Thermally Responsive Membranes

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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.