The structure and properties of poly(hexamethylene terephthalate): 1. The preparation, morphology and unit cells of three allomorphs

Abstract

Poly(hexamethylene terephthalate) is shown to crystallize in one of three possible allomorphs. Two of these have been found in material crystallized from the melt, but neither has been produced in the pure state. One (the α-phase) is favoured when crystallization occurs under stress and is found in fibrillar materials, the other (the β-phase) is favoured by high-temperature, relaxed, annealing, and is found in spherulitic material. Where there is a high proportion of α-phase present, the β-phase appears to grow epitaxially on it. The third phase (the γ-phase) has only been found in material crystallized from solution at room temperature, and is converted to the other phases by annealing or orientation. All three phases have the same crystallographic repeat along the chain axis, which corresponds to a nearly fully extended conformation. They must therefore differ principally in the way chains pack together. The α-phase unit cell is monoclinic, the other two are triclinic. All unit cell parameters are given.