The transition toward “green” alternatives to petroleum-based plastics is driven by the need for “drop-in” replacement materials able to combine characteristics of existing plastics with biodegradability and renewability features. Promising alternatives are the polyhydroxyalkanoates (PHAs), microbial biodegradable polyesters produced by a wide range of microorganisms as carbon, energy, and redox storage material, displaying properties very close to fossil-fuel-derived polyolefins. Among PHAs, polyhydroxybutyrate (PHB) is by far the most well-studied polymer. PHB is a thermoplastic polyester, with very narrow processability window, due to very low resistance to thermal degradation. Since the melting temperature of PHB is around 170–180°C, the processing temperature should be at least 180–190°C. The thermal degradation of PHB at these temperatures proceeds very quickly, causing a rapid decrease in its molecular weight. Moreover, due to its high crystallinity, PHB is stiff and brittle resulting in very poor mechanical properties with low extension at break, which limits its range of application. A further limit to the effective exploitation of these polymers is related to their production costs, which is mostly affected by the costs of the starting feedstocks. Since the first identification of PHB, researchers have faced these issues, and several strategies to improve the processability and reduce brittleness of this polymer have been developed. These approaches range from the in vivo synthesis of PHA copolymers, to the enhancement of post-synthesis PHB-based material performances, thus the addition of additives and plasticizers, acting on the crystallization process as well as on polymer glass transition temperature. In addition, reactive polymer blending with other bio-based polymers represents a versatile approach to modulate polymer properties while preserving its biodegradability. This review examines the state of the art of PHA processing, shedding light on the green and cost-effective tailored strategies aimed at modulating and optimizing polymer performances. Pioneering examples in this field will be examined, and prospects and challenges for their exploitation will be presented. Furthermore, since the establishment of a PHA-based industry passes through the designing of cost-competitive production processes, this review will inspect reported examples assessing this economic aspect, examining the most recent progresses toward process sustainability.

中文翻译：

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增强基于 PHB 的材料性能的体内和合成后策略：综述

向石油基塑料的“绿色”替代品过渡是由于需要能够将现有塑料的特性与生物降解性和可再生性特征相结合的“插入式”替代材料。有前景的替代品是聚羟基链烷酸酯 (PHA)，它是由多种微生物生产的微生物可生物降解聚酯，作为碳、能源和氧化还原存储材料，显示出非常接近化石燃料衍生聚烯烃的特性。在 PHA 中，聚羟基丁酸酯 (PHB) 是迄今为止研究最充分的聚合物。PHB 是一种热塑性聚酯，由于耐热降解性非常低，因此具有非常窄的加工窗口。由于PHB的熔化温度在170-180°C左右，加工温度至少应在180-190°C。PHB 在这些温度下的热降解进行得非常快，导致其分子量迅速降低。此外，由于其高结晶度，PHB 硬且脆，导致机械性能非常差，断裂伸长率低，这限制了其应用范围。对这些聚合物的有效开发的进一步限制与其生产成本有关，其主要受起始原料成本的影响。自从首次发现 PHB 以来，研究人员就面临着这些问题，并已开发出多种策略来提高该聚合物的可加工性并降低其脆性。这些方法的范围从 PHA 共聚物的体内合成，到合成后基于 PHB 的材料性能的增强，从而添加添加剂和增塑剂，作用于结晶过程以及聚合物玻璃化转变温度。此外，反应性聚合物与其他生物基聚合物的共混代表了一种调节聚合物性能同时保持其生物降解性的通用方法。本综述考察了 PHA 加工的最新技术，阐明了旨在调节和优化聚合物性能的绿色且具有成本效益的定制策略。将研究该领域的开创性例子，并介绍其开发的前景和挑战。此外，由于建立基于 PHA 的行业是通过设计具有成本竞争力的生产流程，因此本次审查将检查评估这一经济方面的报告示例，检查流程可持续性方面的最新进展。