Synthesis of layered double hydroxide/poly(N-isopropylacrylamide) nanocomposite hydrogels with excellent mechanical and thermoresponsive performances.

Nanocomposite (NC) hydrogels of positively charged layered double hydroxide (LDH) single-layer nanosheet (SLNS) cross-linked poly(N-isopropylacrylamide) (PNIPAM) were synthesized. Especially, the LDH SLNSs used here were pre-synthesized via an aqueous synthetic route without using organic solvents and modifiers. The obtained LDH/PNIPAM NC hydrogels were characterized using XRD, SEM, TEM, and DSC. The mechanical and thermoresponsive properties were determined using tensile, compression, and swelling/deswelling tests. Interestingly, different network structures are observed for the NC hydrogels along the horizontal and vertical directions; those along the horizontal direction exhibit a fine and uniform sponge-like network structure while those along the vertical direction exhibit a hierarchical layered architecture. Compared with the conventional N,N'-methylene bisacrylamide cross-linked PNIPAM hydrogel, the NC hydrogels exhibit extraordinary deformability and stretchability and obviously improved thermoresponsive swelling/deswelling characteristics. Furthermore, the fracture elongation observed here is obviously higher than those reported for negatively charged clay/PNIPAM NC hydrogels. With the increase in the LDH content from 0.8 to 2.0 wt%, the fracture strength and the compressive strength at an 85% strain increase from 23.5 to 37.2 kPa and from 0.15 to 0.57 MPa, respectively, while the fracture elongation decreases from 2689 to 2202%. The mechanism for the improved mechanical performances of the NC hydrogels is discussed. To the best of our knowledge, this is the first report on LDH/PNIPAM hydrogels. This work provides a green synthesis route for LDH-containing NC hydrogels. The new NC hydrogels may have great potential applications such as in tissue engineering, drug vehicles, and sorbents.