Exploring the role of crystal thickness and secondary crystallization in tie molecule effectiveness and ductility

In most applications, it is immensely desirable for semicrystalline polymers to resist brittle fracture. Tie molecules (TMs) – individual polymer chains connecting adjacent crystalline lamellae – are considered a key structural feature governing this mechanical toughness, yet little research has investigated whether all TMs are equally effective at transmitting stress. Here, we study the effects of comonomer content and thermal history on ductility in semicrystalline hydrogenated polynorbornene and its random copolymers with hexylnorbornene. In quenched specimens, we find that TM content at the brittle-ductile transition (the onset of ductility) is a strong function of crystal thickness, indicating that a TM’s effectiveness depends on how firmly embedded it is within each lamella. However, in low-M_n polymers with high comonomer content, crystallization via slow-cooling promotes the formation of two distinct lamellar stacks. Counterintuitively, the stacks containing the thinner crystals can constitute the majority phase. Because the brittle-ductile transition occurs at the point where brittle fracture stress surpasses yield stress, these secondary stacks with thinner crystals govern ductility by yielding at a lesser tensile stress.