SusChEM: Studies of Molecular Orientation, Degradation and Thermoreversible Gelation in Environmentally Sustainable Polymers: Poly(hydroxybutyrates) and Their Copolymers

摘要：塑料包装通常被填埋或焚烧，加剧了世界上许多地区普遍存在的空气污染。许多第三世界国家太穷，无法进行垃圾回收，几乎没有卫生的垃圾填埋场和焚烧设施。目前用于包装的大多数聚合物都是不可生物降解的，并且在垃圾填埋场中存在数十年。如果包装材料是基于可再生资源的可生物降解聚合物，这些问题就可以得到解决。然而，可生物降解的聚合物在强度和柔韧性方面的机械性能往往会降低。这个研究小组正试图用可生物降解和生物合成的聚合物来解决这些问题。这是一项跨学科的研究，研究一类可持续聚合物，聚羟基丁酸酯（PHB）及其随机共聚物的化学和生物降解，结合了涉及化学合成的方法，使用静电纺丝技术的新处理，以及使用新的仪器组合对所得到的材料进行表征。该小组将评估随机加入第二种成分到PHB聚合物分子中的效果，以改善其机械性能，同时保持降解速度和降解产物的性质。最终目标是一种可持续的聚合物材料，这种材料价格低廉，可加工，在垃圾填埋场中可快速降解（30-45天），同时为包装提供更好的强度和柔韧性。如果成功，这种开发优化的、可持续的、可生物降解聚合物的方法可能会扩展到PHB系统之外，并对新的可持续材料的开发和生产产生重大影响。技术摘要：聚羟基丁酸酯（PHB）是一种可生物降解的脂肪族聚酯，可通过化学工艺或细菌发酵生产。事实上，它可以用热塑性塑料的特性生物生产，并且是自然可生物降解的，这使得它成为世界上许多工业和学术实验室的研究课题。然而，细菌产生的PHB导致聚合物易碎且缺乏柔韧性。为了改善这些性能，加入了3-羟基己酸酯（3HHx）作为共聚物，这些新材料被称为PHB-HHx，其晶体含量显著降低，从而改善了机械性能和加工性能。虽然这种方法肯定会带来重要的改进，但总是存在其他重要特性（如生物降解性）的伴随变化的问题。如果可以优化这些可持续的热塑性聚合物的可降解性，以便在垃圾填埋场中快速酶降解（30-45天），这将对环境和社会做出极其重要的贡献，对土地利用，土壤污染和材料处理的经济性产生影响。在这个项目中，研究人员将使用不同数量的3HHX共聚物（0 mol%（纯PHB）、3.9 mol%、5.8 mol%、6.2 mol%、7.6 mol%、9.4 mol%、11.9 mol%和13 mol%）来探索这一假设。此外，研究人员将探索结构、加工和链取向/结晶度之间的关系，并验证在电纺丝纳米纤维中改善链取向可以增加模量和韧性的假设，即使结晶度被适当的共聚单体成分破坏。他们还将探索替代的加工方法，以生产类似于电纺丝膜的纤维结构，而不会显著改变所需的性能。在PHB和PHB- hhx中已经发现了热可逆凝胶化，当凝胶化之后进行冻干时，提供了一条通往非织造纤维样结构的途径。研究人员假设，这种处理方法可以显著提高吞吐量，同时保持与电纺丝膜相同的性能结构。

NON-TECHNICAL ABSTRACT: Plastic packaging is usually disposed of in landfills or incinerated, adding to the air pollution which is endemic in many regions of the world. Many third world countries, too poor to recycle, have few sanitary landfills and few facilities for incineration. Most of the current polymers used for packaging are not biodegradable and persist in landfills for many decades. These problems could be addressed if packaging materials were based on biodegradable polymers from renewable sources. However, polymers that are biodegradable often suffer from reduced mechanical properties in terms of strength and flexibility. This research group is trying to address these issues using polymers that are biodegradable and biosynthetically produced. This interdisciplinary research on the study of chemical and biological degradation of a class of sustainable polymers, poly(hydroxybutyrates) (PHB) and their random copolymers, combines approaches involving chemical synthesis, novel processing using an electrospinning technique, as well as characterization of the resulting materials using new combinations of instrumentation. The group will assess the effects of randomly incorporating a second component into the PHB polymer molecules in order to improve their mechanical properties while maintaining the rate of degradation and the nature of the degradation products. The ultimate goal is a sustainable polymeric material that is inexpensive, processable, and degrades rapidly (30-45 days) in landfills, while providing improved properties of strength and flexibility for packaging. If successful, this approach to the development of optimized, sustainable, and biodegradable polymers may be extended beyond the PHB systems and have significant impact on development and production of new sustainable materials.TECHNICAL ABSTRACT:Poly(hydroxybutyrate) (PHB) is a biodegradable, aliphatic polyester that can be produced by chemical processes or bacterial fermentation. The facts that it can be produced biologically with the properties of a thermoplastic and is naturally biodegradable have made it a subject of research in many industrial and academic laboratories worldwide. However, bacterially produced PHB results in a polymer that is brittle and lacks flexibility. In order to modify these properties 3-hydroxyhexanoate (3HHx) has been added as a co-monomer and these new materials, referred to as PHB-HHx, exhibit a significantly reduced crystalline content, resulting in improved mechanical properties and processability. While this approach definitely leads to important improvements, there is always the question of concomitant changes in other important properties like biodegradability. If the degradability of these sustainable, thermoplastic polymers can be optimized so as to enzymatically degrade rapidly (30-45 days) in landfills, this would be an extremely important contribution to the environment and society with impacts on land use, soil contamination, and the economics of material disposal. It is this hypothesis that researchers will explore in this project using a wide range of copolymers with varying amounts of 3HHX comonomer (0 mol% (pure PHB), 3.9 mol%, 5.8 mol%, 6.2 mol%, 7.6 mol%, 9.4 mol%, 11.9 mol%, and 13 mol%).In addition, the researchers will explore the correlation between structure, processing, and chain orientation/crystallinity, and test the hypothesis that improved chain orientation in electrospun nanofibers can increase modulus and tenacity even while crystallinity is disrupted by appropriate comonomer compositions. They will also explore alternate processing approaches to produce a fibrous structure similar to an electrospun membrane without significantly changing desired properties. Thermoreversible gelation, which has been discovered for PHB and PHB-HHx, offers a route to a non-woven fiber-like structure when gelation is followed by lyophilization. The researchers hypothesize that this processing method may significantly improve throughput while maintaining structures with the same range of properties as electrospun membranes.