Structures and mechanical properties of zone-drawn-zone-annealed blends of cocrystallizing poly(butylene terephthalate) and a poly(ether ester)

Abstract

Poly(butylene terephthalate) (PBT) and a poly(ether ester) (PEE) based on PBT and poly(ethylene glycol) were melt-blended and extruded as films with quenching. They were then zone-drawn (ZD) and zone-annealed (ZA) at various stresses (between 10 and 50 MPa) at temperatures of 160 and 190° C. The goal was to improve their mechanical properties relative to those of the same blend, but cold-drawn (X = 5) and isothermally annealed with fixed ends at the same temperatures for 6 h. All samples were characterized by DSC, WAXS, SAXS, and static mechanical property measurements. In contrast to the isothermally annealed samples, the zone-drawn and zone-annealed ones exhibit one population of crystallites arising from the homo-PBT, as demonstrated by the DSC and SAXS measurements. In addition, however, the WAXS photographic patterns indicate that zone annealing at 190°C results in isotropization of crystallites originating from the PEE, resulting in the formation of a microfibrillar-reinforced composite. It is assumed that some of the isotropic crystallization occurs on preexisting homo-PBT crystallites, i.e., a partial cocrystallization occurs, improving the adhesion between the components of the blend. The structural features created in the zone-drawn-zone-annealed materials result in higher values of the Young's modulus and tensile strength in comparison to the materials receiving the simple isothermal treatment (1,200 vs. 480 MPa and 213 vs. 113 MPa, respectively).