Thermally Responsive Membranes

CTS-0086961热响应膜德州理工大学Donald R.Paulisity摘要用于气体和液体分离的聚合物膜是众所周知的；然而，这项研究解决了一系列新的应用，例如种子包衣和用于气调包装的膜，这些应用要求膜的渗透性随着温度的升高而异常增加。这些所谓的“热响应膜”的基础是具有可以结晶的长烷基侧链的聚合物。侧链的熔点为流动、粘合或渗透等性质提供了开关效应；熔融温度可以通过连接到聚丙烯酸酯或聚甲基丙烯酸酯主链上的烷基单元的长度来调节。在最简单的形式下，由这种聚合物形成的膜对气体、水和其他渗透剂的渗透性在很窄的温度范围内表现出10到100倍的巨大变化，即渗透性开关。这项研究的目的之一是更好地理解如何操纵这种膜的化学和物理结构，以实现所需的开关效果。一个基本的问题涉及聚合物的化学结构和加工如何影响膜的结晶形态，以及微晶的存在如何改变渗透剂在非晶相中的溶解度和迁移率。第二个目标是开发在更宽的温度范围内扩展渗透开关效应的方法，以获得比大多数聚合物更强的渗透性依赖于温度的膜。这方面的研究包括使用共聚、共混和层压将分布的烷基侧链长度结合到单一膜中。这项工作的科学成果将对至少两项正在发展的技术产生影响。第一种是种子的涂层，通过渗透性开关效应将种子的萌发与土壤温度联系在一起；种子必须有水才能萌发。防止早熟发芽已被证明可以提高作物产量，并给农民带来其他好处。这项研究将使人们更好地了解如何配制这种涂料。第二个应用涉及气调包装，现在使用聚合物薄膜来控制水果和蔬菜包装中的氧气和二氧化碳浓度，使它们比通常可能的保鲜期更长。膜必须以与农产品的呼吸速率相匹配的速率传输这些气体。不幸的是，传统聚合物的渗透性不会像产品的呼吸速率那样随着温度的变化而迅速变化；因此，需要上述类型的热响应性更强的膜。

CTS- 0086961Thermally Responsive MembranesDonald R. PaulUniverisity of Texas AustinAbstractPolymer membranes for separation of gases and liquids are well known; however, this research addresses a new set of applications, such as seed coatings and membranes for modified atmosphere packaging that require membranes with an abnormal increase in penetrant permeability as temperature is increased. The basis for these so-called "thermally responsive membranes" is polymers having long alkyl side-chains that can crystallize. The melting point of the side-chain provides a switch effect for properties like flow, adhesion, or permeation; the temperature of melting can be adjusted by the length of the alkyl unit attached to a polyacrylate or a polymethacrylate backbone. In their simplest form, membranes formed from such polymers exhibit a dramatic change in permeability to gases, water, and other penetrants by a factor of 10 to 100-fold over a narrow temperature range, i.e., a permeability switch. One objective of this research is to develop a better understanding of how the chemical and physical structure of such membranes must be manipulated in order to achieve a desired switch effect. A fundamental problem relates to how the chemical structure and processing of the polymer affects the crystalline morphology of the membrane and, in turn, how the presence of the crystallites alters the solubility and mobility of the penetrant in the amorphous phase. A second objective is to develop methods to spread the permeation switch effect over a broader range of temperature to achieve a membrane whose permeability depends more strongly on temperature than is found for most polymers. This aspect of the research involves combining a distribution of alkyl side-chain lengths into a single membrane using copolymerization, blending, and lamination. The scientific results of this work will have impact in at least two growing technologies. The first is a coating for seeds that ties their germination to the temperature of the soil by the permeability switch effect; water must enter the seed for germination to occur. Prevention of premature germination has been shown to improve crop yields among other benefits for farmers. This research will provide a better understanding of how to formulate such coatings. The second application involves modified-atmosphere packaging, where polymer membranes are now being used to control the oxygen and carbon dioxide concentration in packages containing fruits and vegetables so they remain fresh longer than normally possible. The membranes must transmit these gases at rates that match the respiration rate of the produce. Unfortunately, the permeability of conventional polymers does not change with temperature as rapidly as the respiration rate of produce does; hence, there is a need for more thermally responsive membranes of the type suggested above.