Directing Thermoplastic Elastomer Microdomain Parallel to Fiber Axis: A Model Case of SEBS with Benzoxazine through π-π Stacking

In our previous work, we revealed for the first time the existence of ordered-lamellar microdomains in thermoplastic elastomer based on the case study of an as-spun electrospun polystyrene-b-poly(ethylene-co-l-butene)-b-polystyrene triblock copolymer (SEBS), and also demonstrated how the extensional force initiated a certain level of lamellar-microdomain orientation. The present work shows the approach to direct lamellar-microdomain orientation to be parallel to the fiber axis by simply initiating a specific molecular interaction among thermoplastic elastomer chains. The blend system of SEBS and bisphenol A based benzoxazine monomer (BZ) forms pi-pi interaction as confirmed by the nuclear overhauser effect in the nuclear magnetic resonance spectroscopy (NOESY NMR). The electrospun fibers of this blend under a low take-up velocity (below 310 m/min) show not only microdomain orientation but also (i) the fragmented lamellar grains which orient almost perfectly parallel to the stretching direction (SD) or fiber axis, and (ii) the lamellar angles (mu) are also almost parallel to the fiber axis. When the take-up velocity reaches a certain level (similar to 620 and 1240 m/min), the extensional force overcomes the. pi-pi interaction between SEBS and BZ, and as a result, the regular microdomains oriented to the fiber axis are suddenly diminished. Thermal treatment of the fibers (similar to 170 degrees C) initiates the lamellar microdomain rearrangement to an isotropic one with significant traces of lamellar orientation parallel and perpendicular to the fiber axis. The present work demonstrates how two extreme combined conditions, which are (i) macroscopic external stimulus, i.e., extensional force, and (ii) specific interaction at molecular level, i.e., pi-pi interaction, direct the orientation of rnicrodomains in the confined space of the as-spun electrospun fibers to be a uniaxial microdomain aligned parallel to the fiber axis.