Effect of Diamine Addition on Structural Features and Physical Properties of Polyamide 6 Synthesized by Anionic Ring-Opening Polymerization of ε-Caprolactam

Abstract

Synthesis of ε-caprolactam in a twin screw extruder by anionic ring-opening polymerization was successfully carried out to prepare a novel nylon 6 (polyamide 6 (PA6)) containing a small amount of diamine moiety. The produced PA6 shows a remarkable improvement of the physical properties (mechanical properties as well as rheological properties). Added diamine molecules led to some structural changes in the synthesized PA6. The molar ratio of the additive (diamine) to the initiator appeared to be optimum at ca. 0.1. Although the molar mass of the PA6 has not changed significantly, less than twice the PA6 molar mass, the physical properties of the polymers produced have been markedly improved. The zero shear rate viscosity of the PA6 at the optimum diamine concentration appeared to be increased by almost 30 times that of the pure PA6. We have attributed this remarkable increase in viscosity to structural changes (branching formation during synthesis) and increased molar mass. The storage modulus at a low shear rate increased more than 100 times that of the pure PA6, but the loss modulus increased only 10-fold. This indicates that the elastic properties dominated due to the enhanced chain entanglements. In addition, the mechanical properties were significantly improved at the optimal amount of diamine reagent concentration. The elongation at break for the sample with optimum diamine addition was increased to twice that of neat PA6, whereby the tensile toughness was also doubled. Produced PA6 has a merit of processibility in the extrusion process such as a blow molding process in which stability against sagging under gravity is required and other processes in which elongational properties dominate.

Long-Living Anions Could Dramatically Change the Overall Physical Properties of a Polyamide (Nylon 6) Synthesized by a Novel Process

“…We tested this approach with the aim of improving the physical properties of the PA6 samples. 9 The variation in the physical properties of the synthesized PA6 samples via single extrusion was similar to that observed for the samples prepared with the diepoxides. 8 − 17 The mechanical properties of the samples produced with the two methods were similar; thus, we concluded that the reactivity of the diamine additive was similar to that of the diepoxide additive.…”

“…Diamine is expected to participate in the PA6 synthesis process because the amine group of diamine molecules can be attacked by the anions in the chain growing reactions in the extruder. 9 Depending on the amount of diamine added, the PA6 molecules are cross-linked or form branched PA6. We tested this approach with the aim of improving the physical properties of the PA6 samples.…”

“…Although these PA6 samples synthesized via AROP of ε-caprolactam in the presence of additives (diepoxide or diamine) exhibit improved physical properties (similar to or better than those of commercial PA6, which has a molar mass more than twice that of our PA6 samples), their mechanical properties were not as dramatically improved as their rheological properties, probably because of their low molar mass. 1 , 8 , 9 The AROP was performed in a laboratory-scale twin screw extruder where the additives quench partly the living anion and can stop further growth of the chain. Thus, we altered the reaction scheme in order to increase the molar mass of the synthesized PA6 and improve the reaction with the diamine: PA6 was initially synthesized by performing the AROP of ε-caprolactam in the twin-screw extruder followed by the mixing of pelletized PA6 particles with the additives and a second extrusion process.…”

“…The general trend is similar to that observed for the single-step process found in our previous study, although the viscosity average molar mass is higher. 9 The addition of excessive diamine results in quenching of the growing anions and the cessation of further chain growth in the single-step extrusion process. By contrast, in the two-step process, the living anions of the PA6 chain ends react with diamine molecules added during the second extrusion; thus, further anion transfer to other monomers or initiator species does not occur because most of them already reacted during the first extrusion process.…”

“…Thus, we attempted to improve the physical properties of PA6 by adding to the polymerization mixture a bifunctional molecule that can react with two PA6 molecules and thus double the molar mass. 1 , 8 , 9 A diepoxy compound was synthesized as such a linking agent and was mixed with the commercially available PA6. 1 The chain relaxation dynamics were found to vary over a few orders of magnitude as a result of the addition of the appropriate amount of the linking agent.…”