Effect of CO2on Mesomorphic Structure of Cold-Drawn Poly(ethylene therephthalate) Fibers by Dynamic Mechanical Analysis

Abstract

Dynamic mechanical analysis (DMA) provides interesting information about the dynamic viscoelastic properties and molecular scale motions. This article presents the results of an investigation into the effects of draw ratio and supercritical CO2 (scCO2) exposure on structural changes and mesomorphic transitions in cold-drawn poly(ethylene terephthalate) (PET) fibers by focus on DMA. Samples of partially oriented yarn PET fibers were uniaxially drawn below the glass transition temperature (Tg) using various draw ratios to obtain filaments with different structures for subsequent exposure to scCO2 in the presence and absence of tension at a temperature of 80°C and under a pressure of 220 bar. Microstructural investigation of the samples was carried out by combining the results obtained from DMA, differential scanning calorimetry, as well as density and birefringence measurements, all of which exhibited a good correlation. The rigid amorphous region in the PET samples was considered for interpreting structure–property relations. Results showed that increasing draw ratio leads to increased values of rigid amorphous regions and distribution of molecular orientation in the amorphous regions followed by an increase in the relaxation time distribution. Depending on sample conditions, exposure to scCO2 was found to have a strong effect on the transformation of the transient structure into crystalline or amorphous regions. PET fibers exposed to scCO2 under tension yielded higher values of crystallinity and orientation compared to those under no tension. In the case of PET fibers exposed to scCO2 under no tension, CO2 molecules released from the structure possibly caused a metastable structure to form in the amorphous phase. Therefore, rearrangement of molecular chains during the dynamic mechanical test in the temperature sweep mode is expected to increase the storage modulus of the sample before its glass transition temperature is reached. As a result, an increment is observed in the storage modulus of samples exposed to scCO2 without tension that can be related to the release of stress stored during the depressurization process. scCO2 exposure can be, therefore, claimed to be an environmentally friendly method for inducing structural changes and enhancing crystallinity in PET samples.