Crystallization of Polyamide 66 Copolymers at High Supercoolings

Crystallization kinetics and morphologies of a series of random copolymers of PA 66 (or Nylon 66) have been investigated at high supercoolings. Optical microscopy with rapid cooling apparatus was employed to observe spherulitic morphologies and measure growth rates. Final spherulitic morphologies of PA 66 and copolymers could be changed with increasing supercoolings from impinged spherulites to isolated spherulites with decreasing size until total amorphous. Spherulite growth results indicated that the rates of crystallization of PA 66 copolymers were reduced with increasing content of comonomer, and crystallization was moved to lower temperatures. The melting temperature, crystallinity, crystal structure and lamellar thickness of the PA 66 copolymers from different cooling conditions were studied with Differential Scanning Calorimetry (DSC), Wide Angle X-ray Diffraction (WAXD), and Small Angle X-ray Scattering (SAXS). Even though no temperature plateau is detected in the cooling curve, the spherulite growth of PA 66 at high supercooling is still found to be linear with time. This is attributed to a steady temperature gradient existing at the growth front. The spherulite growth kinetics of PA 66 across the whole supercooling range could be affected by the interaction of chain diffusion rate (into growth front), nucleation rate and latent heat diffusion (from growth front) at different crystallization temperatures. The morphology and melting behavior of PA 66 crystals can be explained by the behavior of H-bonding with increasing temperatures. Dynamic mechanical relaxation behavior of PA 66 copolymers with different spherulitic morphologies were examined and compared with those of polyethylene copolymers to reveal the relationship between morphologies and dynamic mechanical relaxations