Properties of pullulan-based blend films as affected by alginate content and relative humidity

► Mechanical properties of pullulan–alginate films are affected by polymer blend ratio and water activity. ► Glass transition deceases as water activity increases. ► We use FTIR to probe different water molecules exist in pullulan–alginate films as water activity changes. Pullulan–sodium alginate blend films were prepared and characterized as a function of water activity (aw). At low aw, the incorporation of alginate into pullulan film increased the tensile strength and elastic modulus, but decreased the elongation at break of the composite films; the opposite trends were observed at elevated aw. Above 0.43 aw, water exerted a typical plasticization effect upon the biopolymer blends. As aw increased from 0.23 to 0.43, an anti-plasticization effect was observed as tensile strength and elastic modulus increased. The glass transition temperature of all samples decreased substantially as aw increased from 0.23 to 0.84 due to the plasticization effect of water. Within this aw range, one transition temperature was observed for all film specimens. The stretching vibration band of O–H was investigated using attenuated total reflection Fourier transform infrared spectroscopy to identify the various species of water interacting with the polysaccharide films.