Correlation Between Structural and Dynamic-Mechanical Transitions of Different Syndiotactic Polypropylene Polymorphs

We prepared several well-characterized syndiotactic polypropylene (sPP) polymorphs so as to correlate their thermal and dynamic-mechanical behaviors. A sample was crystallized in pure form I at 100°C; a second sample containing only the trans-planar mesophase was prepared by directly drawing a quenched sample at 0°C; a third sample, drawn at room temperature, contained both the trans-planar mesophase and a fraction of the helical form II. By annealing this sample at increasing temperatures, we obtained a series of samples containing either trans-planar mesophase, or form II and form I crystallinities. In the dynamic-mechanical analysis, the sample containing form I crystallinity showed only the amorphous glass transition, at 19°C, before melting at a high temperature. The trans-planar mesophase transformed, at temperatures higher than 50°C, into the helical forms, and this transition was completed at 80°C. The dynamic mechanical curve of the sample containing only the mesophase showed a peak, centered at 50°C, which could be clearly associated to this transition. The sample containing the trans-planar mesophase and the helical form II, showed in the dynamic-mechanical curve a third peak that can be associated with the melting recrystallization of form II into the most stable form I. These results are important, because it was possible to directly correlate the structural transitions of the sPP polymorphs to the dynamic-mechanical behavior. Moreover, a dynamic-mechanical analysis could help recognize the presence of the tran-planar mesophase or of the helical form II in more complex structural organizations.