The deformability of polymers: the role of disordered mesomorphic crystals and of stress-induced phase transformations.

Stereodefective samples of isotactic polypropylene prepared with single-center metallocene catalysts and slowly crystallized from the melt in lamellar gamma form, show unexpected high ductility and deformabilty, much higher than observed in the same samples crystallized in the nodular non-spherulitic mesophase. This behavior has been explained considering that when these samples are crystallized in the gamma form, polymorphic transformations occur during deformation, whereas when they are crystallized in the mesophase no structural transformations occur during stretching. Crystals of the gamma form transforms into the mesomorphic form during deformation. The occurrence of these transformations produces an effective mechanism able to gradually dissipate some amount of mechanical energy and produces a neat increase of the values of the strain at break. During stretching the mechanical energy is, indeed, converted into a latent heat of fusion that induces local melting of crystals, followed by recrystallization at high deformation into the mesophase, that facilitates the further successive deformation up to very high deformation, resulting in high ductility and flexibility.