The influence of temperature on the structure of poly(2,6-hydroxynaphthoic acid)

Abstract

The structure of poly(2,6-hydroxynaphthoic acid) (PHNA) is studied as a function of temperature using X-ray diffractometry. Similarities are observed between its behaviour and that of poly(1,4-hydroxybenzoic acid) (PHBA), including a high temperature transition to a pseudo-hexagonal rotator phase, which has been commented on previously. There are, however, striking differences between the two polymers, most notably the presence of a melt phase in PHNA, above 440°C, which displays ordering similar to that observed in smectic liquid crystals. The phase observed in PHNA at a given temperature depends on thermal history so that, for example, the room temperature orthorhombic phase reported by Mühlebach et al. is seen only after annealing, while the temperature of the transition to the pseudo-hexagonal phase decreases from 395°C, in our as-polymerized samples, to 300°C or lower, after heat treatment. The unit cell determined for the pseudo-hexagonal phase of PHNA has similar a and b parameters to those in the corresponding phase of PHBA. Implications of these findings for the structures of the copolymers of PHBA and PHNA are discussed.