Synthesis and properties of star-branched nylon 6 with hexafunctional cyclotriphosphazene core

A novel star-branched nylon 6 with hexafunctional cyclotriphosphazene core was synthesized using hexa(4-carboxylphenoxy)cyclotriphosphazene (HCPCP) with six carboxyl groups as multifunctional agent in the hydrolytic ring-opening polymerization of epsilon-caprolactam, and then used for the investigation of mechanical properties, crystallization and rheology behaviors. Star-branching structure and molecular weight have great effects on its properties. Compared with linear nylon 6, star-branched nylon 6 has lower relative viscosity and higher melt flow rate while its mechanical properties can be almost retained by the use of star-branching and an appropriate molecular weight. Research on crystallization behavior indicates that the degree of crystallinity (X-c) of star-branched nylon 6 decreases slightly, but its crystal structure still belongs to a form. The peak crystallization temperature (T-c) and crystallization rate (1/t(1/2)) of star-branched nylon 6 are significantly higher than that of linear nylon 6 because of heterogeneous nucleation induced by HCPCP core, which is beneficial for the use of rapid molding process. As the molecular weight increases, the T-c and 1/t(1/2) of star-branched nylon 6 first increase and then decrease. Capillary rheometer measurement exhibits that the shear viscosity of star-branched nylon 6 decreases with decreasing molecular weight, and the shear viscosity of star-branched nylon 6 with an appropriate molecular weight shows a low value and little or no sensitivity to shear rate and temperature. Such rheology behavior allows for processing at low temperature and low pressure and reduces system cost.