The Role of Crystals in the Elasticity of Semicrystalline Thermoplastic Elastomers. Claudio De Rosa,* Finizia Auriemma, and Odda Ruiz de Ballesteros Dipartimento di Chimica, UniVersita ` di Napoli “Federico II”, Complesso Monte S. Angelo, Via Cintia, 80126 Napoli, Italy. ReceiVed February 17, 2006. ReVised Manuscript ReceiVed May 19, 2006 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. Introduction Rubber elasticity is a peculiar property of polymeric materials and is generally defined as being a property of the amorphous phase. Elastomers are high-molecular-weight polymers that possess chemical and/or physical cross-linking. For industrial applications, the temperature at which the elastomers are used must be above the glass transition temperature (to allow for full chain mobility), and in its normal unextended state an elastomer should be amorphous. The restoring force, after elongation, is largely entropic. As the material is stretched, the chains in random coil conforma- tion are forced to occupy more-ordered positions in more- extended conformations. On release of the applied force, the chains tend to return to a more-random state. The gross mobility of entire chains must be low. The cohesive energy forces between chains of elastomers permit rapid, easy extension. In its extended state, an elastomeric chain exhibits a high tensile strength, whereas at low extension, it has a low modulus. Polymers with a low cross-link density usually meet the desired properties requirements. After deformation, the material returns to its original shape because of the cross- linking. 1 Elastomeric materials should, therefore, be composed of long and flexible molecules of high molecular mass, should be amorphous in their stable unstretched state with a glass transition temperature much lower than the ambient tem- perature, and finally, should have a network structure to avoid viscous flow of chains. 1 In some cases (thermoplastic elastomers), the presence of a small level of crystallinity ensures the formation of the elastomeric network, because the small crystalline domains act as physical cross-links. Unusual elastic behavior has also been observed in highly crystalline polymers, such as syndiotactic polypropylene (sPP), 2,3 and some polyesters, such as poly(tetramethylene terephthalate) and poly(trimethylene terephthalate), 4,5 that do not comply with the general requirements described above. This suggests that the elasticity of highly crystalline polymers is compatible only when the crystals actively participate in the elastic recovery of the material. In these cases, and in all crystalline polymers that show unusual elastic properties, both the crystalline and amorphous regions play a key role in determining the mechanical behavior and elasticity. The role of the crystals may show up by the occurrence of either stress-induced phase transitions, as in sPP 2,3 and poly- (tetramethylene terephthalate), 4,5 or reversible strains of the crystalline lattice through a continuum of accessible states, as in poly(trimethylene terephthalate). 4,5 The crystals act as cushion for the elastomeric network, storing the mechanical energy applied during the deformation and releasing the energy upon removal of the strain. The active role of crystals in defining the elastomeric properties of crystalline polymers is discussed in this paper in the case of sPP. It has been recently suggested that polymorphic transformations occurring in the crystalline * To whom correspondence should be addressed. Tel: 39 081 674346. Fax: 39 081 674090. E-mail: claudio.derosa@unina.it. (1) Treolar, L. R. G. The Physics of Rubber Elasticity; Claderon Press: Oxford, U.K., 1975. (2) Auriemma, F.; Ruiz de Ballesteros, O.; De Rosa, C. Macromolecules 2001, 34, 4485. (3) Auriemma, F.; De Rosa, C. J. Am. Chem. Soc. 2003, 125, 13144. (4) Jakeways, R.; Ward, I. M.; Wilding, M. A.; Hall, I. H.; Desborough, I. J.; Pass, M. G. J. Polym. Sci., Part B 1975, 13, 799. (5) Ward, I. M.; Wilding, M. A.; Brody, H. J. Polym. Sci., Part B 1976, 14, 263. 3523 Chem. Mater. 2006, 18, 3523-3530 10.1021/cm060398j CCC: $33.50 © 2006 American Chemical Society Published on Web 06/23/2006