Anisotropic and Mechanically Strong Polyvinyl Alcohol-Tannic Acid Hydrogel

Polyvinyl alcohol-tannic acid (PVA-TA) hydrogels with significant anisotropic structures and high mechanical strengths are prepared by the combination of directional freezing-thawing (DFT) and freezing-thawing under drawing (FTD) methods．Scanning eletron microscope (SEM) and polarized optical microscope observations confirm that the hydrogels have oriented porous structures and crystalline structures, X-ray diffraction (XRD) measurements prove that the addition of TA and the orientation treatments lead to the decrease of the crystallinity of the hydrogels, while Fourier transform infra-red (FTIR) characterizations prove that the orientation treatments are conducive to enhancing the hydrogen bonding interactions in the hydrogels．Tensile testing shows that the hydrogels doubly oriented with DFT and FTD methods have obvious anisotropic mechanical properties, whose tensile strength and elastic modulus are significantly higher in the direction parallel to the orientation direction than those in the perpendicular direction, and their mechanical properties are also significantly better than those of the hydrogels without any orientation treatments or with only one orientation treatment by DFT．The tensile strength and elastic modulus of the hydrogels increase with the increase of drawing ratio．The presence of TA also endows the hydrogels with excellent antibacterial properties, and the antibacterial ratio is up to 100%．