Polymer composites of rigid and flexible molecules: System of wholly aromatic and aliphatic polyamides

Abstract

As an extension of reinforcing principles of composite materials in which macroscopic fibers are dispersed in a ductile matrix, rigid microfibrils of wholly aromatic polyamides such as poly(p-phenylene terephthalamide) (PPTA), poly-p-benzamide (PBA), and their block copolymers with blocks of nylon 6 or nylon 66 were dispersed in a matrix of nylon 6 or nylon 66. On blending polyaramides with aliphatic polyamides, the spherulitic texture disappeared and a locally birefringent homogeneous texture was obtained. Electron microscopy revelaed that microfibrils of PPTA with diameter of 15-30 nm were dispersed in a fractured surface of the polymer composite. Strong interaction between the fibril surface and the matrix was suggested by the fact that the crystallization of matrix polyamides was induced by the rigid polyaramides. Wide-angle x-ray diffraction intensity curves, DSC curves of cooling process from the melt, and isothermal crystallization curves indicated a marked tendency for induced crystallization with polyaramides. Viscoelastic relaxation curves indicated increased fraction of immobilized noncrystalline chains, increased modulus of the blend, and increased heat resistance of the blend as shown in retention of the blend modulus even at 200°C. By applying the modified Halpin-Tsai equation to the relationship between Young's modulus and the fraction of rigid molecules in the blend, the aspect ratio of microfibril was estimated at 15 to 25. Young's modulus and yield stress were remarkably improved by blending the polyaramides as low as 5%. With increasing molecular weight of PPTA, both modulus and yield strength systematically increased, while ultimate strength did not vary and ultimate elongation decreased. Blending the block copolymers of polyaramides and aliphatic nylon blocks with nylon matrix increased ultimate elongation compared with the physical blend of homopolyaramides and aliphatic polyamides. The orientation effect was very effective, as shown in the system of 3% PPTA and 97% nylon 6, in which the ultimate strength increased from 230 MPa of nylon 6 to 340 MPa of the blend. An ultimate strength of the PPTA microfibril of more than 4 GPa is realized in the blend, indicating full reinforcing effect of rigid fibrils at a supramolecular level.