DOUBLE YIELD POINTS IN POLY(TETRAMETHYLENE TEREPHTHALATE) AND ITS COPOLYMERS UNDER TENSILE LOADING

Double yield points before necking: were observed in injection molded specimens of poly(tetramethylene terephthalate) (PTMT) and its copolym ers under tensile loading. The first yield is associated with deformat ion of the amorphous region and the second yield is caused by the alph a to beta transition of PTMT crystallites. The first yield point becam e less apparent with an increase of crystallinity of the specimens. Th e second yield point became more apparent with an increase of crystall inity. Annealing of injection molded specimens increased the crystalli nity and increased the second yield point on stress-strain curves. Cop olymerization decreased the crystallinity and made the first yield poi nt more prominent. Effects of annealing on mechanical and thermal prop erties of the specimens were also measured. The specimens changed thei r properties from ductile to brittle during annealing. Their change du ring annealing was mainly attributed to the increase of crystallinity and not to thermal degradation and/or crosslinking.