Mamiko Narita, Hiromitsu Tanaka, Takuya Morishita
Monomer casting (MC) polyamide 6 (PA6), prepared by the anionic polymerization of ε-caprolactam within a mold, has attracted attention owing to its good mechanical properties, high heat resistance, high dimensional stability, and good processability compared with those of general PA6 (gPA6). However, further improvements in its mechanical properties under harsh environments, such as high temperatures and wet conditions, are required for applications including next-generation automobiles and electronic devices. Herein, the effects of varying the amounts of NaH in the anionic polymerization initiator and metal salts (MgCl 2 or CaCl 2 ) added during polymerization were investigated to enhance the heat resistance of MCPA6. Increasing the amount of NaH increased the glass-transition temperature ( T g ) and improved the storage modulus ( E’ ) at temperatures < 100 °C, while substantially decreasing E’ at higher temperatures (≥ 150 °C). Adding a trace amount of CaCl 2 (0.25–0.75 mol%) to this polymerization system containing 0.87 mol% NaH increased both T g and E’ in the temperature range of 40–150 °C. Even under wet conditions, adding 0.25–0.75 mol% CaCl 2 substantially improved E’ in the range of 40–150 °C; however, this effect was not observed when gPA6 was melt-blended with CaCl 2 . Although CaCl 2 interacted with the amide groups, introducing trace amounts of CaCl 2 did not reduce the crystallinity of MCPA6; instead, it increased the stiffness of the amorphous region. Therefore, MCPA6 with optimized amounts of NaH and CaCl 2 is promising as a heat- and moisture-resistant thermoplastic matrix for various applications, including high-performance fiber-reinforced plastics for the automotive and electronic industries.