Transcrystallinity in fiber-reinforced thermoplastic composites

The presence of transcrystallinity (TC) in bulk composite samples of polypropylene (PP}-aramid fiber and PP-E glass fiber has been studied by etching/scanning electron microscopy, confocal scanning laser microscopy, and differential scanning calorimetry. Its effect on mechanical properties and interfacial shear strength was evaluated. Morphological studies showed that the amount of fiber surface nucleation gradually increases with decreasing crystallization temperature and increasing volume fraction of fiber. In PP-E glass composites, fiber surface nucleation was at a much lower level than in PP-aramid. A distinct boundary temperature, as previously observed in PP-aramid single-fiber specimens, was not found. Since surface nucleation increases gradually with decreasing temperature, it was found that the assessment of its effect on the mechanical properties of otherwise similar composites cannot be separated from other temperature-dependent morphological features, especially the strength of the spherulite boundaries. However, an impression can be obtained by comparing the reinforcing effects of fibers with different levels of surface nucleation. In micromechanical test specimens containing one single fiber (fiber pull-out, fragmentation), the occurrence of TC can be carefully controlled. The effect of crystallization conditions was evaluated. It was found that the interfacial shear strength increased with the crystallization temperature of the transcrystalline layer.