Melts shear-induced crystallization

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

Wassner E, Maier RD (2000) Shear-induced crystallization of polypropylene melts. Proc XIII Int Congr Rheol, Cambridge. 1 183-185... 

As is well-known, the rate of nucleation is enhanced by the application of a deformation to the polymer melt (2/i). But only recently, a theory of shear induced crystallization could be developed (25). This type of crystallization causes highly oriented boundary layers in injection molded articles (cf.ref. 8). Shear induction is an elastico-viscous relaxation phenomenon. So far, however, this perception did not contribute to a simplification of the situation. 

The main problem is as everywhere in this field A lack of experimental data for the model parameters, as there are Critical shear rate ya for the activation of shear induced crystallization, effective relaxation time T of the melt and induction time ti, at infinite (i.e. large enough) rate of shear. For all these parameters also their temperature and molecular weight dependences should be known. 

Shear-Induced Crystallization of the Melt, 186 Crystalhzation upon Heating from the Glassy State, 189... 

Polyethylene can crystallize during extrusion if the conditions are favorable. This phenomenon, known as shear-induced crystallization, occurs when highly oriented melts are extruded at temperatures at or below their melting points (93). 

However, for polymers with low degree of crystallization it was assessed that the melting point was depressed at high shear stresses (4). Shear Induced melting has been theoretically predicted from non-equilibrium molecular dynamic simulations (5,... 

In the filling stage of injection molding, the polymer melt is subjected to high shear rates, which results in the so-caUed flow-induced crystallization (FIC). The main features of FIC are the dramatic enhancement of the crystallization rate, and the formation of oriented morphology. The effect of FIC continues after the cessation of flow. 

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