| Abstract
| - Highly crystalline materials show elastic behavior when crystals actively participate in the elastic recovery. Elasticity in syndiotactic polypropylene has enthalpic or entropic origin depending on stereoregularity and crystallinity. Enthalpic elasticity is due to the transition of the trans-planar form III into the more-stable helical form II during elastic recovery.
- The role of crystals in the elasticity of semicrystalline polymers is discussed in the case of syndiotacticpolypropylene, which provides an example of a thermoplastic elastomer with a degree of crystallinitythat can be tailored by changing and controlling the stereoregularity. This can be achieved using metallocenecatalysts with different structures and stereoselectivity. The comparison of crystallization and physicalproperties of samples of syndiotactic polypropylene of different stereoregularity, with rrrr pentadconcentrations being variable in the wide range 26−96%, prepared with different catalysts, has shownthat syndiotactic polypropylenes present different types of elastic behavior, depending on the degree ofcrystallinity. For the most-stereoregular and crystalline samples with high melting temperatures, crystalsactively participate to the elastic response of the material and elasticity has a mainly enthalpic characterattributable to the metastability of the trans-planar form III that transforms into the more-stable helicalform II during elastic recovery. For less-crystalline samples, with low melting temperatures, elasticityhas instead a pure entropic origin as in conventional thermoplastic elastomers, and crystals act only asknots of the physical elastomeric network.
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