Elongational flow-induced crystallization of poly(ethylene terephthalate) under the supercooled state

Abstract

Elongational flow-induced crystallization in supercooled liquid poly(ethylene terephthalate) (PET) was investigated via elongational flow opto-rheometry (c.f.o.r.) and temperature-modulated differential scanning calorimetry (t.m.d.s.c.) in the temperature range 100–130°C, in which the blow moulding operation on PET is usually conducted. Measurements of tensile stress σ( t) and birefringence Δn( t) as a function of the Hencky strain rate ϵ ̇ 0 and time t with e.f.o.r. revealed the features of their time evolution as being due to spherulite and/or flow-induced oriented crystalline formation during the elongation process. Below 110°C, where spherulite growth was negligible, elongational viscosity η E( t) (= σ(t)/ ϵ ̇ 0) determined at different ϵ ̇ 0 first developed slowly and exhibited ‘up-rising’ or ‘hardening’ after certain periods of time t ηE; this behaviour reduced to a characteristic Hencky strain ϵ ηE (= ϵ ̇ t ηE ) ≅ 1.0 , independent of ϵ ̇ 0 , reflecting the formation of flow-induced oriented crystallites beyond ϵ ηE. At higher temperatures (∼ 120–130° C), however, where spherulites that contributed significantly to η E but little to Δn( t) were formed during the resting period and/or in the early stage of elongation, the critical up-rising strain ϵ ηE varied from ϵ ηE ≅ 0.5 to ϵ ηE ≅ 1.0 as ϵ ̇ 0 was increased. Eventually the spherulites appeared to be deformed or broken down to form oriented crystallites. Analyses via t.m.d.s.c. combined with e.f.o.r. identified spherulites in the specimens elongated at high temperatures but flow-induced oriented crystallites in those elongated at low temperatures.