Polymorphism and structural disorder in melt-crystallized and fiber samples of syndiotactic copolymers of propane with 1-butene

The structural characterization of melt-crystallized samples and oriented f ibers of syndiotactic copolymers between propene and 1-butene is reported. Melt-crystallized samples of copolymers are crystallized in the form I of s yndiotactic 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 ch ains along both axes of the unit cell is present. More ordered modification s, close to the limit ordered, fully antichiral, form I of s-PP are obtaine d by crystallization at high temperatures only for samples containing small contents of 1-butene (1-2 mol %). The presence of 1-butene prevents that t he order in the alternation of right- and left-handed helical chains develo ps 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 conten t of 1-butene (1-2 mol %) present the same behavior of syndiotactic polypro pylene. Stretched fibers are in the trans planar form III of s-PP,which tra nsforms 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 ann ealing. With increasing the 1-butene content only the antichiral helical fo rm 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 co ntents 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 upo n the release of the tension, is a further evidence that the isochiral heli cal 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.