Effect of Pre-Melting Time on Crystallization of Poly(ethylene terephthalate)

In this study poly(ethylene terephthalate) (PET) was melted at 300°C, approximately 46°C above the crystalline melting point, Tm, for different times, i.e., Δtm,=5, 8, and 10 min, and then quenched to different isothermal crystallization temperatures, Tc, ranging from 190°C to 230°C. The effect of pre-melting time, Δtm, at 300°C on the degree of crystallinity and on crystalline morphology were investigated by differential scanning calorimetry (DSC) and polarized-light microscopy (PLM). After crystallization at low Tc, PLM data revealed the PET contained usual, positive, and unringed spherulites. After crystallization at high Tc PET contained unusual, ringed, and double-extinction spherulites. The experimental results reveal that increasing the pre-melting time Δtm at 300°C causes an increment in Tc for usual–unusual, unringed–ringed, and positive–double-extinction transitions of the PET spherulites. The experimental results also show that PET with a pre-melting time Δtm=8 min had higher crystallinity than those with pre-melting times Δtm=5 and 10 min. These crystallization phenomena were attributed to the different numbers of residual unmelted PET crystallites as a result of the variation in pre-melting time, Δtm, at 300°C.