Polymorphism and Structural Disorder in Melt-Crystallized and Fiber Samples of Syndiotactic Copolymers of Propene with 1-Butene

The structural characterization of melt-crystallized samples and oriented fibers of syndiotactic copolymers between propene and 1-butene is reported. Melt-crystallized samples of copolymers are crystallized in the form I of syndiotactic polypropylene (s-PP) up to a content of 1-butene of 60-70 mol %, although disorder in the alternation of right- and left-handed helical chains along both axes of the unit cell is present. More ordered modifications, close to the limit ordered, fully antichiral, form I of s-PP are obtained by crystallization at high temperatures only for samples containing small contents of 1-butene (1-2 mol %). The presence of 1-butene prevents that the order in the alternation of right- and left-handed helical chains develops at high crystallization temperatures. Samples with contents of 1-butene higher than 70 mol % crystallize in structures similar to that of form I of syndiotactic poly(1-butene). Fiber samples of copolymers with small content of 1-butene (1-2 mol %) present the same behavior of syndiotactic polypropylene. Stretched fibers are in the trans planar form III of s-PP, which transforms into the isochiral helical form II of s-PP upon the release of the tension. Mixtures of crystals in forms I and II of s-PP are obtained by annealing. With increasing the 1-butene content only the antichiral helical form I of s-PP is observed in the stretched fibers of the copolymers, as well as upon the release of the tension. The presence of 1-butene units, for contents higher than 4 mol %, prevents the formation of the trans planar form III of s-PP by stretching. For these samples the formation of form I of s-PP, instead of the isochiral form II, either in the stretched fibers or upon the release of the tension, is a further evidence that the isochiral helical form II of s-PP can be obtained only from fibers initially in the trans planar form III, through a cooperative conformational transformation which induce the formation of helical chains having the same chirality.