The effect of dye on molecular motions of poly (ethlene terephthalate) (PET) was investigated by dynamic viscoelastic measurement of PET films dyed with C. I. Disperse Violet 1 (Violet 1) and C. I. Disperse Orange 3 (Orange 3). Both α and β loss modulus peaks of dyed films shifted to lower temperature side with increase of dye concentration. The relaxation intensity of β dispersion decreased with increase of dye concentration. As the dye concentration in the films increased, the number of segments per cross section derived from the relaxation intensiy of β dispersion decreased, and then became constant at a certain dye concentration which was defined as the critical dye concentration. The critical dye concentrations for Violet 1 and Orange 3 were equal, had a same value of 2.0×10
-5 mol/g•film. For the basic dye, Crystal Violet (CV), the critical dye concentration was 0.6×10
-5 mol/g•film. As the dye concentration in PET films increased, the melting points of films were depressed, Interaction parameter between polymer and dye,
X1, was obtained using Flory's equation which indicated the melting point depression of mixed polymer systems. The values of
X1 of CV, Violet 1 and Orange 3 were positive, slightly positive and approximately zero respectively. These results suggest that CV molecules aggregate at low dye concentration because of weak interaction between CV and PET, and that Violet 1 and Orange 3 are in the state of molecular dispersion in PET even at high concentrations.
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