Multi-Scale Investigation of Metastable Phases in Sustainable Polymers

在过去的半个世纪里，塑料材料在人们的生活中扮演了重要的角色。然而，塑料的广泛使用导致了处理数百万吨废旧塑料的严重问题，这些废旧塑料占据了垃圾填埋场、路边以及开放和封闭的水域。因此，为了保持塑料材料的有益方面，探索可再生材料至关重要，即来自自然界和可生物降解的材料。聚羟基丁酸盐（PHBs）满足了这一需求，因为它们可以由微生物合成，并在相应的时间尺度上生物降解形成二氧化碳和水。然而，从石油基塑料到PHBs等材料的转变要求新的可生物降解塑料具有与石油基塑料相似的机械和电气性能。强度、拉伸和刚度等性能必须至少与目前的商品塑料相当。在以往的工作中已经发现，PHB的最终力学性能可以通过PHB的加工方式来改变。在成型过程中，可以通过快速拉伸或压缩材料来提高机械强度，从而使塑料仍然是可生物降解的，但具有所需的性能。如果加工速度足够快，那么PHB塑料可以产生新的性能，例如改进的机械性能，以及压电性（即对机械刺激的电响应，这是一种理想的电子性能）。这项工作的重点是将PHB塑料材料快速加工成具有更高强度、更高韧性和压电响应的纤维和薄膜。这些新特性源于PHB分子在晶格中的不同排列，在纺丝或快速拉伸薄膜时获得。合成的材料可以被“锁定”在这种新的排列中，从而产生更好的性能。该项目旨在寻找适当的条件，使这种新的聚合物结构最大化，从而为包装、环境传感器和电子应用提供新的可能性，从而实现可持续塑料的生产。技术概述：半结晶聚合物生产薄膜、纤维和模塑制品的过程涉及到对二级和三级结构的操纵，以产生预期应用所需的宏观性能。由于大多数加工方法相对于链和节段运动是缓慢的，因此通常可以获得热力学稳定的晶体结构。研究表明，当聚（羟基丁酸酯）（PHB）与羟基己酸酯（Hx）共聚时，利用微生物，形成一种随机共聚物PHBHx，与PHB均聚物相比，它具有非常不同的性能。当这些PHBHx共聚物被加工（例如，从聚合物溶液中静电纺丝成纳米纤维）时，产生亚稳的β晶体形式，其中聚合物骨架采用平面之字形构象。这种晶体改性产生了压电响应和更高的机械强度。本研究的重点是解决以下问题：在什么条件下（溶剂和溶剂蒸发动力学，高压和高温，拉伸变形）可以产生和稳定亚稳晶型，以及这些相如何影响PHBHx的力学性能和压电响应作为Hx浓度的函数？该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:Plastic materials have played a major role in everyone's life for the last half century. However, the widespread use of plastics has led to intense problems with respect to disposal as many millions of tons of used plastics that occupy landfills, roadsides, as well as open and confined waters. In order to keep the beneficial aspects of plastic materials it is therefore critical to explore materials that are renewable, i.e. both sourced from nature and biodegradable. Polyhydroxybutyrates (PHBs) fill this need since they are synthesized by microbes and also biodegrade to form carbon dioxide and water on relevant timescales. However, the switch from petroleum-based plastics to materials like PHBs requires that the new biodegradable plastics possess similar mechanical and electrical properties compared to the petroleum-based plastics. Properties such as strength, stretch, and stiffness must be at least equivalent to current commodity plastics. It has been discovered in previous work that the ultimate mechanical properties of PHB can be changed by how the PHB is processed. Mechanical strength can be increased by stretching or compressing the material rapidly during the formation process, resulting in a plastic that is still biodegradable but has the desired properties. If the processing is fast enough, then new properties can be generated in PHB plastics, such as improved mechanical properties, as well as piezoelectricity (i.e., an electrical response to a mechanical stimulus, which is a desirable electronic property). This work focuses on the rapid processing of PHB plastic materials into fibers and films with higher strength, improved toughness, and a piezoelectric response. These new properties arise from a different arrangement of the PHB molecules in a crystalline lattice, obtained when spinning fibers or rapidly stretching films. The resultant material can be "locked" into this new arrangement, which yields improved properties. This project is directed towards finding the appropriate conditions that maximize this new polymer structure, and could thus enable the production of sustainable plastics with new possibilities for packaging, environmental sensors, and electronic applications. TECHNICAL SUMMARY:The processing of semi-crystalline polymers to produce films, fibers and molded articles involves manipulating the secondary and tertiary structure to produce the required macroscopic properties for the intended application. Since the bulk of the processing methods are slow relative to the chain and segmental motion, generally the thermodynamically stable crystalline structure is obtained. It has been shown that when poly(hydroxybutyrate) (PHB) is copolymerized with hydroxyhexanoate (Hx) comonomers, using microbes, a random copolymer, PHBHx, is formed that exhibits very different properties compared to PHB homopolymer. When these PHBHx copolymers are processed (e.g., electrospun from a polymer solution into nanofibers), a metastable beta crystalline form is produced in which the polymer backbones adopt a planar zig-zag conformation. This crystalline modification gives rise to a piezoelectric response and higher mechanical strength. The focus of this research will address the following question: Under what conditions (solvent and solvent evaporation kinetics, high pressure and temperature, tensile deformation) can metastable crystalline form(s) be produced and stabilized and how do these phases affect the mechanical properties and the piezoelectric response of PHBHx as a function of Hx concentration?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.