In recent decades, the depletion of oil resources and environmental pollution caused by excessive carbon emissions have become exigent problems [1,2]. Biological and biodegradable polymers, such as polylactic acid (PLA), poly (butylene adipate-co-terephthalate) (PBAT), and biobased polyamide 56 (PA56), have attracted considerable attention [[3], [4], [5], [6]]. PA56 is a new biological material with the same excellent mechanical strength and friction resistance as others polyamide compounds. [[7], [8], [9]]. This material is expected to replace traditional petroleum-based plastics [[10], [11], [12], [13], [14], [15]]. However, the low notch impact strength and high moisture absorption of PA56 considerably limit its application [[16], [17], [18]].

Currently, toughening methods used for PA56 include rigid-particle toughening, core–shell polymer toughening, control annealing process toughening [19], and inorganic-filler toughening. Wang. et al. [20] prepared PA56/PA6 blends using the solution method and studied their crystallization behavior and mechanical properties. Their results verified that the breaking elongation of the blends was considerably improved. However, this method is complex and requires a considerable amount of expensive solvent. Zhang. et al. [21] performed PA56 toughening using POE-g-GMA/PP, which is a core–shell rubber, increasing the impact strength of the blend by 560 % compared to the pure sample. Zhang. et al. [22] used dopamine to modify the surface of halloysite nanotubes (HNTs) and demonstrated that the simultaneously formed polydopamine (PDA) layer could facilitate the grafting of the silane coupling agents on the HNT surface. The final modified PA56 material exhibited good biocompatibility and toughness. Compared with these modified polymers, poly (butylene adipate-co-terephthalate) (PBAT) is an environmentally friendly material with excellent toughness [[23], [24], [25]]. It has received extensive attention from academics [[26], [27], [28], [29], [30]]. Therefore, a PA56/PBAT blend is expected to be an environmentally friendly material, which will comply with the principles of sustainable development. In the use of elastic toughened polymer, the nano-scale dispersion phase is introduced into the matrix polymer to considerably improve its toughness [31]. However, PA56 is poorly compatible with PBAT, and it is difficult to achieve a nanoscale dispersive phase.

Epoxy-terminated hyperbranched polyester (EHBP) is a special form of a typical hyperbranched polymer with multiple functional groups at the end group. In recent years, EHBP has been widely used in the field of biomedicine as an effective and environmentally friendly modifier. Han. et al. [32] successfully synthesized EHBP as an additive for a poly (3-hydroxybutyrate-co-3-hydroxyvalerate/biobased engineering polyester elastomer blend. At an EHBP content of 3 resin percent (phr), the breaking elongation and impact strength of the blend increased by 134.2 % and 76.8 %, respectively, compared to those without this additive. In addition, EHBP was used to improve the compatibility of binary blends such as PLA/thermoplastic starch (TPS) and PBAT/TPS [33]. The terminal epoxy group of EHBP readily reacts with the amine group in the polyamide. At the same time, the amine group easily forms hydrogen bonds with the epoxy group. Compared to polyester compounds, EHBP holds great promise in the modification of polyamide compounds. EHBP exhibits several advantages such as high branching, spherical structure, multifunctional groups, and low viscosity. It is expected to realize the nanonization of PBAT dispersed phase in PA56/PBAT blend system. Unfortunately, there are no reports of EHBP being used in PA56.

This study aims to investigate the effect of introducing a hyperbranched structure into a polymer on the dispersed-phase structure and the interface between the two phases as well as to obtain a nanoscale PBAT dispersed phase for achieving ultrahigh PA56 toughing. Herein, an EHBP was successfully synthesized and used to enhance the compatibility of PA56/PBAT blends. PA56/PBAT/EHBP blends were prepared through melt blending using a twin-screw extruder. The effects of EHBP on the PBAT dispersed-phase structure and crystallization behavior as well as mechanical properties of the PA56/PBAT blends were investigated. This study provided a simple and effective toughening method for broadening the PA56 applications.