1. Introduction Incorporation of stereo defects and low molecular weight a- olefins such as ethylene and 1-butene in the chain of isotactic polypropylene (iPP) using metallocene catalysts has been effec- tively utilized to change the mechanical properties of iPP from a rigid thermoplastic to elasto-plastomeric and to elastomeric behavior [1e10]. Adding defects or a comonomer to the iPP macromolecule decreases the crystallizable sequence length in the chain, and hence the degree of crystallinity decreases. The iPP polymorphic behavior also changes with type and defect content as amply studied in the last two decades [11e24]. Conversely, blending different types of iPPs, iPP with iPP-based copolymers, or blending different iPP-based copolymers is also an avenue for en- gineering mechanical properties. Compared to the synthetic approach, iPP blends are often a cost-effective path to change the * Corresponding author. E-mail address: alamo@eng.fsu.edu (R.G. Alamo). http://dx.doi.org/10.1016/j.polymer.2016.08.101 0032-3861/© 2016 Elsevier Ltd. All rights reserved. The mechanical properties of binary blends of propylene-1-hexene random copolymers (with 11 and 21 mol% 1-hexene) are studied in parallel with polymorphic transformations under uniaxial tensile deformation. The modulus, yield stress, and draw ratio of the pure PH copolymers decrease with increasing 1-hexene content, while for the blends the change of mechanical properties with composition is highly non-linear. The addition of just 10 wt % PH11 to PH21 doubles the elastic modulus and yield strength of the blends in reference to the value of PH21, reaching for all blends values close to the performance of pure PH11. The elongation at break and the ultimate tensile strength increase more gradually with content of PH11. On tensile deformation, pure components and blends undergo morphological and polymorphic transformations, such as a reversible lamellar to fibrillar transformation of trigonal PH21, or an irreversible a crystal to mesophase in pure PH11 and the blends. In blends and neat PH11, a fibrillar trigonal morphology that develops under deformation is stabilized by the trans- formation of a to mesophase, and remains after removing the load, explaining the lower elastic recovery of the blends compared to PH21. The formation of stress-induced trigonal crystallites in PH11 and blends after strains >150% is explained as a decrease of the free energy barrier for nucleation of a phase that requires short iPP sequences.

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