Thermal and mechanical behavior of poly(butylene terephthalate) and its blends with an epoxy

The behavior of blends and thermoreversible gels of a liquid diglycidyl ether of bisphenol-A epoxy and poly(butylene terephthalate) (PBT) have been investigated. On cooling moderately concentrated (2-15%) solutions of PBT in epoxy, crystallization of the PBT can lead to either isolated spherulites suspended in liquid epoxy or gelation, which produces a material of infinite viscosity with no structure discernible by optical microscopy. Gels of this type are made by seeding with additional crystal nuclei by heating to just above the dissolution temperature and cooling again. The competition between liquid-liquid demixing and liquid-solid demixing (crystallization) determines which morphology will form. After formation, the gels can be chemically crosslinked to form stable, solid materials. The crosslinked gels display a large fracture energy compared to that of the unmodified epoxy or the blends composed of the spherulitically crystallized PBT and epoxy. The high glass transition temperature and elastic modulus of the neat epoxy is not sacrificed. The principle toughening mechanism in the crosslinked gels is suggested to be enhanced plastic deformation of the material owing to the presence of the crystalline network.; The melting behavior of pure PBT has also been investigated, and a simulation has been performed to determine whether the multiple melting endotherms observed during the thermal analysis of PBT can be explained by the simultaneous melting and recrystallization of an initial distribution of crystal melting temperatures that contains only one maximum and two inflection points. By combining temperature-dependent recrystallization with an initial distribution of melting temperatures, simulated DSC curves were produced that agreed well with experimental DSC curves. In instances where three peaks occurred, the high temperature peak was due to crystals formed during the scanning process, and the middle and low temperature peaks were due to crystals originally present in the material. Satisfactory agreement between the experimental and simulated curves was found without considering additional crystallization from the amorphous regions during the scanning process.