Crystallization behavior and mechanical properties of copolymers of isotactic poly(1-butene) with 1-octene from metallocene catalysts

The crystallization behavior and the mechanical properties of random isotactic buteneeoctene co-polymers (iPBC8) prepared with a metallocene catalyst are presented. Samples with low octene con-centrations, up to nearly 6 mol%, crystallize from the melt in form II. In these samples octene units stabilizethe tetragonal form II, decelerating or even preventing the transformation of form II into the trigonal formI. Octene concentrations higher than 7 mol% prevent crystallization of the samples from the melt and theobtained amorphous samples crystallize by aging at room temperature in mixtures of form II and form I'.The sample with the highest octene concentration of 11.5 mol% crystallizes surprisingly from the amor-phous phase directly in the pure form I'. In these samples, form I' crystallized from the amorphous phasetransforms into form II by stretching. Form II transforms back into form I' upon releasing the tension.Therefore, in samples where form I' is favored in powder samples, form II is favored in stretchedfibers. Thetransformation of form I into form II by stretching and the back transformation of form II into form I afterreleasing the tension is unusual for isotactic polybutene (iPB) and rather unexpected. The presence ofoctene comonomer units also affects the mechanical behavior of iPB and produces increase offlexibilityand ductility with increasing comonomer content, and samples with octene concentration higher than 6e7 mol% show unexpected remarkable elastomeric properties. The formation of crystals of form II or formI' during aging allows development of elastomeric properties and enhancement of mechanical strength.The elastic properties are associated to the reversible crystalecrystal phase transition between form II andform I occurring during stretching and when the tension is removed.