An attempt to mechanistically explain the viscoelastic behavior of transparent epoxy/starch-modified ZnO nanocomposite coatings

The effects of bare and starch-modified ZnO (ZnO-St) nanoparticles on viscoelastic and mechanical properties are studied by dynamic mechanical and tensile analyses. Transparent epoxy-based nanocomposite films are prepared by incorporating bare or starch-modified ZnO particles into the epoxy matrix. The results demonstrated that ZnO particles hindered the curing reactions and hence the final properties of the cured epoxy. As a result, glass-transition temperature (T-g) and crosslinking density demoted. However, starch as a surface modifier compensated for the undesired effects of ZnO in a way that by enhancing the curing reactions through autocatalytic mechanism, T-g and crosslinking density increased. The storage moduli for epoxy, epoxy/ZnO and epoxy/ZnO-St are accordingly as 13.84, 3.95 and 19.54 MPa. Therefore, the molecular weight between the entanglements is calculated as 0.2878, 1.0089 and 0.2039 in the same order. Moreover, considering the peaks of the tanS diagrams, T(g)s for epoxy, epoxy/ZnO and epoxy/ZnO-St are obtained as 95.95, 100.16 and 101.24 degrees C, respectively. Comparing epoxy/ZnO-St nanocomposite to epoxy, it can be inferred that the network becomes tougher in the elastic region and then becomes softer passing this region. Mechanistic sketches of epoxy network formation in the presence of bare and surface-treated nanoparticles are discussed.