TENSILE DRAWING, MORPHOLOGY, AND MECHANICAL-PROPERTIES OF POLY(BUTYLENE TEREPHTHALATE)

The concept of the drawing of a molecular network has been employed to derive a total network draw ratio from the combination of the two def ormations occurring in the production of poly(butylene terephthalate), PBT, fibers by the consecutive processes of melt spinning and cold dr awing. The mechanical properties of PBT can then be more readily expla ined in terms of increases in this total network draw ratio. However, the preorientation and crystallization that occurs in the melt-spinnin g process can occur at different strain rates and temperatures, depend ing on the wind up speed employed, on the extensional viscosity of the polymer, and on the variation of the extensional viscosity with tempe rature. Therefore, for polymers such as poly(butylene terephthalate), which can exist in two crystalline forms, the morphology of the final drawn fiber might be expected to depend on the first melt-spinning sta ge of the process as well as on the total network draw ratio. In this work, density, birefringence, mechanical measurements, and WAXD measur ements, which have been made on the melt-spun fibers and on the drawn fibers, are described. Small differences in some of the drawn yarn mec hanical properties at the same overall network draw ratio are related to the crystallinity and in particular to differences in the proportio n of the alpha and beta phases present in the drawn yarn. These in tur n are related to differences in the temperature and stress during melt spinning and drawing. (C) 1997 John Wiley & Sons, Inc.