Elongational Flow-Induced Crystallization and Structure Development in Supercooled Poly(ethylene naphthalate)

Elongational flow-induced crystallization behavior was investigated on supercooled liquids of two poly(ethylene naphthalate)s (PENs) with different crystallization habits via elongational flow optorheometry (EFOR), temperature-modulated differential scanning calorimetry (TMDSC), and Rayleigh scattering in the temperature range 170−190 °C. The samples were an antimony-catalyzed PEN (Sb-PEN) with a rapid crystallization rate but less well-organized spherulites and a germanium-catalyzed PEN (Ge-PEN) with a temperature sensitive (slow) crystallization rate but better organized spherulites. In the elongation of Sb-PEN at 170 °C, a strong tendency of strain hardening was observed in the early stage and transformation of the spherulites into rodlike morphology occurred in the later stage, accompanying a substantial increase in birefringence. On the other hand, for Ge-PEN at 170 °C, molecular orientation along the flow direction proceeded and flow-induced crystallization took place rather suddenly at a Hencky strain of ε ≈ 2.65, with the crystalline lamellae irregularly growing transverse to the oriented chains. However, in Ge-PEN elongated at 190 °C, especially with a low strain rate ε 0 (<0.01 s -1 ), stable spherulites were formed and their growth dominated the subsequent elongation behavior. In these PENs, the features of the flow-induced structure development were governed by the dimensionless strain rate, which was the ratio of ε 0 to the spherulite growth rate under the quiescent state. Depending on the dimensionless strain rate being above or below the critical value, either the oriented-crystallite formation or the spherulite growth dominated the elongation behavior, respectively.