Submitted to: Macromolecular Chemistry and Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 2007
Publication Date: March 5, 2008
Citation: Willett, J.L. 2008. Humidity-responsive starch-poly (methyl acrylate) films. Macromolecular Chemistry and Physics. 209(1):764-772. Interpretive Summary: There is a growing need to develop functional biobased materials which provide new markets for agricultural products and reduce dependence on nonrenewable resources. We have developed "smart" or responsive films by combining starch, a renewable agricultural product from corn, with a synthetic polymer known as poly(methyl acrylate) or PMA. These starch-PMA films shrink in a controllable manner when exposed to certain relative humidities. By controlling the composition of the starch-PMA film, we can control the amount of shrinkage at a given relative humidity. We also demonstrated that the shrinkage properties are controlled by the starch component. Potential applications for these films include "smart" packaging, actuators, and sensors. These results demonstrate the potential of producing functional starch-based materials which utilize the inherent properties of starch. These starch-based copolymers offer new market opportunities for starch and improve the economic competitiveness of U.S. agriculture.
Technical Abstract: Blown films prepared from starch-poly(methyl acrylate) graft copolymers plasticized with urea and water display shrinkage at relative humidities greater than 50%. Shrinkage at relative humidities below approximately 75% is strongly correlated with the urea/starch weight ratio, which controls the equilibrium moisture content in the films. Above 75% relative humidity, film shrinkage is essentially independent of composition. At relative humidities below 90%, films plasticized with urea and water exhibited greater shrinkage than films plasticized with water only. A master curve can be constructed by shifting shrinkage data with respect to a reference relative humidity, indicating that relaxation processes in the starch phase control film shrinkage. This conclusion is confirmed by the fact that shrinkage data for a wide range of compositions and relative humidities fall on a single curve when plotted against moisture content. Polarized Fourier Transform infra-red spectroscopy indicated some loss of orientation in the starch phase during shrinkage. These results demonstrate that the hydrophilic nature of starch can be exploited to develop responsive polymers which display controllable shrinkages activated by increases in relative humidities.