Morphology of α-crystals of poly (butylene 2,6-naphthalate) crystallized via a liquid crystalline mesophase according to Ostwald's rule of stages

Abstract

Poly(butylene 2,6-naphthalate) (PBN) is a crystallizable aromatic polyester and develops different crystal polymorphs as a function of the crystallization conditions. High-temperature crystallization above about 200 °C yields β′-crystals while α-crystals develop between about 160 and 200 °C, both forming directly from the melt. At lower temperature, crystallization follows Ostwald's rule of stages, with the melt first transforming into a smectic liquid crystalline structure, which then converts into α-crystals. Information about the morphology of the liquid crystalline mesophase at the nanometer-length scale and of α-crystals, formed directly from the melt or from the liquid crystalline mesophase, are not available yet, and is therefore analyzed in the present study using atomic force microscopy (AFM). Direct transformation of the melt into α-crystals proceeds via spherulitic crystal growth. AFM reveals formation of laterally extended lamellae with a thickness of 10–15 nm. The smectic liquid crystalline mesophase, isolated by quenching before its transformation into α-crystals, appears as a close arrangement of clusters with a characteristic spacing of around 10 nm. The overall morphology is preserved on transformation of the mesophase into α-crystals supporting the notion that the mesophase converts directly, that is, within a solid-solid phase transition into α-crystals without intermediate melting, confirming a crystallization pathway according Ostwald's rule of stages.