Unexpected Crystallization Barrier in Partially Miscible Polymer Blends -A New Opportunity for Tailoring Self-Reinforcing Polymer Materials

Abstract

Polymers that can store mechanical energy administered shock-wise would be superior to simply shock absorbing elastic materials due to advanced safety and the ability to simultaneously store energy. Such so-called rapid energy and shock absorbing (RESA) polymeric materials require a rubber-elastic polymer network that exhibits strain-induced formation of highly melting, shape-stabilizing crystals while being intrinsically inhibited against thermal crystallization. So far no such material exists. Our approach to design those is to cross-link partially miscible blends composed of semi-crystalline poly(vinylidene fluoride) (PVDF) and amorphous poly(2-ethyl-2-oxazoline) (PEtOx). Networks with a PVDF content of 65 wt% undergo strain-induced crystallization upon rapid uniaxial deformation, resulting in significant self-reinforcement characterized by strain hardening. Further, the crystallization within these networks is inhibited even above the glass transition temperature (Tg) of the blends. This unusual behavior is attributed to a local compositional segregation during formation of a minor fraction of seed crystals, which increases the local Tg in the amorphous interphase between crystals and mixed amorphous blend that retains the lower mixture Tg. This unexpected effect prevents further crystal growth even in a material with an overall Tg below ambient temperature.