Whiskers of poly(4‐hydroxybenzoic acid) — preparative aspects

Abstract

Abstract Polycondensation of 4‐acetoxybenzoic acid (4‐ABA) at 300–400°C involves side reactions yielding a difficult to be hydrolyzed fraction of modified poly(4‐hydroxybenzoic acid) (poly‐HBA). Such side reactions are the formation of diphenyl ether groups, Fries‐rearrangement, formation of ketene or acetic anhydride and, possibly, isomerization to 3‐hydroxybenzoic acid. These by‐products not only influence the properties of poly‐HBA, they also affect the crystal growth of needle‐like crystals (whiskers). Even small amounts of free 4‐hydroxybenzoic acid poison the crystal growth of whiskers. The reaction conditions required for their preparation were optimized at 400°C. Fibrcus whiskers with lengths ≥ 100 μm and length/diameter ratios > 50 were only obtained at low monomer concentrations (≤0,05 mol/L). It is demonstrated by differential scanning calorimetry, wide‐angle X‐ray scattering and NMR spectroscopy that these whiskers are the chemically and physically most perfect form of poly‐HBA.