Studying the changes in the microstructure of polyester (PET) in water and non-aqueous solvents is important to understand the swelling mechanism of PET, which can help to reduce water pollution during the dyeing process. This study uses molecular models of PET, water, and decamethyl-cyclopentasiloxane (D5) and employs molecular dynamics method to simulate the influence of solvents on the microstructure of PET. The results show that the glass transition temperature (T_g) of the pure PET system is close to the experimental value. The T_g of PET decreases with the addition of water and D5 solvents, and the free volume after adding D5 is higher compared to the free volume after adding water. Through molecular dynamics simulation of PET microstructure, it is found that D5 has a better SWELLING effect on PET than water.

To improve the level dyeing property and colorfastness of natural pigment-dyed cellulose fiber, a study on the structure and properties of natural indigo spun-dyed viscose fiber was carried out systematically. Herein, the natural pigment-dyed cellulose fiber was prepared by wet-spinning technique, and the microstructure of the colored fiber was comprehensively studied. Fabrics with different color depths were obtained by adjusting the color value and the content of indigo pigment. The natural indigo was evenly embedded in the viscose fiber, and the results indicated the existence of a direct ratio relationship between the performance of natural indigo and the color depth of the fiber. The level dyeing property and colorfastness of the fabric were tested. The fabric exhibited excellent dyeing uniformity, as indicated by the relative standard deviation of the surface color depth value on the fabric, which was no more than 2.39%. The colorfastness of natural indigo spun-dyed fiber was outstanding even when mordant was not used in the production process. The colorfastness to artificial light could reach grade 5, the fastness to washing with detergent reached grade 3–4, the fastness to rubbing reached grade 4–5, and that to high temperature reached grade 4–5. These results can possibly promote the future use of natural dyes in the fiber produced by a spun-dyeing technique.