Influence of the Initial Morphology on the Elasticity of Oriented Syndiotactic Polypropylene

The influence of the initial phase composition and crystallite size on the mechanical properties and the elastic behavior of oriented syndiotactic polypropylene (sPP) was investigated. Two samples (A 25 and B 100 ) were crystallized from the melt respectively at 25 and 100 °C, obtaining the same crystalline forms but different morphologies, in terms of crystallinity, crystal dimensions, and phase composition. X-ray diffraction was used to determine the crystallinity and the crystal dimensions, whereas the vapor transport properties allowed to evidence and quantify the presence of an intermediate phase. Sample A 25 shows smaller crystals, lower crystallinity, and a lower amorphous fraction but a larger mesomorphic component as compared to B 100 . The two samples were then stretched to a draw ratio of 7. They showed very different stress−strain curves, which appear to correlate with the initial structure. On releasing the tension both fibers undergo a large shrinkage, reaching λ = 4. The presence of the trans-planar form III and of the trans-planar mesophase characterize the fiber A 25 at λ = 7 while upon relaxation only the trans-planar mesophase and a small fraction of an oriented helical polymorph are apparent. In the fully extended fiber B 100 well-oriented form III and small amounts of a modestly oriented helical form were evidenced. The relaxed fiber B 100 contains an essentially unchanged crystalline helical component along with the trans-planar mesophase. Stress−strain cycles of fibers A 25 and B 100 allowed the determination of the hysteresis curves and the permanent set. The stress−strain data were evaluated in terms of the Mooney−Rivlin equation and show for fiber B 100, with a more substantial, less constrained amorphous fraction and larger crystals, a more traditional elastomeric behavior. The permanent set is similar for the two samples, but A 25, characterized by smaller crystallites, more intimately connected with the amorphous and a larger intermediate phase component, shows both higher modulus and strain hardening. This features appear related to the limited extensibility of the chains forming the elastic network of A 25 and to its capacity to crystallize under stress.