Effect of ultra fine particles on the elastic properties of oriented polypropylene composites

Fine spherical particles with various diameters (70, 160 and 40 nm, and 35, 65 and 125m) were mixed with isotactic polypropylene (PP). For the oriented composites having hexagonal symmetry produced by drawing, the elastic properties were determined by five compliances,S, or stiffness constants,C. Four of these, namely,S₃₃,S₁₁,S₁₃ andS₄₄ (orC₃₃,C₁₁,C₁₃ andC₄₄) were determined for the oriented composites filled with particles whose average diameters were 7 nm and 65m. For the composites filled with the smaller particles (7 nm), all the stiffness constants (C₃₃,C₁₁,C₁₃ andC₄₄) increased with the filler content, whereas for those with larger particles (65m), this relation was reversed. The Young's moduli of the oriented composites filled with relatively small particles (7, 16 and 40 nm) in each re-stretching direction increased with increasing filler content and with decreasing filler size, whereas those of the composites filled with larger filler (35, 65 and 125m) decreased with increasing filler content and size. It was concluded that the modulus of the oriented composite is determined by three factors, namely: (1) molecular orientation of matrix polymer; (2) the volume-fraction and size of filler; and (3) the fraction of void introduced by drawing. The moduli observed for the oriented composites are well explained by an equation derived on an assumption of the independence of the three effects. It was also concluded that extremely small fillers with particle sizes comparable to that of the crystalline region in PP matrix have a prominent reinforcing effect in the oriented polymer matrix.