Structural Changes at Different Length Scales Caused by Mechanical Deformation of SEBS Triblock Copolymers
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Structural Changes at Different Length Scales Caused by Mechanical Deformation of SEBS Triblock Copolymers Kishore K. Indukuri, Edward T. Atkins and Alan J. Lesser Department of Polymer Science and Engineering, Silvio O. Conte National Center for Polymer Research, University of Massachusetts, Amherst, MA. Abstract A Kraton® poly (styrene-ethylene/butylene-styrene) (SEBS) Triblock copolymer, with hexagonally packed PS cylinders embedded within the Poly (ethylene/butylene) (E/B) matrix, has been studied using simultaneous small- and wide-angle X-ray diffraction (SAXD/WAXD) as a function of mechanical deformation. SAXD studies on unstrained samples not only show a series of concentric diffraction rings, that index as the successive orders of a two-dimensional hexagonal lattice of side a ≈ 30 nm, but also indicate the presence of a broader, more diffuse, diffraction ring of a different character centered at a spacing of 7.8 nm. At 65% strain, a four-point X-cross pattern develops and emerges from the {10 1 0} diffraction ring for the unstrained sample. The fundamental spacing of the hexagonal lattice increases, and in addition the first evidence of discrete layer lines appears, orthogonal to the strain direction (SD). At higher strains, both the layer line spacing and the angle of the X-cross, relative to the SD, increase. These changes in layer line spacing as a function of strain provide additional insight into the extent of deformation imposed on the flexible matrix with shearing of PS cylinders layers relative to each other. Also, with increasing strain, the diffuse 7.8 nm diffraction signal transforms into a streak, parallel to the SD and centered on a reciprocal plane orthogonal to the SD. In the simultaneously recorded WAXD (4 nm-0.2 nm) patterns, an initial single diffraction ring observed at no strain, steadily transforms with strain into a symmetric pair of concentrated arcs centered on the equator. These X-ray diffraction results enable a delineation to be made between rotation and relative shear of layers of PS rods and the simultaneous strain-induced crystallization of the flexible matrix. Introduction
Styrenic block copolymers with cylindrical morphologies are one of the most widely used thermoplastic elastomeric (TPE) systems commercially and hence extensively studied 1-8 with respect to their morphology and mechanical properties. Keller and coworkers 9-11 studied the deformation of highly oriented ‘single crystal’ styrenebutadiene-styrene (SBS) TPEs using small angle x-ray diffraction (SAXD), TEM and birefringence measurements. Hashimoto and coworkers 3 using SAXD technique have also studied the deformation behavior of SBS triblock copolymers with cylindrical morphologies. The structural changes in morphology and energy absorption characteristics on deformation of oriented ‘single crystal’, as well as isotropic SBS block copolymers have been studied by Godovsky and coworkers 7,8,12 using SAXD and deformation calorimetry respectively. The ‘zig-zag’ structure or chevron instability in
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