Structure–property relationship in heat-set poly(ethylene terephthalate) fibers. II. Thermal behavior and morphology

Thermal behavior of drawn poly(ethylene terephthalate) (PET) multifilament yarn which had earlier been heat-set at various temperatures between 100°C and 250°C under “free to relax” and “taut at constant length” conditions is studied by differential scanning calorimetric (DSC) measurements. Structural studies are made by X-ray diffraction and infrared absorption spectrophotometry. Two interesting observations are reported: (i) In the taut-annealed samples, the noncrystallizable fraction increases and crystallizble fraction decreases with increasing heat-setting temperature, the free-annealed samples show the opposite effect; and (ii) a well crystallized free-annealed sample melts at a higher temperature than the corresponding taut annealed sample. Both these observations are interpreted in terms of sample morphology; the free-annealed samples have more perfect crystals and a more distrinct separation between the crystalline and amorphous phases which facilitates transport and diffusion resulting in an ease of reorganization.