Effects of tension on mesomorphic transitions and mechanical properties of oriented poly(ethylene terephthalate) fibers under supercritical CO2 exposure

Samples of partially oriented yarn PET fibers were uniaxially drawn below the glass transition temperature (cold-drawing) before exposure to super-critical carbon dioxide (scCO(2)) in the absence and presence of tension at a temperature of 80 degrees C and a pressure of 220 bar. The effects of draw ratio, scCO2 exposure, and tension on structural changes and on mechanical properties in particular were investigated using differential scanning calorimetry, tensile deformation, and birefringence and density measurements. A good correlation was obtained among the results obtained from various techniques. Results indicate that exposure to scCO2 not only induces structural changes but also develops crystallization in the samples. Tension under scCO(2) exposure also produces significant effects in terms of causing structural changes and transforming the oriented chains in the mesophase into the crystalline or noncrystalline domains. PET fibers exposed to scCO(2) under no tension yields lower values of crystallinity, orientation, tenacity, and Young's modulus but higher values of breaking elongation compared to samples exposed under tension. This is an indication of higher plasticity of the chains in the amorphous domains in samples exposed under no tension. It is also found that mechanical measurements confirm the structural changes taking place in exposed PET samples.