Synthetic hydrogels or water-containing polymeric materials are much inferior to biological tissues and solid plastics in many aspects of mechanical properties; it is a great challenge to develop hydrogels with mechanical properties comparable with or even superior to those of biological tissues and plastics. Here, we report a type of super-strong and tough hydrogen-bonded poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hydrogel by immersing as-prepared PVA hydrogels in aqueous PAA solutions and then cold-drawing the hydrogels to different strains. The immersing process introduces PAA chains into the PVA hydrogels, which increases the cross-linking density by hydrogen bonding and hence, much improved mechanical properties and low water contents (35.9–40.2 wt%) are observed. The cold-drawing orients the polymer chains, which enables the formation of more and stronger hydrogen bonds. The mechanical properties of cold-drawn gels are dramatically enhanced, with tensile strength and elastic modulus up to 140 and 100 MPa, respectively; also, super-high toughness (117 MJ m⁻³) and fracture energy (101 kJ m⁻²) are obtained. Very impressively, the ultra-high tensile strengths of the cold-drawn hydrogels are superior to those of biological tissues and most solid engineered plastics. Characterizations and comparative studies prove that the enhancement of mechanical properties is mainly due to the formation of more hydrogen bonding rather than the loss of water or the change in crystallinity. This study provides a new strategy for preparing super-strong physically cross-linked hydrogels and other polymeric materials. This super-strong and tough hydrogel may find potential applications in biomedical and load-bearing fields.

Fabrication of hydrogels with unique microstructures and better mechanical properties through the self-assembly of commercially available synthetic polymers and small molecules is of great scientific and practical importance. A type of physical hydrogels is prepared by the self-assembly of polyvinyl alcohol (PVA) and tannic acid (TA) in aqueous solution with a low PVA–TA concentration (0.5–6.0 wt %) at room temperature. With the increase of the PVA–TA concentration, the water content of the hydrogels increases, and the content of TA in the hydrogels decreases from 23.1 to 6.4%. The driving force for the self-assembly is proven to be the hydrogen bonding between PVA and TA, which also induces the crystallization of PVA chains. The self-assembled PVA–TA hydrogels have diverse morphologies that change from microspheres to oriented porous structures with the increase of the PVA–TA concentration, and these structures are all composed of nanosized particles, fibers, and/or sheets. Most of the self-assembled PVA–TA hydrogels show good mechanical properties. The highest tensile strength and elastic modulus of the PVA–TA hydrogel prepared with 1.0 wt % PVA–TA concentration are about 84 and 30 kPa, respectively. This self-assembly method would lead to the fabrication of more hydrogels with unique microstructures and properties for practical applications.

A panchromatic hybrid crystal of anionic iodoplumbate nanowires and J-aggregated protonated naphthalene diimides has been formed through charge-assisted anion–π and lone pair–π interactions.

Biological hydrogels usually possess anisotropic structures to achieve maximum functionalization at a low material cost. Unfortunately, most conventional synthetic hydrogels are intrinsically isotropic in microstructures and hence in properties. Here, we report a freezing–thawing under drawing (FTD) method to prepare poly(vinyl alcohol) (PVA) hydrogels with an anisotropic microstructure and significantly enhanced mechanical properties. Drawing of an as-prepared PVA hydrogel leads to the orientation of PVA crystallites and PVA chains, which enables the formation of more and even cooperative hydrogen bonding via freezing under drawing. The PVA hydrogels prepared with the FTD method are much stronger than the corresponding PVA hydrogels prepared with a common freezing–thawing (FT) method. For instance, after only one cycle of FTD at a drawing ratio of 100%, the tensile strength of the PVA hydrogel is dramatically increased, about 3.6 times that of the original hydrogel. Both the tensile strength and elastic modulus of the PVA hydrogels increase significantly with the increase of FTD cycles, drawing ratio and PVA content, and they are higher in the direction parallel to the drawing direction than the perpendicular direction, showing significant mechanical anisotropy. For the PVA hydrogel prepared with a PVA content of 16 wt% and via freezing at a drawing ratio of 200%, its tensile strength and elastic modulus in the parallel direction reach 5.00 MPa and 0.50 MPa, respectively. This work provides a simple and convenient strategy to design anisotropic hydrogels with significantly enhanced mechanical properties for potential applications in biomedical materials and soft machinery.