(53f) Extreme Architectural Asymmetry with Miktoarm Star Polymers: Tough Thermoplastic Elastomers and Frank-Kasper Phases

Thermoplastic elastomers (TPE's) are ubiquitous materials based on ABA triblock copolymers that have uses ranging from additives, to inks, to synthetic rubbers. The unique mechanical properties of TPE's are contingent on their self assembly into spherical or cylindrical morphologies composed of discrete glassy "A" domains immersed in a continuous "B" rubber matrix. Unfortunately, the requirement for spherical or cylindrical morphologies has historically set an upper bound on the volume fraction of the "A" domains in the material (<0.3), which has limited the resulting toughness of TPE materials. In principle, increasing the "A" volume fraction while still maintaining morphologies with discrete "A" domains should yield materials that are uniquely hard, tough and elastic.

In recent years, nonlinear A(BA')n miktoarm star polymers have emerged as a promising approach toward achieving TPE's with uniquely hard, tough and elastic materials. In this talk, we employ self-consistent field theory to examine the effect of extreme architectural asymmetry on the phase behavior of these miktoarm stars. We show that increasing the arm count of these miktoarm stars dramatically widens the phase boundaries of the spherical and cylindrical phases, with "A" domain volume fractions exceeding 0.5 and 0.7, respectively. Notably, these regions of sphere stability are shown to contain very large regions where Frank-Kasper sphere phases, such as the σ and A15 phases, are globally stable. We then quantify the prevalence of chain bridging in these morphologies and show that chain bridging is significantly enhanced in highly-asymmetric miktoarm stars. Though the focus of this talk is on TPE's, the results presented here are generally applicable to any engineering scenario where it is desirable to decouple the volume fraction of a component from the resulting morphology.