Towards the toughness-strength balance of poly(vinyl alcohol) films via synergic plasticization

Polyvinyl alcohol (PVA) films have significant rigidity due to the large number of hydrogen bonds within and between molecular chains. This feature hinders their industrial applications. Small molecule plasticizers can effectively improve the toughness of PVA films. However, this improvement decreases their modulus and strength. In this study, the effect of individual plasticizer, namely, diethanolamine (DEA), glycerol (GLY), and H2O, on the mechanical properties was first studied. It was found that adding an individual plasticizer increase the toughness but decrease the modulus for all the three plasticized PVA films, with DEA decreased least. Then, the property changes caused by of H2O and plasticizer content in two synergic plasticizing systems of DEA/H2O and GLY/H2O were systematically investigated and compared to demonstrate further the different effects of DEA and GLY on the toughness and modulus of the PVA film. Results indicated that the modulus and toughness of PVA casted film with a 10% DEA content improved after the addition of 3% H2O for the DEA/H2O plasticizing system. Despite a large improvement in toughness, the strength was always largely reduced with the increase in H2O or GLY content for the GLY/H2O system. The infrared spectrum results showed that H2O could be physically combined with both PVA and DEA to construct a cross-linked hydrogen bond network, because of the relatively strong electronegativity of the N atom in DEA. However, forming such a cross-linked hydrogen bond network in the GLY/H2O system is difficult. Thus, only a plasticizing effect was observed in the PVA/GLY/H2O films. Our work provides a better understanding of the effects of plasticizer and H2O on the hydrogen bonding and properties of PVA films to achieve toughness-strength balance via synergic plasticization.