Study of structural changes and mesomorphic transitions of oriented poly(ethylene therephthalate) fibers in supercritical CO2

Abstract

Supercritical carbon dioxide (scCO2), an environmentally friendly solvent, can change the fine structure of fibers depending on treatment temperature and pressure. Samples of partially oriented yarn (POY) PET fibers were uniaxially drawn below the glass transition temperature (Tg) and then exposed to scCO2 under tension to induce morphological changes in them. The effects of draw ratio and scCO2 exposure on the structural changes and mesomorphic transitions were evaluated. For this purpose, thermal characteristics of the structure, especially the rearrangement of polymer chains in the amorphous phase induced by cold-drawing and exposure to scCO2, were evaluated by differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR). The orientation factor of the fibers was measured using a polarizing microscope. The paracrystalline portion and the crystallite sizes of exposed samples were estimated by wide-angle X-ray diffraction. To evaluate the solubility of CO2 molecules in the samples, density and fractional free volumes were also measured. A good correlation was obtained between the results obtained from various measurement techniques. Results showed that cold-drawing of PET fibers leads to a strain-induced crystallization and that exposure to scCO2 changes the structure of the oriented PET samples. To investigate mesophase transitions, percentage of extended chains in the amorphous regions was calculated by analyzing the results of DSC and FTIR. Results showed that the amount of extended chains developing during cold-drawing in the amorphous regions decreased when exposed to scCO2.