Effect of loading and process conditions on the mechanical behavior in SEBS thermoplastic elastomers (TPEs)

Styrenic block copolymer thermoplastic elastomers are one of the most widely used thermoplastic elastomers (TPEs) today. The focus of this research is to fundamentally understand the structure-process-property relationships in these materials. Deformation behavior of the block copolymers with cylindrical and lamellar morphologies has been investigated in detail using unique techniques like deformation calorimetry, transmission electron microscopy (TEM), combined in-situ small angle x-ray and wide angle x-ray scattering (SAXS/WAXS). The research involves the study of structural changes that occur at different length scales along with the energetics involved upon deformation. The structural changes in the morphology of these systems on deformation have been investigated using combined SAXS/WAXS setup. Small angle x-ray scattering probed the changes at the nano-scale of polystyrene (PS) cylinders, while wide angle x-ray scattering probed the changes at molecular length scales of the amorphous/crystalline domains of the elastomeric mid-block in these systems. TEM analysis of the crosslinked elastomers (by UV curing) further confirms the interpretation of structural details as obtained from SAXS upon deformation. New structural features at both these length scales have been observed and incorporated into the overall deformation mechanisms of the material. Characteristic structural parameters have been correlated to differences in their mechanical response in the commercially relevant cylindrical block copolymers. Effect of various process conditions and thermal treatments has been investigated. The process conditions affect the structure at both micro-scopic (grain size) and nano-scopic (domain size) length scales. A correlation has been obtained between a mechanical property (elastic modulus) and an easily measurable structural parameter (d-spacing). Effect of various phase transitions such as order-to-order transition has been studied. Selective solvents can preferentially swell one phase of the block copolymer relative to other and thus bring a change in morphology. Such kinetically trapped structures when annealed at higher temperature try to achieve their thermodynamic equilibrium state. Such changes in morphology significantly affect their tensile and hysteretic response. In another work it has been shown that by carefully compounding these styrenic block copolymers having different morphologies, it is possible to completely disrupt the local scale order and remove the grain boundaries present in these materials. Finally, a new test technique has been developed, by modifying an existing Charpy device to test polymeric films at a high strain rate. A custom designed load-cell is used for force measurements which imposes harmonic oscillations on a monotonic loading signal. The data obtained from this device can be used to analyze visco-elastic response of polymeric films at frequencies much higher than the conventional dynamic mechanical analyzer (DMA)