Polyesteramides have attracted much attention due to their intriguing properties. However, the synthesis of di- or triblock polyesteramides based on polyamide 6 (PA6) remains challenging owing to their harsh reaction conditions. In this work, we introduced a new strategy to synthesize poly(ε-caprolactone)-block-PA6-block-poly(ε-caprolactone) (PCL-b-PA6-b-PCL) in a mild way. A series of well-designed copolymers were characterized by gel permeation chromatography, 1H, DOSY, and 13C NMR, and their crystallization and melting behavior, degradation rate, and compatibilization effect on PA6/PCL blends were found to be strongly composition dependent. Interestingly, the copolymers exhibited diverse spherulite morphologies, including curving stems and banded spherulites, which also correlated with the different relative content of PA6. Besides, PA6 nanodomains could act as physical cross-linking points to improve the tensile strength and Young’s modulus of PCL. This work fills the synthesis gap of well-defined PA6-based triblock polyesteramides, providing new materials for material modification and elucidation of some basic scientific questions.

Poly(ester amide)s have aroused extensive research interest due to the combination of the degradability of polyester and the higher mechanical properties of polyamide. In this work, a series of poly(ε-caprolactam-co-ε-caprolactone) (P(CLA-co-CLO)) copolymers with different compositions were synthesized by anionic copolymerization. The structure, crystallization behavior, water absorption, and biodegradation behavior of these copolymers were investigated by means of nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized optical micrographs (POM). The results indicated that the composition of P(CLA-co-CLO) copolymers can be adjusted by the molar feed ratio. The PCL blocks decreased the crystallization rate of PA6 blocks but had little effect on the melting behavior of PA6, while the crystallized PA6 acted as a heterogeneous nucleating agent and greatly improved the crystallization rate of PCL. Moreover, the introduction of PCL blocks greatly reduced the water absorption of P(CLA-co-CLO) copolymers and endow them a certain degree of degradability.

Due to the extremely wide crystallization temperature range of chemically jointed PA6 blocks, three types of spherulitic morphologies were found in poly(ε-caprolactone)-b-polyamide 6-b-poly(ε-caprolactone) (PCL-b-PA6-b-PCL), including conventional ring-banded spherulites, special spherulites with a curve-like morphology, and composite spherulites with the above two characteristics. The formation conditions, internal factors, and the lamellar orientation and assembly of different spherulites have been studied in detail. It was found that the PCL spherulites with a curve-like morphology can form only when PCL and PA6 blocks crystallize simultaneously, whereas the ring-banded spherulites can form regardless of whether PA6 is amorphous or crystallized. The morphological results show that the ring-banded spherulites are composed of lamellae with different orientations along the radial direction, while spherulites with a curve-like morphology are composed of different stacked lamellae (flat-on lamellae and tilted flat-on lamellae) arranged alternately along the tangential direction. The appearance of unusual spherulitic morphology may result from different internal stress induced by chemically jointed PA6 blocks under diverse crystallization conditions.