Amplifying the Mechanical Resilience of Chemically Cross-Linked Poly(vinyl alcohol) Films with the Addition of Boric Acid

Mechanical behavior of the cross-linked polymer system is modulated by various cross-linkings, i.e., chemical and physical cross-linking. The poly(vinyl alcohol) (PVA)–boric acid (BA) cross-linking system is known to exhibit cross-linking-induced stiffening behavior. In this study, the step-cycle mechanical tests reveal that the addition of BA not only enhances the chain modulus but also amplifies the mechanical resilience within a BA concentration range of 0.1–2.0 wt %. This structural origin is attributed to the effect of BA on the interpenetrated network consisting of (i) a crystallite-based network and (ii) a chain network in the amorphous domain. For the crystallite-based network, the lamella thickness of crystallites remains nearly constant at middle BA addition levels (0.1 wt % < cBA < 2.0 wt %), whereas the selective destruction of (100) and (200) planes over (101)/(101̅) planes is observed, as evidenced by synchrotron radiation X-ray scattering (wide- and small-angle X-ray scattering). This preserves the skeleton of the crystallite-based network. Various time-domain nuclear magnetic resonance techniques, i.e., 1H T2 relaxometry and 1H double-quantum NMR, quantitatively present the linear relationship between cross-linking density and modulus in the amorphous domain. These findings contribute to the understanding of the deformation mechanism of the PVA–BA cross-linking system during loading–unloading tests.