Shear-induced crystallization of polymorphic poly(butylene 2,6-naphthalate)

Crystal nucleation of poly(butylene 2,6-naphthalate) (PBN) and the subsequent growth of α- or β′-crystals were investigated under different shear and crystallization conditions. The critical shear conditions for the generation of crystal nuclei at different temperatures were determined by analyzing the relationship between the onset time of crystallization, as detected by the increase in the shear modulus, and the shear rate/specific work of flow. Critical shear rates and specific work of flow to cause nuclei formation increase with temperature, being around 0.5s⁻¹ and 0.01MPa at 220°C, and around 3s⁻¹ and slightly above 0.1MPa at 235°C, respectively, for a shearing duration of 10s. As such, PBN exhibits a rather low critical specific work of flow compared to many other polymers, which is attributed to the presence of the rigid naphthalene group in its chemical repeat unit. When the shear conditions exceed critical values, β′-crystals containing molecular segments with a more extended chain conformation than in α-crystals also form at temperatures where α-crystals typically grow in the quiescent melt. At certain (mild) shear and crystallization conditions, compared to crystallization of the relaxed melt, crystallization is slower, pointing to the destruction of pre-existing nuclei. This study provides initial guidelines for tailoring the morphology (molecular orientation, semicrystalline superstructure, crystal polymorphism) of PBN by variation of the industrial processing conditions, ultimately affecting product properties.