纳米复合水凝胶的力学行为与刺激响应性

纳米复合水凝胶（nanocomposite hydrogels,NC凝胶）由于其简便的制备方法、独特的组成、优异的力学性能、高光学透明度以及良好的溶胀/去溶胀性等,引起了广泛的关注.近年来,本课题组在NC凝胶的力学行为及刺激响应性NC凝胶的制备方面取得了可喜的研究成果.观察到新合成的和溶胀平衡的聚N-异丙基丙烯酰胺（PNIPAm）-锂藻土LaponiteNC凝胶的超拉伸性,发现了在大应变下的应变硬化现象;发现利用Mooney-Rivlin方程可以描述NC凝胶的压缩应力-应变,但不能描述较大的应变硬化;Creton模型能很好地描述NC凝胶在大形变下的应力-应变曲线,特别是凝胶的应变硬化;从NC凝胶小应变下的平衡剪切模量得到了有效交联密度,观察到NC凝胶形变-回复过程的迟滞现象,测定了NC凝胶的松弛指数.我们认为NC凝胶具有超拉伸性的原因是其较低的交联密度和适度的松弛速率;在响应性NC凝胶的制备方面,发现了溶胶型Laponite的水分散液的稳定窗口,避免了加入离子性单体引起的聚集沉淀;通过共聚制备了具有超拉伸性、pH响应或温度和pH双响应的透明NC凝胶.本文主要综述了我们课题组在NC凝胶力学行为及响应性NC凝胶领域的一些研究进展,并分析了NC凝胶研究领域仍然未解决的科学问题,以及今后可能的发展方向.

Mechanical behavior and stimuli-response of nanocomposite hydrogels

Nanocomposite hydrogels （NC gels） have been intensively studied up to now among all the high performance hydrogels due to the facile preparation, unique composition, and excellent properties of mechanical, optical, and swelling/deswelling. We have achieved some significant results in the mechanical behavior of the NC gels and the synthesis of stimuli-response NC gels during several years. Ultrahigh extensibility was found from the as-prepared and swelling-equilibrated poly（N-isopropylacrylamide） （PNIPAm）-hectorite Laponite NC gels with strain hardening at large strain region. Mooney-Rivlin function was found available to describe the stress-strain behavior under compression, but failed to predict the high strain hardening. Creton model was adopted to interpret the stress-strain curves for the NC gels under large deformation well, especially the strain hardening in these gels. For the NC gels, the effective cross-linking density was estimated from the equilibrium shear modulus under small strain, the hysteresis was observed during a deformation-recovery cycle, and the relaxation exponent was determined based on the relaxation modulus. According to these results, we suggest that the ultrahigh extensibility of the NC gels is originated from low cross-linking density and moderate relaxation. In respect of stimuli-response NC gel preparation, a stability window was found from aqueous Laponite suspensions for NIPAm copolymerization with ionic monomers to avoid flocculation and precipitation of Laponite platelets. In this way, the pH response and pH/temperature dual response homogeneous NC gels with ultrahigh extensibility were successfully synthesized. Finally, the unsolved scientific problems concerning the NC gels and the potential applications in the near future were discussed.