Humidity-induced structural transitions in amylose and amylopectin films

The effect of humidity on the dynamic mechanical and barrier properties and on the microstructure of amylose and amylopectin films was studied. The films were solutiongel-cast, with and without glycerol as added plasticizer, and dried to equilibrium weight under various relative humidities (RH). The network microstructure of glycerolplasticized amylose films was shown to change from dense and homogeneous to a more open structure with fluctuations in the pore size when the surrounding RH was increased. The structural change was attributed to plasticization of the amorphous areas by increased water content, leading to higher mobility in the network and subsequent inhomogeneous swelling. This structural change had a direct influence on the oxygen permeability, which drastically increased. The onset of the increase occurred at lower surrounding RH for the films containing glycerol, and these films also had higher oxygen permeability. Glycerol-plasticized amylopectin films with gradually increased crystallinity were prepared by varying the RH during film formation. Dynamic mechanical analysis at both varied temperature and RH showed that the increased crystallinity led to higher glass transition temperature, Tg, and a smaller effect of the surrounding humidity on mechanical and barrier properties. The RH during the formation of glycerol-plasticized amylose films did not affect crystallinity or Tg. These films formed at 70% RH were shown to have a marked heterogeneous network.