Drawing, crystallization during

The shear-induced crystallization of heterogeneous systems is a subject that has received much less attention, but there are a few, both experimental [41, 92, 93] and theoretical, studies [71]. 

Several polymer forming processes are dominated by uni- or biaxial drawing, such as FS and film blowing. These forming processes contrast with IM, where there is hardly any control over the physical and mechanical properties, which will vary over the whole product. With 

FS and film blowing, properties can be controlled by varying the process parameters and thus the structure created in the processing step. This shows the importance of such processes. 

All aspects of crystallization dynamics of polymers, the strong dependence on the temperature, cooling rate, and the application of flow, are of relevance to highspeed FS (uniaxial extensional flow) [1-3,13] and during biaxial extensional flow, that is, flhn blowing [28,93-95]. 

FIC models for FS often apply the Nakamura equation [114] to describe the crystallization dynamics (see for example References [115-117]). However, the use of the Nakamura equation is inappropriate because, first, the isokinetic assumption is not satisfied during FIC, and, second, a change in the number density of crystals cannot be described [3]. 

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Fig. 51. Change of modulus (0 and draw ratio (X) for natural rubber The Vidicon pictures show the onset of crystallization during stretching (X,), the maximum of the crystallization (X ) and the melting of the last crystallites upon relaxation (X.,)...

This step is often problematic and should NOT be carried out unless an unacceptable (use your judgement) amount of insoluble material is suspended in the solution. The difficulty here is that the compound tends to crystallize during the filtration so an excess of solvent (ca. 5%) should be added, and the apparatus used for the filtration should be preheated to about the boiling point of the solvent. Use a clean sintered funnel of porosity 2 or 3, or a Hirsch or Buchner funnel, and use the minimum suction needed to draw the solution rapidly through the funnel. If the solution is very dark and/or contains small amounts of tarry impurities, allow it to cool for a few moments, add ca. 2% by weight of decolorising charcoal, reflux for a few minutes, and filter off the charcoal. Charcoal is very finely divided so it is essential to put a... 

The orientation in PLLA films depends on the draw rate, temperature, and ratio. High strain rate, low temperature, and high stretch ratio favor strain-induced crystallization during orientation. PLA has excellent optical properties and high modulus. However, it has low elongation, tear, and burst strengths. To overcome these shortcomings, PLA is often coextruded with other polymers to form multilayer structures to enhance its properties [27]. 

During drawing at 120°C chains pulled out from polypropylene crystals posses much higher mobility and may form assemblies ready to crystallize into well ordered a form crystals. 

The extreme plastic deformation of wires during drawing not only affects the microstructure, but also induces characteristic, material-specific textures (orientation of crystal axis). Since the mechanical properties of drawn wires are equally dependent of strainhardening, grain size and texture, the knowledge of the degree of crystal orientation is of practical importance. 

The relatively low molecular weight of melt-spun fiber, between 30 and 150 kDa, results in high crystallizability especially during drawing. In a double-step process of fiber spinning followed by fiber drawing, crystallization from... 

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