Reinforcement factors in fibers from block copolyamides and polyamide blends

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Abstract
The modulus of a fiber can be increased by plying with it a higher‐modulus fiber. In this case, the modulus of the combination is characterized by a springs‐in‐parallel model, and the modulus of the composite is a linear function of the per cent of the second fiber in the composite. Another method of obtaining reinforcement is to melt‐blend a higher‐modulus polymer with the substrate polymer. With polyamides, this leads to a certain degree of amide interchange and block copolymer formation which depends on the compatibility of the polymers as well as on the usual kinetic factors. If the dispersion of the higher‐modulus polymer is such that aggregate size is relatively large (e.g., ≥500 Å) and if the adhesion between the two polymers is good, a springs‐in‐parallel‐type reinforcement is the best which can be obtained. In melt‐blend polyamides, a “nonclassical” phenomenon in reinforcement has been noted when the diameters of the dispersed aggregates are ≤500 Å and when there are a relatively high number of hydrogen bonding sites on both polymer components. In this case, it appears that moduli appreciably higher than predicted from a springs‐in‐parallel model are obtained as well as higher than expected Tg values. A mechanism is proposed to account for this “nonclassical” behavior along with data to support it. Another type of anomaly is observed when the components of the blend are isomorphous. In this case, the reinforcement is considerably less than expected.

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