Humidity‐Responsive Starch‐Poly(methyl acrylate) Films

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 (MC) 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 MC. Polarized Fourier transform infrared 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. magnified image