Fabrication and characterisation of metal-doped pectin films

Metal-doped pectin films have been fabricated and their thermal, mechanical and microstructural properties were examined by means of complementary physicochemical techniques. Films were fabricated at two pH values, 2.0 and 7.0, with inclusions of metals (Na+, K+, Ca2+, Mg2+ and Al3+) and conditioned in a range of relative humidity environments. Glass transition temperatures (Tg) of water-plasticised films ranged between 54 and 95 °C. Treatment of Tg values with Gordon-Taylor empirical model revealed a spectacular increase (∼25 °C) of the Tg of dry films at pH 2.0 and with the addition of metals. Uniaxial extension measurements revealed that, at pH 2.0, films were stronger with lower extensibility in contrast to their counterparts prepared at pH 7.0. All films were microstructurally inspected and revealed a continuous one-phase microstructure at length scales >100 μm with no significant differences in the surface topography. Changes of the physical properties of films have been attributed to the modulation of the intermolecular interactions that are influenced by the degree of ionisation of carboxyl groups (pH), electrostatic interactions (inclusion of cations), and conformational reorientation of pectin chains. Overall, it has been shown that it is possible to engineer biopolymer films for a range of applications depending on the desired operating environment.