WHISKERS OF POLY(4-HYDROXYBENZOIC ACID) - PREPARATIVE ASPECTS

Polycondensation of 4-acetoxybenzoic acid (4-ABA) at 300-400-degrees-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-degrees-C. Fibrous whiskers with lengths greater-than-or-equal-to 100-mu-m and length/diameter ratios > 50 were only obtained at low monomer concentrations (less-than-or-equal-to 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.