Effect of Crystal Polymorphism on Elastic Modulus and Hardness of Poly(Butylene 2,6‐Naphthalate) (PBN)

The modulus of elasticity and hardness of poly(butylene 2,6‐naphthalate) (PBN) were investigated at different length scales using dynamic‐mechanical analysis (DMA), microhardness testing, and nanoindentation. By variation of the conditions of melt‐crystallization, PBN containing around 40% α‐crystals, β′‐crystals, or liquid crystals, beside glassy amorphous structure, was obtained. The absence of crystals and formation of liquid crystals, achieved by quenching the melt to ambient temperature, yield a material of relatively low stiffness and hardness of around 1/2 to 2/3 of PBN preparations containing crystals. The difference in modulus and hardness of PBN containing α‐ or β′‐crystals is minor, with slightly higher values observed for PBN composed of the amorphous glass and β′‐crystals. The higher values of the elastic modulus and hardness in case of presence of crystals are attributed to the higher packing density of molecular stems in the crystalline phase, compared to their rather loose packing within the layers of the smectic liquid crystals, besides the possible effect of the morphology and superstructure if plastic deformation is involved. The findings of this study provide important insight into structure–property relationships of polymorphic PBN and offer guidance for the design of this material for specific industrial applications.