ϵ-caprolactam was polymerized in the interlayer space of montmorillonite, the clay mineral yielding a nylon-clay hybrid (NCH). X-ray and TEM measurements revealed that each template of the silicate, which was 1 nm thick, was dispersed in the nylon 6 matrix, and that the interlayer distance of clay increased continuously from 1.2 nm for the unintercalated material to 21.4 nm for the intercalated material. Thus, NCH is a polymer-based molecular composite or a nano-composite.
NCH contains 1-15 vol% of monolayer clay. Injection-molded NCH showed excellent mechanical properties compared with nylon 6 in terms of tensile strength, tensile modulus and heat resistance. The tensile modulus of NCH was twice that of Nylon 6, and the heat distortion temperature increased from 65°C for nylon 6 to 145°C for the NCH containing only 1.6 vol% of a clay mineral. It was found that such excellent properties of an NCH system was due to the strong ionic interaction between nylon 6 and the silicate layer.
Since NCH has a high modulus and a heat distortion temperature, it has already been applied to automotive timing belt covers as an engine part.
Polyamides have such fairly good properties that they are widely used as engineering plastics in the automobile industry.[1]* However, their strength and heat resistance are insufficient for the structural parts in an engine room. The mechanical properties of polyamides have been improved so far by reinforcing them with glass fibers or by adding inorganic materials.[2] In such composites with improved mechanical properties, dispersoids are of the size of some micrometers and their separation is also of the same order. It seems that the matrix and dispersoids are bonded to each other mainly through Van der Waals force, and scarcely through chemical binding force.[3] If the molecular-size dispersion of additives could be realized, the mechanical properties of the material would be greatly improved and some exotic features must be added to the blends or the nano-composite. Clay is a candidate material for the additive illustrated schematically in Fig. 1, because it forms intercalations with organic compounds such as toluene and aniline. [4]
Fig. 2 shows the concept of polymerization in the presence of clay. In this study, ϵ-caprolactam was polymerized in the presence of montmorillonite (a clay mineral or a silicate) intercalated with 12-aminolauric acid, and a nano-composite was produced.
Furthermore, in order to study the effect of ionic interaction between nylon 6 and clay on the mechanical properties of NCH, we have synthesized nylon-clay hybrid materials using another layered clay mineral, saponite. The interface between nylon 6 and the two varieties of clay was then analyzed by solid state 15N-NMR method.