Stability and phase transformations of the mesomorphic form of isotactic polypropylene in stereodefective polypropylene

•The mesomorphic form of iPP crystallizes also in stereoirregular samples.•The mesophase of stereoirregular samples transforms into α form at 25 °C.•The mesophase always transforms into α form by annealing at T > 60–70 °C.•The transformation into α form occurs even in samples that crystallize into γ form.•Evidences supporting the structure model of the mesophase of iPP are given.

A study of the thermodynamic stability and the related polymorphic transformations induced by thermal treatments of the mesomorphic form that crystallizes in stereodefective metallocene isotactic polypropylene (iPP) is presented. We show that the mesomorphic form of the more isotactic samples is stable at room temperature, whereas the mesomorphic form crystallizing in the more stereoirregular sample is unstable and crystallizes at room temperature in the crystalline α form. In any case, the mesomorphic form transforms during heating or by annealing at temperatures higher than 60–80 °C always in the α form, regardless of the stereoregularity, even in the case of stereoirregular samples generally crystallizing from the melt in the γ form. These data confirm the proposed model of structure of the mesomorphic form as small aggregates of chains in three-fold helical conformation packed with lateral correlations similar to the α form of iPP.

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