Molecular motion in the homopolyesters of 4-hydroxybenzoic acid

Abstract

The homopolyester of 4-hydroxybenzoic acid and its copolyesters with 6-hydroxy-2-naphthoic acid and with 4,4′-biphenol and terephthalic acid are of commercial and academic interest. From the applications side, the interest stems from the high moduli and high temperature properties of these polymers. From the fundamental side, the interest stems from the main-chain thermotropic liquid crystalline character of these polymers. The homopolymer, p(HBA), itself is a highly crystalline material that has been the subject of a number of structural investigations by X-ray and electron-diffraction. The initial results using multinuclear NMR to probe the motion of p(HBA) provide the following insight into the details of the 350°C DSC transition. Ring motion in the polymer ensues well-below this transition and thus is not tied to it. The ring motion occurs in the form of 180° flips about the C1 - C4 axis of the ring and is augmented by ring oscillations. The onset of motion for some of the rings occurs at ca. 120°C while all the rings show motion by 300°C.