCC and ECC, during the crystallization of strongly oriented melts of flexible-chain polymers. 

Liquid crystalline solutions as such have not yet found any commercial uses, but highly orientated liquid crystal polymer films are used to store information. The liquid crystal melt is held between two conductive glass plates and the side chains are oriented by an electric field to produce a transparent film. The electric field is turned off and the information inscribed on to the film using a laser. The laser has the effect of heating selected areas of the film above the nematic-isotropic transition temperature. These areas thus become isotropic and scatter light when the film is viewed. Such images remain stable below the glass transition temperature of the polymer. 

We can manipulate the properties of nylon products by changing the conditions under which we crystallize them. The degree of crystallinity is increased by slow cooling, annealing, and by crystallization from highly oriented melts. As we increase the crystallinity level, stiffness and yield strength increase at the expense of impact strength. 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

Finally the effect of orientation on the rate of crystallization should be mentioned. In a strained, oriented condition crystallization proceeds much more rapidly. By orientation the ordering is increased the chains approach the crystalline condition more closely. As a matter of fact there is a relation to the increase in melting point, mentioned in 4.2 under strain the amount of undercooling AT is increased, but this gives only a partial explanation of the increase in the rate of crystallization. 

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

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