## 1 Introduction  
Polyhydroxyalkanoate (PHAs) are biodegradable polyesters produced via microbial fermentation using renewable carbon sources, such as sugars, fatty acids, and agricultural by-products. Their biosynthesis is regulated by a sequence of enzymatic reactions, with PHA synthases playing a key role in catalyzing the polymerization of hydroxyalkanoate monomers [1; 2]. By carefully controlling the carbon source, fermentation conditions, and microbial strain, it is possible to produce various types of PHAs with a wide range of monomer compositions and material properties [3; 4; 5; 6; 7; 8; 9].  
Among the different PHAs, poly(3-hydroxybutyrate-\(co\)-3-hydroxyhexanoate) (P3 HBHHx) stands out due to its superior flexibility and mechanical performance compared to poly(3-hydroxybutyrate) (PHB). The incorporation of 3-hydroxyhexanoate (3 HHx) comonomer units increases the elasticity and toughness of the polymer, making P3 HBHHx particularly suitable for applications in packaging, agriculture, and biomedicine [10; 11; 12; 13].  
Despite these promising properties, a key limitation of P3 HBHHx for industrial use is its slow crystallization rate, especially in copolymers with a high 3 HHx content [14]. This slow crystallization behavior can hinder manufacturing processes, such as injection molding and extrusion, leading to longer cycle times, incomplete solidification, and product defects. One effective strategy to accelerate crystallization is the addition of nucleating agents, which facilitate crystal formation and enhance the crystallization rate [15; 16; 17]. These additives have been widely applied to biodegradable polymers, including polylactic acid (PLA) and other PHAs [15; 18; 19; 20; 21].  
In this study, a range of nucleating agents, including boron nitride (BN), poly(3-hydroxybutyrate) (PHB), talc, organic potassium salts (LAK), and ultrafine cellulose (UFC), were incorporated into P3 HBHHx to evaluate their effects on crystallization behavior, thermal stability, and mechanical performance. These nucleating agents were selected based on their distinct chemical properties, morphology, and compatibility with biopolymers. Inorganic fillers, such as BN and talc, are known for their high thermal conductivity and surface activity, which can promote heterogeneous nucleation [22]. Organic nucleating agents, like LAK and PHB, offer potential benefits in terms of biocompatibility and chemical affinity with the P3 HBHHx matrix [23; 24]. UFC, on the other hand, represents a natural fiber-based additive with a high surface area and the potential to act as a reinforcement agent [25; 26].  
Both non-isothermal and isothermal crystallization methods were employed to comprehensively characterize the crystallization behavior. The non-isothermal approach simulates industrial cooling conditions, while isothermal crystallization enables kinetic modeling using the [PERSON] and [PERSON]\(-\)[PERSON] theories. This dual approach provides valuable insights into nucleation efficiency, growth mechanisms, and structure-property relationships under both practical and theoretical conditions.  
In addition to the crystallization behavior, thermal stability was examined using thermogravimetric analysis (TGA), and the molecular degradation was evaluated via gel permeation chromatography (GPC). The mechanical properties were assessed using tensile testing. Polarized optical microscopy (POM) was used to study spherulitic morphology, crystal growth rates, and nucleation density. Self-nucleation experiments were conducted to quantify the nucleation efficiency and provide deeper insight into the effectiveness of each additive.  
By systematically comparing the crystallization behavior and material performance of P3 HBHHx with various nucleating agents, this study aims to identify the most suitable additive for improving the processability and expanding the industrial applicability of P3 HBHHx-based material.