Morphology and physical properties of poly(butylene terephthalate)

Liquid nitrogen-quenched PBT samples produce much larger spherulites of an optic axis orientation different from the of the air-cooled samples. Optical and scanning electron microscopy show that glass fibers in the glass-reinforced PBT sample nucleate the growth of well-defined spherulites along the glass fiber axis. Fracture studies at temperatures below and above the T_g indicate, respectively, brittle and ductile interspherulite boundary fracture. From dynamic mechanical studies, three transitions designated by α (flow transition), β (T_g), and γ (secondary relaxation) are observed. The magnitudes of the β and γ transitions are larger for the more amorphous quenched sample than the air-cooled sample, suggesting their amorphous phase origin. Addition of glass fibers raises the dynamic modulus and flow temperature, but suppresses the γ transition without significantly affecting the melting and glass transition temperatures.