POLYMER WHISKERS OF POLY(4-HYDROXYBENZOATE) - REINFORCEMENT EFFICIENCY IN COMPOSITES WITH POLYAMIDES

The reinforcing efficiency of polymer whiskers of poly(4-hydroxybenzoa te) (PHB) in polyamide-6 (PA-6) and polyamide-11 (PA-11) composites wa s investigated by tensile testing, scanning electron microscopy (SEM), and density measurements. The composites were prepared by mixing the whiskers into the melts of the polyamides. Ductile and stiff PA-6-comp osites with different crystallinities of the PA-6-matrix were investig ated. In the ductile PA-6 (low crystallinity) composites, the elastic modulus E and the yield stress sigma(f) increase more than twice at a whisker content of 2 vol %. The increase in E and sigma(f) in stiff PA -6 (high crystallinity) composites is not as pronounced as in the duct ile PA-6 composites, but is still remarkably higher than in the PA-11 composites, which is about 1.2 times at a whisker content of 2 vol %. At higher whisker content, the PA-6 composites show the opposite of th e PA-11-composite-no further increase in E and sigma(f), which may be due to an agglomoration of whiskers in the high viscous PA-6 melt. The different tensile properties of the composites can be explained by SE M analysis of the fracture surfaces, which shows that the adhesion of PA-6 to the PHB whiskers is better than of PA-11. This is due to a hig her number of hydrogen bonds between the PA-6 and the whisker surface. Density measurements show that the crystallinity of the polyamides is not affected by the PHB whiskers. (C) 1996 John Wiley & Sons, Inc.