The crystallization kinetics of the copolyester

poly(ethylene terephthalate) (PET) modified by sodium salt of 5-sulpho-isophthalic acid(SIPM)

was investigated by means of differential scanning calorimeter. The experimental results and polari-microscopy observation all showed that the introduction of SIPM did not affect the nucleation of crystallization. Within the temperature range between their glass transition temperature Tθ and melting point Tm

the crystallization rate of the copolyester sample decrrased with increasing content of SIPM.The relative crystallization rate constant Z of SIPM/DMT (dimethyl terephthalate)4mol% sample was about 1% pure PET's Z value.For isothermal crystallized copolyester samples

DSC heating curves displayed multi-melting behavior.This was interpreted by molecalar weight fractionation during crystallization and premelting-recrystallization mechanism.This interpretation showed why the second melting point Tm2 will change according to Hoffman-Weeks(H-W) equation[1] and the first melting point Tm1 will increase with increasing SIPM.The principal cause of these phenomena is the high temperature crystallization rate decreases rapidly with increasing SIPM.