owadays, polyester fibres are the most important man-made fibre and the production amount of these fibres is increasing continuously. These fibres have high tenacity, high chemical stability and low water absorption. Due to the lack of enough functional group in the polymeric chain op poly (ethylene terephthalate) fibres, and because these fibres are very crystalline and compact, they mainly dyed with disperse dyes in high temperature(above 120°C); or in boiling temperature and in the presence of a carrier. Conventional poly (ethylene terephthalate) fibres have very low amount of diethylene glycol in their molecular chain. Diethylene glycol are produced by the reaction of two units of ethylene glycol and have etheric group. During the polymerization of poly (ethylene terephthalate), these diethylene glycol units react with terephthalic acid (TPA) and because part of the PET polymeric chain. These have been many researches about the modification of the polyester fibres to improve their different features and/or to give them new characteristics. In this research, the effect of the amount of diethylene glycol, in the polymeric chain of poly (ethylene terephthalate), on the physical, mechanical and dyeing properties of PET fibres is investigated. In this study, poly (ethylene terephthalate) filaments fibres having two different amount of diethylene glycol (0.9% and 1.4%), which have been produced in Zagros Co. were used. The type and amount of functional groups of the polymeric chain of the poly (ethylene terephthalate) fibres were investigated using Furrier Transform Infrared (FTIR). The ratio between crystalline and amorphous region in the poly (ethylene terephthalate) fibres were calculated and compared, using three methods Furrier Transform Infrared (FTIR), Differential Scanning Calorimetry (DSC) and X Ray Diffraction (XRD). In this research, dye absorbtion of the fibres after a specific time and their rate of dyeing with disperse dyes, in different dye methods were studied. The results show that the presence of higher amount of diethylene glycol reduces the crystallinity in the undrawn and low oriented fibres and increase the crystallinity in the drawn fibres. Also increasing the amount of diethylene glycol in the PET polymeric chain reduces the transition temperature (Tg), melting point (Tm) and tensile strength, and increases their elongation and moisture absorption. In the fibres with low and medium orientation, the presence of diethylene glycol increases the dye absorption and the rate of dyeing. These effects are reduced by the increasing of the amount of diethylene glycol in the fibres and even in some of fully drawn fibres this increase can reduce the dye absorption and the rate of dyeing.
