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. A way to obtain such novel material properties is to design a polymeric system that is rubber-elastic and has the ability to form shape-stabilizing crystals upon strain while no such crystals are formed upon thermal crystallization. So far no such material exists. This study investigates, if cross-link partially miscible blends composed of semi-crystalline poly(vinylidene fluoride) (PVDF) and amorphous poly(2-ethyl-2-oxazoline) (PEtOx) shows such behavior. Networks with a PVDF content of 65 wt% were found to inhibit the thermal crystallization of PVDF at ambient temperature by 69% of the maximal degree of crystallinity. Rapid stretching this material affords a strain-induced crystallization, which results in self-inforcement and stabilization of the stretched shape. The crystallization within these networks is inhibited even above the glass transition temperature (T_g) 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 T_g in the amorphous interphase between crystals and mixed amorphous blend that retains the lower mixture T_g. The unexpected effect prevents further crystal growth even in a material with an overall T_g below ambient temperature. The increased local T_g is increasing the stiffness of the material, making it unsuitable as shock absorber, but the blend approach is a promising way towards rapid energy and shock absorbing materials.