Crystallization and Multi-Melting Behavior of Poly(Ethylene Terephthalate) Modified by Sodium Salt of 5-Sulpho-Isophthalic Acid

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 calorimetry. 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 T sub g and melting point T sub m , the crystallization rate of the copolyester sample decreased with increasing content of SIPM. The relative crystallization rate constant Z of SIPM/DMT (dimethyl terephthalate) 4 mol.% sample was about 1% pure PETs Z value. For isothermal crystallized copolyester samples, DSC heating curves displayed multi-melting behavior. This was interpreted by molecular weight fractionation during crystallization and premelting-recrystallization mechanism. This interpretation showed why the second melting point T sub m2 will change according to the Hoffman--Weeks (H--W) equation and the first melting point T sub m1 will increase with increasing SIPM. The principal cause of these phenomena is the high temperature crystallization rate which decreases rapidly with decreasing SIPM. 18 ref.--AA