Morphological study on three kinds of two-dimensional spherulites of PBT

Poly(butylene terephthalate) (PBT) is one of the aromatic polyesters and an excellent engineering thermoplastic. It was reported for PBT that three different types of spherulite (1. usual-positive type, 2. usual-negative type, and 3. un-usual type in which the Maltese cross pattern does not coincide in direction with the crossed polars) are formed under static condition, depending on the crystallization conditions [1,2]. In isothermal melt-crystallization, when crystallization temperature is above 200°C the resulting spherulites are of “usual-positive type”, and below 200°C the resulting ones are of “un-usual type” [1]. In solution-cast crystallization, when crystallization rate is adequately low the resulting spherulites are of “usual-negative type”, and when adequately high the resulting ones are of “un-usual type” [2]. The major object of this study is to clarify their structural origins, each of which determines the optical property of a PBT spherulite, in particular of a two-dimensional one.