Amorphous and Semi-Crystalline Nylon-6: Processing and Properties

Abstract

The influence of processing conditions on the structure and properties of nylon-6 are investigated. Amorphous nylon-6 is obtained and its properties are compared with those of semi-crystalline nylon-6. The kinetics of crystallization of nylon-6 depends strongly on the previous thermal prehistory of the melt. The recently observed cumulative character in the structural memory of nylon-6 is successfully applied for obtaining this polymer in a pure amorphous state. It was demonstrated that a critical temperature exists above which the “memory” of this polymer can be erased, and this critical temperature is close to the equilibrium melting temperature T m°. The value of T m ° ≍280°C was evaluated using an independent technique. Melt annealing above this temperature suppresses the crystallization process due to destruction of all crystal nuclei. Amorphous nylon-6 was obtained by ultraquenching of its melt, previously treated at a temperature above T m °, to a temperature well below the glass transition region. The ultraquenched nylon-6 is characterized by amorphous initial structure and improved mechanical properties. The thermal behaviour, crystallization and mechanical properties are investigated by differential scanning calorimetry (DSC), wide angle X ray scattering (WAXS) and mechanical testing. After drawing and annealing, the tensile strength and elasticity modulus of initially amorphous nylon-6 increase three times and the elongation at break decreases by half as compared with conventionally quenched samples. The thermal treatment in vacuum of semi-crystalline nylon-6 affects both the structure and the molecular weight. It is demonstrated that the most important factor for the improvement of mechanical properties is the orientation in the amorphous regions. The lower die initial degree of crystallinity, the better are the orientation and the mechanical properties of nylon-6. The best mechanical properties of initially amorphous nylon-6 are explained by achievement of better orientation of molecules in the amorphous regions.