Comprehensive effects of metal ions on responsive characteristics of P(NIPAM-co-B18C6Am)

Comprehensive investigations of the effects of species and concentrations of metal ions on the ion-responsive behaviors of poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (P(NIPAM-co-B18C6Am)) are systematically carried out with a series of P(NIPAM-co-B18C6Am) linear copolymers and cross-linked hydrogels containing different crown ether contents. The results show that when the B18C6Am receptors form stable B18C6Am/M(n+) host-guest complexes with special ions (M(n+)), such as K(+), Sr(2+), Ba(2+), Hg(2+), and Pb(2+), the LCST of P(NIPAM-co-B18C6Am) increases due to the repulsion among charged B18C6Am/M(n+) complex groups and the enhancement of hydrophilicity, and the order of the shift degree of LCST of P(NIPAM-co-B18C6Am) is Pb(2+) > Ba(2+) > Sr(2+) > Hg(2+) > K(+). With increasing the content of pendent crown ether groups, the LCST shift degree increases first and then stays unchanged when the B18C6Am content is higher than 20 mol %. Remarkably, it is found for the first time that there exists an optimal ion-responsive concentration for the P(NIPAM-co-B18C6Am) linear copolymer and cross-linked hydrogel in response to special metal ions, at which concentration the P(NIPAM-co-B18C6Am) exhibits the most significant ion-responsivity either in the form of linear copolymers or cross-linked hydrogels. With an increase of the content of crown ether groups, the value of corresponding optimal ion-responsive concentration increases. Interestingly, there exists an optimal molar ratio of metal ion to crown ether for the P(NIPAM-co-B18C6Am) copolymer in response to Pb(2+), which is around 4.5 (mol/mol). If the ion concentration is too high, the ion-responsive behaviors of P(NIPAM-co-B18C6Am) may even become surprisingly unobvious. Therefore, to achieve satisfactory ion-responsive characteristics of P(NIPAM-co-B18C6Am)-based materials, both the operation temperature and the ion concentration should be optimized for the specific ion species. The results in this study provide valuable guidance for designing and applying P(NIPAM-co-B18C6Am)-based ion-responsive materials in various applications.