Mechanism of whisker growth of poly(4‐hydroxybenzoate)

Needle‐like crystals (whiskers) of poly(4‐hydroxybenzoate) were prepared by thermal condensation of 4‐acetoxybenzoic acid in Marlotherm‐S® at 300, 350 and 400°C. In addition to the temperature, also reaction time and concentration were varied. Wide‐angle X‐ray scattering powder patterns show that the “300°C whiskers” contain nearly equimolar fractions of orthorhombic modifications I and II, “350°C whiskers” and “400°C whiskers” only consist of modification I. Differential scanning calorimetry measurements revealed a phase transition at 325, 346 and 359°C for the “300°C”, “350°C” and “400°C whiskers”, respectively. Electron microscopy shows that with increasing conversion the thickness of the whiskers passes through a maximum. The previously proposed whisker growth model along a central screw was not confirmed and a new crystal growth model is proposed.