The role of crystals in the elasticity of semicrystalline thermoplastic elastomers

The role of crystals in the elasticity of semicrystalline polymers is discussed in the case of syndiotactic polypropylene, which provides an example of a thermoplastic elastomer with a degree of crystallinity that can be tailored by changing and controlling the stereoregularity. This can be achieved using metallocene catalysts with different structures and stereoselectivity. The comparison of crystallization and physical properties of samples of syndiotactic polypropylene of different stereoregularity, with rrrr pentad concentrations being variable in the wide range 26-96%, prepared with different catalysts, has shown that syndiotactic polypropylenes present different types of elastic behavior, depending on the degree of crystallinity. For the most-stereoregular and crystalline samples with high melting temperatures, crystals actively participate to the elastic response of the material and elasticity has a mainly enthalpic character attributable to the metastability of the trans-planar form III that transforms into the more-stable helical form II during elastic recovery. For less-crystalline samples, with low melting temperatures, elasticity has instead a pure entropic origin as in conventional thermoplastic elastomers, and crystals act only as knots of the physical elastomeric network.