A Strong Bio‐Inspired Layered PNIPAM–Clay Nanocomposite Hydrogel

Abstract

The demand for robust hydrogels in pharmaceutical, biomedical, and industrial applications has motivated intense research efforts in these wet polymeric materials. Recent advances have resulted in different strategies for creating highly stretchable polymeric hydrogels, including sliding-ring gels (SR gels), nanocomposite gels (NC gels), doublenetwork gels (DN gels), macromolecular microsphere composite gels, and tetra-poly(ethylene glycol) gels. Among them, NC gels have attracted significant interest because of the simplicity of synthesis, high transparency, impressive mechanical properties, large reversible deformation, and excellent swelling and stimuli sensitivities. The high mechanical properties of NC gels are desirable for practical application in many fields. However, it remains a great technical challenge to improve mechanical properties by enhancing the content of inorganic composition. Only a few percent of inorganic clay can be incorporated into NC gels (< 12 wt% at most), even though special mixing or modification of clay are adopted. Further incorporation of clay leads to opacity and lower mechanical properties than the expected theoretical values resulting from the insufficiency of the nanoplatelet dispersion at high viscosity. As natural inorganic–organic nanocomposites, nacre finds its way around this problem by developing a well-ordered brick-and-mortar microstructure and robust interface. Although mostly made of mineral platelets, nacre possesses an excellent combination of elastic modulus, strength, and toughness [10] and it provides a prime microstructural design model for the development of new materials. Recently, the layered microstructure of nacre has been successfully mimicked and nacrelike hard structure materials with high mechanical performance were fabricated. As for soft, wet inorganic–organic composites, NC gels can also benefit from duplication of the micro-/nanoscale structures of nacre. Herein, inspired by the ordered brick-and-mortar arrangement of inorganic and organic layers in nacre, we first demonstrate layered nanocomposite hydrogel (L-NC gel) films with a high clay content. The perfect micro-/nanoscale layered structure results in excellent mechanical properties higher than that of other reported NC hydrogels. We believe that it could offer innovative insights into the design of robust polymeric hydrogels for practical application. In our experiment, we prepared layered poly(N-isopropylacrylamide)–nanoclay (i.e. PNIPAM-nanoclay) hydrogel films with a nacrelike structure (Scheme 1). In a first step, clay platelets with a diameter of 28.4 nm and a thickness of 1.11 nm, the NIPAM monomer and the initiator were assembled into a lamellar structure by vacuum filtration (Scheme 1b). Then, L-NC gels were synthesized easily by in situ radical polymerization of NIPAM initiated through UV light. After polymerization, none of the hydrogel films dissolved when kept in water for a long time or sonicated, implying that PNIPAM and clay successfully formed a kind of network structure. The structural model for L-NC gels is proposed, as shown in Scheme 1c. The L-NC gel films are highly transparent, almost irrespective of the clay content. Light transmittance measurements show about 50–95% of transparency across the visible spectrum of light (Figure S1 in the Supporting Information). The results indicate that an uniform dispersion of clay is achieved. The L-NC gel films did not distinctly change in transparency by altering the temperature across the lower critical solution temperature of PNIPAM. Probably the thermal molecular motion of the PNIPAM chains is restricted through interaction with a high content of clay platelets. The pure clay film was prepared by vacuum-filtration assembly, indicating a densely well-defined layered microstructure as shown in Figure S2 in the Supporting Information. The L-NC gel shows the overall structure with a strikingly alignment of the clay platelets, which are parallel to the film surface (Scheme 1e and Figure S2 in the Supporting Information). The TEM image further shows well-defined and aligned self-assemblies with alternating hard clay and soft polymer layers (Scheme 1 f). The ordered microstructure was further elucidated by small angle X-ray power diffraction measurements (Figure S3 in the Supporting Information). Relative to the d spacing of 1.26 nm of pure clay, for dried L-NC gels the d spacing increases to 2.2 nm, indicating that the PNIPAM layers are very thin. The structure is thus reminiscent of the brick-and-mortar structure of nacre, [*] Dr. J. F. Wang, Prof. Q. F. Cheng, Prof. L. Jiang Key Laboratory of Bio-inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment, BeiHang University Beijing 100191 (P.R. China) E-mail: cheng@buaa.edu.cn