Isotatic polypropylene (i-PP) is a polymorphic material with several known crystal modifications

such as monoclinic (α)

hexagonal (β) and triclinic (γ).Of these polymorphs

theαform is the thermodynamically stable crystalline modification and is predominant under common processing conditions.But iPP may form theβ-phase under shear conditions.By tensile and shear loads

polymeric molecules are stretched and oriented into rows in the molten state

the row nucleus transform into cylindrites or fibril crystallites.The effect of shear on crystallization kinetics is characterized in the following aspects: shear accelerates the crystalline growth rate

decreases the induction time and increases the activated nuclei density.The effect of flow on the kinetics can be understood by considering that the increase of the degree of order due to flow results in an effective change of the melt energy.On the basis of this concept

several empirical approaches have been proposed

but these models have some limits.