Stress-induced polymorphic transformations and mechanical properties of isotactic propylene-hexene copolymers

A study of the relationships between the stress-induced phase transitions and the mechanical properties of isotactic propylene-hexene copolymers with hexene concentration in the range 1-26 mol%, prepared with metallocene catalysts, is reported. Hexene units are included in crystals of R form of isotactic polypropylene (iPP) and produce large disturbance of the crystalline lattice and a consequent decrease of melting temperature, degree of crystallinity, crystallite size, and plastic resistance of the crystals. Defective crystals of R form rapidly transform by stretching into the mesomorphic form of iPP that, in turn, facilitates further stretching up to very high values of deformation of nearly 900-1000%, resulting in high flexibility. This explains the experimental observation that the presence of hexene comonomeric units induces a strong enhancement of ductility, flexibility, and toughness, compared to the highly stereoregular homopolymer prepared with the same catalyst. These copolymers show mechanical properties of highly flexible materials with values of the tensile strength, elastic modulus, and resistance to the plastic deformation that depend on the degree of crystallinity and the occurrence of phase transitions during deformation and can be easily tailored by changing the hexene concentration.