Micromechanical testing of polyisobutylene-polystyrene block-type thermoplastic elastomers

The mechanical properties of a variety of linear triblock copolymers (polysytrene-polyisobutylene-polystyrene, PS-PIB-PS) have been measured. The molecules were synthesized using living carbocationic polymerization of isobutylene, followed by the sequential addition of the second monomer, styrene. The microstructure of the resulting materials was examined as a function of mass fraction of PS using transmission electron microscopy of stained thin films. At small mass fractions, the reinforcing phase (PS) was distributed within the rubbery PIB phase, and the composite could be considered micromechanically using the Bard and Chung model. The comparison of experiment to theory revealed a strong mechanical dependence on Langley-Graessley intermolecular entanglement in the PIB phase. At higher mass fractions the microstructure was seen to consist of interpenetrating networks similar to the double gyroid morphology reported for block copolymers. The experimental approach involved the preparation of solution cast transmission electron microscopy specimens from which the undistorted size, distribution and arrangement of phases could be measured, and the similar preparation of thicker tensile specimens from which the accurate mechanical response could be measured. In this way the correlation between the microstructure and deformation was made accurately. The unique mechanical testing methods and instrumentation to measure tensile properties of thin samples were described.