Nanostructure Evolution of SIS Thermoplastic Elastomers During Straining as Revealed by USAXS and Two-Dimensional Chord Distribution Analysis

Abstract

A new method for the analysis of small-angle X-ray scattering (SAXS) of anisotropic samples is introduced in the field of thermoplastic elastomers. It results in a detailed picture of the complex two-phase nanostructure and its range of order as a function of strain. A linear triblock copolymer from polystyrene and polyisoprene (SIS) and a brominated sample are studied as a function of elongation by ultra-small-angle X-ray scattering (USAXS) from synchrotron. The styrene content is 30%. Two-dimensional scattering patterns are transformed to the Laplacian of the two-dimensional correlation function. Analysis of this chord distribution function (CDF) is demonstrated and compared to direct analysis of the fiber patterns. CDF analysis proves to be superior, especially when the description of lateral arrangement of domains is concerned. Neat isotropic SIS reveals remarkable long-range order. When strained to high elongation, it exhibits elongated cylindrical domains arranged in a macro-lattice, whereas the brominated material shows almost spherical domains arranged in microfibrils with some lateral order. Domain shape and arrangement is determined.