Processing-Structure-Mechanical Property Relationships of Semicrystalline Polyolefin-Based Block Copolymers FANNY DEPLACE, 1,2 ZHIGANG WANG, 1,2 NATHANIEL A. LYND, 1,2 ATSUSHI HOTTA, 1,2 JEFFREY M. ROSE, 3 PHILIP D. HUSTAD, 3 JUN TIAN, 3 HISASHI OHTAKI, 3 GEOFFREY W. COATES, 3 FUMIHIKO SHIMIZU, 4 KOUSOU HIROKANE, 4 FUMIYOSHI YAMADA, 4 YONG-WOO SHIN, 4 LIXIA RONG, 5 JIE ZHU, 5 SHIGEYUKI TOKI, 5 BENJAMIN S. HSIAO, 5 GLENN H. FREDRICKSON, 1,2 EDWARD J. KRAMER 1,2 1 Department of Materials, Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California 2 Department of Chemical Engineering, Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California 3 Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 4 Mitsubishi Chemical Group, Science and Technology Research Center, Inc., Yokohama, Japan 5 Department of Chemistry, SUNY Stony Brook, Stony Brook, New York Received 30 September 2009; revised 2 December 2009; accepted 2 December 2009 DOI: 10.1002/polb.21969 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: The incremental plastic deformation of the crystals of block copolymers made of semicrystalline polypropylene endblocks and amorphous ethylene-r-propylene midblocks occurring during step cycle tensile tests has dramatic effects on the stress-strain curves. This can be understood from the evolution of the morphology and of the microstructure of the crystalline blocks revealed by X-ray scattering experiments. V C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1428–1437, 2010 KEYWORDS: block copolymers; elastomers; mechanical proper- ties; polyolefins; thermoplastics; SAXS; WAXS INTRODUCTION The volume of polyolefins produced dwarfs that of all other classes of polymers, due to the low cost of the monomers and the range of mechanical properties that can be achieved. 1,2 Recent catalyst discoveries have enabled the synthesis of multiblock copolymers with high melting point semicrystalline syndiotactic polypropylene (sPP) blocks and rubbery, low glass transition temperature (T g ) ethylene- r-propylene (EPR) blocks. 3,4 These polymers have remark- able mechanical properties, as neat thermoplastic elastomers, as gels in mineral oil, and as ‘‘dried gels’’ prepared from gels from which the mineral oil has been extracted. They can be used as thermoplastic elastomers and can potentially com- pete in cost and performance with hydrogenated styrenic block copolymers and thermoplastic urethane elastomers. 4,5 The mechanical properties of these semicrystalline block copolymers can be dramatically improved by step cyclic me- chanical deformation. The semicrystalline polyolefins sPP have crystalline regions with high melting temperature (T m ) [T m (sPP) 186  C with [rrrr] ¼ 0.94 6 ] higher than polyethyl- ene [T m (PE) 105–130  C 7 ] but they also have amorphous regions exhibiting a relatively high T g [T g 0  C 8 ]. For this reason alone, they are unsuitable as elastomeric materials. 9 The random copolymerization of propylene with low T g monomers such as 1-hexene lowers the T g but also decreases the crystallinity and, therefore, decreases T m which is not favorable for high temperature applications. The development of new metal centered, single site polymer- ization catalysts have led to the previously unimagined capa- bility to produce multiblock copolymers with high T m stereo- regular polypropylene blocks and low T g EPR blocks. This enables the formation of block copolymers that have both high use temperatures and excellent elastomeric properties for weight percentage of sPP blocks less than 30%. The properties include a low initial Young’s modulus, a high elas- tic recovery, and a good extension ratio to break. 3–5,10,11 One major difference between chemically crosslinked elastomers and those elastomers based on the semicrystalline block copolymers is the fact that once a strand in the crosslinked network has been pulled tight and subsequently broken, it no longer can contribute to either the elasticity or the frac- ture resistance of the network. Because the ‘‘crosslinks’’ in the semicrystalline block copolymer elastomers are crystals that can deform plastically, this plastic deformation could protect the network from total failure and promote a more Correspondence to: E. J. Kramer (E-mail: edkramer@mrl.ucsb.edu) Journal of Polymer Science: Part B: Polymer Physics, Vol. 48, 1428–1437 (2010) V C 2010 Wiley Periodicals, Inc. 1428 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLB