Structure Evolution during Cyclic Deformation of an Elastic Propylene-Based Ethylene-Propylene Copolymer Shigeyuki Toki,* ,† Igors Sics, † Chris Burger, † Dufei Fang, † Lizhi Liu, † Benjamin S. Hsiao,* ,† Sudhin Datta, ‡ and Andy H. Tsou ‡ Department of Chemistry, Stony Brook UniVersity, Stony Brook, New York 11794-3400, and ExxonMobil Chemical Company, Baytown Technology & Engineering Complex-West, Baytown, Texas 77522-5200 ReceiVed January 3, 2006; ReVised Manuscript ReceiVed March 17, 2006 ABSTRACT: In-situ structural evolution during uniaxial extension and subsequent retraction of a thermoplastic elastomer (TPE) based on propylene-dominant ethylene-propylene (EP) copolymer was studied. Combined measurements of time-resolved wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) as well as stress-strain curves revealed molecular mechanism responsible for the elastic behavior. During the first cycle of deformation, a fraction of the crystals was destroyed, while the rest was reoriented. At strains larger than 1.0, strain-induced R-crystals in the lamellar form took place, resulting in the creation of a network with well-oriented lamellae having their normals parallel to the stretching direction. With the increase of strain, more crystals were induced, forming an enhanced network with strain-hardening behavior. During retraction and even after complete relaxation to zero stress, the majority of the strain-induced crystalline network remains in tact as being “permanent set”, where lamellar stacks act as the network points. This strain-induced crystalline network structure is thermally stable at room temperature and is responsible for the elastic behavior during subsequent cyclic deformation, similar to a vulcanized rubber. Introduction Understanding the relationships between structure and physi- cal properties is essential for developing thermoplastic elastomer (TPE) products with elastic properties approaching those of cross-linked rubbers. Since the introduction of elastic ethylene- propylene (EP) copolymers (also referred as thermoplastic polyolefin (TPO)) in the early 1960s, this class of material has become one of the fastest growing TPEs because of its excellent ozone resistance and high extensibility, suitable to replace some vulcanized rubber goods. The typical commercialized TPOs or TPVs (i.e., vulcanized TPOs) are composed of multiple components, including polypropylene (PP), ethylene-propylene rubber (EPDM), and a substantial amount of paraffin oil. The reason why TPV can behave as a vulcanized rubber despite having the stiff PP component has been provided by Inoue et al. 1 They argued that the PP crystallites play the role of tie (cross-link) points for amorphous chains to render the elastic recovery of TPE. In other words, the fragmented crystallites behave as cross-links and the swollen amorphous PP regions behave as the rubbery domains. Recently, Hsiao et al. 2 have studied the relationship between structure and property of an ethylene-based ethylene-propylene copolymer (made with the conventional vanadium catalyst) during tensile deformation using the time-resolved synchrotron small-angle X-ray scattering (SAXS) technique. They found that upon stretching some polyethylene crystallites were destroyed and new crystallites were induced by chain extension, resulting in the creation of a lamellar structure with its principal axis perpendicular (or its normal parallel) to the stretching direction. During retraction, strain-induced crystallites can largely remain intact with only partial relaxation of the oriented state. It is thought that the destruction of the initial PE crystals, which is often termed “mechanical melting”, at relatively low strains (starting from ca. 10%) can be attributed to the high mobility of chains in the ethylene crystals. The development of metallocene catalysis has enabled us to produce a new type of ethylene-propylene (EP) copolymer, having a polypropylene-based crystalline structure. 3-5 The metallocene reaction makes it possible to insert the ethylene unit into the polypropylene backbone randomly (not in block). The random distribution of the ethylene unit hinders the crystallization of the propylene segments. As a result, at the right composition ratio, the propylene (P)-based EP copolymer can attain a microstructure similar to the ethylene (E)-based EP copolymer 2,6-9 and other segmental elastomers such as polyurethanes 10-12 or poly(urethane-urea). 13 The propylene- based EP copolymer is considered more thermally stable (the melting point of PP is higher than that of PE) and mechanically stronger than its ethylene-based counterpart. In this study, we investigated in-situ structure changes during cyclic uniaxial deformation of a propylene-based EP copolymer using combined synchrotron SAXS and wide-angle X-ray diffraction (WAXD) techniques. The objective of this study is to understand the molecular mechanism responsible for the elastic behavior of TPE that can be similar to vulcanized rubber. A particular emphasis was placed on the monitoring of structure evolution, involving destruction and orientation of original crystallites (e.g., mechanical melting), strain-induced new crystallites, and extension of amorphous chain segments, during the first extension and retraction cycle as well as subsequent cycles. It was found that the first cycle induced a “permanent set” crystalline network structure that dictates the resulting elastic properties. Experimental Section The chosen propylene-based ethylene-propylene copolymer was an experimental sample provided by the ExxonMobil Chemical † Stony Brook University. ‡ ExxonMobil Chemical Company. * To whom correspondence should be addressed: e-mail stoki@mail.chem.sunysb.edu, bhsiao@notes.cc.sunysb.edu; Tel (631) 632- 7793; Fax (631) 632-6518. 3588 Macromolecules 2006, 39, 3588-3597 10.1021/ma0600106 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006