Effects of molecular structure on the essential work of fracture of amorphous copolyesters at various deformation rates

The fracture behavior of amorphous copolyesters with different molecular structure was studied with double edge notched tensile loaded specimens (DENT) using the essential work of fracture (EWF) approach. Various deformation rates ranging from 1 to 1000 mm/min were employed. Amorphous poly(ethylene terephthalate) (aPET) exhibited considerably higher specific essential and non-essential work of fracture than the copolyesters containing either cyclohexylenedimethylene (aPET-C) or neopentyl glycols (aPET-N). At high deformation rates, ductile/brittle fracture transition was observed with aPET-C and aPET-N, while aPET always fractured in ductile mode within the entire deformation rate range. These phenomena were ascribed to the different molecular flexibility and entanglement density of the copolyesters. The specific EWF of the aPET as a function of deformation rate went through a minimum. The initial decrease in toughness was caused by the hampered segmental mobility due to the increased deformation rate. The subsequent increase in toughness was attributed to the adiabatic heating induced temperature rise in the process and plastic zones. Strain-induced crystallization of the aPET was observed at ν=500 and 1000 mm/min, which may also contribute to the increase of the specific EWF.