SusChEM: Rheology of Cellulose and other Biopolymers in Ionic Liquids

摘要：纤维素是地球上最丰富的可再生高分子天然资源。多年来，人们对纤维素进行化学改性，使其可溶于水或至少可溶于水，从而使纤维素的溶液加工成为可能。然而，这些改性是昂贵的，并且抑制了纤维素的结晶，而纤维素的结晶赋予了最终的机械性能。离子液体是非挥发性溶剂，最近发现某些离子液体可以溶解天然纤维素。有大量的工业研究工作旨在纤维纺丝和其他未改性纤维素的溶液加工操作，公司开发了纤维素/离子液体溶液的大量专利组合。为了支持这些技术，计划对离子液体中纤维素溶液的流动特性进行基础研究。由于纤维素/离子液体流变学有几个不同寻常的方面，包括微量水的看似巨大的影响，这项研究可能是变革性的和有利的。该研究将为了解天然纤维素的流变学提供基础，使其能够在离子液体溶液中进行加工。这种绿色加工方案允许在不进行化学改性的情况下加工纤维素，这已经被认为可以生产出具有优越机械性能的纤维，而且离子液体是非挥发性的，并且通常从纤维素中提取的水可以在不使用任何挥发性有机溶剂的情况下回收99%。因此，离子液体溶液可能为从非石油基可再生资源中获得高模量聚合物产品提供了一条可行的途径。技术性：在一定浓度下，离子液体溶液粘度高于水溶液，弹性稍强（弛豫时间较长）；两者都有利于涂层和纤维纺丝作业的稳定性。计划中的流变学实验将彻底表征纤维素在三种离子液体（一种似乎是theta溶剂，另一种初步数据表明是良好溶剂）中在广泛的温度和浓度范围内的粘弹性反应。干燥纤维素/离子液体溶液的黏度、松弛时间和末端模量在线性响应中的浓度依赖性具有预期的标度，但线性粘弹性对少量的水相当敏感。此外，在与涂层、纤维纺丝和其他溶液加工相关的较强剪切流中观察到意想不到的结果；剪切粘度明显大于振荡剪切的线性粘度，而传统聚合物溶液要么显示两者相同，要么发现剪切链排列的剪切粘度更低。这将详细探讨使用x射线散射在剪切流检测纤维素链的排列。PI和他的团队还将开始探索其他多糖（几丁质/壳聚糖，地球上第二丰富的天然高分子资源）在离子液体中的溶液流变学。

AbstractNon-Technical: Cellulose is the most abundant renewable polymeric natural resource on our planet. For many years cellulose has been chemically modified to make it water-soluble or at least water-dispersible, enabling solution processing of cellulose. However, the modifications are expensive and inhibit the crystallization of cellulose that imparts final mechanical properties. Ionic liquids are nonvolatile solvents and recently, certain ionic liquids have been found to dissolve native cellulose. There is a large inductrial research effort aimed at fiber spinning and other solution processing operations on unmodified cellulose with companies developing large patent portfolios on cellulose/ionic liquid solutions. In support of such technologies, a fundamental study of the flow properties of cellulose solutions in ionic liquids is planned. Since there are several unusual aspects of cellulose/ionic liquid rheology, including a seemingly large effect of trace amounts of water, this research may be transformative and enabling. This study will provide a basis for understanding the rheology of native cellulose, enabling it to be processed in ionic liquid solutions. This green processing scheme allows cellulose to be processed without chemical modification, which is already known to produce fibers with superior mechanical properties, and ionic liquids are nonvolatile and routinely water-extracted from the cellulose to be 99% recovered without use of any volatile organic solvents. Consequently, ionic liquid solutions may provide a viable pathway to high modulus polymer products from renewable resources that are non-petroleum based.Technical: At a given concentration, ionic liquid solutions have higher viscosity than aqueous solutions and are slightly more elastic (longer relaxation time); both are advantageous for stability of coating and fiber spinning operations. The planned rheology experiments will thoroughly characterize the viscoelastic response of cellulose in three ionic liquids (one that appears to be a theta-solvent and another that preliminary data suggests to be a good solvent) over wide ranges of temperature and concentration. While the concentration dependences of viscosity, relaxation time and terminal modulus of dry cellulose/ionic liquid solutions in linear response have the expected scalings, the linear viscoelasticity is quite sensitive to small quantities of water. Additionally, unexpected results are observed in stronger shear flows relevant to coating, fiber spinning and other solution processing; the shear viscosity is significantly larger than the linear viscosity from oscillatory shear, whereas conventional polymer solutions either show the two to be identical or find the shear viscosity is lower from chain alignment in shear. This will be explored in detail using X-ray scattering in shear flow to detect alignment of cellulose chains. The PI and his team will also begin to explore the solution rheology of other polysaccharides (chitin/chitosan, the second-most abundant polymeric natural resource on the planet) in ionic liquids.