Morphology and fatigue behavior of poly(ether-ester) block copolymers

Poly(ether-ester) multiblock copolymers belong to a general category of materials termed thermoplastic elastomers. These elastomers exhibit elastic properties of rubber while possessing the strength and processability characteristic of thermoplastics. This unique combination of properties is attributed to the presence of a two-phase microstructure consisting of a rubbery soft phase and a glassy or semicrystalline hard phase. Due to their excellent physical and mechanical properties, these materials find wide use in diverse industrial applications in which they are subjected to static as well as dynamic deformation.;Compression molded poly(ether-ester) block copolymers based on hard segments of poly(tetramethylene terephthalate) (4GT) and/or poly(tetramethylene isophthalate) (4GI) and soft segments of poly(tetramethylene oxide) (PTMO) have been prepared. Differential scanning calorimeter, dynamic mechanical testing, tensile testing, and x-ray scattering were used to characterize the morphology of the copolymers. Results showed that hard segment crystallizability is a major factor governing the structure and properties of the poly(ether-ester) copolymers. Thermal analysis showed that the soft phase glass transition temperature, the crystalline fraction and the crystalline melting point increase significantly by increasing hard segment content. For a copolymer series with constant hard segment content of 60 wt%, changing the ratio of 4GT to 4GI alters the ester crystallizability as illustrated by the thermal properties of the copolymers. An intermediate composition H60/35:65 results in minimum crystallizability of both the isomer components as indicated by a minimum in the melting endotherm.;Tensile testing showed a large increase in the tensile strength of the copolymers as the hard segment content is increased. Young's modulus and yield stress show a systematic variation with the 4GT fraction in the mixed isomers H60 series, reaching a minimum at the intermediate composition H60/35:65. The type of crystallinity present (i.e. 4GT vs. 4GI) also affects the mechanical behavior of the copolymers. The 4GT samples have a higher stress-strain curve than those of the 4GI samples through the drawing region. At high elongation, the 4GI materials achieve higher stress levels than those of the 4GT materials.;Fatigue testings on the molded poly(ether-ester) copolymers show an "on-off" behavior with instantaneous stress loading. At a certain dynamic stress level, these polymers either failed on the first deformation cycle or lasted beyond the time limit for the experiment. A typical stress-cycle to failure (S-N) curve was generated for the copolymers using a gradual cyclic loading approach. The fatigue lifetime curves were sensitive to the content and type of hard segment present in the sample. Increasing hard segment content increases the overall fatigue lifetime of the copolymers. The 4GI-rich materials have higher S-N curves than the 4GT-rich materials.