Abstract: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV), is a kind of biomaterial produced by fermentation engineering technology using starch as the raw material. It is not only environmentally friendly, easy to biodegrade, and pollution-free, but also exhibits similar properties to polypropylene. It holds great promise for the development and application of new bio-based fibers. However, PHBV itself also has problems of high brittleness, poor toughness, and poor thermal stability, restricting its large-scale application in textile and other fields. Considering that the physical and mechanical properties of polymers are closely related to the crystalline condensed structure, this paper therefore introduces the crystal structure and properties of PHBV copolymers, summarizes the methods for regulating the crystallisation behaviour of PHBV, concludes the existing studies on the crystalline phase transition mechanism of PHBV copolymers, and provides an outlook on the application prospects of PHBV materials. 
The copolymerization units of PHBV are 3-hydroxybutyric acid (HB) and 3-hydroxyvalerate acid (HV), both of which belong to the same orthogonal crystal system. The common methods for regulating the crystallization behavior of PHBV include chemical modification, physical blending modification, external force field induced crystallization, heat treatment regulation and thermal stress stretching. Chemical modifications include block copolymerization modification, graft copolymerization modification, crosslinking modification, end group chain extension and long chain branching, etc., which can increase the main chain of the PHBV polymer or construct the body structure to improve the mechanical properties of the material. Physical blending modification usually blends PHBV with nanoparticles, plasticizers, and other polymers to increase the nucleation rate by heterogeneous nucleation to improve the tenacity of the material. During the processing, PHBV is subjected to external force to induce the orientation of the polymer molecular chain, which may be accompanied by the phase transition of the polycrystalline polymer, thus affecting the crystal structure and crystallization kinetics of the polymer. Heat treatment is conducive to the generation of thermodynamically stable crystals to improve the brittleness of materials. Thermal stress stretching promotes the formation of βchain conformation and increases the strength of the material.
The β-form of PHB in PHBV is a metastable crystal form formed by stretching, with good elongation of the molecular chain and significantly improved mechanical properties of the material. The thermodynamic stability of α and β crystal forms of PHBV is different, and the two will change under the action of external force strong enough. One-step or two-step stretching is usually used to promote the transformation of α-form to β-form to improve the mechanical properties of materials.
The regulation of PHBV crystallization behavior and the study of phase transition mechanism can improve the toughness, thermal stability and tensile properties of PHBV materials to some extent. And a more convenient and easy-to-operate way to regulate the crystal form and crystalline structure of PHBV can be explored in the future by considering the synergistic effect between various regulation methods, so as to meet the needs of PHBV industrialization production and continuously promote the expansion of PHBV application market.

Key words: poly(3-hydroxybutyrate-co-3-hydroxyvalerate), crystal structure, phase transition, toughening modification, stretching process

摘要： 聚羟基丁酸戊酸酯共聚酯（PHBV）材料是一种可生物降解聚合物，但存在脆性大、韧性差、热稳定性差等诸多问题，在纺织材料加工和应用方面受到了局限。针对性进行结晶调控是改善PHBV材料的脆性问题的有效途径，据此对国内外相关研究进展进行了综述。阐述了PHBV的晶体结构特点以及聚合单体含量对材料结构和性能的影响；具体分析了PHBV结晶行为调控的几种方法，包括化学改性、物理共混改性、外力场诱导结晶，热处理和热应力拉伸；围绕PHBV存在的晶相转变行为，对β晶形成条件和现有的相变机理研究进行了梳理和分析。最后指出未来可综合考虑多种结晶调控方法间的协同效应，以期进一步扩大PHBV材料在纺织领域的应用。

关键词: 聚羟基丁酸戊酸酯共聚酯（PHBV）, 晶体结构, 晶相转变, 增韧改性, 拉伸过程