Multi-Scale Investigation of Metastable Phases in Sustainable Polymers

非技术性总结：塑料材料在过去的半个世纪中在每个人的生活中发挥了重要作用。 然而，塑料的广泛使用已经导致了关于处理占据垃圾填埋场、路边以及开放和封闭沃茨的数百万吨用过的塑料的严重问题。 因此，为了保持塑料材料的有益方面，开发可再生材料至关重要，即既来自自然又可生物降解。 聚羟基丁酸酯（PHBs）填补了这一需求，因为它们是由微生物合成的，并且在相关的时间尺度上也可生物降解形成二氧化碳和水。 然而，从石油基塑料到PHB等材料的转变要求新的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当前的商品塑料相当。 在以前的工作中已经发现，可以通过如何处理PHB来改变PHB的最终机械性能。 机械强度可以通过在成型过程中快速拉伸或压缩材料来增加，从而产生仍然可生物降解但具有所需特性的塑料。 如果加工足够快，则可以在PHB塑料中产生新的性能，例如改进的机械性能以及压电性（即，对机械刺激的电响应，这是期望的电子特性）。 这项工作的重点是快速处理的聚羟基丁酸塑料材料成纤维和薄膜具有更高的强度，改善韧性，和压电响应。 这些新的性质产生于不同的排列的聚羟基丁酸分子在晶格中，获得时，纺丝纤维或快速拉伸膜。 所得到的材料可以被“锁定”到这种新的排列中，这产生了改进的性能。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，从而能够生产可持续塑料，为包装，环境传感器和电子应用提供新的可能性。技术概述：半结晶聚合物生产薄膜、纤维和模塑制品的加工涉及操纵二级和三级结构，以产生预期应用所需的宏观性能。 由于大部分加工方法相对于链和链段运动是缓慢的，因此通常获得结晶稳定的晶体结构。已经表明，当使用微生物使聚（羟基丁酸酯）（PHB）与羟基己酸酯（Hx）共聚单体共聚时，形成无规共聚物PHBHx，其与PHB均聚物相比表现出非常不同的性质。 当这些PHBHx共聚物被加工时（例如，从聚合物溶液静电纺丝成纳米纤维），产生亚稳β晶型，其中聚合物主链采用平面之字形构象。这种结晶改性产生压电响应和更高的机械强度。 本研究的重点将解决以下问题：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）可以亚稳态的结晶形式（S）的生产和稳定，以及这些阶段如何影响的机械性能和PHBHx的压电响应作为Hx浓度的函数？该奖项反映了NSF的法定使命，并被认为是值得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的支持。

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.