The Oriented γ Form of Isotactic Polypropylene

Abstract

A structural analysis of the oriented y form of isotactic polypropylene (iPP) is presented. The X-ray diffraction intensity distribution in the oriented γ form is quantitatively evaluated for the first time. The structural analysis is performed on a new, poorly stereoregular ([mmmm] = 35%) ) polypropylene material, prepared with a novel C 2 -symmetric zirconocene based catalytic system, showing elastomeric properties. The sample, initially amorphous, partially crystallizes upon aging. In the oriented crystalline materials, obtained by stretching the aged sample, the crystallites tend to assume a preferred orientation with the chain axes parallel to the draw direction. Disorder is present in the structure. For low draw ratios (less than twice the initial length) disordered crystalline modifications, very close to the y form of iPP, are obtained; increasing the draw ratio, crystalline modifications, closer to the a form of iPP, are observed. The structural disorder occurring in our stretched samples originates from the random succession of bilayers of chains with chains parallel (like in the a form) or tilted 81° each other (like in the y form). The results of this analysis are the follows. For the sample stretched at a low draw ratio, 75% of the bilayers of chains are faced each other like in the y form so that the fraction of bilayers with chain axes pointing preferentially along the draw direction is on the order of 60-65% (the remaining 35-40%, being oriented with the chain axes tilted 81° to the draw direction). For the samples stretched at higher draw ratios, the fraction of bilayers facing each other like in the γ form is reduced to 20-30%, so that only 10-15% of the bilayers results oriented with the chain axes nearly normal to the draw direction. This is as if, in all cases, the regions of the crystals with chains arranged like in the a form tend to be oriented with the chain axes parallel to the stretching direction rather than tilted 81°. A continuum of disordered modifications intermediate between the pure γ and a forms could exist. The possible inclusion in the crystalline domains of stereo defects (mainly of the kind mrrm), which would favor the y -like packing situations on the local scale, is discussed.