Construction of Dual Orientation Crystalline Structure in Poly(vinyl alcohol)/Graphene Oxide Nano-Composite Hydrogels and Reinforcing Mechanism

Oriented poly­(vinyl alcohol) (PVA)/graphene oxide (GO) nanocomposite hydrogels with different draw ratios were prepared by stretching during a freezing/thawing process. The PVA/GO samples with a draw ratio of 200% had significantly enhanced mechanical properties by formation of a microfibrillar structure, while the tensile strength and compressive modulus respectively reached as high as 11.60 and 4.05 MPa, outperforming the unoriented samples with an increase of 167% and 540%, respectively. During the freezing/thawing process under stress, the orientation factor, crystallinity, and cross-linking density (υe) of PVA/GO samples increased steadily, while the regularly arranged fibrillar bundle structure increasingly formed, and the size of the porous structure decreased. Besides that, additional PVA crystals formed with decreasing long period (L), crystalline region (L c), and lamellae size (L lateral). Due to the synergic orientation of GO sheets and nucleation effect, the oriented PVA/GO samples showed an orientation factor, crystallinity, and υe that were higher with lower crystal size in comparison with PVA samples. Thus, the dense dual orientation crystalline network structure formed, and the highly reinforcing PVA/GO hydrogels were achieved.