Study of Microstructure of Poly(ethylene terephthalate) Fibers by Exposing to Supercritical Carbon Dioxide

POY PET fiber samples were uniaxially drawn below the glass transition temperature (Tg) at various draw ratios to obtain filaments with different transient structures. The transient structure was composed of polymeric chains with para-crystalline or mesomorphic order and played a key role on the ultimate properties of fibers. Therefore, the study of the post-treatment mesomorphic structural changes would be of great interest. Initially the samples were exposed to supercritical CO2 under tension and tension-free conditions to induce morphological changes in their structures. The evolution of micro-structural changes under exposure to supercritical CO2 was investigated by conventional methods such as DSC, FTIR, birefringence and mechanical properties. A good correlation was obtained between the results of various analytical studies. Analyzing the results of DSC and FTIR showed that the amount of extended chains and transient structure developing during cold-drawing increased with the draw ratio. The results showed that exposure to supercritical CO2 led to the absorption of CO2 molecules into the free volumes of amorphous phase of the samples and increasing the crystalline phase and lowering the transient structures in the treated fibers. Sample treatment conditions play important role on the structural changes and in transforming the oriented chains of the mesophase into the crystalline or non-crystalline domains. PET fibers exposed to supercritical CO2 under tension present higher degree of crystallinity and molecular compactness in the amorphous domains but lower values of transient structures than the non-tension exposed samples. Exposure to supercritical CO2 gave rise to such structural changes as crystallization, orientation, and mesomorphic transitions. Hence, this method is a promising approach for tailoring structural changes in PET samples.In this paper, samples of POY PET fibers were uniaxially drawn below the glass transition temperature (T_g) at various draw ratios to obtain filaments with different transient structures. The transient structure composed of polymeric chains with paracrystalline or mesomorphic order and has a key role on the ultimate properties of fibers. So, it will be interesting to learn about the post-treatment mesomorphic changes that occur in this structure. The samples were exposed to scCO₂ under and without tension-conditions to induce morphological changes in them. The evolution of micro structural changes under exposure to scCO₂ was investigated by conventional methods such as DSC, FTIR, birefringence and mechanical propreties. A good correlation was obtained between the results of various analyses. Analyzing the results of DSC and FTIR showed that the amount of extended chains and transient structure developing during cold-drawing increased with the draw ratio. Results showed that exposure to scCO₂ led to the absorption of CO₂ molecules into the free volumes of amorphous phase in the samples as well as increasing their crystallinity and decreasing the amount of transient structure in the treated fibers. Sample treatment conditions have an important role on the structural changes and in transforming the oriented chains in the mesophase into the crystalline or non-crystalline domains. PET fibers exposed to scCO₂ under tension present higher values of crystallinity and molecular compactness in the amorphous domains but lower values of transient structures than the samples exposed without tension. Exposure to scCO₂ gives rise to such structural changes as crystallization, orientation, and mesomorphic transitions. Hence, this method is promising for tailoring structural changes in PET samples.