Elongational flow-induced crystallization in supercooled poly(ethylene terephthalate) with different crystallization habit

Abstract

Elongational flow-induced crystallization behaviour was investigated on supercooled liquids of two poly(ethylene terephthalate)s (PET) with different crystallization habit via elongational flow opto-rheometry (EFOR), temperature-modulated differential scanning calorimetry (TMd.s.c.) and Rayleigh scattering in the temperature range of 100–130°C usually employed for blow moulding operations. The samples were an antimony catalysed PET (Sb-PET) with rapid crystallization rate but less-ordered spherulites, and a germanium catalysed PET (Ge-PET) with temperature sensitive (slow) crystallization rate but better organized spherulites. In the elongation of Sb-PET at 110°C, transformation of the spherulites into rod-like morphology occurred even in the early stage, which was followed by a strong strain hardening tendency, accompanying the increasing of birefringence. On the other hand, for Ge-PET at 110°C, molecular orientation along the flow direction preceded and then flow-induced crystallization with crystalline lamellae growing transverse to the oriented chains took place rather suddenly at a Hencky strain ϵ of ≈2. However, in Ge-PET elongated at 130°C especially with low strain rate ϵ ̇ 0 , spherulite growth dominated the rheology. In these PETs, the features of the flow-induced structure development were governed by the dimensionless strain rate that was the ratio of ϵ ̇ 0 to the spherulite growth rate under quiescent state. Depending on the dimensionless rate being above or below a certain critical value, the oriented crystallite formation or the spherulite growth dominated the behaviour, respectively.